Stratigraphic architecture and controls on a convergent margin deepwater progradational system: Insights from the Ngamring Formation, Xigaze forearc basin, southern Tibet

Submarine progradational systems are widespread along active margins, and their deposits contain valuable insights into perturbations within their upstream systems and long-term geological evolution. However, decoding these records is often challenging due to the scarcity of well-exhumed/preserved and well age-controlled deepwater strata. This study integrates zircon U-Pb geochronology combined with detailed stratigraphy to illustrate the stratigraphic architectural evolution and drivers on the deepwater progradational strata, totaling 3.0 km in thickness, in the Xigaze forearc basin in southern Tibet. The deepwater system preserves an upward-coarsening succession, which prograded from the mid-lower fan lobe deposits, to the base of slope to proximal basin floor, to the middle- to upper-fan channel-levee deposits. Coarse-grained channel deposits are overlain by >100 m of mudstone and siltstone, which we interpret as an abandonment phase deposit. Depositional ages derived from three tuffs and maximum depositional ages from five sandstones range from 99.0 ± 0.59 Ma to 93.8 ± 0.41 Ma. Increased sediment supply induced by a Cretaceous magmatic flare-up (peak at 95 Ma), tectonic contraction by the activity of the Gangdese Retroarc Thrust Belt (beginning in the mid-Cretaceous), and expansion of drainage configuration facilitated the systematic upward change in facies architecture and depositional setting. However, a dramatic sea-level rise during the mid-Cretaceous and/or a deposystem shift interrupted these progradational processes and halted the delivery of coarse sediment into the basin. Our study highlights the linkage between detailed stratigraphic architecture and chrono-stratigraphy that provides a useful analogue for comprehending how Earth’s deepwater systems respond to allogenic controls in ancient active margin settings. Locally, this study enhances our understanding of the tectonic-sedimentary evolution of the arc-basin system that occurred along the margin of southern Tibet before the collision between the Indian and Asian blocks. Broadly, this contribution also can be employed to aid interpretations of geological processes in other regions or deepwater systems where constraining factors are limited.