ALBERT

Description: Deep-sea sediments deposited at or near an actively spreading mid-ocean ridge experience a distinctly different thermal history than sediments deposited in other marine geotectonic settings. For sediments of a given age the impact of high heat flow decreases with increasing distance from the ridge. This simple relation is superimposed in the active spreading ridge along the margin of Southern Chile—the Chile Triple Junction—by the process of sediment accretion. Both processes, heat flow and lateral thrusting, strongly modify the mechanical behavior of sediment accreted. The original intent was to study the mechanical state of accreted sediment recovered during Ocean Drilling Program (ODP) Leg 141, both unaltered and altered diagenetically by hydrothermal fluids, to differentiate the effects of thermal and mechanical diagenesis. The research program included high-pressure oedometer as well as triaxial testing to quantify the relative importance of both diagenetic processes for shear strength and consolidation behavior. Unfortunately, whole-round core samples available from Leg 141 appeared to be inadequately preserved and only a limited number of specimens could be used for mechanical testing. Therefore, most of the pre-cruise goals could not be attained. This paper reports the results of oedometer as well triaxial tests obtained from a restricted testing program.

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: Ocean Drilling Program (ODP) Leg 156 revisited the northern Barbados Ridge, where the previous Deep Sea Drilling Program Leg 78A and ODP Leg 110 studied the frontal part of this accretionary prism. Drilling and logging-while-drilling at Sites 947, 948, and 949 successfully identified major thrust faults and the décollement, which was the target of several downhole experiments. Two of the eight holes drilled were equipped with borehole observatories that will monitor temperature, pressure, and fluid flow over the next years. Coring at Hole 948C recovered 180 m of sediment, centered around the décollement, which was positively identified based on structural information. The aim of this study is the evaluation of the possible correlation of preferred orientation of acoustic properties and the direction of maximum compressive strain in the frontal part of the accretionary prism. For this purpose, shipboard P-wave velocities from Holes 948C and 949B were reoriented. This information was then used to compare the directional properties of accreted and subducted sediments. In Hole 948C, lowest transverse velocities (Tmin ) were observed to be consistently oriented perpendicular to the maximum horizontal compressive stress, believed to be parallel to the convergence vector. In the underthrust domain of Hole 948C, several preferred orientations for Tmin were detected, but no correlation with the geotectonic reference frame could be identified. Acoustic anisotropy does not show a comparable pattern in Hole 948C. It is concluded that the observed directional dependence of P-wave velocity in the accreted sediment domain in Hole 948B is the result of moderate to steeply inclined bedding, although this conclusion can not adequately be tested due to the lack of corrected structural data.

Description: Low-gradient flow tests and one-dimensional consolidation tests were performed on four samples from Ocean Drilling Program (ODP) Leg 156, northern Barbados Ridge, Hole 949B, to provide direct as well as indirect measures of permeability and stress history. All samples tested show a significant degree of underconsolidation, with overconsolidation ratio (OCR) values decreasing with depth from 0.4 to 0.2 to 0.1. These low ratios were used to approximate the excess pore pressures within the accreted sediment and the décollement. The largest estimated excess pore-pressure values lie within the interpreted décollement zone at Site 949 and range from 1600 to 1900 kPa. Combining results from this study with previous data from Leg 110, two generalized functions of void ratio vs. hydraulic conductivity for high and low permeability sediments are developed that are characteristic for sediments of low and high smectite content. By applying these functions to the Site 671 (ODP Leg 110) porosity profile, it is possible to demonstrate the occurrence of a zone of low permeability immediately above the décollement. This is consistent with the concept that low permeability sediments will develop higher excess pore pressures, and therefore, are more susceptible to shear failure.

Description: Borehole logs from the northern Barbados accretionary prism show that the plate-boundary decollement initiates in a low-density radiolarian claystone. With continued thrusting, the decollement zone consolidates, but in a patchy manner. The logs calibrate a three-dimensional seismic reflection image of the decollement zone and indicate which portions are of low density and enriched in fluid, and which portions have consolidated. The seismic image demonstrates that an underconsolidated patch of the decollement zone connects to a fluid-rich conduit extending down the decollement surface. Fluid migration up this conduit probably supports the open pore structure in the underconsolidated patch.

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.