Geological study of the Triassic to Lower Jurassic volcano-sedimentary units in northern Chile: the quest to Andean subduction initiation Verónica Oliveros*, Paulina Vásquez, Christian Creixell, Daniela Vallejos and Ximena Valin Departamento Ciencias de la Tierra, Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C, Concepción. Servicio Nacional de geología y Minería. Avda. Santa María 0104, Providencia, Santiago. *Contact email: [email protected] Abstract. A geological study, integrating structural volumes of volcanic, plutonic and sedimentary rocks of sedimentological, petrological and geochemical data and Carboniferous to Early Cretaceous ages are exceptionally analyses, is aimed to constrain the timing and mechanisms well exposed due to prevalent arid to semi-arid conditions. of the onset of Andean subduction in the southern Central Consequently, this area is suitable for a systematic study of Andes of Chile. The Late Triassic to Early Jurassic is a magmatic and basin evolution linked to changes in the very enigmatic period in the Andean record because it marks the transition between two subduction settings, the tectonic setting of the continental margin and tracking the Gondwana and the Andean. They strongly differ in initiation of Andean subduction. deformation styles, and associated magmatism and metamorphism, reflecting an important contrast in 1.1 Geological framework in Chile and western subduction conditions and likely an important change in the Argentina configuration of the tectonic plates. The present forearc region of north-central Chile (25°-31°S) preserves the During the Late Paleozoic, the subduction of the oceanic geological record from both Gondwana and Andean plate under the continental margin originated extensive subductions systems, and the episode in between. Large magmatism with significant crustal signature and the volumes of volcanic, plutonic and sedimentary rocks of Carboniferous to Early Cretaceous ages are exceptionally subduction system was characterized by an accretionary well exposed. Consequently, this area is suitable for a prism (Mpodozis and Ramos, 1989). At the Permian to systematic study of the magmatic and basin evolution from Late Jurassic times, this episode was followed by events of Triassic to Lower Jurassic. This evolution could be linked to continental extension and some authors proposed a the changes in the tectonic setting of the continental margin lithospheric rifting that took place at the Triassic-Jurassic and tracking the onset of the Andean subduction system. times (Charrier, 1979; Mpodozis and Ramos, 1989). Keywords: Andean subduction, Triassic, northern Chile. From the Late Triassic to the Early Jurassic, in central- western Argentina, several syn-rift depocenters 1 Introduction accumulated intermediate to acidic volcanics and clastic sedimentary material in hemigrabens, with orientation-axis The Andean continental margin has been periodically parallel to adjacent basement terranes (Franzese and active at least since the Early Paleozoic. During this time, Spalleti, 2001). Some of these depocenters were developed two different subductions systems have been recognized in as oblique rift systems (Bechis et al., 2010). At the same north and central Chile and its border region to Argentina: time in northern Chile La Ternera, Estratos del Mono, La one at the Devonian-Permian (Gondwana or Pre-Andean), Totora and Estratos del Verraco formations were deposited and the other at the Jurassic to Recent times (Andean). and crop out in the Precordillera and high Andes (Moscoso Both subduction systems strongly differ in deformation et al. 2010, Blanco, 1996). These are described as volcano- styles, metamorphism and main types of magmatism sedimentary sequences up to 2.5 km in thickness with (Mpodozis and Ramos, 1989; Franzese and Spalletti 2001; silicic and basaltic volcanism and sedimentological Charrier et al., 2007). The nature of the tectonic change features of synrift deposits (Suárez and Bell, 1992). during the Late Triassic-Early Jurassic, the time of transition between the Pre-Andean and the Andean At the same latitude (25°30’S) in the Coastal Cordillera subductions, is still subject of discussion. The onset of the there are the volcano-sedimentary deposits of Agua Chica Andean subduction is much less clear, their timing and and Cifuncho formations concordantly overlied by the mechanisms are not known. Early Jurassic Pan de Azúcar formation. All the aforementioned units rest concordantly below the Jurassic The present forearc region of north-central Chile (25°- La Negra Formation (Suárez et al. 1984). The local 31°S) preserves the geological record from both Pre- absence of unconformities during the Late Triassic to the Andean and Andean subduction systems, and the Early Jurassic suggests that there were basins which have a continental extension episode in between (Fig.1). Large tectonic stability during this period. However, the volcano- 159 sedimentary Canto del Agua Formation, equivalent at 28º Cordillera and the other in the Precordillera and High and 29ºS of the Agua Chica and Cifuncho formations Andes (Fig. 1). These specific units are the subject of the (Suárez et al. 1984, Moscoso and Covacevich, 1982), proposed research study as they are considered the product underlies unconformably below the volcanic rocks of La of changes in the tectonic regime and subduction style Negra Formation (Godoy, 1985, Welkner et al. 2006). during the Late Triassic-Early Jurassic times. These stratigraphic relations show that the tectonic regime was not homogeneous along westernmost part of the continental margin during the transition from the Triassic to the Jurassic. Thus, whereas the shift from continental rifting to back-arc domain during Late Triassic and Early Jurassic seems relatively clear from the geological record in the landward side of the continental plate (now Precordillera and High Andes), the evolution of the region close to the convergent margin (Coastal Cordillera) is much less constrained, and evidence of subduction interruption (and consequently subduction initiation) is not conclusive. In this sense some authors have proposed that those basins developed in a subduction regime and that magmatism was continuous from Permian (or Late Triassic in some places) to Early Cretaceous (Brown, 1991, Suárez and Bell, 1992). Following the Triassic-Jurassic period, at the Andean margin the magmatism is characterized by the emplacement of huge volumes of subduction-related igneous rocks (Fig. 1). 2 General Objective and Proposed Methodology Figure 1. Schematic map showing the main outcrops of each According to the bibliographic information available, three relevant group of geological units and the transects for the main areas of study have been chosen to perform the proposed study. Modified after Franzese and Spalleti (2001), collection of geological data and samples. These areas are Charrier et al. (2007), Kleiman and Japas (2009). 150 km-long x 50 km-wide E-W transects from the High Andean to the Coastal cordillera (25°-31°S) (Fig. 1) that Our working hypothesis is that the onset of Andean cut much of the Triassic to Lower Jurassic volcano- subduction in the southwestern margin of Gondwana sedimentary sequences and plutonic bodies, as well as the started during the Late Triassic to Early Jurassic. The upper and lower stratigraphic units representative of the associated magmatism exposed on the present forearc had Late Paleozoic time, and the Jurassic rocks, which are a distinct evolution and geochemical signature relative to undoubtedly associated. The objective of focusing our partly contemporaneous syn-rift igneous activity occurred work in these transects is to study comparatively the landward, in the continental plate. The latter had an Triassic-Jurassic sequences located at the same latitude in influence of crustal melting associated to asthenospheric the Coastal Cordillera and the High Andes Cordillera. In upwelling followed by continental rifting, whereas the these transects the aim is to determine the tectonic former would have a signature dominated by fluid- conditions and timing of the change from Late Paleozoic induced asthenospheric mantle melting characteristic of to Jurassic (Triassic to Late Jurassic). subduction. 3.2 First results 3 Discussion The Triassic to Lower Jurassic units cropping out in the 3.1 Main working hypotheses southernmost transect (Fig. 1) have been subject of a preliminary geological study. Sedimentary marine and Between 25º30’ and 31º00’ very well preserved geological continental successions are intercalated with or overlying units make this area suitable for geological studies. It is by volcanic rocks of a wide compositional range. A first possible to identify two roughly N-S oriented belts of petrological examination indicated no petrographical Middle Triassic to Early Jurassic volcano-sedimentary bimodality in the magmatism and some textural features successions and plutonic rocks, one in the Coastal indicative of subduction-related rather than rifting-related, 160 volcanism, such as porphyritic lavas with large continental extension in southwestern Gondwana: tectonic phenocrysts and hydrated mineral phases. Whole rock segmentation and pre-break-up rifting. Journal of South chemistry, both elemental and isotopic, and radiometric American Earth Sciences 14: 257-270. Godoy P.-B., Estanislao. 1985. Nuevos antecedentes sobre el dating is expected for a selected group of volcanic basamento metamórfico y la base tectónica