Thermochronologic evidence for late Eocene Andean mountain building at 30°S

ANA LOSSADA, LAURA GIAMBIAGI, GREG HOKE, PAUL FITZGERALD, CHRISTIAN CREIXELL, ISMAEL MURILLO, DIEGO MARDONEZ, RICARDO VEL�SQUEZ Y JULIETA SURIANO

TECTONICS

AMER GEOPHYSICAL UNION

Año: 2017 vol. 36 p. 2693 - 2713

0278-7407

The physiographic transition between the Puna-Altiplano Plateau and the Frontal/Principal Cordillera fold-and-thrust belts to the south is the result of a shift in structural style. Each structural domain had a distinctive response to discrete Cenozoic tectonic phases. Apatite fission track (AFT) thermochronology and apatite (U-Th-Sm)/He (AHe) dating from a vertical profile and a horizontal transect in the core of the Andes reveal the exhumation history of the Frontal Cordillera at 30°S, an area at the heart of this major transition in the Andean orogen. AFT and AHe ages exhibit coherent, positive age-elevation trends. Interpretation of the age-elevation profile, plus inverse thermal modeling indicate that the onset of rapid cooling (~10-20°C/m.y.) was underway by ~35 Ma, followed by a significant decrease in cooling rate at around 30-25 Ma (~3°C/m.y.). AFT thermal models from the horizontal transect in the valley bottom reveal a second episode of rapid cooling in the early Miocene (initiated ca. 18 Ma), resulting in exhumation of rocks to its present position. We interpret observed Eocene rapid exhumation as the product of a previously unrecognized compressive event in this part of the Andes and propose it reflects a southern extension of the widely recognized Eocene orogenic event in the Puna/Altiplano. Renewed early Miocene exhumation indicates that the late Cenozoic compressional stresses responsible for the main phase of uplift of the South Central Andes also impacted the core of the range in this transitional sector.