Hillslope denudation and morphologic response to a rock uplift gradient

Nicolas Espurt,Olivier Bellier,Edward Cushing,Jean-Claude Hippolyte,Jules Fleury,Vincent Ollivier,The Aster Team ,Vincent Godard

doi:10.5194/esurf-8-221-2020

Abstract

Abstract. Documenting the spatial variability of tectonic processes from topography is routinely undertaken through the analysis of river profiles, since a direct relationship between fluvial gradient and rock uplift has been identified by incision models. Similarly, theoretical formulations of hillslope profiles predict a strong dependence on their base-level lowering rate, which in most situations is set by channel incision. However, the reduced sensitivity of near-threshold hillslopes and the limited availability of high-resolution topographic data has often been a major limitation for their use to investigate tectonic gradients. Here we combined high-resolution analysis of hillslope morphology and cosmogenic-nuclide-derived denudation rates to unravel the distribution of rock uplift across a blind thrust system at the southwestern Alpine front in France. Our study is located in the Mio-Pliocene Valensole molassic basin, where a series of folds and thrusts has deformed a plateau surface. We focused on a series of catchments aligned perpendicular to the main structures. Using a 1 m lidar digital terrain model, we extracted hillslope topographic properties such as hilltop curvature CHT and nondimensional erosion rates E∗. We observed systematic variation of these metrics coincident with the location of a major underlying thrust system identified by seismic surveys. Using a simple deformation model, the inversion of the E∗ pattern allows us to propose a location and dip for a blind thrust, which are consistent with available geological and geophysical data. We also sampled clasts from eroding conglomerates at several hilltop locations for 10Be and 26Al concentration measurements. Calculated hilltop denudation rates range from 40 to 120 mm kyr−1. These denudation rates appear to be correlated with E∗ and CHT that were extracted from the morphological analysis, and these rates are used to derive absolute estimates for the fault slip rate. This high-resolution hillslope analysis allows us to resolve short-wavelength variations in rock uplift that would not be possible to unravel using commonly used channel-profile-based methods. Our joint analysis of topography and geochronological data supports the interpretation of active thrusting at the southwestern Alpine front, and such approaches may bring crucial complementary constraints to morphotectonic analysis for the study of slowly slipping faults.

Highlights

The topography of the Earth evolves in response to surface processes driven by external forcing of tectonic and climatic origins (e.g., Champagnac et al, 2012; Whittaker, 2012)
The present-day land surface morphology results from this accumulated actions of tectonic and climatic forcing through time, and a major endeavor of geomorphological research is to interpret measurable topographic properties in terms of space and time variations of either of these tectonic uplift or climatic conditions (e.g., Roberts et al, 2012; Demoulin, 2012; Fox et al, 2014)
The objective of our study is to investigate the changes in hillslope morphology, as observed with a lidar digital terrain models (DTMs), across a rock uplift gradient at the front of the southwestern Alps, France, and to assess what kind of information can be retrieved concerning the underlying tectonic processes

Summary

Introduction

The topography of the Earth evolves in response to surface processes driven by external forcing of tectonic and climatic origins (e.g., Champagnac et al, 2012; Whittaker, 2012). Documenting the spatial variability of tectonic processes from topographic analysis has been a key research focus (Wobus et al, 2006), as changes in topographic gradients could record variations in rock uplift rates at various scales, from regional patterns associated with crustal or lithospheric deformation (e.g., Gallen et al, 2013) down to differential motion across individual faults (e.g., Boulton et al, 2014) Such investigations have often been motivated by the practical concern of identifying high strain zones in tectonically active regions in order to contribute to seismic hazard assessment (e.g., Morell et al, 2015)

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