A unified model for the origin of Himalayan anatexis and inverted metamorphism, Main Central Thrust, Nepal Himalaya

T. Mark Harrison, Marty Grove, Oscar M. Lovera, and E.J. Catlos

Submitted to JGR, Nov. 11, 1997.

Fig. 1 Geological map of the Himalaya and Southern Tibet. Fig. 2 Generalized cross section through the central Himalaya illustrating the juxtaposition of the major lithostratigraphic units across the major Himalayan faults (modified from Schelling and Arita [1991] and Zhao et al. [1993].

Fig. 3 Schematic illustration of tectonic development of the Himalayan Thrust System. Active faults shown as bold black lines while abandoned faults indicated with bold gray lines (a) Possible 25 Ma distribution of the protoliths of Greater Himalayan Crystallines and Lesser Himalayan Formations with respect to Indian cratonic margin after Eohimalayan thickening at ca. 40 Ma. (b) Thrusting begins along the MHT flat and MCT decollement from 25-15 Ma. Note that this fault system forms immediately above refractory rocks of the Indian craton. (c) Thrusting along MHT flat and MBT ramp from 15-8 Ma. Abandonment of the MCT ramp at 15 Ma causes accretion of upper Lesser Himalayan Formations rocks to the hanging wall. (d) Out-of-sequence thrusting in the high Himalaya from 8-6 Ma involving MCT-I thrust ramp. (e) Further development of MCT zone (6-2 Ma). Abandonment of MCT-I at 6 Ma leads to accretion of lower Lesser Himalayan Formations rocks to hanging wall. (f) Abandonment of the MCT zone at 2 Ma. Southward transfer of displacement to MFT ramp/MHT decollement.
Fig. 4 Plot of melt fraction versus temperature for dry melting experiment performed on several different pelitic bulk compositions (see AKNa and AFM projections in inset). The solid line represents the bulk melting relationship used in the model.

Fig. 5 Sequence of isothermal sections at 23 Ma, 15 Ma, 8 Ma, and 2 Ma indicating the position of HHL (solid black) and NHG (stippled pattern) source regions with time. Active faults are shown as bold lines (see Fig. 3). Isothermal contour interval is 100°C.

Fig. 7 Variation in melt fraction vs. source region position in the hanging wall caused by changes in the model parameters. (a) Effect of shifting the melting relationship of Fig.4 downwards 10°C in temperature (dashed). A positive 10°C shift produces the opposite effect (dotted). Solid line represents no shift in temperature (b) Effect of increasing the thickness of the shear zone from 1 km (solid) to 2 km (dashed) and 4 km (dotted). Note that although the fraction of melt produced decreases, the overall melt volume increases significantly. (c) Effect of changing shear stress from 30 MPa (solid) to 10 MPa (dashed) and 50 MPa (dotted).
Fig. 6 Time-cumulative melt fraction-distance predictions from the model. Position of melt source regions in the hanging wall relative to the left boundary of the grid are calculated for 8 Ma (17 m.y. after displacement begins in model). (a) Total melt fraction produced vs. 8 Ma hanging wall position. Age contours represent melt production at the indicate times. (b) Time of cessation of melting vs. 8 Ma hanging wall position. Contours are of melt fraction produced by 8 Ma.

Fig. 8 (a) Positions of MCT zone samples from 8-4 Ma. (b) Cooling histories of MCT zone samples. Fig. 9 Observed and predicted variation of mineral ages with structural distance from MCT. (a) ⁴⁰Ar/³⁹Ar biotite. (b) Th-Pb monazite apparent ages.

TABLES

Table 1: Crystallization Ages of Himalayan Granites.
Table 2: Slip History of Himalayan Thrusts.
Table 3: Model Parameters


Fig. 1 Geological map of the Himalaya and Southern Tibet.	Fig. 2 Generalized cross section through the central Himalaya illustrating the juxtaposition of the major lithostratigraphic units across the major Himalayan faults (modified from Schelling and Arita [1991] and Zhao et al. [1993].

Fig. 3 Schematic illustration of tectonic development of the Himalayan Thrust System. Active faults shown as bold black lines while abandoned faults indicated with bold gray lines (a) Possible 25 Ma distribution of the protoliths of Greater Himalayan Crystallines and Lesser Himalayan Formations with respect to Indian cratonic margin after Eohimalayan thickening at ca. 40 Ma. (b) Thrusting begins along the MHT flat and MCT decollement from 25-15 Ma. Note that this fault system forms immediately above refractory rocks of the Indian craton. (c) Thrusting along MHT flat and MBT ramp from 15-8 Ma. Abandonment of the MCT ramp at 15 Ma causes accretion of upper Lesser Himalayan Formations rocks to the hanging wall. (d) Out-of-sequence thrusting in the high Himalaya from 8-6 Ma involving MCT-I thrust ramp. (e) Further development of MCT zone (6-2 Ma). Abandonment of MCT-I at 6 Ma leads to accretion of lower Lesser Himalayan Formations rocks to hanging wall. (f) Abandonment of the MCT zone at 2 Ma. Southward transfer of displacement to MFT ramp/MHT decollement.	Fig. 4 Plot of melt fraction versus temperature for dry melting experiment performed on several different pelitic bulk compositions (see AKNa and AFM projections in inset). The solid line represents the bulk melting relationship used in the model. Fig. 5 Sequence of isothermal sections at 23 Ma, 15 Ma, 8 Ma, and 2 Ma indicating the position of HHL (solid black) and NHG (stippled pattern) source regions with time. Active faults are shown as bold lines (see Fig. 3). Isothermal contour interval is 100°C.

Fig. 7 Variation in melt fraction vs. source region position in the hanging wall caused by changes in the model parameters. (a) Effect of shifting the melting relationship of Fig.4 downwards 10°C in temperature (dashed). A positive 10°C shift produces the opposite effect (dotted). Solid line represents no shift in temperature (b) Effect of increasing the thickness of the shear zone from 1 km (solid) to 2 km (dashed) and 4 km (dotted). Note that although the fraction of melt produced decreases, the overall melt volume increases significantly. (c) Effect of changing shear stress from 30 MPa (solid) to 10 MPa (dashed) and 50 MPa (dotted).	Fig. 6 Time-cumulative melt fraction-distance predictions from the model. Position of melt source regions in the hanging wall relative to the left boundary of the grid are calculated for 8 Ma (17 m.y. after displacement begins in model). (a) Total melt fraction produced vs. 8 Ma hanging wall position. Age contours represent melt production at the indicate times. (b) Time of cessation of melting vs. 8 Ma hanging wall position. Contours are of melt fraction produced by 8 Ma.

Fig. 8 (a) Positions of MCT zone samples from 8-4 Ma. (b) Cooling histories of MCT zone samples.	Fig. 9 Observed and predicted variation of mineral ages with structural distance from MCT. (a) ⁴⁰Ar/³⁹Ar biotite. (b) Th-Pb monazite apparent ages.