Fluid Flow within the Barbados Ridge Complex, Part II: Permeability Estimates and Numerical Simulations of Flow Velocities and Pore Pressures

Geomechanical tests of three well-characterized, clay-rich, macroscopically undeformed samples document the loading history and permeability of two late Miocene samples above the décollement zone and a late Oligocene sample below the décolle-ment zone of the Barbados accretionary prism. These samples are similar in all aspects studied. About two-thirds of each sample is clay, of which most is mixed-layer illite/smectite. Permeabilities are low, ranging from 5 × 10 -18 to 1 × 10 -19 /m 2 . All samples are underconsolidated and may have been overpressured since shortly after deposition, especially the late Miocene sample near the deformation front (Site 949). Alternatively, the apparent low consolidation state may arise from disruption of grain contacts by early deformation in the accretionary prism, thereby erasing any earlier stress history.

Section: Permeabilitysupporting

confidence: 66%

Hydrostatic consolidation tests of undeformed, clay-rich samples from the Barbados accretionary prism, Leg 156

Vrolijk¹,

Miller²

1998

“…The same impedance to horizontal flow along a 200 km path requires permeability significantly lower than I0" 12 m 2 (1 Darcy), the maximum transient permeability required for the décollement at the Barbados accretionary complex to explain differences in thermal anomalies landward and seaward of the deformation front (Fisher and Hounslow, 1990). Hydraulic impedance of I0 18 m" 1 is attained along a flow path only about 10 km if permeability is 10~1 4 m 2 , however, as modeled by Wuthrich et al (1990) for the Barbados complex.…”

mentioning

confidence: 99%

“…Mechanical modeling (Carson and Berglund, 1986) and hydrological modeling (Shi andWang, 1985,1987;Screaton et al, 1990;Wuthrich et al, 1990) require measured boundary conditions of structural, constitutive, stress, and hydrologic conditions of these active margins to refine continued efforts.To quantitatively evaluate sediment consolidation state, in-situ measurements of both pore pressure and stress are needed; however, most studies of pore pressure and in-situ stress, to date, depend on indirect evidence. The existence of abnormal pore pressures in excess of hydrostatic around accretionary wedges has been inferred both from indirect observations and theoretical modeling (von Huene and Lee, 1983).…”

Section: Introductionmentioning

confidence: 99%

Sediment Permeability at the Nankai Accretionary Prism, Site 808

Taylor¹,

Fisher²

1993

Coring at ODP Site 808 successfully recovered a sequence of accreted and subducted sediments from the toe of the Nankai convergent margin. Detailed physical properties measurements revealed a systematic downhole consolidation trend, punctuated by two main offsets: a thrust fault at 365 mbsf and a décollement near 950 mbsf. Sediment dewatering during the accretion and subduction processes are strongly influenced by the permeability of the sediments. Results of over 60 permeability measurements made on subsamples of Site 808 cores reveal coefficients of permeability ranging from I0" 5 to I0" 10 cm/s (10 to I0" 19 m 2 ) in horizontal and vertical directions. The generally low permeability of sediments at the depth of the décollement must influence the stress-strain behavior of this major structural horizon.

“…For these calculations the frictional coefficient µ in the preferentially oriented planes of weakness is assumed to be 0.8 (Byerlee, 1978) as an upper limit for tectonic stress. In-situ pore pressure measurements in accretionary wedges are rare and have shown that pore pressure can reach near-lithostatic values just above the décollement (Wuthrich et al, 1990). In-situ pore pressure measurements from Leg 134 are not available.…”

Section: Conditions Of Breakout Occurrence and Magnitude Estimationmentioning

confidence: 99%

Borehole Televiewer Data Analysis from the New Hebrides Island Arc: The State of Stress at Holes 829A and 831B

Krammer¹,

Menger²,

Chabernaud³

et al. 1994

Analysis of borehole televiewer and Formation MicroScanner data from Ocean Drilling Program (ODP) Holes 829A and 83 IB, located in the Vanuatu subduction zone, reveals that both boreholes are elongated. However, borehole televiewer data from neither borehole show the classical form of borehole breakouts (enlargements on opposite sides in an otherwise circular wellbore); rather, the data indicate smooth, varying borehole surfaces. The orientation of the borehole elongation in Hole 829A (123° ± 16°) parallels topography and structural lineaments. In Hole 83 IB the orientation of elongation (148° ± 19°) is roughly perpendicular to the direction of plate convergence (76°).Focal mechanism solutions show that in the area of investigation the overriding plate is characterized by an east-northeastwest-southwest directed compressional stress regime. This indicates, that the orientation of the minimum horizontal stress direction, where compressive shear failure occurs, is parallel to the strike of the Australia/Pacific Plate boundary. Calculation of finite element modelling also show that high compressive stresses are assumed in accretionary wedges (Bott et al., 1989).The correlation of the orientation of the elongations in both wellbores with the local and regional stress field implies that the orientation of borehole elongation results from the present day stress field. Orientation of borehole elongation of Hole 83 IB strikes parallel to the Australia/Pacific Plate boundary indicating that it is consistent with the stress field deduced from earthquake focal mechanisms. However, orientation of borehole elongation of Hole 829A is affected by the local rather than the regional stress field. Observed local tectonics and topography superimpose the regional stress field at this drill site and lead to a 43° northward rotation of the maximum horizontal stress field.Additionally, the existence of a smooth borehole surface rather than borehole breakouts implies that despite the assumed high compressive stresses, additional processes, for example decoupling along thrust faults, must exist to reduce these stresses.