RECONCILIATION OF CONFLICTING HYPOTHESES FOR THE PLATE TECTONIC DEVELOPMENT OF THE NORTHERN CORDILLERA DURING THE EARLY CENOZOIC

In the past decade, the literature on the plate tectonics of the northern Cordilleran during the early Cenozoic has expanded with diverse and largely mutually exclusive hypotheses. Here I explore a possible reconciliation with a model using parts of these hypotheses: Resurrection plate (Haeusller et al 2003); Baranof – Leech River correlation (Cowan, 2003); Yellowstone hot spot origin for Siletzia and its subsequent collision (Wells et al 2014). A modification of the Resurrection plate model to a more southeasterly position for the Resurrection – Kula spreading ridge, combined with the Chugach – Prince William terrane reconstruction of Cowan, agrees with previous ideas that the ridge migrated south along the margin from 61 to 50 Ma to produce the Sanak to Baranof near-trench magmatism as the Resurrection plate was overridden by North America. But in this revised model, the Chugach – Prince William terrane lay 1100 km southeast of its present position before 50 Ma, and North America completely overrode the Resurrection plate by 50 Ma. Therefore, the Resurrection – Kula ridge swept south along SE Alaska and British Columbia to end at 50 Ma at the Baranof schist that lay along the Leech River schist on southern Vancouver Island. At the same time, the Yellowstone hot spot lay offshore of the Pacific NW near the Farallon – Resurrection ridge to produce the Sileztia and Yakutat oceanic ridges from 56 to 50 Ma (Wells et al, 2014). A major transition occurred at 50 Ma with the demise of the Resurrection plate: (i) initiation of rapid northward motion of the Chugach – Prince William and Yakutat terranes along the coast of British Columbia and Alaska, and (ii) a large increase in the rate of right-slip faulting across the North Cascades of Washington and in British Columbia, both driven by the highly oblique Kula – North America relative motion, and (iii) the short-lived collision of Siletizia along the NW-trending margin of North America. The period from the demise of the Resurrection plate at 50 Ma to the initiation of the Cascadia arc margin at 42-40 Ma in the Pacific NW included the westward stepping of the subduction zone after Sileztia collision, the complex role of slab window(s), possible spreading ridge jumps along the margin, and an episodic decrease of extensional and transtensional deformation and Kamloops – Challis magmatism.