Geomorphology 180–181 (2013) 33–46 Contents lists available at SciVerse ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Geochronology of pediments and marine terraces in north-central Chile and their implications for Quaternary uplift in the Western Andes María Pía Rodríguez a,⁎, Sébastien Carretier b, Reynaldo Charrier a,1, Marianne Saillard b,2, Vincent Regard b, Gérard Hérail b, Sarah Hall c,3, Dan Farber c, Laurence Audin d a Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Casilla 13518, Correo 21, Santiago, Chile b Université de Toulouse, UPS, IRD, CNRS, GET, 14 Av. Edouard Belin, 31400, Toulouse, France c Earth and Planetary Science Dept., University of California Santa Cruz, CA, CA 95060, USA d Institut des Sciences de la Terre, IRD: UR219, CNRS: UMR5275, IFSTTAR, Université de Savoie, Université Joseph Fourier, Grenoble I, INSU, OSUG, France article info abstract Article history: In north-central Chile, a wide shore platform is morphologically connected with a high fluvial terrace and a Received 15 March 2012 pediment. The eastward extension of Quaternary coastal uplift in the Southern Central Andes is poorly Received in revised form 1 September 2012 constrained since no age correlation between marine and continental landforms has been reported. We Accepted 12 September 2012 use 26Al and 10Be concentrations to constrain the geomorphic evolution of these marine and continental Available online 23 September 2012 landforms near the Choapa valley (31.6° S). 10Be ages for the shore platform indicate that this surface was repeatedly reoccupied during sea-level highstands between ~800 and 500 ka and uplifted after 500 ka. Keywords: ‘ ’ CRN dating While zero erosion ages for the pediment between ~600 and 300 ka only partly overlap the shore platform Uplift age range, more realistic exposure ages calculated for an erosion rate of 1 m/Ma are between ~945 and Chile 475 ka, fitting the age range of the correlated shore platform. 10Be concentrations of the high fluvial terrace Fluvial terraces are highly scattered evidencing vertical mixing of clasts probably due to slow lowering of the surface. Pediment Although it is not possible to determine an age for this landform, the scattering among its 10Be concentrations Shore platform implies that this marker is several hundreds of thousands of years old and that the high fluvial terrace began to form at ~1200 ka or after. Finally, 10Be concentrations of the high fluvial terrace, the pediment and the shore platform are of the same order of magnitude, which is consistent with the clear morphologic correla- tion between these three types of landforms. These data suggest that the marine and continental landforms studied formed synchronously, with some local differences, during a long period of relative tectonic stability between ~(1200?) 800 and 500 ka and uplifted after 500 ka. Our results confirm recent studies showing a post-400±100 ka renewal of uplift along the Pacific coast after a Lower to Middle Pleistocene period of slow uplift. Moreover, the extension of the surfaces suggests that a broad region of ~40 km has been uplifted ca. 150 m during the Quaternary. © 2012 Elsevier B.V. All rights reserved. 1. Introduction and Wehmiller, 1992; Ota et al., 1995; Ortlieb et al., 1996; Marquardt et al., 2004; Quezada et al., 2007; Saillard et al., 2009; Regard et al., Near-coastal deformation in the Southern Central Andes is widely 2010). Initially, the age of these surfaces was constrained through mac- documented by the presence of marine sedimentary sequences and rofossil biostratigraphy and through diverse dating techniques on shells shoreline and fluvial geomorphic features uplifted along the Pacific such as U-series, electron spin resonance, amino-acid racemization and coast. In particular, Quaternary coastal uplift has been inferred from 14C(e.g.Herm, 1969; Leonard and Wehmiller, 1992; Ota et al., 1995; the study of marine terraces and beach ridges (Paskoff, 1970; Leonard Marquardt et al., 2004). However, many of the marine terraces along the Pacific margin of the Southern Central Andes correspond to shore platforms with little or no fossil content. Only recently, has the use of ⁎ Corresponding author. in-situ produced cosmogenic nuclides facilitated better constraint on E-mail address: [email protected] (M.P. Rodríguez). the geomorphic evolution of these surfaces by providing exposure age 1 Current address: Escuela de Ciencias de la Tierra, Universidad Andres Bello, Campus dating of these marine landforms (Marquardt, 2005; Quezada et al., República, Santiago, Chile. 2007; Saillard, 2008). In contrast to previous models of coastal uplift 2 Current address: Géoazur, Université Nice Sophia-Antipolis, Centre National de la which assumed steady uplift in this region (e.g. Ota et al., 1995), recent Recherche Scientifique (UMR 7329), Observatoire de la Côte d’Azur, La Darse B.P. 48, 06235 Villefranche sur-Mer Cedex, France. data suggest that coastal uplift rates in the Southern Central Andes have 3 Current address: Earth and Planetary Sciences, McGill University, Quebec, Canada. been highly variable during the Pleistocene and that periods of rapid 0169-555X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2012.09.003 34 M.P. Rodríguez et al. / Geomorphology 180–181 (2013) 33–46 uplift alternate with periods of relatively slow uplift (Quezada et al., interpret the data as representing a coastal region spanning ~1500 km 2007; Saillard et al., 2009). Previous authors show that periods of from Atico to La Serena that experienced renewed uplift since at least slow uplift favor the development of particularly wide shore platforms 400 ka, corresponding to marine isotopic stage (MIS) 11. This uplift (>1 km) due to superimposition of repeated sea-level highstands on followed a long period of relative quiescence, which is necessary to a relatively stable rocky coast (Anderson et al., 1999; Quezada et al., shape such a wide shore feature. Quaternary uplift could have also af- 2007; Álvarez-Marrón et al., 2008; Saillard et al., 2009; Regard et al., fected the coastal area located further south of La Serena as the presence 2010). This particular type of wide shore platform, named rasa, corre- of rasas has been reported from Nazca, Peru to Valparaíso, Chile sponds to a characteristic and relatively continuous feature of the coast- (Paskoff, 1970; Fig. 1), however no geochronological data have been al morphology along the Pacific margin in southern Peru and northern reported that would support this model. Importantly, it has also been Chile (Fig. 1; Paskoff, 1970; Regard et al., 2010). In order to laterally cor- proposed that subduction-related Quaternary uplift was not solely lo- relate surfaces with similar cliff foot elevations and determine the cated at the coast but has affected all the forearc of southern Peru and timing of uplift, Regard et al. (2010) compiled cliff foot elevations and northern Chile (Mortimer, 1973; Tosdal et al., 1984; Clark et al., 1990; extrapolated cliff foot ages using the available chronological data for Hartley et al., 2000, 2005; Hall et al., 2008). Recently a number of studies the rasas spanning the region from Atico, Peru to La Serena, Chile have applied cosmogenic nuclide dating to constrain the geomorphic (Fig. 1). The data compiled thus include a range of diverse dating tech- evolution of planation surfaces located inland from the coast niques on shells already mentioned above (e.g. Leonard and Wehmiller, (Nishiizumi et al., 2005; Kober et al., 2007; Hall et al., 2008; Evenstar 1992; Ota et al., 1995; Marquardt et al., 2004), the faunal assemblage et al., 2009; Placzek et al., 2010). These studies have documented the (e.g. Herm, 1969) and cosmogenic nuclides to determine the age of existence of Neogene and Quaternary aged surfaces. However, many the rasas (Marquardt, 2005; Quezada et al., 2007; Saillard, 2008). of these studies have interpreted cosmogenic concentrations to reflect Regard et al. (2010) recognized three rasa levels in southern Peru and surface erosion rates rather than the age of landform abandonment fol- northern Chile, the main one of which has a cliff-foot formation age of lowing surface uplift. Moreover, the planation surfaces studied in these 400±100 ka and lies at 110±20 m (above mean sea-level). They works have no specific geomorphic correlation with marine landforms. Fig. 1. Geodynamic setting of the study region. Dashed lines mark the limits of the Central Depression. Topography and bathymetry based in NASA elevation model with a resolution of 30 and 90 m respectively. CC=Coastal Cordillera; CD=Central Depression; CP=Principal Cordillera; AP=Altiplano-Puna. M.P. Rodríguez et al. / Geomorphology 180–181 (2013) 33–46 35 Thus, it remains unclear whether the uplift documented by marine ter- races corresponds to a local uplift of the coastal area or a more regional one involving planation surfaces inland. To address this issue, we have collected new data from a key location in north-central Chile between 30 and 32.5° S, which document the geomorphic connections of Quater- nary tectonics in the western Andes. The study area is bounded to the north by the region studied by Saillard et al. (2009) and Regard et al. (2010) where rasas have been described in detail and, importantly, our geomorphic analysis suggests lateral connections between diverse geomorphic elements such as a shore platforms, a high fluvial terrace and a pediment uplifted in a west–east direction over a broad region (~40 km). We use 10Be concentrations of the shore platform and 10Be/26Al concentrations of the high fluvial terrace and the pediment to constrain the geomorphic surface evolution and the timing of uplift near the Choapa valley (~31.6° S; Figs. 1, 2). 2. Regional setting The study area between 30 and 32.5° S (Fig. 1) is located within a segment of the Southern Central Andes where the subduction angle between the Nazca and South American Plates is ~10°, usually re- ferred as the Chilean or Pampean flat subduction segment (Cahill and Isacks, 1992).