Upper Cretaceous-Lower Eocene Dajiweng Formation in the Zhada area, southern Tibet : Implications for the Trans-Himalayan forearc baisn evolution

The marine strata in the Gangdese forearc area play a significant role in the study of the closure of Neo-Tethyan Ocean and the subsequent India-Asia collision. The Dajiweng Formation exposed in the Zhada area, southern Tibet, is bounded by the southern Gangdese magmatic arc to the north and the Indus-Yarlung Zangbo suture zone to the south. It is mainly composed of conglomerate, sandstone, mudstone and limestone. Sedimentary analysis revealed that the Dajiweng Formation had been deposited in the fan-delta environment. A combination of the tuff zircon U-Pb dating, youngest detrital zircon ages and large benthic foraminiferal assemblage yielded that the Dajiweng Formation formed during the Late Cretaceous-Early Eocene period (i.e., ca. 73 similar to 49Ma). The provenance data showed that the Dajiweng Formation is similar to that of the Xigaze forearc basin sediments, which were mostly derived directly from the Gangdese magmatic arc. In contrast to the Gangdese-Ladakh forearc deposits, the forearc marine regression was diachronous along the strike, i.e. the sea water regressing gradually from east to west and disappearing at similar to 49 Ma in the western Gangdese-Ladakh forearc area.