Loa-Geo1: A field regional transect to unravel the structure of the western Central Andes

Highlights

• Thin-skinned fold-and-thrust belt.

• The Andean forearc of northern Chile.

• Basin inversion of Mesozoic rift systems.

• Restoration of Andean structures.

Abstract

The structure of the Central Andes forearc of northern Chile has been an enigma for many years. Along this region, the preservation of a long pediplain constituted by gravels and unconsolidated sediments has hidden the structure of the region, making it difficult to understand its tectonic evolution. The traditional and regional-scale structural models are mainly supported by outcrop-scale and local observations. To unravel the structure of this Andes region, we carried out a regional-scale study based on local and regional observations, geological mapping, and the elaboration of local and regional balanced cross-sections, thereby illustrating the geometry and distribution of the first-order structural styles. We examined four specific sectors along a regional transect: the Loa and San Salvador rivers (parallel to each other), Sierra de Limón Verde, and Cerros de Tuina. Three structural styles were identified in the hybrid thick- and thin-skinned and doubly verging fold-and-thrust belt. These styles comprise partially inverted Permian to Jurassic normal faults, thick-skinned reverse faults affecting Paleozoic crystalline basement blocks, and shallow and thin-skinned fault-related folds. The shallow and thin-skinned fault-related folds usually result from the propagation of large, thick-skinned thrust ramps into stratigraphic successions. The restoration of a regionally balanced cross section constrained by field data and the previously interpreted two-dimensional (2-D) seismic profiles suggest that much of the crustal shortenings are accommodated by the reverse reactivation of Triassic and Jurassic normal faults bounding half-graben structures. It is also accommodated by blind thrust faults, whose geometry, kinematic, and depth are debatable. However, we have proposed a large, thick-skinned thrust ramp as the structure responsible for the greater shortening in the region, along which a significant thick- to thin-skinned transition occurs. The age of the synorogenic deposits identified here is unknown, specially those exposed at the Loa and San Salvador rives, however, the basal successions exposed at the Tuina sector reported Upper Cretaceous ages, thus suggesting that the orogeny in the forearc could have started during this time spam. Regional correlations with similar synorogenic deposits recognized in neighboring regions (e.g., Salar de Atacama Basin and Coastal Cordillera) also affirm an Upper Cretaceous–Paleocene age for the initiation of the crustal contraction in the region.

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.