Localized deformation, uplift and fluvial processes of the proximal Bermejo megafan in the Andean foreland: Topographic growth of the Lomas de Olmedo anticline of NW Argentina

Abstract

The Andean foreland hosts fluvial megafans, which are sensitive sedimentary archives of tectonic and climatic forcing. This study investigates the Quaternary growth of an anticline in the Olmedo Range within the Bermejo megafan, in the northernmost sector of broken-foreland deformation in northern Argentina. The Olmedo Range is located at the transition between wedge-top and foredeep, approximately 250 km west of the forebulge. By integrating satellite imagery, UAV-derived topography, differential GPS profiles, 30-m digital elevation models, geological and soil mapping, as well as the integration of geochronological data, we constrain the kinematics and timing of deformation across the east-vergent, doubly plunging Olmedo anticline, a fault-propagation fold. The anticline results from contractional inversion of buried Cretaceous extensional faults and records neotectonic activity along the El Turco and El Retiro faults, documented by fluvial incision and terrace uplift, the formation of knickpoints, and syntectonic sedimentation. A ~25 m escarpment formed by incision of the Bermejo River at the northern terminus of the anticline and records at least 6.2 m of vertical displacement on the El Retiro fault since 9090 y BP, yielding a minimum slip rate of 0.68 mm/y. OSL and 14C dating across uplifted terrace surfaces constrains shortening rates to ~2.34 mm/y, comparable to present-day geodetic E-W shortening rates of 2.64 mm/y. Downstream incision beyond the fold hinge of the Olmedo anticline suggests recent eastward propagation of deformation, potentially along active shortcut splays associated with reactivated extensional faults; this documents how ongoing deformation along inherited structures has influenced deposition, erosion, and fluvial networks during megafan evolution.