Oligocene to present shallow subduction beneath the southern Puna plateau

Abstract

The southern Puna plateau is a conspicuous example of a high-elevation orogenic plateau in a non-collisional setting. This orogenic sector is currently located above an anomalously shallow subduction segment, in which timing and relation to upper-plate tectonics have been widely overlooked. This subduction segment, here referred to as the southern Puna shallow subduction (SPSS), is characterized by a ~200 km wide shallow area located at ~300 km from the trench at a depth of ~100–120 km and dipping 10–12° to the east. To determine the onset of the SPSS and its link to the tectonic and magmatic activity in this region, we analyzed the tectonomagmatic record of the southern Puna plateau from preexisting datasets. Also, we present a new approach based on global subduction data that provides a straightforward methodology to extract potential paleo-slab angles from the bedrock arc record. This analysis reveals that a pronounced eastward arc-front migration and magmatic broadening took place at ~26 Ma and was preceded by ~4 Ma of reduced magmatic activity, which we link to the inception of the SPSS. As expected in shallow subduction settings, a change to basement-cored distributed deformation south of 25°S in the southern Puna plateau coincides with the beginning of shallow subduction. Also, the SPSS is coincident with the enigmatic post-Eocene intraplate deformation of the Otumpa Hills located at ~950 km from the trench. We suggest that this succession of events is not fortuitous and that the development of the SPSS impacted directly the overriding plate since the Oligocene contributing to the building of one of the largest topographies (>3 km) and thickest orogenic crusts (~70–60 km) on Earth. The shallow subduction would have acted jointly with Cenozoic changes in plate kinematics and climate enhancing Andean orogenesis at studied latitudes.