Sedimentological responses to initial continental collision: Triggering of sand injection and onset of mass movement in a syn-collisional trench basin, saga, southern tibet

A continuous Late Cretaceous–Paleocene sedimentary succession within the India–Asia collision suture zone in Xigaze, Tibet contains a c. 80 m thick sand injection complex immediately overlain by a c. 60 m thick mass transport deposit (MTD; the first of several) with the first evidence of Asian provenance, immediately followed by a c. 61 Ma tuff. The youngest in situ strata with unequivocal Indian provenance are probably the source beds of the sand intrusions, separated from the first MTD by c. 50 m of pelagic deposits that potentially represent an interval of several million years; the collision could thus have occurred at any time within this interval. However, the uppermost limit of the sand intrusions closely coinciding with the MTD suggests that they occurred penecontemporaneously, possibly associated with the initial continental collision. This may provide an additional constraint of initial collision onset at c. 61 Ma. The co-occurrence of MTDs and sand injections are possibly good sedimentary indicators of the onset of continental collision and are characteristic of syn-collisional trench basins. Because neither the youngest Indian nor the oldest Asian provenance sediments are in their original stratigraphic position, this study shows that detailed sedimentological work combined with provenance study can better constrain the timing of continental collision.