Landscape evolution of the Bhutan Himalaya - insights from tectonic geomorphology and low-temperature thermochronology

In contrast to the steep, rugged terrain that characterizes the southern slope of the Himalaya in most areas, discontinuous regions of low-relief and low-slopes occur perched at high elevations (~3500 m) in the middle latitudes of Bhutan. These surfaces are marked by subdued landforms, thick soils and saprolites, massive alluvial and colluvial fills, and presumably slow erosion rates. It is likely that these landscapes were more continuous east to west across Bhutan and possibly extended south toward the foreland. However, due to active river incision, only isolated patches remain. These enigmatic surfaces pose two problems: (1) how and why did low-relief surfaces develop? and (2) how and why did they become perched at high elevations above rugged mountains and deep river gorges? Although robust answers to these questions require the integration of many modes of investigation, the age of low-relief surface development and the age of its uplift to higher elevations are crucial to our understanding of how climate and tectonics have shaped the Bhutan Himalaya. Our first approach to obtaining such information involved sampling an elevation transect beneath the low-relief region in eastern Bhutan (from the Yotang La to Trongsa) for single-crystal apatite (U-Th)/He dating. The dates for most samples decrease with distance below the low-relief surface from 5.79 +/- 0.08 to 4.47 +/- 0.04 Ma (2 SE), down to a depth of ~1600 m below the surface. They define a linear array on an apparent age vs. elevation plot suggestive of steady, relatively rapid exhumation at a rate of ~2 mm/yr from around 6 to 5 Ma. The lowermost sample of the transect was younger than predicted by the ~2 mm/yr trend of higher elevation samples. Although more data are needed, this preliminary result implies a deceleration in exhumation rate after ~ 5 Ma, which may suggest establishment of the low-relief surface at about that time. If the surface was established by ~ 5 Ma at low elevations, then uplift of the surface to its current elevation of ~ 3500 m was a Pliocene-Quaternary process.