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

LOSSADA, A. C.; HOKE, G. D.; GIAMBIAGI, L. B.; FITZGERALD, P. G.; MESCUA, J. F.; SURIANO, J.; AGUILAR, A.

TECTONICS

AMER GEOPHYSICAL UNION

Año: 2020 vol. 39

0278-7407

The Cordón del Plata and Cordón del Portillo (32.6º-33.8ºS) 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 provide constraints on regional exhumation histories. AFT data from each catchment typically contain multiple age peaks, but all have a prominent ca. 16 Ma age peak, while one catchment, Las Tunas River, contains only this 16Ma peak. Confined track lengths from Las Tunas detrital apatites indicate rapid cooling, thus the 16 Ma peak reflects an episode of rapid exhumation related to rock uplift of the Frontal Cordillera. These results, combined with provenance analysis in the adjacent Cacheuta Basin, indicate that the main phase of rock uplift in the Cordón del Plata and the northern sector of the Cordón del Portillo occurred in the early to middle Miocene, prior to the onset of a flat slab geometry at these latitudes (~11 Ma, Ramos et al., 2009), but immediately after the main east-vergent contractional event in the adjacent Principal Cordillera. Such Frontal Cordillera exhumation fits in a sequence of deformation of the different morphostructural Andean units at ~33º-34ºS that gets youngest to the foreland (eastward), arguing against the recently proposed west-vergent orogenic system that claims for an initial exhumation of Frontal Cordillera.