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doi:10.2204/iodp.proc.311.109.2006

General plate tectonic regime and structure of accretionary prism

Comprehensive summaries of the regional geological framework and tectonics of the northern Cascadia margin were previously given by Hyndman et al. (1994) and Hyndman (1995). Extensive reference lists therein provide further detailed information. In this paper we describe only the most significant elements as they relate to the Expedition 311 objectives.

The Juan de Fuca subduction zone, with its large accretionary prism, is part of the mainly right-lateral transform boundary of the continental margin of western North America, which includes the San Andreas fault system extending from the Gulf of California to Cape Mendocino off northern California and the Queen Charlotte fault system extending from just north of Vancouver Island to the Aleutian Trench off Alaska (Fig. F1). The present Juan de Fuca plate configuration had its origin in a major reorganization of the northeast Pacific plate regime in the Eocene, ~43 m.y. ago. Since this time, convergence has been continuous and approximately orthogonal to the coast along the northern portion of the Cascadia margin. The present rate of convergence is ~46 mm/y off southern Vancouver Island (Riddihough, 1984). Two narrow exotic terranes were emplaced against the Cascadia margin at ~43 Ma: the Eocene marine volcanic Crescent Terrane and the Mesozoic mainly marine sedimentary Pacific Rim Terrane (Fig. F2). The Crescent Terrane provides the landward-dipping backstop to the accretionary prism that has accumulated since the Eocene (e.g., Davis and Hyndman, 1989; Hyndman et al., 1990). The Tofino Basin, containing as much as 4 km of gently deformed Eocene and Holocene sediments, was deposited over the accretionary wedge and the two accreted terranes beneath the continental shelf of southern Vancouver Island (e.g., Tiffin et al., 1972; MacLeod et al., 1977; Dehler and Clowes, 1992). The results of petroleum exploration wells drilled into the shelf in the 1960s were described by Shouldice (1971, 1973).

In 1985, widely spaced marine MCS lines were acquired across the continental shelf and slope off Vancouver Island as part of the Frontier Geoscience program of the Geological Survey of Canada. As part of the site survey for Leg 146, additional MCS data were acquired in 1989, out of which Lines 89-07, 89-08, 89-10, and 89-11 have direct relevance to the drilling transect of Expedition 311 (Fig. F3). Site survey seismic data especially acquired for Expedition 311 and a review of the older seismic data are given in the report by Scherwath et al. (this volume).

The incoming 2.3–3.0 km thick sediment section on oceanic crust of the Juan de Fuca plate consists of an upper 1.5–2 km of acoustically layered turbidite sediment interval overlying a section of more seismically transparent sediments, interpreted to be pre-Pleistocene hemipelagic sediments deposited on the flanks of the Juan de Fuca Ridge (e.g., Hayes and Ewing, 1970; Davis and Hyndman, 1989). No significant bathymetric trench is present adjacent to the Cascadia accretionary prism. At the deformation front, thrusts reach close to the top of the oceanic crust and the sediments of the Cascadia Basin are almost completely scraped off the downward-moving oceanic crust. The sediments are folded and faulted into anticlinal ridges that stand as high as 700 m above the adjacent seafloor. Individual ridges are typically 20–30 km in length, a few kilometers in width, and generally parallel to the margin. The internal structure of the ridges is continuous along strike, but their shape is usually asymmetric with the steep flank facing seaward (e.g., Davis and Hyndman, 1989; Rohr, 1987; Hyndman et al., 1994).

An important process in the development of accretionary prisms is load-induced tectonic consolidation and pore fluid expulsion. There is a progressive transformation, both with depth and landward, of the incoming high-porosity unconsolidated sediments into low-porosity sedimentary rock. The consolidation mainly involves loading-induced physical compaction and porosity loss. Deformation-induced changes in the grain-scale sediment structure, grain cementation processes, and chemical clay-mineral changes resulting from heating are also important. The primary data to define pore fluid expulsion associated with sediment prism accretion are seismic velocities from MCS data (e.g., Westbrook, 1991; Moore and Vrolijk, 1992; Yuan et al., 1994). Detailed velocity/porosity cross sections across the accretionary prism were constructed along seismic Lines 89-04 and 89-07 by Yuan et al. (1994). To a first order, the velocity changes are closely associated with porosity changes. For both Cascadia Basin and the accretionary prism, inferred porosity decreases approximately exponentially with depth, as expected for sediment loading consolidation in a relatively homogeneous section. Yuan et al. (1994) found that in areas where the prism has thickened substantially, the porosities are much higher at any given depth than beneath the Cascadia Basin. This landward porosity increase and underconsolidation are inferred to be a consequence of horizontal shortening and vertical stretching of the porosity-depth profile. Sediment elements do not increase in porosity; rather, they simply move to greater depths where the porosity would normally be lower. The tectonic thickening occurs faster than the loading and pore fluid expulsion can reestablish a normally consolidated depth section.

In summary, the sediment accretion process can be represented by a combination of two processes:

1. Bulk shortening and vertical thickening of the section and
2. Thrust faulting that emplaces high-porosity surface sediments at greater depth.

The subsequent process of reestablishing the porosity-depth profile by consolidation and fluid expulsion has large implication for the formation of gas hydrate in accretionary prisms, as outlined in greater detail below.

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