Plio–Pleistocene stratigraphy of the Horcón Basin (central Chile, ~33°S): Transgressive–regressive cycles and forearc deformation above the Juan Fernández Ridge

This paper analyzes the marine and continental successions of the Neogene to Quaternary Horcón Basin (32°–33°S), a forearc basin of the Central Andes, based on sedimentological, stratigraphic, and UPb detrital zircon geochronological data. The basin lies above the present-day projection of the Juan Fernández Ridge (JFR), which has been subducting beneath the South American Plate at a fixed piercing point over the past ~10 million years. An initial transgressive episode (Late Pliocene) is recorded in successions that grade from middle transitional to deeper offshore settings. A prominent erosional surface overlying these deposits marks a subsequent regressive phase. This was followed by a second transgression (Early Pleistocene?), preserved in a deepening-upward sequence dominated by gravelly upper shoreface to foreshore facies. A later regression (Early to Middle Pleistocene?) is expressed by debris flow deposits and extensive alluvial braidplains progradationally advancing toward the coast. Detrital zircon UPb ages and gravel composition indicate sediment sources from Jurassic and Cretaceous arc rocks of the Coastal Cordillera throughout the basin's evolution, with a significant contribution from the Principal Cordillera possibly linked to tectonic activity upstream of the Aconcagua River. We suggest that the first transgressive episode may reflect regional subsidence, whereas the subsequent regression could be associated with regional uplift of the South American margin at these latitudes. Additionally, the transgressive–regressive cycles documented in the Horcón Basin likely record localized vertical motions driven by coastal normal faulting, resulting in high-relief coastal blocks and embayments. While further evidence is needed, we propose that this complex deformation history, occurring within an overall compressive regime, may have been modulated by the subduction of JFR seamounts, contributing to localized subsidence and uplift along the margin of the overriding plate.