ALBERT

Description: Logs collected while drilling measured density in situ, through the accretionary prism and decollement zone of the northern Barbados Ridge. Consolidation tests relate void ratio (derived from density) to effective stress and predict a fluid pressure profile, assuming that the upper 100 m of the prism is at a hydrostatic pressure gradient. The calculated fluid pressure curve rises to 〉90% of lithostatic below thrusts in the prism, presumably due to the increase in overburden and lateral tectonic loading. Thin (0.5–2.0 m) intervals of anomalously low density and resistivity in the logs through the basal decollement zone suggest dilation and perhaps hydrofracturing. A peak in hydraulic head in the upper half of the decollement zone requires lateral influx of fluid, a conclusion consistent with previous geochemical studies. Although the calculated fluid-pressure profile is model dependent, its inherent character ties to major structural features.

Description: Structural, physical property, and magnetic data from Ocean Drilling Program Site 808 in the Nankai Trough, Japan, indicate that both sediment loading and plate convergence have driven dewatering and consolidation in this region. Evidence for this interpretation is provided by a migrated seismic reflection profile, computed tomography of core-scale structures, magnetic susceptibility and P-wave velocity data, and experimental studies of Site 808 samples. These data also show that the strain (including volume change) resulting from plate convergence is partitioned into both penetrative deformation structures as well as more discrete, core-scale structures with finite displacements. These core-scale structures range from relatively subtle, kink-like deflections of the primary phyllosilicate fabric to sharp discontinuities with probable displacements much greater than the dimensions of the core barrel. Although all of the structures acted at least in part as dewatering conduits, evidence of concentrated fluid flow in this region of the prism is limited to a narrow interval almost 150 m above the décollement (located at between 946 and 965 mbsf). This interval correlates with the middle of a hemipelagic sequence above the décollement that appears to have thinned, apparently through dewatering, relative to a more seaward section. Thinning and dewatering appear to have been induced by deposition of a more clastic sedimentary sequence (the outer marginal trench-wedge sediments) that grades upward into a coarse-grained trench-fill sequence. Importantly, the hemipelagic sequence below the décollement appears to have thinned very little, suggesting that these sediments are underconsolidated and overpressured. This interpretation is consistent with porosity measurements from below the décollement. Microscopic and submicroscopic studies of sediments from within the décollement record a cyclic deformation sequence of displacement-brecciation-porosity collapse and compaction that may also reflect deformation of an overpressured sequence. Finally, the structural, physical property, and magnetic data also yield kinematic and geometric results consistent with the present convergent vector between the Philippine Sea Plate and Eurasia. These data indicate a shortening direction that trends between 308° and 315°, consistent with plate convergent vectors that trend between 310° and 314°.

Description: Logs collected while drilling measured density in situ, through the accretionary prism and decollement zone of the northern Barbados Ridge. Consolidation tests relate void ratio (derived from density) to effective stress and predict a fluid pressure profile, assuming that the upper 100 m of the prism is at a hydrostatic pressure gradient. The calculated fluid pressure curve rises to 〉90% of lithostatic below thrusts in the prism, presumably due to the increase in overburden and lateral tectonic loading. Thin (0.5–2.0 m) intervals of anomalously low density and resistivity in the logs through the basal decollement zone suggest dilation and perhaps hydrofracturing. A peak in hydraulic head in the upper half of the decollement zone requires lateral influx of fluid, a conclusion consistent with previous geochemical studies. Although the calculated fluid-pressure profile is model dependent, its inherent character ties to major structural features.

Description: The interrelation between deformation styles and behavior of fluids in accretionary prisms is under debate, particularly the possibility that overpressuring within the basal decollement may enable mechanical decoupling of the prism from the subducting material. Anisotropy of magnetic susceptibility (AMS) data from sediments spanning the basal decollement of the Barbados accretionary prism show a striking progression across this structure that strongly supports the hypothesis that it is markedly overpressured. In the accretionary prism, above the decollement, the minimum AMS axes are subhorizontal and oriented nearly east-west, whereas the maximum AMS axes are oriented nearly north-south and shallowly inclined. At the top of the decollement, the minimum AMS axes orientations abruptly change to nearly vertical; this orientation is maintained throughout the decollement and in the underthrust sediments below. The AMS orientations in the prism sediments above the decollement are consistent with lateral shortening due to regional tectonic stress, as the minimum axes generally parallel the convergence vector of the subducting South American plate and the maximum axes are trench-parallel. Because the orientations of the AMS axes in deformed sediments usually parallel the orientations of the principal strains, the AMS results indicate that the incremental strain state in the Barbados prism is one dominated by subhorizontal shortening. In contrast, the AMS axes within and below the decollement are consistent with a strain state dominated by vertical shortening (compaction). This abrupt change in AMS orientations at the top of the decollement at Site 948 is a direct manifestation of mechanical decoupling of the off-scraped prism sediments from the underthrust sediments. The decoupling horizon occurs at the top of the decollement zone, coinciding with the location of flowing, high-pressure fluids.

Description: Geological sediments tend to strengthen during progressive burial but the interplay of porosity and permeability, strain and effective stress gives rise to numerous circumstances in which the strength increase can be temporarily reversed. The sediment becomes capable of bulk move- ment - sediment mobilization. Most explanations involve overpressuring, which results from addi- tional loading being sustained by pore-fluid that is unable to dissipate adequately, leading to frictional strength reduction. The processes are highly heterogeneous, areally and with depth. The loads can be external ('dynamic') and both monotonic (e.g. a rapidly added suprajacent mass) and cyclic (e.g. the passage of waves), internal (e.g. the result of mineral reactions) and hydraulic (e.g. injection of external fluid). The sediments may become liquidized- that is, lose strength completely and behave as a fluid - through temporary fabric collapse (sensitive sediments) because loads are borne entirely by the pore-fluid (liquefaction), or by the grains becoming buoyant (fluidization), typically due to the ingress of externally derived fluids. In response to hydraulic gradients, buoy- ancy forces and reversed viscosity or density gradients, the weakened sediment may undergo bulk movement, though this requires failure of the enclosing material and sustained gradients. Mobilized but non-liquidized sediments retain some residual strength but can attain large shear displacements under critical state conditions.

Description: In situ and laboratory studies of permeability, conducted by Ocean Drilling Program scientists from Leg 156, provide constraints on parameters controlling the hydrogeologic system in the Barbados accretionary prism. Results from these studies indicate that core-scale and formation-scale permeability values differ by at least several orders of magnitude and are dependent on pore-fluid pressure and effective stress conditions. Direct measurement from packer experiments and indirect evidence from consolidation tests suggest that pore-fluid pressures are commonly above hydrostatic values and approach lithostatic values within the décollement zone. Permeability and fluid pressure conditions in the Barbados accretionary prism reflect the complexity of the hydrogeologic system of such an active tectonic environment.