The impact of erosion on fault segmentation in thrust belts: Insights from thermochronology and fluvial shear stress analysis (southern Longmen Shan, eastern Tibet)

Abstract

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, such as a recess vs. a salient structure at the range front, and higher vs. lower erosion rate than that of the adjacent areas along strike in the hinterland, respectively. 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 that flow across the faults in the southern Longmen Shan. These two methods yield consistent mean erosion rates, implying an exhumational steady state since at least 5 Ma. Intriguingly, 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. Moreover, the Dayi subsegment could have acted as a barrier during the recent great earthquakes, posing substantial seismic potential to this region.