G.D. Spence1, R.D. Hyndman2'1, N.R. Chapman1, M. Riedel1, N. Edwards3 and J. Yuan3
2Pacific Geoscience Centre Geological Survey of Canada Sidney, B.C., Canada V8L 4B2
Natural gas hydrate was first recognized on the Cascadia margin in 1985 through the characteristic bottom-simulating reflector (BSR) on conventional multichannel seismic data (Davis and Hyndman, 1989, Davis et al., 1990). Since then, the Cascadia accretionary margin has received the most intensive studies of any convergent margin for determination of the in-situ properties of marine gas hydrate. Key control for understanding the properties and formation processes of hydrate has been derived from drill holes of the Ocean Drilling Program (ODP) Leg 146, carried out in 1992. Estimates of hydrate concentration were provided through analysis of downhole seismic and resistivity logs and through measurement of chlorinity in pore fluids from recovered sediment core samples.
Most information on the areal and depth distribution of hydrate and on its concentration is obtained using remotely-sensed investigations, primarily geophysical. The region off Vancouver Island has been the target for an exceptionally broad range of such studies, including seismic, heat flow, seafloor electrical sounding, seafloor compliance, and physical property measurements of seafloor piston cores. The primary objective of this chapter is to review briefly the principal geophysical methods used off the northern Cascadia margin, and to provide examples of what we have learned about the broad regional distribution of hydrate and its fine vertical structure.
Although not the focus of this paper, geochemical and biological studies have contributed significantly to understanding the sources of methane in
School of Earth and Ocean Sciences University of Victoria Victoria, B.C., Canada V6T 1W5
Department of Physics University of Toronto Toronto, Ont., Canada hydrate, as well as hydrate formation and dissociation processes. Examples can be found in the work of Whiticar et al. (1995), who studied the carbon isotope signature of methane dissociated from hydrate and determined that its origin is clearly bacterial and not thermogenic. In a study to determine the nature of the bacterial source of the hydrate, Cragg et al. (1995) found significantly increased bacterial populations and activities in the 10 m interval above the base of hydrate at Sites 889/890.
Seafloor hydrate has been observed at sites off Oregon, particularly in the region known as Hydrate Ridge. In ODP drilling, solid macrocrystalline hydrate was recovered from near the seafloor (2-19 mbsf) at Site 892. It is associated with seafloor venting, or "cold seeps", along with related features such as pockmarks, carbonate pavement and unique biological communities at the vent sites. Over the past 15 years, the region has been the focus of extensive sampling and coring programs, submersible investigations and geochemical analyses. For a review of recent exciting results, the reader is referred to Suess et al. (1999a, 1999b). For information on geophysical investigations on the Cascadia margin off Oregon, see MacKay et al. (1994), Trehu et al. (1995, 1999) and Goldfinger et al. (1999).
2. TECTONIC AND GEOLOGICAL SETTING
The oceanic Juan de Fuca plate has been subducting beneath the North American plate since the Eocene, approximately normal to the Vancouver
Islandmargin at a current rate of 45 mm/yr (Fig. 1). This is also the time of emplacement of the Eocene marine volcanic Crescent terrane (and the equivalent Siletz terrane to the south). The Crescent terrane outcrops on southern Vancouver Island and on the Olympic Peninsula. Magnetic, gravity and multichannel seismic data (Hyndman et al., 1990; Dehler and Clowes, 1992) indicate that the Crescent terrane extends northwest of the Olympic Peninsula along the central portion of the continental shelf off Vancouver Island, that it dips landward, and that it acts as the backstop to the accretionary wedge formed by sediments scraped off the underthrust oceanic plate.
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