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Evidence from Geochronology, Stratigraphy, and Geochemistry Geoscience Frontiers 10 (2019) 1139e1165 HOSTED BY Contents lists available at ScienceDirect China University of Geosciences (Beijing) Geoscience Frontiers journal homepage: www.elsevier.com/locate/gsf Research Paper Cenozoic basin evolution of the Central Patagonian Andes: Evidence from geochronology, stratigraphy, and geochemistry A. Encinas a,*, A. Folguera b, R. Riffo a, P. Molina a, L. Fernández Paz b,c, V.D. Litvak b,c, D.A. Colwyn d, V.A. Valencia e, M. Carrasco a a Departamento de Ciencias de la Tierra, Facultad de Ciencias Químicas, Universidad de Concepción, Víctor Lamas 1290, Concepción, 4030000, Chile b Instituto de Estudios Andinos, Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Departamento de Ciencias Geológicas, Buenos Aires, Argentina c CONICET-Universidad de Buenos Aires, Instituto de Estudios Andinos Don Pablo Groeber (IDEAN), Buenos Aires, Argentina d Department of Geology & Geophysics, Yale University, 210 Whitney Ave., New Haven, CT, 06511, USA e School of the Environment, Washington State University, Pullman, WA, 99164, USA article info abstract Article history: The Central Patagonian Andes is a particular segment of the Andean Cordillera that has been subjected to Received 26 September 2017 the subduction of two spreading ridges during Eocene and Neogene times. In order to understand the Received in revised form Cenozoic geologic evolution of the Central Patagonian Andes, we carried out geochronologic (U-Pb and 25 April 2018 40Ar/39Ar), provenance, stratigraphic, sedimentologic, and geochemical studies on the sedimentary and Accepted 22 July 2018 volcanic Cenozoic deposits that crop out in the Meseta Guadal and Chile Chico areas (w47S). Our data Available online 11 August 2018 Handling Editor: R. Damian Nance indicate the presence of a nearly complete Cenozoic record, which refutes previous interpretations of a hiatus during the middle Eoceneelate Oligocene in the Central Patagonian Andes. Our study suggests fl fl Keywords: that the uvial strata of the Ligorio Márquez Formation and the ood basalts of the Basaltos Inferiores de Central Patagonian Andes la Meseta Chile Chico Formation were deposited in an extensional setting related to the subduction of the U-Pb geochronology Aluk-Farallon spreading ridge during the late PaleoceneeEocene. Geochemical data on volcanic rocks Ar-Ar geochronology interbedded with fluvial strata of the San José Formation suggest that this unit was deposited in an Geochemistry extensional setting during the middle Eocene to late Oligocene. Progressive crustal thinning allowed the Stratigraphy transgression of marine waters of Atlantic origin and deposition of the upper Oligoceneelower Miocene Sedimentology Guadal Formation. The fluvial synorogenic strata of the Santa Cruz Formation were deposited as a consequence of an important phase of compressive deformation and Andean uplift during the early emiddle Miocene. Finally, alkali flood basalts of the late middle to late Miocene Basaltos Superiores de la Meseta Chile Chico Formation were extruded in the area in response to the suduction of the Chile Ridge under an extensional regime. Our studies indicate that the tectonic evolution of the Central Patagonian Andes is similar to that of the North Patagonian Andes and appears to differ from that of the Southern Patagonian Andes, which is thought to have been the subject of continuous compressive deformation since the late Early Cretaceous. Ó 2018, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/). 1. Introduction and Antarctic plates beneath South America (Mpodozis and Ramos, 1989). Although plate velocities at the trench are rather uniform The Andean Cordillera is the longest and highest subaerial north of the Nazca-Antarctica-South America triple junction, the mountain belt formed in a convergent setting. Along the Chilean Andes show striking latitudinal differences in width, height, and margin, this range is related to the subduction of the oceanic Nazca shortening (Mpodozis and Ramos, 1989; Martinod et al., 2010). It appears that the tectonic regimes operating along the Andean range have varied widely along strike and across time. For example, compressional tectonics have dominated the Andes of southern * Corresponding author. Peru and northern Chile (w15Se27S) throughout most of the E-mail address: [email protected] (A. Encinas). Cenozoic (Oncken et al., 2006, and references therein), whereas Peer-review under responsibility of China University of Geosciences (Beijing). https://doi.org/10.1016/j.gsf.2018.07.004 1674-9871/Ó 2018, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC- ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1140 A. Encinas et al. / Geoscience Frontiers 10 (2019) 1139e1165 A. Encinas et al. / Geoscience Frontiers 10 (2019) 1139e1165 1141 alternating stages of compressional extensional, or neutral tec- deformation is not restricted to the time of ridge subduction and tonics have characterized the Andes of south-central Chile expressed doubts about the cause-effect relationship between (w33Se46S) during the same period (e.g., Mpodozis and Ramos, these two processes (Ray, 1996; Suárez and De La Cruz, 2000). In 1989; Jordan et al., 2001; Charrier et al., 2002; Orts et al., 2012; fact, Suárez et al. (2000), Lagabrielle et al. (2007), and Scalabrino Encinas et al., 2016a; Horton, 2018b). These temporal and lat- et al. (2010) proposed that the main result of the Chile Ridge sub- itudinal changes in the tectonic regime affected not only Andean duction has not been compressional deformation, but fast regional growth but also other geological characteristics such as the distri- uplift related to extensional or transtensional tectonism. bution of volcanic centers across the upper plate, the geochemistry The study of the Cenozoic geological evolution of the Patagonian of magmatism, the distribution and characteristics of sedimentary Andes is hampered by the erosion of most of the MesozoiceCenozoic basins, and the occurrence of marine transgressions of Pacific and volcano-sedimentary cover, which resulted in extensive exposure of Atlantic origin that have periodically flooded parts of the Chilean the plutonic and metamorphic basement (Thomson, 2002). However, margin (e.g., Mpodozis and Ramos, 1989; Jordan et al., 2001). the hinterland area at w47S contains an almost complete Cenozoic The causes for these latitudinal and temporal variations in the record of stratified rocks, which crop out east of the present volcanic tectonic regime are debated. The exact mechanism is controversial, arc in the axial part of the Andean Cordillera at Meseta Guadal and in as it has been ascribed to different factors such as changes in the eastern flank of this chain south of Chile Chico (Niemeyer, 1975; interplate coupling (Lamb and Davis, 2003; Yanez and Cembrano, Charrier et al., 1979; Flint et al., 1994; Suárez et al., 2000; De la Cruz 2004), the dip angle of the subducting slab (Martinod et al., et al., 2004; Espinoza et al., 2005; De La Cruz and Suárez, 2006; De 2010), the convergence rate and angle (e.g., Pardo-Casas and la Cruz and Suárez, 2008)(Figs. 1e3). Sedimentary and volcanic Molnar, 1987), or the absolute westward motion of South America rocks in these areas are comprised of upper Paleoceneelower Eocene (e.g., Silver et al., 1998). Another important process that influences fluvial strata of the Ligorio Márquez Formation, Eocene flood basalts of the geologic evolution of convergent margins is the subduction of the Basaltos Inferiores de la Meseta Chile Chico Formation, middle active oceanic ridges, because they represent first order heteroge- Eoceneeupper Oligocene fluvial strata of the San José Formation, neities that affect the tectonic and magmatic development of the upper Oligoceneelower Miocene marine strata of the Guadal For- upper plate (Scalabrino et al., 2010). Subduction of spreading ridges mation, lower-middle Miocene fluvial strata of the Santa Cruz For- can cause important effects in crustal deformation and surface mation, and middle-late Miocene flood basalts of the Basaltos morphology (Scalabrino et al., 2010). Their subduction may also Superiores de la Meseta Chile Chico Formation (Fig. 4). The tectonic lead to the opening of a slab window, which can trigger significant setting of these strata, which constitute the most complete and well changes in the mantle dynamics beneath the orogenic wedge preserved Cenozoic succession of the entire Patagonian Andes, is not (Scalabrino et al., 2010, and references therein). well understood, as noted by previous authors (e.g., Suárez and De La The Central Patagonian Andes at w47S(Figs. 1 and 2)isan Cruz, 2000; De la Cruz et al., 2003). Interestingly, the flood basalts of excellent area to study the geological effects of alternating episodes the Basaltos Inferiores and Superiores de la Meseta Chile Chico for- of compressional and extensional tectonics, as well as the influence mations have been attributed to slab windows opened during the of the subduction of spreading ridges on the geological evolution of subduction of the Aluk-Farallón and Chile ridges during the Eocene a convergent margin. This area is of particular interest because it is and Miocene respectively (Espinoza et al., 2005). Also relevant is the transitional between
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