Spatial variations of the décollement/protodécollement zone and their implications: A 3‐D seismic inversion study of the northern Barbados accretionary prism

Abstract

We conducted a 3‐D seismic inversion study to investigate spatial variations of physical properties of the décollement zone (DZ) and protodécollement zone (PDZ) under the northern Barbados accretionary prism. Significant spatial variations of physical properties were observed in the PDZ seaward of the thrust front from the inversion data. The density generally increases southward with a few localized low‐density patches. A lower density commonly corresponds to a thicker PDZ, suggesting that the paleomorphology may at least partially control the variations of the physical properties. Similar low‐density patches were also found in the DZ. These features may be inherited from those of the PDZ and enhanced after subduction through localized arrested consolidation. Under the prism toe, the density of the DZ increases landward. This trend may mainly result from shear‐induced consolidation of the DZ but may also be related to landward increasing tectonic loading. Significant north–south differences in density and, thus, porosity and strength of the PDZ, are observed and these differences may continue into the DZ. A stronger DZ is likely responsible for a larger prism taper observed in the southern area of the prism toe. The larger taper, thus more horizontal shortening, coupled with a thinner sediment sheet above the PDZ in the southern area, may cause a relative retreat of the thrust front and a pronounced change in strike of the sequence thrusts south of seismic Line 690. The north–south differences may ultimately have originated in the approach of a structurally higher segment of the Tiburon Rise. The Tiburon Rise affects regional morphology and, thus, it controls the sedimentation and physical properties of the PDZ. It may also control sediment accumulation above the PDZ. Therefore, the sedimentational change induced by the structural high of the Tiburon Rise, in turn, resulted in structural change of the prism in the southern area.

The structure and fault activity of the Makran accretionary prism

“…Low taper, fluid‐rich subduction zone forearcs are often inferred to have weak basal surfaces, commonly observed as high‐amplitude, negative polarity décollement reflectors in seismic data suggesting reduced bulk density, increased porosity and likely increased pore fluid pressure [e.g., Moore et al , 1995; Zhao et al , 2000; Bangs et al , 2004]. Prism fault geometry analysis of Kukowski et al [2001] suggest that the décollement in the Makran is >3 times weaker than the overlying prism sediments, however they also note that the slight observed overpressure [ Fruehn et al , 1997] may only account for ∼30% of the inferred décollement weakness, with the remainder a function of lithology or fracturing.…”

Effective stress and pore pressure in the Nankai accretionary prism off the Muroto Peninsula, southwestern Japan

“…[3] Seismic reflection data have shown that the décolle-ment characteristics of the Nankai and Barbados accretionary prisms can be recognized on the basis of their reflection polarities, seismic velocities, and other seismic attributes [e.g., Moore and Shipley, 1993;Shipley et al, 1994;Bangs et al, 1996Bangs et al, , 1999Bangs et al, , 2004Zhao et al, 2000;Tsuji et al, 2005a]. These studies using seismic data have focused mainly on the décollement reflection only in order to reveal its characteristics including pore pressure, although some studies have used seismic velocity data to predict pore pressure distributions within some accretionary wedges: the Nankai accretionary prism [Tobin and Saffer, 2003], the Barbados accretionary prism [Westbrook, 1991;Hayward et al, 2003], and the Oregon accretionary prism [Cochrane et al, 1994].…”

Northern Barbados accretionary prism: Structure, deformation, and fluid flow interpreted from 3D seismic and well‐log data

“…(2000), based on 3D seismic inversion, suggested that the southward increase in density and thus strength of the proto‐décollement zone continues into the décollement. Zhao et al. (2000) further proposed that a stronger décollement zone is responsible for a larger prism taper, thus more horizontal shortening, in the southern part of the prism toe.…”

“…On the other hand, those in the western part of the area show largely NNE‐trending lineations. Zhao et al. (2000), based on 3D seismic inversion, suggested that the southward increase in density and thus strength of the proto‐décollement zone continues into the décollement.…”

“…At the toe of the Middle America Trench offshore Mexico, subducting normal fault blocks in the oceanic crust cause a ramp in the décollement (Moore & Shipley 1988). The oceanic basement at the toe of the northern Barbados accretionary prism is characterized by structural highs with relief of about 300 m (Zhao et al. 2000), whereas the relief of the décollement appears to be very small.…”