The Dellwood knolls and their role in triple junction tectonics off northern Vancouver Island

Abstract

The Dellwood knolls are two small topographic features on the ocean floor off northern Vancouver Island. They have been proposed as a spreading centre connecting the Explorer ridge to the Queen Charlotte fault and the location of a triple junction between the Pacific, American, and Juan de Fuca plate systems.Detailed geophysical surveys and ocean-bottom seismometer deployments confirm that they are the site of active seismicity and recent volcanism. Modelling of the magnetic anomaly field shows that it is almost entirely produced by normally magnetized material, supporting geological evidence that the knolls are probably less than 1 Ma old. Although the two knolls are separated by a narrow valley with some downfaulting, they do not form a clearly linear spreading rift.Assessment of their role in the plate tectonics of the region suggests that spreading at the knolls was initiated around 1 Ma ago in crust now 4.5 Ma old as part of a complex, northwesterly ridge migration process at the northern end of the Explorer ridge. Reconstruction of this process, which involves asymmetric spreading and ridge jumping, provides an explanation for the creation of the associated Paul Revere and Winona ridges.

The Winona Basin: structure and tectonics

“…The conditions under which such interfingering may be produced are demonstated in the Dellwood Basin (see profile "A" of Riddihough et al 1980; profiles 2 and 3 of Davis 1982). There, thin basalt flows from the Dellwood Rift Valley are seen to extend over the present turbidite surface; similar flows are seen buried by subsequent sedimentation farther to the southwest.…”

“…The reflectors occur throughout the sediment section in this northwestern region down to a depth of at least 0.9 s, and this suggests coevolution of the Dellwood Knolls with the Winona Basin sediment fill back at least to the time equivalent of that stratigraphic depth. If the Knolls are less than 1 Ma old (Riddihough et al 1980) a limit for the post-uplift turbidite accumulation rate can be estimated. Using the sediment velocity established by E. E. Davis and R. M. Clowes (manuscript in preparation, 1982), a minimum average accumulation rate in the outer basin would be about 1 km ~a -' .…”

“…For this most recent period of time, the vertical component of deformation must have been small compared with the accumulation rate. If a maximum age estimate of 0.7 MaBP (Riddihough et al 1980) for the deepest reflector is used, and if it is assumed that sedimentation has been steady on average, we can estimate that deformation of this small part of the Winona Ridge began about 0.4 Ma BP and was complete by about 0.1-0.2 Ma BP.…”

“…972. and Tiffin 1974;Chase et al 1975;Kulm et al 1973; beneath the basin, the history of the basin development, Riddihough et al 1980) and some of its main features and the manner in which this history fits into the established. Among these are the facts that the basin evolution of the triple plate junction that dominates the contains a considerable amount of young (Pliocene-region.…”

Gas hydrate within the Winona Basin, offshore western Canada

“…Ridges in Winona basin are 20e95 km long, up to km 15 wide and linear, whereas thrusted ridges in Cascadia are 20e30 km long, a few km wide and often arcuate . The Winona ridge is so linear it was once considered to be the locus of the paleo-Queen Charlotte fault (Riddihough et al, 1980). In map view, fold crests can be observed along the entire Cascadia accretionary prism whereas in Winona basin a cluster of four ridges lies at the southern end of the basin and the Winona ridge alone extends northward.…”