DEPOSITIONAL SYSTEMS OF THE DUBA BASIN DURING CENOZOIC THRUSTING ALONG THE SOUTHERN MARGIN OF THE LUNPOLA BASIN SYSTEM, SOUTH-CENTRAL TIBETAN PLATEAU

The Lunpola basin system of the south-central Tibetan plateau preserves a deformation record of Cenozoic and possibly latest Cretaceous age in the northern Lhasa terrane. The Duba basin, representing the southern part of the Lunpola basin system, contains up to 5 km of principally conglomerate and sandstone deposited in alluvial fan and fluvial environments. Duba basin fill is continuously exposed along a series of northwest-plunging structures in the footwall of a northeast-directed thrust fault that carries a Cretaceous succession of carbonate and clastic rocks. Paleocurrent and sediment compositional data indicate variable sediment source areas during basin development. Upsection changes in conglomerate clast composition and sandstone modal mineralogy indicate a shift in source areas from the ~120 Ma Baingoin granite (exposed to the east) to a mid-Cretaceous, fossiliferous marine carbonate (exposed in the thrust hanging wall to the south). Provenance data and lateral facies changes suggest that a northwest-flowing axial drainage system parallel to the basin-bounding northeast-directed thrust was derived from the thrust hanging wall during syndepositional faulting. A younger gravel deposit containing a diverse clast population overlies the Duba basin in angular unconformity; this gravel may be linked to youthful fluvial strath terraces 100-200 m above the modern drainage system. These relationships suggest relatively sustained deformation (generally north-south shortening) along the southern basin-bounding thrust throughout the Cenozoic history of the Duba basin, and perhaps recent incision in this region of presently closed drainage.