Neotectonics of the Thakkhola graben and implications for recent activity on the South Tibetan fault system in the central Nepal Himalaya

The Thakkhola graben is one of many north-trending rifts that define the Ne ogene structural pattern of the southern Tibetan Plateau, Lying at the sout hern margin of the plateau and extending to the crest of the Himalaya, the graben provides an opportunity to evaluate the kinematic relationships betw een east-west extensional strain in southern Tibet and north-south extensio nal strain in the Himalaya. Neotectonic and structural mapping of the Danga rdzong fault along the western margin of the graben reveals a southward-dec reasing component of normal slip coupled with a southward-increasing compon ent of right-lateral slip that affects Pleistocene basin-fill sediments. We present C-14 ages for river terraces in the Thakkhola graben that provide a ca, 17.2 ka minimum age on the latest stage of Dangardzong fault movement . Near the southern termination of the graben, the Dangardzong fault appare ntly offsets the Annapurna detachment, an early (Miocene) strand of the eas t-striking South Tibetan fault system. However, the Dangardzong fault itsel f terminates against a young (i.e., younger than ca. 17.2 ka) strand of the South Tibetan fault system, the Dhumpu detachment. Structural relationship s among the Dangardzong, Annapurna, and Dhumpu faults suggest that the Dang ardzong structure is a tear fault in the South Tibetan allochthon that acco mmodates differential amounts and rates of displacement along the South Tib etan fault system. Thus, although the South Tibetan fault system first deve loped as part of the structural architecture of the Himalaya in Miocene tim e, at least some strands have been active as recently as the Pleistocene, I n a regional contest, the South Tibetan fault system serves to accommodate the strain gradient between extension in Tibet and shortening in the Himala ya.