Exhumation timing of the Cordón del Portillo, Frontal Cordillera, supports the east-vergent model for the growth of the southern central Andes

The timing of contraction, uplift and exhumation of the Cordón del Portillo (33.5–34°S), Frontal Cordillera, southern Central Andes, remains unresolved. This fact has led previous studies to postulate opposite west- and east-growing orogen models. Here, we present the first apatite fission-track (AFT) ages from samples at the Cordón del Portillo, and apatite (U–Th)/He dates (AHe), which allow us to clearly recognize a ca. 15 to 5 Ma period of rapid cooling, interpreted as the record of exhumation of the Frontal Cordillera basement. We present a kinematic evolution of the area that supports these timing of exhumation and a tectonic transport towards the east. Our results agree with previous AHe dating and detrital AFT, which suggest ongoing exhumation pre-14 Ma and peak of exhumation at ca.15 Ma, respectively. It is also in concordance with regional geology and thermochronology pointing to a similar onset of exhumation of the Cordón del Plata, Frontal Cordillera, further north. Moreover, a rising Cordón del Portillo during the early to middle Miocene, contemporaneous with the beginning of sedimentation in the intermontane Alto Tunuyán Basin, implies that Miocene sedimentary rocks were not deposited in a contiguous foreland basin but in an isolated basin separated by topographic highs in the Frontal Cordillera. Finally, our data indicate an eastward shift in the focus of Andean deformation during the Miocene, jumping into the Frontal Cordillera after contraction started in the fold-and-thrust belt of the Principal Cordillera (west of the Cordón del Portillo) at ca.18 Ma, standing for the east-vergent model of tectonic evolution proposed for the Andes.