Paleomagnetism of the Jurassic Island Intrusions of Vancouver Island, British Columbia

Granodioritic batholiths of the mid-Jurassic (15 9 ±. 10 m. y.) Island Intrusions were sampled at 24 sites (218 specimens) along the length of Vancouver Island. After AF demagnetization and statistical consistency tests, 1 7 sites were judged to have stable primary TRM with 16 sites reflecting a normal mid-Jurassic paleomagnetic field and one site a reversed field. Use of the fold test indicates that the broad Island-spanning anticlinorium found in the Triassic Karmutsen basalts was formed prior to cooling of the Island Intrusions. The calculated geomagnetic pole position for the Island Intrusions of 79°N, 240°E (10°, 11°)is consistent with positions for other North American Jurassic rock formations indicating either no post-intrusion movement of Vancouver Island relative to the North American continental block or at most very little movement. If minor relative movement has occurred it can best be explained by a few degrees (""1. 5° ) of clockwise rotation of Vancouver Island either during the mid-Cretaceous Nevadan Orogeny or during the later Tertiary through sea-floor spreading off the adjacent Fast Pacific Rise, and by a few degrees (~5°) of the northeastward tilt of panels of rock between block faults during the Tertiary.