Along-strike tectonic evolution of the Neogene Bermejo foreland basin and Eastern Precordillera thrust front, Argentina (30-32°S)

Abstract

The Bermejo retroarc foreland basin system formed in flexural response to Cenozoic crustal thickening in the Andean orogenic system, specifically, the eastward propagation of the Precordillera fold-thrust belt and the basement-involved uplift of the Sierras Pampeanas. Previous work in the region has mainly focused on the mechanisms and expression of flat slab subduction and the structural geometry of the basement-involved Sierras Pampeanas and east-directed Precordillera fold-thrust belt at depth, advancing our understanding of the Bermejo basin history north of 31°S. However, the along strike evolution of the basin system to the south remains unresolved and contrasting tectonic models have proposed the Bermejo basin evolved synchronously versus asynchronously through time. Our study seeks to constrain the along-strike Neogene tectonics of the Bermejo basin via well-exposed Miocene fluvial stratigraphic intervals along the Eastern Precordillera, integrated with detrital zircon U–Pb geochronology provenance and detrital apatite (U–Th)/He thermochronology datasets. New data from two Neogene stratigraphic sections in the southern Bermejo basin constrain deposition between 13 and 6 Ma. Dominant fluvial-lacustrine mudstones, siltstones, and sandstones transition into fluvial-megafan deposits capped by alluvial fan conglomerate facies, tracking the eastward migration of Precordillera deformation. The cessation of sedimentation, and thermal history models of apatite (U–Th-Sm)/He thermochronology ages, indicates basin incorporation into the orogenic wedge by 6 Ma. When we compare our southern datasets with previous constraints from the northern Bermejo basin, we observe, from north to south: (1) a time-transgressive trend in basin initiation, (2) a ∼3 km decrease in stratigraphic thicknesses, and (3) older exhumation along the thrust front. These trends highlight the asynchronous nature of the Bermejo foreland basin system and along-strike variability of Precordillera thrust-front evolution.