Metamorphic field gradients across the Himachal Himalaya, northwest India: Implications for the emplacement of the Himalayan crystalline core

Abstract

New constraints on pressures and temperatures experienced by rocks of the Himachal Himalaya are presented in order to test models for the emplacement of the Himalayan crystalline core here. A variety of methods were employed: petrographic analysis referenced to a petrogenetic grid, exchange and net‐transfer thermobarometry, Ti‐in‐biotite thermometry, and analysis of quartz recrystallization textures. Rocks along three transects (the northern Beas, Pabbar, and southern Beas transects) were investigated. Results reveal spatially coherent metamorphic field gradients across amphibolite‐grade and migmatitic metamorphic rocks. Along the northern Beas transect, rocks record peak temperatures of ~650–780°C at low elevations to the north of ~32°10' N; rocks in other structural positions along this transect record peak temperatures of <640°C that decrease with increasing structural elevation. Rocks of the Pabbar transect dominantly record ~650–700°C peak temperatures to the east of ~77°55' E and ~450–620°C peak temperatures farther west. Peak temperatures of ~450–600°C along the southern Beas transect record a right‐way‐up metamorphic field gradient. Results are integrated with literature data to determine a metamorphic isograd map of the Himachal Himalaya. This map is compared to metamorphic isograd map pattern predictions of different models for Himalayan crystalline core emplacement. This analysis excludes models involving large magnitude (>20–30 km) extrusion and permits (1) models involving small magnitude (<20–30 km) extrusion that is discontinuous along the orogen and (2) tectonic wedging models, in which the crystalline core was emplaced at depth between a sole thrust and a back thrust in the Early‐Middle Miocene.