Impact of fault damage on eastern Tibet topography

Tectonic deformation can influence spatiotemporal patterns of erosion by changing both base level and the mechanical state of bedrock. Although base-level change and the resulting erosion are well understood, the impact of tectonic damage on bedrock erodibility has rarely been quantified. Eastern Tibet, a tectonically active region with diverse lithologies and multiple active fault zones, provides a suitable field site to understand how tectonic deformation controls erosion and topography. In this study, we quantified erosion coefficients using the relationship between millennial erosion rates and the corresponding channel steepness. Our work shows a twofold increase in erosion coefficients between basins within 15 km of major faults compared to those beyond 15 km, suggesting that tectonic deformation through seismic shaking and rock damage significantly affects eastern Tibet erosion and topography. This work demonstrates a field-based, quantitative relationship between rock erodibility and fault damage, which has important implications for improving landscape evolution models. INTRODUCTION Relationships between topography and millennial erosion rates derived from cosmogenic 10Be isotopes are typically interpreted to be controlled by tectonics or climate (von Blanckenburg, 2005; Portenga and Bierman, 2011). The topography of the eastern margin of the Tibetan Plateau, which exhibits a >4000 m elevation change over a lateral distance of <100 km (Fig. 1), has also been attributed to interactions among tectonic uplift, climate, and surface processes (Burchfiel et al., 1995; Ouimet et al., 2009; Kirby and Ouimet, 2011; Scherler et al., 2017). The important role of lithologic compositions and bedrock weathering on erosion and landscape evolution in eastern Tibet was emphasized by Godard et al. (2010) and Gallen et al. (2015). However, impacts of tectonic deformation through seismic shaking and bedrock damage (Faulkner et al., 2010; Huang et al., 2014; Ben-Zion and Zalipian, 2019), which can potentially lead to enhanced erosion (Molnar et al., 2007; Koons et al., 2012; Gallen et al., 2015; Roy et al., 2015, 2016; Duvall et al., 2020), have not been quantified in this tectonically active region. Here, we addressed this issue by showing how erosion coefficients, which quantify the relationship between topography and millennial-averaged erosion rates, vary as a function of distance to faults in eastern Tibet. To do so, we used cosmogenic 10Be-derived erosion rates compiled from the literature and our own new measurements. We found a systematic increase in erosion coefficients within 15 km of major faults, and this signal was stronger than that induced by lithologic variation alone. METHODS We recalculated previously published 10Bederived erosion rates (E) from 100 basins across eastern Tibet (Ouimet et al., 2009; Godard et al., 2010; Ansberque et al., 2015) and measured E from an additional 11 basins in the Min Shan area of eastern Tibet. These additional samples came from a tectonically active region underlain mostly by sedimentary rocks (Fig. 1; see the Supplemental Material1 and Figure S1 and Tables S1–S2 therein). Please see the Supplemental Material for details about sample preparation and analysis. We quantified erosion coefficients based on the relationship between E and normalized channel steepness (ksn). We assumed that (1) river incision into bedrock controls erosion, and (2) detachment-limited bedrock erosion is a function of shear stress or stream power (Howard and Kerby, 1983; Howard et al., 1994; Whipple and Tucker, 1999). Then, we related erosion rate as a function of measurable topographic attributes by: