Detrital Thermochronology Reveals Major Middle Miocene Exhumation of the Eastern Flank ofthe Andes That Predates the PampeanFlat Slab (33°–33.5°S)

The Cordón del Plata and Cordón del Portillo (32.6°–33.8°S) are the portions of the Frontal Cordillera that straddle the transition zone between the Pampean flat‐slab subduction segment to the north and the normal subduction segment to the south. A complete understanding of how the Frontal Cordillera developed is necessary in order to evaluate different tectonic models for the Andes along with their relation to subduction dynamics and contractional upper‐crustal deformation. Detrital apatite fission track thermochronology of modern river sediments that drain the eastern and western slopes of the Cordón del Plata and the northern Cordón del Portillo provides constraints on regional exhumation histories. AFT data from each catchment typically contain multiple age peaks, but all have a prominent 15.3–17 Ma age peak. One catchment, the Las Tunas River has a unimodal distribution at 17.0 ± 1.1 Ma, with a confined track length distribution indicative of rapid cooling at that time. These results, combined with provenance analysis in the adjacent Cacheuta Basin, indicate significant early to middle Miocene (~16 Ma) exhumation in the Cordón del Plata and the northern sector of the Cordón del Portillo. Exhumation related to rock uplift occurred prior to the ~11 Ma onset of a flat slab geometry at these latitudes, but immediately after the main east vergent contractional event in the adjacent Principal Cordillera. Such Frontal Cordillera exhumation fits in an eastward, youngest to the foreland sequence of deformation of the different morphostructural Andean units at ~33°–34°S, arguing against the recently proposed west vergent orogenic system at these latitudes. Key Points Modern river sand thermochronometry constrains timing of exhumation of the Andes in the flat to normal subduction transition Main exhumation of Frontal Cordillera in this region occurred at 15.3–17 Ma, prior to the onset of flat‐slab conditions Timing of exhumation is consistent with an eastward progression of deformation