Paleomagnetic evidence for poleward transport of Upper Jurassic Rocks in the Decatur terrane, San Juan Islands, Washington

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Abstract
The San Juan Islands are underlain by a west‐vergent Late Cretaceous thrust and nappe system, part of a larger thrust belt exposed in the northwest Cascades of Washington and the southwestern Coast Mountains of British Columbia. Published paleomagnetic data from Cretaceous plutons (for example, the Coast Plutonic Complex) intruding the thrust belt suggest that it has been displaced northward by as much as 2400 km since 90–100 Ma. This study focuses on the preintrusive travel history of the Decatur terrane, one of the units involved in the San Juan thrusting. The Decatur terrane is of special interest because it may be correlative with the Coast Range ophiolite (CRO) and Great Valley sequence of the southern California Coast Ranges. We report new paleomagnetic data from the James Island Formation, an Upper Jurassic, preintrusive turbidite unit, and from the Obstruction Formation, a terrigenous sandstone unit deposited on the Decatur terrane in the Late Cretaceous. Nine sites in the Obstruction Formation yield a single remanence direction (grand mean D = 90.7°, I = 67.9°) that is clearly a postfolding overprint, possibly of Late Cretaceous age. The six site mean remanence directions from the James Island Formation cluster well only after restoring the beds to horizontal by a scheme that takes account of their complex structure. This positive fold test and the presence of two polarities provide evidence that the remanence is primary. The mean paleolatitude corresponding to the corrected remanence direction from the James Island Formation is 0°±9.2°. The data thus corroborate equatorial paleolatitudes derived from parts of the CRO by others and suggest that the Decatur terrane and CRO represent fragments of a larger terrane originally located near the Jurassic equator. Unlike the CRO, however, the Decatur terrane was still near the equator at 145 Ma. If the plutons intruding the San Juan‐north Cascades thrust belt formed 2400 km to the south, then the paleomagnetism of the Decatur terrane suggests it moved 20° farther poleward in the interval 145 Ma to 97 Ma than did an equivalent site on North America. A much smaller amount of pre‐Late Cretaceous motion is allowed by error limits on the paleolatitudes, but more is required if tilting is partly responsible for the shallow paleomagnetism of the Cretaceous plutons.