thermochronological analysis of siwalik sediments … · lh crystalline nappe and crystalline nappe...

1
(1) Laboratoire de Géodynamique des Chaînes Alpines, UMR 5025, UJF, Grenoble, France (email: [email protected]) (2) IFP, 92852 Rueil-Malmaison Cedex, France (3) Géosciences Rennes, F-35042 Rennes Cedex, France X. Robert (1), P. Van der Beek (1), J.-L. Mugnier (1), E. Labrin (1), W. Sassy (2), J. Braun (3) Thermochronological analysis of Siwalik sediments from the Karnali River section (western Nepal): Constraints on the kinematics of the frontal Himalayan prism. Programme : RELIEFS DE LA TERRE ; Projet : « Flux tectonique et relief de l'Himalaya : une approche par thermo-chronologie détritique et géomorphologie » Problems The relief of mountain belts results from the interaction between tectonics and surface processes. Here, we develop a tool to follow the evolution of the thermal structure of a mountain belt by combining low-temperature thermochronometry and thermo-kinematic modeling, in order to better understand relief development in mountain belts. This is the first stage of a project that aims to elucidate the kinematics of shortening of the central Himalayan mountain belt and focuses on the currently active outermost thrust (the Main Frontal Thrust). The work will continue with a study of the more internal fold-and-thrust belt deformation along the Main Boundary thrust and Main Central Thrust as well as assessing possible out of sequence faulting 84° TIBET INDIA Kali G. Bhairawa Piuthan Nepalganj Jajarkot Dunai Dhangadhi Bajhang Kar nali 30° 30° 28° 28° 80° MCT MBT MBT MT MT 25 25 50 75 100 km 0 MFT MFT Jumla Rapti b LESSER HIMALAYAS HIMALAYAS TIBET PLATEAU GANGA PLAIN HIGHER M C T M B T M B T M F T M C T SUTURE KARAKORUM FAULT 30° N 85°E 75°E 80°E 25°N 90°E 95°E a Tinau Surai Karnali Terai Duns & recent sediments Siwaliks Lesser Himalaya zone Greater Himalaya zone LH crystalline nappe and crystalline nappe GH leucogranite Thrust Active fault Coupe Study area and sample section 0 -2000 2000 -4000 MBT Karnali canyon MFT3 ID MDT3 OF 0 5 10 km Sampling zone C Figure 1: a) Tectonic sketch map of central Himalaya showing location of the study area (inset); b) Geological map of Western Nepal showing locations of sampled section (KAR: Karnali); Modified from Mugnier et al. (2004). c) Balanced cross section along the Kar- nali River showing sample locations and relationship with local structure. Location of cross-section is indicated on Figure b. Modified from Mugnier et al. (1999). AFT age (Ma) 0 1000 2000 3000 4000 5000 6000 0 2 4 6 8 10 12 14 Stratigraphic depth (m) KAR-3 N = 36 ; μ = 9.52 ; e = 1.85 0 1 2 3 4 5 6 7 8 9 5 6 7 8 9 10 11 12 13 14 15 16 17 KAR-6 N = 51 ; μ = 9.39 ; e = 1.87 0 2 4 6 8 10 12 14 5 6 7 8 9 10 11 12 13 14 15 16 17 KAR-7 N = 80 ; μ = 10.74 ; e = 2.07 0 2 4 6 8 10 12 14 16 18 20 5 6 7 8 9 10 11 12 13 14 15 16 17 KAR-9 N = 54 ; μ = 11.22 ; e = 2.50 0 2 4 6 8 10 12 14 5 6 7 8 9 10 11 12 13 14 15 16 17 KAR-10 N = 68 ; μ = 12.11 ; e = 2.02 0 2 4 6 8 10 12 14 16 18 5 6 7 8 9 10 11 12 13 14 15 16 17 KAR-11 N = 78 ; μ = 11.32 ; e =2.61 0 2 4 6 8 10 12 14 5 6 7 8 9 10 11 12 13 14 15 16 17 KAR-12 N = 75 ; μ = 10.97 ; e = 2.92 0 2 4 6 8 10 12 14 5 6 7 8 9 10 11 12 13 14 15 16 17 KAR-14 N = 54 ; μ = 11.69 ; e = 2.00 0 2 4 6 8 10 12 5 6 7 8 9 10 11 12 13 14 15 16 17 Number Number Number Number Number Number Number Number Length (μm) Length (μm) Length (μm) Length (μm) Length (μm) Length (μm) Length (μm) Length (μm) AFT ages and track length distribution Figure 2: Main panel shows plot of AFT central ages (black diamonds) and minimum ages (i.e., age of the youngest popula- tion; white squares) against depth for samples from the Karnali River section. The solid line corresponds to the stratigraphic age. Stratigraphic ages for this section are from Gautam and Fujiwara (2000). Small panels show track-length histograms for Karnali section samples (N = number of tracks lengths measured; μ = Mean track length, e = standard deviation). Minimum ages are younger than stratigraphic ages under the depth 2000 m, which corresponds to the top of the PAZ (60°C). The mean thermal gradient is around 20°C/km. Mean track lengths decrease with increasing depth, as a result of increasing partial annealing. Note that even deepest samples are not yet fully annealed. Inversion of AFT data 0 20 40 60 80 100 120 0 2 4 6 8 10 12 14 16 KAR3 KAR3-rev KAR6-re2 KAR-7-rev KAR6-r22 KAR7-re2 KAR10-1 Ages (Ma) Temperature (°C) 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 0 20 40 60 80 100 120 140 Maximal temperature (°C) Depth (m) ~9 °C/km r² = 0.8448 ~31 °C/km KAR-3 KAR6-re-2 KAR6-r22 KAR7-res KAR7-re2 KAR10-1 KAR11 ~20 °C/km Figure 3: Inversion of AFT data of partially annealed samples from the Karnali River with AFTSolve (Ketcham et al., 2000). For clarity, confidence intervals and pre sedi- mentary histories are not represented. The heating from 14 Ma to ~3 Ma is interpreted as burial in the Siwalik foreland. The cooling from ~2 Ma to today corresponds to the exumation of rocks linked to the activty of the MFT. The timing of the initiation of the MFT is difficult to determine however. Figure 4: Maximum paleotempera- ture reached by the partially annealed samples from the Karnali River sec- tion, as predicted by the AFTSolve inversions, plotted as a function of stratigraphic depth. Dots indicate maximum temperature of best-fit in- version; error bars correspond to maximum and minimum peak tem- peratures allowed by the acceptable model fits. The peak temperatures from AFT inversions are consistent with a geothermal gradient of ~9 °C/km (95% confidence limit; r2 = 0.84) between ~2300 and ~4600 m depth and require a gradient of ~30 °C.km above that depth range. A linear 20 °C/km gradient is plotted for comparison. Constraints on tectonic evolution from thermal model ID MDT3 MBT MFT3 OF 0 5 10 km 0 -2000 2000 -4000 0 0,1 0 5 10 15 20 Length (μm) Frequency 0 20 40 60 80 100 120 140 0 1 2 3 4 5 Time (Ma) Temperature (°C) 1 2 3 4 5 6 Model 1 : 20 mm/an shortening for the last 0.3 Ma 3.7 Ma ; L=11.1+/-4.1 μm 0 0,1 0 5 10 15 20 Length (μm) Frequency 1.1 Ma ; L=10.6+/-3.4 μm 0 20 40 60 80 100 120 0 1 2 3 4 5 Time (Ma) Temperature (°C) 1 2 3 4 5 6 Model 2 : 3 mm/an shortening for the last 2 Ma 0 0,1 0 5 10 15 20 Length (μm) Frequency 0 20 40 60 80 100 120 0 1 2 3 4 5 Time (Ma) Temperature (°C) 1 2 3 4 5 6 Model 3 : 2 mm/an shortening from 2 Ma to 0.1 Ma 20 mm/an shortening from 0.1 Ma to today { 0 0,1 0 5 10 15 20 Length (μm) Frequency 0 20 40 60 80 100 120 140 0 1 2 3 4 5 Time (Ma) Temperature (°C) 1 2 3 4 5 6 6.3 Ma ; L=8.7+/-3.2 μm Model 4 : 15 mm/an shortening from 2 Ma to 1.8 Ma 20 mm/an shortening from 0.15 Ma to today { KAR-3 N = 36 ; μ = 9.52 ; e = 1.85 0 1 2 3 4 5 6 7 8 9 5 6 7 8 9 10 11 12 13 14 15 16 17 Length (μm) Number Lesser Himalayas Upper Siwaliks Lower Siwaliks Middle Siwaliks Eroded surface 2.8 Ma ; L=8.8+/-3.3 μm Figure 5: Restored cross section of the Karnali River used as input for the tectono-thermal modeling with Thrustpack (Institut Français du Pétrole package). The other inputs are lithology and associated thermal parameters; Modified from Mugnier et al. (1999). Figure 7: 4 end-member shortening models, with associated thermal histories and AFT data calculated for point 1 with the AFTSolve program. For comparison, we plot the track length distribution for sample KAR-3 witch corresponds to this point (L = Mean track length). A recent cooling underpredicts low mean track length. Hybrid models better fit the data. Mean Age : 4 +/- 0.5 Ma Min. Age : 1.8 +/- 0.7 Ma Figure 6: a) Initial state of the 2D models, and b) final state of model 3,showing teh temperature distri- bution. a) b) Further work Figure 8: PECUBE modeling output with one fault for a transect along the Trisuli River, cen- tral Nepal. There are some incoherences in the ages in the fault hangingwall. This work is in progress (modeling and data acquisition). -5 0 5 10 15 20 25 30 Depth (km) 27 28 29 Latitude ( o ) NS Profile 0 50 100 150 200 250 300 350 400 450 500 550 600 650 Temperature (C) Figure 9: North-South temperature transect along the Trisuli River profile from the PECUBE modeling in figure 8. Litterature cited: CARTER, A. & GALLAGHER, K. (2004) Characterizing the significance of provenance on the inference of thermal history models from apatite fission-track data - a synthetic data study. In: Detrital thermochronology - Provenance analysis, exhumation and landscape evolution of mountain belts (Ed. by Bernet, M. & Spiegel, C.). Geological Society of America Special Paper 378, Boulder Colorado, 7-24. GAUTAM, P. & FUJIWARA, Y. (2000) Magnetic polarity stratigraphy of Siwalik Group sediments of Karnali River section in western Nepal. Geophysical Journal International 142, 812-824. KETCHAM, R. A., DONELICK, R. A. & DONELICK, M. B. (2000) AFTSolve: A program for multi-kinetic modeling of apatite fission-track data. Geological Materials Research 2, http://gmr.minsocam.org/Papers/v2/v2n1/v2n1abs.html. MUGNIER, J. L., HUYGHE, P., LETURMY, P. & JOUANNE, F. (2004) Episodicity and rates of thrust sheet motion in the Himalayas (western Nepal). In: Thrust Tectonics and Petroleum Systems (Ed. by McClay, K. C.). American Association of Petroleum Geologists Memoir 82, 1-24. MUGNIER, J. L., LETURMY, P., MASCLE, G., HUYGHE, P., CHALARON, E., VIDAL, G., HUSSON, L. & DELCAILLAU, B. (1999) The Siwaliks of western Nepal: I - Geometry and kinematics. Journal of Asian Earth Sciences 17, 629-642. ROBERT, X. (2005) Analyse thermochronologique des sédiments Siwaliks : implications pour la séquence d’activité des failles et la mécanique du prisme frontal de l’Himalaya, Unpublished MSc. Thesis, Université Joseph Fourier, Grenoble, 36 pp.

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Page 1: Thermochronological analysis of Siwalik sediments … · LH crystalline nappe and crystalline nappe GH ... LETURMY, P. & JOUANNE, F. (2004) Episodicity and rates of thrust sheet motion

(1) Laboratoire de Géodynamique des Chaînes Alpines, UMR 5025, UJF, Grenoble, France (email: [email protected])(2) IFP, 92852 Rueil-Malmaison Cedex, France (3) Géosciences Rennes, F-35042 Rennes Cedex, France

X. Robert (1), P. Van der Beek (1), J.-L. Mugnier (1), E. Labrin (1), W. Sassy (2), J. Braun (3)

Thermochronological analysis of Siwalik sediments from the Karnali River section (western Nepal): Constraints on the kinematics of the frontal Himalayan prism.

Programme : RELIEFS DE LA TERRE ; Projet : « Flux tectonique et relief de l'Himalaya : une approche par thermo-chronologie détritique et géomorphologie »

ProblemsThe relief of mountain belts results from the interaction between tectonics and surface processes. Here, we develop a tool to follow the evolution of the thermal structure of a mountain belt by combining low-temperature thermochronometry and thermo-kinematic modeling, in order to better understand relief development in mountain belts. This is the first stage of a project that aims to elucidate the kinematics of shortening of the central Himalayan mountain belt and focuses on the currently active outermost thrust (the Main Frontal Thrust). The work will continue with a study of the more internal fold-and-thrust belt deformation along the Main Boundary thrust and Main Central Thrust as well as assessing possible out of sequence faulting

84°

TIBET

INDIAKali G.

Bhairawa

PiuthanNepalganj

JajarkotDunai

Dhangadhi

Bajhang

Karnali

30° 30°

28° 28°

80°

MCT

MBT

MBT

MT

MT

25 25 50 75 100 km0

MFT

MFT

Jumla

Rapti

b

LESSERHIMALAYAS

HIMALAYAS

TIBET PLATEAU

GANGA PLAIN

HIGHER M C T

M B TM B T

M F T

M C T

SUTURE

KARAKORUM FAULT

30° N 85°E75°E 80°E 25°N

90°E

95°E

a

TinauSurai

Karnali

Terai

Duns & recent sediments

Siwaliks

Lesser Himalaya zone

Greater Himalaya zone

LH crystalline nappe

and crystalline nappe

GH leucogranite

Thrust

Active faultCoupe

Study area and sample section

0

-2000

2000

-4000

MBT

Karnali canyon

MFT3 IDMDT3 OF

0 5 10 km

Sampling zone

C

Figure 1: a) Tectonic sketch map of central Himalaya showing location of the study area (inset); b) Geological map of Western Nepal showing locations of sampled section (KAR: Karnali); Modified from Mugnier et al. (2004). c) Balanced cross section along the Kar-nali River showing sample locations and relationship with local structure. Location of cross-section is indicated on Figure b. Modified from Mugnier et al. (1999).

AFT age (Ma)0

1000

2000

3000

4000

5000

6000

0 2 4 6 8 10 12 14

Stra

tigra

phic

dep

th (m

)

KAR-3N = 36 ; µ = 9.52 ; e = 1.85

0

1

2

3

45

6

7

8

9

5 6 7 8 9 10 11 12 13 14 15 16 17

KAR-6N = 51 ; µ = 9.39 ; e = 1.87

0

2

4

6

8

10

12

14

5 6 7 8 9 10 11 12 13 14 15 16 17

KAR-7N = 80 ; µ = 10.74 ; e = 2.07

02468

101214161820

5 6 7 8 9 10 11 12 13 14 15 16 17

KAR-9N = 54 ; µ = 11.22 ; e = 2.50

0

2

4

6

8

10

12

14

5 6 7 8 9 10 11 12 13 14 15 16 17

KAR-10N = 68 ; µ = 12.11 ; e = 2.02

0

2

46

8

10

1214

16

18

5 6 7 8 9 10 11 12 13 14 15 16 17

KAR-11N = 78 ; µ = 11.32 ; e =2.61

0

2

4

6

8

10

12

14

5 6 7 8 9 10 11 12 13 14 15 16 17

KAR-12N = 75 ; µ = 10.97 ; e = 2.92

0

2

4

6

8

10

12

14

5 6 7 8 9 10 11 12 13 14 15 16 17

KAR-14N = 54 ; µ = 11.69 ; e = 2.00

0

2

4

6

8

10

12

5 6 7 8 9 10 11 12 13 14 15 16 17

Num

ber

Num

ber

Num

ber

Num

ber

Num

ber

Num

ber

Num

ber

Num

ber

Length (µm)

Length (µm)

Length (µm) Length (µm) Length (µm)

Length (µm)

Length (µm)Length (µm)

AFT ages and track length distribution

Figure 2: Main panel shows plot of AFT central ages (black diamonds) and minimum ages (i.e., age of the youngest popula-tion; white squares) against depth for samples from the Karnali River section. The solid line corresponds to the stratigraphic age. Stratigraphic ages for this section are from Gautam and Fujiwara (2000).

Small panels show track-length histograms for Karnali section samples (N = number of tracks lengths measured; µ = Mean track length, e = standard deviation).

Minimum ages are younger than stratigraphic ages under the depth 2000 m, which corresponds to the top of the PAZ (60°C). The mean thermal gradient is around 20°C/km.

Mean track lengths decrease with increasing depth, as a result of increasing partial annealing. Note that even deepest samples are not yet fully annealed.

Inversion of AFT data

0

20

40

60

80

100

120

0246810121416

KAR3KAR3-revKAR6-re2KAR-7-revKAR6-r22KAR7-re2KAR10-1

Ages (Ma)

Temp

erature (°C

)

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 20 40 60 80 100 120 140

Maximal temperature (°C)

Dep

th (m

)

~9 °C/kmr² = 0.8448

~31 °C/km

KAR-3KAR6-re-2KAR6-r22KAR7-resKAR7-re2KAR10-1KAR11

~20 °C/km

Figure 3: Inversion of AFT data of partially annealed samples from the Karnali River with AFTSolve (Ketcham et al., 2000). For clarity, confidence intervals and pre sedi-mentary histories are not represented. The heating from 14 Ma to ~3 Ma is interpreted as burial in the Siwalik foreland. The cooling from ~2 Ma to today corresponds to the exumation of rocks linked to the activty of the MFT. The timing of the initiation of the MFT is difficult to determine however.

Figure 4: Maximum paleotempera-ture reached by the partially annealed samples from the Karnali River sec-tion, as predicted by the AFTSolve inversions, plotted as a function of stratigraphic depth. Dots indicate maximum temperature of best-fit in-version; error bars correspond to maximum and minimum peak tem-peratures allowed by the acceptable model fits. The peak temperatures from AFT inversions are consistent with a geothermal gradient of ~9 °C/km (95% confidence limit; r2 = 0.84) between ~2300 and ~4600 m depth and require a gradient of ~30 °C.km above that depth range. A linear 20 °C/km gradient is plotted for comparison.

Constraints on tectonic evolution from thermal model

IDMDT3 MBT

MFT3 OF

0 5 10 km

0

-2000

2000

-4000

0

0,1

0,2

0 5 10 15 20

Length (µm)

Freq

uenc

y

020406080100120140

012345

Time (Ma)

Tem

pera

ture

(°C

) 123456

Model 1 : 20 mm/an shortening for the last 0.3 Ma

3.7 Ma ; L=11.1+/-4.1 µm

0

0,1

0,2

0 5 10 15 20

Length (µm)

Freq

uenc

y

1.1 Ma ; L=10.6+/-3.4 µm

0

20

40

60

80

100

120

012345

Time (Ma)

Tem

pera

ture

(°C

) 123456

Model 2 : 3 mm/an shortening for the last 2 Ma

0

0,1

0,2

0 5 10 15 20

Length (µm)

Freq

uenc

y

0

20

40

60

80

100

120

012345

Time (Ma)

Tem

pera

ture

(°C

) 123456

Model 3 : 2 mm/an shortening from 2 Ma to 0.1 Ma 20 mm/an shortening from 0.1 Ma to today{

0

0,1

0,2

0 5 10 15 20

Length (µm)

Freq

uenc

y

020406080100120140

012345

Time (Ma)

Tem

pera

ture

(°C

) 123456

6.3 Ma ; L=8.7+/-3.2 µm

Model 4 : 15 mm/an shortening from 2 Ma to 1.8 Ma 20 mm/an shortening from 0.15 Ma to today{

KAR-3N = 36 ; µ = 9.52 ; e = 1.85

0

1

2

3

45

6

7

8

9

5 6 7 8 9 10 11 12 13 14 15 16 17

Length (µm)

Num

ber

Lesser Himalayas

Upper Siwaliks Lower Siwaliks

Middle Siwaliks

Eroded surface

2.8 Ma ; L=8.8+/-3.3 µm

Figure 5: Restored cross section of the Karnali River used as input for the tectono-thermal modeling with Thrustpack (Institut Français du Pétrole package). The other inputs are lithology and associated thermal parameters; Modified from Mugnier et al. (1999).

Figure 7: 4 end-member shortening models, with associated thermal histories and AFT data calculated for point 1 with the AFTSolve program. For comparison, we plot the track length distribution for sample KAR-3 witch corresponds to this point (L = Mean track length). A recent cooling underpredicts low mean track length. Hybrid models better fit the data.

Mean Age : 4 +/- 0.5 MaMin. Age : 1.8 +/- 0.7 Ma

Figure 6: a) Initial state of the 2D models, and b) final state of model 3,showing teh temperature distri-bution.

a)

b)

Further work

Figure 8: PECUBE modeling output with one fault for a transect along the Trisuli River, cen-tral Nepal. There are some incoherences in the ages in the fault hangingwall. This work is in progress (modeling and data acquisition).

−5

0

5

10

15

20

25

30

Dep

th (k

m)

272829

Latitude (o)

NS Pro�le

0 50 100 150 200 250 300 350 400 450 500 550 600 650

Temperature (C)

Figure 9: North-South temperature transect along the Trisuli River profile from the PECUBE modeling in figure 8.

Litterature cited:CARTER, A. & GALLAGHER, K. (2004) Characterizing the significance of provenance on the inference of thermal history models from apatite fission-track data - a synthetic data study. In: Detrital thermochronology - Provenance analysis, exhumation and landscape evolution of mountain belts (Ed. by Bernet, M. & Spiegel, C.). Geological Society of America Special Paper 378, Boulder Colorado, 7-24.GAUTAM, P. & FUJIWARA, Y. (2000) Magnetic polarity stratigraphy of Siwalik Group sediments of Karnali River section in western Nepal. Geophysical Journal International 142, 812-824.KETCHAM, R. A., DONELICK, R. A. & DONELICK, M. B. (2000) AFTSolve: A program for multi-kinetic modeling of apatite fission-track data. Geological Materials Research 2, http://gmr.minsocam.org/Papers/v2/v2n1/v2n1abs.html.MUGNIER, J. L., HUYGHE, P., LETURMY, P. & JOUANNE, F. (2004) Episodicity and rates of thrust sheet motion in the Himalayas (western Nepal). In: Thrust Tectonics and Petroleum Systems (Ed. by McClay, K. C.). American Association of Petroleum Geologists Memoir 82, 1-24.MUGNIER, J. L., LETURMY, P., MASCLE, G., HUYGHE, P., CHALARON, E., VIDAL, G., HUSSON, L. & DELCAILLAU, B. (1999) The Siwaliks of western Nepal: I - Geometry and kinematics. Journal of Asian Earth Sciences 17, 629-642.ROBERT, X. (2005) Analyse thermochronologique des sédiments Siwaliks : implications pour la séquence d’activité des failles et la mécanique du prisme frontal de l’Himalaya, Unpublished MSc. Thesis, Université Joseph Fourier, Grenoble, 36 pp.