Geologic interpretation of magnetic and gravity anomalies in the Strait of Juan de Fuca, U.S.–Canada

Abstract

A gravity and magnetic survey of the Strait of Juan de Fuca and adjacent Pacific continental shelf was conducted to define the tectonic framework in this 20 to 35 km wide seaway and its relation to that of Vancouver Island and the Olympic Peninsula. The offshore extensions of large onshore faults are delineated by linear magnetic and gravity anomalies. One of these, the Leech River fault of southern Vancouver Island, marks the northern limit of oceanic-type basaltic basement present in western Washington and Oregon. This fault probably continues southeast-ward from Vancouver Island across the strait to near the northeastern coast of the Olympic Peninsula, and westward across the strait to the continental shelf off Cape Flattery. The Calawah fault, which extends northwestward from near Cape Flattery onto the Pacific shelf, terminates the Leech River fault. Northwest of the Leech River fault on the shelf, the Calawah fault probably is the contact between oceanic and continental crustal types. The gravity and magnetic data also indicate the location of folds, other faults, and areas of shallow basement rocks.

Quaternary Rupture of a Crustal Fault beneath Victoria, British Columbia, Canada

“…Several previous authors suggest that this fault, which places Jurassic-Cretaceous schists of the Leech River Complex to the north against Eocene basalts of the Metchosin Formation to the south ( Fig. 2A) (Fairchild and Cowan, 1982;Rusmore and Cowan, 1985), was last active in the Eocene (MacLeod et al, 1977;Johnston and Acton, 2003). Yet, trenching, coring, and geophysical studies indicate multiple Quaternary ruptures of adjacent fault systems in Washington state, USA, including the Southern Whidbey Island fault, the Utsalady Point fault, and the DarringtonDevil's Mountain fault ( Fig.…”

“…The potential Quaternary activity of the Leech River fault, an ~60-km-long terrane-bounding fault in the southern Vancouver Island forearc (Muller, 1977;MacLeod et al, 1977), has drawn significant attention in recent years because of the seismic hazard it may pose to the nearby population of Victoria, British Columbia (Figs. 1 and 2A) (see Cassidy et al, 2000;Mosher et al, 2000;Balfour et al, 2011).…”

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

“…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). The results of petroleum exploration wells drilled into the shelf in the 1960s were described by Shouldice (1971Shouldice ( , 1973.…”

Neogene strontium isotope stratigraphy, foraminifer biostratigraphy, and lithostratigraphy from offshore wells, Queen Charlotte Basin, British Columbia, Canada

“…In Haida Gwaii and the QCB, volcanic rocks may be as old as Jurassic, and younger volcanism is represented by the Masset Formation deposited during Late Oligocene to Early Pliocene time (Anderson and Reichenbach 1991;Hickson 1991). Volcanic units in the TB are primarily known on and offshore as the Eocene-Paleocene Crescent Formation and Metchosin Igneous Complex, and as volcanic units and intrusions on Vancouver Island (MacLeod et al 1977;Muller 1980;Massey 1986;Madsen et al 2006). The significance of the volcanic units in both basins is their timing, potential (or not) for significant heating, and their possible porosity for fluid migrations.…”