Fluid accumulation and channeling along the northern Barbados Ridge decollement thrust

Abstract

Abstract. A volume of three-dimensional seismic reflection data, acquired in 1992, imaged the decollement beneath the northern Barbados Ridge accretionary prism revealing reflection amplitude and waveform variations attributed to fluid accumulations along the plate boundary fault. We model the seismic reflection by inversion for seismic impedance (the product of velocity and density) throughout the 5 x 25 km survey area and thus map physical property variations. In 1997, Ocean Drilling Program Leg 171A penetrated the protodecollement and decollement at five sites with a logging-while-drilling (LWD) tool to log density and other physical properties of the decollement. We construct a regional map of density, and inferred porosity, within the decollement from seismic models calibrated with LWD density data. In the sediments out in front of the trench the protodecollement forms in a radiolarian-rich Miocene mudstone with an anomalously high porosity (70-75%) that appears as a pervasive, inherent characteristic of this interval seaward of the deformation front. In the decollement beneath the wedge a consolidation trend of decreasing porosity runs perpendicular to the deformation front with porosity decreasing from 70% at the wedge toe to 50% 4 km from the wedge toe. A second, distinct trend also forms along a 10-km-long, 1-to 2-km-wide, NE-SW zone in which porosity is 70%, as high as it is in the protodecollement. This zone can be explained as an area of the decollement where fluid accumulations develop by maintaining high fluid content. We postulate that high fluid content is maintained by continuous recharge flowing into and along this channel. This porosity distribution within the decollement also strongly influences fluid migration into the overlying accretionary wedge and is directly associated with fluid charging of ramps and out-of-sequence thrusts above the decollement.

Evolution of the Nankai Trough décollement from the trench into the seismogenic zone: Inferences from three-dimensional seismic reflection imaging

“…Décollement reflection amplitudes are most likely due to fluids and unlikely affected by minimal lithology changes or changes in décollement stratigraphic level. (2) The dé-collement reflection has predominantly reversed seismic wave polarity, which has been explained as due to underconsolidation and overpressures within a subduction-zone décol-lement (e.g., Bangs et al, 1999). Here this condition persists to 30 km landward of the deformation front, beyond which the porosity contrast across the décollement may decrease beneath the thickening prism.…”

“…Numerous observations show that subduction-related plate-boundary faults are inherently weak (Davis et al, 1983), fluid rich (Shipley et al, 1994;Bangs et al, 1999), and aseismic (Byrne et al, 1988) along their updip parts beneath accretionary wedges. However, subduction-zone thrusts produce some of the world's largest and most devastating earthquakes, implying significant downdip transformation of properties from the trench into the seismogenic zone.…”

The Integrated Ocean Drilling Program: Utilizing New Drilling Platforms for Ocean Research

“…The first MSP effort, the Arctic Cor-FIGURE 3 Figure 3. Perspective view of a portion of the accretionary prism east of Barbados; these 3D seismic data were acquired there in 1992 (refer to Bangs et al, 1999). The back-and side-walls of the cube are seismic profiles; inline 635 is a dip profile of the accretionary prism, while cross-line 1260 is a strike profile.…”

Control of internal structure and fluid-migration pathways within the Barbados Ridge decollement zone by strike-slip faulting: Evidence from coherence and three-dimensional seismic amplitude imaging: Discussion

“…Different seismic refl ection characteristics have been associated with fl uid distribution within the décollement zone (Shipley et al, 1994;Bangs et al, 1996;Moore et al, 1998;Bangs et al, 1990Bangs et al, , 1999. The more fl uid-rich regions of the décollement are typifi ed by a composite waveform of two positive refl ections bounding an intervening high-amplitude negative refl ection (Shipley et al, 1997).…”

“…Using various seismic refl ection data volumes to investigate basement-controlled strike-slip faults below the décollement, they inferred an ENE grain defi ned by faults that propagate upward from the ocean crust but then largely become detached at the décollement. Although earlier studies identifi ed these faults in the ocean crust (Shipley et al, 1994;Bangs et al, 1996Bangs et al, , 1999Zhao et al, 2000), the combined use of amplitude (both polarity and intensity attributes) and coherency volumes by DiLeo nardo et al (2002) further elucidated these faults and their impact on the subducting sedimentary column and the décollement zone. DiLeonardo et al (2002) further suggested that this ENE-fault pattern created a component of margin-oblique fl uid fl ow.…”