Along- and across-strike variations in erosion in southern Longmen Shan thrust belt based on low-temperature thermochronology and fluvial shear stress analyses

Along the southern Longmen Shan thrust belt in eastern Tibet, the Dayi subsegment stands out as it remained unruptured in both the 2008 Wenchuan and the 2013 Lushan earthquakes. Whether localized weak materials or differential erosion caused this fault segmentation is unclear. According to the critical-taper wedge theory, these two mechanisms would generate distinct morphotectonic features and erosional distributions. We integrated two independent methods to evaluate these mechanisms. We obtained 10 apatite fission track and two apatite (U-Th)/He dates from the Dayi subsegment, which yield ages of ~3-44 Ma, and deduced an average erosion rate of 0.5-0.6 mm/yr and 0.2-0.4 mm/yr since ~6-8 Ma in the hanging-wall and footwall of the Shuangshi-Dachuan fault, respectively. We also calculated the fluvial shear stress and erosion rate at 708 sites along five rivers in the southern Longmen Shan. These two methods yield consistent mean erosion rates, implying an exhumational steady state since at least 5 Ma. A systematic, heterogeneous pattern of erosion is found on a 3-by-3 grid along and across the Dayi subsegment. In the hinterland, the erosion rate in the Dayi subsegment is lower than that of the adjacent areas along strike, whereas at the range front, the erosion rate in the Dayi subsegment is greater. The spatial patterns of erosion and topography suggest that differential erosion plays a major, but not exclusive, role in regulating the fault segmentation and earthquakes in the Longmen Shan. This study shows that in a fold-and-thrust belt with heterogeneous erosion, sampling along a few transects may not adequately capture the entire pattern of erosion. Along-strike variations in geometry of a thrust belt could be influenced by the lateral variations in erosion.