Hillslope denudation and morphologic response across a rock uplift gradient

Abstract

Documenting the spatial variability of tectonic processes from topography is routinely undertaken through the analysis of river profiles, as 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 uplift rates. 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 5 and cosmogenic nuclides 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 Valensole Mio-Pliocene molassic basin, where a series of fold and thrust 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 hillslopes topographic properties such as hilltop curvature C HT or non-dimensional erosion rates E * . We observed a systematic variation of these metrics coincident with the location of a major 10 underlying thrust system identified by seismic surveys. Using a simple deformation model, the inversion of the E * pattern allows 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 conglomerate at several hilltops locations for 10 Be and 26 Al concentration measurement.Calculated hilltops denudation rates range from 40 to 120 mm/ka. These denudation rates appear to be correlated with E * and C HT extracted from the morphological analysis, and are used to derive absolute estimates for the fault slip rate. This 15 high resolution hillslope analysis allows to resolve short wavelength variations in rock uplift, that would not be possible to unravel using commonly used channel profiles based methods. Our joint analysis of topography and geochronological data supports active thrusting at the Southwestern alpine front, and such approaches may bring crucial complementary constraints to morphotectonic analysis for slip rates on slow active faults.Similarly to river profiles, theoretical formulations of hillslope denudation predict a strong dependence of morphological parameters, such as slope or relief, on uplift rates. However, key elements of hillslope behavior such as the threshold stability angle for hillslope material and the non-linear relationship between sediment fluxes and topographic gradient (Roering et al., 1999(Roering et al., , 2001(Roering et al., , 2007, implies that for fixed valley spacing, hillslope morphology can be insensitive to changes in erosion or rock uplift rates over large range values (Burbank et al., 1996;Ouimet et al., 2009). This behavior is a major limitation for the use 50 of hillslope morphology to retrieve information about tectonic gradients. Furth...