What is the structure of the forearc region in the Central Andes of northern Chile?: An approach from field data and 2-D reflection seismic data

Abstract

The tectonic evolution of the Andean forearc in the Central Andes is understood by the superimposition of deformational events associated with different tectonic regimes, which control the main morpho-tectonic units of northern Chile. Varied interpretations have opened a great discussion about the deep structure of the Andean forearc and its implications in the evolution of the Central Andes. These ideas are commonly accompanied by the lack of regional tectonic models that explain the geometry and kinematics of first-order structures. The main limitations are due to the unknown origin of the structural styles in the substrate and their relationships with those exposed in the field. In order to solve this problem, we present a new structural model of the forearc region of northern Chile, specifically along the Central Depression (22°-23°S) supported by field observations and the structural interpretations of 2D seismic data. Four major tectono-sequences have been recognized related to (a) a Paleozoic-Triassic basement rocks, (b) a Jurassic-Lower Cretaceous synrift deposits, (c) Upper Cretaceous-Paleocene volcano synorogenic deposits and (d) a Late Cenozoic covered gravel. The first-order structural styles identified correspond to inverted half-graben basement structures related to the Jurassic Tarapacá Basin. The hanging wall blocks of the master faults are usually composed of large inversion anticlines along which, Jurassic and Lower Cretaceous siliciclastic deposits were expelled. Other basement contractional structures consist of short-cut and by-pass faults that modified the previous geometry of extensional systems. The second-order thin-skinned fold and thrust exposed in the region accommodated the shortening experienced by the Mesozoic synrift deposits during the basin inversion. The Upper Cretaceous-Paleocene deposits usually unconformably overlie the folded Mesozoic synrift deposits. This structural feature is first-order evidence of the positive reactivation of the Mesozoic rift basins and therefore is a regional marker of the onset of the Andean uplift in the region.