Structural Setting of the Leg 156 Area, Northern Barbados Ridge Accretionary Prism

Abstract

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.

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).…”

“…[8] The Barbados Ridge complex (Figure 1) has long been identified as an accretionary complex and is well studied [e.g., Westbrook et al, 1984;Pindell et al, 1988;Pindell and Barrett, 1990;Shipley et al, 1995;Moore et al, 1995Moore et al, , 1998]. This subduction zone forms the eastern boundary of the Caribbean plate, where Atlantic oceanic lithosphere is subducted westward.…”

“…(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].…”

Site 1044

“…The four main seismic units on Line 751, identified by Moore et al (1995) are reasonably well correlated with the synthetic seismogram (Fig. 18).…”

“…Black is positive polarity; white is negative polarity. The seismic stratigraphic units shown to the right of the interpreted panel are after Moore et al (1995). itive shift in the gamma-ray and potassium logs.…”