Constraints on Cenozoic tectonics in the southwestern Longmen Shan from low-temperature thermochronology

The Longmen Shan, located at the boundary between the Tibetan Plateau and the Sichuan Basin, has received considerable attention following the 2008 Wenchuan earthquake. However, the tectonic history of the southwestern segment of the range has remained poorly constrained. We present zircon fission-track, zircon (U-Th)/He, and apatite (U-Th)/He data from the Baoxing region in the southwestern Longmen Shan that provide the first constraints on the cooling and exhumation history of the region. All of the measured ages are Cenozoic, and the data suggest that exhumation of the Baoxing region was ongoing by ca. 15 Ma. Zircon (U-Th)/He ages from several samples appear to be affected by radiation damage, suggesting that damage may be a concern even in samples with Cenozoic cooling ages. Samples were collected from two bodies of Precambrian crystalline rocks separated by the Wulong fault, and for all three thermochronometers, ages west of the Wulong fault are systematically younger than ages to the east, indicating that the fault has accommodated differential exhumation since 8-10 Ma. The regions east and west of the Wulong fault have experienced 7-13 km and at least 7-10 km of exhumation, respectively. The magnitude of exhumation in the southwestern Longmen Shan is similar to that reported in the central Longmen Shan, indicating consistency along strike. The thermochronology data also suggest that the Erwangmiao fault in the southwestern Longmen Shan is analogous to the Beichuan fault in the central Longmen Shan, and therefore may represent a source of seismic hazard.