The tectonometamorphic evolution of the Greater Himalayan sequence as exposed in central Nepal and adjacent south-central Tibet

Abstract

Understanding the development of the Himalaya is critical to elucidating continental collisional processes. The Greater Himalayan sequence (GHS), the exhumed mid-crust of the Himalayan orogen, records the tectonometamorphic evolution of the Himalaya from its deep hinterland to its foreland. The GHS in central Nepal and adjacent Tibet is deformed pervasively; quartz c-axis orientation fabrics from across the GHS indicate that it was deformed at high temperatures (~550-650˚C). The asymmetries of these quartz c-axis fabrics confirm field observations that define a reversal in shear sense from top-south shear near the bottom and middle of the GHS to top-north shear near the top of the package proximal to the South Tibetan detachment system (STDS). Estimates of mean kinematic vorticity from across the GHS indicate a pure shear contribution between 33% and 67%. U-Pb geochronologic data from the upper GHS exposed in the Changgo culmination in south Tibet indicate that melt crystallization and metamorphism related to crustal thickening occurred at ca. 35 Ma and was succeeded by a second metamorphic episode and syn-kinematic voluminous anatexis at ca. 22 Ma. The upper GHS was thinned vertically by 50% and extended horizontally during and immediately after the second metamorphic event, in a manner typical of the deep hinterland regions of orogens. In central Nepal, the ductile lateral extrusion of the upper GHS between the Main Central thrust (MCT) below and STDS above ceased by ~19 Ma. The cessation of lateral extrusion followed the collapse of the orogenic wedge and a reduction in the gravitational potential necessary to drive the propagation of deformation southwards towards the foreland. To restore the geometry of the wedge, deformation stepped out-of-sequence into southern Tibet, with the exhumation of the Changgo culmination and the North Himalayan antiform, before migrating incrementally back toward the foreland. Subsequently, the MCT migrated downward structurally adding material to the lower GHS as thrust slices, characteristic of foreland-style deformation. Thus, the transition between the upper and lower GHS in central Nepal records the transition from hinterland-style deformation to foreland-style deformation.