MICROSTRUCTURAL AND THERMOBAROMETRIC DATA DEFINE DISTRIBUTED NORTH-VERGENT SHEARING IN EASTERNMOST BHUTAN: AN ALTERNATIVE STYLE FOR THE SOUTH TIBETAN DETACHMENT SYSTEM

The South Tibetan Detachment System (STDS) is a top-to-north, normal-sense structure that places Tethyan Himalayan (TH) sedimentary rocks over high-metamorphic grade Greater Himalayan (GH) rocks. Across Bhutan, several isolated exposures of the STDS have been mapped at the base of interpreted TH klippen. However, the location and deformation style of the STDS are disputed; some studies map discrete STDS exposures at structural levels varying between 2 and 10 km above the Main Central thrust (MCT), while others interpret a 10 km-thick zone of distributed north-vergent shearing. To help resolve this debate, we collected microstructural and P-T data from a transect at Sakteng, easternmost Bhutan, that crosses 6 km of GH rocks, the inferred STDS at the GH-TH contact, and 7 km of overlying TH rocks. Between 1 km below and 5 km above the GH-TH contact, top-to-north shear-sense indicators are prevalent, finite strain analyses define foliation-normal flattening (Rs ~2-3) with no apparent trend with structural height, kinematic vorticity ranges between 0.0-0.3 (pure shear-dominant), and quartz fabric intensity (cylindricity) is overall low (0.1-0.3) and decreases gradually structurally upward. RSCM and garnet-biotite thermometry define a nearly linear upward decrease from ~750 °C at 4-5 km below the GH-TH contact (1-2 km above the MCT) to ~500 °C at 4 km above the contact (~30 °C/km field gradient). Peak pressures decrease upward from ~10 kbar at 4-5 km below the GH-TH contact, to ~7-8 kbar between 2 km below and 0.2 km above the contact, to ~5 kbar at 1 km above, defining a distributed, super-lithostatic field gradient of ~1 kbar/km. The Sakteng transect lacks any discrete structural or P-T discontinuities that are hallmarks of other studied exposures of the STDS. Collectively, our structural and metamorphic data indicate that north-vergent, pure shear-dominant shearing was distributed through a ~10 km-thick section approximately centered on the GH-TH contact, with post-peak metamorphic flattening strain acting to yield a super-lithostatic pressure gradient. This is similar to the scenario of distributed shearing that has been documented in central Bhutan. Our work demonstrates the utility of applying a combination of techniques to evaluate the structural style, thickness, and strain gradients across shear zones.