Analysis of the magnetic anomalies of the Juan de Fuca plate system allows instantaneous poles of rotation relative to the Pacific plate to be calculated from 7 Ma to the present. By combining these with global solutions for Pacific/America and “absolute” (relative to hot spot) motions, a plate motion sequence can be constructed. This sequence shows that both absolute motions and motions relative to America are characterized by slower velocities where younger and more buoyant material enters the convergence zone: “pivoting subduction.” The resistance provided by the youngest portion of the Juan de Fuca plate apparently resulted in its detachment at 4 Ma as the independent Explorer plate. In relation to the hot spot framework, this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased. After 4 Ma the remainder of the Juan de Fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south. Differences in seismicity and recent uplift between northern and southern Vancouver Island may reflect a distinction in tectonic style between the “normal” subduction of the Juan de Fuca plate to the south and a complex “underplating” occurring as the Explorer plate is overridden by the continent. The history of the Explorer plate may exemplify the conditions under which the self‐driving forces of small subducting plates are overcome by the influence of larger, adjacent plates. The recent rapid migration of the absolute pole of rotation of the Juan de Fuca plate toward the plate suggests that it, too, may be nearing this condition.

Detailed re-examination of existing magnetic anomaly data reveals the fine structure of variations in spreading rates and directions at the Juan de Fuca and Explorer Ridges during the last 10 million years. A geometrical model using these variations delineates the theoretical history of the interactions between the lithospheric plates involved. These interactions demonstrate the independent movement of the Juan de Fuca and Explorer plates and the development of the Sovanco Fracture Zone. The latter was apparently initiated E–W at 7 Ma, rotated clockwise to 120° and may have been the site of up to 50 km of crustal shortening. The model demonstrates that subduction rates at the Canadian continental margin declined from 5 cm/yr to a present 1.5 cm/yr and that recent relative movements are compatible with the N–S compression observed from earthquakes. It also suggests that the existence of both E–W and NE trending faults in the downgoing lithosphere beneath Vancouver Island, shows that a triple junction remained static near the northern end of Vancouver Island from 10–4 Ma, and predicts a buried northern edge of subducted material striking NE in this area.

A compilation of published and new geophysical data from the Winona Basin off northern Vancouver Island has allowed a detailed interpretation of the sedimentary and tectonic history of the region to be made. The basin is forming as a result of the asymmetric subsidence of a recently isolated lithospheric block that is slowly converging with the continental margin. The crust beneath the basin is young (1 -5 Ma, increasing in age from southeast to northwest) and of normal oceanic thickness. It is virtually non-magnetic, however, probably because of its having been rapidly buried1 by turbidite sedimentation. Subsidence of the basin and uplift of the Paul Revere Ridge began in the Early Pleistocene (ca. 1 ..8 Ma) and, since that time, up to 8 km of turbidite sediments has accumulated in the basin. The nature of the fanning of the deposits suggests that the basin has been kept full throughout its history; the minimum average supply rate necessary to ac:complish this is about 70 x 106Mg year-'. This Pleistocene average is considerably greater than the present discharge rates of any of the major rivers in the area. Subsidence, indicated by the large gravity anomaly over the basin (-130 x lo-' m s-2 (-130 mGal)) and by the tilting of sediment layers at depth, and convergence, indicated by folding of sediments throughout the blasin fill, appear to be continuing at the present time. From the timing of various events associated with the formation of the b,asin, we conclude that the recent reorganization of spreading and the recent relocation of the Pacific-Explorer-America triple jlunction have occurred in response to the demands of local small plate motions that are controlled by the interaction of the small1 plates with the continental margin.Une compilation de donnees nouvelles ou dejh publiks de geophysique sur le Bassin Winona au nord de 1'Ile Vancouver a permis de formuler une interpretation trapnt 1'Bvolution ddimentaire et tectonique de la region. Le bassin s'est form6 grLe h une subsidence asymetrique d'un bloc lithospherique recemment isole lequel converge lentement vers la marge continentale. La crohte terrestre sous le bassin est jeune (1-5 Ma, l'hge croissant du sud-estvers le nord-ouest) et elleest d'une Bpaisseur oceanique normale. Elle semble etre non-magnetique, neanmoins, probablement parce: qu'elle fut rapidement recouverte par des turbidites. La subsidence du bassin et le soul&vement de la Paul Revere Ridge ont comnnence h se manifester dhs le debut du PlBistoc&ne (ca.1,8 Ma) et depuis ce temps il s'est accumul6 dans le bassin jusqu'h 8 km de turbidites. La nature des c6nes de dejection des depots sugghre que le bassin fut constamment bien rempli dwant toute son evolution historique; la quantitk minimum moyenne de s6diments apportes au bassin pour le maintenir comble est environ 70 x 1 0 6~g annee-'. Cette valeur moyenne durant le Pleistoc&ne depasse consid6rablement les taux de dtcharge actuellemen~t observes dans n'importe laquelle des principales rivi&res de la dgion. La subsidence est revel& par une ...

Summary. Relative motion across a boundary between the main Juan de Fuca plate and its northern extension, the Explorer plate, had earlier been suggested from sea‐floor magnetic anomaly analysis and from earthquakes recorded on the western Canada land seismic network. The location of the boundary, called the Nootka fault zone, and the motion across it have been examined through seismic reflection profiles, accurate location of earthquakes with an array of ocean bottom seismometers and through analysis of magnetic, gravity and bathymetric data. The fault zone extends from a ridge‐fault—fault triple point at the northern end of the Juan de Fuca ridge to a fault—trench—trench triple junction at the margin off north‐central Vancouver Island. The active portion of the fault zone is about 20 km wide, and has produced extensive disturbance in the 0.5 to 1 km of overlying sediments. Magnetic anomaly analysis suggests present left‐lateral strike slip motion of about 3 cm/yr, with convergence at the margin being more rapid to the south than to the north of the fault zone. Because of rapidly changing spreading parameters on the Explorer and Juan de Fuca ridges over the past 5 Myr the Nootka fault zone has had a very complex history.

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