168 Preliminary Report

Geological Setting

The Juan de Fuca Ridge is a seafloor-spreading center several hundred kilometers off the coast of North America (Fig. 1). It is creating the crust and lithosphere of the Juan de Fuca plate at a rate of about 29 mm/yr. The topographic relief of the ridge produces a barrier to terrigenous turbidite sediment supplied from Pleistocene glacial sources along the continental margin, primarily at Queen Charlotte Sound, Juan de Fuca Strait, and the Grays Harbor and Columbia River estuaries. This situation has resulted in the accumulation of an onlapping layer of sediment that buries the eastern flank of the Juan de Fuca Ridge. This sedimented region known as the Cascadia Basin, extends from the base of the continental margin, where accretion of the sediment entering the Cascadia subduction zone begins, to within a few tens of kilometers of the ridge crest, where sediment laps onto crust that is younger than 1 Ma at some locations (Fig. 2). Along the deformation front of the northern Cascadia accretionary prism the sediment layer is more than 3 km thick; in the northern part of the basin, the fill is sufficient to completely bury most of the relief of the igneous crust of the Juan de Fuca plate. The local basement relief beneath the nearly continuous, flat-lying sediment cover is dominated by linear ridges and troughs that were produced by normal faulting and variations in volcanic supply at the time the crust was created. The amplitude of this relief varies across the basin, from less than 100 m in places to over 700 m in others with the exception of only a few isolated volcanic cones and seamounts (see Leg 168 Scientific Prospectus). A few of these penetrate the sediment section.

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