Two-phase Himalayan extension recorded in the Late Miocene-Pleistocene Gyirong Basin, south Tibet

Abstract

The Late Miocene-Pleistocene Gyirong Basin in the central Himalaya provides a record of basin evolution in response to tectonic extension within the active Himalaya orogen. The up to ~550-m-thick basin fill can be subdivided, from bottom to top, into four stratigraphic units with distinctly different lithofacies associations: alluvial fan, debris flow, lacustrine-fan delta, and braided river. Analysis of the stratigraphic pattern, provenance, and detrital zircon UPb geochronology of the basin fill indicates significant changes of depositional sources and tectonic boundary conditions through time. The alluvial fan deposits in the Oma sub-basin, which unconformably overlie the Tethyan strata, show northward paleoflow directions, detrital zircon UPb ages >400 Ma, and low subsidence rates. The unconformably overlying debris conglomerates show westward paleoflow directions, younger detrital zircon UPb ages of 15–37 Ma, and high subsidence rates. The abrupt changes in depositional source and tectonic subsidence indicate that the basin evolved from a supradetachment basin related to east-trending northward normal faults in the hanging wall of the South Tibetan Detachment to a rift basin that was mainly controlled by a major north-trending normal fault along the eastern basin margin from ~10–9 Ma onwards. We propose that initial basin formation is the result of N-S extension related to faulting along the South Tibetan Detachment, which was immediately followed by E-W extension of the Himalaya orogen in response to decoupling of the Himalayan crust related to India underthrusting beneath Asia.