UNRAVELING LATE CRETACEOUS TO MIOCENE OROGENESIS IN THE MALARGÜE FOLD-THRUST BELT AT 36°S (SOUTHERN CENTRAL ANDES)

The Andes represent the archetype of orogenesis resulting from ocean-continent convergence, placed at a long-lived subduction margin since at least the Mesozoic. At our study latitude (36°S), the onset of the contractional regime took place in the Late Cretaceous. Following this period of contraction, the main phase of shortening that led to the formation of the present-day cordillera occurred in the Miocene.

The Late Cretaceous phase remains enigmatic, as some researchers have proposed that the orogenic front reached almost its present-day position. This hypothesis is supported by alleged Upper Cretaceous synorogenic growth strata related to faults close to that position. Conversely, other contributions suggest that the Late Cretaceous orogen was restricted to a more westerly position, near the present-day watershed. In this contribution, we provide new structural and thermochronological analysis to enrich the ongoing debate.

We surveyed outcrops at the easternmost position of the 36°S transect, where synorogenic growth strata, represented by the Neuquén Group, were reported. None of these outcrops show features to support this interpretation. The bedding does not exhibit any systematic change in dip or thickness, nor are there progressive unconformities. Furthermore, we examined one of the outcrops at the eastern frontal limb of the Sierra Azul anticline. This structure was interpreted as being uplifted by a west-dipping fault during Late Cretaceous, based on the presence of alleged growth strata dipping to the east. We measure the bedding attitude, finding dips ranging from 38-44° to NW, without significant thickness changes. This challenges the interpretation of these as growth strata related to a west-dipping fault.

We collected three samples for apatite fission track (AFT) thermochronology along the 36°S transect. These yielded ages indicative of a Neogene exhumation. Considering their position, it is challenging to interpret that after the Late Cretaceous exhumation, these samples were reheated due to burial and completely reset, recording only the Neogene exhumation.

Although preliminary, our results pose a challenge in reconciling a Late Cretaceous orogen as wide as the present-day mountain chain. Thus, we consider it more likely that this shortening phase was restricted to a more western position.