SERPENTINIZATION OF THE CASCADIA FOREARC MANTLE

In 1993-94 researchers at the Oregon State University conducted an IRIS-PASSCAL passive seismic experiment across the Cascadia subduction zone in central Oregon. The sampling density afforded by this experiment allows for detailed analysis of forearc crust and mantle shear-velocity structure using scattered teleseismic wavefields. We have employed this data set in a formal waveform inversion to image fine-scale structure of the subduction zone. The Moho of the subducting oceanic plate dips shallowly beneath the coast, more steeply below the Willamette valley where its signature becomes less pronounced, and can be traced to depths of 90 km as it approaches the arc. The continental Moho is evident as a boundary near 36 km depth, from the eastern end of the profile to approximately -122.3W, wherepon it apparently disappears. To the west, a horizontal boundary at similar depth with ``inverted'' shear-velocity contrast appears and persists to its intersection with dipping oceanic crust. We interpret this observation to manifest the occurrence of an inverted continental Moho resulting from very low mantle shear-velocity. This interpretation implies the presence of a hydrated and serpentinized forearc mantle wedge, an inference consistent with both thermal and petrologic models. Much of the water contributing to serpentinization is thought to be supplied through eclogitization of basalt which in turn reduces the the seismic signature of the oceanic crust at depths below 40 km. The identification of these structures has important implications for our understanding of: 1) the downdip rupture limit of great thrust earthquakes, 2) the nature of flow in the mantle wedge, 3) the evolution of continental margins, and 4) structural controls on ground motions from intra-slab events.