Loa-Geo1: A field regional transect to unravel the structure of the western Central Andes
Introduction
The Central Andes is the largest orogenic system related to the subduction developed in western South America at 20°–24°S (Fig. 1). Its western section comprises the forearc region, mainly established in northern Chile. Along with this, several N–S oriented morphotectonic units with variable topography represent the Andean orogen's main tectonic pieces. From west to east, the outer forearc comprises the Coastal Cordillera and the Central Depression, while the inner region comprises the Domeyko Cordillera (also called the Chilean Precordillera), Pre-Andean basins, and present-day volcanic arc (Fig. 1). At these latitudes, the most refined tectonic studies have been developed in the back-arc regions of Argentina, Perú, and Bolivia, specifically on the eastern, Interandean, and Sub-Andean cordilleras (Roeder, 1988; Kley, 1996; Baby et al., 1995, Baby et al., 1997; Gil Rodriguez et al., 2001; McQuarrie, 2002, Espurt et al., 2011; McClay et al., 2018; Horton, 2018, among others).
Numerous works supported by multiscale data including two-dimensional (2-D) and three-dimensional (3-D) industrial seismic data, oil and gas borehole break-cuts, field and thermochronological data, and the restoration of kilometric-scale balanced cross-sections (Roeder, 1988; ANCORP Working Group, 2003; Carrapa et al., 2011; McGroeder et al., 2014; Carrapa and DeCelles, 2015; Eichelberger and McQuarrie, 2015; Pérez et al., 2016; Calderon et al., 2017; Rojas Vera et al., 2019; Zamora et al., 2019; Cristallini et al., 2020) have revealed the detailed structure and tectonic evolution of the eastern Central Andes. It has been interpreted as an east-verging orogenic system that result of a combination of thick-skinned and thin-skinned tectonic styles.
In Chile, major advances in understanding the structure of the western Central Andes mainly come from regional and local-scale geological mapping, which only in some sectors (Pre-Andean basins) has been integrated with 2-D seismic data (Pananont et al., 2004; Arriagada et al., 2006; Jordan et al., 2007; Henríquez et al., 2018; Bascuñan et al., 2019; Martínez et al., 2018, 2021). The main structural studies have been developed in the Coastal Cordillera and some localities of the Domeyko Cordillera (Tomlinson et al., 1993; Lindsay et al., 1995; Victor et al., 2004; Mpodozis et al., 2005; Amilibia et al., 2008; Bascunan et al., 2015; Espinoza et al., 2018; López et al., 2019). In general, these were always focused on understanding local fault arrays' kinematics, the timing of crustal uplift, and the relationship between faulting and mineralization.
The hyperarid conditions of northern Chile created favorable conditions to preserve the good natural exposure of the geological units and also created an extensive plain formed by gravels and unconsolidated sediments (pediplains) unformally named “pampas”, which cover more than 70% of the surface (Blanco, 2008; Nester and Jordan, 2012). Thus, preventing a full observation of major structures, this cover represent the major problem in generating regional tectonic models. Frequently, the superficial correlations between those structures exposed on isolated outcrops are made in an empirical form and are weakly constrained, hindering the construction of robust structural models. Some of the tectonic mechanisms invoked (described in Section 3) to explain the uplift of the forearc present opposite interpretations of the regional-scale structural styles exposed on the surface and even with the recognized from seismic data (Flint et al., 1993; Buddin et al., 1993; Tomlinson and Blanco, 1997; Jordan et al., 2007; Blanco, 2008; Duhart et al., 2018; Henríquez et al., 2018). Many of the faults and folds have geometrically and kinematically been interpreted in different ways, thus allowing propose extensional, contractional or strike-slip models. At the latitude of study, López et al. (2020) presented an updated approach integrating previous (Arriagada et al., 2006; Henríquez et al., 2018; Bascunan et al., 2015; among others) geological interpretations with new mesoscopic, regional, and subsurface geological and geophysical data. However, they concluded that the region's structure and style are much more complex than is thought, because many of the structures previously interpreted, such as large Cenozoic normal and strike-slip faults (Tomlinson and Blanco, 1997; Blanco, 2008; Rubilar et al., 2017; Duhart et al., 2018) are poorly constrained by the seismic data. This has triggered extensive debates on the geometry and kinematics of the structures responsible for the construction of the western Central Andes.
The Loa-Geo1 is a regional field transect, approximately 120 km in length, and covering more than half of the Andean forearc (Fig. 2). The favorable orientation of this region make it possible to infer and constrain the tectonic style of the first-order structures exposed on the Domeyko Cordillera and the Central Depression, in order to better document the crustal shortening mechanisms responsible for the development of the Andean forearc and the western Central Andes. The transect forms part of a research project developed between the Chilean and Brazilian academic institutes. This research project is focused on determining the structural configuration and timing of the uplift of the Andean orogen. This contribution presents the results of recent field surveys carried out along the Domeyko Cordillera, the Loa and San Salvador rivers and in other neighboring regions, such as the Sierra de Limón Verde and the Tuina sector (Fig. 2). We chose these sectors because they exhibit a good preservation of Paleozoic to Cenozoic structural and stratigraphic relationships, which will help understand how the Andean orogenesis occurred. This work is supported by the geological mapping of satellite images, field observations, dip and strike measurements of faults, folds and stratigraphic beds, and the construction of a regionally balanced cross-section.
Section snippets
Geological setting
The Central Andean forearc represents one of the most outstanding morphotectonic units and the entire western flank of the Central Andes. Its inner region comprises the segment from the Domeyko Cordillera to the Pre-Andean Depressions (Fig. 1). Precisely, the Domeyko Cordillera comprises Upper Paleozoic basement blocks and Mesozoic syn-rift sequences. A distinctive physiographic feature immersed in the Domeyko Cordillera corresponds to the Calama Basin (Fig. 1, Fig. 2), an intramontane basin
Stratigraphy
The stratigraphic record of the Andean forearc in northern Chile at 22°30′–22°50′ can be recognized on the isolated ranges and hills of the Domeyko Cordillera and from the canyons of the Loa and San Salvador rivers (Fig. 2), which cut the entire forearc transversally. The oldest units, preferably exposed at the Sierra de Limón Verde (Fig. 2), comprise Paleozoic rocks composed of stratified Devonian quartzites and conglomerates (with a low-grade of metamorphism) intruded by a series of
Main tectonic models
Numerous geological studies have been developed in the Andean forearc of northern Chile for many years. However, these concentrated their efforts to determine the origin of the shallow structures and the tectonic evolution of the Domeyko Cordillera (or Chilean Precordillera), as well as the relationship between the structures and mineral bodies. Considering its great attractiveness from an economic viewpoint, this region concentrates the largest copper deposits in South America (e.g.,
Loa-Geo1 transect and methods
As indicated in the Introduction, the Loa-Geo1 transect was designed to recognize and document the main structural styles along the Andean forearc of northern Chile, determining the mechanisms of crustal uplift that operated in the region. This design was configurated as one W–E-oriented transect, along with the Loa and San Salvador rivers and Cerros de Tuina (Fig. 2). The canyons of both rivers represent favorable places for analyzing the Andean forearc structure because the definition of many
Eastern structures
The eastern structures correspond to those exposed at the Cerros de Tuina (Fig. 2, Fig. 4). Here a series of NNW-striking (Tuina, San Jorge, San Martin faults) faults involve Permo–Triassic syn-rift deposits (Tuina Fm.) and Middle–Upper Cretaceous units. These mainly comprise kilometric-scale reverse faults, along which the hanging wall blocks were displaced westward that in some places are intercalated with east-directed, reverse-reactivated normal faults. Along with the west-directed
Discussions
The first results of this transect show a structuration for the upper plate of the Central Andes forearc at 22°–22°30′S dominated by an important interaction of thick- and thin-skinned fold-and-thrust styles affecting Paleozoic–Cenozoic rocks (Fig. 13). They form a doubly directed thrust system where most of structures lie hidden under a large cover made of undeformed Mio–Pliocene sedimentary successions (Fig. 13), making it difficult for their areal observation. From the restoration of one
Conclusions
New field data combined with previous 2-D seismic interpretations have allowed the unraveling of the structure of the upper crust of the Central Andes forearc of northern Chile. The results of the Geo-Loa1 indicate that this region comprises a doubly verging and hybrid thin- and thick-skinned fold-and-thrust belt involving Paleozoic to Cenozoic crystalline to stratified rocks. Three structural styles were preferably recognized, which include partially inverted normal faults, along which
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledge
This work was supported by the scientific project PCI-2019/133349-8 “Tectono-stratigraphic evolution of intermontane basins related to forearc tectonic settings using the Chilean Pre-Andean Depression of the Central Andes as a case study” financed by ANID-Chile and directed by the first author of this contribution. The authors thank Mary Rogan for facilitating the academic license of the StructureSolver program used to construct and restore the balanced cross section presented in this work.
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