Paleomagnetism of the Upper Cretaceous Nanaimo Group, southwestern Canadian Cordillera

The Baja B.C. model has the Insular Superterrane and related entities of th e Canadian Cordillera subject to >3000 km of northward displacement with re spect to cratonic North America from similar to 90 to similar to 50 Ma. The Upper Cretaceous Nanaimo Group (on and about Vancouver Island, British Col umbia) is a prime target to test the model paleomagnetically because of its locality and age. We have widely sampled the basin (67 sites from seven is lands spread over 150 km, Santonian to Maastrichtian age). Most samples hav e low unblocking temperatures (<450<degrees>C) and coercivities (similar to 10 mT) and strong present-field contamination, forcing us to reject three quarters of the collection. Beds are insufficiently tilted to provide a con clusive fold test, and we see evidence of relative vertical axis rotations. However, inclination-only analysis indicates pretilting remanence is prese rved for many samples. Both polarities are observed, and reversals correlat e well to paleontological data, proving that primary remanence is observed. The mean inclination, 55 +/- 3 degrees, is 13 +/- 4 degrees steeper than p reviously published results. Our new paleolatitude, 35.7 +/- 2.6 degrees is identical to that determined from the slightly older Silverquick and Powel l Creek formations at Mount Tatlow, yet the inferred displacement is smalle r (2300 +/- 400 km versus 3000 +/- 500 km) because North America was drifti ng southward starting around 90 Ma. The interpreted paleolatitude conflicts with sedimentologic and paleontologic evidence that the Nanaimo Basin was deposited near its present northern position.