SEISMIC IMPEDANCE AND SHEAR VELOCITIES OF THE COSTA RICAN SUBDUCTION ZONE THROUGH RECEIVER FUNCTION ANALYSIS AND AMBIENT NOISE TOMOGRAPHY

Flat-slab subduction, a unique form of subduction where the downgoing oceanic plate assumes a sub-horizontal angle, is thought to have played an important role in the tectonic and volcanic evolution of several regions including the North, Central and South American Cordilleras. While the effects of this process are well known and commonly include a migration of deformation inboard from the plate margin, increased upper-plate seismicity, and the shut-off of the volcanic arc, the driving mechanisms behind flat-slab subduction are still debated. In order to better understand this atypical form of subduction, the geometry of these settings must be well defined. We use Ps receiver functions and ambient noise tomography to examine flat-slab subduction in Costa Rica. We specifically calculate crustal thickness and shear velocities of the crust and upper mantle. Initial results show continental crustal thicknesses range from ~25-50 km across the region with the thickest crust observed beneath the volcanic arc. We also interpret a slab tear based on Wadati-Benioff zone seismicity located at the transition between the steep (~45°) angle subduction in northwestern Costa Rica, and the flat-slab (~7°) region in the southeast. High shear velocities observed in the flat-slab forearc at >25 km depth are interpreted as the downgoing Cocos slab. Costa Rica’s subduction geometry is defined by steep subduction in the northwest, and shallow in the southeast divided by a slab tear. The presence of a slab tear in Costa Rica’s flat-slab subduction region, along with work in Peru and the Pampean region, suggests that slab tears may be more prevalent than originally thought in flat-slab settings.