The northern Cascadia subduction zone at Vancouver Island: seismic structure and tectonic history

Abstract

The structure and Tertiary tectonic history of the northern Cascadia subduction zone have been delineated by a series of new multichannel seismic lines acquired across the continental shelf to the deep sea, combined with adjacent land multichannel seismic data and results from a wide range of other geophysical and geological studies. The top of the downgoing oceanic crust is imaged for a remarkable distance downdip from the deep ocean basin to a depth of 40 km beneath Vancouver Island. The reflection depths are in good agreement with seismic refraction models and Benioff–Wadati seismicity. Two broad reflective bands imaged as dipping gently landward at depths of about 15 and 30 km on the land lines merge to a single reflector band offshore. They may represent underplated oceanic material or, alternatively, they may not be structural but may be zones of contrasting physical properties, perhaps representing trapped fluid. Two narrow terranes, the Mesozoic marine sedimentary Pacific Rim Terrane and the Eocene marine volcanic Crescent Terrane, have been thrust beneath, and accreted to, the margin in the Eocene, about 42 Ma, near the start of the present phase of subduction. They provide a landward-dipping backstop to the large sediment wedge accreted since that time. The deformation front is characterized by mainly landward-dipping thrust faults that cut close to basement. This result and the mass balance of the incoming sediment compared with that present in the accreted wedge suggest that there is little subduction of sediment into the mantle. The Tofino Basin sediments, up to 4 km in thickness, have been deposited on the continental shelf over the accreted terranes and the developing accretionary wedge.

The fate of fluids released from subducting slab in northern Cascadia

“…The regional structure of the forearc is quite well defined by seismic and other geophysical data (e.g. Hyndman et al, 1990). The depth to the Cascadia forearc Moho beneath southwestern British Columbia is ∼36 km (Cassidy and Ellis, 1993;Zhao 2001Zhao et al, 2001Ramachandran et al, 2006, Brocher et al, 2003.…”

“…The global ocean may be cycled through subduction zones about every billion years (e.g. Ingebritsen and Manning, 2002;Bounama et al, 2001;Jambon, 1994). With increasing temperature and pressure in the downdip side, dehydration reactions release water upward into the overlying mantle and crust (e.g.…”

“…With increasing temperature and pressure in the downdip side, dehydration reactions release water upward into the overlying mantle and crust (e.g. Ingebritsen and Manning, 2002;Peacock, 1990;Schmidt and Poli, 1998;Rüpke et al, 2004). For old cold subducting plates, most water is inferred to be expelled under the arc and backarc; for young and hot subduction zones, most water is concluded to be expelled upward beneath the forearc (e.g.…”

Gas hydrate on the northern Cascadia margin: regional geophysics and structural framework

“…F2). The Crescent Terrane provides the landwarddipping 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).…”

IODP Expedition 328: Early Results of Cascadia Subduction Zone ACORK Observatory

“…1, IODP Site U1364 lies 18 km landward of the toe of the Cascadia subduction zone accretionary prism, where much of the thick section of turbidite and hemipelagic sediments deposited on the eastern flank of the Juan de Fuca Ridge are scraped off the underthrusting oceanic crust (Davis and Hyndman, 1989;Hyndman et al, 1990;Westbrook et al, 1994). Convergence of the Juan de Fuca oceanic plate relative to the North American continental plate occurs in a direction roughly normal to the continental margin and at a rate of roughly 42 mm y -1 (DeMets et al, 1990).…”