23. Frontal Thrust, Oregon Accretionary Prism: Geometry, Physical Properties, and Fluid Pressure

The frontal thrust of the Oregon accretionary prism scrapes off about 2 km of incoming sedimentary section and underlies a fault-bend fold of the first ridge landward of the abyssal plain. Scarps with associated chemosynthetic biological communities at the surface and pore-water anomalies at depth indicate that the frontal thrust is tectonically and hydrologically active. At Site 891, near seismic line OR-5, the thrust splits into at least three splays from 375 to 500 mbsf. The upper splay is defined in the cores by strongly developed scaly fabric and is mostly positive polarity on seismic reflection lines. The middle splay shows low to moderate deformation in the cores. Drilling did not penetrate the lower splay. The middle and lower splays are mostly negative polarity on the seismic reflection data. At Ocean Drilling Program (ODP) Site 891 the upper splay is marked by increased velocity and density on borehole logs, which explains its positive polarity reflection. In core samples the middle splay is associated with a velocity and density decrease about 10 m thick that explains its negative polarity reflection. Four techniques were used to estimate fluid pressure from the anomalously porous interval associated with the middle splay of the frontal thrust: (1) comparison of porosity and effective overburden to an undeformed reference section in Cascadia Basin; (2) comparison of the porosity-depth profile over this interval with the normally consolidated porosity-depth function determined from one-dimensional consolidation tests on recovered samples; (3) comparison of the undrained shear strength/ overburden pressure ratio (Su/P0') with the same ratio for normally consolidated clays; and (4) direct measurement of sediment stress history from consolidation tests. These four methods result in a range of fluid pressure ratios (λ) from 0.88 to 0.99 that indicates nearly lithostatic fluid pressures associated with the middle splay of the frontal thrust. Overall the frontal thrust at Site 891 evolves from a fluid-rich, high-fluid-pressure state, as exemplified by the middle and lower splays, to the dense, collapsed mature state of the upper splay. The frontal thrust is thus undergoing "in sequence" or seaward migration into the footwall, associated with dewatering and densification. Rapid burial of the highly porous sediment associated with the middle splay preserved near-surface effective stresses by high fluid pressures, which were perhaps temporarily sealed by the low transverse permeability of the enveloping fault strands.