Seismic velocities from the Barbados Ridge Complex: Indicators of high pore fluid pressures in an accretionary complex

Abstract

Three wide‐aperture (0–5 km offset) seismic profiles were shot perpendicular to the strike of the Barbados Ridge accretionary complex at 16°12′N (line 465), 14°23′N (line 480), and 13°20′N (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 km, has increased to greater than 3.0 km/s. However, the velocities within the leading margins and within undeithrust 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.