TECTONIC GEOMORPHOLOGY OF THE PUNA TSANG CHHU VALLEY, BHUTAN

We use tectonic geomorphologic methods to investigate Quaternary deformation along the Puna Tsang Chhu (river valley) of western Bhutan. The Puna Tsang Chhu contains a set of alluvial cut-and-fill and bedrock river terraces with associated sediments that record a prolonged history of aggradation and incision in response to climatic changes, surface uplift, and erosional exhumation. Geomorphic and geologic mapping along a 70-km stretch of the Puna Tsang Chhu between the towns of Uma and Amrimo was completed in March and April 2009. We identified a flight of 8 fluvial terrace levels cut into a complex succession of fluvial sands and conglomerates, landslide deposits, and lacustrine units, the latter two of which may be associated with episodic damming resulting from glacial lake outburst floods (GLOFs). Our mapping shows that some of the oldest fluvial gravels have experienced localized backtilting of up to 10 degrees. High-precision real-time-kinematic (RTK) GPS surveys of the modern river profile, terrace tread surfaces, dry waterfalls, and other geomorphic features in the Puna Tsang Chhu were also carried out. We integrate the survey data with our mapping and correlate the various terrace surfaces throughout the Puna Tsang Chhu to produce a 3D geometric model of the geomorphology of the area in which we identify stretches of oversteepening in the modern river profile and localized warping of interpolated terrace tread profiles. These observations indicate at least two zones of localized deformation in the Puna Tsang Chhu. A suite of samples for geochronologic analysis consisting of eight optical luminescence (OSL) samples and three bedrock cosmogenic radionuclide (CRN) exposure age samples were also collected to place numerical constraints on the ages of the terrace tread surfaces and of the underlying sedimentary packages. Ongoing work will incorporate the geochronologic data to determine rates of aggradation and incision associated with tectonic activity, from which rates of surface uplift may be inferred. The geochronology may also place constraints on the timing of past GLOF events.