72-1 Lithospheric Structure of the Downgoing Plates along the Cascadia Margin

Session: Toe to Toe: Cordilleran Systems from Trench to Retroarc Domains (Posters)

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Abstract:

The Lithosphere-Asthenosphere Boundary (LAB) plays a fundamental role in plate tectonics and may also contribute to subduction processes. Subduction of young (~4-13 Ma) oceanic crust, such as is found at the Cascadia subduction zone, implies that the depth to the LAB on the incoming plates is relatively shallow and may influence how it subducts under the North American plate. We use marine active-source seismic data to investigate the deep lithosphere of the incoming plates at the Cascadia subduction zone and provide insight on the structure of the LAB. The CAscadia Seismic Imaging Experiment 2021 (CASIE21) collected multichannel seismic data throughout Cascadia aboard R/V Marcus G. Langseth using a 6,600 in³ tuned air gun array and an ultra-long (12/15 km) streamer. Seismic data was processed through Kirchoff pre-stack depth migration. P-wave velocity building used MCS constraints for the upper crust, previous velocities down to the Moho, and a constant 8 km/s for beneath the Moho. Pre-migrated gathers and stacks were also used to verify imaging. In many lines, we find a deep reflector located well beneath the Moho. We focus on regions where we can tie the deep reflector with other lines and where the reflector is strongest, which are largely found in northern Cascadia. Other regions do have some signal, albeit weaker, and may indicate lateral variation in the sharpness of the boundary along the margin, since data processing was consistent for all the lines. The reflector has a dominant low-frequency signal (~4-14 Hz), and reverse polarity compared with the seafloor. The depth of the reflector varies from ~23-29 km. These interpretations suggest that this could be the LAB, and its observed depth would be comparable to previous results in the region. While we do not discern a direct correlation between depth and lithospheric age, we note that age variations within each line are relatively small. Instead, we see a heterogeneous LAB and melt channel for Cascadia, which may be associated to a thermal anomaly emanating from the nearby hotspot upwelling. Given the imminent subduction of this lithosphere, better understanding of the LAB and its impact on subduction-related processes is still needed.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11138

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