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