Insights into Cordilleran collisional processes: Early shared history of the Eocene Siletzia and Yakutat terranes as a ridge-centered oceanic plateau

A regional network of dextral strike-slip faults along the northwestern margin of North America separates crustal fragments of early Eocene oceanic plateau crust by ∼1600 km. In this study, we test the hypothesis that both the Siletzia terrane (Pacific Northwest) and Yakutat terrane (southern Alaska, USA) had a shared origin and early history prior to strike-slip separation. New high-precision U-Pb zircon geochronology (chemical abrasion−isotope dilution−thermal ionization mass spectrometry) from the volcanic strata of the Yakutat oceanic plateau (Hubbs Creek volcanics; HCV) yield an eruption date of 56.26 ± 0.12 Ma, matching the age of the oldest part of Siletzia volcanic strata. The pelagic siltstone of Oily Lake overlies the HCV and is interbedded with a tuff that yields an eruption date of 55.672 ± 0.079 Ma. These strata are coeval with and have similar depositional settings as the precollisional strata of Siletzia. Our findings are consistent with the initial construction of both terranes as conjugate oceanic plateaus that formed on different sides of an Eocene spreading ridge offshore the Pacific Northwest.