No channel flow in the Longmen Shan: evidence from the Maoxian-Wenchuan fault Cenozoic kinematics (SE Tibet)

The NE striking Longmen Shan (LMS) mountains are located at the eastern margin of the Tibetan plateau, and towers nearly 5000m above the Sichuan basin, which is considered to be the greatest relief than anywhere else around the plateau. From west to east, three major sub-parallel faults straddle the Longmen Shan: Wenchuan-Maoxian fault (WMF), Yingxiu-Beichuan fault and Guanxian-Anxian fault. Several models have been proposed to explain the Cenozoic uplift of the Longmen Shan. The major two models are lower crustal channel flow and upper crustal shortening, which imply different movement sense on the Wenchuan-Maoxian fault. The former suggests that the LMS were uplifted above a lower crustal flow expulsed from below the Tibetan plateau and would require a normal sense movement on the MWF. The latter implies that a series of upper crustal thrusts controlled the uplift of the LMS, and the WMF should have a reverse sense. Here we present field observations, fault gouge structural analysis and authigenic illite K-Ar geochronology data of fault gouge in the Wenchuan-Maoxian fault, showing that the Maoxian-Wenchuan fault was dextral with a reverse component at ~7Ma. Reconstruction of offsets of river valleys along the Wenchuan-Maoxian fault suggests that the corresponding total horizontal dextral offset is ~25km. Analysis of the thermochronology data acquired on both side of the fault suggest that dextral-reverse faulting started at ~13 Ma and possibly lasted until today. Our conclusions support the upper crustal shortening model and suggest the channel model maybe not applicable to Longmen Shan uplifting in the Miocene.