CRUSTAL THICKENING, BARROVIAN METAMORPHISM, AND MID-CRUSTAL FLOW: INSIGHTS FROM THE HIMALAYA, NW INDIA

In many cases, shear zones and detachments that formed during crustal thickening in the Himalaya (i.e., Eocene and Oligocene) controlled the subsequent distribution of rocks that experienced different exhumation histories, pressure-temperature-time paths, and degrees of partial melting during the Miocene. Inherited structures can become boundaries for localized zones of partial melting and mid-crustal flow during doming and/or channel flow. New micro- and meso-scale structural analysis, combined with pressure-temperature estimates from the Haimanta Group exposed in the Sutlej Valley, NW India indicate that the rocks experienced Barrovian metamorphism that culminated at ~30 Ma. Crustal thickening created a metamorphic field gradient that increases from garnet-grade (567 ± 105°C and 6.7 ± 1.6 kbar to 499±99ºC and 4.5±1.4 kbar) through staurolite-grade (571 ± 92°C and 7.8 ± 1.4 kbar) and kyanite-grade (650-750ºC, 6-8 kbar) adjacent to the Leo Pargil dome. While at deeper structural positions rocks of the Haimanta Group were folded into recumbent nappes, passive Indian margin sedimentary rocks formed a fold and thrust belt in the upper crust. The base of the thrust wedge coincided with the top of the subducting Indian Plate where conditions were conducive to partial melting. A zone of partial melting fed an extensive leucogranite injection complex (23-19 Ma) that is preserved within the Leo Pargil dome. Sillimanite and cordierite overgrowths on porphyroblasts that grew during Barrovian metamorphism record near isothermal decompression, constrained by pressure-temperature estimates that were coeval with monazite growth at ~23 Ma. The overlap between the onset of decompression and timing of the injection complex suggests that crustal melting contributed buoyancy-driven flow into areas of extension that were localized near the dome. A distributed zone of interlayered schist, quartzite, marble, and leucogranite within the Leo Pargil shear zone records top-to-the-west shearing that partially accommodated doming. The coincidence of Leo Pargil shear zone with the base of the fold and thrust belt within passive Indian margin rocks supports models where detachments that were inherited from earlier stages of crustal thickening contributed to the localization of deformation and mid-crustal flow during doming.