NEW PERSPECTIVES ON THE TECTONIC AND PALEOTOPOGRAPHIC EVOLUTION OF THE SOUTHERN CENTRAL ANDES

The tectonic evolution of the Southern Central Andes of Argentina is still nowadays a matter of hot debate, with varied and contrasting proposals regarding timing of main deformational phases, mountain-building mechanisms and sediment accumulation processes. The Andes at the latitudes of the study area straddle the transition between the Chilean-Pampean flat-slab segment to the north and the normal subduction segment to the south, which despite their striking differences, are commonly viewed as a series of Miocene structurally uplifted mountain ranges associated with purely flexural foreland basins. However, a novel type of dataset has been extracted from the Malargüe basin at 35°S, that challenges these long-standing notions and offers a new set of perspectives on the evolution of this Andean segment (Fennell et al., 2023, EPSL). A continuous paleotopographic record of the last 55 My based on hydrogen stable isotopes measured from hydrated volcanic glass obtained from 107 tuffaceous sediment samples suggests a similar to modern magnitude of isotopic fractionation during most of the Cenozoic. A drastic 50% decrease in fractionation was detected in middle Miocene times, which then recovered to present values, as indicated by an exhaustive analysis of the present climatic and isotopic setting. This analysis consisted of the prediction of the relationship between volcanic glass, precipitation waters and orographic lifting obtained via the Orographic Precipitation Isotope numerical model (OPI, Brandon et al., 2022), which was based on the isotopic measurements of 11 Quaternary tuffs and 197 modern meteoric water samples. As a result, we propose the existence of stable topography between 55 and 15 Ma, the subsidence of the entire foreland between 15 and 10 Ma and a rebound towards modern elevations during the last 10 My. Complemented with several other datasets, these results support the idea of an old origin for the several mountain ranges that compose the Southern Central Andes and underscores the role of deep mantelic processes as modulators of topography through time. These new perspectives help reconcile many of the contrasting previous proposals, highlighting the importance of paleotopographic approaches to unravel the long-term evolution of orogenic systems such as the Andes.