Mid‐Cretaceous Exhumation of the Central Qiangtang Mountain Range Metamorphic Rocks as Evidenced by the Abushan Continental Redbeds

Abstract

AbstractMetamorphic rocks and surrounding Paleozoic strata in central Tibet compose the core of an anticlinorium associated with the Central Qiangtang Mountain Range. The analysis of Mesozoic strata flanking the anticlinorium provides insights into the geological and landscape evolution of the range, which is important in determining the Mesozoic evolution of central Tibet and the paleo‐elevation of the Tibetan Plateau. Here we present a detailed stratigraphic study of the Shuanghu Conglomerate and over 1‐km‐thick mid‐Cretaceous continental redbeds of the Abushan Formation, which non‐conformably overlie the Qiangtang metamorphic belt. Pollen assemblages and detrital zircon ages along with published magnetostratigraphic results indicate a mid‐Cretaceous depositional age for the Abushan Formation. The Shuanghu Conglomerate is undated but may have been coeval with the Abushan Formation as suggested by foraminiferal assemblages contained within limestone clasts. The Abushan Formation shows a fining‐ and deepening‐upward megasequence, changing from braided river delta‐plain conglomerate and sandstone to lacustrine delta‐plain and delta‐front sandstone, siltstone and carbonate rocks. Both the Abushan Formation and Shuanghu Conglomerate received metamorphic detritus, indicating that the exhumation of the Qiangtang metamorphic belt to the surface took place at or before mid‐Cretaceous time. The lower member of the Abushan Formation contains dominant limestone clasts, whereas quartz, metamorphic, and volcanic clasts increase in percentage up‐section reflecting an unroofing sequence. The stratigraphic, sedimentologic, and provenance evolution suggests that exhumation of the Central Qiangtang Mountain Range metamorphic rocks may have occurred primarily by tectonic extension after the end of a long period of late Mesozoic Lhasa‐Qiangtang convergence.