seismic reflection profiling in the proterozoic arunta

Tectonophysics, 173 (1990) 257-268

Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

257

Seismic reflection profiling in the Proterozoic Arunta Block, central Australia: processing for testing models

of tectonic evolution

B R GOLEBY ‘3 *, B.L.N. KENNETT *, C. WRIGHT r, R.D. SHAW 1 and K. LAMBECK * . .

’ Bureau of Mineral Resources, G. P.O. Box 378, Canberra, A. C. T. 2601 (Awtraha)

.’ Research School of Earth Sciences, Australian National Uniuersity, G.P.O. Box 4, Canberra, A.C. T. 2601 (Austrd~a)

(Received September 30, 1988; accepted April 13, 1989)

Abstract

Goleby, B.R., Kennett, B.L.N.. Wright, C., Shaw, R.D. and Lambeck, K., 1990. Seismic reflection profiling .n the

Proterozoic Arunta Block, central Australia: processing for testing models of tectonic evolution. In: J.H. Leven.

D.M. Finlayson, C. Wright, J.C. Dooley and B.L.N. Kennett (Editors), Seismic Probing of Continents anti their

Margins. Tectonophysics, 173: 257-268.

The region extending from the northern Amadeus Basin to the Northern Arunta Province is characterised by z large

change in Bouguer gravity (1400 Frn sm2 ), abrupt changes in teleseismic travel-time residuals, and large thrust

structures in which rocks from the lower crust are now exposed at the surface. Deep seismic reflection data, other

geophysical data and geological mapping within this region have contributed to the development of a crustal model

that would result from “thick-skinned” tectonic processes, in which deformation occurred along moderately d pping

thrusts or shear zones that extended from the surface down to at least the crust-mantle boundary. Two important

features of this model have emerged with the help of unconventional stacking techniques. The first is a series of

reflections from the Redbank Deformed Zone that can be traced from surface outcrop to depths of more than 110 km.

The second is evidence of a fault extending through the crust and displacing the Moho.

Introduction

Central Australia is characterized by a series of

late Proterozoic-Palaeozoic intracratonic basins

(Amadeus and Ngalia Basins) and exposed base-

ment blocks of middle Proterozoic age (Musgrave

and Arunta Blocks). Major east-west trending

gravity anomalies exceeding 1400 pm ss2 (Fig.

la), which are clearly related to the basin and

block structures, also occur within the region.

* Present address: Research School of Earth Sciences,

Australian National University, G.P.O. Box 4, Canberra,

A.C.T. 2601, Australia.

Teleseismic travel-time anomalies in excess of 1.5

s over distances of a few tens of kilometres have

also been observed across the Arunta Block

(Lambeck and Penney, 1984; Lambeck et al., 1988)

(Fig. la). Both the gravity anomaly and the tele-

seismic travel-time anomalies within the Arunta

Block relate to the location of the Redbank De-

formed Zone (RDZ, Fig. lb) (Shaw el al., 1984;

Stewart et al., 1984) one of a series of thrust zones

that have been mapped within the Arunta Block

(Fig. lc). The geological, structural and geophysi-

cal data from this region are sufficiently anoma-

lous to have attracted the attention of several

workers over the last twenty years. To date, mod-

els for the type of deformation for the region

include a crustal compressional model. “thin- skinned” in style, with faults terminating on a mid-crustal sole thrust (Teyssier, 1985), a mechanical folding and thrusting model (Lambeck, 1983), and a “thick-skinned” faulted block model (Shaw, 1987; Lambeck et al., 1988).

This paper concentrates on the interpretation of 130 km of deep seismic reflection data obtained from north to south across the Proterozoic Anmta Block (A-A’ of Fig. 1). (Details of the seismic survey have been given by Goleby et al., 1986, 1988.) As part of this interpretation, unconventional methods of data processing and display were used to discriminate between “ thick-skinned” and “thin-skinned” styles of deformation, and

have enabled the RDZ to be imaged to depths of at least 35 km.

The Arunta Block is a region of exposed Pro- terozoic crust that has been interpreted as a major ensialic mobile belt (Shaw et al., 1984; Stewart et al., 1984). It has been divided into three tectonic provinces (Northern, Central and Southern: Fig. lb), each of which has undergone separate histo- ries of deformation and metamorphism. In the vicinity of the seismic line, the Southern Arunta Province is an amphibolite facies terrane made up of granitic gneiss, whereas the Central Arunta Province to the north is a granulite facies terrane composed of felsic and relatively minor mafic granulite. The granulites of the Central Arunta Province were metamorphosed at depths of around 25 km at about 1750-1800 Ma (Black et al., 1983). These two provinces are separated by the RDZ (Fig. 1, b and c), the major structural feature within the Arunta Block. The RDZ occurs as a 7-10 km wide easterly trending zone of anastomosing mylonites dipping northwards at around 45”. This zone has been interpreted as a major Proterozoic crustal suture zone that has been reactivated during the Alice Springs Orogeny between 300 and 400 Ma (Shaw, 1987). Some thirteen kilometres to the south, and running sub- parallel to the RDZ, is the Ormiston Nappe and Thrust Zone (ONTZ), another thrust zone that appears to have dominated deformation during

the last stages of the Alice Springs Orogcny. Iht Northern Arunta Province, separated lrom tht Central Arunta Province by large granitoid intru- sions and by moderate to steep faults, consists 01 low-grade metasediments of amphibolite and greenschist facies. The Ngalia Basin (Wells and Moss, 1983) lies within the Arunta Block (Fig. la). and its northern margin is also fault-controlled; it is not an important feature on the seismic reflection section because the ma~mum sedimentary thickness along the profile A-A’ does not exceed 1 km.

Processing techniques for enhancement of deep

crustal reflections

The use of explosives as the energy source and the limited fold of recording proved highly successful in producing a strong high-frequency signal that stacked well. Indi~dual reflection seg- ments, however, were often short (typically lo-30 traces or 400-1200 m) and show a patchy coherency, both within a single-shot gather and across several shots. This variation in reflection character raised serious doubts as to the validity of conventional stacking for the data (Goleby et al., 1987). The fragmented nature of the reflections also makes effective migration difficult.

In order to develop a crustal structure for the region, we had to be confident of the geological significance of the events observed on the seismic sections. We therefore investigated a number of different stacking procedures to generate a work- ing section with minimum processing artifacts, and in a form suitable for migration.

All processing was done using pseudo true-am- plitude techniques. Although the processed seismic reflection section did not require major efforts to enhance the deep signals, the various stacking techniques applied did emphasise different attri- butes of the data, thus assisting inte~r~ta~on~ The deep seismic reflection data therefore enable a well constrained crustal model to be developed to depths of about 40 km.

The display techniques we have investigated include a single-fold display, the standard CMP stack, median stack, coherency stack, Nth root stack, and an energy stack. Each stacking tech-

PROCESSING FOR TESTING MODELS OF TECTONIC EVOLUTION

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~ CENTRALARUNTA PROVIN 4

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Fig. 1. Location diagram showing the principal tectonic elements of the central Australian region (b) and the location of the central

Australian seismic traverse, LI. Gravity and teleseismic travel-time profiles from Lambeck et al.. 1988 (a) and a schematic geological

cross-section (A-A’, in c) are also shown. The travel-time anomalies are for earthquakes from two different regions: Japan from the

north and Fiji-Tonga from the east. L2, L3 and L4 are the positions of shorter seismic traverses recorded during 1985.

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PROCESSING FOR TESTING MODELS OF TECTONIC EVOLUTION 261

nique showed some advantages over the conven-

tional CMP stack, and, as a combined processing

package, aided the interpretation of data from the

deep crust. The effects on the data after using

these techniques are discussed below.

Single-fold display

Single-fold displays along the entire section

were used to gauge the effect of the stacking

procedure; this display highlights variations in the

surface conditions and the degree of energy

penetration, and comparison with the single-fold

display showed where the signal was degraded or

destroyed by the stacking process. In most cases

the cause for this could be established. All stack-

ing methods showed some improvement in data

clarity.

CMP (common mid-point) stack

The effectiveness of the CMP stack depends on

having approximately flat-lying reflectors, as

smearing occurs if the reflecting surfaces depart

significantly from the horizontal. In addition there

is relatively little improvement in the signal-

to-noise ratio with a low fold of cover, and this

method, like the others discussed here, is degraded

by near-surface complexities. Within the Arunta

Block, the dips are steep and the fold is relatively

low. Nevertheless, in spite of the predominance of

steep dips, results obtained using CMP stacks are

good (Fig. 2). The CMP stack for the Arunta

Block shows an improvement over any of the

single-fold displays, but there is a reduction in the

frequency bandwidth, with loss of high-frequency

information.

Coherency stack

This coherency stack weights each stacked trace

by a measure of the coherency of each of the

traces within each CMP gather. This is a one-di-

mensional coherency, based solely on traces within

each CMP. The technique enhances small-ampli-

tude coherent events over incoherent noise. It is

very susceptible to any misalignment caused by

near-surface-related effects or residual dip-related

effects. The coherency stack for the Arunta Block

data showed a preferential enhancement of the

shallow-dipping to flat reflectors over the very

steep events, and produced a marked reduction in

the background noise. There was a decrease in

continuity resulting from amplification of residual

static effects.

Median stack

The median stack was very success ‘ul in pro-

ducing a quality section for display (Fig. 3). The n

highest and n lowest samples (n is a positive

integer) are rejected, and the remaining data are

stacked. This approach was excellent in removing

noisy portions of trace, or a single trace incon-

sistent with the rest of the traces of the same CMP

gather. Although the final fold may be dramati-

cally reduced, the results were often found to be

well worth it. If taken to the extreme, only the

median value is retained, giving a single-fold dis-

play that “simulates” a stacked section.

Nth root stack

The Nth root stack technique (Muirhead, 1968;

McFadden et al., 1986) has the ability to enhance

the signal-to-noise ratio in the stack. It is good in

reducing random noise events and in suppressing

noise spikes. Fourth, eighth and sixteenth root

stacks were tried. Each Nth root stack of the

Arunta Block data showed a marked decrease in

the background noise where the signal was low.

Where reflected energy was strong, no improve-

ment in contrast between events and noise was

found. Improvements in continuity varied over the

section, ranging from a good improvement in the

northern portion at shallower depths, tll a marked

decrease in continuity at the southern end.

Energy stack

The energy stack was developed to take ad-

vantage of the high level of reflected signal and is

similar to “reflection strength displays” (Taner et

al., 1979), but instead of modulating the stack

with the envelope of the energy density. the energy

within a specified time gate is displayed. The

energy within a sliding gate is computed for each

Q co 0 N 3- *I

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m .2

264

trace, then summed for each trace in the stack. By varying the gate length to match particular frequencies, an estimate of the variation in frequency content across the section could be obtained. The central value of each gate is assigned the computed energy level for that gate. An example of an energy stack is shown in Fig. 4. The energy stack has a number of advantages, since it allows the interpreter to look at packets of energy rather than the normal display of signal ~plitudes, and is not affected by signal polarity. It also enables an easier correlation of events, because they stand out more clearly over the generally noisy background on the seismic sections.

Summary of display procedures

No single stacking method is significantly bet- ter than the others. However, we found there were many advantages in using several types of stack, because each assisted in identifying different events. The persistence of many reflections through the various stacked sections provide confidence in their validity. The improvement in coherency ob- tamed with some of the stacking techniques over the original CMP stack allowed the successful migration of the data. In particular, the energy stack of the Arunta Block data (Fig. 4) ~~~t~ the coherent events and greatly improved continuity, especially for the northerly dipping sequence of faults. Because of this improvement, these “pseudo line-drawing” sections can be success- fully migrated. The best migration results were obtained after either median or energy stacking and display processes. The results compared favourably with the results from migration of line-drawings, and were a great improvement over the original migration of the CMP stacked data.

Deep seismic reflection results

A migrated line-drawing of the deep seismic reflection section from the Arunta Block is shown in Fig. 5. The migration was accomplished with a velocity-depth function ranging from 5.5 km s- ’ at the surface to 6.2 km s-’ at the greatest depths. We are confident that the line-drawing shown here is a faithful representation of the seismic reflec-

tion data, because of the similar characteristic? emphasised by the various stacks. Ihis deep seismic section shows several unusual features

The first of these features is the exceptionally high strength and depth of penetration of the reflected deep seismic energy, particularly in the Northern Arunta Province. This is seen in Fig. 4, where the signal is still strong at reflection times of 14 s, while frequencies up to 80 Hz at reflection times of 5-6 s and to 60 Hz at 6-10 s reflection time are common, particularly within the North- ern Arunta Province. This high-frequency content implies that the rocks at these depths (15-18 km) have a very high Q (low attenuation) (Goleby et al., 1987).

The second feature is the series of northerly dipping reflections across the section. These reflections correspond to the surface outcrop of either mapped faults or shear zones, or to the positions of faults inferred from aerumagnetic surveys (Mutton and Shaw, 1979). One group of these dipping reflections corresponds to the RDZ, and can be traced with confidence to depths of about 30-35 km, and may extend to 50 km depth (Fig. 3). Seismological expression of this fault

zone is further discussed by Cao et al. (1990). There are several shallower dipping reflectors within the Southern Arunta Province. The stron- gest of these corresponds to the ONTZ, although possible interaction between this thrust and the RDZ is not clear.

A third feature of the data is the marked dif- ference in the character of the deep seismic reflections between the Southern Arunta Province and the region to the north (Fig. 3). Beneath the Southern Arunta Province, the reflectors down to 30 km are predominantly sub-horizontal, but beneath the Central and Northern Arunta Provinces they generally dip northwards at around 30-35”. The boundary between this change of dip of reflectors coincides with the position of the RDZ.

Tectonic implications

Figure 5 summarizes the principal features of the seismic sections. The surface geology of the Central and Northern Arunta Provinces is characterized by a series of moderately dipping faults


S~~~~~~N

t CENTRALARUNTA PROVINCE

Is!00 -t-

NORTHERN ARUNTA PROVlNCE---- PROVINCE. +-. NGALIA BASIN 180

--* - ___

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.

.*

Fig. 5. Interpreted crustal structure (B) of the Arunta Block superimposed on a depth-migrated line-drawing of the seismic section.

This section has a V : H ratio of 1 : 1 at a velocity of 6 km s-l. RDZ refers to the Redbank Deformed Zone and ~ON7’2 to the

Ormiston Nappe and Thrust Zone. In this interpretation there is a marked offset on the crust-mantle boundary of some 15 km, with

mantle material upthrust with part of the Central Arunta Province. This high-density region is part cause for both the gravity

anomalies and the teleseismic travel-time residual anomalies. The uninterpreted line diagram is shown above i A).

or shear zones. The seismic sections show that sional models of a faulted crust have shown that

these faults have an apparent dip of about 40°, are such faults can generate recoverable reflected sig-

traceable to depths of around 35 km (Fig. 5), and nals (Cao et al., 1990).

appear to divide the crust of this region into a Sub-horizontal reflections within the crustal

series of “blocks”, each with a similar seismic “blocks” occur (Fig. 5) and are interpreted as

character and bounded by the dipping faults. Syn- compositional changes that indicate the present

thetic seismograms calculated for three-dimen- attitude of the original crustal layering (Goleby et

Computed Gravity

Residual Profile +ISO ymS*

S:;;i;;N -I

-CENTRAL AiUNTA PROVINCE PROVINCE 16000 ---t--

NORTHERN ARUNTA PROVINCE +-, NGALIA BASIN 18Ol

. _ L - . . . 0

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t

SEISMIC REFLECTION MODEL

Fig. 6. Observed and computed gravity profiles along LI (Fig. I). l&e computed profiile is based on the interpreted deep seismic reflection model (A) for the Arunta B&k. Ia B, crosses represent the computed profile and closed circles represent the the residual

profile 4 150 pm s-*. Numbers superimposed on A are the assumed rock densities, in g cmd3, used in the gravity catc&tions.

al., 1989). Within some blocks these sub-horizontal events are truncated against dipping faults, suggesting that the deformation of the crust occurred by movement aIong these faults, with minimal rotation of the crustal blocks.

North of the IlIogwa Creek Schist 2one (Shaw et al., 1984) (the eastern extension of the Palaeo- zoic continuation of the RDZ), erogenic high- strain zones older than 2600 Ma appear to be flat-lying (Ding and James, 19g5). M~~b~s and Black (1974) suggest that the RD2 may origi-

nally have been a thrust zone, dating from about 1600 Ma. However, Shaw (1987) has suggested a separate evolution for the crustal blocks north and south of the RDZ until 1450 Ma and possibly until 1100 Ma, based on Rb/Sr and 40Ar/39Ar isotopic data, bulk lithology, structural Mstory and rnet~o~~c grade. This suggests that the RDZ may originally have been a suture (Shaw, 1987).

The crust-mantle bound&y beneath the North- em Arunta Province is interpreted to be. at the


base of the zone of strong reflections at about

35-40 km, whereas beneath the Southern Arunta

Province it is most likely at the base of the lowest

reflectors seen at about 50 km depth (Fig. 5).

There may be a local crust-mantle transition zone

of about 20 km thickness, extending from 30 km

to at least 50 km depth. This is consistent with the

seismic refraction results along an orthogonal

east-west profile that crosses the reflection line

near CDP 15300 of Fig. 5 (Goleby et al., 1988;

Wright et al., 1990). This implies an abnormally

thick crust beneath the Southern Arunta Province,

with significant variation in crustal thickness be-

tween the Southern and Northern Arunta Pro-

vinces, which creates a problem in explaining the

long-term mechanical stability of the region (dis-

cussed by Goleby et al., 1989). The Amadeus

Basin to the south has up to 14 km of sedimentary

rocks (Lindsay and Korsch, 1989) and the gravity,

teleseismic travel-time anomalies, and deep crustal

reflection data, do not indicate any discontinuity

in the crustal structure between the southern part

of the Arunta Block and the Amadeus Basin. A

current crustal thickness of about 50 km, inferred

from an east-west refraction profile across the

northern part of the Amadeus Basin (Wright et al.,

1990) implies that the sediments were deposited

on an approximately normal thickness crust (30

km), consistent with a predominantly compres-

sional origin for the basin, particularly in the final

stages. This is also consistent with both the ob-

served gravity field and the teleseismic travel-time

anomalies (Fig. la). The zone of seismic trans-

parency (between 25 and 30 km depth) below

CDP 16000-16500 (Figs. 3 and 4), just above the

RDZ, is interpreted as a region of mantle material

(Goleby et al., 1989); this suggests that significant

thrusting has occurred on both the RDZ and

ONTZ, and that high-velocity and high-density

mantle material has been emplaced relatively near

to the surface beneath the Central Arunta Pro-

vince, as has previously been inferred from other

geophysical data (Lambeck, 1983; Lambeck et al.,

1988).

Gravity modelling

Measured densities of near-surface rocks vary

from 2.5 g cm-j for the Palaeozoic sedimentary

rocks of the Ngalia Basin to 2.95 g crn3 for the

granulite facies rocks north of the RDZ (Lambeck,

1983; Shaw, 1987). Even if these lateral variations

persisted to depths of 10 km, they would account

for only about 50% of the observed gravity

anomaly, and would have a much shorter wave-

length (Goleby et al., 1989). This suggests a deeper

origin for the major contribution to the observed

gravity field over the southern part of the Arunta

Block. Figure 6 shows the observed Bouguer grav-

ity anomalies and those calculated from a density

model of the crust, which is displayed in the lower

half of Fig. 6, and which is consistent with the

“faulted block” model of Fig. 5. Most im-

portantly, the model clearly predicts both the am-

plitude and the wavelength of the observed

anomaly (Goleby et al., 1989).

Conclusions

When unconventional processing methods were

used to process deep seismic reflection data re-

corded within the Arunta Block, the fine details of

the deep crust were clarified and then used to

confidently infer the style of crustal deformation.

Our experience with the seismic data from the

Arunta Block leads us to recommend experimen-

tation with processing techniques for similar data

from other regions in order to obtain the best

results. The combination of seismic reflection and

refraction profiles, gravity observations and

surface geological mapping has enable:d the con-

struction of a model of the crust for the Arunta

Block that is consistent with a recent interpreta-

tion of teleseismic travel-time residuals (Lambeck

et al., 1988). These combined data sets suggest

that the crust of the Southern Arunta Province is

underthrust beneath the overriding lower crust

and uppermost mantle of the Central Arunta Pro-

vince. The model of the crust is also consistent

with new geological, structural, metamorphic and

radiometric data (Shaw, 1987) which indicate a

complex uplift and deformation history which oc-

curred by movement on moderately dipping faults.

The RDZ is the main structural boundary within

the Arunta Block, and its planar geometry to

depths of about 35 km, together with the likeli-

hood that mantle material forms the hanging wall

26X

of the main thrust zone, supports a “thick-

skinned” model for the tectonic evolution of the

region (Shaw, 1987; Goleby et al., 1989). Isotopic

data (Shaw, 1987) show that a substantial part of

this structure developed at a time leading up to

and during the Alice Springs Orogeny, producing

a stacking or imbrication of the crust and the

exposure of lower crustal material north of the

RDZ.

Adtnmkdgements

The authors acknowledge the efforts of the

seismic team involved in the data acquisition. We

thank Dr. J. Leven for the median- and

coherency-stack programs, and Dr. S. Kravis for

the Nth-root-stack program. Mr. I. Chertok

drafted the figures. Rb-Sr isotopic studies to be

reported elsewhere were carried out in conjunction

with Dr. L. Black (BMR), and 40Ar/39Ar and

K/Ar isotopic studies with assistance from Dr. P.

Zeitler and Dr. I. McDougall (Australian National

University). We thank Dr. B. Drummond for his

constructive comments on an earlier draft of this

paper. This paper is published with the permission

of the Director, Bureau of Mineral Resources.

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seismic reflection profiling in the proterozoic arunta

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