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

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 similar to 50-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 similar to 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. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

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 basin fill can be subdivided into four stratigraphic units with distinctly different lithofacies associations, reflecting significant changes of depositional sources and tectonic boundary conditions through time. From alluvial fan to braided river, the different lithofacies associations indicate the evolution of depositional environments during the basin's evolution.