Cenozoic morphotectonic evolution of the northeasternmost Tibetan Plateau: Evidence from detrital thermochronology

摘要

Despite much research we still lack a comprehensive understanding of the formation and evolution of the northeastern (NE) Tibetan Plateau. The West Qinling is regarded as the northeastward front of the transmission of the compression induced by the India-Asia collision and convergence, and thus its Cenozoic morphotectonic evolution can be used to elucidate the growth history of the northeastern plateau. The Cenozoic deposits of the intermontane Wushan basin of the West Qinling provide a long-term and continuous archive of information about the morphotectonic evolution of the plateau on its northeastern margin. Based on magnetostratigraphy and mammalian biostratigraphy, these deposits are well dated to similar to 34-6 Ma, and here we present the results of apatite fission-track thermochronology (AFT), sedimentation rates and facies, color reflectance and magnetic susceptibility, and structural and provenance analyses. Our results indicate a series of exhumation events, likely related to tectonics, with ages ranging from Paleocene through Late Miocene. Morphologic analyses indicate two levels of relict low-relief surfaces at high elevations in the source regions of the Wushan basin sediments. Based mainly on analyses of bedrock AFT and regional deformation signals, we speculate that these landscape remnants developed at low elevations during quiescent phases of orogenic construction, with the age ranges of Middle Eocene and Early-Middle Miocene, respectively. We use a synthesis of our data with existing records of deformation and paleo-altimetry to produce a morphotectonic scenario for West Qinling during the Cenozoic. This suggests that far-field stress due to the India-Asia collision was propagated to the modern northeasternmost boundary as early as the Paleocene. Subsequently, the topography of the West Qinling developed via discrete episodes of tectonic uplift activity/quiescence, or by enhanced denudation, until the Late Miocene, when the modern morphology of the northeasternmost plateau finally developed. Our accurate chronology of surface processes related to tectonic uplift-exhumation-denudation-sedimentation for the West Qinling provides key morphotectonic criteria for a comprehensive understanding of the pulsed growth history of the NE Tibetan Plateau.