The Winona Basin: structure and tectonics

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 buried 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 accomplish this is about 70 × 106 Mg year−1. 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 × 10−5 m s−2 (−130 mGal)) and by the tilting of sediment layers at depth, and convergence, indicated by folding of sediments throughout the basin fill, appear to be continuing at the present time. From the timing of various events associated with the formation of the basin, we conclude that the recent reorganization of spreading and the recent relocation of the Pacific–Explorer–America triple junction have occurred in response to the demands of local small plate motions that are controlled by the interaction of the small plates with the continental margin.