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Introduction. - The evolution of continental landforms is mainly modulated by the impact of climatic and tectonic processes. Because of their distinctive morphology and the periodicity of their deposition, climatically induced landforms such as alluvial fans or terraces are well suited to infer rates of tectonic and continental climatic processes, Within tectonically active regions, an important step consists in dating displaced geomorphic features to calculate slip rates on active faults. Dating is probably the most critical tool because it is generally much more simplier to measure deformation resulting from tectonic activity than it is to accurately date when that deformation occurred.

Recent advances in analytical chemistry and nuclear physics (accelerator mass spectrometry) now allow quantitative abundance measurements of the extremely rare isotopes produced by the interaction of cosmic rays with surface rocks and soils, the so-called in situ-produced cosmogenic nuclides (He-3, Be-10, Ne-21, Al-26,Cl-36), and allow to directly date the duration that a landform has been exposed to cosmic rays at the Earth’s surface [Lal, 1991; Nishiizumi ct al., 1993; Cerling and Craig, 1993; Clark ct al., 1995]. In fact, the abundance of these cosmonuclides is proportional to landscape stability and, under favorable circumstances, their abundance within surface rocks can be used as a proxy for erosion rate or exposure age. These cosmonuclides thus provide geomorphologists with the opportunity to constrain rates of landscape evolution.

This paper presents a new approach that combines cosmic ray exposure (CRE) dating using ill situ-produced Be-10 and geomorphic as well as structural analyse. This approach has been applied on two active strike-slip and reverse faults located in the Andean foreland of western Argentina. These two case studies illustrate how CRE dating using in situ-produced Be-10 is particularly well suited for geomorphic studies that aim to estimate the respective control of climate and tectonics on morphogenesis.

Cosmic ray exposure dating. The application of in situ-produced cosmogenic nuclides in quantitative geomorphology is based on their continuous production in the upper few meters of rocks exposed to cosmic rays at the Earth’s surface. At a given site, the cosmogenic production rate is controlled by the location (i.e., latitude and altitude) and by the exposure geometry (i.e., topography). The largest cosmonuclide accumulation would thus be expected for a flat surface that has experienced no aggradation and no or little erosion. Abandoned alluvial features in arid climatic conditions can present such characteristics of static, non-evolving surfaces. They are geomorphic features particularly well sui red to decipher the shape and history of the earth’s surface in the context of tectonic and climatic forces [Ritz er (II., 1995, Brown ct al., 1998; Siame, 1998]. Cosmogenic Be-10 (t(1/2) = 1.5 Ma) is produced in surficial quartz minerals exposed to cosmic rays. This nuclide is particularly well suited because its ill situ-production is essentially limited to neutron-induced spallation, and thus to the upper few meters of surficial material. In addition, the build-up of Be-10 content does not suffer from loss by diffusion and its radiogenic formation is negligible within quartz minerals. In situ-produced Be-10 nuclide accumulates with time until its concentration reaches a steady-state balance between production and loss by erosion and radioactive decay. The Be-10 content of a given surficial boulder reflects the entire exposure history of the surface and integrates the exposure in source region, during transport as well as the exposure since abandonment in its current position [Siame ct at, 2000].

The Be-10 concentration C (atom.g(-1)) within a surficial embedded boulder as a function of time t lye) is given by :

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where epsilon is a mass erosion rate (g.cm(-2).yr(-1)), PO is the local production rate (atom.g(-1).yr(-1)), rho is the rock density (g.cm(-3)), ii is the attenuation length of cosmic rays (similar to 150 g.cm(-2) for production of Be-10 by neutron-induced spallation), lambda is the radioactive decay constant (yr(-1)) and C(D) is the cosmogenic nuclide concentration at the initiation of the present surface exposure episode. This equation assumes constant rates of production and erosion and has two unknowns :epsilon, t. To calculate a lower limit for the exposure age of a surface from this equation, one must check that exposure prior to the present exposure episode was minimal and assume that the mass erosion rate is negligible. In this study, in situ-produced cosmogenic Be-10 concentrations in surface boulders exposed to cosmic rays have been measured at the Tandetron AMS facility, Gif-sur-Yvette, France [Raisbeck ct a]., 1987; Raisbeck ct al., 1994].

Application to a strike-slip fault : the EI Tigre fault (ETF), - The "El Tigre" Fault (ETF is a spectacular 120 km-long right-lateral strike-slip fault with a N010 degreesE-trending located on the western slope of the Argentine Precordillera (Fig. 1, 2). To analyse its geometry and characterize the active deformation along the ETF, detailed geomorphic SPOT image and fieldwork studies have been performed. The high resolution (10 m a pixel) panchromatic SPOT images provide evidence of recent tectonic activity such as deviated stream channels incised onto a series of six alluvial fan units (fig. 2) [Siame, 1997a]. To establish quantitatively the timing of emplacement and abandonment of these alluvial units, Be-10 concentrations in surface boulders exposed to cosmic rays have been measured. The results show that the alluvial fans have been emplaced during successive and different major paleoclimatic events. The calculated minimum exposure ages date fan abandonment from 41.0 +/- 8.5 ka for the youngest alluvial fan deposit up to 670 +/- 140 ka for the oldest relic deposit. When linked to measured cumulative tectonic displacements, these exposure ages yield horizontal slip rates ranging from 0.5 to 2 mm/yr.

Application to a reverse fault : the Las Tapias fault (LTF), - The 18 km-long reverse "Las Tapias" Fault (LTF), is located at about 31 degreesS latitude in the eastern part of the Argentine Precordillera (fig. 1). A W-facing scarp that can be followed all along its entire length characterizes the LTF (fig. 3). The LTF crosses three alluvial fan units which are stepped on the hangingwall while they are superposed on the footwall of the LTF [Siame, 1998]. Using topographic profiles constructed across the fault scarp, cumulative vertical displacements for the upper and intermediate alluvial fan have been estimated at 6 +/- 1 m and 11 +/- 2 m, respectively. Using ill situ-produced Be-10 concentrations, minimum surface exposure ages have been calculated at 1.0 +/- 0.5 ka for the: lowermost fan, 6.0 +/- 1.4 Ira for the intermediate fan and 16.0 +/- 3.6 ka for the uppermost fan. These data lead to a Holocene vertical slip rate of 0.8 +/- 0.4 mm/yr on the LTF.

Conclusions. - The evolution of continental landforms being modulated by the impact of climatic and tectonic processes. climatically induced landforms such as alluvial fans and terraces are well suited to infer rates of tectonic processes. Within tectonically active regions, CRE dating of alluvial fan surfaces allows the quantification of crustal deformation rates and earthquake-related landform development as well as the assessment of seismic hazard parameters. We have studied two series of Quaternary alluvial fans and terraces located along active faults (the right-lateral strike-slip ETF and the reverse LTF) in similar climatic settings (Andean desert at similar to 31 degreesS). Along the ETF, the CRE ages suggest that the alluvial fans were abandoned during major paleoclimatic events and preserved over a long time period (from 41 +/- 8 Ira up to 670 +/- 140 ka). In contrast, the exposure ages of alluvial fans along the LTF indicate that they were emplaced and abandoned during the last 16.0 +/- 3.6 ka. In absence of direct chronological data, these two series of alluvial fans would have been probably correlated to the same climatic events and the resultant slip rates would have been in error. This study demonstrates that in a low erosional environment, in situ-produced cosmogenic Be-10 dating call be regarded as a valuable tool to determine minimum CRE ages of tectonically displaced morphological features and associated slip rates.