There are two boundaries, or physiographic transitions, between three distinct topographic regions in Bhutan. The southern and northern regions have relatively steep hillslopes and high relief, while the central region has gentle hillslopes and low relief. The geomorphology of a 70-km stretch along the Puna Tsang Chhu valley between the towns of Uma and Amrimo was studied in March and April of 2009 to provide constraints on the active tectonics of western Bhutan and the nature of these physiographic transitions.

The Puna Tsang Chhu valley contains a set of alluvial cut-and-fill and bedrock (strath) river terraces with associated sediments – younger than ca. 18.8 ka – that record a prolonged history of aggradation – ca. 7.5 mm/yr – and incision – ca. 7.7 mm/yr – in response to climatic changes and surface uplift. The presence of preserved geologic markers, particularly river terraces that are younger than ca. 10.4 ka, makes this valley an ideal candidate for tectonic geomorphology research. Methods used in order to investigate the physiographic transitions in the Puna Tsang Chuu valley include: geologic field mapping, surveying, OSL geochronology, and quantitative geomorphic modeling. Geologic observations are incorporated with the geochronology control to investigate the cut-and-fill history, uplift rates, and temporal uplift patterns along the Puna Tsang Chhu.

This research suggests that these physiographic transitions are tectonic in nature. Observations associated with the northern physiographic transition (PT2a) include a steep, concave river profile, an increase in relief to the north, a fluvial knickpoint, and an increase in terrace riser height to the north. Evidence for an active fault at PT2a includes abrupt changes in modern stream length-gradient (SL), slope gradient (S), and area of relief (A_r) indices. The SL and S indices, calculated for paleo-river profiles defined by retro-deformed fluvial terrace profiles, indicate that activity at PT2a is steady. The fault at PT2a could be the Kakhtang Thrust (KT) because the KT is close to PT2a and both have the same trend. PT2b may be caused by recent activity along a ramp on the Main Central Thrust (MCT) rather than the surface trace of an active fault. Observations associated with PT2b include: a steep, convex river profile, an increase in relief to the south, a fluvial knickpoint, a gradual increase in surface uplift to the south, and an increase in terrace riser heights to the south. The results of the SL, S, and A_r analyses suggest that recent, yet diminishing tectonic activity formed PT2b. The recent, out-of-sequence thrust activity along the KT and MCT suggests that exhumation of the GHS as the leading edge of an intracrustal channel may be an ongoing process in the Bhutan Himalaya.