| Title | Fieldwork in the Sylvester allochthon, Cassiar Mountains, British Columbia: Investigations of the Rapid River tectonite and the Slide Mountain terrane |
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| Author | Ryan, J J; Nelson, J; van Staal, C |
| Source | Geological 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 |
| Links | Online - En ligne (PDF, 4.7 MB)
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| Links | Online - En ligne (PDF, 30
MB; complete volume / volume complet)
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| Image |  |
| Year | 2015 |
| Alt Series | Earth Sciences Sector, Contribution Series 20140351 |
| Publisher | British Columbia Ministry of Energy and Mines |
| Document | serial |
| Lang. | English |
| Media | on-line; digital |
| File format | pdf |
| Province | British Columbia |
| NTS | 104I; 104P |
| Area | Dease 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 |
| Subjects | tectonics; 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 |
| Illustrations | geological sketch maps; photographs |
| Program | GEM2: Geo-mapping for Energy and Minerals Western Cordillera, Redefinition of crustal blocks |
| Released | 2015 01 01 |
| Abstract | Back-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. |
| GEOSCAN ID | 295808 |
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