Seismic tomography of the Pampean flat-slab subduction zone and its implications for volcanism and seismicity in the Central Andes

• 3-D shear-wave tomography of the Pampean flat-slab subduction zone is obtained. • A subducting ridge and a hole are revealed within the slab. • Anomalously hydrated mantle wedge is revealed beneath the Pampean volcanic gap. To clarify the deep structure of a flat-slab subduction zone, we determine new high-resolution 3-D shear-wave velocity (Vs) tomography of the crust and upper mantle beneath the Pampean region of the Central Andes by jointly inverting a large number of local P and S wave travel times, teleseismic S-wave relative travel-time residuals, and fundamental mode Rayleigh-wave phase and amplitude data at periods of 20–100 s. Our Vs tomography clearly captures the flat-slab subduction and associated structures beneath the Pampean region. The subducting slab is imaged clearly as a high-Vs zone with a distinct Wadati-Benioff deep seismic zone extending from the trench to the area beneath the foreland basin, leading to the establishment of a new slab geometry model beneath the study region. We further find Vs variations along the subducting Juan Fernandez hotspot chain, suggesting ridge segmentation, and a slab hole beneath the Cerro Blanco volcano. In addition, our tomography reveals obvious low-Vs anomalies in the crust and mantle wedge beneath not only the active arc volcanoes but also the volcanic gap formed by slab flattening due to the ridge subduction. These Vs anomalies may indicate source zones of potential arc magma with high water content, resulting from slab dehydration and/or upwelling of subslab mantle materials through the slab hole. The ridge segmentation along the hotspot chain probably facilitates fluids from the normal oceanic crust released into the crust and mantle wedge above the Pampean flat slab, leading to anomalous hydration beneath the volcanic gap in the Central Andes.