NordCalifornie

West Canada Offshore

Vancouver Island Offshore

Between the Permian and Miocene several plate geological terranes accreted to one another and to the North American margin between Permian and Miocene time. In fact three accreted terranes : (i) Wrangellia ; (ii) Pacific Rim and (iii) Crescent are recognized in Vancouver Island an its offshore. The Wrangellia terrane, the largest one, is formed of rocks ranging from Devonian to Jurassic age. The Pacific Rim terrane, displaced in the latest Cretaceous to early Tertiary, is bounded to the northeast by the Westcoast Fault that seems to to have been the loci of major, pre-Eocene displacement, is interpreted, often , as an olistostrome melange produced by submarine-slumping that was deposited on an arc basement. The Crescent terrane, probably originated as a series of seamounts formed above a hotspot, is a Paleocene to Early Eocene oceanic assemblage of basalt flows, breccia, tuff and volcanic sandstones cut by gabbro and diabase intrusions. (https://pubs.usgs.gov/of/1999/of99-422/norpac/data/terranes/nor_pac/explanat/terranes/PR.txt) .

On this sector of the Cascadia subduction zone, which extends from northern Vancouver to Cape Mendocino in northern California, the overriding plate, i.e., the western part of the North America lithospheric plate, seems to be formed by an accretionary prism with forearc sedimentary basins overlying an hypothetical oceanic crust, while the plunging plate is, here, the oceanic Explorer plate (see location map). Notice the Cascadia subduction zone separates the plunging Pacific oceanic crust (Juan de Fuca, Explorer and Gorda plates) from the continental North American Plate. In fact, according the USGS, the oceanic crust of the Pacific Ocean has been sinking beneath the continent for about 200 Ma and presently at a rate of 40 mm/y. The pop-up structure, which corresponds to a deformed zone characterized by thrust- and backthrust-related anticlines, developed in the hanging-wall of frontal thrust sheet of the accretionary prism is known as Halda ridge.