Pre-Neogene tectonics and early broken foreland basin stages in the Bermejo basin

Abstract

Broken foreland basins develop in response to far-field compressional stresses from active margins, resulting in deep-seated intraplate contractional structures that fragment the original foreland basin. This study investigates the tectono-sedimentary evolution of the Bermejo Basin, the first recognized broken foreland basin in South America, situated between the Eastern Precordillera and Western Pampean Ranges. Through the reprocessing of seismic data and gravity analyses, we identify three main tectono-sedimentary stages: Stage 1 (Pre-Neogene), Stage 2 (19-4 Ma), and Stage 3 (4–2.5 Ma to Present). Stage 1 generated structures that were buried and sub¬ sequently reactivated during Stage 2. This latter stage marked a significant reconfiguration of the basin with progressive northward propagation of compressional structures and a major shift in basin polarity around 6-5 Ma. These stages correspond to varying configurations of the Andean subduction zone, with strong foreland compression linked to flat subduction. However, the mild initial compression of a key structure active since 19- 17 Ma, as evidenced by syntectonic strata, suggests that the foreland remained responsive to compressional forces transmitted from the plate margin during episodes of normal subduction. Our findings underscore the previously unrecognized role of inherited (or pre-Andean) compression in the evolution of the Bermejo Basin and provide evidence for a hybrid deformation style affecting the Eastern Precordillera, impacting the western margin of this basin. This study emphasizes the need for updated regional kinematic models that account for the interplay of ancient and more recent tectonic processes in the Bermejo Basin. Finally, our observations indicate that inherited compressional structures, such as the Carboniferous-Permian faults and the early Miocene Alto del Jumeal Range, have played a significant role in guiding subsequent deformation and sedimentation within the Bermejo basin.