Various plate reconstructions predict that the Alexander terrane, a Neoproterozoic–Jurassic crustal fragment now located in the North American Cordillera, evolved in proximity to the northern Appalachian-Caledonian convergent margin during assembly of supercontinent Laurussia. To test stratigraphic connections with Laurussia that are implied by these plate reconstructions, we measured the Hf isotopic compositions of 176 detrital zircons from two relevant sedimentary sequences of the Alexander terrane. An older, Upper Silurian–Lower Devonian terrestrial to shallow-marine molasse sequence yields 405–490 Ma detrital zircons with negative εHf(t) values and Mesoproterozoic to Paleoproterozoic Hf model ages. In combination with paleomagnetic and biogeographic constraints, these Hf data argue for the molasse strata to be now-displaced equivalents of the Old Red Sandstone and primarily sourced from crustally contaminated granitoids in the Greenland, Svalbard, or British Caledonides. Late Silurian–Early Devonian orogenesis in the Alexander terrane is therefore likely related to the Scandian-Salinic phase of Appalachian-Caledonian mountain building. Younger, Middle Devonian sequences of the Alexander terrane are endowed in 390–490 Ma detrital zircons with positive εHf(t) values and Neoproterozoic Hf model ages. These isotopic signatures are consistent with the erosion of local basement rocks during the opening of the Slide Mountain–Angayucham backarc rift and tectonic separation of the Alexander terrane from northern Laurussia.
Detrital zircon U–Pb geochronology and Hf isotope geochemistry allow us to decipher the crustal provenance of Cambrian–Ordovician backarc basin strata of the Alexander terrane, North American Cordillera, and evaluate models for its origin and displacement history relative to Baltica, Gondwana, Siberia, and Laurentia. Quartzose shallow-marine sandstones of the Alexander terrane contain a range of Neoproterozoic to Neoarchaean detrital zircons with the most dominant age groupings c . 565–760, 1000–1250, 1450, and 1650 Ma. Subordinate volcaniclastic sandstones yield Cambrian and Ordovician detrital zircons with a prominent age peak at 477 Ma. The detrital zircon age signatures resemble coeval strata in the Eurasian high Arctic, and in combination with faunal and palaeomagnetic constraints suggest provenance from local magmatic rocks and the Timanide orogenic belt and Fennoscandian Shield of NE Baltica. The Hf isotopic compositions of Palaeozoic to Neoarchaean detrital zircons strongly favour Baltican crustal sources instead of similar-aged domains of Gondwana. The Alexander terrane formed part of an arc system that fringed the Uralian passive margin, and its position in the Uralian Seaway allowed faunal exchange between the Siberian and Baltican platforms. The available evidence suggests that the Alexander terrane originated in the Northern Hemisphere and migrated to the palaeo-Pacific Ocean by travelling around northern Laurentia. Supplementary material: U–Pb and Lu–Hf data tables are available at www.geolsoc.org.uk/SUP18557 .
Exotic and far‐traveled oceanic crustal rocks of the Cache Creek terrane (CC) are bordered by less exotic Quesnel (QN) and Stikine (ST) arc terranes to the east, north, and west. All of these terranes are enveloped by an arcuate belt of displaced continental margin rocks; the Kootenay (KO), Nisling (NS), and parts of the Yukon‐Tanana (YTT) terranes, that have indirect ties to ancestral North America (NA). Initial 87 Sr/ 86 Sr isopleths conform to this arcuate pattern. Such a pattern of concentric belts presents a geological conundrum: How did the QN, ST, and CC come to be virtually enveloped by terranes with ties to NA? Past and current models that explain assembly of the Canadian Cordillera are deficient in their treatment of this problem. We propose that Early Mesozoic QN and ST were joined through their northern ends as two adjacent arc festoons that faced south toward the Cache Creek ocean (Panthalassa?). Oceanic plateau remnants within the CC today were transported from the Tethyan realm and collided with these arcs during subduction of the Cache Creek ocean. Counterclockwise oroclinal rotation of ST and NS terranes in the Late Triassic to Early Jurassic caused enclosure of the CC. Rotation continued until these terranes collided with QN in the Middle Jurassic. Paleomagnetic declination data provide support for this model in the form of large average anticlockwise rotations for Permian to Early Jurassic sites in ST but moderate clockwise rotations for sites in QN. Specific modern analogues for the Cordilleran orocline include the Yap trench, where the Caroline rise is colliding end‐on with the Mariana Arc and the Banda Arc, located on the southeastern “tail” of the Asian plate, which is being deformed into a tight loop by interactions with the Australian and Pacific plates.
Research Article| February 01, 1993 Cache Creek ocean: Closure or enclosure? JoAnne Nelson; JoAnne Nelson 1British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Survey Branch, 553 Superior Street, Victoria, British Columbia V8V 1X4, Canada Search for other works by this author on: GSW Google Scholar Mitch Mihalynuk Mitch Mihalynuk 1British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Survey Branch, 553 Superior Street, Victoria, British Columbia V8V 1X4, Canada Search for other works by this author on: GSW Google Scholar Geology (1993) 21 (2): 173–176. https://doi.org/10.1130/0091-7613(1993)021<0173:CCOCOE>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JoAnne Nelson, Mitch Mihalynuk; Cache Creek ocean: Closure or enclosure?. Geology 1993;; 21 (2): 173–176. doi: https://doi.org/10.1130/0091-7613(1993)021<0173:CCOCOE>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Exotic Tethyan faunas within the Cache Creek terrane contrast markedly with faunas and lithologic associations in the adjacent Quesnel and Stikine terranes. In northern British Columbia and southeast Yukon, all three terranes are enveloped in the north by pericontinental rocks of the Yukon-Tanana terrane, a geometry that imposes severe constraints on terrane assembly models for the northern Canadian Cordillera. Our solution to the problem invokes a northern join between the Stikinia and Quesnellia arcs through the Yukon-Tanana terrane, forming an orocline that encloses the Cache Creek terrane. This model involves (1) collision of a linear oceanic plateau at the cusp between Quesnellia and Stikinia, (2) anticlockwise rotation of Stikinia about an axis in the Yukon-Tanana terrane, (3) simultaneous enclosure of the Cache Creek ocean, and (4) emplacement of Quesnellia onto the margin of ancestral North America and the Cache Creek terrane onto Stikinia during final closure of the orocline. Early Mesozoic Paleomagnetic declinations in Stikinia are permissive of the large anticlockwise rotations predicted by the model. Similar large-scale rotations and ocean-basin enclosure are common features in the southwest Pacific. This model accounts for Paleozoic and younger linkages between Yukon-Tanana and both northern Stikinia and Quesnellia, the striking similarity between Triassic-Jurassic arcs east and west of the Cache Creek terrane, and the profound early Mesozoic deformational event in the Yukon-Tanana terrane. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Detrital zircon U–Pb geochronology and Hf isotope geochemistry allow us to decipher the crustal provenance of Cambrian–Ordovician backarc basin strata of the Alexander terrane, North American Cordillera, and evaluate models for its origin and displacement history relative to Baltica, Gondwana, Siberia, and Laurentia. Quartzose shallow-marine sandstones of the Alexander terrane contain a range of Neoproterozoic to Neoarchaean detrital zircons with the most dominant age groupings c. 565–760, 1000–1250, 1450, and 1650 Ma. Subordinate volcaniclastic sandstones yield Cambrian and Ordovician detrital zircons with a prominent age peak at 477 Ma. The detrital zircon age signatures resemble coeval strata in the Eurasian high Arctic, and in combination with faunal and palaeomagnetic constraints suggest provenance from local magmatic rocks and the Timanide orogenic belt and Fennoscandian Shield of NE Baltica. The Hf isotopic compositions of Palaeozoic to Neoarchaean detrital zircons strongly favour Baltican crustal sources instead of similar-aged domains of Gondwana. The Alexander terrane formed part of an arc system that fringed the Uralian passive margin, and its position in the Uralian Seaway allowed faunal exchange between the Siberian and Baltican platforms. The available evidence suggests that the Alexander terrane originated in the Northern Hemisphere and migrated to the palaeo-Pacific Ocean by travelling around northern Laurentia.
