Ridge asymmetry and deep aqueous alteration at the trench observed from Rayleigh wave tomography of the Juan de Fuca plate. Issue 10 (15th October 2016)

• Record Type: Journal Article
• Authors: Bell, Samuel; Ruan, Youyi; Forsyth, Donald W.
• Is Part Of: Journal of geophysical research. Volume 121:Issue 10(2016:Oct.)
• Page Start: 7298
• Page End: 7321
• Publication Date: 2016-10-15
• Subjects: spreading ridge asymmetry -- subduction zone -- tomography -- surface wave -- serpentinization -- noise; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1002/2016JB012990
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Using Rayleigh wave tomography of noise‐removed ocean bottom seismometer data from the Cascadia Initiative, we illuminate the structure of the upper mantle beneath the Juan de Fuca plate. Beneath the Juan de Fuca ridge, there is strong asymmetry, with a pronounced low‐velocity zone in the 25–65 km depth range. Extending to the west from the spreading axis, this anomaly has velocities low enough to indicate the presence of melt. The asymmetry in velocity structure and the much greater abundance of seamounts on the west flank of the ridge suggest that dynamic, buoyant upwelling is important, perhaps triggered by thermal or compositional anomalies beneath Axial Seamount. In contrast, there is no evidence for asymmetry in the axial zone or lower than expected velocities beneath the Gorda ridge. On the eastern flank of the Juan de Fuca ridge, the shear velocity in the 25–65 depth range is higher than expected; the lithosphere appears to be colder and thicker than predicted by standard plate cooling models, perhaps caused by the downwelling counterpart of the upwelling on the west side of the ridge. Close to the trench, there is a sharp decrease in shear velocity. We interpret this as aqueous alteration caused by hydrothermal circulation through deep normal faults associated with bending of the plate. Beneath the Astoria and Nitinat fans, where abyssal plain sediment is thickest, the velocity decrease is much smaller, which is consistent with a thick sediment cap that prevents hydrothermal alteration of the plate. Key Points: We have detected an asymmetric low‐velocity zone beneath the Juan de Fuca ridge, probably due to partial melt We have detected reduced shear velocities in the downgoing Juan de Fuca slab, which we interpret as evidence of aqueous alteration The aqueous alteration is reduced beneath the area around the Columbia River outflow, probably indicating sediment shielding