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SUMMARY Following reanalysis of data from eight seismic networks that operated in the region surrounding the Pampean flat slab during the past several decades, we generated 3-D images of Vp, Vs and $$V_{\rm p}/V_{\rm s}$$ from a combination of arrival times of P and S waves from local earthquakes, and Rayleigh wave dispersion curves from both ambient noise and existing shear wave models. Among the robust features in these images is a low velocity, root-like structure that extends beneath the high Andes to a deflection in the flat slab, which suggests the presence of an overthickened Andean crust rather than a hypothesized continental lithospheric root. Most of the larger scale features observed in both the subducted Nazca plate and the overriding continental lithosphere are related to the intense seismic activity in and around the Juan Fernandez Ridge Seismic Zone (JFRSZ). $$V_{\rm p}/V_{\rm s}$$ ratios beneath, within and above the JFRSZ are generally lower (~1.65–1.68) than those in the surrounding Nazca and continental lithosphere (~1.74–1.80). While the higher continental lithosphere ratios are due to reduced Vs and likely a result of hydration, the lower JFRSZ related ratios are due to reduced Vp and can be explained by increased silica and CO2 originating from beneath the slab, perhaps in concert with supercritical fluid located within the fracture and fault networks associated with the JFR. These and related features such as a region of high Vp and Vs observed at the leading edge of the JFRSZ are consistent with a basal displacement model previously proposed for the Laramide flat-slab event, in which the eroded base of the continental lithosphere accumulates as a keel at the front end of the flat slab while compressional horizontal stresses cause it to buckle. An initial concave up bend in the slab facilitates the infiltration of silica and CO2-rich melts from beneath the slab in a manner analogous to petit spot volcanism, while a second, concave down bend, releases CO2 and supercritical fluid into the overlying continental lithosphere. 

Abstract Seamounts and ridges are often invoked to explain subduction‐related phenomena such as flat slab generation, but the extent of their involvement remains controversial. An analysis of seismicity in the region of the Pampean flat slab through an application of an automated catalog generation algorithm resulted in 35,924 well constrained local earthquake hypocenters and a total of 12,172 focal mechanisms. Several new features related to the subduction of the Juan Fernandez Ridge (JFR) were discovered, including (a) a series of parallel lineaments of seismicity in the subducted Nazca plate separated by about 50 km and trending about 20°, and (b) a strong spatial correlation between these deeper (>80 km depth) regions of intense seismicity and concentrations of activity in the crust almost directly above it. Focal mechanisms of the deeper events are almost exclusively (∼81%) normal, while those in the crust are predominantly (∼70%) reverse. The deeper lineaments mirror the orientation and spacing of several seamount chains seen on the Nazca plate, suggesting that these patterns are caused by the same types of features at depth. This would imply that relatively minor features persist as slab anomalies long after they are subducted. The correlation of the deeper seismicity that defines these features with seismicity in the mid to lower crust suggests a genetic relation between the two. We postulate that volatiles from the subducted ridges percolate into the South American crust and induce seismicity essentially by fracking it.