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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. 

Abstract Oceanic plateaus are common in modern oceanic basins and will ultimately collide with continental subduction zones. Despite the frequency of these events, complete sedimentary records of oceanic plateau collision and accretion have remained limited to only a few Cenozoic examples with excellent exposure and tectonic context. Our study focuses on building a stratigraphic record of plateau collision using the sedimentary strata deposited on the Siletzia oceanic plateau, which accreted to the Pacific Northwest at ca. 50 Ma. By combining previously published provenance and stratigraphic data with new lithofacies and geologic mapping, measured stratigraphic sections, conglomerate clast counts, and U-Pb zircon geochronology, we were able to divide the strata of the northern Olympic Peninsula in Washington, USA, into precollisional, syn-collisional, and postcollisional stages. Precollisional strata include early Eocene deep-marine hemipelagic to pelagic mudstones of the Aldwell Formation that were deposited directly on Siletzia basalts. These strata were deformed during collision and are separated from the overlying syn-collisional middle Eocene sandstone and conglomerate of the marine (?) Lyre Formation by an angular unconformity. Postcollisional strata were deposited by submarine fans and include interbedded sandstone and siltstone of the Hoko River and Makah formations. These units initially record the filling of isolated trench-slope basins by late Eocene time before eventual integration into an Oligocene regional forearc basin as the accreted Siletzia plateau began to subside. Our chronostratigraphy permits the correlation of basin strata across tectonic domains and provides more general insight into how forearc sedimentary systems evolve following the accretion of a young, buoyant oceanic plateau. 

Analysis of the late Miocene to Holocene McCallum sedimentary basin, located along the south side of the eastern Denali fault system, provides a better understanding of strike-slip basin evolution, timing of displacement on the Denali fault, and tectonics of the southern Alaska convergent margin. Analysis of the McCallum basin utilizing measured stratigraphic sections, lithofacies analyses, and 40Ar/39Ar tephra ages documented a 564-m-thick, two-member stratigraphy. Fine-grained, lacustrine-dominated environments characterized deposition of the lower member, and coarse-grained, stream-dominated alluvial-fan environments characterized deposition of the upper member. The 40Ar/39Ar dating of tephras indicated that the lower member was deposited from 6.1 to 5.0 Ma, and the upper member was deposited from 5.0 to 3.8 Ma. Our stratigraphic analysis of the McCallum basin illuminates the development of a composite strike-slip basin, with the deposition of the lower member occurring along a transtensional fault section, and deposition of the upper member occurring along a transpressional fault section. This change in depositional and tectonic settings is interpreted to reflect ~79–90 km of transport of the basin along the Denali fault system based on Pleistocene–Holocene slip rates. Previous studies of the timing of Cenozoic displacement on the Denali fault system utilizing sedimentary records emphasized a Paleogene component; our findings, however, also require a significant Neogene component. Neogene strike-slip displacement and basin development along the Denali fault system were broadly coeval with development of high topography and related clastic wedges across southern Alaska in response to flat slab subduction of the Yakutat microplate.