High-resolution seismic tomography of the transition zone from normal to flat slab subduction in central Chile: implications for volcanoes, plate coupling and flat subduction

The subduction zone in South America spans approximately 7500 km from around 10°N to about 60°S, making it the world’s longest continuous subduction zone. In this study, we focus on the northernmost part of the central Chile flat slab subduction zone, which stands out as one of the most prominent flat slabs in South America. Within this region, the Nazca plate subducts beneath the central Chile with a direction of N78°E at a convergence rate of 6.7 ± 0.2 cm a-1 and with a direction of N 78°E. To better understand factors controlling the distribution of volcanoes, plate coupling along the subducting plate interface, and the transition from normal to flat slab subduction, we have determined high-resolution Vp, Vs and Vp/Vs models in the central Chile subduction zone where normal slab subduction transits to flat slab subduction. In the study region spanning latitudes of 22° to 31°S, volcanoes to the north of latitude 25.5°S are underlaid by intensive intermediate-depth earthquakes, but those to the south are correlated with very few. Based on velocity features, we proposed that volcanoes to the north are likely caused by partial melting of mantle wedge by incorporation of fluids released during the dehydration reactions of various hydrous minerals in the slab that are responsible for inducing intermediate-depth earthquakes, while volcanoes to the south are likely caused by sub-slab hot materials migrating upwards through the tear or gap due to the transition from normal subduction to flat subduction. Along the plate surface constructed based on our inverted velocity models and relocated earthquakes, higher plate coupling is spatially correlated with lower Vp/Vs values and fewer earthquakes, whereas lower plate coupling is correlated with relatively higher Vp/Vs values and intensive small earthquakes. These features suggest that the plate coupling state is controlled by the existence of fluids along the plate interface, with high degree of fluids reducing plate coupling and causing the creep deformation. In the region where the flat slab subduction is evident, there exist apparent high velocity anomalies above the intraslab seismicity. This indicates that some buoyant materials such as oceanic plateaus, aseismic ridges and seamount chains that featured high velocity anomalies were subducted with the slab and caused the nominal flat subduction.