Comparison of the Lithosphere-Asthenosphere Boundary Beneath Oceanic and Continental Lithosphere in the Southern California Continental Margin

Oceanic lithosphere is compared to continental lithosphere with an amphibious seismic array offshore southern California. We consider seafloor age between 15-35 Ma offshore the continental margin of California. This complex tectonic region experienced the subduction of an oceanic spreading center 30 Ma and nearly 100o rotation of the Western Transverse Ranges (WTR) resulting in opening of the Borderlands. Here, we invert Rayleigh wave data obtained from the ALBACORE seismic array and permanent land stations, for 1D and 2D shear wave velocity structure. Inversion results for the oceanic lithosphere indicate average shear velocities of 4.60 ±0.20 km/s. Immediately west of the Patton Escarpment the oceanic lithosphere-asthenosphere boundary (LAB) is observed at 65 ±15 km depth. We observe a strong well resolved asthenospheric low velocity zone reaching a minimum of 4.10 ±0.25 km/s. The vertical velocity contrast between the lithosphere and asthenosphere is strongest in the oceanic plate at ~11%. This contrast is reduced everywhere else in our study including the Borderlands, and continental regions to approximately 2 - 7%. Continental lithospheric velocities at 4.3 ±0.2 km/s are ~7% lower than the oceanic lithosphere. The continental LAB near the coast is identified at 60 ±15 km depth and shows a sloping boundary that is shallower near the coast and deepens inland to 75 ±15 km depth. The lithospheric high velocities beneath the WTR extend only to 50 ±15 km depth. We observe a step jump in the LAB at 33o N from 65 km depth in the oceanic plate to ~100 km depth at the Inner Borderland. We suggest this steep LAB offset indicates the presence of underplating by an oceanic plate or slab fragment. We observe a high velocity anomaly at ~34o N near Palos Verdes with a nearly vertical dip offshore near the continental coastline observed to extend from 90 km depth to at least 250 km depth. The Palos Verdes high velocity anomaly, located within the Borderlands, may indicate a lithospheric downwelling, delamination, or slab fragment that has been captured by the Pacific plate.