ACTIVE UPLIFT IN THE BHUTAN HIMALAYA AS INDICATED BY BEDROCK RIVER PROFILES

The topography of the Bhutan Himalaya is unlike that of the central Nepal Himalaya. The Bhutan Himalaya is characterized by two “lobes” of steep topography, separated by a step of low slope and diminished relief which forms an east-west trending, anomalously flat belt. The northern and southern boundaries of this belt are abrupt physiographic transitions that may be due to spatial gradients in tectonic uplift. This study investigates the spatial distribution of bedrock channel gradients and concavity in the Bhutan Himalaya to understand the nature and degree of active deformation. Analyses of longitudinal profiles of major rivers in Bhutan using a bedrock incision model indicate two zones of steep channels adjacent to the physiographic transitions. Channel profiles are steep in their headwaters and lower reaches but are less steep within the central, anomalously flat belt. Since the region has homogeneous lithology and invariant orographic precipitation, we argue that these patterns of channel response cannot be easily explained by differential erosion and therefore likely reflect tectonic uplift along these physiographic transitions. Recent deformation at these physiographic transitions is further supported by field mapping of brittle faults and surveys of deformed river terraces.