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Tectonic and stratigraphic relations between the Coast Plutonic Complex and Intermontane Belt, west-central Whitesail Lake map area, British Columbia

University of British Columbia

The upper Tsaytis River area, about 150 km² in extent, straddles the Coast Plutonic Complex-Intermontane Belt boundary about 580 km northwest of Vancouver, British Columbia. The boundary, here, is the Sandifer Lake Fault Zone, a highly disrupted, northeastward directed, imbricated thrust complex of middle to Late Cretaceous age. Rocks of the Coast Plutonic Complex occur in imbricate sheets above a frontal thrust. In this gently dipping, homoclinal sequence of tectonic sheets, amphibolites, amphibolite-grade granitoid gneisses, migmatites and metacarbonates of the Central Gneiss Complex tectonically overlie greenschist facies, metavolcanic and metaplutonic schists and mylonites of the Gamsby Group. The Central Gneiss Complex and the Gamsby Group appear to be stratigraphically equivalent units, metamorphosed to different grade and structurally juxtaposed in the thrust complex. Major and trace element chemistry of metavolcanic rocks in the Gamsby Group indicates that protoliths were tholeiitic and calc-alkaline basalt-andesite and calc-alkaline dacite-rhyolite which originated in a mature island arc setting. A minimum, Upper Triassic protolith age for the Gamsby Group is provided by a 210 Ma, near concordant U/Pb zircon date for metarhyolite, and a 230±39 Ma Rb/Sr whole rock isochron date. Mylonitic granite in the Gamsby Group gives a 160±24 Ma Rb/Sr whole rock isochron date, and amphibole from a related, deformed injection agmatite gives a 145±5 Ma K-Ar date. Hornblende from a dyke with chilled margins, which intrudes the Gamsby Group, gives a 66±2 Ma K-Ar date, indicating that the metamorphic complex was cold and close to the surface before the end of the Cretaceous. These results and geology reported for the Prince Rupert area (Crawford and Hollister 1982, and unpublished GSC and UBC zircon data) indicate that at this latitude in the Coast Plutonic Complex, regional metamorphism, polyphase ductile deformation, and intrusion of granitoid material occured in a two-sided, Jurassic and Cretaceous orogenic welt. The orogen was superimposed on the pre-Upper Triassic island arc in the western edge of Stikinia, as a consequence of initial suturing of Stikinia with the allochtonous Wrangellia-Alexander terrane at an unknown distance to the southwest of the thesis area. A Lower Cretaceous volcanic-plutonic complex forms the eastern and lowest thrust sheet of the Coast Plutonic Complex. The volcanic rocks may be correlative with the Gambier Group of the southern Coast Plutonic Complex, and were perhaps deposited unconformably on the uplifted Jurassic orogen. They were invaded and hornfelsed by Cretaceous granitic stocks before being thrust northeastward over strata of the Intermontane Belt. Middle-Upper Cretaceous shortening and associated brittle shearing along the Sandifer Lake Fault Zone occured in a high heat flow, back-arc setting. The Central Gneiss Complex, Gamsby Group, and Gambier Group(?) were imbricated and thrust over Intermontane Belt rocks of the Telkwa Formation. The imbricate tectonic front was disrupted, successively, by strike-slip and dip-slip faults in Late Cretaceous to early Cenozoic time. The latest movement on high angle faults postdates Eocene intrusions, nearby, and Eocene strata, Regionally, but predates Miocene Plateau basalts (Woodsworth 1979, 1980). Several dated, cross-cutting intrusive stocks in the Whitesail Lake map area (Woodsworth 1980), indicate that the Sandifer Lake Fault Zone, juxtaposing the Coast Plutonic Complex and Intermontane Belt, had definitely ceased movement by Eocene time and quite possibly by Late Cretaceous time.

Text

2010-03-31

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http://hdl.handle.net/2429/23174

Geological Sciences

University of British Columbia

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