AGE-ELEVATION RELATIONSHIPS FROM CERRO MERCEDARIO (6720M) RECORD EXHUMATION OF THE LA RAMADA MASSIF, HIGH ANDES OF WESTERN ARGENTINA

The Frontal Cordillera (FC) basement culmination is a first-order geologic feature of the southern central Andes, hosting the highest hinterland topography above the modern Pampean flat-slab segment. Yet, the timing of uplift and exhumation of the FC remains uncertain, and few efforts have been made to map range-bounding structures in detail. We conducted a multi-thermochronometric (apatite and zircon U-Th/He) study of seven samples collected along an ~4.3 km age-elevation transect from the northeast ridge of Cerro Mercedario, the eighth-highest peak in the Andes and the tallest peak in the La Ramada Massif at ~31°S. ZHe dates between 2 and 3 km asl are dispersed but approach the crystallization age of host rocks with increasing elevation. These dates are interpreted to represent partial resetting of the ZHe system either by 1) residence of the lowest samples near the zircon partial retention zone at the onset of Andean exhumation, or 2) reheating by sediment and/or thrust sheet burial from the impinging thrust belt to the west. Single grain AHe dates range from 8.53 ± 0.76 to 35.81 ± 1.85 Ma, with median dates of ~10.47 to ~17.34 Ma with increasing elevation. Reset AHe dates from the summit of Cerro Mercedario suggest a relatively high degree of rock exhumation in this portion of the FC relative to existing datasets to the south. Integrated with new geologic mapping and field observations, thermochronology data and inverse thermal history models are consistent with Miocene thrust driven exhumation along the Santa Cruz and Espanacito faults, two regional scale reverse faults responsible for uplift of the La Ramada massif. The latter stage of middle to late Miocene cooling and exhumation of the FC at this latitude records out-of-sequence deformation coeval with contraction in the eastward adjacent Precordillera.