SURFACE WAVE TOMOGRAPHY STUDYING OFFSHORE SLAB FRAGMENTS REMNANT FROM FARALLON PLATE SUBDUCTION
We consider deformation and plate breakup caused by the tectonic process of plate boundary collision between a divergent spreading center a convergent subduction zone. The mid-ocean East Pacific Rise (EPR) spreading center steadily approached the Farallon-North American subduction zone 50-35 Ma because the convergence rates exceeded the spreading rate. The complex history of this region experienced subduction not only of the Farallon oceanic plate, but also subduction of the advancing EPR spreading center, as well as opening and rifting of the Borderlands and Baja. We present preliminary surface wave tomography results that will address an active debate that argues high velocities lithospheric anomalies observed beneath central California are caused by continental delamination versus an oceanic slab fragment remnant from Farallon subduction 35 Ma. We invert Rayleigh wave data obtained from the offshore ALBACORE broadband array and permanent land stations, for 1D and 2D shear wave velocity structure. We solve for shear wave velocities with the best fit between observed and predicted phase velocity data in a least square sense. Preliminary results from seismic tomography images will be presented with coverage in the central California continental margin showing a remnant slab fragment originating offshore and dipping eastward beneath the coastline. We suggest this oceanic slab fragment was caused by microplate capture during the collision of the EPR with the western North American convergent margin, terminating subduction and initiating continental rifting in Baja and in southern California via the San Andreas transform plate boundary.