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 degrees, 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 (similar to 81%) normal, while those in the crust are predominantly (similar to 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.