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

Xu Q, Li Z, Wolff R, Tan X, Hetzel R, Yue Y, Tang X

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 U-Pb 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 U-Pb ages >400 Ma, and low subsidence rates. The unconformably overlying debris conglomerates show westward paleoflow directions, younger detrital zircon U-Pb 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.