Tectonic rotations and translations in western Canada; new evidence from Jurassic rocks of Vancouver Island

Abstract

The Lower Jurassic Bonanza Group (Sinemurian and Pliensbachian lavas and tuffs about 205 to 195 Ma) on northwestern Vancouver Island possesses a stable remanent magnetization with a mean direction (D, I) of 276°, 42° (13 sites distributed through 1800 m of section, 128 specimens, kB= 50, α95= 6°) palaeopole 22° N, 154° E (dm, dp= 9°, 6°). Reversals are present, but one polarity is dominant and most directions are westerly. The palaeolatitude is 24 ± 6°, with the usual ambiguity as to whether it is north or south of the palaeoequator. To determine the relative motions between ancestral North America and Vancouver Island, these data are compared with determinations from peri‐Atlantic igneous rocks dated radiometrically in the same time interval. The use of model apparent polar wander paths for this purpose is criticized. A net, post‐Jurassic northerly motion of 9 ± 7° or 57 ± 7° is indicated, depending on the chosen hemisphere of origin. The net post‐Jurassic rotations, relative to cratonic North America, are 66 ± 7° anticlockwise or 114 ± 7° clockwise for the northern and southern hemispheres, respectively. Such large rotations are, apparently, a common feature of early Mesozoic rocks of the Canadian Cordillera. It is suggested that these large and variable rotations characterize the deformation occurring during the pre‐mid‐Cretaceous amalgamation of the various terranes of British Columbia. Copyright © 1987, Wiley Blackwell. All rights reserved