A KINEMATIC MODEL FOR THE FORMATION OF THE SILETZ, CRESCENT, AND YAKUTAT TERRANES BY CAPTURE OF COHERENT FRAGMENTS OF THE FARALLON AND RESURRECTION OCEANIC PLATES

Our kinematic model of Kula, Resurrection (RES), Farallon (FAR), and Juan de Fuca plate motion relative to the Pacific plate builds on previous work of Ray Wells and colleagues [see Wells et al., 2014, Geosphere] to reconcile previously conflicting interpretations regarding the origin of Paleogene oceanic terranes currently stranded against western North America. Interpretation of the volcanic basement of the Oregon and Washington Coast Ranges—as representing a pair of Yellowstone hotspot tracks formed at a mid-ocean ridge during the early Cenozoic [e.g., Duncan, 1982, JGR]—has been questioned on many grounds, especially that the range of ages does not match the offshore spreading rates and the presence of continental coarse clastic sediments is difficult to reconcile with fast convergence rates between the oceanic plates and North America. Updates to basement geochronology and plate motion history reveal that these objections are much less serious than when first raised. Forward plate kinematic modeling reveals that predicted basement ages are consistent with the observed range of about 55–49 Ma, and that the entire volcanic basement can form within about 300 km of clastic sediment sources. This model indicates no firm reason to reject the near-ridge, hotspot hypothesis on the basis of plate motions.

A novel element of the model is that by including a previously proposed, pre-40-Ma Resurrection plate in our reconstruction, we are able to account for the Crescent and Siletz basalts as remnants of a RES-FAR ridge segment that transferred to North America ~53 to 42 Ma. The similarity in age, thickness, and geochemistry between Yakutat and Crescent basalts [Davis and Plafker, 1986, Geology] can be explained by a common Resurrection origin near the Yellowstone hotspot. Though currently separated by ~1500 km, the distance may have been a few hundred km at 50 Ma if the Resurrection and Kula plates were both captured by the Pacific plate at ~40 Ma, with the Yakutat terrane moving northward with the Pacific plate since then. Our kinematic model of mobile Paleogene terranes, including accretion of Farallon fragments and partial subduction of Resurrection fragments, provides a basis for reinterpretation of the complex three-dimensional structure of the modern Aleutian and Cascadia forearcs.