Hillslope denudation and morphologic response to a rock uplift gradient

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.

Transient hillslope erosion in slow evolution landscapes

“…Combining the analysis of detailed high-resolution hillslope morphometry with TCN data is a promising way to investigate landscape dynamics but has been used only in a limited number of situations (DiBiase et al, 2012;Godard et al, 2019Godard et al, , 2020Hurst et al, 2012).…”

Influence of Climate‐Forcing Frequency on Hillslope Response

“…Data used in this study are available through Codilean et al (2018), Godard et al (2019Godard et al ( , 2020, Grieve et al (2016), Hurst et al (2012), Richardson et al (2019), andRoering et al (2007).…”

“…OR: Oregon Coast Range, OR (Grieve et al, 2016;Roering et al, 2007). VA: Valensole Plateau, Provence, France (Godard et al, 2020) and A2), and thus their dynamics involve significant nonlinear contributions to the hillslope sediment flux, with response times ranging from 10 ka (OR and GM) to several 100s of ka.…”

Assessing post-pliocene deformation in a context of slow tectonic deformation: insights from paleoseismology, remote sensing and shallow geophysics in Provence, France

“…First, we are unable to estimate horizontal cumulative displacements due to the lack of preserved offset markers.Besides, it is currently not possible to estimate when and over how long this cumulative slip has been recorded.Nevertheless, this vertical rate is coherent with published slip rates in western Provence. For example,Godard et al (2020) identified similar rock uplift rates in the Digne-Valensole basin only 35 km to the north of Maragrate. There, the authors deduced a ∼0.021 mm/yr differential uplift velocity along a 10 km-transect, which is interpreted as related to active thrusting.…”

“…years. In addition, morphological evidence of activity along active fault segments is subdued because denudation rates are comparable or faster than fault velocities (about 30-60 mm/ka, Siame et al, 2004;Molliex et al, 2011;Godard et al, 2016;Thomas et al, 2017;Thomas et al, 2018;Godard et al, 2020). This implies that tectonic geomorphic markers tend to be rapidly eroded away and that no single stand-alone method can be used to accurately evaluate seismic deformation.…”