METAMORPHIC AND DEFORMATION TEMPERATURE TRENDS IN THE HIMALAYAN THRUST BELT IN EASTERNMOST BHUTAN: PRELIMINARY INSIGHTS FROM RSCM THERMOMETRY AND QUARTZ PETROFABRICS

The temperature conditions of deformation exert a 1st-order control on the evolution of orogenic systems. To better understand how the thermal architecture of an orogenic belt can affect the style and magnitude of deformation, we are investigating trends in peak metamorphic and deformation temperatures in the Himalayan orogen in easternmost Bhutan. Our studied transect extends from dominantly brittle deformation in the frontal thrust belt to penetratively strained rocks deformed by ductile shear zones in the hinterland. Here, we present preliminary temperature data from rocks that span from ~22 km structurally below to ~10 km structurally above the Main Central thrust (MCT), including peak temperatures from Raman spectroscopy of carbonaceous material (RSCM; n=17), deformation temperatures from quartz fabric opening angles (n=5), and semi-quantitative deformation temperature ranges from quartz recrystallization microstructures (n=60).

Rocks 22 km below the MCT yield an RSCM temperature of ~190±30 °C and contain non-recrystallized quartz. Rocks between 21-17 km below the MCT yield opening angles that correspond to temperatures of ~350±50 °C and quartz recrystallization textures consistent with temperatures of ~350-450 °C. Between ~17-2 km below the MCT, RSCM temperatures range from ~470 to 570 °C, which overlap with quartz opening angle (~450-520 °C) and recrystallization texture (~450-550 °C) estimates. Between 2 km below and 5 km above the MCT, RSCM temperatures increase upward from ~470 °C to ~550-650 °C, and apparent quartz recrystallization temperatures increase upward from ~450-550 °C to ~550-750 °C. Between 5-10 km above the MCT, RSCM temperatures decrease upward from ~630-680 °C to ~500-550 °C, and apparent quartz recrystallization temperatures decrease from ~550-750 °C to ~450-550 °C.

In summary, temperatures increase sharply at 17 km below the MCT, then remain relatively constant between 17-2 km below the MCT. A trend of apparent inverted temperature is defined across the MCT zone, with the highest temperatures located 5 km above the MCT. An apparent upright temperature gradient is defined between 5-10 km above the MCT. Additional temperature data, fabric analyses, metamorphic pressures, and finite strain data will be collected to further define these preliminary trends.