Abstract
The tectonic significance, or lack thereof, of the southern margin of the Tibetan Plateau is the source of much controversy. The margin is defined by a distinct morphologic change from the low-relief Tibetan Plateau to the rugged topography and deep gorges of the Himalaya, and lies at the transition from N-directed convergence in the Himalayan zone to E-W extension that is pervasive across the Tibetan Plateau. Some workers have suggested the transition may be structurally controlled, in a similar role to that played by the South Tibetan fault system (STFS), a family of predominantly east-west trending, low-angle normal faults with a known trace of over 2,000 km along the Himalayan crest. The majority of mapped strands linked to the STFS are demonstrably inactive since the Middle Miocene, and thus, modern-day control of the physiographic transition by this fault system seems unlikely. Despite this, workers across the Himalaya have documented Quaternary activity on faults with a similar structural character and slip direction as the STFS, although initiation and timing are often poorly constrained. In this work, we examine the nature of the southern plateau margin in two locations. New structural mapping and (U-Th)/He apatite and zircon dates in the Annapurna and Dhaulagiri Himalaya of central Nepal reveals the existence of a low-angle detachment sub-parallel to, but structurally higher than, the basal detachment of the STFS, that coincides with an abrupt break in thermochronologic cooling ages, juxtaposing Miocene dates against Pleistocene. These data, combined with previous fission track data from the region, document for the first time clear thermochronologic evidence for significant N-S extension as recently as the Early Pleistocene. In the Nyalam region of south-central Tibet, multiple thermochronologic datasets (with a temperature range of approximately 70 degrees C-300 degrees C) paired with river incision patterns inferred by the longitudinal profile of the Bhote Kosi River reveal a drastic change in cooling rate at approximately 3.5 Ma at a location corresponding to a sharp increase in river gradient, and presumably rock uplift (where rock uplift congruent to incision rate). These data strongly point to young (Plio-Pleistocene) N-S extension controlling the position and nature of the southern plateau margin in several locations along the orogenic system.