NEW SEISMIC IMAGES OF THE CASCADIA SUBDUCTION ZONE FROM CRUISE SO108-ORWELL

In April and May 1996, a geophysical study of the Cascadia continental margin off Oregon and Washington was conducted aboard the German R/V Sonne. This cooperative experiment by GEOMAR and the USGS acquired wid e-angle reflection and refraction seismic data, using ocean-bottom sei smometers (OBS) and hydrophones (OBH), and multichannel seismic reflec tion (MCS) data. The main goal of this experiment was to investigate t he internal structure and associated earthquake hazard of the Cascadia subduction zone and to image the downgoing plate. Coincident MCS and wide-angle profiles along two tracks are presented here. The plate bou ndary has been imaged precisely beneath the wide accretionary wedge cl ose to shore at ca. 13 km depth. Thus, the downgoing plate dips more s hallowly than previously assumed. The dip of the plate changes from 2 degrees to 4 degrees at the eastern boundary of the wedge on the north ern profile, where approximately 3 km of sediment is entering the subd uction zone. On the southern profile, where the incoming sedimentary s ection is about 2.2 km thick, the plate dips about 0.5 degrees to 1.5 degrees near the deformation front and increases to 3.5 degrees furthe r landwards. On both profiles, the deformation of the accretionary wed ge has produced six ridges on the seafloor, three of which represent a ctive faulting, as indicated by growth folding. The ridges are bordere d by landward verging faults which reach as deep as the top of the oce anic basement. Thus the entire incoming sediment package is being accr eted, At least two phases of accretion are evident, and the rocks of t he older accretionary phase(s) forms the backstop for the younger phas e, which started around 1.5 Ma ago. This documents that the 30 to 50 k m wide frontal part of the accretionary wedge, which is characterized by landward vergent thrusts, is a Pleistocene feature which was formed in response to the high input of sediment building the fans during gl acial periods. Velocities increase quite rapidly within the wedge, bot h landward and downward. At the toe of the deformation front, velociti es are higher than 4.0 km/s, indicating extensive dewatering of deep, oceanic sediment. Further landward, considerable velocity variation is found, which indicates major breaks throughout the accretionary histo ry. (C) 1998 Elsevier Science B.V. All rights reserved.