escape tectonics along major shear zones in western...

FACULTY OF SCIENCES

Master of Science in geology

Academic year 2015–2016

Master’s dissertation submitted in partial fulfillment of the requirements for the degree of Master in Science in Geology

Promotor: Prof. Dr. J. De Grave Jury: Prof. Dr. P. Van den haute, Prof. Dr. D. Vandenberghe

Escape tectonics along major shear zones in Western Thailand induced by India-Asia

convergence: constraints from basement thermochronology

Cai-Hua Yao

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ACKONWLEDGEMENTS

Hereby I would like to express my gratitude to all those that have supported me along the writing

process of this Master’s dissertation.

First of all and foremost, I would like to thank my thesis promotor prof. Dr. Johan De Grave for giving

me the opportunity to work on this thesis project and guiding me through it. The time he has spent

to read and re-read my manuscript, the constructive feedback and the help with the many questions

and concerns I had, certainly improved the quality of my thesis dissertation. I am also very grateful to

him for his understanding and patience in difficult times.

I am also grateful to Dr. Kerry Gallagher who has taken the time to introduce me to the QTQt-

programme and its application and underlying principles. Also, Elien De Pelsmaeker is thanked for

her persistent enthusiasm and advice whenever I had questions during the microscopic analyses.

Thanks should be mentioned to Ann-Eline Debeer for her assistance during the sample preparations

and to Jan Jurceka for cutting the whole rocks, which allowed me to further crush the samples. I

would also like to gratitude the entire MINPET-group for using their laboratories and equipment.

Special thanks goes to the staff of the SCK-CEN for the irradiation of the samples.

I am also indebted to prof Dr. Johan De Grave and Dr. Stijn Glorie for the fieldwork they have

performed and the samples they provided to me that made this thesis project possible.

Last but not least, I am grateful to my sister and boyfriend for their emotional support and they have

always encouraged me to go on with writing this thesis. I also thank my sister for reading the

manuscript and for her linguistic advice.

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TABLE OF CONTENTS

CHAPTER 1: INTRODUCTION ......................................................................................................... 1

CHAPTER 2: THE GEOLOGY OF THAILAND ...................................................................................... 3

2.1 GENERAL EVOLUTION ...................................................................................................................... 3

2.2 TECTONIC AND STRUCTURAL HISTORY ............................................................................................ 5

2.2.1 The Paleozoic and Mesozoic evolution of Sibumasu and Indochina..................................... 5

2.2.2 The Cenozoic evolution of Western Thailand ....................................................................... 8

2.2.2.1 Evolution of the Mae Ping and Three Pagodas fault zones ...................................... 9

2.2.2.2 Ductile shear zones and exhumation ..................................................................... 10

2.2.3 The Cenozoic evolution of peninsular Thailand .................................................................. 14

2.3 INDIA-EURASIA COLLISION ............................................................................................................ 15

2.4 IGNEOUS HISTORY ......................................................................................................................... 16

2.4.1 Granite Provinces ................................................................................................................ 16

2.4.1.1 Emplacement ages.................................................................................................. 18

2.4.2 I-Type versus S-type granites .............................................................................................. 18

CHAPTER 3: THE APATITE FISSION TRACK DATING METHOD ........................................................ 19

3.1 FISSION TRACKS ............................................................................................................................. 19

3.1.1 Definition, structure and formation .................................................................................... 19

3.1.2 Revelation ............................................................................................................................ 20

3.1.3 Apatite fission tracks ........................................................................................................... 23

3.2 THE FISSION TRACK DATING METHOD .......................................................................................... 23

3.2.1 Fundamental and practical age equation ............................................................................ 23

3.2.2 The thermal neutron fluence .............................................................................................. 25

3.2.3 Dating procedures and techniques ..................................................................................... 26

3.2.4 The Zeta-calibration method ............................................................................................... 27

3.2.4.1 Principles ................................................................................................................ 27

3.2.4.2 Apatite age standards ............................................................................................. 29

3.2.5 Applicability of the fission track dating method ................................................................. 29

3.3 THE THERMAL STABILITY OF FISSION TRACKS ............................................................................... 30

3.3.1 Annealing kinetics and Arrhenius diagram .......................................................................... 31

3.3.2 The effects of chemical composition .................................................................................. 33

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3.3.3 The effects of crystallographic orientation ......................................................................... 33

3.3.4 Annealing models ................................................................................................................ 34

3.3.5 The QTQt-modelling software ............................................................................................. 35

3.4 APATITE FISSION TRACK THERMOCHRONOLOGY .......................................................................... 36

3.4.1 General aspects ................................................................................................................... 36

3.4.2 The closure temperature concept ....................................................................................... 37

3.4.3 The Apatite Partial Annealing Zone concept ....................................................................... 38

3.4.4 Geological interpretation of apatite fission track ages ....................................................... 40

3.4.4.1 Cooling through denudation .................................................................................. 40

3.4.4.2 Apatite fission track ages along horizontal profiles ............................................... 41

3.4.4.3 The influence of topography .................................................................................. 42

CHAPTER 4: SAMPLES AND METHODOLOY .................................................................................. 43

4.1 OVERIEW OF THE SAMPLES ........................................................................................................... 43

4.2 SAMPLE PREPARATION .................................................................................................................. 45

4.2.1 Separation of apatite ........................................................................................................... 45

4.2.2 Preparation for irradiation .................................................................................................. 46

4.2.3 Irradiation ............................................................................................................................ 48

4.2.4 Preparation after irradiation ............................................................................................... 48

4.3 APATITE FISSION TRACK ANALYSIS ................................................................................................ 48

4.3.1 Counting procedure............................................................................................................. 48

4.3.1.1 Zeta-calibration ...................................................................................................... 49

4.3.2 Length measurements ......................................................................................................... 50


4.4.1 Modelling with QTQt ........................................................................................................... 51

CHAPTER 5: RESULTS .................................................................................................................. 52


5.1.1 Counting procedure............................................................................................................. 52

5.1.1.1 The glass monitor interpolation curve ................................................................... 52

5.1.1.2 ζ-calibration factor .................................................................................................. 53

5.1.1.3 Apatite fission track ages ........................................................................................ 56

5.1.2 Length measurements ......................................................................................................... 59

5.1.2.1 Track length calibration .......................................................................................... 59

5.1.2.2 Track length distribution of apatite samples .......................................................... 59


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5.2.1 Thermal history models ....................................................................................................... 60

CHAPTER 6: INTERPRETATION AND DISCUSSION ......................................................................... 64


6.1.1 Track length calibration ....................................................................................................... 64

6.1.2 Track length distribution ..................................................................................................... 64

6.2 THE CENOZOIC STRUCTURAL HISTORY OF NORTH-WESTERN THAILAND ..................................... 64

6.2.1 Comparison with previous apatite fission track studies ..................................................... 66

6.3 THE CENOZOIC THERMAL HISTORY OF WESTERN THAILAND ....................................................... 68

6.3.1 Comparison with previous apatite fission track studies ..................................................... 71

6.4 IMPROVEMENTS ............................................................................................................................ 73

CHAPTER 7: CONCLUSION ........................................................................................................... 74

REFERENCES ............................................................................................................................... 76

APPENDIX .................................................................................................................................. 85

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CHAPTER 1: INTRODUCTION

Thailand is bordered by Myanmar, Laos, Vietnam and Cambodia in the north and by Malaysia in the

south. Together with these countries, the basement rocks of Thailand form the continental core of

Southeast Asia, which is tectonically-geodynamically referred to as Sundaland. During the Cenozoic,

this tectonic entity was located in the convergence zone between the Indian-Australian, the

Philippine and the Eurasian plates. Moreover, in the Early Eocene India and Eurasia collided and

resulted in a persistent indentation of India into Eurasia. This ongoing indentation induced strike-slip

movements (or escape tectonics) along the Mae Ping and Three Pagodas shear zones in Western

Thailand. The strike-slip movements were associated with the exhumation of granitic rocks. These

granitoids form important basement rocks of Thailand and were intruded during the Mesozoic due to

the closure of the Tethys Ocean. Consequently, the several granitic rocks that were exhumed along

the Mae Ping and Three Pagodas fault zones, provide deeper insights into the Meso-Cenozoic

(structural) evolution of Thailand and hence the evolution of mainland Southeast Asia.

The study area of this thesis project is mainly situated in Western Thailand along the Mae Ping and

Three Pagodas fault zones, where several granitic rocks were sampled. The granitic rocks are then

used to date the exact timing of the basement exhumation by means of the apatite fission track (AFT)

dating method. Additionally, AFT length distributions are constructed by performing AFT length

measurements to establish the mean track lengths and standard deviations. These AFT data are then

inserted in the QTQt thermal modelling programme to reconstruct the thermal history of the

samples. On the basis of these results, the aim of this thesis project is to reconstruct the general

geodynamic evolution of the study area within the absolute time frame of the India-Eurasia

convergence. As previous studies by Upton et al. (1997), Upton (1999) and Morley et al. (2007) were

also situated in our study area, the results will thus be used for comparison. Recently the MINPET

group of the Ghent University has also performed AFT studies on Thailand. Among these, the thesis

projects of Blomme (2013) and De Clercq (2016) can be mentioned, which were focused on the

Khlong Marui and Ranong faults of peninsular Thailand. Also this thesis project is part of this larger

project. And as the faults in this study show a similar evolution as the ones of Blomme (2013) and De

Clercq (2016), the results can also be compared to their results and vice versa.

This thesis dissertation is composed of seven chapters. The first chapter, this introduction, is

followed by a detailed overview of the tectonic, structural and igneous history that are relevant to

Chapter 1: Introduction

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our study area and constitute the second chapter. The third chapter is devoted to the basic principles

of the apatite fission track method and its application as a thermochronological tool. An overview of

the samples is given in chapter four that also outlines the preparation steps prior to the analysis as

well as the analysis procedure. The results of the analyses are then given in chapter five and are

interpreted and discussed in chapter six. Finally, the seventh chapter summarizes the important

findings of this thesis research.

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CHAPTER 2: THE GEOLOGY OF THAILAND

In order to make interpretations on certain geological features (strike-slip faults) and distribution of

rock-types occurring in Thailand (granitic and gneissic rocks), it is of major importance to understand

its geological history. Therefore this chapter will provide a detailed overview of the tectonic,

structural and igneous history of Thailand, more specifically of Northern and Western Thailand to

which our study area belongs.

2.1 GENERAL EVOLUTION

Geographically Thailand belongs to Southeast Asia along with Vietnam, Laos, Cambodia, Myanmar,

Peninsular Malaysia, Sumatra, Java and part of Borneo. This assembly of continental blocks and their

associated continental shelves form the large geological entity named Sundaland (Metcalfe, 2011;

Ridd et al., 2011) (Fig. 2.1). In addition, two tectonic terranes are of importance to this work: This is

(1) the continental block of Indochina and comprises Vietnam, Laos, Cambodia and the eastern part

of Thailand and (2) the Sibumasu block that covers the western part of Thailand (Barber et al., 2011;

Metcalfe, 2011; Ridd et al., 2011) (Fig. 2.2). Both terranes were derived from the northern margin of

Gondwana and drifted northwards at different times (Ridd et al., 2011), resulting in the opening of

multiple Tethyan ocean basins in the Late Paleozoic, Mesozoic and Cenozoic (Metcalfe, 2013). The

Sukhothai Zone and the suture zones located in between the aforementioned blocks (Fig. 2.2) reflect

the subduction and closure of the Paleotethys Ocean, which were accompanied by granite plutonism

of either S- or I- type (Barber et al., 2011; Ridd et al., 2011). A period of non-deposition, deformation,

uplift and erosion immediately followed whereafter the sedimentation resumed. In the Cretaceous a

deformation event intensifying into the Paleogene was associated with a phase of granite plutonism

(Ridd et al., 2011). Morley et al. (2011) and Searle & Morley (2011) interpreted both events as the

eastward subduction beneath the margin of Sundaland. In addition to the Cretaceous plutonism

dextral strike-slip movements occurred on the Ranong and Khlong Marui faults (Fig. 2.2). The latter

was considered as the southwards continuation of a strand of the Three Pagodas Fault (Fig.2.2) (Ridd,

2012). In the Eocene-Oligocene ductile sinistral strike-slip movements appeared on the Three

Pagodas Fault as well as on the Mae Ping Fault (Fig. 2.2) and were linked to the India-Eurasia collision

(Morley et al., 2011). By the Oligocene, an east-west extension took over with the development of

north-south grabens and half-grabens. Later on, recent tectonic movements appeared on a number

of faults (Ridd et al. 2011).

Chapter 2: The geology of Thailand

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Figure 2.1: Topography and plate motions (black arrows) of Southeast Asia (Metcalfe, 2011).

Figure 2.2: Major structural elements of Thailand. Orange oval indicating the study area. XXXX representing tectonic lines or suture zones. MYF: Mae Yuam Fault; MPFB: Mae Ping Fault Belt; TPFB: Three Pagodas fault Belt; TMF: Tha Mai Faut; RF: Ranong fault; KMF Khlong Marui Fault; BRS Bentong Raub Suture (Ridd et al., 2011).


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2.2 TECTONIC AND STRUCTURAL HISTORY

2.2.1 The Paleozoic and Mesozoic evolution of Sibumasu and Indochina

The evolution of the several crustal blocks that make up Sundaland goes back to the Precambrian

and Lower Paleozoic times, when they were still in close proximity and attached to the northern

margin of the supercontinent Gondwana (Barber et al., 2011). During the Latest Silurian, this margin

underwent rifting and oceanic spreading opened the Paleotethys Ocean separating North China,

South China and Indochina from Gondwana in the Early to Middle Devonian (Metcalfe, 1996a;

Metcalfe, 1996b, Metcalfe, 2013). Later on, South China and Indochina fused to form Cathaysia

(Gatinsky et al., 1984), a continent that laid north of Gondwana (Fig. 2.3a). The Early Permian

subduction of the Paleotethys beneath the southern margin of Cathaysia (Indochina) generated the

Sukhothai Volcanic arc and its associated accretionary complex. However, continued subduction led

to back-arc spreading, which opened the Nan-Back arc Basin and separated the Sukhothai Arc from

Indochina (Barber et al., 2011; Metcalfe, 2013) (Fig. 2.3b, c & Fig. 2.4a).

By the mid-Carboniferous to Late Permian, Gondwana experienced extensional tectonics and rifts

opened along the north-western margin of Australian Gondwana (Eyles et al., 2003). The subsequent

broadening of these rifts, filled with glacial deposits, eventually initiated the opening of the

Mesotethys Ocean and the separation of the Cimmerian continent, comprising Sibumasu at its east

(Sengor et al., 1988) (Fig. 2.3c). This oceanic basin expanded and provoked three subsequent events

being: the northwards movement of Sibumasu towards Cathaysia (Metcalfe, 2011; Metcalfe, 2013)

and the further subduction of the Paleotethys Ocean, followed by the later subduction of Sibumasu

beneath the Sukhothai Arc in the Late Permian (Fig. 2.3c). The subduction of the advancing

continental margin sediments of Sibumasu were first thrusted beneath the accretionary complex (Fig.

2.4b), resulting in thrust and fold structures that detached along decollement surfaces due to the

prevailing stresses. The detachment of the thrust and fold structures, in turn, produced a nappe that

thrusted back over Sibumasu and gave rise to a foreland fold-and-thrust belt. Eventually, Sibumasu

collided with the Sukhothai Arc in the Triassic and the foreland fold-and-thrust belt subducted

beneath the overriding plate. This forced the accretionary complex to overthrust the belt and

thereby created an ophiolite nappe (Inthanon Zone) consisting of remnants of the Paleotethyan

ocean floor (Fig. 2.4c). Afterwards the ophiolite nappe was largely removed by uplift and erosion

(Barber et al., 2011; Metcalfe, 2013).

During the Triassic the northern stretch of the Paleotethys Ocean subducted southwards beneath

Indochina resulting in its final closure (Cai & Zhang, 2009). Meanwhile, the simultaneous collapse of


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Figure 2.3: Plate reconstructions for the Late Paleozoic until Middle Mesozoic (Barber et al. , 2011).


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the Nan-Marginal Basin, induced by the Sibumasu-Sukhothai Arc collision, generated the Nan-

Uttaradit Suture and pronounced the amalgamation between Sibumasu and Indochina (Sone &

Metcalfe, 2008; Barber et al., 2011) (Fig. 2.3d & Fig. 2.4c). This in turn produced the Indosinian

Orogeny (Barber et al., 2011). Nevertheless, the definite timing for the amalgamation of Sibumasu

and Indochina is controversial with proposals of Late Permian to Early Triassic (Metcalfe, 1996b;

Metcalfe, 2000; Barber & Crow, 2009) and Late Triassic (Kamata et al., 2009).

Figure 2.4: Schematic illustration for the tectonic evolution of Sibumasu and Indochina during the Paleozoic and Mesozoic (Barber et al., 2011).


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In addition to the final tectonic evolution of the Sibumasu Block, it was suggested that the West

Burma Block accreted on its western margin. Nonetheless, it was disputed whether this collision

occurred during the Cretaceous involving movements on the strike-slip Mae Ping Fault in western

Thailand (Charusiri et al., 1993) or whether the collision took place in the Latest Permian to Earliest

Triassic by strike-slip movements, which emplaced West Burma and West Sumatra adjacent to

Sibumasu (Barber & Crow, 2008; Barber & Crow, 2009) (Fig. 2.3d). Subsequently, during the Mid-

Cretaceous the offshore oceanic island arc of Mawgyi thrusted over and accreted to the western

margin of Sibumasu as a result of the eastward subduction of the intervening Cenotethys Ocean

(Mitchell et al., 2007) (Fig. 2.4d & Fig. 2.5).

Fig. 2.5: Plate reconstructions for the Cretaceous. EM= East Malaya, I= Indochina, L= Lhasa, PA = Incipient East

Philippine arc, S = Sibumasu, SC = South China, SWB = Southwest Borneo, WB = West Burma, WSu = West Sumatra (Metcalfe, 2013).

2.2.2 The Cenozoic evolution of Western Thailand

After a long history (400 Ma) of continental rifting and plate tectonic convergence, collision and

amalgamation, Thailand was finally constructed on the northern edge of Sundaland (Morley et al.,

2011; Metcalfe, 2013). From then onwards, the region was dominated by Cenozoic deformation due

to its position in the convergence zone between the Indian-Australian, the Philippine and the

Eurasian plates (Simons et al., 2007) with a south-eastwards movement relative to Eurasia (Searle &

Morley, 2011) (Fig. 2.1). Earlier, a similar scenario was proposed by the continental extrusion model


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also known as the continental escape tectonic model (Molnar & Tapponnier, 1975; Tapponnier &

Molnar 1977). This model explained the Cenozoic structural evolution of Southeast Asia as a result of

the India-Asia collision whereby a south-eastward extrusion of continental blocks occurred. More

specifically, the extrusion was induced by the persistent northward indentation of India into Eurasia.

It was assumed that the extrusion operated by major strike-slip faults (Mae Ping, Three Pagodas)

involving large offsets and that the continental blocks behaved rigidly. But based on the much

smaller offsets along the faults, the disappearance of the faults towards the south and the non-

coincident timing of the strike slip motion and India-Asia collision, Morley (2002), Green et al. (2008)

and Hall et al. (2008) argued that the model was impossible. However, the escape tectonic model still

works to some extent but for sure cannot be seen as the sole mechanism responsible for the

extrusion and extensional tectonics affecting the Thai basement during the Cenozoic. Therefore, this

thesis research will attempt to gain more insights in the structural evolution of the region and

especially provide age constraints on the crustal movements.

The Cenozoic structural style in the north-western part of Thailand was dominated by strike-slip

faulting and extension (Morley et al., 2011). Many authors (Bunopas, 1981; O’Leary & Hill, 1989;

Polachan et al., 1991; Kozar et al., 1992; Booth 1998; Morley et al, 2004; Ridd, 2009) recognized that

these Cenozoic structures were influenced by the inheritance of older fabrics and as such the suture

zones (Nan-Uttaradit, Chiang Mai), detachment zones (Inthanon Zone) and terrane boundaries

(eastern boundary of the Phuket terrane) of Thailand were reactivated as they acted as the

preferential location for strike-slip activity throughout younger events (Morley et al., 2011).

2.2.2.1 Evolution of the Mae Ping and Three Pagodas fault zones

The Early Cenozoic deformation in Western Thailand was responsible for the development of the

NW-SE to north-south trending Mae Ping and Three Pagodas fault zones. Dependant on the sense of

motion on the NW-SE trends, the north-south trends were either placed under compression or

extension and hence acted as restraining or releasing bends, respectively (Morley et al., 2011).

During the Paleogene the sense of motion along these faults was sinistral and characterized by

ductile, transpressional deformation (Lacassin et al., 1993; Lacassin et al., 1997; Morley, 2004;

Morley, 2007). The origin of these transpressional fault systems was possibly related to the closure of

the Neotethys Ocean during the Paleogene (see section 2.4.1) but remained controversial. In

addition, the sinistral transpressional motions along the Mae Ping Fault Zone (MPFZ) at the

restraining bend resulted in the development of the outstanding north-south trending Chainat

Duplex (Morley et al., 2011) (Fig. 2.6). From the Late Oligocene-Early Miocene onwards, the north-

south trending Mae Ping and Three Pagodas fault strands were no longer restraining bends but


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instead basins were formed at releasing bends, reflecting a reversal towards brittle dextral motion in

a transtensional regime. The relatively minor dextral strike-slip activity was considered as the

consequence of the India-West Burma coupling and their subsequent northward movement relative

to Sundaland (Morley et al., 2011). This brittle dextral strike-slip deformation was associated with a

lateral extrusion of crustal blocks in Southeast Asia. In Central and Northern Thailand, dextral strike-

slip activity decreased and gave way to east-west extension forming rift basins bounded by normal

faults, while the dextral strike-slip activity became focused upon the Sagaing Fault in Myanmar.

However, the east-west extension reactivated the Nan-Uttaradit Suture zone along the NE-SW

Uttaradit Fault Zone from the Oligocene to Recent. During the Late Miocene to Early Pliocene,

extension decreased and alternated with inversion events due to the varying stress field derived

from the nearby Himalaya Orogen and active Indian-Australian plates (Morley et al., 2011).

Eventually, the dextral strike-slip faults were reactivated during the Late Miocene to Recent in a

transpressional regime (Rhodes et al., 2005).

2.2.2.2 Ductile shear zones and exhumation

The ductile sinistral shear initiated possibly around the Late Cretaceous-Early Paleogene and

overprintend the Indosinian orogeny-related fabrics by mylonitization (Morley et al, 2011), resulting

in ductile shear zones. For example, the Thabsila metamorphic complex (Fig. 2.6) along the Three

Pagodas Fault Zone (TPFZ) contains mixed linear and schistose tectonics indicative of the ductile

sinistral strike-slip deformation (Lacassin et al., 1997; Morley, 2002). Similarly, ductile shear sense

indicators were found in the Lan Sang mylonitic gneisses along the MPFZ (Lacassin et al., 1993;

Lacassin et al., 1997). Later, the Thabsila metamorphic complex was brought up from mid-crustal

levels to shallow depths by means of an exhumation process induced by sinistral ductile strike-slip

motions along the TPFZ in a transpressional regime. This exhumation occurred between 36 and 32

Ma (Late Eocene-Oligocene) and was interpreted as indicating the end of sinistral shearing along the

TPFZ (Nantasin et al., 2012). During the reversal of motion from sinistral to dextral strike-slip faulting

around 24 Ma (Late-Oligocene) (Lacassin et al., 1997), the ductile fault core was further exhumed

and was accompanied by a lateral extrusion of crustal blocks in Southeast Asia as mentioned in the

previous paragraph. Later on, the brittle dextral strike-slip faulting decreased or even ceased due to

the stress fields derived from the nearby Himalaya Orogen and active Indian-Australian plates that

resulted in an increasing east-west extension that alternated with inversion events (Morley et al.,

2011). During the Late Miocene to Recent, the Himalaya Orogen extended and hence reactivated the

dextral strike-slip movements whereby the ductile fault core exhumed and finally exposed by

denudational processes, i.e. by weathering and erosion (Nantasin et al., 2012). A similar exhumation


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history was established along the MPFZ with cooling ages of 33-30 Ma (Lacassin et al., 1997) and 24-

19 Ma (Upton et al., 1997), which were interpreted in a same way (Morley, 2002). Nevertheless, the

exhumation history of ductile shear zones along the MPFZ was much more complicated as shown by

their varying range of cooling ages. As such, cooling ages northwest and southeast of the Lan Sang

ductile fault core (Fig. 2.6) appeared to become older (Eocene-Cretaceous) compared to the cooling

ages within the ductile shear zone (Oligocene). It was considered as a regional and unusual

exhumation event related to the evolution of the Chainat Duplex (Morley et al., 2007). Figure 2.7

shows the evolution of the Chainat Duplex and can be explained as follows: during the Eocene (50-40

Ma) the Umphang Gneiss was rapidly exhumed at the restraining bend of the sinistral MPFZ, while

the Lan Sang Gneiss was located east of it. A progressive simple shear along the MPFZ provoked a

translation of the Khlong Lhan Gneiss within the restraining bend from its southern end towards its

northern end as well as a translation of the Lan Sang Gneiss. In the meanwhile, the exhumed and

relatively strong Umphang gneiss acted as an anvil against which the Lan Sang Gneisses were uplifted,

flattened and sheared during the Middle Eocene to Early Oligocene (40-30 Ma). As a result, the Lan

Sang Gneisses display retrograde metamorphism that reflect the uplift during the progressive shear

at 31 Ma (Oligocene) (Lacassin et al., 1997). This sequence of events created the Chainat Duplex,

whose geometry was then modified during the Early Miocene (22-18 Ma) by a short phase of minor

sinsitral motion that uplifted the Khlong Lhan Gneisses within the duplex (Morley et al. 2007).

Anyhow, the apatite fission track data from Upton (1999) revealed a long-term slow exhumation

during the Cretaceous-Early Cenozoic in eastern, central and northern Thailand, and a more rapid

Oligocene-Early Miocene uplift in western Thailand.


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Figure 2.6: Map of Thailand showing the distribution of Mesozoic rocks and structural

features (Morley et al., 2011).


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Figure 2.7: Schematic representation of the evolution of the Chainat Duplex (Morley et al., 2007).


14

The exhumation of ductile shear complexes along strike-slip faults is a quite complex mechanism and

involves uplift in order to expose the ductile fault cores at the surface. For the exhumation of the

fault cores along the MPFZ or TPFZ, either an independent uplift mechanism or a significant

component of pure shear along with the sinistral shear deformation is required. The component of

pure shear can originate from the geometry of the shear zone, such as restraining or releasing bends.

Hence, the compressional stresses at the restraining bend could cause an uplift and erosion of the

ductile shear zone (Fig. 2.8a), while the transtensional stresses at the releasing bend could remove

the cover rocks (Fig. 2.8b). Another source of pure shear can result from transpressional and

transtensional movements along the fault zone (Fig. 2.8c,d). In the case that the ductile core was not

completely exhumed during the ductile shear activity, a two-stage exhumation mechanism (Fig. 2.8e)

could take place. The first stage of this process involves one of the mechanisms mentioned above.

While in the second stage a reactivation of the fault zone results in brittle faulting that disrupts the

ductile shear complex and brings it to the surface as slivers (Morley et al., 2011). This two-stage

exhumation mechanism is recognized along the MPFZ and TPFZ as described in the previous section.

Figure 2.8: Mechanisms for the exhumation of a ductile shear complex (Morley et al., 2011).

2.2.3 The Cenozoic evolution of peninsular Thailand

In this paragraph the structural and thermal evolution of the Khlong Marui (KM) and Ranong (RN)

fault zones will be described, because they show a similar evolution with the Mae Ping and Three

Pagodas fault. According to Watkinson et al. (2008) both faults were considered as conjugate faults

of the Mae Ping and Three Pagodas faults. The findings of this thesis work will therefore be

compared to those of the literature concerning the fault zones in peninsular Thailand. But as this


15

does not belong to our study area, the evolution of the KM and RN faults will only be discussed

briefly.

According to Ridd et al. (2011) and Barber et al. (2011), the KM and RN faults developed during the

Mesozoic (Late-Jurassic) as a result of the subducting Neotethys Ocean beneath the western margin

of Sundaland (West Burma and Western Thailand). Both faults were accompanied by ductile dextral

movements during the Late Cretaceous-Early Paleogene (72-56 Ma) whereafter they became inactive.

In the Late Eocene-Oligocene (52-23 Ma) the faults were reactivated in a brittle sinistral regime,

which was caused by extensional tectonics related to the subduction of the Sunda Trench (Fig. 2.1).

From the Late Oligocene-Early Miocene (23 Ma) the brittle sinistral motions along the KM and RN

faults ceased and reversed to brittle dextral motions. Subsequently, the Miocene-Pliocene period

was characterized by the opening of the Andaman Basin (Fig. 2.1) that initiated around 11 Ma and

reactivated the faults again (Watkinson et al., 2008).

Apatite fission track studies along the KM and RN faults from Blomme (2013) and De Clercq (2016)

yielded ages around 30-16 Ma and 33-11 Ma (Oligocene-Miocene), respectively.

Thermochronologically, their studies revealed that the crystalline basement rocks of peninsular

Thailand experienced a three-phase thermal history: First they were rapidly cooled and hence

exhumed by the strike-slip movements during the Early Cenozoic until the Oligocene (30-25 Ma).

Afterwards, during the Oligocene (30-25 Ma) and Middle Miocene (13-12 Ma) a tectonically quiet

period without activity along the faults prevailed. It was only during the Middle Miocene-Pliocene

(13-5 Ma) that the faults were reactivated by the opening of the Andaman Basin. It was accompanied

by a base level drop induced by erosion and resulted in a rapid cooling of the crystalline basement

rocks. Hence, these rocks were exhumed and brought to the surface where they finally got exposed

by further denudational processes. This exhumation event is still active at the present day.

2.3 INDIA-EURASIA COLLISION

From the previous sections, it is obvious that the role of the India-Eurasia collision cannot be

neglected as it plays an important role in the Cenozoic geodynamical evolution of Asia as a whole,

and of Thailand in particular. This continent-continent collision was responsible for the rise of the

Himalaya-Tibet Orogen, by crustal shortening and thickening. The subsequent continued

convergence was largely accommodated by the lateral extrusion of Southeast Asian crustal blocks

along major strike-slip faults (e.g. extrusion of Indochina along the Mae Ping and Three Pagodas

faults in Thailand) (Yin, 2010; Chatterjee et al., 2013).


16

Several observations established the timing of the India-Eurasia collision at around 57-50 Ma

(Paleocene-Eocene) (Yin, 2010; Searle & Morley et al., 2011). As such, the decelerated northward

translation of India around 57-55 Ma (Klootwijk et al., 1992; Acton, 1999) was considered as the

result of the initial collision with Eurasia (Chatterjee et al., 2013), while the closure of the Neotethys

Ocean around 51-50 Ma (Rowley, 1998; Zhu et al., 2005; Green et al., 2008) was regarded as the

direct consequence of the collision. Nevertheless, the exact timing of the India-Eurasia collision

remains controversial. For example, the proposed collision age of 57-55 Ma was refuted by Aitchison

et al. (2007) based on paleomagnetic reconstructions showing a significant distance between Eurasia

and India at that time. These researchers argued that these ages were linked to the collision of

Greater India with an intra-oceanic island arc, namely the Kohistan-Ladakh Arc (Fig. 2.5b). This

collision was thought to represent the first contact with Eurasia and was referred to as the soft

collision (Ali & Aitchison, 2008; Chatterjee et al., 2013). The actual continent-continent collision or

hard collision, on the contrary, was assumed to take place much earlier around 35 Ma when the

Neotethys Ocean completely subducted beneath the southern margin of Eurasia (Aitchison et al.,

2007; Ali & Aitchison, 2008). Though, this last assumption of Aitchison et al (2007) and Ali & Aitchison

(2008) is not supported by many other researchers (Yin, 2010).

2.4 IGNEOUS HISTORY

2.4.1 Granite Provinces

Throughout the geological history of Thailand various phases of plutonism occurred and resulted in

the emplacement of three roughly north-south linear belts known as the Western, Central and

Eastern Granite Provinces. Each province was characterized by its own distinctive batholiths, plutons,

pegmatites and granitic units, which were associated with tin-mineralizations (Cobbing, 2011). The

distribution of the Granite Provinces is shown in figure 2.9 and their characteristics, origin and

emplacement will be explained in a chronological order in the following paragraph.

As previously mentioned (see section 2.2.1), during the Permian, the Paleotethys Ocean subducted

beneath Indochina whereby the Sukhothai Volcanic Arc and Sibumasu accreted. This collision

resulted in the generation and emplacement of I-type granitoids of the Eastern Granite Province

(Barber et al., 2011). As a consequence the granites of the Eastern Province are typically all of I-type

affinity, Permo-Triassic in age and largely confined to Southeast Thailand (Cobbing, 2011).

Additionally, the initial 87Sr/86Sr ratios of the granites are indicative of a relatively small crustal

influence. Finally, during the Triassic, the Paleotethys Ocean closed entirely and invoked the collision

between Sibumasu and Indochina. This brought a new phase of granite plutonism with the

emplacement of S-type granitoids of the Central Granite Province (Cobbing, 2011). As can be seen on


17

figure 2.9, the Central Province is divided into the undeformed Main Range Granites and strongly

deformed North Thai Granites. Only the granites of the North Thai Province are relevant in this study

and are largely dominated by migmatites with a characteristic banded texture and subhorizontal

foliations. It is due to this banded texture, that the rocks were named granite gneisses (Cobbing,

2011). Dunning et al. (1995) dated and interpreted this deformation event as caused by the crustal

extension during the Late Cretaceous-Early Miocene. Unlike the granites of the Eastern Province, the

granites of the North Thai Province have a very clear crustal origin (Cobbing, 2011). The Late

Cretaceous plutonism was responsible for the emplacement of I- and S-type granitoids of the

Western Granite province. They were attributed to the subduction of the Neotethys and subsequent

collision of the Mawgyi Nappe with the western margin of Sibumasu (Western Thailand). This

subduction had already begun in the Mid-Cretaceous and resulted in an overthrusting of the Mawgyi

nappe with compression and thickening of the crust. Hence, the rising pressures, temperatures and

volatile components induced partial melting of the continental crust of Sibumasu and thereby

generated the I- and S-type granitoids in the Late Cretaceous-Paleocene (Barber et al., 2011). In

contrast to the North Thai Granites, the granites of the Western Province have low initial 87Sr/86Sr

ratios that reflect a mantle signature (Cobbing, 2011).

Figure 2.9: Map showing the three Granite Provinces and tectonic units of Thailand.

White dots indicate the location of the analysed samples in this study (Cobbing, 2011).


18

2.4.1.1 Emplacement ages

Insights on the timing of the emplacement of the different granite provinces as discussed above was

mainly obtained by using several isotopic dating methods. In this way, U-Pb zircon dating generally

provided Triassic ages between 248 and 224 Ma for the granites of the Eastern Province (Cobbing,

2011) reflecting the timing of the Indosinian orogeny. Salam et al. (2007) found a similar age of 244 ±

1 Ma by using the Re-Os method. The Permo-Triassic ages of the gneissic granites from the North

Thai Province were affirmed by Cobbing et al. (1992) and Dunning et al. (1995). They established a

Rb-Sr age of 269 ± 18 Ma and 241 ± 9 Ma, and U-Pb age of 205 Ma, respectively. The Rb-Sr method

was also predominantly used to date the emplacement of the Western Granite Province. Thereby,

Cobbing et al. (2011) achieved Cretaceous ages ranging between 114 ± 10 Ma and 79 ± 4 Ma. These

ages represent the closure of the Neotethys Ocean and the subsequent collision of the offshore

island arc of Mawgyi with the western margin of Thailand (Sibumasu). More specifically, they indicate

the approaching Indian plate towards Thailand.

2.4.2 I- type versus S-type granites

Two types of granites were generated throughout the igneous history of Thailand: I-type and S-type

granites. They differ from one another in that they originate from another kind of granitoid precursor

(Frost et al. 2001). As such, I-types granites are defined as the derivation of melted igneous source

rocks, while S-type granites are thought to originate from melted sedimentary rocks (Searle et al.,

2012). Generally, I-type granites are typically metaluminous and characterised by low initial 87Sr/86Sr

ratios and the occurrence of hornblende, biotite and titanite. On the contrary, S-type granites are

peraliminous and are mainly marked by the presence of muscovite, biotite, garnet and tourmaline.

Additionally, high silica contents and high initial 87Sr/86Sr ratios are found in the S-type granites.

Moreover, I-type granites are associated with magmatism in a subduction zone at an Andean-type

continental margin, while S-type granites are related to post-collisional settings (Cobbing, 2011;

Searle et al., 2012). Thus, the occurrence of I- and S-type granites provides implications on the

geodynamical setting wherein the granites were emplaced.

19

CHAPTER 3: THE APATITE FISSION TRACK DATING METHOD

Apatite fission track dating is a widely used and successful technique that is used to understand the

thermal structure of the upper crust (Garver, 2008) and constitutes the main methodology used in

this thesis. This chapter will thus be devoted to the basics of that particular dating technique and

mainly the work of Wagner and Van den haute (1992) will be used as a theoretical framework.

3.1 FISSION TRACKS

3.1.1 Definition, structure and formation

Nuclear fission is a type of radioactive decay where the unstable parent isotope disintegrates by

splitting into two daughter fragments. The fission reaction is restricted to the heavy nuclides of the

actinide series (Z ≥ 90 and A ≥ 230). These heavy nuclides can be unstable and disintegrate resulting

in a spontaneous fission, while the bombardment with particles (neutrons, protons) or irradiation

with γ-rays can result in induced fission. Since the parent nuclide splits into two fission fragments

with unequal mass and atomic number, the fission reaction is binary and asymmetric. In most cases,

the daughter fragments are unstable and will thus further disintegrate to stable daughters by β-

emission. Besides the emission of β-particles, several neutrons are emitted and a large amount of

energy (~210 MeV) is released, mostly in the form of kinetic energy. The neutrons can produce new

induced fission reactions, in turn, leading to a nuclear chain reaction on which the nuclear reactor is

based. Due to the kinetic energy (~170 MeV) of the two fission fragments and their strong positive

charge, they repel each other and thereby travel in opposite directions at high velocities through the

host medium. Consequently, each fission fragment will create a narrow linear trail of damage along

its trajectory in a solid medium or detector (mineral, glass, plastic) and eventually the trails combine

at the place of the fissioned parent isotope to result in a latent fission track. Although, several

models exist for the formation of a fission track, only the most widely accepted Ion Explosion Spike

model from Fleischer et al. (1975) will be explained here. The model states that in a first stage, the

strong positively charged fission fragment causes an ionization and thus creates an array of positively

charged ions along its trajectory. It is attributed to the abduction of electrons from the lattice atoms

by the fission fragment (Fig. 3.1a). In a second stage, the positive ions along the trajectory repel each

other by electrostatic forces, forming vacancies and interstitials (Fig. 3.1b). The last stage is

characterized by an elastic relaxation that distributes the local stress more evenly (Fig. 3.1c).

However, the kinetic energy of the fission fragment will be lost along the trajectory and hence its

velocity decreases due to the interaction with the detector. This is known as the stopping power of

Chapter 3: Apatite fission track dating method

20

the solid medium and influences the distance travelled by the fission fragment, which is referred to

as the range of the fission fragment. The length of the fission track is defined by the range of both

fission fragments and depends on the composition of the detector and energy of the fission

fragments. Generally, the length of fission tracks varies between less than 1 µm to several mm, while

the width is about a few nm. As a consequence, they are submicroscopic and invisible under an

optical microscope. These tracks are known as latent tracks and only reveal when etched with a

chemical agent. The revelation technique will be discussed in section 3.1.2.

Figure 3.1: The Ion Explosion Spike Model (Wagner & Van den haute, 1992).

There are four major naturally occurring nuclides, being 232Th, 234U, 235U and 238U, that allow a

spontaneous fission in terrestrial materials. Among these, 232Th, 234U and 235U have too long

spontaneous half-lives and too low abundances that are unable to cause a significant amount of

fission tracks in the terrestrial materials. Thus, this means that nearly all the fission tracks are

produced by the spontaneous fission of 238U (Table 3.1).

Table 3.1: Abundances, total half-lives and spontaneous fission half-lives of the four major naturally occurring nuclides that allow spontaneous fission (Wagner & Van den haute, 1992). *Geochemical average

Relative abundance Total half-life Spontaneous fission half-life (compared to 238U) (years) (years)

232Th 4* 1.40 x 1010 1.0 x 1021 234U 5.44 x 10-5 2.46 x 105 1.5 x 1016 235U 7.25 x10-3 7.04 x 108 1.0 x 1019 238U 1 4.47 x 109 8.2 x 1015

3.1.2 Revelation

Latent tracks are invisible under an optical microscope. Chemical etching offers the possibility to

reveal and observe them with an ordinary optical microscope. The revelation technique that is most


21

widely used is where the detector is etched with an appropriate chemical agent for a significant

amount of time. Since, fission tracks are actually damage zones and hence lattice defects, they form

the preferential site for the chemical attack. As a result, part of the detector material is removed and

thereby the fission track is enlarged. Practically, chemical etching affects the size (width and length)

and shape of the fission tracks. As such, the etchable length of the latent fission track is smaller

compared to the combined range of the two fission fragments and this difference is termed the

range deficit. That is because both fission fragments travel through the detector and thereby interact

with it. As a result their energy will decrease and hence the fission fragments will have an energy that

is smaller than the registration threshold. As a consequence, the ends of the fission fragments are

unetchable (Fleischer et al., 1975) (Fig. 3.2). In addition, the shape of the fission tracks is dependent

on the etching conditions (type of etchant, concentration of the etchant, temperature, etch time)

and nature of the detector. The track channel is the most diagnostic feature of a fission track. Where

the track channel intersects the etched crystal surface, an etch pit occurs. In some cases, the etch pit

has a characteristic geometry that represents the symmetry and crystallographic orientation of the

etched crystal, giving the fission tracks a funnel shape (Fig. 3.3).

Figure 3.2: The energy scheme along the trajectory of a fission fragment through a solid medium. The etchable range or etchable length (Re) of the latent fission track is defined by the registration threshold and the type of detector. The difference between the actual range of the latent track (Rl) and Re is the range deficit, which is the reason that the ends of the fission fragments cannot be revealed by chemical etching (Jonckheere, 1995).

Unlike surface tracks that are connected with the etched surface, confined tracks are entirely located

within the detector and do not intersect the etched surface. Nevertheless, they are revealed as well

because they become etched when surface tracks or cleavages within the crystal traverse them.

Therefore, they are also referred to as TINTs (Track IN Tracks) or TINCLEs (Tracks IN CLEavage)

(Bhandari et al., 1971) (Fig. 3.4). Confined tracks are particularly important in thermochronological

purposes, where measurements of their length are used to establish length distributions, which are


22

then interpreted to gain information on the thermal history of the sample (see section 3.4). On the

other hand, surface tracks are relevant in the age determination of a sample and can be combined

with the fission track length data of confined tracks to reconstruct the thermal history (see section

3.3.5). In uranium poor or young samples, however, the spontaneous track densities are too low to

measure enough confined tracks for the construction of track length distributions. In these cases, the

samples can be irradiated with heavy ions (Jonckheere et al., 2007; Min et al., 2007) or with a 252Cf-

fission source that create host tracks. Thereby, the possibility that the etchant will reveal confined

tracks will be increased.

Figure 3.3: Funnel shaped fission tracks that display their

narrow channels and hexagonal etch pits on the basal plane in apatite (Wagner & Van den haute, 1992).

Figure 3.4: Illustration of confined tracks. (a) etching of confined tracks through surface tracks (semi-tracks)

or through a cleavage plane within the crystal surface. (b) horizontal confined track that is revealed by a crack in apatite from Fish Canyon tuff (Colorado, U.S.A.) (after Wagner & Van den haute, 1992).


23

3.1.3 Apatite fission tracks

Apatite (Ca5(PO4)3(F, Cl, OH)) is one of the most frequently used minerals in fission track analyses,

because it is an accessory mineral that commonly occurs in sedimentary rocks, granites and

metamorphic equivalents (Garver, 2008). Regularly, uranium replaces the calcium atoms in the

apatite lattice in the order of 1 to 100 ppm (De Grave, 2003) and hence it decays to form fission

tracks in apatite. The latent apatite fission tracks generally have diameters between 5 to 10 nm (Paul

& Fitzgerald, 1992) and are revealed by treating them with HNO3. Here, the apatites are etched with

a HNO3-solution of 2.5 during 70 seconds at temperatures of 20 °C. Gleadow et al. (1986) stated that

etched induced apatite fission tracks have observed length of 16.3 µm on average.

3.2 THE FISSION TRACK DATING METHOD

3.2.1 Fundamental and practical age equation

The radioactive decay of an unstable parent isotope to a stable daughter isotope at a fixed rate,

known as the decay constant, forms the basic principle of dating methods. For example, a mineral

containing uranium such as apatite, will accumulate spontaneous fission tracks at a fixed rate λf due

to the spontaneous fission of 238U. Similarly, 238U can also disintegrate by α-emission to 206Pb at a

fixed rate λα. However, the fission track dating method differs from other dating methods, as it

measures the spontaneous fission tracks instead of daughter isotopes (Garver, 2008). Besides the

number of spontaneous fission tracks, the actual number of 238U atoms is required as well to

establish a fission track age. In order to do that, the mineral is irradiated in a nuclear reactor with a

fluence of thermal or slow neutrons (see section 3.2.2). The irradiation induces fission of the 235U

atoms of the mineral and gives rise to a number of induced fission tracks. Because of a constant

235U/238U ratio in nature, the induced tracks from the 235U are an indirect measure for 238U as well.

Subsequently, the fission track age of the mineral can be calculated by determining the thermal

neutron fluence as well as the ratio of the number of spontaneous tracks to the number of induced

tracks. This finally yields the fundamental age equation (Price & Walker, 1963) and is given by:

t =1

λα ln (

λα

λf Ns

Ni Iσфth + 1) 3.1

where: λα = disintegration constant for α-decay of 238U (a-1)

λf = the disintegration constant for spontaneous fission of 238U (a-1)

Ns = number of spontaneous tracks per unit volume (cm³)

Ni = number of induced tracks per unit volume (cm³)

I = natural 235U/238U-ratio


24

Фth = thermal neutron fluence to which the sample is exposed (neutrons/ cm²)

σ = conventional effective cross-section for fission of 235U induced by thermal neutrons.

It is specific to a nuclear reaction with a particular neutron energy and corresponds

to the ratio of the number of neutrons that effectively produces a fission reaction

per unit of time to the thermal neutron fluence.

The number of spontaneous and induced fission tracks is derived by a microscopic counting of both

on an etched and polished surface of the mineral. This means that the number of tracks is expressed

as surface densities and not as units of volume. The theoretical relationship between the planimetric

(cm²) and volumetric (cm³) fission track densities is given by the following equation (Wagner & Van

den haute, 1992):

ρs,i = gs,iNs,i(Re)𝑠,𝑖ηs,iqs,i 3.2

The subscripts s and i refer to the spontaneous and induced fission tracks, respectively.

Implementing equation 3.2 in the Ns/Ni-ratio of the fundamental age equation gives (Wagner & Van

den haute, 1992):

Ns

Ni=

ρs

ρi

giRe,iηiqi

gsRe,sηsqs 3.3

Consequently, the geometry ratio (G) and procedure factor (Q) can be defined (Wagner & Van den

haute, 1992):

G = gi

gs 3.4

Q = Re,iηiqi

Re,sηsqs 3.5

where: gs,i = geometry factor, depends on the registration method of the fission tracks and can

either be 1 (internal surface) or 0.5 (external surface)

Rs,i = average etchable range of the fission track

ηs,i = etching efficiency factor, refers to the fact that not all fission tracks can be revealed

by etching and is defined by the angle between the surface and the fission track

(Fleisher & Price, 1964; Fleischer et al., 1975)

qs,i = observation factor, depends on the fission track researcher and optical

characteristics of the microscope


25

By using these equations (3.4 and 3.5), the Ns/Ni-ratio can be rewritten as:

Ns

Ni=

ρs

ρi GQ 3.6

Finally, combining equation 3.6 with the fundamental age equation yields the practical age equation

for fission tracks:

t = 1

λα ln (

λα

λf ρs

ρi QGIσфth + 1) 3.7

The procedure factor Q is dependent on the following three factors: the optics of the microscope,

the fission track researcher and the etching conditions that were used to reveal the spontaneous and

induced fission tracks. Note that an accurate quantification of this factor is difficult and that choosing

the appropriate dating procedure can facilitate this process. As such, the population method (see

section 3.2.3) yields a value of 1 and hence the Q-factor can be eliminated from the practical age

equation. Since, the determination of the Q-factor is impossible in the external detector method (Q ≠

1) (see section 3.2.3), one can opt to use age standards (see section 3.2.4). Therefore, the procedure

factor is also known as a calibration factor. Similarly, the choice of the dating procedure influences

the geometry ratio G, whereby the population method produces a value of 1 (Green & Durrani, 1978)

and the external detector method yields a value of 0.5 (Gleadow & Lovering, 1977).

Calculating the fission track age of the mineral by using the practical age equation, requires

knowledge of the microscopically counted spontaneous and induced fission track densities, the

thermal neutron fluence, the procedure factor, as well as the following empirical nuclear constants:

λα = 1.55125 x 10-10 a-1 (Jaffey et al., 1971; Steiger & Jäger, 1977)

λf = 8.46 x 10-17 a-1 (e.g. Galliker et al., 1970)

I = 7.2527 x 10-3 (Cowan & Adler, 1976; Steiger & Jäger, 1977)

σ = 570.8 barn (1 barn = 10-24 cm²) (Wagner & Van den haute, 1992)

3.2.2 The thermal neutron fluence

Thermal neutrons are used to irradiate fission track samples for fission track dating in order to

determine the actual 238U concentration and hence to establish the fission track age as described in

the previous section. These thermal neutrons are often also termed slow neutrons because they

have low energies between 0 and 0.25 MeV (Fig. 3.5) and are thereby capable of inducing a fission of

only 235U. Higher energetic neutrons as fast and epithermal neutrons, on the contrary, can also

induce fission of 238U and 232Th besides the fission of 235U. Fast neutrons have high energies around


26

0.5 and 10 MeV (Fig. 3.5) and are produced by the fission reaction. These fast neutrons are, in turn,

slowed down as a consequence of interactions with the reactor’s moderator (see section 4.2.3.) that

absorbs their energy. As a consequence they are converted into epithermal neutrons, which have

energies intermediate between those of fast and slow neutrons (Fig. 3.5). The further absorption of

energy will finally generate thermal or slow neutrons that are in thermal equilibrium with the

moderator. In other words, the irradiation of fission track samples with thermal neutrons occurs in a

well-thermalized channel of the nuclear reactor. Since, fast and epithermal neutrons can also induce

fission of 235U, the fluence ratio of thermal and epithermal neutrons must be higher than 50 in order

to neglect the influence of fission reactions produced by fast and epithermal neutrons (Wagner &

Van den haute, 1992).

Fig. 3.5 : The energy (E) of thermal, epithermal and fast neutrons in function of the neutron flux, ϕ(E), i.e. the number of neutrons that cross a certain unit area at a given unit of time (De Grave, 2003).

3.2.3 Dating procedures and techniques

Several procedures exist to determine the ratio of spontaneous to induced fission track densities.

The distinction between the different procedures is based on the strategy that is used to count the

induced tracks and can be divided in two groups: single-grain methods and multi-grain methods. In

the former, only one population of grains is used to establish both spontaneous and induced track

densities. The latter, on the contrary, requires two populations of grains where the first is used to

establish the spontaneous track densities and the second for the induced track densities. Therefore,

it is also called the population method. Shortly, in this method the etching, counting and observation

conditions are identical for both spontaneous and induced tracks.


27

The external detector method was described by Price & Walker (1963) and is an example of a multi-

grain method. It is nowadays the most widely used technique in fission track dating and is commonly

associated with the zeta-calibration method (see section 3.2.3). The external detector method owes

its name to the use of an external detector (ED), which has a low to negligible uranium content, such

as plastic foils and muscovite. During the irradiation, the ED is closely attached to the polished and

etched surface of the mineral mount which contains spontaneous tracks. The irradiation produces

fission tracks in the ED that originate from the induced fission of 235U of the mineral mount, since the

ED has a low to negligible uranium content. Moreover, the induced fission tracks can only originate

from depths very close to the sample/ED interface. After irradiation, the ED is etched and then the

induced tracks are counted in the ED, while the spontaneous tracks are counted in the mineral

mount. Consequently, the gi/gs-ratio equals 0.5/1, giving a geometry ratio G of 0.5 (Gleadow &

Lovering, 1977).

3.2.4 The Zeta-calibration method

The zeta-calibration method was established as a result of the disagreement about the value of the

fission decay constant (λf) and the difficulties concerning an accurate quantification of the thermal

neutron fluence (фth). Nowadays, these parameters are well-known or can be determined in a

straightforward manner. Nevertheless, the zeta-method is still widely used. This dating system was

developed by Hurford & Green (1982, 1983) to determine the age of a sample by using a

comparative analysis with age standards. The basic principles of the method were proposed by

Fleischer & Hart (1972).

3.2.4.1 Principles

The zeta-calibration method requires a co-irradiation of age standards and uranium doped glass

monitors in order to define the ζ-calibration factor. This factor is independent from the irradiation

and its determination is explained hereafter.

In a first step, fission track age standards (S) (apatites in this thesis) are irradiated together with a

specific type of glass monitor (IRMM-540 in this thesis). Due to the co-irradiation, induced fission

tracks accumulate in the glass monitor as in the attached ED and are subsequently etched and

counted. This gives an induced fission track density (ρd)S that can be related to the thermal neutron

fluence фth by a proportionality constant B, which is expressed as neutrons/track. The B-factor is

dependent on the type of glass monitor and the conditions of observation and etching (De Grave,

2003). The relation is given by the following equation:


28

фth = B(ρd)𝑆 3.8

Combining this with the practical age equation (3.7) gives the age equation for the age standard (S):

tS = 1

λαln [(

λα

λf) (

ρs

ρi)

SQGIσB(ρd)S + 1] 3.9

Consequently, the ζ-calibration factor can be defined as:

ζ = QIσB

λf 3.10

Incorporating the ζ-calibration factor in equation 3.9 simplifies the age equation for the standard to:

tS = 1

λαln [λα (

ρs

ρi)

SGζ(ρd)S + 1] 3.11

Thus, knowing the age of the standard, the ratio of the spontaneous to induced track densities in the

standard and the induced track densities in the glass monitor enables a determination of the ζ-

calibration factor:

ζ = eλαtS − 1

λα(ρs/ρi)SG(ρd)S 3.12

The ζ-calibration factor is expressed as an age multiplied by a surface (a cm²) and encompasses the

poorly known parameters such as Q, σ and λf. Its value may vary depending on the fission track

researcher, the laboratory conditions, the type of mineral and uranium content of the glass monitor

(Green, 1985; Tagami, 1987; Shin & Nishimura, 1991). In addition, the ζ-values of age standards from

the same mineral can also slightly deviate from each other (Green, 1985). Therefore, it is

recommended to work with a weighted average ζ-factor. When a weighted average ζ-factor is

calculated for a specific age standard, it is referred to as the sample weighted mean zeta (SWMZ).

This way, a SWMZ-value can be obtained for each specific age standard, using the same glass

monitors under the same etching and observation conditions. When the several SWMZ-values are

averaged they produce an overall weighted mean zeta (OWMZ) (Hurford & Green, 1983). Commonly,

the OWMZ-value is used because a good and accurate calibration requires several experiments.

In a second step, the age of the unknown sample (u) is determined by irradiating the unknown

sample together with the same type of glass monitor. Again, induced fission tracks accumulate in the


29

glass monitor. Etching and counting them gives an induced fission track density (ρd)u. The age of the

unknown sample is then given by:

tu = 1

λαln [λα (

ρs

ρi)

uGζ(ρd)u + 1] 3.13

3.2.4.2 Apatite age standards

Age standards are substantial in the ζ-calibration method and that is why they must meet the

following essential requirements (Hurford & Green, 1981): (1) the standard should originate from a

geologically well documented rock formation; (2) a fair amount of the proposed standard should be

present in the rock unit; (3) the standard should be homogeneous in fission track age, which means

that only minerals from one age population may be present; (4) the sample ages should be known

precisely from relative (stratigraphy) and absolute dating methods; (5) the fission track age should

date the formation event instead of a cooling event; (6) no correction for track fading (see section 3.3)

should be required.

The IUGS Subcommission on Geochronology (Hurford, 1990) presented two age standards for apatite:

the Fish Canyon Tuff and Durango. The former is an Oligocene volcanic tuff from Colorado, USA and

was dated by three independent decay systems (K-Ar, Rb-Sr and U-Pb). These dating systems

suggested an age of 27.51 ± 0.60 Ma for the Fish Canyon Tuff (Lanphere & Baadsgaard, 2001). The

Durango apatite is derived from the Durango martite (hematite variety) ore body in the Tertiary

Carpintero volcanic group in Mexico and an age of 31.4 ± 0.3 Ma was obtained from K-Ar dating

(McDowell & Keizer, 1977; Green, 1985).

3.2.5 Applicability of the fission track dating method

The fission track method can be used to date man-made objects as young as the Holocene up to

geological materials as old as hundreds of millions of years. Moreover, the applicability of the

method is determined by the product of the age and the uranium content (in ppm) that is

incorporated in the lattice of the mineral sample. This product is limited by a lower and upper

boundary, meaning that it may not be too low nor too high (Fig. 3.6). In other words, a very young

material generally has too low track densities that will result in a counting process that demands a lot

of time and effort. As a consequence this increases the uncertainty of the determined age. The

uranium content of the sample should therefore be high enough to allow a fission track dating. On

the other hand, a very old sample with a very high uranium content will accumulate too much fission

tracks that will overlap one another, making the counting process impossible. Thus, if one wants to


30

perform a relatively fast and efficient counting of apatite fission tracks, it is imperative that the

apatite minerals have track densities between 105 and 107 tracks/cm² as depicted in figure 3.6.

Additionally, it is recommended that at least thousands of tracks are counted to obtain an accurate

dating.

Figure 3.6 : Applicability of the fission track dating method, which is determined by the uranium content of the

sample and its age (de Grave, 2003).

3.3 THE THERMAL STABILITY OF FISSION TRACKS

From the previous section (3.2.5) it is clear that the fission track age of a sample is dependent of its

uranium content, but also the thermal stability of the tracks plays an important role. For this reason

this section will be devoted to the thermal stability of fission tracks.

As already known, fission tracks represent damage zones within the detector. The damage zones are

formed by the passage of a fission fragment that distorts the crystal lattice and results in a

metastable state of the detector. With time the detector will be restored by the intervention of

external factors. This process is known as the fading of fission tracks and causes a reduction of the

etchable length and/or etching rate and thereby reduces the areal fission track density. In turn, this

will lower the apparent fission track age, so that the sample will be dated too young and hence there

will be no correspondence with the formation age. The phenomenon of track fading reflects the

stability of the fission tracks as well as the fact that the fission track dating method does not meet


31

the condition of a closed system, which is a requirement in radiometric dating systems. Moreover,

the phenomenon of fission track fading forms the base for the geological interpretation of fission

track data.

As previously mentioned, with time and under influence of external factors (mainly temperature) the

detector will be restored and thus fission tracks will fade. These external factors may be hydrostatic

and shock wave pressure, plastic deformation, ionizing radiation, time and especially temperature.

Among these, time and indeed temperature are the most influencing factors and their collective

effect on the fading of fission tracks is generally known as the annealing of fission tracks.

The degree of annealing can be expressed by the retention factor r (l/l0 or ρ/ρ0). This factor is defined

as the reduction in fission track length (l) or the fission track density (ρ) relative to the original length

(l0) or density (ρ0) before annealing. When the retention factor equals 1 (l = l0 or ρ = ρ0 and r= 100%),

all the fission tracks are retained indicating that no annealing occurred. While, a zero retention factor

means that no single track is retained and thus a complete annealing occurred. Retention values in

between, reflect a progressive annealing whereby fission tracks will fade and hence less tracks will be

retained. Commonly, the retention factor is expressed as a length reduction (l/l0), because it is more

precise than the density reduction (ρ/ρ0), which is actually a secondary response to the length

reduction.

3.3.1 Annealing kinetics and Arrhenius diagram

Experimental studies were carried out to understand the kinetics of fission track annealing. These

experiments showed that the thermal influence was of major importance for the reactivation of the

displaced lattice atoms along the fission track. As they overcome the activation energy by the

thermal influence, the lattice atoms returned to their original lattice sites by diffusive processes and

as a consequence the lattice defects were restored. In other words, the annealing of fission tracks

was considered as a diffusion process with first order kinetics and the following equation was derived

(Märk et al., 1973):

ln t = Ea

kT+ ln [−ln (

l

l0)] − lnα0 3.14

where: t = age of sample (expressed in a)

T = absolute temperature (K)

Ea = activation energy for diffusion process (J)

k = Boltzmann constant (= 8.616 x 10-5 eV/K)


32

l/l0 = retention factor, ratio of reduced track length to original length

α0 = material specific resonance frequency

From this equation it is obvious that a linear relationship between ln t and 1/T exists and as a result

equal annealing degrees or equal retention factors form straight lines in an Arrhenius diagram (Fig.

3.7). The slope of these lines becomes steeper from the line representing 100% retention (no

annealing) to the line that represents 0% retention (total annealing) and varies by a factor 2 or 3. The

increase in slope reflects an increase in activation energy, causing a fanning of the annealing lines in

the Arrhenius diagram (De Grave, 2003). The annealing fan was attributed to an artefact resulting

from the superposition of several Arrhenius plots, whereby each plot corresponds with a different

chemical composition of apatite (Green et al., 1985). Since Ea and α0 are material specific, Arrhenius

plots were derived for each type of mineral. These diagrams generally served to plot the

experimental data and were extrapolated over geological time scales. In the case of apatite, the

fission track annealing lines were calibrated towards geological timescales using borehole data.

Figure 3.7: An Arrhenius diagram showing the annealing fan of fission tracks in the Libyan Desert Glass (Wagner & Van den haute, 1992).

The diffusion process with first order kinetics cannot entirely account for the annealing of apatite

fission tracks (AFT) due to an oversimplification. The AFT fading is also influenced by the chemical


33

composition and orientation of the fission track relative to the c-axis of the crystal. Their effects on

the annealing process will be explained in the next two sections.

3.3.2 The effects of chemical composition

Apatite is a mineral that consist of three end members: hydroxy apatite (Ca5(PO4)3(OH)), fluorapatite

(Ca5(PO4)3F) and chlorine apatite (Ca5(PO4)3Cl). Fluorapatite is the predominant rock-forming variant

while hydroxy apatite is very rare in nature. The effects of the chemical composition of apatite on the

annealing properties was studied by Gleadow & Duddy (1981) and Green et al. (1985, 1986). They

stated that the Cl/F or Cl/(F + Cl) ratio determines the annealing degree of AFT. As such, apatites rich

in chlorine are more resistant to annealing than apatites rich in fluorine. Furthermore, the chlorine

content of apatites can be indirectly estimated by using the kinetic parameter Dpar, which depends on

the etching conditions and therefore also on the chemical composition. It is defined as the mean

maximum etch pit diameter of an AFT etch figure parallel to the crystallographic c-axis. A high Dpar is

usually indicative of a high chlorine content.

3.3.3 The effects of crystallographic orientation

According to Green & Durrani (1977) and Donelick (1991), the AFT annealing process is anisotropic,

meaning that the orientation of a fission track relative to the c-axis of the crystal lattice influences

the annealing properties. That is why tracks parallel to the c-axis are longer than tracks perpendicular

to the c-axis, because the latter are less resistant to annealing. In other words, the larger the angle

between the AFT and crystallographic c-axis, the stronger the AFT fading and thus the shorter the

fission track will be. This anisotropy effect appears to become stronger with increasing degrees of

annealing. Due to the development of the elliptical model of Donelick (1991) the anisotropy can be

quantified. In this model, an ellipse is constructed with its maximum and minimum semi-axes,

representing the mean AFT length parallel to the c-axis (lc) and the mean AFT length perpendicular to

the c-axis (la), respectively. Therefrom the equation for the mean etchable AFT length (lθi) at an angle

θi to the crystallographic c-axis can be derived:

lθi= √

sin² θi

la2 +

cos² θi

lc2 3.15

The elliptical model, however, is invalid for mean track lengths below 11 µm, because at higher

annealing degrees an accelerated track length reduction occurs for tracks at high angles to the c-axis.

This problem was solved by the c-axis projection model (Donelick et al., 1999), which is an extension

of the elliptical model and accounts for all annealing degrees.


34

3.3.4 Annealing models

The effects of chemical composition and crystallographic orientation can be accounted for by using

an annealing model. Several thermal annealing models were developed for apatite, whereby those of

Laslett et al. (1987), Ketcham et al. (1999) and Ketcham et al. (2007) are the most widely used. For

this thesis project, the annealing model of Ketcham et al. (2007) is employed.

The empirical model of Laslett et al. (1987) was developed by means of laboratory annealing

experiments on the mono-compositional Durango apatite, using the length retention (r=l/l0) to

describe the annealing of fission tracks. It was established to investigate the annealing fans in

Arrhenius plots and to account for the compositional variation in apatites (De Grave, 2003). In their

model, an initial mean track length parameter of 16.35 µm was used, which was contested by

Ketcham et al. (1999). They stated that this model parameter was too high, since such lengths are

rarely observed in natural samples.

The Ketcham et al. (2007) model is an empirical model and basically uses the Ketcham et al. (1999)

equations to estimate the mean track length and the standard deviation from the track length

distribution. The multi-kinetic Ketcham et al. (1999) equations were defined by experiments in which

apatites with compositional variations were annealed concurrently. Unlike the Laslett (1987) model,

the effects of anisotropy were also included in the annealing model of Ketcham et al. (2007) by

normalizing the track lengths, using the c-axis projection technique. In both Ketcham models, the

effects of compositional variation between different apatites were accounted for and expressed as:

rlr = (rmr−rmr0

l−rmr0

)κ

3.16

where: rlr,mr = reduced length of less annealing-resistant apatites (lr) and more annealing-

resistant apatites (mr)

rmr0, κ = empirical parameters

The kinetic parameters that are used in the Ketcham et al. (2007) model are the chlorine content, the

etch figure dimension (Dpar). When these measurable kinetic parameters are related to the model

parameters using equation 3.16, the annealing kinetics could be estimated.

Moreover, the annealing model of Ketcham et al. (2007) allows a reconstruction of time-temperature

or t-T paths by means of the QTQt-modelling software. These modelled t-T paths, in turn provide


35

deeper insights into the thermal history that enables a geological interpretation of the AFT data (see

section 3.4).

3.3.5 The QTQt-modelling software

The QTQt program is a thermal history modelling software package developed by Gallagher (2012)

and based on the Laslett (1987) and Ketcham (1999, 2007) fission track annealing models. The

programme owes its name to the ability of quantitative thermochronology (QT) and convenient

observation of time-temperature paths (Qt – ‘cute’) from complex datasets. These datasets may

comprise multiple samples as well as data obtained from several dating methods (fission tracks, U-

Th/He, 40Ar/39Ar, vitrinite reflectance) (Vermeesch & Tian, 2014). Both forward and inverse modelling

are implemented. The inverse thermal history modelling in QTQt tries to find a range of acceptable

thermal histories (posterior probability distribution) given the inserted data (AFT lengths and ages)

by means of the Bayesian trans-dimensional Markov Chain Monte Carlo (MCMC) algorithm. In order

to establish the posterior probability distribution, the MCMC sampling approach requires a prior

probability distribution. This prior distribution includes information on time, temperature and

annealing kinetics and is specified by the user by geological constraints and additional information.

The modelled thermal histories are associated with a likelihood function that is a measure of how

well the model fits the data (Gallagher, 2012).

The MCMC approach samples the time-temperature model space through which the thermal

histories must pass. This approach is iterative wherefore ‘burn-in’ and ‘post-burn-in’ series are

defined. The former serves to explore the model space and is thereafter rejected, while the latter is

accepted and serves to fit the model parameters of the posterior distribution as close as possible.

Thus, given a certain amount of iterations, the MCMC approach randomly samples the prior to

generate a model. After each iteration, the model is either rejected or accepted. At the end of all the

iterations, the accepted models are collected to fit the posterior distribution as close as possible. The

decision to accept or reject a model is made by the Bayesian approach that tends to simplify complex

models and can be defined by the acceptance rate criterion, i.e. the number of accepted models with

respect to the total number of proposed models. Acceptance rates for time and temperature around

20-60% are recommended as well as similar acceptance rates for birth (addition of time-

temperature-offset point) and death (removal of time-temperature-offset point). Finally, the output

in QTQt provides a range of acceptable thermal histories (posterior probability distribution) together

with the maximum likelihood (data fit) and expected model (weighted mean posterior distribution)

(Gallagher, 2012). The latter usually forms the basis for thermal history interpretation.


36

3.4 APATITE FISSION TRACK THERMOCHRONOLOGY

3.4.1 General aspects

With the development of fission track annealing models and due to the low-temperature sensitivity

of apatites, AFT data are now increasingly often applied as a thermochronological tool to infer the

thermal history and to make valuable geological interpretations. More specifically, it is the analysis of

confined track lengths that forms the base of AFT thermochronology. In other words, an AFT age is

only meaningful when a fission track length distribution is provided. That is because annealing

shortens the fission tracks, resulting in a lower track density and hence the fission track age is

lowered. This reflects the dependence of the AFT age on the length distribution, which in turn holds

information on the thermal history of the sample.

Inferring the thermal history from an AFT length distribution also requires knowledge of the mean

track length (lm) and standard deviation (σ), as they provide information on the thermal history as

well. For example, when an apatite sample experiences a progressive annealing, the AFT lengths will

shorten. And as they become shorter the standard deviation will increase, generating a broad length

distribution. Theoretically, five types of length distributions can be distinguished (Gleadow et al.,

1986) as shown in figure 3.8. The induced apatite contains freshly induced fission tracks with a large

lm of 16.3 µm and σ of 0.9 µm. The shape of the length distribution is narrow and symmetrically

around lm. This type of samples are rarely observed in nature. The undisturbed volcanic type apatite

experienced a rapid cooling after its formation and is not affected by a later thermal event. The

length distribution is also narrow and symmetrical but centred around a shorter lm of 14.0 to 15.7 µm

with σ between 0.8 and 1.3 µm. The undisturbed basement type apatite is characterized by a slow

cooling and the absence of later thermal events. The slow cooling is reflected by a slightly broader

length distribution with lm varying between 12.5 and 13.5 µm and σ being 1.3 to 1.7 µm. The length

distribution displays a skewness towards shorter track lengths, which is attributed to older tracks.

Apatites of this type are associated with an uplift of basement rocks such as intrusive and

metamorphic rocks. Apatites from the bimodal distribution type exhibit two distinct populations of

track lengths with a very broad length distribution having a lm < 13 µm and σ > 2 µm. The mixed

distribution is characterized by apatites that accumulated fission tracks during two or more events

and thereby show two or more populations of track lengths in the length distribution, which has a lm

< 11.5 µm and σ around 2.0 µm. Such apatites are hard to interpret since the mixed AFT ages have no

significant geological meaning.


37

Figure 3.8: Track length distributions of confined AFT. (a) freshly induced tracks in apatite, (b) undisturbed volcanic type apatites, (c) undisturbed basement type apatites, (d) apatites with bimodal distribution, (e)

apatites showing mixed distribution (Gleadow et al. 1986).

In order to extend the interpretation of AFT ages to geological events, two important concepts were

developed: the closure temperature and the partial annealing zone concept.

3.4.2 The closure temperature concept

The closure temperature, also known as the blocking temperature or retention temperature. Dodson

(1973, 1979) defined the closure temperature as the temperature of a more or less stable cooling

isotopic system at the time given by its apparent age.

For AFT dating, the closure temperature concept defines three discrete temperature thresholds (De

Grave, 2003): (1) minimum temperature of total annealing (TA), (2) temperature of total retention (TR)

and (3) closure temperature (TC). Each of these temperature thresholds is characterized by an abrupt

change in the stability behaviour of the fission tracks. TA represents the high-temperature threshold

above which no tracks are retained, while TR represents the opposite extreme, i.e. the low-

temperature threshold (De Grave, 2003). Note that fission tracks are continuously annealed even at

ambient surface temperatures (Donelick et al., 1990; Vrolijk et al., 1992), so that a total retention

cannot be obtained in geological conditions. For this reason, Wagner & Van den haute (1992) defined

the TR threshold as the temperature above which a significant increase in annealing rate appears. The

transitional zone between TA and TR is characterized by a partial annealing and shortening of fission

tracks wherein TC is located. TC represents the temperature threshold where 50% of the tracks are

annealed or retained (Wagner & Reimer, 1972). However, the values of TA,C,R are not constant but

vary depending on the cooling rate: the higher the cooling rate, the higher the values. Wagner & Van

den haute (1992) suggested a TC-value of 100 ± 20°C for apatites at normal cooling rates of 10°C/Ma.

The values of TA and TR also vary in function of the composition of the apatites. For the most common


38

types of apatites (fluorapatites), values of 120°C to 125°C and 60°C were considered for TA and TR at

normal cooling rates, respectively.

It must be borne in mind that apatites have several cooling rates and not just a single cooling rate as

they experience several phases of cooling and heating in the upper part of the crust (De Grave, 2003).

Furthermore, a discrete closure temperature at which tracks will suddenly be retained does not exist.

Thus, the interpretation of AFT ages by means of the closure temperature concept is only valid for

more or less linearly cooling systems. For more complex cooling systems, the interpretation of AFT

ages will be based on the partial annealing zone concept.

3.4.3 The Apatite Partial Annealing Zone concept

The annealing of AFT is considered as a gradual process that is reflected in the Earth’s crust as a wide

temperature range. Consequently, the Earth’s crust can be divided in three geothermal depth zones

based on the aforementioned TA and TR thresholds. With decreasing temperature and depth these

are: (1) the Total Annealing Zone (TAZ) that forms the bottom zone where high temperatures

completely anneal the newly formed AFT. As a result, no tracks have accumulated and the fission

track age of the apatite samples in this zone will be 0. The upper boundary of the TAZ is defined by TA.

(2) the Apatite Partial Annealing Zone (APAZ) is predominated by intermediate temperatures and

partial stability. As the temperature decreases from bottom to top, the stability of the tracks

significantly increases with retention factors (r) ranging from 0 at the bottom to 1 at the top. Hence,

more tracks will accumulate and track lengths will increase towards the top. Yet, a certain degree of

annealing is present, which shortens the tracks. It is therefore difficult to date the time at which the

sample entered the APAZ and the apparent fission track age will rather date the time that the apatite

sample passed through the APAZ. The closure temperature TC is located centrally within the APAZ,

which is bounded by TA and TR being ~120°C and ~60°C at normal cooling rates, respectively. When a

normal geothermal gradient of 30°C/km is assumed, it can be inferred that the APAZ is located at a

depth between 2 km and 4 km (De Grave, 2003). (3) the Total Retention Zone (TRZ) or Total Stability

Zone (TSZ) forms the top zone and is bounded at the top by the ambient surface temperature and at

the bottom by TR. This zone exhibits low temperatures and hence stable conditions. As a

consequence, all fission tracks are no longer affected by annealing or shortening and will thus be

retained. When an apatite sample resides in the TSZ, it may comprise unannealed as well as partially

annealed fission tracks, whereby the latter reflects an origin from the APAZ or TAZ.

With respect to the APAZ concept, four potential cooling models can be presented (Wagner, 1981;

Wagner & Van den haute, 1992; De Grave, 2003) (Fig. 3.9): (1) A rock that cools quickly through the


39

APAZ after its formation and is thereafter unaffected by thermal events, will accumulate AFT

immediately after the rock had crystallized. The accumulation continues at a constant rate and

annealing is neglectable due to the fast cooling. As a result, track lengths are nearly unaltered so that

the AFT age can be used to approach the formation age. Such samples will exhibit a length

distribution of the undisturbed volcanic type (Fig. 3.8b). (2) A rock that cools slowly through the APAZ,

will accumulate AFT that experience a greater thermal influence. Hence, the tracks are partially

annealed, resulting in smaller track lengths and thus the AFT age represents the cooling age, i.e. the

time at which the sample crossed the TC isotherm. This cooling model corresponds with the

undisturbed basement type length distribution (Fig. 3.8c). (3) A rock that is suddenly reheated by a

short event, which is unable to completely anneal all the pre-existing AFT, will produce a thermal

overpint. In other words, the thermal event annealed and shortened the previous accumulated AFT

and thereafter accumulated newly formed AFT. Therefore, the AFT age will produce a mixed age,

meaning that the age is located in between the moment where it was formed and the point in time

where it reached the TSZ. Although these type of ages are geologically insignificant, they may

contribute to the quantification of the intensity of the thermal overprint. This thermal event can be

derived from tectonic or sedimentary burial and hydrothermal activity. The associated track length

distribution is either of mixed or bimodal type (Fig. 3.8d,e). (4) The last model is similar as the third

one, but this time the rock suddenly reheated by an intense event, which completely annealed all the

pre-existing AFT. In the case the thermal event is followed by a fast cooling, the thermal history will

be similar as the first model and the AFT age will yield the age of the thermal overprint. In the case a

slow cooling followed, the thermal history will be similar as the second model, producing a cooling

age.

Figure 3.9: Four potential cooling histories of apatite. (1) fast cooling through APAZ, (2) slow cooling through APAZ, (3) thermal overprint reaching APAZ, (4) thermal overprint reaching TSZ (De Grave, 2003).


40

3.4.4 Geological interpretation of apatite fission track ages

The type of an AFT age, as mentioned in previous sections, is reflected by the samples’ length

distribution as it depends on the thermal history. Whether the AFT age represents a cooling age can

thus be determined from the length distribution. Independent geological information and

radiometric ages (of co-existing minerals) can also contribute to the determination of a cooling age.

3.4.4.1 Cooling through denudation

Once the AFT age (tAFT) is considered as a cooling age, the mean cooling rate (uc) of the sample can be

derived immediately from its AFT age (tAFT) and is defined as (De Grave, 2003):

uc =(TC−T0)

tAFT 3.17

Where TC is the effective closure temperature at cooling age tAFT and T0 is the ambient surface

temperature. The cooling of rocks is mostly considered as the result of uplift, in which the rock

column is lifted towards the surface. The uplift itself may be driven by isostasy or tectonic forces. In

the latter case, the uplift may induce denudation, i.e. erosion of surface layers. Note that denudation

is not necessarily induced by tectonic uplift and that rifting processes and climatically controlled

base-level drops can also trigger denudation. However, uplift alone cannot cause cooling, because

the geothermal gradient stays constant and thus the thermal structure of the rock column remains

the same. On the contrary, denudation compresses the geothermal gradient and thereby disrupts

the thermal structure of the uplifted rock column, resulting in the actual cooling (Fig. 3.10).

When the rock column is uplifted towards the surface, followed by denudation given a constant

geothermal gradient (dT/dx), the mean uplift rate (ux) can be calculated from the mean cooling rate

(uc) as follows:

ux =uc

(dT dx⁄ ) 3.18

In this case, the AFT age represents the time that the rock column was uplifted and crossed the TC-

isotherm. On geological time scales, however, uplift, denudation and thermal relaxation of the

thermal isotherms may not occur consecutively as there is a certain time lag (Fig. 3.10). As a

consequence, the AFT age will be interpreted in terms of denudation. Moreover, when there is no

evidence for uplift, the AFT cooling ages are also interpreted in terms of denudation instead of uplift.

It is obvious from this that the AFT ages do not only hold information on the thermal history of a

sample, but also on the tectonic evolution.


41

Figure 3.10: illustration of the relationship between uplift, denudation and thermal relaxation of the isotherms

(Wagner & Van den haute, 1992).

3.4.4.2 Apatite fission track ages along horizontal profiles

When performing AFT studies, rocks may be sampled around structural features (e.g. faults) that lay

perpendicular to the horizontal sampling profile. Due to the horizontal sampling, AFT ages within the

same region may be different and can be interpreted based on two models: tectonic model and

thermal model. In the tectonic model, a constant geothermal gradient is assumed and the different

AFT ages are interpreted as the result of a differential uplift/denudation. As such, the higher the AFT

age, the lower the uplift/denudation rate (Fig. 3.11a). On the other hand, the thermal model

assumes a constant rate of uplift/denudation. Here the difference in AFT ages is interpreted as

caused by a variation in the geothermal gradient and thus the higher the AFT ages, the higher the

geothermal gradients (Fig. 3.11b).

Figure 3.11: the interpretation of AFT ages along horizontal profiles in terms of a (a) tectonic model and (b) thermal model (De Grave, 2003).


42

3.4.4.3 The Influence of topography

The Earth’s crust comprises a set of isotherms that can be influenced depending on the surface

topography, which varies as a function of the denudation and erosion rates as well as the wavelength

of the topography. Parallel isotherms occur during periods of erosional stability and no tectonic

activity. While periods of active erosion and denudation disturb the isotherms by compressing them

beneath valleys and stretching them out under hilltops (Fig. 3.12). The perturbation of the isotherms

is shown to decrease exponentially with depth, so that mainly the low-temperature isotherms are

influenced (Braun, 2002). Since the AFT dating method is a low-temperature geochronometer, the

perturbation of isotherms will influence the interpretation of AFT ages. Stüwe et al. (1994) stated

that the perturbation of the isotherms on the AFT TC-isotherm becomes significant when the

denudation rates exceed 500 to 1000 m/Ma and when the topography has amplitudes larger than 3

km and wavelengths greater than 20 km.

Figure 3.12: Cartoon illustrating the influence of topography and active erosion (U) on the thermal isotherms within the Earth’s crust. z represents the depth in the crust; TL,H are the low and high isotherms and TS the surface isotherm or surface temperature; y0 indicates the normal situation and y1,2 the situation under hilly regions and valleys (De Grave, 2003).

43

CHAPTER 4: SAMPLES AND METHODOLOGY

4.1 OVERVIEW OF THE SAMPLES

This thesis research is based on 16 crystalline basement samples that were sampled by prof Dr. Johan

De Grave and Dr. Stijn Glorie along the Uttaradit, Mae Ping and Three Pagodas fault zones (Fig. 4.1).

The South Thailand samples (ST-set) were obtained in January-February 2011, the North Thailand

samples (NT-set) in December 2013 and the Khorat-Mekong samples (KM-set) in December 2014 -

January 2015. The majority of the samples belong to the Central and Western Granite Provinces,

which were emplaced during the Triassic and Cretaceous magmatic phases, respectively. A small

sample set belongs to the Permo-Triassic Eastern Granite Province. More details on the samples can

be found in table 4.1 and on the geological map in the appendix.

Figure 4.1: (a) Topographic map of Thailand with indication of the sample locations and the Uttaradadit (UFZ), Mae Ping (MPZ) and Three Pagodas Fault Zones (TPFZ); (b) Simplified geological map of Thailand (Ridd et al., 2011) denoting the study area with a black circle.

Chapter 4: Samples and methodology

44

Table 4.1: Overview of sample location and lithology.

Sample Latitude Longitude Altitude Location Lithology

Uttaradit Fault Zone

KM-25 N17°29’40.1” E100°18’07.2” 88 m near Thong Saen Khan; route 1245 (grano) diorite KM-26 N17°47’31.9” E100°07’48.3” 221 m near Uttaradit; Wat Huai Ha; junction 11/1105 (grano) diorite

Mae Ping Fault Zone

KM-40A N16°12’57.2” E099°16’42.8” 185 m Khlong Nam Lai waterfall (Khlong Lan); route 1117 granite KM-40B N16°12’18.2” E099°15’56.4” 208 m Khlong Nam Lai waterfall (Khlong Lan); route 1117 gneiss KM-41 N16°02’42.1” E099°17’24.3” 163 m Mae Wong NP entrance: route 1117 (meta) granite KM-42 N14°54’02.6” E100°37’51.8” 65 m Wat Siri Chanthonnimit Worawihan, Lopburi; hwy 1 skarn/felsic dyke NT-09 N16°59’32.9” E099°16’34.9” 140 m near Pong Daeng/route 111/Tak granite complex diorite – monzonite NT-10 N16°59’32.9” E099°16’34.9” 140 m near Pong Daeng/route 111/Tak granite complex (leuco) granite NT-11 N16°55’16.1” E099°23’19.0” 150 m Sukothai – Tak highway/Tak granite complex granite NT-12 N16°26’12.2” E009°24’49.8” 130 m near Nong Bua (Kamphaeng Phet)/route 1116 granodiorite

Three Pagodas Fault Zone

ST-03 N14°34’05.1” E099°12’13.9” 217 m road 3199 towards Si Sawat granite ST-07 N14°07’44.4” E099°23’40.4” 47 m Khao Chon Kai military camp mylonite – gneiss ST-08 N13°52’32.6” E099°39’50.7” 51 m along road 323, close to tha Maka (grano) diorite ST-09 N13°32’23.6” E099°25’29.2” 163 m Quarry close to Suang Peng (rd. 3087) diorite ST-10 N13°18’27.2” E099°29’13.9” 306 m near Khao Krathing Bon (rd. 3313) (grano) diorite – anorthosite ST-11 N13°20’29.9” E099°31’22.6” 317 m further east along rd. 331 granite


45

4.2 SAMPLE PREPERATION

4.2.1 Separation of apatite

In order to perform AFT analyses, the apatites must be separated from the whole rock samples.

Therefore the whole rock samples are first crushed, grinded and sieved into a fraction comparable to

the size of an apatite mineral. The jaw-crusher consists of two jaws, between which the sample is

crushed. The distance between the jaws is continually decreased by moving one jaw with respect to a

fixed one. With decreasing distance, the sample is crushed into smaller sizes until a size of ± 5 mm is

obtained. The sample is further crushed by means of a disc mill. Similarly as the jaw-crusher, two

adjustable rotating discs will crush the sample by decreasing the distance between them. After this

crushing step the sample is fine enough for dry sieving between 250 µm and 65 µm. To ensure that

this fraction does not contain any smaller fraction (dust), the samples are washed on a 65 µm sieve

and decanted. Subsequently, the samples are dried and made ready for further separation steps.

Since apatites are diamagnetic, they are separated by means of the Frantz Isodynamic Magnetic

Barrier Separator. As shown in figure 4.2, the separator consists of an electromagnet, between which

there is a two-piece ramp. Due to the magnetic field generated by the electromagnet, the sample is

separated into a magnetic and non-magnetic fraction. This separation is done in 5 steps by adjusting

the electrical current and hence the magnetic field starting from 0.1 A to 0.5 A, 0.8 A, 1.0 A and 1.2 A.

Based on the density of apatite, being 3.1 to 3.2 g/cm3, the non-magnetic fraction (> 1.2 A) is further

separated by treating it with heavy liquids to separate the lighter minerals from the heavier ones. By

using the LST (Low Sodium Toxicity heteropolytungstate) Fastfloat with a density of 2.81 g/cm3 as

heavy liquid, minerals heavier (e.g. apatite) than 2.81 g/cm² will sink and those that are lighter (e.g.

quartz) will float or are kept in suspension. The resulting heavy separate is washed, dried and ready

to be handpicked under a stereomicroscope. Approximately 100 apatite grains are picked on the

basis of their hexagonal, prismatic habit and clear to transparent appearance and applied on a

double sided sticky tape (Fig. 4.3a). Separates that comprise insufficient amounts of apatite are

excluded in the next preparation step, such as KM-41.

Figure 4.2: The Frantz Isodynamic Magnetic Separator from the MINPET laboratory at the Ghent University.


46

4.2.2 Preparation for irradiation

For irradiation and later AFT analysis, the apatites are embedded in epoxy and mounted. To this end,

small cylindrical plastic tubes are attached to the double sided sticky tape comprising the handpicked

apatites. By mixing Struers EpoFix resin and EpoFix hardner in a 15/2 ratio at room temperature, an

epoxy liquid is created and poured in the small plastic tubes (Fig. 4.3b). After 2 days of hardening, the

plastic tubes are removed and the grain free side of the epoxy discs (Fig. 4.3c) are cut off to yield 2

mm thick apatite-epoxy mounts (Fig. 4.3d). Thereafter, the internal surfaces of the apatite minerals

are revealed by grounding and polishing the grain side of the epoxy mounts. This is achieved by

grounding the mounts on three successive SiC-grinding papers (#500, #1000, #2400 mesh size)

followed by three successive polishing steps using the Struers DP-U4 polishing machine. For each

polishing step, a polishing pad with the corresponding Struers diamond paste (6 µm, 3 µm and 1 µm)

is used. At this point, the mounts are etched in a 2.5% HNO3 solution for 70 seconds at a temperature

of 20°C to reveal the spontaneous tracks for microscopic observation. In the last preparation step

before the irradiation, the grain side of the apatite mounts are covered with an external detector (i.e.

Goodfellow clear ruby muscovite). Thereafter, the apatite mounts, apatite age standards, uranium

doped glass monitors and their attached ED are stacked together to form an irradiation package (Fig.

4.4), and sent for irradiation. Sample KM-25 was not sent for irradiation because it was badly

polished and did not contain sufficient grains for later analysis.

Figure 4.3: Preparation for irradiation: (a) double sided sticky tape with handpicked apatites; (b) apatites embedded in epoxy resin; (c) apatite-epoxy discs mounted on epoxy stubs; (d) 2mm thick apatite-epoxy

mounts.


47

Figure 4.4: Irradiation packages. The boxes indicate the for this thesis analysed uranium doped glasses (green), standards (red) and apatite mounts (purple).


48

4.2.3 Irradiation

In this research project four irradiation packages are used: M7, M8, M10 and M11 (Fig. 4.4). The

three former irradiation packages were already irradiated and ready for analysis prior to the onset of

this thesis work, while the latter is sent to the Belgian Nuclear Research Centre (or SCK-EN) at Mol for

irradiation during the course of the thesis lab work. The irradiation with thermal neutrons took place

in the Belgian Reactor 1 (BR1), which is a research reactor that uses natural uranium as fuel, graphite

as moderator and air as cooling system. The exterior of the reactor consists of a 2 m thick concrete

construction that protects the surroundings from the radioactive radiation. The moderator is located

in the centre of the reactor consisting of graphite blocks and it surrounds the fuel channels that

comprise the natural uranium. Between the concrete shielding and the graphite moderator, cold air

cools the system through a forced convection: cool air is circulated using a fan, while warm air

generated by the convection is discharged through a chimney. More specifically, the samples are

irradiated in the well-thermalized channel X26 with a thermal/epithermal fluence ratio of 98 ± 3 until

a fluence of ~2x1015 neutrons/cm² is attained.

4.2.4 Preparation after irradiation

After irradiation, the irradiation package is left to cool for approximately three to four weeks until

the radioactivity caused by short lived isotopes is below the safety levels. The ED are then detached

from each mount (apatite samples, apatite standards, uranium doped glass monitors) and etched in a

40% HF solution for 40 minutes at a temperature of 20°C. This etching process reveals the induced

tracks for the observation under the microscope.

4.3 APATITE FISSION TRACK ANALYSIS

4.3.1 Counting procedure

The ages of the samples are determined by counting the ratio of spontaneous to induced AFT

densities by using the ED method that is linked with the zeta-calibration method. Counting of the AFT

is performed with the Olympus BH2 binocular microscope at a magnification of 1250x. This

magnification is attained by a 10x objective, a 100x dry objective and a 1.25x drawing tube module.

The objective contains a counting grid with 100 counting squares. The counting grid is calibrated with

a micrometre scale and it yields a surface area of 6400 µm² (80 µm x 80 µm). Consequently, the

surface area wherein the fission tracks are counted is known and the fission track densities can be

derived. When counting the tracks, care is taken that they show etch pits, straight track channels,

have a homogenous distribution and that other similar features (cleavages, scratches, bubbles) are

not mistaken for tracks.


49

4.3.1.1 Zeta-calibration

As mentioned in section 3.2.3.1, uranium doped glass monitors are irradiated together with apatite

age standards in order to derive the ζ-calibration factor. In this case, eleven IRMM-540 reference

glasses being FCT 1 - M7, FCT 2 - M7, GL5 - M7, GL6 - M7, GL7 - M7, GL1 - M8, GL2 - M8, GL3 - M10,

GL4 - M10, GL5 - M11 and GL6 - M11 are counted (Fig.4.4). The IRMM-540 references glasses were

particularly designed for the AFT method and have a certified uranium content of 13.9 ±0.5 ppm and

natural 235U/238U-isotopic ratio of (7.277 ± 0.0007) x 10-3 (De Corte et al., 1998). For the age standards,

three Durango apatites (DUS 16, DUR 31, DUR 34) and two Fish Canyon Tuff apatites (FCT 1 - M7, FCT

2 - M7) are counted (Fig. 4.4). Note that FCT 1 - M7 and FCT 2 - M7 are used as glass monitors as well

as age standards, because the glass shards were embedded together with the apatite age standards

as shown in figure 4.4.

For each of the glass monitors, a total of 70 grids is counted in the associated ED and gives the

induced fission track densitiy from the glass monitor (ρd). In turn, the ρd-values are plotted in

function of the relative position of the glass monitors within the irradiation package. The relative

position of the glass monitor is defined from an arbitrary chosen starting point, in this case the base

of each irradiation package. The ρd-calibration curve is then obtained by linear regression and can be

used to interpolate the ρd-value for any individual apatite sample or standard within the irradiation

package. Moreover, ρd is related to the thermal neutron fluence as mentioned in section 3.2.3.1. As a

result, the slope of the ρd-calibration curve is a measure of the axial thermal neutron fluence. For the

age standards on the contrary, counting is performed in the ED as well as in the standards by means

of the repositioning method (Jonckheere et al., 2003). In this method, the etched ED is placed back

on the apatite mount in a way that the induced fission tracks of a certain grain exactly coincide with

the spontaneous fission tracks of that same grain. The spontaneous tracks are then counted by

turning the focus of the microscope to the apatite standard mount, while the induced tracks are

counted by changing the focus to the ED. Wherever possible, a total of 1000 spontaneous tracks is

counted for statistical reasons. This counting procedure provides the ρs/ρi-ratio. An inhouse

spreadsheet is used to derive the ratios and relevant statistical parameters (De Grave, press. Comm.)

Finally, these ρs/ρi-ratios, ρd-value and reference age of the standards enable to calculate the ζ-

calibration factor.

Additionally, the apatite mounts are also co-irradiated with the same glass monitors as those used

for the apatite age standards. As a result the ρd-value for the apatite mounts can be interpolated

from the ρd-calibration curve. Similar as the age standards, the spontaneous and induced fission

tracks from the apatite mounts are counted using the reposition method: switching the focus the to


50

the apatite mount enables a counting of the spontaneous fission tracks, while switching the focus to

the ED allows a counting of the induced fission tracks. Also here, the aim is to count approximately

1000 spontaneous tracks if possible and the counting procedure yields the ρs/ρi-ratio. Eventually,

knowing the ρs/ρi-ratio and ρd-value of the samples, and the ζ-calibration factor, the age of the

samples can be calculated.

4.3.2 Length measurements

Similar as to the counting procedure, length measurements are carried out with the Olympus BH2

binocular microscope at a magnification of 1250x. In addition, the KONTRON-MOP-AMO3 image

analysing system is employed that comprising a processing unit, a digitizing tablet and a pointer with

a LED light (Fig. 4.5). This LED light is projected into the microscopic image by the drawing tube

attachment, enabling the measurement of track lengths by marking the end points of the tracks with

the projected red laser light.

Figure 4.5: Installation of the Olympus BH2 binocular microscope and KONTRON-MOP-AMO3 image analysing system for the AFT analysis.


51

Prior to the confined track length measurements, a certain distance is measured against a

micrometre scale. This process is repeated ten times and the measured distances in centimetre are

then converted to micrometre wherefrom the calibration factor is calculated. Subsequently, a

confined track length calibration is performed by means of two reference samples (Hurford 1 and

Hurford 2). For each reference sample, a total of 50 horizontal confined tracks are measured and

their length distribution is then constructed. Note that horizontal tracks refer to tracks whereof both

endpoints stay in focus. The track length calibration provides a measure of internal control and

reference, since length measurements carried out by different fission track researchers may slightly

differ. The determination of horizontal confined fission tracks is facilitated by using reflected light

that enables to see the interaction with the surface. Thus, non-confined tracks and other features are

easily distinguished from confined tracks, because the latter ones do not intersect the surface. After

the length measurements are executed, the results are used to construct AFT length distributions.

These are histograms that indicate how often a certain AFT length occurs and from which the

thermal history of the samples can be inferred.


4.4.1 Modelling with QTQt

As is known, AFT data are applied as a thermochronological tool to reconstruct the thermal history.

Hence, the AFT ages and length measurements are inserted in the QTQt programme (Gallagher,

2012). However, only one sample (ST-07) had enough confined tracks to enable modelling.

The QTQt data file is built by inserting the spontaneous and induced track counts, and length

measurements in µm. As for the annealing model the Ketcham et al. 2007 is chosen with a

compositional Cl (Wt %) parameter set as the default value 0.040. The prior is specified as one

general time-temperature box: 30 ± 30 Ma and 75 ± 75 °C. After a few trials, the proposal moves are

set to 17.000 and 65.000 for time and temperature, respectively. This was chosen because it yielded

acceptance rates between the recommended range of 0.2-0.6, nearly similar acceptance rates for

death and birth, and a stationary likelihood chain. Finally 500000 iterations were employed to

improve the quality.

52

CHAPTER 5: RESULTS

5.1 APATITE FISSION TRACK ANAYSIS

In the AFT analysis, surface tracks are counted to establish the AFT ages of the samples, while

confined tracks are employed to measure the length and to construct AFT length distributions that

can elucidate the thermal history of the sample. From the 16 samples only 13 samples were analysed

because KM-41, KM-25 and ST-11 contained insufficient apatite grains to be counted and hence were

omitted from this work.

5.1.1 Counting procedure

5.1.1.1 The glass monitor interpolation curve

The co-irradiation of uranium doped glass monitors with apatite age standards produces induced AFT

in the glass monitors and their associated ED. Counting these tracks gives the number of induced

tracks (Nd) and induced track densities (ρd) from the glass monitor. Plotting the ρd-values in function

of the relative position of the glass monitors in the irradiation package, generates the ρd-

interpolation curve by means of linear regression. The interpolation curves obtained for the four

irradiation packages are depicted in figure 5.1 and the counting results are represented in table 5.1.

Figure 5.1: Interpolation curves obtained for the four irradiation packages by linear regression.

Chapter 5: Results

53

Table 5.1: Counting results for induced fission tracks from the glass monitors for each of the four irradiation packages. FCT glasses are in fact glass shards that are embedded with the Fish Canyon Tuff apatite age standard, while GL-glasses are separately embedded glass shards. The relative position of the glass monitor is defined from an arbitrary starting point, in this case the base of the irradiation package. Nd is the number of induced fission tracks counted in the ED attached to the glass monitor, and ρd is the induced fission track density in the ED.

Irradiation package Glass monitor Position (mm) Nd ρd ± 1σ (105 tracks/cm²)

M7 FCT 2 0.00 1684 3.759 ± 0.092

GL 7 16.10 1921 4.288 ± 0.098

GL 6 30.55 1903 4.248 ± 0.097

GL 5 42.95 1695 3.783 ± 0.092

FCT 1 49.80 1656 3.696 ± 0.091

M8 GL 1 46.80 1636 3.652 ± 0.090

GL 2 31.80 1741 3.886 ± 0.093

M10 GL 3 20.80 2220 4.955 ± 0.105

GL 4 0.00 2066 4.612 ± 0.101

M11 GL 5 53.55 2556 5.705 ± 0.113

GL 6 37.85 2544 5.679 ± 0.113

From figure 5.1 it can be remarked that the ρd-interpolation curve obtained for irradiation package

M7 by linear regression does not fit very well with the data points (R² = 0.065). This affects the

calculated value for the zeta calibration factor and thereby also the obtained AFT ages. In order to

attain a better fit, the outlier (glass monitor FCT 2) should be recounted. Instead, the very good fit for

the other interpolation curves (R² = 1) is straightforward as only two data points were used for the

linear regression since the apatite sample mounts were only located between two glass monitors.

5.1.1.2 ζ-Calibration factor

After constructing the ρd-calibration curve, the ρd-value of the apatite age standards can be

interpolated from the curve. The spontaneous and induced fission tracks are counted for each

apatite age standard by means of the reposition method as described in section 4.3.1.1. This yields

the spontaneous and induced fission track densities from which the ρs/ρi-ratio of the standards is

determined. Then, the only unknown parameter needed to calculate the ζ-calibration factor is the

reference age of the apatite standards. The equation to calculate the ζ-calibration factor is given by

(equation 3.12):

ζ = eλαtS − 1

λα(ρs/ρi)SG(ρd)S

Chapter 5: Results

54

Where: λα = 1.55125 x 10-10 a-1 (Jaffey et al., 1971; Steiger & jäger, 1977)

G = 0.5 (ED method)

tS = 27.51 ± 0.60 Ma for Fish Canyon Tuff (FCT) (Lanphere & Baadsgaard, 2001) and 31.4 ±

0.3 Ma for Durango apatite (DUR/DUS) (McDowell & Keizer, 1977; Green, 1985)

The statistical uncertainty σ(ζ) associated with ζ, is calculated by assuming that the fission process is a

stochastic process that is described by Poissonian statistics (Green, 1981). The standard error of the

ζ-values is calculated as follows (Green, 1981):

σζ = √(σtS

tS)

2+

1

Ns+

1

Ni+

1

Nd 5.1

Where: tS = reference age of the apatite age standard

σtS= error on the reference age

Ns,i= number of spontaneous and induced fission tracks counted in the apatite age

standard and associated ED, respectively

Nd = number of induced fission tracks counted in the ED attached to the glass monitor, i.e.

an interpolated value from the ρd-calibration curve

The counting results for the apatite age standards and resulting ζ-calibration factors are summarized

in table 5.2.

Table 5.2: AFT results for the Fish Canyon Tuff (FCT) and Durango (DUR/DUS) apatite age standards with ζ-calibration factors. These results were obtained from one irradiation package (M7). n is the number of counted grains, Ns,i is the number of counted spontaneous and induced tracks. ρs,i is the spontaneous and induced track densities and ρs/ ρi is their ratio. Nd is the number of induced tracks in an ED co-irradiated with an IRMM-540 glass monitor and ρd is the corresponding track density. Both are interpolated values except for those of FCT 1 and FCT 2. Track densities are expressed in 10

5 tracks/cm² and are associated with a statistical error (σ). ζ-

calibration factors are expressed in a x cm² and indicated with their statistical uncertainty σ(ζ).

Standard n Ns ρs (± 1σ) Ni ρi (± 1σ) ρs/ ρi Nd ρd (± 1σ) ζ σ(ζ)

FCT 2 100 1348 2.106 1931 3.017 0.698 1684 3.759 210.12 9.21

(0.057) (0.069) (0.092)

FCT 1 100 1456 2.275 2406 3.759 0.605 1656 3.696 246.49 10.38

(0.060) (0.077) (0.091)

DUR 31 107 1267 1.850 2089 3.051 0.607 1745 3.895 266.47 11.71

(0.052) (0.067) (0.093)

Chapter 5: Results

55

Table 5.2 continued.

Standard n Ns ρs (± 1σ) Ni ρi (± 1σ) ρs/ ρi Nd ρd (± 1σ) ζ σ(ζ)

DUR 34 100 1076 1.681 1780 2.781 0.604 1478 3.903 266.86 12.39

(0.051) (0.066) (0.093)

DUS 16 100 1016 1.588 1701 2.658 0.597 1797 4.011 262.78 12.38

(0.050) (0.064) (0.095)

From the calculated ζ-calibration factors, the SWMZ-values (sample weighted mean zeta) are

obtained for each of the two apatite age standards (Durango and Fish Canyon Tuff apatite). These

two SWMZ-values are then used to determine the OWMZ-value (overall weighed mean zeta)

(Hurford & Green, 1983). The underlying principle behind the calculation of the weighted mean zeta-

values is that the more precise ζ-factors (smaller statistical uncertainty) are given a larger weight

than the less precise ζ-factors (larger statistical uncertainty). The following equations are used for

calculating the SWMZ- and OWMZ-values (Hurford & Green, 1983):

SWMZ =

∑ζj

σj2j

∑1

σj2j 5.2

OWMZ =

ζDURσDUR

2 + ζFCT

σFCT2

1

σDUR2 +

1

σFCT2

5.3

The weighted uncertainties are given by:

σSWMZ = √1

∑1

σj2

5.4

σOWMZ = √1

1

σDUR2 +

1

σFCT2

5.5

Incorporating the data from table 5.2 in the equations gives:

SWMZDUR/DUS = 265.4 ± 7.0 a cm²

Chapter 5: Results

56

SWMZFCT = 226.1 ± 6.8 a cm²

OWMZ = 245.4 ± 4.9 a cm²

The obtained weighted mean zeta-values are user specific for a certain optical set-up and can only be

used by the individual fission track researcher. Note that the values are also dependent on the

laboratory and observation conditions, and the type of glass monitors. The obtained OWMZ-value is

in fact the ζ-calibration factor, which is used to calculate the AFT ζ-ages.

5.1.1.3 Apatite fission track ages

Because the apatite mounts are also co-irradiated with the same glass monitors as those used for the

apatite age standards, the ρd-calibration curve can also be used to interpolate the ρd-value of any

apatite mount of unknown age within the irradiation packages. As outlined in section 4.3.1.1, the

counting procedure provides the ρs/ρi-ratios of the apatite samples. Incorporating these ratios, the

obtained OWMZ-value and the ρd-values in the following equation (equation 3.13) yields the AFT ζ-

ages of the samples:

tu(ζ) = 1

λα

ln [λα (ρs

ρi

)u

Gζ(ρd)u + 1]

The relative (RE(t)) and absolute uncertainty (ABS(t)) on the AFT ζ-ages are calculated by the

following equations (Green, 1981):

RE(t) = √(σ(ζ)

ζ)

2+

1

Ns+

1

Ni+

1

Nd 5.6

ABS(t) = t x RE(t) 5.7

Where: Ns,i = number of spontaneous and induced fission tracks counted in the apatite sample

and associated ED, respectively

Nd = number of induced fission tracks counted in the ED attached to the glass monitor,

i.e. an interpolated value from the ρd-calibration curve

The counting results and AFT ζ-ages for the apatite samples are summarized in figure 5.2 and table

5.3. The ages along both the Mae Ping and Three Pagodas fault zones range from Late Eocene to

Middle Miocene (39-14 Ma). Along the Mae Ping fault zone, KM-40A, KM-40B and KM-42 show

Chapter 5: Results

57

slightly younger ages of Late Miocene. KM-26, which is located along the Uttaradit Fault Zone gives

an Early Miocene age.

Figure 5.2: Map showing the Cenozoic strike-slip faults of Thailand and Myanmar (yellow) with indication of the obtained AFT ages for the analysed samples (excluding sample KM-42) (adapted from Morley et al., 2011).

Chapter 5: Results

58

Table 5.3: AFT results for the apatite samples along the Uttaradit Fault Zone (UFZ), Mae Ping Fault Zone (MPFZ) and Three Pagodas Fault Zone (TPFZ). n is the number of counted grains. ρs,i is the spontaneous and induced track densities. ρd is the induced track density in an ED co-irradiated with an IRMM-540 glass monitor, i.e. an interpolated value. Track densities are expressed in 10

5 tracks/cm² and are associated with a statistical error (σ). Ns,i is the number of counted spontaneous and induced

tracks. Nd is the number of induced tracks counted in an ED co-irradiated with an IRMM-540 glass monitor, i.e. an interpolated value. P(χ²) is the chi-squared probability that the dated grains have a constant ρs/ρi-ratio. t(ζ) is the AFT-age expressed in Ma. nl is the number of measured horizontal confined tracks. lm is the mean track length expressed in µm and is associated with a standard deviation (σ).

Location Sample n ρs (± 1σ) Ns ρi (± 1σ) Ni ρd (± 1σ) Nd ρs/ρi P(χ²) t(ζ) lm nl σ

UFZ KM-26 77 1.090 (0.052) 435 3.531 (0.093) 1431 5.697 (0.113) 2552 0.346 0.6025 24.1 ± 1.5 -- -- --

MPFZ KM-40A 43 1.794 (0.090) 395 14.054 (0.255) 3050 5.694 (0.113) 2551 0.142 0.5537 8.9 ± 0.5 -- -- --

KM-40B 38 3.459 (0.122) 827 26.494 (0.336) 6226 5.692 (0.113) 2550 0.134 0.2001 9.3 ± 0.4 -- -- --

KM-42 90 1.752 (0.060) 844 13.034 (0.163) 6414 5.346 (0.109) 2395 0.140 0.6549 9.2 ± 0.4 -- -- --

NT-09 11 0.739 (0.102) 52 2.074 (0.172) 146 4.879 (0.104) 2186 0.432 0.2216 25.8 ± 4.2 -- -- --

NT-10 2 3.047 (0.488) 39 7.422 (0.762) 95 4.860 (0.104) 2177 0.434 0.1425 25.8 ± 5.0 -- -- --

NT-11 40 2.718 (0.170) 256 4.918 (0.231) 453 4.829 (0.104) 2163 0.592 0.9722 35.0 ± 2.9 -- -- --

NT-12 8 5.295 (0.358) 219 20.642 (0.717) 829 4.804 (0.104) 2152 0.248 0.6547 14.6 ± 1.2 -- -- --

TPFZ ST-03 3 3.229 (0.410) 62 8.281 (0.657) 159 3.681 (0.091) 1649 0.508 0.0001 22.9 ± 3.5 -- -- --

ST-07 18 11.910 (0.322) 1372 22.474 (0.442) 2589 3.704 (0.091) 1659 0.524 0.1368 23.8 ± 1.1 9.8 67 3.0

ST-08 40 4.125 (0.127) 1056 4.813 (0.137) 1232 3.729 (0.091) 1671 0.864 0.9494 39.4 ± 2.1 -- -- --

ST-09 40 4.316 (0.130) 1105 5.305 (0.144) 1358 3.749 (0.091) 1680 0.839 0.5731 38.5 ± 2.0 -- -- --

ST-10 28 5.943 (0.182) 1065 7.785 (0.208) 1395 3.767 (0.092) 1688 0.801 0.1459 36.9 ± 1.9 -- -- --

Chapter 5: Results

59

It should be remarked that the performed AFT counts in order to establish the AFT ages must be

statistically representative. As outlined in the methods (section 4.3.1), it is the aim to count at least

1000 spontaneous tracks. However, the very young samples yielded insufficient tracks to be counted

and hence the results are not that representative. In the case of NT-10, NT-12 and ST-03 less than 10

measurements were performed, which is not representative at all. Yet, they will be included in the

interpretation and discussion as their ρs/ρi-ratios yield ages that are consistent with the neighbouring

samples. Although, they are correlated with the other samples, they must be taken with prudence

when performing an interpretation individually. In addition, some samples were badly polished (NT-

09, KM-26, KM-40A), which made the analysis much more difficult. This influences the quality of the

AFT analysis.

5.1.2 Length measurements

5.1.2.1 Track length calibration

Before onset of track length analysis of unknowns, a track length calibration is carried out on specific

length standard mounts. Therefore, two Hurford reference samples are analysed as described in

section 4.3.2. The obtained mean confined track length for Hurford 1 is 11.6 ± 1.4 µm and 15.9 ± 1.2

µm for Hurford 2. Their track length distributions are shown in figure 5.3. These values are slightly

lower than the average values obtained on these samples by other researcher of the MINPET group,

with mean track lengths of 12.45 ± 0.10 µm for Hurford 1 and 16.38 ± 0.08 µm for Hurford 2,

respectively.

Figure 5.3: Confined track length distribution for Hurford 1 and Hurford 2 reference samples. n is the number of

measured horizontal confined tracks, lm is the mean track length associated with a standard deviation σ.

5.1.2.2 Track length distribution of apatite samples

Unfortunately, the very young ages of the samples and their low uranium content yield low track

densities and hence an insufficient number of horizontal confined tracks can be measured. As a

result only one sample was qualified for length measurements, namely ST-07. Even so, the quality of

these length data for this sample is also low. In other words, the very short track lengths of 4-5 µm

Chapter 5: Results

60

and long tack length of 19 µm possibly do not represent fission tracks. The track length distribution

of this sample is depicted in figure 5.4 and has a very short mean track length lm of 9.8 µm and high

standard deviation σ of 3.0 µm.

Figure 5.4: Confined track length distribution of sample ST-07. n is the number of measured horizontal confined tracks, lm is the

mean track length associated with a standard deviation σ.


5.2.1 Thermal history models

Inserting the AFT data of sample ST-07 in the QTQt thermal modelling programme, as outlined in

section 4.4.1, yields a modelled AFT age of 28.9 Ma and mean track length of 12.8 ± 1.1 µm. Thereby,

the AFT age is not very well predicted and the very short mean track length of this sample is difficult

to model and probably indicates that too low lengths were measured. Figure 5.5 shows the thermal

history model and displays a straightforward rapid cooling from Late Eocene until present. Note that

the probability of the model is quite low (0-0.05) whereby the red/magenta t-T envelope has the

highest probability. When taking a closer look at the t-T paths in figure 5.5, the expected model

(black line) reveals three phases: a rapid cooling phase from ca. 32 Ma to ca. 24-22 Ma (Oligocene)

followed by a relative decreased cooling phase lasting until ca. 7-4 Ma (Late Miocene-Pliocene) and a

final rapid cooling phase until present. Additionally, the modelled track length distribution is

relatively small and centred around long track lengths (11-16 µm) as can be seen in figure 5.6.

Chapter 5: Results

61

Figure 5.5: Modelled t-T paths for sample ST-07 by means of the QTQt programme.

Figure 5.6: Modelled track length distribution for ST-07 by means of the QTQt programme. MTL = mean track length, O= observed, P = predicted.

Chapter 5: Results

62

Since the track length distribution did not reveal a histogram, a second modelling attempt was

undertaken. This resulted in a different thermal history model (Fig. 5.7) that shows a nearly tectonic

stable phase from the Late Eocene until Late Miocene (30-8 Ma) followed by a rapid cooling until

present. Similar as the previous history model, the probability of the model is quite low (0.09). The

associated track length distribution (Fig. 5.8) is broad and has a modelled mean track length of 10.9 ±

0.2 µm and corresponds with a modelled AFT age of 32.1 Ma. Also in this case, the AFT age is not

very well predicted. The mean track length, on the contrary, is approximated much better than the

previous model. Nevertheless, the track length distribution still comprises too short (4-5 µm) and too

long (19 µm) track lengths (Fig. 5.8) that probably indicates that too low lengths were measured.

Figure 5.7: Modelled t-T paths for sample ST-07 by means of the QTQt programme.

Both history models are obtained from a single sample ST-07 that contained a relative little amount

of horizontal confined tracks for length measurements. Due to this, the models are statistically not

representative and cannot be extended to the entire study area. Thereby, the thermal history models

are qualitative insignificant and the resulting interpretations must be taken with caution.

Chapter 5: Results

63

Figure 5.8: Modelled track length distribution for ST-07 by means of the QTQt programme. MTL = mean track length, O= observed, P = predicted.

64

CHAPTER 6: INTERPRETATION AND DISCUSSION

6.1 APATITE FISSION TRACK ANALYSIS

6.1.1 Track length calibration

Based on the results from the track length calibration by means of the two Hurford reference

samples, it is obvious that the performed length measurements are slightly lower compared to those

obtained by the MINPET group. As this length calibration provides a measure of internal control and

reference, it means that in general the resulting length measurements on the AFT samples will be

lower too.

6.1.2 Track length distribution

The track length distribution of sample ST-07 (Fig. 5.4) is quite broad and is indicated by its standard

deviation σ being 3.0 µm. This broad distribution does not completely reflect the thermal stability of

the tracks during their residence and passage through the APAZ. For example the very short track

lengths (4-5 µm) can be ascribed to the segmentation of tracks whereby unetchable gaps are created

so that segments of tracks are measured instead of complete tracks. The long track length (19 µm),

on the other hand, does not originate from tracks but probably from other features resembling

tracks. Thus, the amount of measured track segments contributes to the very short mean track

length lm of 9.8 µm. And as mentioned in the previous paragraph, the length calibration has shown

that the length measurements of confined tracks is generally lower and thereby also contributes to

the very short mean track length. Hence, this means that the actual track length distribution will be

less broad and will have a relative longer mean track length. This would correspond with the

modelled track length distributions in figure 5.6 and 5.8. Due to the incorrect measurements and the

relative little amount of confined tracks, the length distribution cannot be used to make

interpretations about the thermal history of the sample and hence the study area.

6.2 THE CENOZOIC STRUCTURAL HISTORY OF NORTH-WESTERN THAILAND

As indicated in the results, most of the AFT ages along the Mae Ping and Three Pagodas fault zones

range between Late Eocene and Early Miocene (39-22 Ma). These ages correspond with the timing of

the ductile sinistral and brittle dextral motions along these investigated fault zones and thus reflect

the Cenozoic crustal movements that affected the Thai basement.

Chapter 6: Interpretation and discussion

65

For the interpretation of the AFT ages along the MPFZ, three groups can be distinguished. The first

group comprises NT-11, which has an AFT age of 35 ± 2.9 Ma. This age corresponds relatively with

the proposed Oligocene (33-30 Ma) exhumation by means of ductile sinistral shearing that initiated

around the Late Cretaceous-Early Paleogene as a result of the approaching Indian plate and

subducting Neotethys Ocean. Thereby, it reflects the uplift of the granitic basement from mid-crustal

levels towards shallower depths. The second group includes NT-09 and NT-10 with AFT ages of 25.8 ±

4.2 Ma and 25.8 ± 5.0 Ma, respectively. These ages can be correlated with the further exhumation

during the Late Oligocene to Early Miocene (24-19 Ma) when brittle dextral motions reactivated the

MPFZ as a consequence of the India-West Burma collision and their subsequent northward

movement relative to Sundaland. The amalgamation of India and West Burma and their northwards

movement was caused by the persistent northward indentation of India into Eurasia since the Early

Eocene. Although, NT-12 yields an AFT age of 14.6 ± 1.2 Ma that falls outside the age range of further

exhumation (24-19 Ma), it is considered as the late Middle Miocene exhumation when brittle dextral

faulting decreased and became dominated by east-west extension. The third group contains the very

young KM-40A, KM-40B and KM-42 samples that have ages around 9 Ma. These three ages are in

agreement with the timing (Late Miocene to Recent) of reactivation of the MPFZ in a dextral

transpressional regime. The reactivation of this fault zone was related to the active Indian-Australian

plates and extending Himalaya Orogen that in turn resulted from the continued indentation of India

into Eurasia. Consequently, the dextral transpressional motions lead to the final exhumation and

denudation of the granitic basement by means of weathering and erosion of cover rocks. Thus, the

AFT ages of these samples are interpreted to indicate this reactivation event.

The AFT ages along the TPFZ can be interpreted in the same way as those along the MPFZ. As such,

the older AFT ages of ST-08, ST-09 and ST-10 (Fig. 6.1 & Table 5.3) refer to the exhumation of the

granitic bodies during the Late Eocene-Oligocene (36-32 Ma) through sinistral transpressional shear

induced by the closure of the Neotethys (India-Eurasia collision). The further exhumation of these

granitoids during the Late Oligocene-Early Miocene (24-19 Ma) operated by means of brittle dextral

faulting, which was provoked by the accretion and northward translation of West Burma and India

relative to Sundaland. This exhumation event is reflected by the younger AFT ages of ST-03 and ST-07

(Fig. 6.1 & Table 5.3).

The brittle dextral strike-slip motion along the Mae Ping and Three Pagodas fault zones decreased

around the Early-Middle Miocene due to the predomination of east-west extension with the

formation of rift basins bounded by normal faults. During the latest Oligocene to Recent the east-

west extension reactivated the Nan-Uttaradit Suture zone, i.e. a zone that represents the collapse of


66

the Nan-Marginal Basin and hence the collision between Sibumasu and Sukhothai as a result of the

closure of the Paleotethys Ocean. The reactivation of this suture zone is reflected by sample KM-26,

which is located near the Uttaradit Fault zone and yielded an AFT age of 24.1 ± 1.5 Ma that coincides

with the timing of reactivation and is therefore interpreted to represent this event.

Figure 6.1: Map of Thailand showing apatite central ages (Upton et al., 1997; Upton, 1999) and other isotopic

ages (Laccasin et al., 1997; Barr et al., 2002) with indication of AFT ages (in Ma) of analysed samples in this study.

6.2.1 Comparison with previous apatite fission track studies

When comparing the AFT ages established in this thesis work with those obtained by Upton (1999)

and Morley et al. (2007) (Fig. 6.1 & Fig. 6.2), only NT-09, NT-10, ST-03 and ST-07 are consistent. NT-

11, ST-08, ST-09 and ST-10 have relative higher AFT ages, while NT-12 has a relative lower AFT age.

The remaining samples (KM-26, KM-40A, KM-40B and KM-42) are inconsistent. Thus, globally there is


67

still a relative correspondence with the data from the literature. The inconsistency between the AFT

ages can be attributed to the different sampling places compared to those of Upton (1999) and

Morley et al. (2007) and thereby the AFT thermo-chronometers will be affected by a differential

uplift/denudation rate due to the presence of the fault structure as described by the tectonic model

in section 3.4.4.2. Another explanation for the differences in AFT age can be related to the influence

of topography. As shown in the topographic maps in chapter 4 and 5, there is clearly a topographic

difference but the difference is too small to have a significant influence on the closure temperature

isotherm.

Figure 6.2.: Map of Thailand showing cooling age data (Morley et al., 2007) with indication

AFT ages (In Ma) of analysed samples in this study.


68

The AFT ages obtained by Blomme (2013) and De Clercq (2016) along the KM and RN faults range

between 30 Ma and 16 Ma and 33 Ma and 11 Ma (Oligocene-Middle Miocene), respectively. These

ages correspond well with those obtained in this work (39-14 Ma) (Fig. 6.3). The consistency between

these ages confirm the assumption of Watkinson et al. (2008) that both faults are conjugate to the

MPFZ and TPFZ and hence experienced a similar evolution. This means that the previous

interpretation on the structural evolution of the MPFZ and TPFZ can most likely be extended to the

KM and RN faults as well. Thus, the two latter faults developed during the Late Cretaceous as a

consequence of the approaching Indian plate and the resulting subduction of the Neotethys Ocean.

The brittle sinsitral shearing and associated exhumation along the KM and RN faults during the Late

Eocene-Oligocene (33-25 Ma) can be related to ductile sinistral deformation and exhumation along

the MPFZ and TPFZ. Therefrom, it can be inferred that the reactivation of the KM and RN faults was

induced by the closure of the Neotethys Ocean and consequently by the India-Eurasia collision. The

subsequent reversal of motion corresponds with the timing of the reversal of motion along the MPFZ

and TPFZ. Hence, the brittle dextral strike-slip deformation and exhumation along the KM and RN

faults during the Late Oligocene-Early Miocene (20-11 Ma) can be attributed to the collision between

India and West-Burma and their northwards translation. The final deformation and exhumation

phase (13-5 Ma) along the KM and RN faults, which occurred during the Miocene-Pliocene was

assumed as the consequence of the opening of the Andaman Basin. The opening of this (back-arc)

oceanic basin is related to the active plate motions of the Indian-Australian plates that were

considered as the cause of the reactivation of the MPFZ and TPFZ during the Miocene and Pliocene.

6.3 THE CENOZOIC THERMAL HISTORY OF WESTERN THAILAND

The thermal history that will be outlined here below is based on a single sample (ST-07) and as a

consequence it cannot be applied to the entire study area. Note that the sample itself contained a

relative low amount of confined tracks for length measurements and that the number of counted

grains (18) for the age determination is also low. The latter may partly explain the weak accordance

with the modelled AFT ages of 28.9 Ma and 32.1 Ma. So, statistically the sample is not representative

to perform significant interpretations and must be taken with prudence. Since, the QTQt modelling

programme generated two different thermal history models, they will be considered as two

endmembers.


69

Figure 6.3: Geological map of Thailand with indication of AFT ages obtained in this thesis work (black) and those

from Blomme (2013) and De Clercq (2016) (Red) For more details on the geological map see appendix.


70

The thermal history model that displays a straightforward rapid cooling reveals three phases (Fig.

5.5). The first rapid cooling phase during the Oligocene (32 Ma to 24-22 Ma) is considered to result

from the rapid exhumation of the crystalline basement rocks from mid-crustal levels in the APAZ

towards shallower depths due to ductile sinistral and brittle dextral strike-slip motions along the TPFZ.

As indicated in the previous paragraph, these movements are related to the India-Eurasia collision

and the further indentation of India into Eurasia. Consequently, the rapid cooling and hence rapid

uplift within the APAZ just slightly affected or annealed the track lengths. This first rapid cooling

phase is in agreement with the findings of Upton (1999), who stated that western Thailand is

characterized by a rapid uplift during the Oligocene-Early Miocene. The second phase, which is

characterized by a relative decreased cooling rate is interpreted as caused by the decreasing dextral

motions along the TPFZ that became dominated by east-west extension during the Early and Middle

Miocene. This gradual cooling phase partially annealed and shortened the fission tracks, explaining

the slightly broader track length distribution (Fig. 5.6) and shortened mean track length of 12.8 µm.

The subsequent rapid cooling phase during the Late Miocene-Pliocene (7-4 Ma) to present reflects

the final exhumation that exposed the crystalline basement rocks by denudation. This phase of

exhumation was induced by the stresses derived from the nearby spreading Himalaya Orogen that

reactivated the older dextral TPFZ. Since this cooling phase was rapid, the track lengths were nearly

unaffected and thus remained relatively long.

The other thermal history model shows two distinct phases (Fig.5.7): A first tectonically stable phase

from Late Eocene until Late Miocene and a second rapid cooling phase until present. During the

tectonic stable phase, the crystalline basement rocks reside in the APAZ for a quite long period

whereby the tracks are annealed significantly, producing a broader track length distribution (Fig. 5.8)

with a shorter mean track length (10.9 µm) than the aforementioned model where the rocks

experienced a gradual cooling. Relating this phase with the structural evolution of the TPFZ makes us

suggest two phases instead of one as modelled here. The tectonic stable phase is assumed to start

around the Early-Middle Miocene and is considered to reflect the inactivity of the fault zone when

east-west extension prevailed throughout Thailand. This stable phase is assumed to be preceded by a

rapid cooling triggered by ductile sinsitral and brittle dextral displacements along the fault zone,

which exhumed the crystalline basement to shallow crustal depths. These strike-slip motions were

related to the India-Eurasia collision and their continued convergence. The last rapid cooling phase

from Late Miocene to present can be interpreted in the same way as the previous model.

Thus, both thermal history models are statistically not representative and are therefore in the broad

scheme probably qualitatively insignificant. Due to this reason none of the models can be praised as


71

the preferred or ideal one in their own right. However, comparing both models with the thermal

history models obtained along the KM and RN faults by Blomme (2013) (Fig. 6.4) and De Clercq (2013)

(Fig. 6.5), indicates a good accordance with the second model (Fig. 5.7) due to the tectonic stable

phase from Oligocene until Middle Miocene in their models. It should also be noted that during this

tectonically quiet period, the granitoids reside in the APAZ at temperatures around 100 °C and 60 °C

for our second model as well as for the models from Blomme (2013) and De Clercq (2016) (Fig. 6.4).

Therefrom, it can be assumed that the second model is more plausible. But, the structural evolution

as outlined above (section 6.2) clearly indicates an active dextral deformation along the TPFZ and

MPFZ during the Late Oligocene to Early Miocene (24-19 Ma). Although the dextral strike-slip activity

was relatively minor as it decreased around the Middle Miocene, the precise timing of the cessation

is not yet constrained. Therefore, the first thermal history model that displays a decreased cooling

rate from Oligocene until Late Miocene-Pliocene can also be assumed as the preferred one. A more

detailed comparison of the thermal history models in this thesis work with those from Blomme (2013)

and De Clercq (2016) will be discussed in the next section.

Figure 6.4: Thermal history model for sample ST-41 along the Khlong Marui fault obtained by

Blomme (2013).

6.3.1 Comparison with previous apatite fission track studies

The thermal history models in this work are in agreement with those of Blomme (2013) and De

Clercq (2013) as they also reveal three phases during their thermal evolution (Fig. 6.4 & Fig. 6.5). The

timing of the three phases and the associated degree of cooling in our models coincide relatively

good with that of their models. Namely, a first rapid cooling phase lasting until the Oligocene (30-25

Ma) followed by a tectonically quiet period without any detectable activity, which persist until the


72

Middle Miocene (13-12 Ma). Eventually, both faults reactivated during the Middle Miocene (13-12

Ma) and are still active at the present day. This once again confirms the statement made by

Watskinson et al. (2008) that both sets of faults are conjugate. Hence, our thermal history model can

be interpreted in a similar way as Blomme (2013) and De Clercq (2016). Note that their interpretation

corresponds with the interpretation outlined in this work whereby the phase of tectonic quiescence

can be interpreted as described in the second model. Although the interpretation of the thermal

history model is qualitatively insignificant and must be taken with caution, the accordance with the

findings of Blomme (2013) and De Clercq (2016) makes the interpretation more valuable. Yet, the

individual interpretation must still be taken with prudence.

Figure 6.5: Thermal history models for samples along the Khlong Marui and Ranong faults obtained by De

Clercq (2016).


73

The confined track length analysis from Blomme (2013) and De Clercq (2016) yielded track length

distributions with mean track lengths mainly between 10.4 µm and 12.4 µm and standard deviations

ranging from 1.5 µm to 2.5 µm. These data do not correlate with the obtained mean track length (9.8

µm), but closely corresponds with the modelled mean track lengths (10.9 µm and 12.8 µm). Since the

thermal history models from both study areas are comparable, it means that the crystalline

basement rocks experienced a similar annealing so that the track length distributions would not

differ that much and hence the mean track length as well. This clearly indicates that the obtained

mean track length of 9.8 µm mainly results from the measurements of fission track segments as

mentioned in section 6.1.2.

6.4 IMPROVEMENTS

Petrographical and geochemical analysis of the granitic rocks can support this thesis research as they

may improve our understanding: they can aid in the interpretation of the AFT ages and can provide

implications for the geodynamical setting of the study area.

The AFT thermochronological analysis of the samples was very limited due to an insufficient amount

of confined tracks derived from the young AFT samples. Therefore they can be irradiated with heavy

ions in order to reveal more confined tracks. This will enable us to construct more constrained

thermal history models wherefrom more significant information can be obtained. Additionally, the

apatite (U-Th)/He dating technique can be applied as well. It is a lower temperature system than the

AFT method and can thus be combined to better constrain the cooling history of the study area.

74

CHAPTER 7: CONCLUSION

The several granitic rocks that have been sampled along the major fault zones of North-Western

Thailand (Mae Ping and Three Pagodas) in this thesis work were intruded during the Mesozoic as a

consequence of the closure of the Paleotethys Ocean and are now identified as the Thai basement.

Both fault zones developed during the Late Cretaceous-Early Paleogene by the tectonic influence of

the approaching Indian Plate and resulting subduction of the Neotethys Ocean. Afterwards, they

were reactivated due to the India-Eurasia collision and the continued northward indentation of India

against Eurasia. The reactivation of the fault zones was accompanied by exhumation and denudation

that exposed the crystalline basement, which enabled us to improve our understanding of the

Cenozoic structural evolution of the study area. Based on the apatite fission track method, the exact

timing of the basement exhumation was established and attempts were undertaken to reconstruct

the thermal history of the study area. From the obtained results the following conclusions can be

drawn, concerning the Cenozoic structural and thermal evolution of North-Western Thailand:

o The ductile sinistral deformation along the Mae Ping and Three Pagodas fault zones initiated

around the Late Cretaceous-Early Paleogene and were responsible for the first exhumation

event that brought the granitic basement rocks from mid-crustal levels towards shallow crustal

depths during the Late Eocene-Oligocene (36-30 Ma). This Late Eocene-Oligocene exhumation

was induced by the closure of the Neotethys Ocean and consequently by the India-Earasia

collision and represents the end of ductile sinistral srike-slip motion.

o The reversal from ductile sinistral to brittle dextral strike-slip motion reactivated both fault

zones whereby the exhumation continued during the Late Oligocene-Early Miocene (24-19 Ma).

Since the brittle dextral displacement was quickly dominated by an east-west extension, its

activity was relatively minor to enable a complete exhumation of the granitic bodies. This first

reactivation event was caused by the ongoing convergence between India and Eurasia whereby

West Burma and India collided and drifted northwards relative to Sundaland.

o The east-west extension prevailed throughout Thailand during the Early and Middle Miocene

and reactivated the older Nan-Uttaradit Suture Zone that marked the collision of the Sibumasu

terrane and Sukhothai Volcanic Arc and hence the closure of the Paleotethys Ocean. This

reactivation generated the NE-SW trending Uttaradit Fault Zone, which is still active.

Chapter 7: Conclusion

75

o During the Late Miocene to Early Pliocene east-west extension decreased and alternated with

inversion events as a result of the influence from the nearby stress fields of the Himalaya

Orogen and active Indian-Australian plates. The subsequent spreading of the Himalaya Orogen,

in turn, reactivated the Mae Ping and Three Pagodas fault zones for a second time but in a

dextral transpressional regime. These dextral transpressional motions completely exhumed and

exposed the granitoids by denudational processes that removed the cover rock.

o The thermal history of sample ST-07 is statistically not representative and cannot be extended to

the entire study area. It only implies a rather rapid cooling in North-Western Thailand during the

Cenozoic and is in agreement with the findings of Upton (1999). However, there is a good

correlation with the thermal history models obtained by Blomme (2013) and De Clercq (2016),

which make the interpretation more valuable. The thermal history model from sample ST-07

displays three phases: (1) A first rapid cooling phase during the Oligocene resulting from a rapid

exhumation of the crystalline basement rocks due to ductile sinistral and brittle strike-slip

motions. These strike-slip movements are related to the India-Eurasia collision and their ongoing

convergence. (2) A second phase characterized by either a relative decreased cooling rate or a

tectonic inactivity during the Oligocene-Miocene. This phase reflects the decreasing or even the

cessation of the dextral movement along the fault and is attributed to the predomination of

east-west extension during the Early and Middle Miocene. (3) The final rapid cooling phase

reflects the final exhumation and denudation of the crystalline basement rocks during the Late

Miocene-Pliocene to present. This last phase is induced by the stresses derived from the nearby

spreading Himalaya Orogen and active Indian-Australian plates.

The second aim of this thesis project was to compare the results with those from the literature

(Upton, 1999; Morley et al., 2007) and previous thesis projects of the MINPET group. Generally, a

relative good agreement exist with the data from Upton (1999) and Morley et al. (2007). The

inconsistency between the apatite fission track ages could be attributed to the different sampling

sites compared to those of Upton (1999) and Morley et al. (2007). Due to the different sampling sites

along the fault zones, the apatite fission track thermo-chronometers are affected by a differential

uplift/denudation rate and hence results in different apatite fission track ages. Additionally, the

compatibility with the data from Blomme (2013) and De Clercq (2016) confirmed the similar

evolution between the Mae Ping-Three Pagodas faults in our study area and Khlong Marui-Ranong

faults in peninsular Thailand.

76

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85

APPENDIX

Geological map of northern part of Thailand

Appendix

86

Legend of the geological map

escape tectonics along major shear zones in western...

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