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TitleFieldwork in the Sylvester allochthon, Cassiar Mountains, British Columbia: Investigations of the Rapid River tectonite and the Slide Mountain terrane
AuthorRyan, J J; Nelson, J; van Staal, CORCID logo
SourceGeological Fieldwork 2014; by British Columbia Geological Survey; British Columbia Ministry of Energy and Mines, British Columbia Geological Survey Paper pt. 2015-1, 2015 p. 113-128 Open
Access logo Open Access
LinksOnline - En ligne (Complete volume / volume complet)
Alt SeriesEarth Sciences Sector, Contribution Series 20140351
PublisherBritish Columbia Ministry of Energy and Mines
Mediaon-line; digital
File formatpdf
ProvinceBritish Columbia
NTS104I; 104P
AreaDease Lake; Dease River; Cassiar; McDame; Cry Lake; Zus Mountain; Dalton Creek; Four Mile River
Lat/Long WENS-130.0000 -128.0000 60.0000 58.5000
Subjectstectonics; structural geology; field work; lithology; tectonites; depositional environment; mylonites; mafic rocks; ultramafic rocks; plutonic rocks; displacement; faults, thrust; basalts; deformation; sedimentary rocks; conglomerates; tectonic interpretations; subduction; dykes; marbles; field relations; structural interpretations; Eocene; oceanic crust; peridotites; gabbros; dunites; ophiolites; Sylvester allochthon; Rapid River tectonite; Slide Mountain terrane; Tertiary; Cretaceous; Devonian; Permian; Mississippian
Illustrationsgeological sketch maps; photographs
ProgramGEM2: Geo-mapping for Energy and Minerals Western Cordillera, Redefinition of crustal blocks
Released2015 01 01
AbstractBack-arc extension during Devonian-Mississippian eastward subduction beneath the western flank of ancestral North America led to separation of Yukon-Tanana terrane from Laurentia, and opening of the Slide Mountain marginal ocean basin. Rocks exposed in imbricated thrust sheets of the Sylvester allochthon record the opening and closing of this marginal basin. At the highest structural level in the allochthon, the Rapid River tectonite is interpreted as a remnant of Yukon-Tanana basement. Pre-Late Devonian deformation in the Rapid River tectonite may represent collision of an exotic arc terrane with the outer peri-Laurentian margin that was the precursor to east-dipping subduction. Preliminary observations indicate that at least part of the Rapid River tectonite has a protolith of mafic mylonite and marble, possibly representing parts of a primitive arc. At two localities the tectonite is intruded by late synkinematic diorite/gabbro-tonalite-trondjemite plutons, one of which has been previously dated (ca. 362 Ma). Limited shear-sense indicators in the mylonite show top-to-the-northwest displacement.Thrust panels in the lower part of the Sylvester allochthon expose ultramafic-gabbro-supracrustal complexes that represent the youngest pre-accretionary phase in the YTT-Slide Mountain system. Near Zus Mountain northeast of the Cassiar townsite, and near Blue Dome 20 kilometres to the north, partly serpentinized harzburgite tectonites form the lower parts of thrust panels within the Slide Mountain terrane. These tectonites are overlain by interlayered peridotite tectonite, lherzolite, dunite, and gabbro; all are cut by trondhjemite dikes. The trondhjemite dikes cut previously serpentinized hosts, suggesting their emplacement during or after exhumation. At Zus Mountain, a large gabbro body overlies the ultramafites. At the Blue Dome section, seafl oor deposits of locally pillowed basalt and radiolarian chert directly overlie the ultramafic rocks. This section also contains polymictic conglomerates with mainly 1 to 5 cm-sized angular clasts derived from subjacent units. Mafic clasts contain an internal ductile deformation fabric. The conglomerates indicate exhumation and erosional unroofi ng of previously deformed rocks, possibly along penecontemporaneous faults. Notably lacking are the sheeted dike complexes that intervene between ultramafic and supracrustal sections found in classical ophiolites. The Sylvester rocks may not have formed through ‘normal’ sea-fl oor spreading. Instead, they more closely resemble sections formed at slow-spreading ridges or hyperextending margins where the subcontinental mantle is exhumed by low-angle detachment faults. A previously dated (ca. 268 Ma) trondhjemite dike from the Zus Mountain area is coeval with similar dikes in Slide Mountain ophiolites in the Yukon, but significantly younger than supracrustal successions elsewhere in the Slide Mountain terrane. The late ocean opening documented in the Sylvester allochthon and the Yukon was broadly coeval with ocean closing elsewhere, as ocean crust was consumed by westerly subduction beneath the Yukon-Tanana terrane.
Summary(Plain Language Summary, not published)
This open file report presents a summary of 2014 reconnaissance fielwork in the northwestern Cordillera that was focused on establishing constraints on major crustal breaks within and between two major geological terranes. Field work was carried out in the Cassiar Mountains of northern BC. Representative rock samples collected for specific analysis for geochemical signatures, and age dating that will help generate new tectonic models of the northern cordillera.

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