Linking Offshore Seismic Tomography and Surface Volcanology to Study Subduction Termination and Controversial Lithospheric Fragments in the Central California Continental Margin

The University Corporation, Northridge, Northridge CA

Investigators

Abstract

The Earth is the only planet where there is evidence that tectonic plates converge, and heavy oceanic plates sink below buoyant continental plates in a process called subduction. Subduction of one tectonic plate beneath another is the site of unique geologic properties, rich mineral resources, and vital elements that support life on Earth, making this process an active area of interest and research. Once a plate begins subducting, gravitational pull acts to perpetuate the subduction process between two tectonic plates for millions of years as plates move continuously towards each other on the surface of the Earth. This project will consider the process of ‘subduction termination’ and investigate the forces necessary to stop the subduction of a plate by investigating a case at the central California coastline where this may be occurring today. The proposed work will use data from seismic instruments on land and offshore to image the subducting oceanic plate fragments in the Earth’s interior in collaboration with geochemical studies of volcanic rocks which may have been formed in coastal California over the subducting plate. Understanding the cessation of subduction at convergent tectonic margins is critical because this process creates and modifies established plate boundaries, initiates faults, is the location of the largest and most dangerous earthquakes in the world, tsunamis, magmatic centers, and sedimentary basins. Formation of the major faults such as the San Andreas and minor surrounding faults by this process is also a demonstrated source of continental rifting and breakup. This work will recruit and support the scientific development of students underrepresented in the geoscientists, while mentoring a postdoctoral scholar in multi-disciplinary work. To study the process of subduction termination, this project will focus on a case in the central California continental margin where transformation of the North American continental margin from a convergent boundary to a transform boundary began in the Oligocene and continues to the present. This project will address the highly debated origin and significance of a key geophysical feature in this region, the ‘Isabella anomaly,’ an anomalously seismically high-velocity structure beneath the Great Valley, California. This feature is controversial due in part to the absence of marine seismic data and a notable gap in geochemical data from coastal California lavas. This work will use a new configuration of existing offshore and onshore seismic data, integrated with new geochemical and isotope analysis of surface volcanic rocks near the continental margin at locations informed by seismic tomography, to test the hypothesis that the Isabella anomaly is a slab fragment from the subducted Farallon plate. This proposed work will address the following important science questions. 1.) How does subduction terminate when consuming a spreading center? 2.) Is slab fragmentation at continental margins a common process during subduction termination? 3.) Can we differentiate volcanism from slab fragmentation versus arc melting? This project will provide a new pattern of offshore crossing ray paths using a nearby previously deployed marine seismic array combined in a new configuration with central CA stations. Inversions of Love and Rayleigh waves at 20 s – 120 s period will provide new 3D resolution of velocity structure at lithospheric depths and latitudes to test the offshore extension of this anomaly. Volcanic rocks will be analyzed at the coastal margin to link surface magmatic processes with geophysical imaging of the underlying lithosphere and differentiate surface volcanism caused by melting of the slab, the mantle wedge, the crust, and asthenospheric upwelling through fragmented slab windows. At least 70% of the personnel slated to work on this project are underrepresented marginalized or BIPOC students and scientists. These personnel will engage in on-going outreach activities at California State University, an MSI and HSI, such as the CSUN GEMS (Geological Experience for Marginalized Students) and LA-ROCS (Research Opportunities for CSUN Students) programs in the Department of Geological Sciences. This work is co-funded by the Geophysics, Petrology and Geochemistry, and Marine Geology and Geophysics programs. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.