THE ROLE OF SUBDUCTED RIDGES IN THE EVOLUTION OF FLAT SLABS

Flat slab subduction has long been considered as a potential contributing factor in the formation of inland basement cored uplifts, volcanic sweeps and gaps, and widespread ignimbrite volcanism. However, the processes responsible for the formation of flat slabs are poorly constrained. This is of particular importance when evaluating the putative Farallon flat slab, which would have been significantly larger in scale than any flat slab observed today. We present new constraints on the role of ridge subduction in the formation of flat slabs from recent seismic data collected in southern Peru and northernmost Bolivia as part of the PULSE, CAUGHT, and PERUSE deployments. We find several lines of evidence that strongly suggest that the ridge is a necessary, if not sufficient, condition for the formation and stability of the flat slab in southern Peru. Specifically, we find that the flat slab is shallowest directly adjacent to the ridge, and that north of the southward migrating ridge, the flat slab that had formed earlier when the ridge was present is now resuming a normal subduction geometry due to sagging and tearing of the plate. Our results suggest that in Peru, the contributing factors in the formation of the flat slab include 1) trench retreat and rapid overriding plate motion, 2) suction forces between established flat slab and the base of the continent and 3) ridge subduction. However, when the ridge is removed, the first two contributing factors are not sufficient to maintain the flat slab geometry. This has important implications for the tectonic evolution of the Western United States where the buoyant bathymetric feature was not a ridge, but rather a discrete plateau.