Leg 164 of the Ocean Drilling Program was designed to investigate the occurrence of gas hydrate in the sedimentary section beneath the Blake Ridge on the southeastern continental margin of North America. Sites 994, 995, and 997 were drilled on the Blake Ridge to refine our understanding of the in situ characteristics of natural gas hydrate. Because gas hydrate is unstable at surface pressure and temperature conditions, a major emphasis was placed on the downhole logging program to determine the in situ physical properties of the gas hydrate-bearing sediments. Downhole logging tool strings deployed on Leg 164 included the Schlumberger quad-combination tool (NGT, LSS/SDT, DIT, CNT-G, HLDT), the Formation MicroScanner (FMS), and the Geochemical Combination Tool (GST).Electrical resistivity (DIT) and acoustic transit-time (LSS/SDT) downhole logs from Sites 994, 995, and 997 indicate the presence of gas hydrate in the depth interval between 185 and 450 mbsf on the Blake Ridge. Electrical resistivity log calculations suggest that the gas hydrate-bearing sedimentary section on the Blake Ridge may contain between 2 and 11 percent bulk volume (vol%) gas hydrate. We have determined that the log-inferred gas hydrates and underlying free-gas accumulations on the Blake Ridge may contain as much as 57 trillion m 3 of gas.

Three wide-aperture (0-5 lan offset) seismic profiles were shot perpendicular to the strike of the Barbados Ridge accretionary complex at 16ø122q (line 465), 14ø232q (line 480), and 13ø202q (line 484) to image the deep structure of the accretionary complex and obtain seismic velocity data. Velocity analyses on full-fold common depth point (CDP) gathers were used to provide velocity/depth functions at 1 km separation along each line to map the variation in velocity with depth and distance across the accretionary complex. The sediments within the complex show consolidation increasing laterally with greater age of the complex as indicated by the increase in seismic velocity and the inferred decrease in porosity. The velocity at the base of the wedge along line 480 is -2.4 km[s at the toe. The velocity at the wedge base at -60 km from the toe, where the wedge thickness has increased from 1.8 km to -5.0 kin, has increased to greater than 3.0 km[s. However, the velocities within the leading mffrgins and within underthrust sequences in each of the three sections are low, as much as 1.0 km[s less than velocities found in undeformed sedimentary sequences of equivalent thickness. The low-velocity regions are interpreted as sediments with high porosity. Where porosity is inferred to be high, pore fluid pressures are also inferred to exceed hydrostatic and pore fluids actually support the increased lithostatic load produced by recent thrusting and thickening of the complex. The correlation between the most rapidly thickened regions and the velocity anomalies suggests that a rapid rate of loading relative to the rate of pore fluid expulsion produced these overpressured sediments. Velocities in the underthrust sequences, where the differences between loading histories of wedge sediments and the underthrust sequences are largest, are sufficiently low to produce velocity inversions. Low velocities also occur in regions adjacent to the complex where the pore fluid pressures may be elevated by the pressure produced in the adjacent wedge. West of the wedge, in the forearc basin along line 480, and near a seismically imaged mud diapir, velocities at a depth of 3.5 km below the seafloor are 0.5 km[s less than what is expected in a more normal sedimentary section. Immediately seaward of the complex, the deepest undeformed sequences have a velocity anomaly of -0.5 km[s. Velocities remain low for at least 12 km seaward of the complex, but they become closer to a more normal sediment section farther from the wedge. INTRODUCWION a redistribution of soluble chemical tracers in the wedge [Moore et al., 1988], a broad distribution of mud volcanoes on the The Barbados Ridge accretionary complex has formed at the surface of the wedge and seaward of it [Stride et al., 1982; eastern boundary of the Caribbean plate. This boundary is a Brown and Westbrook, 1988; Langseth et al., 1988], and convergent margin that is presently subducting lithosphere of the reductions of bulk porosity in accreted and underthrust sediments Atlantic oceanic floor. [Moo...

Many major geological terranes are interpreted as accretionary complexes, and there are several speculative models for their structure and mode of formation. The seismic reflection section across the Barbados Ridge complex at lat 16°12'N presented here shows, for the first time, the entire cross-sectional shape of a large accretionary wedge and its forearc basin. Atlantic oceanic crust underlies 122 km of the wedge and then passes beneath the crust of the forearc of the Caribbean plate, where it can be traced 15 km farther; it dips landward at 9°. The forearc basement dips seaward to meet the ocean crust. The maximum thickness of the wedge is about 10 km. A layer of sediments, 1 km thick, is drawn beneath the accretionary wedge on the surface of the oceanic crust, with little disturbance, for a distance of 70 km, and some sediments still appear to adhere to the ocean crust to where it passes beneath the forearc basement. It is not clear whether sediment is subducted deeper, but it appears probable. The principal resistance to landward motion of the accretionary wedge is provided by the weight of up to 6 km of forearc-basin sediments on the seaward-dipping forearc basement. Both the forearc sediments and the basement have been deformed as a consequence of the horizontal compression produced by the subduction of ocean crust.

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