Abstract
Crustal thickening by horizontal shortening and associated deformation have been broadly considered as prime mechanisms for mountain building in the Central Andes of western South America. However, timing and structural style of Andean orogeny in northernmost Chile remains to be fully understood. By means of this contribution we attempt to unravel the Cenozoic tectonostratigraphy and structural architecture of a narrow segment within the Western Cordillera (western flank of the Altiplano plateau), based on detailed structural mapping, available and new geochronological data gathered from the Aquine region (~19°15’ S). The geology of this area indicates that compressive tectonics dominated for ~21 Myr, between 27 Ma and 6 Ma, and that onset of deformation probably occurred in the middle Eocene (after ca. 49 Ma). According to angular unconformities registered throughout the region, three principal compressive episodes have been determined: (1) middle Eocene-late Oligocene (ca. 49-27 Ma); (2) late Oligocene-middle Miocene (ca. 27-14 Ma); and (3) middle-latest Miocene, 14-6 Ma. Paleogene and Neogene structural development within the Aquine region was concomitant to activity on the bordering West and East Vergent Thrust Systems (WTS and ETS), which controlled uplift of the western Altiplano since the late Oligocene-early Miocene, located at the foot of the Precordillera and within the eastern part of the Western Cordillera, respectively. Major structures in the western segment of the region are high-angle, east-vergent, substratum-involving thrusts that affect late Mesozoic and Cenozoic volcanic and sedimentary intra-arc deposits. Unalike, a west-vergent fold-and-thrust belt is developed in the southeastern segment of the region, involving only Miocene deposits. These contrasting structural styles and opposite vergences indicate a combination of thin- and thick-skinned tectonics in a “pop-up” like structural array. Growth strata and sedimentological features in alluvial and fluvial sandstones and conglomerates of the upper Miocene Mauque Formation suggest the occurrence of syntectonic deposition. In addition, gentle angular unconformities within upper Oligocene-upper Miocene arc deposits suggest that sedimentation and volcanism developed synchronic to compressive tectonics throughout the Miocene and coeval to deposition in the active bordering thrust systems (WTS and ETS). The interpreted tectonostratigraphic development of the study area is similar to but slightly diachronic with the Cenozoic evolution in the Belen region (18°-19° S) of northernmost Chile. We interpret that east- and west-vergent thrusts within the Western Cordillera, are deeply rooted in a crustal-scale “pop-up” structure that accommodated Cenozoic deformation on the western flank of the Altiplano. Uplift as a consequence of contraction along the Western Cordillera was coeval to late Paleogene block exhumation and subsequent Neogene development of fold-and-thrust belts on the eastern border of the Altiplano. The main structural characteristics within the Aquine region suggest that inversion of an Upper Cretaceous intra-arc basin was of major relevance for the Cenozoic structural development of the Western Cordillera.
Highlights
We have focused on a region of the Western Cordillera in the Central Andes between the WTS and ETS (Fig. 2), which exhibits a narrow (~25 km) but elongate area in which deformed Mesozoic and Cenozoic volcanic and sedimentary rocks are well exposed
The lithostratigraphy of the Aquine region consists of a Mesozoic substratum composed of Upper Cretaceous volcanic and sedimentary rocks, and a Cenozoic cover consisting of late Oligocene-early Miocene ignimbrites and Neogene sedimentary and volcanic deposits
The syntectonic deposition of the El Diablo Formation in the Precordillera at ~19°15’ S, directly related to the development of the WTS (Pinto et al, 2004; Farías et al, 2005), and roughly coeval to the deposition of the Mauque Formation, suggests that sedimentation and volcanism developed throughout the Precordillera and Western Cordillera under active compressive tectonic conditions (Figs. 10 and 11c)
Summary
In a constant attempt to unravel the evolution of Andean uplift, numerous studies have taken place along this long-lived, subduction-related, cordillerantype mountain belt (sensu Dewy and Bird, 1970), of which many have focused on the formation of the Altiplano plateau (e.g., Isacks, 1988; Allmendinger et al, 1997; Lamb et al, 1997; Gregory-Wodzicki, 2000; Elger et al, 2005; McQuarrie et al, 2005; Barnes et al, 2008; Barnes and Ehlers, 2009; Hartley and Evenstar, 2010; Jordan et al, 2010; Charrier et al, 2013; Garzione et al, 2014). 30-20 Ma (Pardo-Casas and Molnar, 1987; Somoza, 1998) Since this time, two deeply-rooted, orogen-scale thrust systems, with opposite vergences and NNW-SSE to N-S trends, developed aligned and along both the Western Cordillera and Precordillera (Charrier et al, 2013) (Fig. 2), with the West-Vergent Thrust System (WTS) located within the Precordillera (Muñoz and Charrier, 1996; Victor et al, 2004; Farías et al, 2005; García and Hérail, 2005; García et al, 2011) and the East-Vergent Thrust System (ETS) located within the Western Cordillera, in the presentday arc (Charrier et al, 2005; Farías et al, 2005; Cortés et al, 2012a).
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