Basin formation in the High Himalaya by arc-parallel extension and tectonic damming: Zhada basin, southwestern Tibet

[1] The late Miocene–Pleistocene Zhada basin in southwestern Tibet provides a record of subsidence and basin formation within an active collisional thrust belt. The >800 m thick basin fill is undeformed and was deposited along an angular unconformity on top of Tethyan strata that were previously shortened in the Himalayan fold-thrust belt. Modal sandstone petrographic data, conglomerate clast count data, and detrital zircon U-Pb age spectra indicate a transition from detritus dominated by a distal, northern source to a local, southern source. This transition was accompanied by a change in paleocurrent directions from uniformly northwestward to basin-centric. At the same time the depositional environment in the Zhada basin changed from a large, braided river to a closed-basin lake. Sedimentation in the Zhada basin was synchronous with displacement on the Qusum and Gurla Mandhata detachment faults, which root beneath the basin and exhume midcrustal rocks along the northwestern and southeastern flanks of the basin, respectively. These observations indicate that accommodation for Zhada basin fill was produced by a combination of tectonic subsidence and damming, as midcrustal rocks were evacuated from beneath the Zhada basin in response to arc-parallel slip on crustal-scale detachment faults.