RESOLVING THE CONTROVERSY OF THE ISABELLA LITHOSPHERIC ANOMALY IN CENTRAL CALIFORNIA USING SURFACE WAVE TOMOGRAPHY
We present new surface wave tomography results to address an active debate about high velocity lithospheric anomalies interpreted to be continental lithospheric delamination versus oceanic slab fragmentation in central California. The East Pacific Rise (EPR) spreading center, located in the central Pacific ocean ~50 Ma, steadily moved eastward and approached the North American subduction zone on the west coastline because the rate of Farallon subduction exceeded the EPR spreading rate. Irregularity in the EPR spreading center axis caused the subduction zone to begin consuming the EPR spreading center in modern day southern California ~30 Ma and later proceeded both northward along the California, USA coastline and southward along the Baja California, Mexico coastline. Several studies have suggested high velocity anomalies observed in the upper mantle beneath the Isabella region between the Sierra Nevada mountain range and the Monterey coastline are due to a slab window, break up of the Farallon oceanic plate and capture by the Pacific plate leaving an oceanic slab fragment in central California. A competing view suggests the high velocity anomaly is due to continental lithospheric delamination beneath the Sierra Nevada mountains. We invert Rayleigh wave data obtained from the ALBACORE marine seismic array and permanent land stations, for 1D and 2D shear wave velocity structure. We divide the study area into several regions along major fault lines as well as the inner Borderland, outer Borderland, and oceanic seafloor by age bins. A unique starting shear velocity model is used for each region incorporating the appropriate crustal, sediment, and lithospheric layer thicknesses, densities, and P and S velocities which predicts Rayleigh phase velocities that are compared to observed phase velocities in each region. We solve for shear wave velocities with the best fit between observed and predicted phase velocity data in a least square sense. Preliminary results indicate that lithospheric velocities in the oceanic mantle are higher than the continental region by at least 2%. The lithosphere-asthenosphere boundary is observed at 50 ± 20 km beneath 15-35 Ma oceanic seafloor. Asthenospheric low velocities reach a minimum of 4.2 km/s in all regions, but have a steeper positive velocity gradient at the base of the oceanic asthenosphere compared to the continent. Preliminary results from seismic tomography images will be presented with coverage in the central California continental margin showing a remnant slab fragment at the coastline. We investigate the possibility that the two competing hypotheses may have been conflated in the literature and both may be valid when considered within a wider framework including oceanic mantle structure and continental arc volcanism.