QUANTIFYING CONTINENTAL DEFORMATIONAL PROCESSES THROUGH DOCUMENTING CHANGES IN TEMPERATURES AND RATES: EXAMPLES FROM THE BHUTAN HIMALAYA

The Bhutan Himalaya is a unique location to evaluate the relative contributions of both channel flow and critical taper deformation mechanisms. Here, Greater Himalayan (GH) rocks are exposed over a north-south distance of 120-150 km. The Main Central Thrust (MCT), the main bounding structure between GH and Lesser Himalayan (LH) rocks, is exposed within 20 km of the Main Frontal Thrust, the southern most structure in the Himalayan fold-thrust belt. In addition, Bhutan hosts a series of erosionally preserved Tethyan Himalayan (TH) remnants, all associated with pervasive top to the north shear indicators. By combining an emerging suite of geochronologic and thermochronologic data, we can document burial age, peak temperatures and complete cooling history of GH, LH and TH rocks from 26 Ma to the present across a distance of ~125 km. These integrated cooling curves highlight windows of fast exhumation that vary spatially and temporally and post-date motion on both the MCT and the Southern Tibetan Detachment (STD). Timing of motion on the MCT and STD is constrained by growth and chemistry of monazite, which indicates burial of GH rocks from 26-20 Ma and crystallization of melt near the STD from 20-15 Ma. Rapid cooling from 700° C to 200° C south of the STD is linked to duplex formation that folds both the MCT and the STD. Linking pulses of fast exhumation to structures that facilitate the vertical motion of material allows us to document the magnitude and timing of displacement on thrust sheets and to highlight temporal variability in shortening rates. The MCT, Paro thrust and STD were active from 20-15 Ma, with thrusting rates at or in excess of plate tectonic rates (4.2 + 4.4/-1.5 cm/yr). Rates in excess of plate tectonic rates provide insight into permissible magnitudes of STD displacement. Although displacement on the STD ceased at ~15 Ma, thrusting continued with the emplacement of the Shumar thrust at 15-12 Ma, and growth of the Lesser Himalayan duplex from 12-9 Ma. Duplex growth accommodated shortening rates of 44 ± 24 mm/yr. The southernmost thrust sheets, including the Main Boundary and Main Frontal thrusts, were emplaced between 9 and 0 Ma. 9 to ~2 Ma marked a period of limited convergence accommodated on the Main Himalayan thrust, with the shortening rate possibly increasing in the last 2 Myr.