MIOCENE INITIATION OF EAST-WEST EXTENSION ALONG THE SHANXI RIFT, NORTH CHINA: IMPLICATIONS FOR LATE CENOZOIC EXTENSION IN TIBET AND ACROSS EASTERN ASIA

Active extension occurs across East Asia: in the Himalaya, Tibet, Baikal, the Altai Mountains, and North China. Extension in Tibet is hypothesized to result from gravitational collapse that initiated when the Tibetan Plateau achieved its maximum elevation, while extension beyond Tibet is commonly attributed to the dynamic interplay of both the India-Asia collision and the Pacific subduction boundary. Alternatively, these extensional systems may share a common dynamic cause because they are all extending in an east-west direction, they may have initiated coevally, and some of these extensional systems may be kinematically linked. Himalayan-Tibetan extension initiated between 18 and 8 Ma. Other east Asian extensional systems initiated in the Late Miocene – Early Pliocene, with the possible exception of grabens bordering the Ordos Block in North China, including the Shanxi rift along the south and east margins. These extensional systems have been interpreted to initiate in the Paleogene based on the occurrence of Paleogene sedimentary rocks in the rift basins, but similar rocks occur along the rift shoulders, suggesting that these rocks represent pre-rift sediments. 40Ar/39Ar thermochronology of muscovite, biotite, and potassium feldspar of samples from rift shoulders of the Shanxi rift yields uniformly pre-Cenozoic ages. Apatite (U-Th)/He thermochronology along a transect across the footwall of the northern Hua Shan frontal normal fault in the southern Shanxi rift shows that significant extension here initiated no earlier than ~18 Ma. Apatite (U-Th)/He and fission-track thermochronology along a transect across the footwall of the Hen Shan frontal normal fault in the northern Shanxi rift yields dominantly Cretaceous ages. Modeling of fission-track data suggests a Neogene initiation of extension. Hen Shan footwall basalt dipping ~15° to the southeast yields 40Ar/39Ar whole rock ages of 28-30 Ma, which indicates that significant extensional tilting occurred after the Oligocene basaltic eruption. Thus all east-west extending rifts in Asia may share a Miocene initiation age, supporting a common dynamic cause for their formation.