Unlike foreland or rift basins, mechanisms driving subsidence during the life cycle of forearc basins are poorly understood. This project seeks to define a holistic model for forearc evolution by studying the Xigaze Forearc, a Cretaceous to early-Cenozoic basin that developed as Neo-Tethyan oceanic lithosphere subducted northward beneath it. Preserved on land as ~ 5-8 km thick marine and non-marine strata, it extends laterally ~550 km across southern Tibet. Stratigraphic and sedimentologic analyses of unexplored sequences north of Lhatse, reveal 5 km of Aptian-Turonian deposits, with 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, thus suggesting a change in sediment source at the contact between these two depositional environments. Although prior work on younger strata revealed the source of detritus in the Xigaze Forearc to be derived solely from the neighboring magmatic arc, I hypothesize that older quartzose rich sources farther north on the Tibetan Plateau may have provided additional sediment thereby driving subsidence in the basin during its initial development. This implies the magmatic arc did not act as a drainage divide and thus addition of crustal material was not the mechanism driving subsidence. To test this, I will determine provenance using sandstone petrographic and U-Pb detrital zircon analyses. I predict a shift to more quartz rich sandstone, the appearance of older detritus, and an increase in sediment flux at the transition from shallow to deep marine deposition.