Response of modern fluvial sediments to regional tectonic activity along the Min River, Eastern Tibet

Abstract

Abstract. The deposition of fluvial sediments in tectonically active areas is mainly controlled by tectonics, climate, and associated Earth surface processes; consequently, fluvial sediments can provide a valuable record of changes in regional climate and tectonic activity. In this study, we conducted a detailed analysis of the grain‒size distribution in modern fluvial sediments from the upper Min River, Eastern Tibet. These data were combined with regional information about climate, vegetation, hydrology, geomorphology, lithology, and fault slip rate, and together indicate that modern regional tectonic activity along upper Min River can be divided into three segments. Specifically, fluvial sediments in the segment Ⅰ are dominated by fine silts (<63 μm: 70.2 %), agreeing with a low‒runoff and low‒rainfall in this segment and revealing a windblown origin influenced by the arid and windy climate. These observations are consistent with the segment’s low hillslope angle and low relief, all indicating weak activity along the Minjiang Fault. The coarse‒grained fraction (>250 μm) of fluvial sediments in the segments Ⅱ ‒ Ⅲ increases in a stepwise fashion (A = 6.2 %, B = 19.4 %, C = 33.8 %) downstream, although runoff and rainfall do not change significantly from segment Ⅱ to segment Ⅲ. These patterns correlate well with an increase in both regional relief and hillslope angles. Together, these observations imply that regional tectonic activity along Maoxian‒Wenchuan Fault becomes more pervasive downstream along the Min River. Fluvial sediments in segment Ⅳ are well sorted and well rounded, which is expected due to significant increases in rainfall and runoff in this segment. This study marks the first development of a new and important research approach that can characterize regional tectonic activity by analysis of fluvial sediments collected from tectonically active regions, combined with regional conditions in geology and geography.