Abstract

We analyze records of ambient seismicity from Central Chile to investigate mechanisms of tectonic erosion and accretion in an Andean margin. The seismograms were recorded by a temporary network of 39 stations deployed between 32.5 degrees S and 34.0 degrees S from April to November 2017. We employ an automatic earthquake catalogue generator to estimate 8463 P-wave and 5482 S-wave arrival times from 924 hypocenters, which were then used to jointly determine hypocenters and 3D models of Vp and Vp/Vs. Previous investigations of the nearby Illapel region suggest that the Coastal Cordillera acts as a subduction wedge, at the bottom of which are accreted crustal slices detached from the overriding plate by tectonic erosion. These slices may be subducted beyond the downdip frictional limit in a way that differs from a traditional orogenic wedge (type A-subduction). Our results reveal a wedge geometry with: (1) body wavespeed anomalies near the interplate contact below 30 km depth, which we interpret as accretionary complexes formed by basal accretion of crustal slices, and (2) latitudinal differences of those anomalies, which we attribute to eroded material from both plates that changes the basalt-eclogite transition depth in the subducted Nazca plate. Based on these, we infer that the development of this basal accretionary complex by subduction erosion could be responsible for the uplift of Coastal Cordillera in Central Chile, and that the retroshear zone of the subduction wedge is controlled by local conditions inside the mantle wedge. Moreover, we suggest that the accretionary complex beneath the Central Chile subduction wedge behaves as seismic barrier that can explain the 1985 Mw 8.0 Valparaiso earthquake slip distribution and the southeastward migration of aftershocks of 2017 Mw 6.9 Valparaiso earthquake. Finally, we suggest that the subduction wedge influences continental structures on a scale of millions of years.