Tectonic Reconstruction of the 4D Geometric Evolution of the Subducting Lithosphere Beneath South America

Abstract

The interaction between oceanic features on the subducting slab and the Andean Orocline influences ore deposits along the western margin of South America. In this study, we construct a 3D+time reconstruction of the Nazca/Farallon plate and the South American plate. The slab geometry was generated by incorporating velocity and thermal gradients from the UUP07 P-wave tomographic model, as well as earthquake hypocentres, to accurately trace the mid-slab surface down to ~2500 km. Prominent slab features, such as slab holes and tears, were compared with other tomographic models for validation. We also estimated the high-angle bend at the mantle transition zone, considering the compressive stress orientation of deep-focus earthquakes. The slab geometry was restored to the Earth's surface using the Orpheus Routine in Pplates to evaluate the distortion of the preserved slab. This resurrected slab model, integrated with the kinematic model in the Global Moving Hotspot Reference Frame (Doubrovine et al., 2012), suggests that the subduction of the preserved slab began in the Late Eocene. Our interpretation of the reconstructed 3D slab geometry reveals downdip tears, almost perpendicular to the subduction megathrust, across the entire slab, along with hotspot-transverse tears, subparallel to the subduction megathrust beneath the Southern Central Andes. Potential rips in the slab were interpreted based on residual distortions in the floated slab. These tears spatially coincide with the location of giant copper porphyry deposits from the Miocene and Pliocene. Based on our 4-D modelling, it appears that the Pampean flat slab evolution, following the arrival of the Juan de Fernandez Hotspot chain, involved the slab jamming at the transition zone. It then kinked, and the deep slab segment underthrust the shallow segment. The shallow segment continued subducting into the asthenosphere, maintaining its current geometry, while the deep segment tore along the 23 Ma hotspot chain bend. This segment then detached from the main slab, and now rests at the transition zone. Unlike the Pampean flat slab, the Peruvian flat slab does not exhibit similar characteristics but likely shows downdip tears that coincides with the inferred Nazca Fracture Zone and the inferred Mendana Fracture Zone. The inferred Nazca Fracture Zone tear may separate the northern and southern slab segments, resulting in depth discrepancies in subparallel subduction at and below the transition zone. To explore how the subducting slab connects with the shallow structures of the orocline, we integrate seismotectonics to link the characteristics of the seismogenic zone with transferred structures that correspond to surface expressions and lineaments, which may have contributed to ore formation in the overriding plate.