Seaward of the deformation front , the Cascadia basin consists of pre-Pleistocene hemipelagic sediments overlain by a rapidly deposited Pleistocene turbidite. Seismic sections image the oceanic crust dipping gently at 3-4° beneath the deformation front (Fig. 2), where the total sediment thickness is 2.53 km. The water depths are only about 2500 m due to the young age (5-7 Ma) of the oceanic lithosphere at the deformation front and the thickening sediment section. Initial deformation is accomplished by landward-dipping thrust faults and margin-parallel folds (Hyndman et al., 1994). On MCS Line 85-02 near ODP Site 889 (Fig. 2), a single frontal anticline has grown to a height of over
700 m above the basin floor. The anticline developed over a thrust ramp extending down to near the top of the oceanic crust. With similar observations on several seismic profiles, Davis and Hyndman (1989) argue that the present decollement is near the base of the sediment section and most of the sediments are scraped off the incoming oceanic crust. This contrasts with the Oregon margin where the detachment is higher in the section (MacKay et al., 1992).
As the sediment section grows rapidly in thickness farther landward, the accreted sediments undergo severe tectonic compaction and distributed small-scale deformation. From cores recovered at ODP Site 889, sediments recovered in the upper 128 m below the seafloor (mbsf) are silty clays and clayey silts interbedded with fine sand turbidites. These were interpreted as slope basin sediments (Westbrook et al., 1994). Sediments below 128 mbsf are more deformed and compacted, and were interpreted as accreted Cascadia basin sediments. On multichannel seismic sections, slope basin sediments are recognized as well-layered events (e.g., Fig. 3a, CDP 3000-2850), whereas few continuous reflections are observed within the accreted sediments.
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