Southward-directed subduction of the Farallon-Aluk spreading ridge and its impact on subduction mechanics and Andean arc magmatism: Insights from geochemical and seismic tomographic data

Since the initial proposal of the past existence of a southward-directed mid-ocean ridge-subduction interaction in the Andes during Late Cretaceous-Paleogene times, several studies have been devoted to uncover the tectonomagmatic evidence of this process. The collision of a spreading ridge against a subduction margin provokes important tectonomagmatic changes, including, between them, variations in arc-related magmatic activity and in the plate-margin stress regime. However, the cryptic nature of the geological record often hampers assessing the influence and along-strike evolution of this process. In this study, we integrate new isotopic data with previous field and geochemical data on Andean arc-related magmatism, together with seismic tomography to track the main tectonic changes that affected the Andes between 35 degrees and 42 degrees S from Latest Cretaceous to early Miocene times. In particular, we carry out a new tomotectonic analysis combining the regional bedrock record of the Late Cretaceous-early Miocene arc with upper-lower mantle seismic tomography. This analysis allowed us to unravel the main geodynamic changes that affected the Andean active-margin when the Farallon-Aluk spreading ridge was subducting. Besides, new isotopic analyses reveal the variable nature of the mantle source that fed the Late Cretaceous-early Miocene arc. Hence, the integration of geological, geochemical, and geophysical data, together with new isotopic data studying the geochemical composition of the main Andean arc-related magmatic units in three main periods - (1) Latest Cretaceous-early Paleocene, (2) Early Paleocene-late Eocene, and (3) Late Eocene-early Miocene - allow us to understand with an unprecedented detail the geochemical and spatiotemporal evolution of the passage of this spreading ridge along the Andean margin.