MAPPING SUBDUCTED ASEISMIC RIDGES WITHIN THE NAZCA SLAB FROM MANTLE SEISMIC TOMOGRAPHY

Eastward subduction of aseismic ridges under western South America (e.g., Carnegie, Nazca, and Juan Fernandez ridges) has likely driven Andean uplift and exhumation, influenced earthquake seismicity, and profoundly shaped the spatiotemporal distribution of arc volcanism, ore deposits, and possible flat slab subduction. However, the downdip geometry of the subducted aseismic ridges is debated and has significant implications for their reconstructed spatiotemporal position under the Andes during the late Cenozoic. Here we attempt to structurally characterize the subducted aseismic ridges from recent regional P-wave mantle tomography SAM5_P_2019 (Portner et al., 2020) and adjoint waveform tomography SAAM23 (Ciardelli et al., 2022). Results are compared to a suite of other global and regional tomography and previous studies.

The Nazca slab downdip of the subducting Carnegie and Juan Fernandez ridges generally exhibits anomalous, slower velocity anomalies at > 150 km depths in SAM5_P_2019 tomography whereas the subducted Nazca ridge does not show a noticeable anomaly. We also identify a slower velocity anomaly within the Nazca slab near the hypothesized, wholly-subducted ‘Inca plateau’. SAAM23 tomography shows slow velocity perturbations downdip of all three aseismic ridges within the Nazca slab. We interpret that our mapped slow tomographic anomalies are warmer slab gaps or tears due to buoyancy contrasts between the aseismic ridges and surrounding Nazca slab; hence, they are possibly indirect indicators of the aseismic ridges. We compare our mapped extent of our subducted aseismic ridge anomalies to previous studies and discuss their implications for late Cenozoic Andean tectonics.