Stage de recherche de Master 2 Master Sciences de la Terre Physique et Chimie de la Terre et des Planètes Author: Louise JEANDET Assessing dynamic topography from rivers characteristics in Patagonia forelands ABSTRACT In Patagonia, south of 46◦S, the Miocene opening of a slab window related to the subduction of the Chile ridge have potentially induced important dynamic topography on the continent. However, the link between mantle flow underneath Patagonia and the post-Miocene relief evolution in the Andean chain and its forelands is not well constrained, and climatic and tectonic factors could also play a role in Patagonia Miocene uplift. To investigate the role of slab window in Patagonia topography, I used two morphometric methods based on the idea that dynamic topography can be eroded away and provide geomorphological record. Stream profile analysis and knickpoints mapping shows a zone between 46 and 49◦S where erosion is important, correlating with the present-day location of dynamic topography previously modelled. However, concavity and steepness data can not be exploited because of important errors on their calculation, probably coming from the quality of the data and the smoothing parameters used in this study. Calculation of Sr index, a new morphometric method based on the catchments geometry, shows a strong and recent uplift in this same zone. This result is likely to be explained by dynamic topography, because this uplifted zone is too far from the belt to be mainly influenced by glacial and tectonic factors. Further SIG analysis combined to fluvial terraces dating and numerical modelling are necessary to precise the delimitation of the zone mainly influenced by dynamic topography and quantify this perturbation. Supervisors: Xavier ROBERT and Laurence AUDIN Institut des Sciences de la Terre (ISTerre), IRD, Grenoble June 4, 2014 Remerciements Je remercie toute l’équipe TRB d’ISTerre pour leur accueil chaleureux. Je remercie tout particulièrement Xavier Robert et Laurence Audin, pour leur disponibilité, et leurs conseils. Merci également à Laurent Husson pour ses idées intéressantes sur mon travail, ainsi qu’à Jean Braun. Merci à Karim, Mallory et Cyril, pour leur bonne humeur quotidienne et pour les gateaux du Jeudi. Cécile, merci pour tes précieux conseils, pour tes contacts, et pour l’expérience des tribulation savantes, qui était très enrichissante. Egalement merci à Laura, Etienne, Olivier, Jérémy, et Amandine, pour ces quelques mois passés ensemble à ISTerre. 1 Introduction According to plate tectonics theory and isostasy principle, the present day topography of the Earth is the result of horizontal motion of the plates, their interactions leading to vertical variations of crustal and lithospheric thickness. How- ever, a growing amount of evidences suggests that the mantle flow creates an important proportion of Earth topography. This low amplitude (1 km) and long wavelength (hundreds to thousands kilometres) topography (Braun, 2010) is the surface expression of the mantle flow generated by density anomalies. This flow interacts with the base of the litho- sphere, inducing viscous stresses balanced by the gravitational stresses generated by the deflection of the Earth surface (Hager et al., 1985, Braun, 2010), generating dynamic topography (fig.1). This boundary deformation takes place on a time scale set by asthenospheric viscosity, similar to the post-glacial rebound characteristic time (1-5 104 years). Thus, dynamic topography can be considered as instantaneous in relation to mantle flow, and moving with it (Hager et al., 1985). In subduction zones, high topography is created by plate convergence. These relief results, at first order, from lithospheric thickening and isostasy. But we have to consider dynamic topography because in subduction zones, density anomalies are very important under the overriding plates. However, dynamic topography is dif- ficult to constrain in these zones, because it is often ... hidden by high tectonic structures. Patagonian Andes is an interesting place to study the surface effects of mantle upwelling in a subduction. The Andean chain borders the western margin of the South American plate. It is a belt of thickened crust, result- ing from the late Mezosoic and Cenosoic subduction of several oceanic plates. Figure 1: Sketch comparing how mantle flow and horizontal plates motion creates topography, from Braun(2010). In Patagonia (part of the continent south of 41◦S), middle Miocene corresponds to an important geodynamical tran- sition, from a widespread marine transgression to a generalized uplift (Lagabrielle et al., 2004). This also corresponds to the opening and northward enlargement of an asthenospheric window (fig.2) through the subducting plates, inducing an upwelling of mantle material (Breitsprecher and Thorkelson, 2009). Thermochronological, numerical, and geomorpho- logical evidences (Guillaume et al., 2009, 2013) suggest that the opening of this slab window induced a wave of dynamic topography in Patagonia, propagating northward since 18 Ma. However, the link between dynamic topography and the Miocene uplift is not well constrained, and glacial isostatic rebound (Lange et al., 2014) or tectonic mechanism (Lagabrielle et al., 2004) have been proposed to explain it. Thus, what is dynamic topography contribution to the Miocene uplift of Patagonia ? Constraining present and past dynamic topography is rather difficult and has been the focus of many recent research (e.g. Braun, 2010, Barnett-Moore et al., 2014, Flament et al., 2014, Moucha et al., 2008). In the Patagonian Andes, it is hidden by tectonic structures, much higher in amplitude, pre-existing to the opening of the slab window. Moreover, quan- tifying current dynamic topography requires a good knowledge of crustal structures and lithospheric thickness, necessary to compute the proportion of isostatically compensated topography in the global Earth topography (Braun, 2010). The best way to infer dynamic topography in the Patagonian Andes is to look at its supposed past situation, looking for a sedimentary, thermochronological, or geomorphological record. This requires that this wave of dynamic topography have have been eroded efficiently. Because of its nature (long wavelength, small amplitude and slow changing), dynamic topography was thought unlikely to be eroded, fluvial erosion needing hight slopes and rapid uplift rate to be efficient. However, recent studies based on 1 numerical modelling shown that dynamic topography can be eroded away (Braun, 2010, Braun et al., 2013). In many cases, dynamic topography directly plays a role on surface processes. Braun et al.(2013) show, using surface processes models, that a passing wave of low amplitude, long wavelength topography can disturb enough the large scale geometry of drainage network to produce an amplification of erosion, the efficiency of fluvial erosion depending on drainage area and slope. Moreover, even if it is eroded away, dynamic topography should be sustained by mantle flow, because viscous forces induced on the lithosphere are balanced by gravitational forces arising from surface deflection (Hager et al., 1985, Braun et al., 2013). Thus, a dynamic topography with a one kilometre amplitude could lead to several kilometres of erosion (Braun, 2010). Geological evidences have also shown that an uplift caused by the passing of a wave of dynamic topography can disturb the drainage network efficiently to provide evidences in the sedimentary and geomorphological record: for example on the Colorado plateau (Moucha et al., 2008, Robert et al., 2011), or on the Brasilian margin (Barreto et al., 2002, Flament et al., 2014). To quantify uplift and topography history, thermochronology can lead to misinterpretation (Guillaume et al., 2013) because it records temperature history of rocks. The link with uplift history is not obvious, because other parameters affect temperature history (erosion, burial...). But we can expect (Braun et al., 2013) that this topography have been efficiently eroded to disturb the drainage network and provide geomorphological record. For more than one century, geomorphologists have tried to understand the response of fluvial drainage network to perturbations (uplift, or subsidence) (Penck, 1919, Ahnert, 1970, Molnar and England, 1990, Whipple and Tucker, 1999). Ultimately, modelling these processes would be useful to go back to the uplift history of a region from the landscape morphology. If many factors controlling the erosion and transport processes still poorly constrained, and vary from one location to another, efficient geomorphological methods have been developed in order to get, at least, regional uplift and subsidence stories. Thus, I use geomorphology to infer the northward propagation of dynamic topography from drainage network char- acteristics. I used Geographic Information System (SIG) analysis to extract rivers and catchments morphometry in tree different areas (fig.2), in order to: − find if the drainage network is responding to a broad uplift of the continent by a wave of erosion; − find if, qualitatively, there are differences in the age of this perturbation, from south to north; − quantify this potential uplift and find an absolute age to the beginning of this perturbation in each zone. 2 Geodynamical context 2.1 A late Cenozoic exhumation During the middle Miocene, the southern portion of the Chile ridge (separating Nazca plate from Antarctic plate, see fig.2) started subducting (Breitsprecher and Thorkelson, 2009). The subduction of this spreading ridge led to the