Imaging Variations within the South Central Chilean Fore-Arc and Subducting Nazca Plate from Receiver Functions

Fore-arcs represent structurally complex parts of subduction systems that contain significant along strike variations. Understanding how these variations at depth relate, or fail to relate, to surface structure and the deeper structure of subduction zones will improve our ability to model subduction processes. To contribute to this effort, we present results from 1137 P-wave Receiver Functions (RFs) calculated for P-, PP-, and PKP-phases recorded at 123 temporary and permanent seismic stations deployed in central Chile between the coastline and western edge of the Andean volcanic arc between 33°S and 38.5°S where the Nazca oceanic plate subducts beneath the South American continental plate. We have used a previously published S-wave velocity model obtained for the region from ambient noise tomography to migrate our RFs to depth through common conversion point stacking, then used the resulting stacked RFs to identify and map the laterally continuous subducted oceanic Moho arrival of the Nazca Plate and a number of features within the South American fore-arc. We have found that the subducted oceanic Moho of the Nazca Plate exhibits variations in depth that are not present in larger scale slab models. The slab's Moho geometry is relatively simple above 40 km depth, however below 40 km depth the slab exhibits noticeable strike perpendicular buckling with an approximate 50-100 km spacing between individual features. These buckling features are not easily relatable to features in the unsubducted plate and may have been imposed on the slab during the subduction process. We have also found that the South American fore-arc Moho is complicated in much of the region and is represented by a negative amplitude RF arrival up to approximately 30 km to 50 km inboard from the coast line. This suggests either that the fore-arc mantle well inboard of the coastline is significantly hydrated or that South American continental crust in this location lies directly above the subduction channel. Finally, we observe a negative arrival approximately 55 to 60 km below and subparallel to the subducted oceanic Moho that we relate to the subducting plate's Lithosphere-Asthenosphere Boundary (LAB). Observation of this feature with P-wave RFs suggests the LAB of the Nazca Plate is a relatively sharp velocity discontinuity.