Compaction-corrected paleomagnetic paleolatitudes for Late Cretaceous rudists along the Cretaceous California Margin:Evidence for les than 1500 km of post-Late Cretaceous offset for Baja British Columbia

摘要

The paleolatitudinal distribution of bivalve rndists has important significance for the Baja British Columbia (Baja BC) hypothesis that western Canadian superterranes from British Columbia have been displaced 3000 km since Cretaceous time. Rudists are not observed in Baja BC sedimentary rocks, yet they are common in Late Creta-ceous strata in California and Baja California, which have the same paleomagnetically determined paleolatitudes (approximately 25°N) as Baja BC rocks of Late Cretaceous age. In order to resolve this con- tradiction and to delimit more exactly the southern paleolatitudes of Baja BC, paleomagnetic inclinations corrected for the effects of burial compaction were used to determine the paleolatitudinal distribution of rndists along the California margin. Compaction-corrected paleo-magnetic data from the Peninsular Ranges and Salinia terranes in- dicate that rudists were restricted to paleolatitudes between 34° and 40°N. Evidence of coastal upwelling in the latest Cretaceous Marca Shale may explain the northern limit of the rndist distribution. These data suggest that Baja BC was no farther south than 4OoN in the Late Cretaceous, thus limiting its post-Cretaceous displacement to less than 1500 km, and that burial compaction has also affected the paleomag-netism of Nanairno Group sedimentary rocks from Vancouver Island. This result also helps resolve the conflict between paleomagnetic re-sults, which show 1500 km of post-Late Cretaceous offset between the lnsular-Coast Plutonic Complex superterrane and the Intermon-tane superterrane and geologic observations, which can allow only tens of kilometers of offset between these terranes in the Methow-Tyaughton basin.