Structural evolution of the Oregon accretionary prism

Geophysical data from an extensive multichannel seismic (MCS) survey and Ocean Drilling Program (ODP) Leg 146 are used to examine: (1) the origin of bottom-simulating reflectors (BSRs); (2) The relationship of seismic velocity to changes in porosity and; (3) the variation in structural style and development of landward-vergence in the Cascadia accretionary prism.; Velocities obtained from vertical seismic profiles (VSPs) and downhole logs at two ODP drill sites on the Cascadia margin are lower than that of water beneath the BSR, strong evidence for the presence of 1%-5% free gas. Velocities in the hydrate stability field above the BSR are only slightly elevated, suggesting that low-velocity free gas, rather than hydrate, is primarily responsible for the velocity contrast that forms the BSR at these sites.; Sediments in stratigraphically-defined intervals decrease in velocity during deformation as they are incorporated into the first thrust sheet of the landward-vergent Oregon accretionary prism, resulting in velocities that are lower in the deforming thrust sheet than in the equivalent undeformed basin sediments. It is possible that the velocity decrease indicates a porosity increase in the upper sediments due to extensional failure during fold development; however, a more geologically reasonable explanation is that the rigidity of the sediments decreases due to loss of incipient cementation during deformation.; The Oregon margin spans a regional transition in structural style from seaward-vergence in the south, to landward-vergence in the north. Landward-vergence requires both low basal shear stress and an arcward-dipping decollement; the regional variation probably reflects a change in the potential for overpressure, and low effective stress, in the sediments. Superimposed on the regional transition are abrupt changes in structural style that correlate with NW-striking strike-slip faults in the Cascadia Basin.; Each of these three cases illustrates the interaction of sediment/fluid properties and deformation. Geophysical data are used to constrain the influence of changing physical properties on structural style, the effect of deformation on the sediments and flow paths, and the effect of both physical properties and pore fluids on geophysical measurements such as velocity.