Seismic Image of the Central to Southern Andean Subduction Zone Through
Finite-Frequency Tomography
Abstract
This study presents new seismic imaging of the Andean subduction zone
through P-wave hybrid finite-frequency and ray-theoretical tomography.
We measured both differential and absolute traveltimes using broadband
seismic waveforms from stations in an array of ocean-bottom seismographs
near the Chile Triple Junction (CTJ) and stations within 30° from the
array. These data were combined with the global traveltime dataset to
obtain a global P-wave velocity structure with a focus on central to
southern South America. The new tomographic image showed the Nazca slab
geometry as a continuous fast anomaly, which is consistent with seismic
activity and prior slab models. Furthermore, two notable structures were
observed: a broad extension of the fast anomaly beneath the Nazca slab
at 26–35° S and a slow anomaly east of the CTJ. The checkerboard
resolution and recovery tests confirmed the reliability of these
large-scale features. The fast anomaly, isolated from the Nazca slab,
was interpreted as a relic Nazca slab segment based on its strong
amplitude and spatial coincidence with the current Pampean and past
Payenia flat slab segments. The slow anomaly near the CTJ was consistent
with the previously inferred extent of the Patagonian slab window.
Moreover, the active adakitic volcanoes are aligned with the southern
edge of the anomaly, and the plateau basalts are located within the
anomaly. Our model showed that the slow anomaly extended to a depth of
up to 250 km, suggesting a depth limit that the asthenospheric window
can influence.