Impacts of focused erosion on crustal dynamics along the Sutlej River, NW Indian Himalaya : implications from apatite (U-Th)/He thermochronology

Abstract

Focused erosion is hypothesized as a major driver of localized rock uplift and exhumation in contractional orogenic systems. However, the three previously-identified locations for maximal rapid exhumation that responds to spatially focused erosion (via major rivers and a glacier) all occur above localized tectonic anomalies. This complicates efforts to isolate the exhumation signal of spatially-focused erosion. Therefore, the actual upper limits for the impact of focused erosion in tectonic development and landform formation are still uncertain. The study of the Sutlej River valley in the NW Indian Himalaya has been carried out to address this problem. This study area contains the third-largest focused erosive system in the Himalayas and the most extensive with a quasi-homogeneous tectonic setting. It is therefore suitable for examining the crustal response to focused erosion. We acquired low-temperature apatite (U-Th)/He thermochronology data to explore the exhumation variations in this study area. According to the spatial distribution of the apatite (U-Th)/He data, age gradually increases as it moves away from the core of the Sutlej River anticline, which is close to the confluence of the Sutlej and Baspa Rivers. The mean exhumation/erosion rate is ~ 3.15 to 5.30 mm/year from the early Pleistocene provided by eroding half-space model for the region within about 25-30 km from the Sutlej River anticline core centre, including the Sutlej-Baspa-Wangtu area. Thermal history models that yield the exhumation rates have a similar pattern to the distribution of ages. The most rapid exhumation occurs along the river anticline core. After combining the data, it shows that rocks in the Sutlej River anticline core may exhume up to ~19-20 mm/year, reaching the exhumation rate of the syntaxes. Integrating the new apatite (U-Th)/He data with published higher-closure-temperature thermochronometry, exhumation must have accelerated in the last 2 Ma. However, the Sutlej River region may not have reached its deformation peak because of the short duration of the rapid exhumation compared to the syntaxial region. Therefore, the zone of rapid exhumation is most likely related to the distribution of rivers and influenced by erosive power. Focused erosion along the Sutlej River appears to produce localized exhumation at rates comparable to previously explored, tectonically heterogeneous sites.