EARLY CRETACEOUS DEPOSITIONAL ENVIRONMENTS, PROVENANCE, AND SUBSIDENCE HISTORY OF THE BASAL XIGAZE FOREARC BASIN, SOUTHERN TIBET

The Xigaze forearc basin in southern Tibet records Cretaceous-Eocene sedimentation along the southern margin of Asia prior to and during the initial stages of continental collision with India in the Early Eocene. Despite this rich record, the Early Cretaceous phase of forearc basin subsidence is poorly understood. This study presents new structural, stratigraphic and sedimentologic data from an unexplored region of the Xigaze forearc, north of Lhatse, Tibet. Our results provide constraints on the evolution of the southern margin of Asia during Early Cretaceous Neo-Tethyan subduction.

Structural investigation of the basal contact between the Xigaze forearc and ophiolitic fragments expressed as serpentinite mélange reveals that the contact is depositional where it has not been overprinted by north-directed thrusting across the Great Counter Thrust system. In these localities, forearc strata immediately above the contact vary from deep marine turbidites to carbonate reef facies. Stratigraphic and sedimentologic analyses of unexplored sequences in the region reveals ~5 km of Aptian-Turonian strata. These include 1.1 km of carbonate reef and shallow marine facies conformably overlain by 3.9 km of deep marine turbidites. Outcrop analysis of feldspatholitic sandstones reveals an increase in quartz abundance upsection suggesting a change in sediment source at the contact between these two depositional environments. Although prior work on younger strata revealed that detritus in the Xigaze forearc was derived from the proximal Gangdese magmatic arc, we interpret that 120-140 Ma grains in Early Cretaceous samples were derived from quartz-rich sources further north, perhaps in the northern Lhasa terrane. If this interpretation is correct, the Gangdese magmatic arc was not a significant topographic barrier at the time of deposition. This scenario implies that topographic loading from the arc was limited and that it was likely not the driving force behind Early Cretaceous forearc basin subsidence. The appearance of older detritus and an increase in sediment flux at the transition from shallow to deep marine deposition during the Early Cretaceous further supports this model.