Journal of Geophysical Research
Journal of Geophysical Research
Volume XX, Issue XX Abstract
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   Geochronologic Data
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Copyright 2001 American Geophysical Union. All rights reserved.

Geochronologic and Thermobarometric Constraints on the Evolution of the Main Central Thrust, central Nepal Himalaya

E.J. Catloscorrespondence information, send an email, 1, T. Mark Harrison1, Matthew J. Kohn2, Marty Grove1, F.J. Ryerson3, Craig E. Manning1, B.N. Upreti4

1.Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, 90095-1567, USA

2.Department of Geological Sciences, University of South Carolina, Columbia, South Carolina, 29208, USA

3.Institute for Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, Livermore, California, 94550, USA

4.Department of Geology, Tri-Chandra Campus, Tribhuvan University, Kathmandu, Nepal

Received April 20, 2000; revised September 22, 2000; accepted October 10, 2000.


Abstract

The Main Central Thrust (MCT) juxtaposes the high-grade Greater Himalayan Crystallines over the lower-grade Lesser Himalaya Formations; an apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust. Garnet-bearing assemblages sampled along the Marysandi River and Darondi Khola in the Annapurna region of central Nepal show striking differences in garnet zoning of Mn, Ca, Mg, and Fe above and below the MCT. Thermobarometry of MCT footwall rocks yield apparent inverted temperature and pressure gradients of ~18 C/km and ~0.06 km/MPa, respectively. Pressure-temperature (P-T) paths calculated for upper Lesser Himalaya samples that preserve prograde compositions show evidence of decompression during heating, whereas garnets from the structurally lower sequences grew during an increase in both pressure and temperature. In situ (i.e., analyzed in thin section) ion microprobe ages of monazites from rocks immediately beneath the Greater Himalayan Crystallines yield ages from 18-22 Ma whereas Late Miocene and Pliocene monazite ages characterize rocks within the apparent inverted metamorphic sequence. A Lesser Himalayan sample collected near the garnet isograd along the Marysandi River transect contains 3.30.1 Ma monazite grains (P 0.72 GPa, T 535C). This remarkably young age suggests that this portion of the MCT shear zone accommodated a minimum of ~30 km of slip over the last 3 m.y. (i.e., a slip rate of >10 mm/yr) and thus could account for nearly half of the convergence across the Himalaya in this period. The distribution of ages and P-T histories reported here are consistent with a thermo-kinematic model in which the inverted metamorphic sequences underlying the MCT formed by the transposition of right-way-up metamorphic sequences during Late Miocene-Pliocene shearing.


Data Repository

All data tables and figures are given in PDF format and can be accessed if you download Adobe Acrobat Reader .  This  program is free of charge.

1. Thermobarometric data

Detailed electron microprobe analyses of Marysandi River samples:

Brief summary of mineral compositions:

MA24

MA27

MA33

MA43

Marysandi River

MA45

MA49

MA58

MA61

Darondi Khola

MA64

MA65

MA68

MA79

MA83

MA86

Mineral compositions used in P-T paths modeling

X-ray element maps of Marysandi River garnets:

MA24

MA25(grt1) MA25(grt2)

MA25(grt3)

MA49(grt1) MA49(grt2) Ma45(grt1) MA45(grt2)
MA45(grts3&4) MA27(grt1) MA27(grts2&3) MA27(grt4)
MA27(grt5) MA43(grt1) MA43(grt2) MA43(grt3)

MA81

MA79(grt1)

MA79(grt2)

MA33

MA83

MA61(grt1) MA61(grt2) MA65
MA74(grt1) MA74(grt2) MA86
Adjust the brightness/contrast of these maps yourself.  For the scale bars, see the pdf files above.  Just download the .tif or .jpg file and open them using Image J (PC) or NIH Image (Mac). These programs are free of charge.

Sample

Mn Ca Mg Fe

MA24

174KB 174KB 170KB 174KB
MA25(grt1) 143KB 143KB 138KB 143KB
MA25(grt2) 392KB 392KB 392KB 291KB
MA25(grt3) 198KB 198KB 198KB 198KB
MA49(grt1) 127KB 127KB 127KB 127KB
MA49(grt2) 479KB 479KB 479KB 479KB
MA45(grt1) 140KB 140KB 140KB 140KB
MA45(grt2) 223KB 223KB 223KB 223KB
MA45(grts3&4) 232KB 232KB 232KB 232KB
MA27(grt1) 158KB 158KB 158KB 158KB
MA27(grts2&3) 404KB 404KB 404KB 404KB
MA27(grt4) 264KB 264KB 264KB 264KB
MA27(grt5) 173KB 173KB 173KB 173KB
MA43(grt1) 81KB 81KB 81KB 81KB
MA43(grt2) 350KB 350KB 350KB 350KB
MA43(grt3) 95KB 95KB 95KB 95KB
MA81 446KB 446KB 446KB 446KB
MA79(grt1) 343KB 343KB 343KB 343KB
MA79(grt2) 340KB 340KB 340KB 340KB

MA33

142KB 142KB 142KB 142KB

MA83

100KB 100KB 100KB 100KB
MA61 138KB 135KB 130KB 264KB
MA65 802KB 802KB 802KB 802KB
MA74 801KB 801KB 801KB 801KB
MA86 227KB 212KB 573KB 150KB

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2. Geochronologic data

Detailed Th-Pb  ion microprobe analyses of Himalayan monazite grains

Greater Himalayan Crystallines

Upper Lesser Himalaya

Lower Lesser Himalaya

BSE images of dated central Nepal monazite grains
MA11 MA15 MA18 MA19
MA25 MA27 MA33 MA45
MA48 MA65 MA83 MA84
MA86 (P1) (P2) DH30 DH39 DH51
DH58(reflected lt) DH71 DH73 DH75
40Ar/39Ar analyses of Himalayan muscovite

Greater Himalayan Crystallines

Upper Lesser Himalaya

Lower Lesser Himalaya

Inverse isochron plots

Greater Himalayan Crystallines
Upper Lesser Himalaya (P1) (P2) (P3)
Lower Lesser Himalaya (P1) (P2)
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3. Full manuscript text
Manuscript text
Figure Captions
Manuscript tables
Table 1. Mineral compositions used for P-T path modeling.
Table 2. Th-Pb ages of spots on central Nepal monazite grains.
Table 3. Summary of muscovite K-Ar analyses.
Manuscript figures
Figure 1. Geologic map of the Himalayan range
Figure 2. Geologic map of the Annapurna-Manaslu-Ganesh region of central Nepal.
Figure 3. Sample location map from rocks collected along the Marysandi River.
Figure 4. Sample location map from rocks collected along the Darondi Khola.
Figure 5. Geologic cross-sections along the Marysandi River and Darondi Khola transects.
Figure 6. Particle and P-T path predicted for a Lesser Himalaya rock if the MCT experienced a single episode of slip.
Figure 7. Particle and P-T path predicted for a Lesser Himalaya Formation rock if the footwall experienced two episodes faulting.
Figure 8. Electron-microprobe X-ray element maps of a Domain 1 garnet.
Figure 9. Electron-microprobe X-ray element maps of Domain 2 garnets.
Figure 10. Electron-microprobe X-ray element maps of a Domain 3 garnet.
Figure 11. Mole fractions of spessartine, grossular, and Fe/(Fe+Mg) across garnets from Domain 1, 2, and 3.
Figure 12. Thermobarometric results from central Nepal samples plotted versus structural distance from the MCT.
Figure 13. Metamorphic pressures and temperatures and P-T paths calculated for central Nepal garnet-bearing assemblages.
Figure 14. BSE images of Domain 1 sample MA45 with monazite age information (1s) indicated.
Figure 15. BSE images of Domain 2 samples MA33 and MA83 with monazite age information (1s) indicated.
Figure 16. BSE image of Domain 3 sample MA84 with monazite age information (1s) indicated.
Figure 17. BSE images of Domain 3 sample MA86 with monazite age information (1s) indicated.
Figure 18. Th-Pb monazite and 40Ar/39Ar mica age distribution map along the Marysandi River.
Figure 19. Weighted mean Th-Pb monazite and 40Ar/39Ar mica age distribution map along the Darondi Khola.
Figure 20. Th-Pb monazite ages of central Nepal samples plotted versus structural distance normal to MCT.
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mail1.gif (129 bytes) Corresponding author. Tel.: (310) 206-2940; Fax: (310) 825-2779; Email: catlos@argon.ess.ucla.edu; Web page: http://oro.ess.ucla.edu/~catlos/


Journal of Geophysical Research Summary
Article
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Volume XX, Issue XX
In press
Pages XX-XX

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