INTEGRATING TEMPERATURE, FABRIC, STRAIN, AND KINEMATIC DATA TO ILLUMINATE THE STRUCTURAL EVOLUTION OF THE SOUTH TIBETAN DETACHMENT SYSTEM IN NORTHWESTERN BHUTAN

The north-vergent South Tibetan Detachment System (STDS) places low-grade Tethyan Himalayan (TH) sedimentary rocks against mid-crustal Greater Himalayan (GH) metamorphic rocks. Despite playing a critical role in all models of Himalayan tectonics, minimal data informing the structural evolution and magnitude of offset along the STDS are available. Here, we integrate petrofabric, finite strain, kinematic, and temperature data collected in northwestern Bhutan, in order to define the boundaries, internal architecture, and offset magnitude of the STDS. We divide the STDS into lower and upper structural levels. The lower level consists of the uppermost ~2 km of GH rocks, with the base defined by the occurrence of top-to-NW shear-sense indicators and an upward-increasing gradient in quartz fabric intensity. Integrating kinematic vorticity with published pressures indicates ~6-13 km of apparent structural thinning (86-93% shortening) in the lower level, accommodated via ≥30-76 km of simple shear-dominant (W_m = 0.74-0.88), top-to-NW displacement. Peak temperatures within the lower level of the STDS zone are ~650-750 °C. The upper level of the STDS zone consists of the lowest ~4.5 km of TH rocks, which accommodated ≥21 km of top-to-NW displacement via an upward decrease in pure shear-dominant (W_m = 0.00-0.45), transport-parallel lengthening (from 44% to 2%). Two distinct intervals of telescoped isotherms in the upper level define a cumulative upward decrease from ~700 to ~325 °C, defining ~160 and ~260 °C/km metamorphic field gradients. These intervals are separated by an abrupt upward increase from ~450 to ~620 °C, which we interpret as the result of post-STDS thrust repetition. Telescoped isotherms lie entirely above the highly thinned lower level of the STDS zone, which we attribute to progressive elevation of isotherms during protracted intrusion of granite sills. The top of the STDS zone is defined by an abrupt upward decrease in finite strain. Above the STDS zone, peak temperatures decrease upward from ~320 to ~250 °C through a 7 km-thick section of overlying TH rocks, defining a field gradient of 13 °C/km. This study demonstrates the utility of using gradients in fabric intensity and thin section-scale finite strain to define shear zone boundaries when field criteria that delineate strain gradients are not apparent.