A two‐dimensional model of seismic velocity derived from multichannel seismic data collected off Oregon in 1989 shows that as sediments are carried from Cascadia Basin into the accretionary prism, there are measurable changes in velocity‐depth profiles. In the seaward most area of the basin, where no thrust faults are observed, there is a landward (and downward) increase of velocity in the sedimentary section. We attribute the velocity increase in the basin to a reduction of porosity resulting from consolidation and cementation, accompanied by diffusive flow of pore water driven by lateral tectonic as well as gravitational stress. Near the base of the slope there is an area of incipient thrusting (the protothrust zone) where protothrusts sole out into a protodécollement. Synthetic seismogram modeling of the reverse‐polarity reflection from the protodécollement shows a 100‐m‐thick layer with a slightly lower velocity relative to the sediments above it. Above the protodécollement, velocity continues to increase landward. We suggest that in this area the diffusive flow of pore water out of the sediment is augmented above the protodécollement by fault‐focused flow. Below the protodécollement a reversal in velocity may be due to an increase in porosity resulting from overpressuring of pore fluid trapped by reduction of the permeability of the sediment above the protodécollement. Farther landward, where thrusting has formed a fault‐bend fold, velocity values are lower in the accreted section of sediments relative to the velocity at a comparable subbottom depth in the protothrust zone. The decrease in velocity is a result of microfracturing of the highly consolidated sediments accompanying uplift and folding and reflects the increasing role of fracturing and faulting in the control of dewatering of the sediments.

The structural framework of the Leg 156 area is provided by a regional two-dimensional seismic reflection line, multibeam bathymetry, and a three-dimensional seismic data set that images the structure beneath a 125-km 2 area of the lower slope of the Barbados accretionary prism. The prism is formed of oceanic plate sediments that are off scraped at the toe of the slope and accreted to the overriding plate. The prism thickens from about 200 m at the thrust front to at least 4.5 km at a distance 75 km landward of the thrust front. A regional décollement separates the accretionary prism from underthrust sediments carried on the subducting oceanic crust. The oceanic crust is cut by normal faults with offsets as great as 250-300 m. The resulting horst and graben topography has a wavelength of 1-3 km and trends east-northeast. Strata seaward of the thrust front are folded and cut by numerous normal faults. The upper 200 m of oceanic plate sediment is accreted at the thrust front, and the remaining sedimentary section is underthrust beneath the prism. The thickness of underthrust sediment is usually about 500 m, but can be as little as 200 m depending on the underlying basement topography. The accretionary prism is cut by numerous thrusts and back thrusts. Thrusts near the toe of the prism trend north-northwest with spacing of 100 to 750 m. The orientation of the thrusts does not correlate with either the convergence direction (~east-west) or the trend of underlying basement topography. Out-of-sequence thrusts trend east-northeast and dominate the surface structure of a 5-km-wide zone approximately 5-8 km landward of the thrust front. Individual thrusts can be traced laterally in horizontal slices through the three dimensional data volume. Lateral ramps are easily recognized in cross lines. All thrusts sole out on the décollement. The décollement is typically located near a common stratigraphic horizon, but locally cuts up-or downsection where the stratigraphy is folded or faulted. The décollement generally is imaged as a simple reversed-phase reflection that has been modeled as a low-velocity, high-porosity zone about 10-14 m thick. We infer that this thin, high-porosity zone is an undercompacted, high-fluid-pressure section, and that mapped variations in décollement amplitude and polarity reflect décollement fluid content and fluid migration paths.

The stratigraphic framework of the Lima and Yaquina forearc basins offshore Peru, determined from multichannel seismic (MCS) data, reveals significantly different Neogene histories in the two basins. Miocene deposition in the Lima Basin was controlled mainly by variations in the relative subsidence rates. Depositional processes, particularly contour currents, may have had a major influence during the Pliocene-Pleistocene. In contrast, the Yaquina Basin shows no evidence of assumed contourites. Severe disruption of the reflectors indicates active tectonism throughout the Neogene in the Yaquina Basin, while reflectors representing late Miocene and younger strata in the Lima Basin are largely undeformed, which indicates relative quiescence. Most sequences in the Lima Basin demonstrate the presence of a hinge line that separates the relatively thin, wedgeshaped landward part from a much thicker, lens-shaped seaward part. Generally, this hinge appears to represent a paleoslope break. Such hinge lines are not evident in the Yaquina Basin. Both basins exhibit migration of the depocenters of the various sequences through time. The movement is to the south and landward in the Lima Basin, while migration is northward in the Yaquina Basin. These migrations appear to be the result of variations in the relative subsidence rates within the basins. In the Lima Basin these movements are closely related to structural features in the basement. In addition to a structural trend that is oriented parallel to the margin, we observed a secondary structural trend that is oriented east-west. Development of structural features along this trend led to the development of two distinct depocenters in most of the stratigraphic sequences in the Lima Basin.

Interpretation of data from a 1989 multichannel seismic (MCS) survey of the central Oregon Margin indicates that pore fluids escaping from the accretionary prism follow paths that are largely fracture controlled and fault guided. MCS evidence of this control includes reversed-polarity, fault-plane reflections, P-wave-velocity reversals, and upward deflection of gas hydrate reflections where they cross fault traces on some seismic sections. Site 891 was located on the frontal thrust of the accretionary prism. At this site the thrust dips 8° landward and produces a high-amplitude, reversed-polarity reflection that suggests the thrust may be a conduit for fluids from the décollement. Site 892 is situated on an out-of-sequence thrust within the prism that is known to be hydrogeologically active. The thrust dips 14° landward. Upward deflection of the gas hydrate reflection is present where it crosses the fault trace.In Cascadia Basin, west of the Leg 146 Oregon drill sites, velocity measurements increase within all strata in a landward direction, suggesting that diffusive flow is the dominant dewatering process. In the area of the basin nearest the prism, where incipient deformation was observed, proto-thrusts may augment diffusive flow in the section above a proto-décollement. A velocity inversion in the sedimentary section occurs below the proto-décollement. The depth of the proto-décollement may be at the top of an overpressured sedimentary section below a layer with low permeability.

Ocean Drilling Program Legs 190 and 196 were a two-part program to study deformation and fluid flow in the central Nankai Trough off Shikoku Island. During Leg 190 two reference sites were drilled outboard of the trench (Sites 1173 and 1178), one site into the protothrust zone (Site 1174), two sites into a trench slope basin above a major outof-sequence thrust (Sites 1175, 1176), and one site into an older portion of the accretionary prism. During Leg 196, Sites 1173 and 808 (drilled through the frontal thrust during Leg 131) were revisited, employing logging while drilling and installing two Advanced Circulation Obviation Retrofit Kits (ACORKs). Our reference sites defined the stratigraphic framework and physical properties baseline of the accreting/ subducting Shikoku Basin sedimentary section. The proto-thrust and frontal thrust sites documented the dewatering and deformation processes at the toe of the accretionary prism. Porosity comparisons between Sites 1173 and 808 suggest that elevated fluid pressures occur beneath the décollement at Site 808. Initial measurements from the ACORK at Site 808 indicate a pressure pulse apparently from the décollement. Negative chloride anomalies at Sites 1174 and 808 could be due to fluid flow from deeper in the prism, but active smectite dehydration could also be responsible for the anomalies. Resistivity imaging of the frontal thrust shows borehole breakouts with principal stress orientations consistent with core-scale structures and plate convergence