by Chad Seigel
April 2005

Table of Contents

1. Introduction 4. Omineca Belt 7. Insular Belt
2. Ancient Margin 5. Intermontaine Belt 8. Summary
3. Foreland Belt 6. Coast Belt 9. References
Click on thumbnail images to see a larger image.

Image from Geological Survey of Canada, 2005

Image from Geological Survey of Canada, 2005


Northern British Columbia is mountainous area composed of various faults and orogenic belts which are part of a larger geographic area known as the Canadian Cordillera. This area is composed of ancient cratonic basement, and a collage of terranes which later acreted to this basement. This report will focus on a transect cut through northern British Columbia from east to west, starting on the BC/Alberta border, running along the 60th parallel and ending on the Alaskan coast. Although much more complex, the five morphological belts and the ancient cratonic basement which are located along this transect will be briefly summarized in terms of the terranes of which each belt is composed and tectonic origin and time of acccretion.

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Ancient Continental Margin

The ancient contental margin of North America was formed during late Proterozoic (750 Ma.), when continental rifting tore the former supercontinent Rodina apart creating ancestral North America (Laurentia). This resulted in the formation of a passive continental margin much like the present day east coast of North America which persisted until middle Devonian (~390 Ma.) (Monger and Price, 2002). During this time a thick miogeoclinal deposit of sediment which was eroded from the Canadian Shield to the east was deposited on the continental shelf of the proto Pacific Ocean known as Panthalassa. This supracrustal wedge increases in thickness from east to west where it is approximately 5 km thick at the edge of the Foreland Belt.

Ancient continental margin near Flin Flon, Manitoba. Lower section is where Ordovician limestone contacts Precambrian basement.

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Foreland Belt

Moving from east to west along the transect the Foreland Belt is encountered. Although this belt is the next in the sequence of orogenic belts it was the last to form. During the break up of Pangea during the Jurassic (~180 Ma.) the North American continent began to move westward like a giant bulldozer accreting terranes which lay just off the coast of the continent. In doing so these terranes were squeezed in a giant vice between the subducting oceanic lithosphere off the coast of North America, and the wedge shaped North American Craton. These terranes were squeezed upwards and downwards which resulted in the detachment of the thick miogeoclinal sequences deposited during the late Proterozoic and Paleozoic from the cratonic basement. These sequences were upthrust onto the edge of the North American craton forming the Foreland Belt (Welford et al., 2001).

Foreland Belt near Wokpash Lake, BC. Note the contorted folds.

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Omineca Belt

Next along the transect is the Omenica Belt. The Omenica Belt is the region of overlap between the Intermontaine Belt to the west and the Foreland Belt to the east. During the late Devonian (390-355Ma), it is thought that an oceanic trench was created at the continent ocean boundary caused by the enourmous weight of the old dense oceanic crust combined with the weight of sediments deposited on the continental shelf. Stratigraphic and U-Pb age constraints indicate that this episode of magmatisim was Devonian-Mississippian and has been interpreted as subduction related magmatisim at a convergent margin (Creaser et al., 1999). Today, nowhere on Earth is there evidence of subduction being initiated, and the cause of change from a passive, intra-plate margin to a convergent, inter-plate margin is uncertain (Monger and Price, 2002). This newly formed convergent boundary created magmatic island arcs which formed on the edge of the North American craton not far from the continental margin. These arcs are the Slide Mountain, Cassiar, and Yukon-Tanana Terranes which make up the Omenica Belt. These two arcs with their associated back arc basins have gone through numerous metamorphic events throughout their history, and today represents the once deeply burried roots of this ancient magmatic arc assemblage.

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Intermontaine Belt

The Intermontaine Belt is composed of the Cache Creek, Nisling, and Stikine terranes. The Cache Creek and Nisling terranes were formed in the western Pacific during the Permian to Middle Triassic (280-230 Ma). Sedimentary rocks of the Cache Creek Terrane contain a particular assemblage of fossils that are found in Asia and not in ancient North American rocks. This suggests that the Cache Creek Terrane likley originated far from North America and may have existed on the other side of the Pacific Ocean (Hart, 2002). The Stikine Terrane is a Carboniferous to Early Jurassic (320-190 Ma) island arc which was formed in the east Pacific. The Stikine Terrane is believed to have evolved in the east Pacific of the Northern Hemisphere and moved northward to dock with ancestral North America sometime during the Middle Jurassic (Macintyre et al., 2001). During the Early to Middle Jurassic (190-178 Ma) the two terranes joined to form the Intermontaine Belt. After this time arc related magmatic activity continued into the Tertiary. Late Triassic through Tertiary plutons intrude structurally imbricated Stikine and Cache Creek terranes in the Atlin-Bennett Lake area of northern British Columbia (Macintyre et al., 2001).

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Coast Belt

The Coast Belt is the suture zone between the Intermontaine Belt and the Insular Belt and is composed of plutonic and metamorphic rock. During the mid Cretaceous, the exotic Insular superterrane collided with North America, further deforming the Intermontaine terranes. The deformation compressed the upper crust of the Nisling and Stikine terranes by more than 160 km, approximately the width of the Coast Belt (Hammer et al., 2000).

Coast Belt near Bennett Lake, Yukon.

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Insular Belt

Westernmost is the Insular Belt which is composed of the Wrangel, Alexander, Chugach, and Yukatat terranes. The Wrangel and Alexander terranes are island arcs having formed in the Pacific during the Devonian (~400 Ma). These arcs amalgamated during the Carboniferous (~320 Ma) and colided with the continent during the Cretaceous (~115 Ma). It is uncertain exactly when the Insular Islands finally arrived on North American shores. Some evidence suggests they persisted as an offshore volcanic chain for some time, much like the islands of Japan do today. In any event, the final collision between the islands and the continent did not occur until mid Cretaceous time, perhaps 115 million years ago (Burke Museum, 2004). The Chugach and Yukatat terranes are composed of sedimentary rock of Tertiary age. Products of erosion were deposited in Pacific Ocean floor and carried northward on the oceanic plate to be accreted as major components of Chugach and younger accretionary terranes southern Alaska (Monger and Price, 2002).

Insular Belt at Hanes, Alaska.

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The collage of terranes and faults that make up the continental crust of northern British Columbia are extremely complex and have been simplified here for clarity. Much is currently known about the complex geology of this area and many questions still remain unanswered. With new theories and geoscience techniques more questions and answers will be brought to light, once again changing our understanding of the tectonic evolution of northern British Columbia.

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Burke Museum. 2005. The Coast Range Episode. The Burke Museum of Natural History and Culture. http://www.washington.edu/burkemuseum/

Clowes, R.M. and Hammer, T.C. 2004. Comparison of lithospheric structures across the Alaskan and Canadian Cordillera. UBC Research Bulliten.

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Creaser, R.A., Goodwin-Bell, J.S. and Erdmer, P. 1999. Geochemical and Nd isotopic constraints for the origin of eclogite protoliths, northern Cordillera: implications for the Paleozoic tectonic evolution of the Yukon-Tanana terrane. Canadian Journal of Earth Sciences. 36: 1697-1709

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Hammer, P.T.C., R.M. Clowes, and R.M. Ellis 2000. Crustal structure of N.W. British Columbia and S.E. Alaska from seismic wide-angle studies: Coast Plutonic Complex to Stikinia. Journal of Geophysical Research. 105: 7961-7981

Harris, M.J., D.T.A. Symons, C.J.R. Hart and W.H. Blackburn 1998. Jurassic plate motions of the Stikine Terrane, southern Yukon: a paleomagnetic and geothermometric study of the Teslin Crossing Pluton (NTS 105E/7). Yukon Exploration and Geology Report 1998.

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Macintyre, D.G.,Villeneuve, M.E.,and Schiarizza 2001. Timing and tectonic setting of the Stikine Terrane magmatisim, Babine-Takla lakes area, central British Columbia. Canadian Journal of Earth Sciences. 38: 579-601

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This web presentation prepared for
Earth Science 767 ES 767 : global tectonics
by Chad Seigel (April 2005)