Structural style of the Guañacos Fold and Thrust Belt (southern Central Andes): A tectonic setting for the Cura Mallín Basin revisited

Abstract

The tectonic setting of the Oligo-Miocene Cura Mallín Basin in the southern Central Andes has been a topic of debate in recent years. Proposed models include extensional, compressional, and a hybrid-sequential model with an initial extension (~20 Ma) followed by contraction (~19 Ma). We conducted a detailed structural analysis to understand the tectonic evolution of the Cura Mallín Basin at the eastern Andean slope in the Guañacos Fold and Thrust Belt (FTB) (37◦-38◦S). This tectonic element provides exceptional exposure of the upper sedimentary infill of the Cura Mallín Basin. Despite detailed studies about composition, the structural configuration of this basin remains poorly understood. This study refines the age of the upper Cura Mallín Formation on the eastern Andean slope with a new detrital U/Pb date of 15.47 ± 0.28 Ma from the basal portion of the sedimentary sequence, indicating a maximum Middle Miocene deposition, in contrasts with the previous Oligo-Miocene age. This work presents the first balanced cross-sections for the Gua˜nacos FTB. Structural analysis reveals a thin-skinned deformation style, contrasting with previous interpretations of thick-skinned deformation, with shortenings of 9.6 km (29 %) and 8.5 km (24 %). Therefore, significant Andean contraction at these latitudes was absorbed by the Guañacos FTB. Our model assumes that all deformation in the Cura Mallín Formation postdates deposition. Contractional deformation began on the Chilean Andean slope by ~18.7 Ma, jumped to the Argentinean side at ~12.5 Ma, and persisted beyond 6.5 Ma, with progressively diminishing intensity. Combined evidence from the Gua˜nacos FTB and the eastern Chos Malal FTB, suggests synchronous deformation throughout the entire orogen during the Late Miocene. Additionally, these two belts record the reactivation of Miocene thrust faults during the Quaternary. This reactivation is potentially linked to the apparent persistence of the regional stress field since the Late Miocene.