Middle to Late Miocene tectonic forcing of intensified humidity in the Xunhua Basin, NE Tibetan Plateau

Abstract

The mechanisms and controls of the Middle-Late Miocene paleoenvironmental evolution of the northeastern Tibetan Plateau remain elusive. Here, we present an integrated multiproxy study of total organic carbon isotopes, lipid biomarkers, petrographic components and micropaleontological records in the well-dated Xigou section of the Xunhua Basin. Our data reveal a large decrease (−1.1 ‰) in bulk organic carbon isotopes (δ13Corg) at ∼12.7 Ma, a broad minimum at ∼12.7–8.0 Ma, and a large increase (+1.3 ‰) at ∼8.0 Ma. The decline in δ13Corg at 12.7 Ma was accompanied by pronounced changes in n-alkane indices, clay minerals, palynology, and ostracods, recording a major rise of lake level of paleo-Lake Xunhua due to increased precipitation in the regional catchment. Our inference of sustained elevated humidity in the Xunhua Basin at 12.7–8.0 Ma is contrary to previously reported cooling and drying trends on the NE Tibetan Plateau for the Middle-Late Miocene (∼14–8 Ma), thus precluding the role of global climate change as the dominant driver. Rather, we attribute the paleoenvironmental evolution of the Xunhua Basin to changes in regional rainfall patterns related to orogenic uplift. The Jishi Shan mountain range reached a critical threshold elevation at ∼12.7 Ma, becoming an orographic barrier to Westerlies-transported moisture and thus enhancing intensified precipitation within the Xunhua Basin. By ∼8 Ma, further uplift caused this orogen to become a barrier to the East Asian monsoon, resulting in aridification of the Xunhua Basin.