PLACING LIMITS ON CHANNEL FLOW: IS CENTRAL BHUTAN STD-FREE?

Along the majority of the Himalayan orogenic belt, high-grade metamorphic rocks of the Greater Himalayan (GH) sequence are separated from lower-grade metasedimentary rocks by two shear zones, the basal, top-to-the-south Main Central Thrust (MCT), and the upper, top-to-the-north South Tibetan Detachment (STD). Seemingly pervasive ductile deformation within GH rocks and a lack of exposed hanging wall or footwall cut-offs for the MCT or STD have permitted models that predict 100’s of km of coeval displacement on both structures, extruding the GH section via gravitational loading from Tibet and focused erosion along the Himalayan topographic front. New mapping, stratigraphic columns, mineral assemblages, U/Pb zircon ages, and kinematic and strain data from GH and Tethyan Himalayan (TH) rocks in eastern and central Bhutan highlight dramatic along-strike changes in the P-T conditions recorded by GH rocks. In eastern Bhutan, TH rocks are separated from GH rocks displaying ubiquitous partial melt textures by a top-to-the-north sense shear zone correlated with the STD. However, in central Bhutan, limited partial melt textures are only present near the base of the GH section, the same biotite-muscovite-garnet mineral assemblage persists from 0.2-3 km above the MCT through the GH and TH sections, and distinct lithologies interfinger at the GH-TH contact, which suggests that TH strata are in depositional contact above GH strata. The same strata exposed on both sides of the STD limits slip along this structure to ~20 km. Thin-section scale top-to-the-south shear is focused in a 2 km thick zone above the MCT while the overlying 11 km thick section displays top-to-the-north shear, defining the profile of an asymmetric channel. Quartz microstructure allows us to document 20-34 km of top-to-the-north shear. Our data suggest that the GH-TH section in central Bhutan acted as a cool, viscous, low-displacement channel. When compared to higher-grade sections observed in eastern Bhutan, this highlights dramatic gradients in P-T conditions, viscosity and possibly displacement along strike. However, we suggest that the smooth, arcuate Himalayan front limits the magnitude of channel flow even in regions containing significant partial melt, and that the magnitude of channel flow is small compared to the total mass balance of the system.