Volume XX, Issue XX	Abstract
In press	Data Repository
Pages XX-XX	Thermobarometric Data
	Geochronologic Data
	Full text
Copyright © 2001 American Geophysical Union. All rights reserved.

E.J. Catlos^{, , 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 3.3±0.1 Ma monazite grains (P » 0.72 GPa, T » 535°C). 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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Corresponding author. Tel.: (310) 206-2940; Fax: (310) 825-2779; Email: catlos@argon.ess.ucla.edu; Web page: http://oro.ess.ucla.edu/~catlos/

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