Western cordillera when was it formed

Price noted that the west flank of the Purcell Anticlinorium is basically a crustal-scale monoclinal flexure marking a change in level of exposure involving an aggregate thickness of about 20 km of stratigraphic section from the lower part of the Middle Proterozoic Purcell Belt Supergroup on the east side to the Triassic-Jurassic volcanogenic rocks of Quesnellia on the west side, and that it coincides with a strong gradient in the Bouguer gravity anomaly field and a westward decrease in the depth to the base of the crust, as well as the palinspastically restored position of the 'hinge zone' between the platform and the miogeocline.

On the basis of these relations Price suggested that the west flank of the Purcell Anticlinorium is a crustal-scale fault-bend fold along the ramp marking the early Paleozoic rifted margin of the North American craton, that the thick section of lower Paleozoic miogeoclinal strata accumulated outboard of this ramp on a basement of tectonically attenuated continental crust or oceanic crust, and that the thin section of lower Paleozoic platformal strata accumulated inboard of the ramp on a normal thickness of unfaulted continental crust Figs.

There is a close correlation between the location of the miogeocline-platform 'hinge zone' and the eastern limit of Upper Proterozoic Windermere Supergroup rocks. This indicates that the locus of the early Paleozoic margin of the North American continent was defined by the Late Proterozoic rifting that was responsible for the accumulation of the Windermere Supergroup. Northward from section W-E Fig.

This southwestward deflection in the locus of the hinge zone separating the early Paleozoic miogeocline and platform coincides with a southwestward deflection in the Kootenay Arc, which follows the west flank of the Purcell Anticlinorium Fig. It marks the northwest margin of Montania Benvenuto and Price, , which encompassed most of the area of the Middle Proterozoic Belt Purcell basin and was a tectonically positive element that behaved like part of the North American craton during Paleozoic time.

Thus, although elsewhere the locus of the Rocky Mountain Foreland Thrust and Fold Belt generally follows the 'hinge zone' between the miogeocline and the platform, south of Crowsnest Pass which extends from Pincher Creek to Cranbrook in Fig. Bond and Kominz used tectonic subsidence curves from the Cordilleran miogeocline to argue convincingly that the rifting that created a new continental margin and initiated the Cordilleran miogeocline occurred in latest Proterozoic or earliest Cambrian time at about Ma , and not with the beginning of Windermere sedimentation at about Ma as suggested by Stewart The tectonic subsidence curves were based upon the variation as a function of geological age of the cumulative thickness of fully lithified lower Paleozoic sediments.

They were prepared by estimating and compensating for the effects of sediment compaction, of eustatic sea-level variation and isostatic subsidence of the lithosphere in response to the weight of overlying sediments and water. New global paleogeographic reconstructions of Gondwana Dalziel, ; Moores, , which indicate that southeastern Australia was adjacent to western North America in latest Proterozoic time, provide support for the correlation by Bell and Jefferson , Jefferson , and Eisbacher of the Belt-Purcell and Windermere rocks of the North American Cordillera with coeval rocks in south-central Australia, and for the conclusion Bond and Kominz, that the rift-drift transition that marked the initiation of sea-floor spreading and the development of an early Paleozoic ocean basin adjacent to western North America occurred at the end of the Proterozoic.

Ross argued that the Windermere Supergroup may record one or more earlier cycles of crustal extension and thermal subsidence of attenuated lithosphere prior to the episode that marked the beginning of the Paleozoic transgression over the stable craton the Sauk Sequence ; however, similarities in Upper Proterozoic stratigraphy between the Cordilleran miogeocline and south-central Australia presumably indicate that the rift-drift transition that separated the two Precambrian cratons occurred after deposition of the Windermere Supergroup.

The Windermere Supergroup appears to record a unusually long interval of intermittent? In this respect the evolution of the pre-Cordilleran margin of North America may resemble the evolution of the continental margins of the North Atlantic, where a protracted interval of rifting dating back to the mid-Paleozoic and early Mesozoic preceded the final continental separation and drift that occurred in the Late Cretaceous and Early Tertiary Ziegler, The transgressive overlap of the North American craton by the Sauk Sequence, which marked the birth of the Western Canada Sedimentary Basin, can be attributed to regional isostatic response of the lithosphere to loads imposed on it at the newly formed continental margin Bond and Kominz, These 'loads' include both the effects of cooling and thermal contraction of hot lithosphere that had been emplaced beneath and adjacent to the newly formed margin during rifting and sea-floor spreading, and of the weight of the sediments that were deposited along the continental margin during and after the rifting.

Significant but enigmatic episodes of block faulting and volcanism occurred along the Cordilleran miogeocline after the rift-drift transition, particularly during the Middle Cambrian in the northern Rocky Mountains and Mackenzie Mountains Fritz et al.

These episodes of block faulting and volcanism point to significant differences in tectonic setting between the Cordilleran miogeocline and passive margins such as those bordering the modern Atlantic ocean basin. It was on this basis that Monger and Price argued that an oceanic volcanic arc and marginal basin probably lay outboard from the continental margin during early Paleozoic time. Block faulting and volcanism along the miogeocline occurred intermittently throughout early Paleozoic time.

However they were most important during Middle and? Early Devonian time, just prior to a second major episode of tectonic subsidence and transgressive overlap along the Cordilleran miogeocline in Late Devonian and Carboniferous time Kaskaskia Sequence , which also dominated the stratigraphy of the cratonic interior.

Extensional block faulting occurred during the formation of the Liard Basin of northern British Columbia and adjacent Northwest Territories Wright et al. In the case of the latter there is about 7 km of stratigraphic relief on the sub-Fairholme Group sub-Upper Devonian unconformity from the south side to the north side of the northeast-trending Moyie-Dibble Creek Fault, which extends into the Rocky Mountains from the Purcell Mountains just south of Cranbrook, British Columbia Fig.

Much of this sub-Fairholme stratigraphic relief is due to differential vertical movements across the northern margin of Montania during Cambrian and?

Ordovician time. South of the Moyie-Dibble Creek Fault, both the thin, platformal Cambrian sequence that is characteristic of Montania, and the upper part of the underlying Middle Proterozoic Purcell Belt Supergroup are unconformably overlapped northward by the Fairholme Group, whereas north of the Moyie-Dibble Creek Fault, the Fairholme Group is underlain by about 6 km of lower Paleozoic strata comprising the Western Main Ranges shaly facies of the Cordilleran miogeocline.

This crustal-scale, pre-Late Devonian deformation appears to have been mainly extensional in the Cordilleran miogeocline in Canada Gordey et al. Whatever the cause of the pre-Late Devonian deformation in the miogeocline, it was essentially contemporaneous with the compressional deformation and the obduction of continental slope and rise deposits over the cratonic margin in the southwestern U.

Cordillera during the Antler Orogeny Turner et al. The apparent synchroneity between the epeirogenic deformation that defined the basins and arches on the stable craton, and the orogenic deformation that is expressed as faulting and volcanism along a "passive" margin that wasn't really tectonically passive is a long-standing enigma Porter et al.

Recently, Kominz and Bond used basin modelling to argue for a nearly synchronous episode of rapid subsidence between Middle Devonian and earliest Mississippian time in almost all pre-existing margins and interior basins of North America and an accompanying large sea-level rise relative to the stable cratonic platform. They suggested that the timing and magnitude of basin subsidence and sea-level change are explicable in terms of the predictions of mantle-convection models for the assembly of supercontinents Gurnis, , in this instance the onset of the accretion of Pangea above a major zone of mantle downwelling Kominz and Bond, Subsidence of the Western Canada Sedimentary Basin during the foreland basin stage can be ascribed to isostatic flexure of the North American lithosphere under the weight of the tectonically thickened supracrustal rocks of the foreland thrust and fold belt Price, As in the case of the Pleistocene ice sheets with their proglacial lakes, the isostatic flexure of the lithosphere induced by the weight of the tectonically prograding foreland accretionary prism produced a migrating moat that trapped the detrital outwash eroded from the emerging foreland thrust and fold belt Fig.

Beaumont used a mathematical model of the process to demonstrate, on the basis of the record of sedimentation and erosion preserved in the foreland basin, that large volumes of rock have been eroded from the thrust and fold belt since thrusting began and that up to 10 km of rock has been eroded since the thrusting stopped.

The pattern of growth and evolution of the foreland basin can be illustrated by comparing the records of subsidence that are preserved in a series of stratigraphic sections of the foreland basin sequence across the Rockies in the Crowsnest Pass area. The cumulative subsidence curves in Figure 2. They do not take into account the effects of post-depositional compaction of the sediments, which has reduced the thickness of the sediment and therefore the apparent amount of subsidence, or the isostatic effect of the weight of the sediment itself, which has amplified the 'tectonic' subsidence; nevertheless, the curves do illustrate the basic pattern of time and space variations of subsidence across the deformed part of the foreland basin.

The initial phase of subsidence in the westernmost preserved part of the foreland basin, which was marked by the deposition of the upper part of the Fernie Formation and the overlying Kootenay Group Fig. The initiation of subsidence in the foreland basin probably can be correlated with the change in style of deformation at the suture zone, from westward subduction under Intermontane Superterrane of oceanic lithosphere that lay outboard of the North American continental margin to "collision" between Intermontane Superterrane and the outboard part of the miogeocline.

The "collision" initiated the development of the accretionary prism that eventually expanded to become the Rocky Mountain Foreland Thrust and Fold Belt Fig. After a brief hiatus, marked by the sub-Blairmore unconformity, a second major episode of subsidence of the foreland basin occurred with the deposition of the Blairmore Group in late Early Cretaceous time.

The areas of greatest subsidence associated with both of these earlier episodes of foreland basin subsidence were restricted to relatively narrow belts immediately adjacent to the advancing thrust front, which was still situated outboard from the rifted margin of the North American continent Fig. The amount of tectonic thickening by folding and thrusting of the supracrustal wedge during the development of the accretionary prism is, in general, uncertain because a large but indeterminate part of the accretionary prism has been removed by later erosion.

The preservation of Lower Oligocene fanglomerates along the down-dropped hanging wall of the Flathead Normal Fault in southeastern British Columbia Fig. Lower Cretaceous strata of the Blairmore Group that are preserved beneath the Lower Oligocene fanglomerates in the hanging wall of the Flathead Fault imply that in Early Oligocene time, when the initial displacement on the normal fault was underway, the Lewis Thrust Sheet in the uplifted footwall block of the normal faults still contained a stratigraphic section extending from the Middle Proterozoic Purcell Belt Supergroup through the Paleozoic and lower Mesozoic to the Lower Cretaceous Blairmore Group, and, therefore, that the Lewis Thrust Sheet was still at least 8 km thick.

Some additional, but indeterminate thickness of Mesozoic strata had already been eroded from this part of the thrust and fold belt in the interval of more than 20 Ma since earliest Eocene time, when the accretionary prism had stopped growing because of the onset of regional extension in the interior of the Cordillera Parrish et al.

Accordingly, the time-depth curve of Figure 2. The third cumulative subsidence curve Fig. The autochthonous Exshaw strata at this locality are overlain by tectonically thickened Mesozoic strata beneath the west-verging Waldron Fault Fig. On the basis of variations in the moisture content of near-surface coal seams, Nurkowski estimated that between 1 and 2 km more sediment has been eroded from this area than from areas farther east in the Interior Plains.

The cumulative subsidence curve of Figure 2. Superimposition of the three cumulative subsidence curves Fig. It shows how the locus of maximum subsidence migrated northeastward with time as the accretionary prism was tectonically prograded over the flank of the continental craton. The collisions between North America and the accreted superterranes were driven by oblique convergence with adjacent oceanic lithosphere Engebretson et al.

They involved transpressional and transtensional deformation, including the development of large strike-slip faults within the Cordillera, and lateral transformations between strike-slip faulting and either thrust faulting and folding or extension faulting Monger and Price, ; Price, ; Price and Carmichael, ; Price et al.

Lateral transformations between strike-slip faulting and thrust faulting and folding during mid-Cretaceous to Late Eocene right-lateral transpression had a profound influence on lateral variations in the size of the Rocky Mountain Foreland Thrust and Fold Belt and in the amount of subsidence in the foreland basin.

This deflection in the surface trace of the TT-NRMT fault zone coincides with a conspicuous change in 1 the amount of Late Cretaceous and Paleocene horizontal shortening across the foreland thrust and fold belt, 2 the width and height of the Rockies, and 3 the amount of subsidence in the foreland basin.

Price Price et al. Early and Middle Eocene right-lateral slip on the Yalakom-Ross Lake fault system has recently been documented by Coleman and Parrish These relations are illustrated in Figure 2. The southward transformation of right-lateral strike-slip on the TT-NRMT fault zone into compressional deformation in the southern Canadian Rockies was the dominant influence on the evolution of the Cordilleran foreland basin during Late Cretaceous and Paleocene time because it controlled the amount of horizontal compression and vertical thickening of the supracrustal rocks in the foreland thrust and fold belt and thereby the amount of isostatic subsidence imposed on the North American lithosphere by the weight of the tectonically thickened supracrustal rocks.

The net subsidence of the basement beneath the Rocky Mountain Foreland Thrust and Fold Belt since the Late Jurassic, and the net uplift of the surface can be estimated by comparing a palinspastic section of the thrust and fold belt, in which the horizontal datum is the boundary between the marine strata of the Fernie Formation and the non-marine strata of the Kootenay Group, with the corresponding structure section Price, The palinspastic section gives the depth to the basement, as it was in the Late Jurassic, just prior to the beginning of isostatic subsidence in response to the weight of tectonically thickened supracrustal rocks and of the weight of the sediment in the foreland basin, whereas the structure section gives the present depth to the basement and the present elevation of the surface along the same line of section after about 60 Ma of erosion and isostatic uplift that followed the termination of thrusting and folding in the foreland belt at the beginning of the Eocene.

The net subsidence of the basement and the net uplift of the surface are clearly illustrated by superimposing the palinspastic section on the structure section, using sea level as the common datum. The southward transformation of right-lateral strike-slip on the TT-NRMT fault zone into compressional deformation, and the consequent deeper basement subsidence in the southern Canadian Rockies, is matched by a commensurate southward increase in the amount of Late Cretaceous and Paleocene subsidence of the foreland basin see Figs.

The conspicuous change in the pattern of subsidence in the foreland basin that occurred between the Cenomanian and the Campanian see Leckie et al. The palinspastic map of Figure 2. The map was prepared by compensating for the effects of: 1 about km of east-west crustal stretching in south-central British Columbia Price, ; Tempelman-Kluit and Parkinson ; Parrish et al.

The arrows on the palinspastic map of Figure 2. These relations characterize a distinct tectonic regime of right-lateral transpression that dominated the evolution of the Canadian Cordillera and the Western Canada Sedimentary Basin in Late Cretaceous and Paleocene time.

The map was prepared by compensating for the effects of 1 an additional km of right-lateral strike-slip on the TT-NRMT fault system, part of which was transformed southeastward into oblique compression, but part of which apparently was linked via the Finlay and Ingenika faults into the suture zone between Quesnellia and Stikinia Gabrielse, , and 2 about km of Late Cretaceous and Paleocene convergence between the displaced terranes and North America in the southern Canadian Rockies Bally et al.

The thickest accumulation of Late Jurassic and Early Cretaceous deposits during this stage in the evolution of the foreland basin occurred in and adjacent to the northern Canadian Rockies in northeastern British Columbia Stott, , adjacent to the zone in which Stikinia appears to have been driven into Quesnellia as an indenter Fig. Three stages in the tectonic co-evolution of the southern Canadian Cordillera and the foreland basin component of the Western Canada Sedimentary Basin are illustrated schematically in Figure 2.

Late Jurassic stage of terrane collision, indentation, and lateral escape Fig. The major shift in the locus of foreland basin subsidence that occurred between the first and the second stages was due to a change in patterns of relative movement of the accreted terranes and the resulting major shift in the locus compression, tectonic thickening and northeasterly displacement of the miogeoclinal rocks.

At the beginning of the Eocene, subsidence of the foreland basin was replaced by isostatic uplift and erosion because the change from transpressional deformation to transtensional deformation between North America and the accreted terranes shut off the convergence between the accreted terranes and North America that was driving the growth of the foreland thrust and fold belt and, therefore, of the foreland basin.

Displacement between North America and the main mass of the Cordilleran accreted terranes ended in the Middle Eocene at about 42 Ma, presumably because of global rearrangement in the pattern of relative movement of lithospheric plates that is clearly marked by the pronounced bend in the Hawaii-Emperor seamount chain.

The Alberta Geological Survey and, particularly, M. Madunicky assisted with the preparation of the figures. The overall interpretation of the relations between terrane accretion and the evolution of the Rocky Mountain Foreland Thrust and Fold Belt are the result of a long-standing collaboration with J.

The subsidence curves for the southern Canadian Rockies are the product of a collaborative project with P. Suggestions made by J. Wheeler and L. Lane have helped improve the presentation and documentation. Chapter 2 - Cordilleran Tectonics Atlas. Price - Queen's University, Kingston Introduction The Cordilleran Connection The connection between the Western Canada Sedimentary Basin and global plate tectonics lies in the Cordillera, because the origin and evolution of the Western Canada Sedimentary Basin was linked inextricably to the origin and evolution of the Cordillera, and thereby to the global plate tectonic processes that produced the Cordillera.

The Supracrustal Wedge The Western Canada Sedimentary Basin, as viewed from the perspective of a cross section of the continental lithosphere, is a very thin, northeastward-tapering wedge of supracrustal rocks overlapping the Precambrian crystalline rocks that form the core of the North American craton. Stages of Tectonic Evolution Two main stages in the development of the Western Canada Sedimentary Basin are distinguished by a profound change in provenance of the clastic sediment preserved within the supracrustal wedge Bally et al.

Retun to top Plate Tectonics and Cordilleran Tectonic Evolution Contemporary Cordilleran Plate Tectonics The present-day plate tectonic regime provides an actualistic model for outlining the role of plate tectonics in the orogenic evolution of the Cordillera. Reconstructions of Late Mesozoic-Tertiary Plate Motions Reconstructions of Late Mesozoic-Tertiary plate motions that are based on the analysis of sea-floor magnetic anomaly patterns and the tracks of mantle hot spots Engebretson et al.

Cordilleran Terranes Most of the Canadian Cordillera consists of a tectonic "collage" of allochthonous terranes Fig. Displacements of Terranes In the mobilist world of plate tectonics disparate terranes must be suspected of being far-travelled with respect to North America and to each other until proven otherwise Coney et al. Retun to top Foreland Basin Isostatic Flexure Subsidence of the Western Canada Sedimentary Basin during the foreland basin stage can be ascribed to isostatic flexure of the North American lithosphere under the weight of the tectonically thickened supracrustal rocks of the foreland thrust and fold belt Price, Migrating Depo-axis The pattern of growth and evolution of the foreland basin can be illustrated by comparing the records of subsidence that are preserved in a series of stratigraphic sections of the foreland basin sequence across the Rockies in the Crowsnest Pass area.

Retun to top Oblique Collision, Transpression and Transtension Mid-Cretaceous to Early Eocene Transpression The collisions between North America and the accreted superterranes were driven by oblique convergence with adjacent oceanic lithosphere Engebretson et al.

Retun to top References Archibald, D. Geochronology and tectonic implications of magmatism and metamorphism, southern Kootenay arc and neighbouring regions, southeastern British Columbia. Part 1: Jurassic to mid-Cretaceous. Canadian Journal of Earth Sciences, v. Bally, A. Structure, seismic data and orogenic evolution of the southern Canadian Rockies. Bulletin of Canadian Petroleum Geology, v. Beaumont, C. Foreland basins. Geophysical Journal of the Royal Astronomical Society , v.

Bell, R. In: Proceedings, Pacific Rim Congress ' Ben-Avraham, Z. Continental accretion: From oceanic plateaus to allochthonous terranes. Science, v. Benvenuto, G. Structural evolution of the Hosmer thrust sheet, southeastern British Columbia. Bhattacharya, J. Mossop and I. Shetsen comps. Bond, G. Construction of tectonic subsidence curves for the early Paleozoic miogeocline, southern Canadian Rocky Mountains - implications for subsidence mechanisms, age of breakup, and crustal thinning.

Geological Society of America, Bulletin v. Brown, R. Obduction, backfolding and piggyback-thrusting in the metamorphic hinterland of the southeastern Canadian Cordillera. Journal of Structural Geology, v. Butler, R. Discordant paleomagnetic poles from the Canadian Coast Plutonic Complex: Regional tilt rather than large-scale displacement?

Geology, v. Cant, D. Regional structure and development of the Peace River Arch, Alberta. Coleman, M. Eocene dextral strike-slip and extensional faulting in the Bridge River Terrane, southwest British Columbia. Tectonics, v. Coney, P. Cordilleran suspect terranes. Nature, London, v. Dalziel, I. Pacific margins of Laurentia and East Antarctica-Australia as a conjugate pair: Evidence and implications for an Eocambrian supercontinent. Davis, G.

Mesozoic construction of the Cordilleran "collage", central British Columbia to central California. In: Mesozoic paleogeography of the western United States. Pacific Coast Paleogeography Symposium 2. Howell, D. Dawson, F. Debiche, M. The motion of allochthonous terranes across the North Pacific basin. Geological Society of America Special Paper, no. Eisbacher, G. Sedimentology of the Dezadeash flysch and its implications for strike-slip faulting along the Denali Fault, Yukon Territory.

Late Proterozoic rifting, glacial sedimentation, and sedimentary cycles in the light of Windermere deposition, western Canada. Paleogeography, Paleoclimatology, Paleoecology, v. Engebretson, D. Estimates for the age, geometry, and amount of oceanic lithosphere subducted along western North America since the Jurassic.

Relative motions between oceanic and continental plates in the Pacific basin. Geological Society of America, Special Paper, no. Ewing, T. Paleogene tectonic evolution of the Pacific Northwest. Journal of Geology, v. Fritz, W. Chapter 7: Cambrian to Middle Devonian assemblages. In: The Cordilleran Orogen: Canada. Gabrielse and C. Yorath eds. Wheeler ed. Gabrielse, H. Major dextral transcurrent displacements along the Northern Rocky Mountain Trench and related lineaments in north-central British Columbia.

Geological Society of America, Bulletin, v. Late Paleozoic and Mesozoic terrane interaction in north-central British Columbia. Geological Survey of Canada, Open File Geoscience Canada, v. Gardner, M. Gordey, S. Structure section - Mackenzie fold belt. Gordy, P. Geological guide for the C. Calgary, Canadian Society of Petroleum Geologists, 93p. Gurnis, M. Large-scale mantle convection and the aggregation and dispersal of supercontinents. Nature, v. Irving, E. New paleomagnetic evidence for displaced terranes in B.

Geological Association of Canada Special Paper, v. Paleomagnetism: review and tectonic implications. Jefferson, C. Correlation of Middle and Upper Proterozoic strata between northwestern Canada and south and central Australia. Geological Society of America, Abstracts with Programs, v. Unpublished Ph. Klepacki, D. Kominz, M. Unusually large subsidence and sea level events during middle Paleozoic time: New evidence supporting mantle convection models for supercontinent assembly.

Krause, F. All of these areas underwent a major invasion of the sea at some time between about and million years ago, which lasted until late in the Devonian period, about million years ago. As a result a gigantic inland sea developed, probably covering almost the entire continent. In it, some of the earliest invertebrate life forms flourished, including trilobites, brachiopods and corals. The limestones that were deposited at this time are well seen in the front ranges of the Rocky Mountains , in the Niagara Gorge and underlying Parliament Hill in Ottawa.

Reefs formed by primitive colonial organisms flourished in the area corresponding to present-day Alberta, and became the host for much of the oil and gas that forms the foundation for the modern economy of that province. At the time the marine invasion of Canada was taking place, the giant continent, Rodinia, was beginning to break up, forming new oceans.

Evidence of this can be seen along the continental margins of interior British Columbia and in Newfoundland , where the edge of the Canadian Shield is thin and faulted, and covered with shallow-marine sedimentary rocks.

Off what is now eastern North America an ocean developed that probably rivalled the modern Atlantic Ocean in size, but it did not last long. By the end of Cambrian time about million years ago a process called subduction began. During subduction, oceanic crust bends down and is swallowed up beneath the continental margin, and the tectonic processes that result include the building of mountain belts and chains of volcanoes, resulting in much disruption of the local geology.

The oceanic crust that undergoes subduction is reabsorbed into the mantle. Geological evidence indicates that the island of Newfoundland contains remnants of the destruction by subduction of an entire ocean. The Avalon Peninsula , in the east, represents a sliver of a continent that formerly lay somewhere between Britain and Africa.

The belt of deformed rocks that developed during this oceanic closure extends southwestwards through the Atlantic provinces and down the eastern margin of the United States, and is collectively referred to as the Appalachian Orogen.

Its formation took a long time, and was only completed about million years ago, when the tectonic plates corresponding to modern Europe and Africa finally collided with and welded against the eastern North American margin.

These collisions were the culminating events in the generation of a new supercontinent, Pangea. North of the Arctic Platform, in the area that corresponds to the Queen Elizabeth Islands , lie two overlapping sedimentary basins Franklin Basin and Sverdrup basin making up the Innuitian Orogen. The two basins have been accumulating sedimentary deposits since the Cambrian period, million years ago.

Until about million years ago, two small continental plates corresponding to parts of modern Alaska and Siberia lay just to the north, and shed sediment into these Arctic basins. In the late Paleozoic era, reverberations from the plate collisions that generated Pangea were transmitted across Greenland, and led to folding and faulting in Ellesmere and Axel Heiberg Islands. In the Cretaceous period, some million years ago, sea-floor spreading created the Arctic Ocean as Alaska and Siberia rotated counter-clockwise into their present positions.

The modern continental configurations began to take shape about million years ago, with the breakup of Pangea. The Atlantic Ocean began to open in the east, and the North American continent began a long, slow drift in a northwesterly direction. This took Canada from equatorial to more northern latitudes for the first time, and the westward drift across the ancient Pacific Ocean caused several large landmasses that were located out on the Pacific oceanic crust to collide and become amalgamated with the western edge of the continent.

Most of British Columbia was formed in this way and the succession of collisions, volcanic episodes, and periods of metamorphism and folding are what largely account for the rugged nature of the Cordilleran belt that extends from Alaska down through the western United States.

One important effect of the collisions was to push giant slabs of rock lying at the edge of the Interior Platform of Alberta and the Northwest Territories upwards and eastwards along thrust faults to form the front ranges of the present Rocky Mountains. Gravel, sand , silt and mud shed eastward from the uplift and erosion of these rising mountains, beginning about million years ago, form thick deposits of sandstone, siltstone and shale that lie on the Paleozoic limestones of the Interior Platform from the Yukon North Slope through Alberta and as far south as Texas.

The youngest rocks that form the topmost sedimentary layer immediately to the east of the Rockies , such as the Paskapoo Sandstone of Calgary , originated as debris carried eastward by the ancestors of the present-day Bow and Athabasca rivers.

Subduction of the Pacific Ocean plate continues offshore west of Vancouver Island. Volcanoes such as Mount St. Helens in Washington State are created by this process. Volcanic eruptions occurred near the Whistler Ski Centre only a few 1, years ago and could occur again, though geologists think that a major earthquake somewhere along the west coast is a more immediate danger.

The present-day east coast represents a ragged tear across the Appalachian Orogen. Part of this belt of rocks was left behind on the North American side by the opening of the Atlantic Ocean, and part was carried eastward and now underlies Britain and Scandinavia. As the continental margins were stretched and broken to form the margins of North America and Europe, beginning about million years ago, they subsided and became sites for the accumulation of thick sedimentary deposits derived from the adjacent continents.

In places, most notably beneath the Grand Banks off Newfoundland , this process was followed by the generation, migration and trapping of large volumes of oil which now form the Hibernia and other oil fields of that region. Eyles and A. Miall , Canada Rocks Douglas, ed, Geology and Economic Minerals of Canada 2 vols, Tour Canada from Space This website features an interactive map linked to spectacular images of various Canadian locations taken by Canadian research satellites.

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