PALEOMAGNETIC CONSTRAINT ON EVOLUTION OF THE SAN JUAN-NORTHWEST CASCADES THRUST SYSTEM

Paleomagnetic directions from the eastern San Juan Islands are dominantly downward and fail fold tests. This suggests remagnetization, likely during the Cretaceous normal superchron (118-83 Ma), which spans the ages of the youngest rocks sampled and of the Nanaimo Group that contains detritus from the thrust system. Directions are typically east with moderate inclinations, which is anomalous compared to the NNW and steeper direction predicted for this location in the Cretaceous from the North American apparent polar wander path. In the Cascades, the Twin Sisters dunite and a 90 Ma mylonitic fault near it share this anomalous direction. Implications of this are that the area of anomalous magnetizations extends across most of the San Juan-Northwest Cascades system, and tectonism that might account for the direction is younger than 90 Ma.

Extreme fabric development (mylonitization, pressure solution) might bias directions, but neighboring rocks that have different protoliths and significantly different anisotropies of magnetic susceptibility have similar directions. Variation of directions between areas could be accounted for by relative block rotation, but system-wide bookshelf faulting to explain the overall discrepancy is undocumented and is inconsistent with vertical orientation of P and T axes deduced from brittle structures. In the absence of a viable local explanation it is worth while entertaining the possibility that the whole system rotated together after magnetization. This seems easier to accomplish with lots of elbow room. A model that includes formation and thrust stacking of the rocks independent of North America with rotation, possibly during northward translation, after 90 Ma and before being incorporated into the North American continent might satisfy other constraints as well.