Canada consists of 6 geological regions. Five of them are of Phanerozoic age (less than 570 million years old) and are arranged roughly concentrically around and partly on the sixth, the Canadian SHIELD (of Precambrian age, more than 570 million years old).
Canada consists of 6 geological regions. Five of them are of Phanerozoic age (less than 570 million years old) and are arranged roughly concentrically around and partly on the sixth, the Canadian SHIELD (of Precambrian age, more than 570 million years old). The Phanerozoic regions, which overlap in age, are the Innuitian Orogen, the Appalachian Orogen, the Interior Platform, the Canadian Cordillera and the continental shelves.
Each region has a distinctive architecture and is composed of building blocks of characteristic types and sequences of rocks (tectonic assemblages) that record its development (see alsoPLATE TECTONICS). Probably the most important developmental events in GEOLOGICAL HISTORY were the orogenies, periods during which compressive deformation, metamorphism, granitic intrusion and other processes formed MOUNTAIN RANGES.
The Canadian Shield is a mosaic of geological provinces in which those of Archean age (more than 2.5 billion years old) are surrounded by a matrix of Proterozoic rocks (2.5-0.57 billion years old). The Archean provinces (Superior, Slave, Kaminak and Nutak) were cratons (ie, stable, low-lying continental parts of the Earth's crust) by the end of the Kenoran Orogeny (2.5 billion years ago); now they are mainly metamorphic and granitic rocks with irregular belts of turbidites and "greenstone" (metamorphic basic volcanics) 18 km in thickness. The Proterozoic provinces (Bear, Churchill, Labrador and Southern) are marked by orogenic trends developed during the Hudsonian Orogeny (1.9-1.8 billion years ago). The Shield was finally completed on the SE by the addition of Grenville Province about 1 billion years ago.
Interior and Related Platforms
At the end of Precambrian time (0.57 billion years ago), the Canadian Shield was a stable craton of low relief forming the foundation of the North American continent. In early Palaeozoic time the Shield was depressed slightly, and much of it was gradually covered by warm, shallow seas. This inundation resulted in deposition of interrupted platformal sequences (1-3 km thick), now preserved in the Interior, Arctic, Hudson and St Lawrence platforms. These sequences generally consist of quartz-rich sandstone, overlain by interbedded carbonate and shale.
By mid-Palaeozoic time, differential uplift and subsidence across the Shield had created a network of arches and intervening basins which led to interruptions in the depositional sequences. The uneven relief, together with the growth of carbonate reefs, produced enclosed basins in the central Interior Platform, in Hudson Platform and in Michigan Basin where evaporites (eg, salt, potash) accumulated.
Platformal sequences of the Interior Platform, containing rocks from Cambrian to Jurassic age, are overlain by a foreland basin which migrated eastward and covered much of the platform ahead of advancing pulses of deformation and uplift in the Cordillera. Sandstone, conglomerate, shale and coal accumulated in 2 major clastic wedges of late Jurassic-early Cretaceous and late Cretaceous-Palaeocene age respectively. The lower wedge reflects several lesser, northward-migrating pulses.
The 2 major wedges are separated by mid-Cretaceous marine shales deposited in a shallow sea that covered Interior Platform. A clastic wedge in the Mackenzie Basin accumulated in pulses from early Cretaceous to Eocene time. Lower Cretaceous coal-bearing clastics were deposited in the Moose River Basin on the Hudson Platform.
The Appalachian Orogen records the latest Precambrian rifting of the ancestral Canadian Shield, the earliest Palaeozoic opening of the Iapetus Ocean to the east, its mid-Ordovician closing and related deformation, and mid- and late-Palaeozoic folding and faulting. The orogen consists of several zones of which only the innermost, Humber Zone, was deposited directly on ancestral North America. There, Grenville Province basement is overlain by Upper Proterozoic rift clastics and volcanics covered by a more extensive Cambrian-Ordovician sandstone-carbonate shelf assemblage. Succeeding clastics were derived from the southeast, heralding the early Palaeozoic arrival of a slice of off-shelf assemblage and higher slices of Ordovician oceanic crust thrust northwestward from Dunnage Zone and now stacked above the clastics.
Dunnage Zone consists of rock complexes which floored the Iapetus Ocean: pieces of upper mantle-oceanic crustal sequences overlain by Lower Ordovician arc volcanics, related clastics and local masses of jumbled blocks of volcanics and clastic rocks surrounded by Ordovician shale. The ocean's eastern margin may be represented in Gander Zone by pre-Middle Ordovician clastics, associated with gneisses in Newfoundland and overlain by arc volcanics in NB. Zonal boundaries are straddled by overlying Upper Ordovician-Silurian marine and continental sediments and volcanics, indicating that Dunnage and Gander zones were accreted to North America during the mid-Ordovician Taconian Orogeny.
Avalon Zone is a foreign fragment composed of Proterozoic partly glacial sediments and volcanics, overlain by Cambrian sediments containing FOSSILS different from those in Humber Zone and St Lawrence Platform, and capped by Ordovician clastics with iron formation. Avalon Zone accreted to Gander Zone during mid-Palaeozoic Acadian Orogeny, resulting in steeply inclined folds and faults and extensive granitic intrusion across the entire orogen, and disruption along steep faults in nearby platforms.
Meguma Zone is also foreign because it includes folded Cambrian-Ordovician turbidites, derived from an unknown source to the southeast, and distinctive granites of Devonian age unlike those associated with Acadian Orogeny. Meguma Zone was accreted to Avalon Zone along steep faults with horizontal slip before both zones were partly covered by continental, coal-bearing Carboniferous clastics. The region contains evidence recording the final assembly of the supercontinent Pangea. The development of rifts in Triassic time (245-208 million years ago), the deposition in them of rift clastics and basalt and the intrusion of alkaline rocks in the Cretaceous are associated with the breakup of Pangea and the subsequent opening of the Atlantic Ocean in the Mesozoic.
The Innuitian Orogen began with earliest Palaeozoic rifting of northern ancestral North America, followed by early Palaeozoic deposition of shelf and off-shelf sediments, the latter in a deep trough bounded on the north by foreign continental fragments. A younger, overlapping clastic wedge was deformed in the mid-Palaeozoic and, finally, the orogen was partly superimposed by a mainly Mesozoic clastic basin, which was itself deformed in latest Mesozoic and early Tertiary time.
The Arctic Platform merges northward into a Cambrian-Lower Devonian southern shelf assemblage of 5 km of carbonate, shale and evaporite. It is adjoined by Hazen Trough, containing Lower Cambrian rift clastics overlain by deep-water shale and 3 km of turbidites. Hazen Trough is bounded on the north by the Northern Belt which comprises continental fragments (largely volcanics and granites). Northerly derived turbidites indicate that the Northern Belt was uplifted, following the collision (along a zone marked by oceanic ultramafic rocks) of Proterozoic and Middle Ordovician fragments in mid-Ordovician time.
This collision ended arc volcanism in the Northern Belt. It was followed by pulses of folding and uplift in Silurian and Devonian times and by granitic intrusion in the Devonian. Uplifts in northeastern and eastern parts of the orogen shed detritus to a southeastward-advancing Middle and Upper Devonian clastic wedge. It was compressively folded during the Ellesmerian Orogeny (about 345 million years ago) which migrated from the Northern Belt to the Southern Shelf.
Subsequently, stretching and subsidence led to the deposition of 12 km of sediments in Sverdrup Basin, mainly of Mesozoic rift clastics and basaltic lava and intrusion, and some Upper Palaeozoic evaporites that were locally squeezed upward into overlying sediments. The latest Cretaceous uplift in the eastern Sverdrup Basin shed a clastic wedge across the basin to the continental margin. This was succeeded by the Eurekan Orogeny (about 65 million years ago) which created broad folds and thrust faults directed south and east. At the same time, more than 3 km of clastics accumulated in local basins on land and in narrow, marine fault-troughs. Late Cenozoic vertical faulting produced the present topography.
In the Canadian Cordillera rifting and continental shelf development took place from Proterozoic to mid-Mesozoic time in the Eastern Belt. Elsewhere, foreign fragments were accreted successively to western North America in mid- and late-Mesozoic time. Each accretion was followed by orogeny and deposition of a clastic wedge; related metamorphism and granitic intrusion were restricted mainly to zones straddling the boundaries of accreting blocks. Since the late Mesozoic, the Pacific Ocean Basin has slid under and horizontally past the Cordillera along northwest-trending transform faults.
A long-lived phase of continental margin sedimentation from Proterozoic to Jurassic time (2.5 billion-144 million years ago) is recorded in the Eastern Belt in at least 3 rift and shelf assemblages, totalling 25 km in thickness. The 2 Middle Proterozoic assemblages reflect continental rifting and separation 1.5 billion years ago, and deposition of 2 clastic-carbonate shelf sequences, the first of which was deformed 1.35 billion years ago. They were accompanied by development of connecting, easterly trending fault-troughs that protruded into the continent.
The third assemblage comprises partly glacial Upper Proterozoic rift clastic overlain by Cambrian to Jurassic shelf sequences continuous with thinner sequences in Interior Platform but passing westward off the shelf to shale and turbidites. The northern Yukon underwent Devonian-Mississippian folding, granitic intrusion and uplift. This and other northern Cordilleran uplifted areas shed clastics southeast across the continental margin.
An accretionary phase in late Jurassic to Cretaceous time resulted from the westward drift of North America and its collision with several northward-moving Pacific plates. Several foreign blocks were amalgamated into 2 large composite fragments (Intermontane and Insular belts) which, as a result of this collision, were then accreted successively to North America. The foreign blocks show fossil and other evidence of displacement through 30° or more of latitude.
The Intermontane Belt consists of 2 extensive Upper Triassic-Middle Jurassic volcanic arcs, each built largely on an Upper Palaeozoic foundation. They are separated by an oceanic terrane which includes subduction complexes of repetitively stacked accretionary wedges and of jumbled blocks of basalt, ultramafic, chert and blueschist. The eastern arc is bounded on the east by a region of Upper Palaeozoic oceanic crust, telescoped over a small Palaeozoic terrane that was deformed before Mississippian time.
The Omineca Crystalline Belt embraces the region where the Intermontane composite fragment collided with and partly overrode the Eastern Belt. This mid-Jurassic collision gave rise to the Columbian Orogeny. Rocks of the outer part of the continental margin and the adjacent Intermontane fragment were compressed, metamorphosed and displaced into stacked sheets transported mainly eastward, but also partly westward, from a central metamorphic core zone. From the resulting uplift, clastics were shed mainly eastward into a foredeep and partly westward into a backdeep. Eastward displacement over the Canadian Shield resulted, in effect, in a westward subduction of continental crust which, when partially melted, gave rise to mid-Cretaceous granites in eastern Intermontane and Eastern belts.
Insular Belt comprises 2 large terranes amalgamated in late Jurassic time. One is mainly of Palaeozoic sediments and volcanics; the other a Triassic to mid-Jurassic sequence of basalt, carbonate and arc volcanics, overlying an Upper Palaeozoic basement. The outer margins of the belt contain remnants of Upper Mesozoic and Cenozoic subduction complexes, suggesting prolonged east-sloping subduction of oceanic crust beneath the Insular Belt.
In late Cretaceous time an Insular composite fragment collided with and accreted to the Intermontane Belt which was by then part of North America. This process created the Coast Plutonic Complex, a belt, uplifted in Cenozoic time, consisting of extensive granitic plutons and of medium-grade metamorphic rocks that straddle the boundary between these composite belts.
Contemporaneously, arc volcanism and related intrusions of granitic plugs took place in the Intermontane Belt while in the Eastern Belt further thrust faulting and uplift during Laramide Orogeny completed the building of the Rocky Mountains. A related clastic wedge was deposited in a foredeep extending into the western Interior Platform. In the late Cenozoic, extensive crustal stretching resulted in extrusion of widespread sheets of fluid basalt over much of the Intermontane Belt and of local rift and arc volcanics in its western part.
The Canadian continental shelves are the youngest additions to Canada's landmass. The arctic and eastern shelves developed along continental margins adjacent to opening oceans; the Pacific Shelf formed along a continental margin that slides over or horizontally past oceanic crust. The shelves form submarine terraces 5-700 km wide and up to 300 m deep. With the exception of the Pacific Shelf, they are underlain by relatively undeformed Mesozoic and Cenozoic prisms of sedimentary rocks (3-13 km thick), composed of materials eroded from the continent, draped over older rocks along its margin.
The Arctic Shelf evolved following opening of the Arctic Ocean basin, possibly from the counterclockwise rotation of Alaska 120 million years ago. Subsequently, 4 km of shelf sediments were superimposed by 9 km of Upper Cretaceous-Cenozoic clastics of the Mackenzie Delta which progressively expanded northward. The eastern shelves built outward as the Atlantic Ocean basin opened and as North America separated from Africa 165 million years ago, from Europe 100-90 million years ago, and from Greenland 70-60 million years ago.
Eastern shelves comprise 2 or 3 tectonic assemblages: a lower rift assemblage in fault troughs containing clastics, evaporites and volcanics lying on Precambrian and Palaeozoic rocks; an upper, more extensive clastic-carbonate shelf assemblage deposited during a continental drifting phase after the continents separated; and a final capping of Pleistocene glacial deposits. Continental crust is 35 km thick under the inner shelf, thinning to 15-20 km under the outer shelf.
The narrow Pacific Shelf lies along an active margin marked by numerous EARTHQUAKES. West of Vancouver I, the oceanic Juan de Fuca and Explorer plates are descending beneath the continent along a northeasterly sloping subduction zone. This has resulted in 2 modern assemblages: an active volcanic arc 300 km northeast of the foot of the Pacific Shelf, where the subduction zone emerges; and Upper Mesozoic and Cenozoic accretionary prisms of clastics and oceanic volcanics scraped off the descending plate and stacked in easterly tilted wedges beneath the narrow shelf. Northwest of Vancouver Island, however, oceanic crust of the Pacific Plate is sliding horizontally northwest past the continent along a transform fault.
Superior Province is the largest Archean province. Its southern part comprises easterly trending belts, alternating between gneisses and greenstone. Some 3.8-billion-year-old gneisses occur in the extensions of Superior Province into Minnesota and of Nutak into Greenland. Slave and Kaminak cratons resemble Superior craton in overall aspect, age of contained rocks, and nature and timing of structural development. In Slave craton, however, weakly metamorphosed turbidites (ie, SEDIMENTARY ROCKS formed in water) predominate.
Churchill Province, the largest of the Lower Proterozoic provinces, includes the sinuous, intensely deformed, metamorphosed and intruded "Reindeer-Circum-Ungava" belt and the reworked fragments and edges of Archean cratons in the "Northwest Churchill" domain. The southern margin of the "Circum-Ungava" belt contains 6-10 km of Lower Proterozoic layered rocks overlying the Archean basement rocks of Superior Province.
This rock succession, reflecting the separation of parts of Superior Province, comprises local clastic-volcanic rift assemblages overlain by a clastic-carbonate shelf assemblage, succeeded by iron formation (seeMINERAL RESOURCES). The shelf sediments pass outward into oceanic basalt with some ultramafic lava (ie, lava rich in iron and magnesium). Convergence and orogeny are manifested along the southern margin of the "Circum-Ungava" belt by folds and thrust faults. The northern margin of the belt is a rough mirror image of the southern margin.
The "Reindeer" belt is a region of complexly deformed metamorphic rocks that includes the Flin Flon and Lynn Lake volcanic arcs separated by a turbidite basin. The northwest margin contains metamorphosed remnants of shelf sediments interfolded with Archean basement that extends into the latter and was probably a semistable extension of Slave Craton, variably affected by the Kenoran and Hudsonian orogenies. It contains remnants of a thin veneer of Lower Proterozoic rocks consisting of 1.5 km of quartz-rich clastics deposited in restricted basins, succeeded by a more extensive, 3-km-thick, platformal assemblage of quartz-rich sandstone, carbonate, local clastics and volcanics. Much of the domain is characterized by the jostling of large blocks of Archean basement, brought together along major faults. In the Foxe Fold Belt, Archean basement and the overlying Proterozoic sediments are tightly folded together; elsewhere they are more broadly folded.
Southern Province contains a southeasterly thickening wedge of Lower Proterozoic, quartz-rich, shelf clastic sediments and subordinate mixed volcanics (7-11 km thick). The wedge accumulated along the southern edge of Superior Province. Its lowest part includes uranium-bearing conglomerate; its upper (northeast) part contains sediments deposited during an ancient ICE AGE. The southeast part of the wedge was variably metamorphosed and deformed by folds and thrust faults, pushed north to form the Penokean Fold Belt. The intensity of metamorphism and deformation increases southward, reflecting the convergence of a 3.5-billion-year-old Archean cratonic block northward against Superior Craton (2.2-1.7 billion years ago). Sudbury Basin and its associated nickel-rich intrusion may have resulted from meteorite impact about 1.85 billion years ago (seeMETEOR).
Bear Province consists of the northerly trending Wopmay Orogen overlapped (in the north) by younger Proterozoic rocks. Westward from Slave Craton the orogen comprises 4 zones, all cut by northeast-trending strike-slip faults. The first zone is a thin veneer of platform carbonate and sandstone superimposed by the outer part of a westerly derived clastic wedge. An eastward thrust and folded assemblage (5 km thick) of shelf clastics and carbonate is overlain by the wedge. A clastic-volcanic rift assemblage (10 km thick) overlain by fine-grained off-shelf clastics, then by early clastic wedge rocks, was deformed, metamorphosed and intruded by 1.89-billion-year-old granites.
Finally, in Great Bear Belt, 1.87-billion-year-old arc volcanics (8 km thick) were deposited on 1.92-billion-year-old basement and are overlapped by younger, more extensive arc volcanics. The orogen began its complex evolution 2.1 billion years ago with the splitting of Slave Craton along northerly and east-northeasterly trending rifts. This led to the deposition of a continental margin assemblage along the western edge of Slave Craton. The assemblage was deformed, metamorphosed and intruded by granites 1.9 billion years ago in response to the collision of a microcontinent from the west. Subsequently, east-sloping subduction led to arc volcanism which extended onto the continent.
A second collision farther west generated northeast- and southeast-trending strike-slip faults across the entire orogen. The indentation of Churchill Province by the rigid Slave Craton cracked the Churchill crust and possibly gave rise to 1.8-billion-year-old alkaline volcanics in the Thelon Basin.
The middle and late Proterozoic record is fragmentary but nevertheless indicates stability of the Canadian Shield from 1.8-0.57 billion years ago. In Middle Proterozoic time, an extensive 1.5-billion-year-old continental and marine sandstone-carbonate assemblage (preserved in separate, partly fault-bounded basins, eg, Athabasca Basin) was deposited over much of the Shield, overlapping 1.8-billion-year-old alkaline volcanics.
Concurrently, from Labrador southwest into Grenville Province, huge masses of anorthosite (composed almost entirely of plagioclase FELDSPAR) and related granitic rocks were intruded. Concurrent and later widespread stretching across much of the Shield resulted in rifting, dike intrusion and volcanism. About 1.3 billion years ago, rifting, accompanied by basaltic volcanism and intrusion of alkaline rocks, began in Labrador parallel to Grenville Front. Enormous amounts of basalt were extruded in the Lake Superior region, accompanied by alkaline intrusions nearby, during the formation of a major continental rift that extended northeast from central Kansas to Lake Superior and southeast into Michigan (1-1.3 billion years ago). Depression of the crust from the weight of the volcanics created a basin in which clastic sediments were deposited until the end of Precambrian time.
Concurrently, swarms of basic dikes 1.2 billion years old were intruded along northwesterly and northeasterly trends across much of the Shield; related basalts were extruded into parallel northwesterly trending sediment-filled rifts on Baffin Island and as lava plateaus in northern Bear Province.
The convergence of Grenville Province against the adjacent provinces to the northwest completed assembly of the Canadian Shield (0.9-1.2 billion years ago). Near Grenville Front this resulted in thickened crust and northeasterly trending structures pushed towards the northwest. Subsequent uplift and erosion reveals widespread metamorphic rocks.