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

Abstract

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...

Décollement depth versus accretionary prism dimension in the Apennines and the Barbados

“…The original data consist of seismic lines (TWT or depth converted), and of geological cross sections, based on unpublished seismic data. For the Northern Barbados Ridge seismic lines 465, 480, and 484 [Bangs et al, 1990] and line 688 [Moore et al, 1995] have been interpreted and measured (Figure 4). For the Apennines we considered two seismic lines for the northern part [Pieri, 1983] and two geological cross sections for the central part of the belt [Bally et al, 1986]; for the southern sector, we used a seismic section crossing the offshore Calabrian arc and a geological section crossing Sicily [Bello et al, 2000] (Figure 4).…”

“…For the Apennines we considered two seismic lines for the northern part [Pieri, 1983] and two geological cross sections for the central part of the belt [Bally et al, 1986]; for the southern sector, we used a seismic section crossing the offshore Calabrian arc and a geological section crossing Sicily [Bello et al, 2000] (Figure 4). TWT sections were depth converted using the Bangs et al [1990] seismic velocity model (for the Barbados seismic lines 465, 480 and 484) and Catalano et al [2000] and Monaco et al [1996] velocity models for the Calabrian arc profile.…”

“…(a) Structural-geological framework of the Northern Barbados Ridge with location of the studied sections and (b) representative cross section. Sections 1B, 3B, and 4B are from Bangs et al [1990]; 2B is from Moore et al [1995].…”

Fluid flow in accretionary prisms: Evidence for focused, time‐variable discharge

“…However, several studies [Bangs et al, 1990;Lewis, 1992; Hyndman, 1995] report an initial increase in porosity (decrease in velocity) just landward of the deformation front. This, porosity increase is cited as a response to early development of a secondary fracture porosity [Lewis, 1992] or to tectonic burial of high-porosity elements more rapidly than they can consolidate and dewater [Hyndman, 1995].…”

CORK: A Hydrologic Seal and Downhole Observatory for Deep-Ocean Boreholes

“…Locally high fluid pressures are inferred to be produced when sections of normally consolidated sediments are tectonically thickened and dewatered during accretion (e.g. Moore et al, 1990;Bangs et al, 1990). If strain rates are sufficiently high and effective permeability is low, pressures approaching lithostatic may be generated which can dramatically alter the mechanical behavior of the sediment (Davis et al, 1983).…”