An asymmetric Late Cretaceous back-arc basin south of Tibet?

The Indus-Yarlung suture zone (southern Tibet) between the Indian plate−derived Tibetan Himalaya and the Asian continental crust intruded by the Gangdese magmatic arc of southern Tibet hosts a <20-km-wide band of ophiolites overlain by Asia-derived clastic sedimentary rocks of the Xigaze forearc basin. How wide this basin was prior to India-Asia collision is unknown: it may have been a typical forearc basin width, i.e., ∼150−200 km, but was also proposed to have become separated from Tibet in the Late Cretaceous by a back-arc basin thousands of kilometers wide. To test this, we present the first paleomagnetic study of upper Cretaceous redbeds of the 71.2−69.3 Ma Padana Formation in the Xigaze forearc basin, near Sangsang town, Ngamring county, Tibet, China (∼86°E). High-temperature magnetic components were isolated at 580 °C or 600−680 °C and passed reversal and fold tests demonstrating a primary magnetization that we corrected for inclination shallowing. Our results reveal that the Xigaze forearc basin at ∼86°E was situated at 18.4° ± 3.6°N at ca. 70 Ma, indicating a separation from Lhasa of <500 km. To reconcile our new data with coeval, much lower, paleolatitudes from the Ladakh arc to the west (∼77°E) we reconstruct a Late Cretaceous−Paleogene opening and closure of an asymmetric back-arc basin in the western Neotethys. This suggests that the distribution of India-Asia convergence was laterally unevenly partitioned over the pre-collisional plate boundaries.