Abstract

Seismic reflection data show that a regionally extensive band of landward-dipping reflections exists above the subducting Juan de Fuca plate at the western Canadian continental margin. The reflections truncate at depth a major terrane boundary mapped near the surface, and must have a structural origin. Many reflections are very strong, having reflection coefficients locally as high as 10%-20%.These amplitudes are consistent with theoretical calculations of reflections from sharp poirosity contrasts, if the pores are very thin; however, seismic data from the accreted sedimentary melange farther seaward, from which much of the deep reflective zone is derived, suggest that significant porosity contrasts do not occur alone. Because higher reflection amplitudes are found in regions of inferred high strain, both geometric and amplitude constraints imply that the deep reflective layering represents intensely sheared rocks, possibly originating close to the subduction decollement.

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