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

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, combined with
information on regional climate, vegetation, hydrology, geomorphology,
lithology, and fault slip rate, indicate that modern regional tectonic
activity along upper Min River can be divided into three segments.
Specifically, fluvial sediments in the Minjiangyuan–Diexi segment are
dominated by silts (< 63 µm, 70.2 %), agreeing with
low runoff, low rainfall, and high vegetation cover and revealing a windblown
origin influenced by the arid and windy climate. These observations are
consistent with the 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 Diexi–Wenchuan and Wenchuan–Dujiangyan segments increases stepwise downstream, although runoff and
rainfall do not change significantly. These patterns correlate well with
increases in both regional relief and hillslope angles. Together, these
observations imply that regional tectonic activity along the Maoxian–Wenchuan
Fault becomes more pervasive downstream along the Min River. The occurrence
of well-sorted and well-rounded pebbles of fluvial sediments downstream
of Dujiangyan must be related to the long-time scouring and sorting by
rivers. This study marks the first development of a new research approach
that can characterize regional tectonic activity by analysis of grain-size
distribution of fluvial sediments collected from tectonically active
regions.
