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