THE SEISMIC EXPRESSION OF HYDRATION IN THE CRUST AND MANTLE OF THE CASCADIA MARGIN

Lateral variations in the seismic properties of the Cascadia margin are fundamentally tied to fluid-mediated processes like non-volcanic tremor, intermediate-depth seismicity, and volcanism. Using a newly developed 3D shear wave velocity model, we implement corrections to the conversion of P-wave receiver functions to depth to create an improved model of the discontinuity structure beneath the Cascadia margin. This model allows us to better differentiate between variations in the depth and seismic properties of the Moho as well as in crustal and upper mantle shear wave velocities.

In the forearc lower crust, very low shear wave velocity zones correlate with regions of high tremor density and attenuation, indicating the presence of fluids likely derived from metamorphic dehydration reactions within the downgoing slab. The spatial extent of tremor and lower crustal low velocity zones is mainly constrained to depths where the downgoing oceanic crust is in contact with the overriding crust, terminating near where the slab underthrusts below the forearc mantle (i.e., the “mantle wedge corner”). This correlation suggests that the forearc mantle may react quickly to form serpentinite if fluids are present and/or may be too permeable to allow fluid pressures to reach the near-lithostatic conditions necessary to cause non-volcanic tremor. Outboard of the forearc mantle corner, velocity contrasts across the Moho and uppermost mantle shear wave velocities indicate much of the forearc mantle is serpentinized, consistent with previous studies. However, there is considerable variability in the extent of mantle serpentinization both along and across strike of the Cascadia margin, possibly indicating variations in the volume and/or depth of fluid release along the margin. Beneath the Cascadia volcanic arc, crustal thickness appears relatively consistent at ~38 km with the exception of ~46 N, where the crust appears much thicker beneath the St. Helens, Adams, and Indian Heaven volcanic fields. Variations in lower crustal and upper mantle shear velocities are also apparent beneath the Cascade volcanoes, which could be related to magmatic ponding at different depths beneath volcanic centers.