PhD defense of Valeria Zavala

Valeria Zavala, a PhD student of the GET,  defended her PhD of the University Toulouse III  at GET the 19th of June.

The PhD was entitled “Geomorphological evolution of coupled mountain-piedmont systems: numerical and experimental modelling of climate impact and study of the incision of the western Andean piedmont (Chiza and Tana canyons, Chile)”

and supervised by Stéphane Bonnet (UT III-GET) and Sébastien Carretier (IRD-GET) with the collaboration of Rodrigo Riquelme (UCN).

The jury was composed of

• Magali DELMAS (CNRS/MNHN/Université de Perpignan, Rapporteuse)
• Peter VAN DER BEEK (ISTerre, Rapporteur)
• Marc DE RAFELIS (UPS/GET, Examinateur)
• Joseph MARTINOD (UFR Science et Montagne, Examinateur)
• Stéphane BONNET  (UPS/GET, Directeur de thèse)
• Sébastien CARRETIER  (IRD/GET, co-Directeur de thèse)

Abstract:

A climate change may trigger a response in the Earth’s surface in the form of erosion, process that changes its topography shape. In turn, the relief can represent a topographic barrier to atmospheric circulation causing orographic precipitation. To understand this complex interaction and feedback between climate and erosion it is important to determine how, why and when the relief has changed in the past and how that climate forcing is registered in erosional landscapes. In this study we present the results of three methodologies that address the effect of climate change on the topographic and denudational evolution of the landscape during long and intermediate timescales (10^4-10^6 yr). Firstly, we studied the effect of orographic precipitation on a growing mountain by numerical modeling using the landscape evolution model CIDRE. The numerical models revealed that precipitation peak elevation appears to be a crucial factor that controls the denudation and topographic mountain history. A peak in the spatial distribution of precipitation located at a low or medium elevation can produce a pulse or acceleration of denudation, while if the peak of rainfall is at the top of the mountain, it limits the growth of the mountain but the denudation history is not affected. These numerical results show that it is possible to produce a pulse or acceleration in the denudation in the absence of climatic or tectonic variations. Secondly, we studied the effect of cyclical variations in precipitation rates over an experimental coupled mountain- piedmont landscape, considering a periodicity that could correspond to climatic forcing on an intermediate timescale (10^4-10^5 yr). Our work especially focused on defining the conditions that drive changes from aggradation to incision in the piedmont. We show that there are two necessary conditions for an incision. First, the ratio between water discharge and sediment  flux induced by precipitation variations must drive a decrease in sediment concentration at the outlet of the catchments. Second, changes in sediment concentration only drive incision when the mean slope of the piedmont, which inversely depends on the mean precipitation rate, is above a threshold value. We finally propose a phase diagram that synthesizes how the relationship between fan slope and sediment concentration controls the passage from deposition to incision in the piedmont at the foot of a mountain relief. Finally, to understand how the incision evolves and the factors that control the valley widening we focus on the geomorphological characterization of two canyons located in northern Chile. For the first time, we use 10Be cosmogenic isotope concentration measured in colluvial deposits at foot of hillslope to quantify longitudinal variations of valley flank erosion rate. We observed an inverse relation between valley flank erosion rate and valley width allowing to infer that the channel lateral mobility is the first-order factor controlling the valley widening. The main results of this thesis highlight the role of climate in the evolution of the landscape at different timescales.