Shear wave velocity structure in the lithosphere and asthenosphere across the Southern California continent and Pacific plate margin using inversion of Rayleigh wave data from the ALBACORE project.

Abstract:

The North American and Pacific plate boundary is a unqiue example of past subduction of an oceanic spreading center which has involved oceanic plate capture and inception of a continental transform boundary that juxtaposes continental and oceanic lithosphere on a single plate. The amphibious ALBACORE seismic project (Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment) deployed 34 ocean bottom seismometers (OBS) on 15-35 Ma seafloor and offers a unique opportunity to compare the LAB in continental and oceanic lithosphere in one seismic study. Rayleigh waves were recorded simultaneously by our offshore array and 82 CISN network land stations from 2010-2011. Here we predict phase velocities for a starting shear wave velocity model for each of 5 regions in our study area and compare to observed phase velocities from our array in a least-squares sense that produces the best fit 1-D shear wave velocity structure for each region. Preliminary results for the deep ocean (seafloor 25-32 Ma) indicates high velocities reaching 4.5 km/s at depths of 50 km associated with the lithosphere for seafloor 25-32 Ma. A negative velocity gradient is observed below this which reaches a minimum of 4.0 km/s at 160 km depth. The mid-ocean region (age 13-25 Ma) indicates a slightly lower magnitude and shallower LVZ. The Inner Borderland displays the highest lithospheric velocities offshore reaching 4.8 km/s at 40 km depth indicating underplating. The base of the LVZ in the Borderland increases sharply from 4.0 km/s to 4.5 km/s at 80-150 km depth indicating partial melt and compositional changes. The LVZ displays a very gradual positive velocity gradient in all other regions such as the deep seafloor and continent reaching 4.5 km/s at 300 km depth. The deep ocean, Borderlands, and continental region each have unique lithospheric velocities, LAB depths, and LVZ character that indicate stark differences in mantle structure that occur on a single plate as well as across the continental margin.