Influence of lithology on hillslope morphology and response to tectonic forcing in the northern Sierra Nevada of California

Martin D Hurst,Kyungsoo Yoo,Rachel Walcott,Mikael Attal,Simon M Mudd

doi:10.1002/jgrf.20049

Abstract

Many geomorphic studies assume that bedrock geology is not a first‐order control on landscape form in order to isolate drivers of geomorphic change (e.g., climate or tectonics). Yet underlying geology may influence the efficacy of soil production and sediment transport on hillslopes. We performed quantitative analysis of LiDAR digital terrain models to examine the topographic form of hillslopes in two distinct lithologies in the Feather River catchment in northern California, a granodiorite pluton and metamorphosed volcanics. The two sites, separated by <2 km and spanning similar elevations, were assumed to have similar climatic histories and are experiencing a transience in landscape evolution characterized by a propagating incision wave in response to accelerated surface uplift c. 5 Ma. Responding to increased incision rates, hillslopes in granodiorite tend to have morphology similar to model predictions for steady state hillslopes, suggesting that they adjust rapidly to keep pace with the incision wave. By contrast, hillslopes in metavolcanics exhibit high gradients but lower hilltop curvature indicative of ongoing transient adjustment to incision. We used existing erosion rate data and the curvature of hilltops proximal to the main channels (where hillslopes have most likely adjusted to accelerated erosion rates) to demonstrate that the sediment transport coefficient is higher in granodiorite (8.8 m2 ka−1) than in metavolcanics (4.8 m2 ka−1). Hillslopes in both lithologies get shorter (i.e., drainage density increases) with increasing erosion rates.

Highlights

[2] Climate and tectonics act in concert to control the morphology of the Earth’s surface
Links between bedrock lithology and topography have been explored in bedrock landscapes, no studies have explored the role that lithology may play in controlling topography in soil-mantled landscapes
[31] To compare estimates of the sediment transport coefficient D between lithologies, we sampled hilltop curvature on all ridges within 500 m of reaches of the Feather River, Cascade River, or Little North Fork River that are downstream of knickpoints (Figure 4) where the long-term erosion rate is estimated to be c. 250 mm kaÀ1 [Riebe et al, 2000; Wakabayashi and Sawyer 2001; Hurst et al, 2012]

Summary

Introduction

[2] Climate and tectonics act in concert to control the morphology of the Earth’s surface. The ability to quantify relationships between topography and climatic or tectonic driving processes is dependent on understanding how efficiently, and by which processes, sediment is generated and transported on hillslopes and in valleys [e.g., Ahnert, 1970; Dietrich et al, 2003] Such knowledge is vital for ongoing [3] Tectonic processes redistribute rock mass within the lithosphere and control the type and flux of rock material exhumed to the surface. McKean et al [1993] showed that the sediment transport coefficient D, which relates hillslope gradient to sediment flux, is an order of magnitude larger in weak clay-rich soils than in strong, granular soils, presumably due to variation in the efficiency of shrink-swell cycles as a transport process This is at least partially controlled by the parent lithology through the nature of jointing and susceptibility to weathering. Gurvan Bugd fault system, Mongolia Lake Bonneville, UT, USA Simulated Douglas Fir Forest Southern Arava Valley, Israel Transverse Ranges, CA, USA

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Results

Conclusion