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

The paleolatitudinal distribution of bivalve rudists has important signific ance for the Baja British Columbia (Baja BC) hypothesis that western Canadi an superterranes from British Columbia have been displaced 3000 km since Cr etaceous time. Rudists, are not observed in Baja BC sedimentary rocks, yet they are common in Late Cretaceous strata in California and Baja California , which have the same paleomagnetically determined paleolatitudes (approxim ately 25 degreesN) as Baja BC rocks of Late Cretaceous age. In order to res olve this contradiction and to delimit more exactly the southern paleolatit udes of Baja BC, paleomagnetic inclinations corrected for the effects of bu rial compaction were used to determine the paleolatitudinal distribution of rudists along the California margin. Compaction-corrected paleomagnetic da ta from the Peninsular Ranges and Salinia terranes indicate that rudists we re restricted to paleolatitudes between 34 degrees and 40 degreesN. Evidenc e of coastal upwelling in the latest Cretaceous Marca Shale may explain the northern limit of the rudist distribution. These data suggest that Baja BC was no farther south than 40 degreesN in the Late Cretaceous, thus limitin g its post-Cretaceous displacement to less than 1500 km, and that burial co mpaction has also affected the paleomagnetism of Nanaimo Group sedimentary rocks from Vancouver Island. This result also helps resolve the conflict be tween paleomagnetic results, which show 1500 km of post-Late Cretaceous off set between the Insular-Coast Plutonic Complex superterrane and the Intermo ntane superterrane and geologic observations, which can allow only tens of kilometers of offset between these terranes in the Methow-Tyaughton basin.