Harold J. Tobin

Abnormal fluid pressures and fault-zone dilation in the Barbados accretionary prism: Evidence from logging while drilling

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.

Strain decoupling across the decollement of the Barbados accretionary prism

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.

Fluid pressure in the frontal thrust of the Oregon accretionary prism: Experimental constraints

Seismic reflection profiles of many accretionary prisms, including the Oregon prism, exhibit high-amplitude, reversed-polarity reflections from the decollement and other thrusts. It has been suggested that these reflectors image fault zones with enhanced fluid content due to dilation by very high fluid pressure. We present measurements of velocity as a function of effective stress on samples of the Oregon frontal thrust that show that velocity decreases by up to 15% as fluid pressure rises from hydrostatic to lithostatic conditions, under constant confining stress, in fault zone and wall rock alike. Synthetic seismic modeling shows that the frontal-thrust reflector at this location is the result of a thin low-velocity zone in the plane of the fault, 100-300 m/s slower than the walls. Combining models with experimental results, we conclude that fluid pressure of 86% to 98% of lithostatic stress reduces velocity enough to generate the reflections.