PLATE CONFIGURATIONS FOR THE EARLIEST CASCADIA ARC: WAS THE KULA PLATE INVOLVED?

Subduction was quickly re-established in the Pacific Northwest following accretion of the Siletzia terrane at 50 Ma. Evidence for renewed subduction includes calc-alkaline plutonic and volcanic rocks that were emplaced within the pre-accretion forearc and adakitic magmatism emplaced within Siletzia. Geochronologic evidence suggests that this new subduction regime was fully established by 45 Ma. Large (>100 km) displacements accumulated on dextral strike-slip faults at the latitude of Washington immediately following the 50 Ma accretion of Siletzia. Motion on these faults was complete by 35 Ma, when the Straight Creek-Fraser River fault was sealed by an intrusion. Dextral motion on these faults suggests that the Pacific Northwest was juxtaposed with the Kula (or Resurrection) oceanic plate during this time, since the motion of this plate would impart a much stronger dextral shear along the plate boundary than the Farallon plate. We discuss whether the first ten million years of the Cascadia arc resulted from the subduction of the Kula (or Resurrection) oceanic plate. We propose that the current configuration, in which subduction of the Farallon plate and its derivatives occurs off in the Pacific Northwest, did not form until the latest Eocene.