GRAVITY AND STRUCTURE OF AN ACTIVE MARGIN - BRITISH-COLUMBIA AND WASHINGTON

Active subduction zone around the world have a gravity expression characterized by a linear negative gravity anomaly over the trench and a parallel, linear positive anomaly some 100 km 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. 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 downgoing 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.