Abstract
Abstract The origin and displacement history of terranes emplaced along the northern margin of North America remain contentious. One of these terranes is the North Slope subterrane of the Arctic Alaska-Chukotka microplate, which is separated from the northwestern margin of Laurentia (Yukon block) by the Porcupine Shear Zone of Alaska and Yukon. Here, we present new field observations, geological mapping, detrital zircon U-Pb geochronology, and sedimentary/igneous geochemistry to elucidate the stratigraphic architecture of deformed pre-Mississippian rocks exposed within the Porcupine Shear Zone, which we distinguish herein as the newly defined Ch’oodeenjìk succession. The oldest rocks in the Ch’oodeenjìk succession consist of siliciclastic strata of the Lahchah and Sunaghun formations (new names), which yield detrital zircon U-Pb age populations of ca. 1050-1250, 1350-1450, 1600-1650, and 2500-2800 Ma (n =800). This succession is overlain by chert-bearing dolostone and limestone of the Caribou Bar formation (new name) that contains vase-shaped microfossils and yields carbonate carbon (δ13Ccarb) and strontium (87Sr/86Sr) isotopic data that range from ca. -3‰ to +3‰ and 0.70636 to 0.70714, respectively. These data suggest that Lahchah, Sunaghun, and Caribou Bar formations are late Tonian in age. These Neoproterozoic rocks are intruded by Late Devonian (Frasnian-Famennian) felsic plutons and mafic dikes, one of which yielded a sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) U-Pb age of 380 ± 4 Ma. Neoproterozoic strata of the Ch’oodeenjìk succession are also unconformably overlain by Upper Devonian-Carboniferous (?) siliciclastic rocks of the Darcy Creek formation (new name), which yields detrital zircon populations of ca. 365–385, 420-470 and 625-835 Ma, in addition to Proterozoic age populations similar to the underlying Tonian strata. Together, these new stratigraphic, geochronological, geochemical, and micropaleontological data indicate that pre-Mississippian rocks exposed within the Porcupine Shear Zone most likely represent a peri-Laurentian crustal fragment that differs from the adjacent Yukon block and North Slope subterrane; thus, the Porcupine Shear Zone represents a fundamental tectonic boundary separating autochthonous Laurentia from various accreted peri-Laurentian crustal fragments.
Highlights
Tectonic models for the evolution of the Arctic margin of North America (Laurentia) either support, disregard, or purport to contradict large-scale middle Paleozoic strike-slip terrane displacement [1,2,3,4,5]
Given lack of post-Devonian zircons, which are common in the Arctic Alaska-Chukotka microplate (e.g., [14, 135, 136]), we suggest that Darcy Creek formation is most likely Upper Devonian–Carboniferous in age and occupies a similar stratigraphic position to the Endicott Group of the North Slope subterrane
The most fundamental question this study aimed to address is whether deformed Proterozoic and Paleozoic rocks of the Porcupine Shear Zone (PSZ) belonged to the Yukon block or the North Slope subterrane (Figure 1)
Summary
Tectonic models for the evolution of the Arctic margin of North America (Laurentia) either support, disregard, or purport to contradict large-scale middle Paleozoic strike-slip terrane displacement [1,2,3,4,5]. Stratigraphic, paleontological, and geochronological data from Neoproterozoic to early Paleozoic strata in various allochthonous and parautochthonous terranes within the North American Cordillera suggest origins adjacent to Baltica, Siberia or the peripheries of Laurentia (Figure 1) ([1, 6] and references therein); the exact position and displacement history of each of these terranes is widely debated. North Slope subterrane CB Pearya Arctic Ocean. Major strike-slip fault system North Slope subterrane Arctic Alaska-Chukotka (a)
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