Evaluating the role of stratigraphy and inherited basement structures on the evolution of thick and thin-skinned related folds: Insights from the Salar de Atacama Basin in the Central Andes

Abstract

We investigated the role of pre-existing basement normal faults, and ductile layers present within post-rift sequences on the final geometry and kinematics of hybrid (thick and thin-skinned) fold and thrust belts that result of tectonic inversion, using as an example the northern region of the Salar de Atacama Basin in the Central Andes. The study was developed integrating field data consisting of regional and local observations and geological mapping, and the seismic and structural interpretation of 2-D industrial seismic profiles to understand the current structure of the basin. A regional-scale balanced cross-section was constructed and restored to validate our interpretations and determine the pre-shortening tectonic configuration of the basin. Our results suggest that low angle normal faults result in favorable structures for the development of thick-skinned thrust ramps from their full tectonic inversion during the growth of fold and thrust belts. In the Salar de Atacama Basin, these structures allowed for the uplift and exhumation of large Paleozoic stratified and crystalline rock on its western section. The existence of ductile strata within post-rift sequences commonly facilitates the transition from thick to thin-skinned structural style during the progressive tectonic inversion of pre-existing half-graben structures. This mechanism allowed for the creation of prominent east-verging basement cored anticlines and doubly verging and narrow, shallow thin-skinned folds and thrusts at the northernmost part of the Salar de Atacama Basin. Pieces of evidence from the basin suggest that the initial areal extension (length) of the post-rift ductile strata exert a fundamental role in the lateral extension of thin-skinned thrusts. Finally, the existence or not of important ductile strata in the overburden stratigraphy conditions the progression of hybrid fold and thrust belts after pre-existing normal faults are reactivated.