EOCENE ORIGIN OF THE YAKUTAT TERRANE AS A SPREADING-RIDGE CENTERED OCEANIC PLATEAU WITH THE SILETZIA TERRANE OFFSHORE PACIFIC NORTHWEST AND BRITISH COLUMBIA (Invited Presentation)

Two fragments of thick basaltic crust are presently exposed along the margin of western North American in southern Alaska (Yakutat terrane) and the Pacific Northwest (Siletzia terrane). Presently, the Yakutat terrane is subducting beneath southeastern Alaska and sits outboard of major northern Cordilleran strike-slip faults. This has led to debate about the translation history of the terrane, with models suggesting different starting points ranging between southeastern AK and northern CA. We suggest it shares an early pre- to syn-collisional history with the Siletzia terrane as an Eocene conjugate oceanic plateau that developed on opposite sides of a spreading center offshore the Pacific Northwest.

Our new high-precision U-Pb zircon geochronology and sedimentologic analyses from Eocene sedimentary and volcanic strata exposed within both terranes focuses on resolving the origin and early history of the Yakutat terrane. Volcanic basement rocks of the Yakutat terrane are known as the Hubbs Creek Volcanics, and a tuff from this unit yields an eruption age estimate of ~56.3 Ma, similar to the ~56-48 Ma age of basement basalts of Siletzia. Overlying and interfingering with these basalts on both terranes is a pre-accretion pelagic to hemipelagic siltstone sequence. These are known as the siltstone of Oily Lake (~55.7 Ma) on the Yakutat terrane and as the Aldwell Fm. (~52.2 Ma) on Siletzia in WA. The pre-accretion sequence is separated from overlying conglomerate of the syn-collisional lowermost Kulthieth Fm. (referred to as the Marvitz Creek Unit) and Lyre Fm. by an angular unconformity on both the Yakutat and Siletzia terranes, respectively.

Both terranes record a similar sedimentologic sequence of deformation of pre-accretion siltstone and unconformity development followed by an influx of coarse-grained detritus during collision. These geological relationships and their timing are interpreted as strong evidence for a composite Siletzia-Yakutat oceanic plateau model, and account for northward translation of the Yakutat terrane after separation from Siletzia. Due to the irregular shape and size of the conjugate plateau fragments on either side of the spreading ridge, the initial collision of the Siletzia-Yakutat composite terrane was likely diachronous along strike and explains subtle differences in age of pre-, syn- and post-collisional strata on both terranes.