Cenozoic extension within the Tibetan Plateau is expressed as a series of prominent N-S trending rift valleys. Recent GPS studies show that these graben systems and associated conjugate strike-slip systems accommodate a significant portion of internal N-S shortening and E-W expansion of the Tibetan Plateau. The development of these structures represents a significant shift in deformational style and marks the transition to a constrictional strain field within the Tibetan crust. Determining the timing of the formation of these structures is a critical step in understanding the evolution of the plateau. In order to test existing hypotheses proposed to explain these N-S trending rifts and to understand the geodynamic significance of Cenozoic E-W extension in Tibet, the PI needs to answer two fundamental questions. (1) What is the timing of initiation and spatial distribution of E-W extension in Tibet? (2) What is the temporal and kinematic interplay between conjugate strike-slip deformation and E-W extension within central Tibet? A regionally synchronous versus diachronous onset of rifting or progressive migration versus discrete episodes of fault initiation have very different implications for the evolution of the intraplate strain field and the driving forces responsible for rift formation. Currently, the different geodynamic models for the Cenozoic evolution of Tibet are difficult to evaluate due to the lack of sufficient temporal constraints on the onset of rifting. The PI proposes to use an integrated approach, combining structural mapping with apatite (U-Th)/He and fission track dating, to elucidate the timing and magnitude of faulting associated with the Cenozoic strain pattern across central Tibet. Thermochronological methods directly dating the cooling of rocks in exhumed fault blocks will complement field-based kinematic analysis of the rift systems and should help resolve outstanding questions regarding the timing and geodynamic significance of the Tibetan rifts. This proposed study will have far-reaching implications for our understanding of how the lithosphere behaves during continent-continent collision. New data will yield insights into the fundamental problem of how and when Tibet extended after thickening, information that is critical for evaluating the many different tectonic models for this entire region. Furthermore, results will shed light on the hotly debated question of whether there is a temporal link between E-W extension and surface uplift of the Tibetan plateau. Given the implications of the uplift of the Tibetan plateau for regional and global climate, the PI expects his results to be of interest to a wide spectrum of geoscientists. This project also has substantial educational and societal merit. The proposed international collaboration with scientists from the Chinese Academy of Science will permit an exchange of scientific ideas and knowledge. This exchange will greatly benefit KU students, exposing them to the classic Himalayan-Tibetan orogenic system. Two KU graduate students will be deeply involved in several mapping expeditions and will work closely with scientists from UCLA and the Chinese Academy of Science.
