TEMPERATURE AND STRAIN GRADIENTS ACROSS THE MAIN CENTRAL THRUST IN SOUTH-CENTRAL BHUTAN: IMPLICATIONS FOR THE ORIGIN OF INVERTED METAMORPHISM AND THE CONTRIBUTION OF TRANSPORT-PARALLEL STRETCHING TO CUMULATIVE MASS TRANSFER

Assessment of the mechanisms that drive mass transfer during orogenesis, and of their effects on the structural and thermal architecture of a mountain belt, is critical for understanding orogenic evolution. In order to evaluate mass transfer processes that operated during Himalayan orogenesis, we collected temperature, finite strain, and kinematic vorticity data through a 5 km thickness of Lesser and Greater Himalayan rocks in southern Bhutan. The transect crosses two major shear zones, the Main Central thrust (MCT) and Shumar thrust (ST). Raman spectroscopy on carbonaceous material and garnet-biotite thermometry are integrated with deformation temperatures from quartz petrofabrics. These data define inverted field gradients that correspond in structural position with the MCT and ST, which are both separated by sections in which temperatures remain essentially constant. Temperatures increase from ~400-500°C to ~700-750°C between 675 m below and 200 m above the MCT. This defines a 269±44 °C/km inverted gradient, interpreted to have formed via high-magnitude (~100-250 km) displacement through a thermal regime of sigmoidally-folded isotherms produced from protracted continental underthrusting. The boundaries of the MCT zone are interpreted to be delineated by the limits of inverted metamorphism, which defines the MCT as a discrete, ~0.9 km-thick shear zone within the >35 km-thick Bhutan thrust belt. Temperatures increase from ~300-400°C to ~400-530°C across the ST, which is attributed to differences in maximum burial depth of hanging wall and footwall rocks. Strain and vorticity data indicate that Lesser and Greater Himalayan rocks were deformed by pure shear-dominant (Wm ~0.0-0.3), layer-normal flattening strain. Rocks below the ST experienced 14% transport-parallel stretching and 22% foliation-normal shortening. Above the ST, transport-parallel stretching increases from 38 to 71%, and foliation-normal shortening increases from 36 to 49%, between 2.3 and 1.0 km below the MCT. The MCT acted as a stretching fault, with translation on the order of 100’s of km accompanied by transport-parallel stretching of footwall and hanging wall rocks on the order of 10’s of km. This demonstrates that stretching accommodated between major shear zones can make a significant contribution to cumulative mass transfer.