Shear wave velocity structure of a remnant slab beneath the western Transverse Ranges offshore southern California

Dayanthie S. Weeraratne, Kaitlyn Amodeo, Sampath Rathnayaka, Escobar Lennin, Carlos D. Gomez, & Monica D. Kohler

Published August 15, 2018, SCEC Contribution #8722, 2018 SCEC Annual Meeting Poster #110

Data from the amphibious ALBACORE (Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment) seismic array across the southern California continental margin allows us to simultaneously study oceanic and continental lithosphere in one seismic study. The complexity of this region allows us to study limits in the rigidity and strength of the lithosphere, as it has undergone spreading center subduction, translation through the mantle, and block rotation. We invert Rayleigh wave phase velocity data for shear velocity structure. Our study area is divided into oceanic regions ranging from 35-10 Ma, the Borderlands, and fault bounded continental regions. A unique starting shear wave velocity (Vs) model is used for each region with detailed crust and mantle velocities based on previous studies in each area. Phase velocities predicted from these Vs models are compared to observed phase velocities from the ALBACORE array to solve for the best fit Vs in a least square sense. Combined data from 100 marine and land based seismometers with good earthquake distribution produce excellent resolution from 20 - 200 km depth. These results are then used to create 1D velocity profiles and integrated into 3D tomographic cross sections. High velocities associated with the lithosphere-asthenosphere boundary (LAB) are clearly observed beneath oceanic lithosphere at 75 km depth +/-15 km and beneath continental lithosphere west of the San Andreas Fault (SAF) at 65 km depth. The continental LAB is deeper east of the SAF at 75 km. In the Borderlands between 33.5 N and 34.5 N (Western Transverse Ranges, WTR) demonstrate anomalous velocities with a horizontal orientation that are 11% higher than ambient mantle at 100-130 km depth. This high velocity anomaly bends sharply dipping northeastward towards the continental margin at an angle of 65o +/- 25o extending to depths of at least 250 km. We suggest this dipping high velocity structure may indicate a remnant slab fragment beneath the WTR left behind from complexities in the subduction of the East Pacific Rise spreading center 10-25 Ma and later rotation of the Transverse Ranges. Azimuthal anisotropy at sublithospheric depths below 90 km is consistent with NW Pacific plate motion direction everywhere except beneath the WTR where fast directions are oriented in a NE-SW direction consistent with a remnant slab fragment oriented in the direction of the ancient Farallon plate subduction.

Key Words
shear wave velocity, Transverse Ranges, remnant slab

Citation
Weeraratne, D. S., Amodeo, K., Rathnayaka, S., Lennin, E., Gomez, C. D., & Kohler, M. D. (2018, 08). Shear wave velocity structure of a remnant slab beneath the western Transverse Ranges offshore southern California. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology