Basin Evolution of the Cretaceous-Early Eocene Xigaze Forearc, Southern Tibet

An understanding of the processes which control the evolution of forearc basins is important for deciphering the tectonic development of a convergent margin prior to continent-continent suturing. This study presents sedimentologic, modal petrographic and geo-thermochronologic data from the Xigaze forearc basin, preserved along ~ 600 km of the Indus-Yarlung Suture Zone in southern Tibet. From late Cretaceous to early Cenozoic time, subduction of Neo-Tethyan oceanic crust beneath the southern margin of Asia accommodated the northward motion of the Indian craton and formed the Xigaze forearc basin. Following collision with India in the early Cenozoic, the basin transitioned from predominantly marine to non-marine sedimentation and was subsequently uplifted to a mean elevation of 5000 m. Thus, the sedimentary record in the Xigaze forearc preserves information regarding the tectonic evolution of the Indo-Asia continental margin prior to and following collision. We present new measured sections and geo-thermochronologic data from Early Cretaceous to Early Eocene clastic and carbonate sedimentary rocks, preserved in two previously unexplored regions of the forearc, (1) at its western most extent, northwest of Saga, and (2) north of Lhatse. In turn, we compare our results with previously published data in order to synthesize our current understanding of forearc evolution. Strata preserved in the Lhaste region record an initial shallow marine phase of forearc sedimentation (Aptian), but quickly transition to deep marine slope and distal fan turbidite facies (Albian-Campanian). In contrast, facies preserved in the Saga region record a younger shoaling upward marine sequence (Maastrichtian-Ypresian), with the uppermost ~ 400 m consisting of fluvial channel sandstones and red-green paleosols. Facies and depositional environments in the Saga region are highly variable along strike, with turbidites, shelf limestones, estuarine siliciclastics and thick paleosols sequences all present along a ~ 60 km corridor of the basin. Strata younger than Campanian are not observed in the Lhatse region. Our results show that the Xigaze forearc basin evolved along an irregular southern Asian margin demonstrating a transition from deep marine sedimentation to shallow marine carbonates prior to filling and possibly extending southward as an integrated terrestrial basin in the early Eocene. Our results place first order constraints on the paleogeography of the basin in from the middle Cretaceous until the early Eocene. In addition, we utilize zircon (U-Th)/He thermochronology coupled with U-Pb geochronology to determine the timing of exhumation of the Xigaze forearc basin. Preliminary double-dated (U-Th/He and U-Pb) zircon crystals from late Cretaceous age deposits suggest basin exhumation from 30 to 10 Ma, with the majority of grains clustering around 19 Ma. These ages are similar to AFT and ZHe ages recorded in post-collisional basins along the suture zone, suggesting a regional exhumation event in the Miocene.