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

Abstract

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 (S u /P 0 ') 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.

Ridge subduction at an erosive margin: The collision zone of the Nazca Ridge in southern Peru

“…The important prerequisite of little sediment input to prevent massive accretion is provided by the Nazca Ridge, with its sedimentary cover not exceeding 400 m. The lack of a thick sedimentary cover contributes to seafloor roughness, which is further intensified by underthrusting of seamounts, volcanic ridges, and fault scarps. Accretive margins for example Makran, Cascadia or Nankai are underthrusted by oceanic crust buried beneath a thick sediment pile [ Moore and Biju‐Duval , 1984; Davis and Hyndman , 1989; Minshull and White , 1989; Moore et al , 1990, 1995; Kukowski et al , 2001a]. As revealed by our Peru seismic reflection data, the complete sequence of sediment on the Nazca Ridge is dragged beneath the toe of the continental slope (Figure 6a).…”

Fluid flow in accretionary prisms: Evidence for focused, time‐variable discharge

“…The faults are shear zones that exhibit evidence of large strains and fabric modification. Fabric collapse associated with fault movement and fluid escape may lead to higher density, whereas faulted zones that retain water remain underconsolidated, exhibit anomalously low density, and are overpressured [Moore' et al, 1995b]. Conversion of density determinations from borehole samples and downhole logs to fluid pressure [Moore et al, 1995c] provides indirect evidence that fluids in low-density fault zones may approach lithostatic pressure.…”

Tectonic compaction shortening in toe region of isolated listric normal fault, North Taranaki Basin, New Zealand

“…The role of compaction in deformation of poorly lithified sedimentary rocks has long been considered as a significant and often difficult to quantify shortening mechanism in addition to folding and thrusting (e.g. Cochrane et al ., ; Moore et al ., ; Evans et al ., ; Bangs & Gulick, ; Gonzalez‐Mieres & Suppe, ). In deepwater gravity‐driven systems where ideally up‐dip extension is balanced by down‐dip contraction, mismatches in shortening/extension amounts between the two structural regimes in regional restored sections have been documented (see discussions in Butler & Paton, and Morley et al ., ).…”