Field Evidence of Active Uplift in the Central Bhutan Himalaya

The Bhutan Himalaya is distinguished by an east-west trending anomalously flat swath of topography bounded by abrupt physiographic transitions (PT2a and PT2b) to the north and south, respectively. These physiographic transitions manifest themselves as prominent knickpoints on the longitudinal profiles of major rivers in Bhutan. Analyses of longitudinal profiles of rivers using a bedrock incision model indicate discrete zones of steep channels adjacent to the physiographic transitions. Mapping of bedrock geology shows that these knickpoints do not mark lithologic contacts, and therefore, patterns in channel response may not reflect differential erosion. In addition, climatic factors, such as precipitation, are spatially invariant in Bhutan. Thus, the morphology of rivers in Bhutan at the physiographic transitions may reflect spatial gradients in tectonic uplift. Kinematic analysis of brittle faults and shear zones indicate potentially recent deformation with thrust sense and normal sense movement along PT2a and PT2b, respectively. Surveys of fluvial terraces show deformation of Quaternary deposits and provide direct evidence for local and regional tectonic uplift. The age of the organic material within the terraces obtained by 14C method will place constraints on the age of the terraces and the rate of Quaternary deformation in the region.