NEW KINEMATIC MODEL OF THE YAKUTAT, CRESCENT, AND SILETZ TERRANES AND ITS IMPLICATIONS FOR EARTHQUAKE HAZARDS

Our kinematic model of Kula, Farallon (FAR), and Juan de Fuca (JDF) plate motion relative to the Pacific (PAC) plate reconciles previously conflicting interpretations regarding the origin of Paleogene oceanic terranes currently stranded against western North America (NAM). By including a defunct Resurrection (RES) plate in our model, we are able to account for the Yakutat terrane in Alaska as well as the Siletz and Crescent terranes in the Pacific Northwest as remnants of a FAR-RES ridge segment that transferred to NAM ~53 to 42 Ma. The similarities between Yakutat and Crescent basalts [Davis & Plafker, 1986, Geology] can be explained by their having previously been adjacent parts of RES. If RES was captured by PAC at the same time as the Kula plate ~40 Ma, the distance between Yakutat and Crescent terranes may have been a few hundred km at 50 Ma. Currently separated by more than 1500 km, the Yakutat must have been moving northward with PAC for most of the time since its formation. An important implication of early capture of Yakutat by PAC is that the Transition Fault does not represent a locus of former motion between Yakutat and PAC, but instead a suture where the intervening JDF plate has disappeared by subduction. Near Prince William sound, this suture may have been healing since 35 Ma, explaining the strong coupling between the Yakutat and the main Pacific plate.

Partial subduction of the thinner N Yakutat and Crescent (both RES) plate fragments and accretion of the thicker Siletz (FAR) and S Yakutat (RES) fragments complicate the 3D structure of the Aleutian and Cascadia forearcs. In particular, the N Yakutat is associated with the asperity that broke during the M9.2 1964 Alaskan earthquake, raising significant concerns about the role of mobile oceanic fragments in promoting damaging earthquakes in Alaska as well as along tectonically active margins elsewhere. Subduction of the relatively buoyant Yakutat terrane, piggybacked on PAC, likely enhances plate coupling beneath SE Alaska. A crustal fragment stranded within the southern Cascadia forearc is associated with the M7.2 1992 Cape Mendocino earthquake, indicating that plate fragments within forearcs can also serve as asperities. Our kinematic model of mobile terranes provides insights into the 3D structure of forearcs and its relationship to ongoing seismotectonic processes.