Cenozoic sedimentation and exhumation of the foreland basin system in the Precordillera fold-thrust belt (31-32°S), southern central Andes, Argentina

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Levina, Mariya

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Andean retroarc shortening associated with flattening of the Pampean segment of the subducting Nazca plate has resulted in a thin-skinned, east-directed thrust system that partitioned and uplifted Cenozoic foreland basin fill in the Precordillera of west-central Argentina. The temporal and kinematic evolution of the Precordillera fold-thrust belt can be approached through detailed analyses of the clastic sedimentary deposits now preserved in intermontane regions between major thrust faults. In this project, we focus on the uppermost Oligocene–Miocene basin fill exposed in the axial and eastern Precordillera along the San Juan River (Quebrada Albarracín and Pachaco regions) and western flank of the frontal structure (Sierra Talacasto). The nonmarine successions exposed in these regions record hinterland construction of the Frontal Cordillera, regional arc volcanism, and initial exhumation of the Precordillera thrust sheets. Measured stratigraphic sections and lithofacies analyses of the preserved stratigraphic successions reveal initial development at ~24 Ma of an eolian depositional system influenced by regional volcanism and fluvial interactions, becoming a fully eolian system by 21-19 Ma. This system transitioned to a distributary fluvial system in which regions closer to the deformation front recorded sandy-gravelly braided stream sedimentation and regions farther east recorded more-distal floodplain-dominated deposition of thin-bedded mudstone and sandstone. The youngest sedimentary record is preserved in the Albarracin basin, a zone strongly influenced by explosive volcanism of nearby eruptive centers around 14 Ma, followed by a progradational alluvial-fan succession of pebbly, cross-stratified sandstone and thick, pebble to cobble conglomerate. Provenance changes recorded by detrital zircon U-Pb age populations suggest that initial deformation in the Frontal Cordillera coincided with the early Miocene transition from eolian to fluvial deposition in the adjacent foreland basin. The overall upward coarsening nature of the fluvial succession and increased presence of Paleozoic clasts reflect the eastward progression of thin-skinned deformation in the Precordillera and resultant structural partitioning of the synorogenic foreland successions. Using apatite (U-Th)/He thermochronometry we are able to further constrain the age of uplift-induced exhumation and cooling of several Precordillera thrust sheets to 12-9 Ma. This apparent pulse of exhumation is evident in all three sections, suggesting rapid, large-scale exhumation by synchronous thrusting above a single décollement linking major structures of the eastern Precordillera.



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