A seismic reflection profile across the Cascadia subduction zone offshore central Oregon: New constraints on methane distribution and crystal structure

Copyrighted by American Geophysical Union.In 1989, we conducted an onshore/offshore seismic experiment to image the crustal structure of the Cascadia forearc. In this paper, we discuss the processing and interpretation of a multichannel seismic reflection profile across the continental margin that was collected as part of this effort. This profile reveals several features of the forearc that were not apparent in an earlier, coincident relection profile. One of the most important of these features is a very strong bottom simulating reflection (BSR) beneath the midslope region that is nearly continuous from water depths of about 1500 m to 600 m, where it appears to crop out on the seafloor. The pressure and temperature conditions at the BSR derived from our observations are remarkably consistent with the experimentally determined phase diagram for a methane hydrate/seawater system over a broad range of temperatures and pressures, assuming hydrostatic pressure and temperature gradiant measured near the base of the continental slope during Ocean Drilling Program (ODP) leg 146. Interval velocities and reflection coefficients derived from the data indicate that the BSR represents a contrast between sediment with a small amount of hydrate overlying sediment containing free gas, consistent with results obtained during leg 146. Although the regional distribution of the anomalously strong BSR beneath the midslope is poorly known, we speculate that it may be related to apparent slop instability. The data also provide constraints on the thickness and geometry of the Siletz terrane, which is the basement beneath the shelf and acts as the subduction zone backstop. A deep reflection, which might mistakenly be interpreted to be Moho if coincident large-aperture data were not available, is interpreted to be the base of the Siletz terrane. A "recently" active strike-slip (?) fault zone that overlies the seaward edge of the Siletz terrane suggests that the Siletz terrane controls the location of decoupling of the subduction complex from the rest of the forearc