Miocene Construction of the High Andes Recorded by Exhumation of the Frontal Cordillera, La Ramada Massif of Western Argentina (32°S).

The Frontal Cordillera is a first‐order geologic feature of the southern central Andes, hosting the highest hinterland topography above the modern Pampean flat‐slab segment. The timing of Frontal Cordillera exhumation is important for testing models of Andean tectonics, yet large latitudinal gaps exist between structural and thermochronological constraints for the region. We conducted a thermochronometric study using a 4.4 km age‐elevation transect along the northeast ridge of Cerro Mercedario, the highest peak in the La Ramada massif at ∼32°S. Zircon (U‐Th)/He dates indicate partial resetting, supporting a limited magnitude of exhumation in even the most extreme Andean topography. Single grain apatite (U‐Th‐Sm)/He dates range from 8.5 ± 0.9 to 35.8 ± 3.6 Ma, with median dates of ∼10.5 to ∼15.7 Ma with increasing elevation. Integrated with geologic mapping and thermal history modeling, these data suggest Early to Middle Miocene exhumation along the Santa Cruz and Espinacito faults concomitant with uplift of the La Ramada massif. New apatite helium data from the Cordillera del Tigre segment of the Frontal Cordillera are partially reset and preferred modeling interpretations suggest exhumation ca. 11–9 Ma, coeval with shortening in the eastward adjacent Precordillera. These data add to accumulating regional evidence for out‐of‐sequence deformation during the Miocene, consistent with internal (hinterland) growth of a subcritical orogenic wedge contemporaneous with surface uplift and crustal thickening in the south‐central Andes. Plain Language Summary: The convergence of tectonic plates causes rocks to bend, break, pile up, and rise to high elevations. As mountains like the Andes of South America rise, they get worn down by rain, wind, ice, and gravity. Geologists can analyze elements in rocks to determine when erosion happened, which tells us when mountains were growing. This is important because the timing of topographic growth is critical to understanding things like the evolution of life and the long‐term behavior of Earth's climate. For this study, we collected rock samples from one of the highest mountains in the Andes (Argentina's Cerro Mercedario) to figure out when uplift and erosion were happening. Sampling at high elevation is a powerful approach because the highest rocks usually reach Earth's surface first, recording the earliest phases of mountain building. We found that the high peaks started rising around 20 million years ago, coeval with regional thickening and uplift of South American crust. In addition, our data show that Cerro Mercedario was still being uplifted after contractional deformation had moved east to near the city of San Juan, adding evidence that the Earth's crust doesn't always break in a sequential, migrating fashion as predicted by idealized models of mountain belts. Key Points: Highest age‐elevation thermochronology profile from the Andes at Cerro MercedarioApatite and zircon (U‐Th‐Sm)/He dates and inverse thermal history modeling show 22–10 Ma thrust driven exhumation of the La Ramada massifGrowth of Frontal Cordillera basement culmination feeds slip into eastward adjacent Precordillera during propagation of an east‐vergent orogenic wedge [ABSTRACT FROM AUTHOR]