Gravity and structure of an active margin—British Columbia and Washington

Abstract

Active subduction zones around the world have a gravity expression characterized by a linear negative gravity anomaly over the trench and a parallel, linear positive anomaly some 100km inland. Although there are local modifications, the same pattern is present in the Pacific northwest across the zone of interaction between the Juan de Fuca and American plates.Previous geophysical interpretations of this region have not specifically used a subduction model but have exposed an apparent conflict between seismic and gravity interpretations of the thickness of the crust under Vancouver Island. The position of Vancouver Island in the arc-trench gap of an active margin suggests that a compromise can be achieved by considering the wedge of material overlying the down-going plate to be of high density and low compressional velocity. Such materials have been documented in the laboratory and are typically amphibolite to granulite facies mafic rocks.Proceeding on this assumption, four structural sections across the margin in southern British Columbia and northern Washington show that both seismic and gravity data can be simply incorporated into models fulfilling the main criteria of a subduction zone. Among the features suggested by the construction of these sections are (1) the density of the material of the down-going slab need not increase beyond that of normal lithosphere to satisfy the gravity observations and (2) the down-going slab may increase in dip approximately beneath Georgia Strait and Puget Sound

Tomographic image of low P velocity anomalies above slab in northern Cascadia subduction zone

“…Seismic surveys conducted on Vancouver Island and the adjacent mainland have identified a number of faults that divide this region into different terranes and significant variations in crustal velocity (Zelt et al, 1993Hyndman, 1995;Clowes et al, 1996;Spence and McLean, 1999). Gravity modeling studies have provided constraints on density contrasts between different terranes (Riddihough, 1979;Clowes et al, 1997).…”

Regional Geophysics and Structural Framework of the Vancouver Island Margin Accretionary Prism

“…In the intervening region, however, the main portion of the Juan de Fuca Plate has been converging toward the northeast and underthrusting the continent since the Eocene. The present rate of convergence increases slightly northward and is about 45 mm/yr off southern Vancouver Island; the direction is approximately orthogonal to the margin off southern Vancouver Island and about 20° from orthogonal off Washington and Oregon (Riddihough, 1984;Nishimura et al, 1984;DeMets et al, 1990). The North America Plate has been advancing westward over the asthenosphere and overriding the Pacific Plate at a rate of about 20 mm/yr for at least the past 10 m.y.…”

“…The Juan de Fuca Plate is the remnant of the large Farallon Plate that has fragmented and diminished in size as a result of the convergence of the East Pacific Rise spreading center and the North American continent, and the northward movement of the southern (Mendocino) triple junction (Atwater, 1970). The northern triple junction appears to have remained stable off northern Vancouver Island since the Eocene (e.g., Riddihough, 1984;Stock and Molnar, 1988;Engebretson et al, 1984Engebretson et al, , 1985Atwater, 1989; Hynd- 3, 6, 7, and 18. man and Hamilton, 1993). The present plate configuration had its origin in a major reorganization of the northeast Pacific Plate regime in the Eocene at about 43 Ma.…”

“…The North America Plate has been advancing westward over the asthenosphere and overriding the Pacific Plate at a rate of about 20 mm/yr for at least the past 10 m.y. and probably at a similar rate throughout the Cenozoic (e.g., Riddihough, 1984;Pollitz, 1988), resulting in a relatively shallow angle of subduction.…”

“…2); (3) the characteristic pattern of low heat flow in the forearc inland from the margin, with an abrupt increase just seaward of the volcanic arc (Lewis et al, , 1991Blackwell, 1991) (Fig. 3); (4) the characteristic low-high gravity couple represented by low values over the outer margin (trench) and high values over the edge of the continent (e.g., Riddihough, 1979) (Fig. 4); and (5) an extensive accretionary sedimentary prism (discussed below).…”