français

Explanatory Notes

Time Scale

Colour Design

Onshore geology

An effort has been made to choose colours for Precambrian units not used for the Phanerozoic: violet and lavender for Paleoproterozoic supracrustal rocks; yellowish-green for a Paleo-Mesoproterozoic unit; orange hues for the Mesoproterozoic; and shades of brown for the Neoproterozoic. Some colours, however, are repeated. Shades of rose and pink are used for the Archean plutonic rocks that dominate the Canadian Shield but, in general, are paler than the colours of the Paleozoic granitoid units. Grey is repeated in representing Archean sedimentary and paragneiss units but the green colours showing Archean greenstone belts are different from those depicting Mesozoic units. The most obvious colour repetition is the use of bold red colours for both the Mesozoic and Paleoproterozoic granitoid units. Bold red colours are important for the latter to highlight the magmatism associated with the Paleoproterozoic zones of convergence which resulted in the present assembly of Canadian Shield, except for the Grenville Province (Hoffman, 1989).

Ice caps are shown only locally, as on Baffin Island and in the eastern Arctic islands. Elsewhere icefields are not shown because they interfere with a clear portrayal of the bedrock geology.

Offshore geology

Beyond the limit of continental crust, the offshore geology is represented by age of oceanic crust rather than the sedimentary cover, although a few isopachs are shown to indicate the thickness of the cover (Tucholke, 1986; Jackson and Oakey, 1988). The intention is to highlight the plate tectonic framework within the oceans surrounding Canada, thereby displaying the contrasting relations between the convergent and transform margin of the Pacific Ocean with the passive continental margins of the North Atlantic and Arctic oceans which formed during ocean opening and crustal spreading.

Rifting between North America and Africa began in the Late Triassic resulting in a rifted margin off Nova Scotia and a transform margin south of Grand Banks. Seafloor spreading, thereafter, produced Jurassic and Cretaceous oceanic crust south of Grand Banks (Keen et al., 1990). Stretching and rifting of the crust between North America and Greenland began in the Early Cretaceous accompanied by formation of fault troughs, some of which contain Cretaceous sediments, in the region surrounding Labrador Sea and Baffin Bay (Balkwill et al., 1990). East-northeast seafloor spreading in Labrador Sea began in Late Cretaceous time but changed to north-northeast in the Early Eocene. Seafloor spreading is thought to have started in Baffin Bay in the latest Cretaceous or Paleocene, but it ceased in both Baffin Bay and Labrador Sea in the Late Eocene (Roest and Srivastava, 1989). On the other hand, seafloor spreading continued during the Cretaceous and Cenozoic in the North Atlantic where the Mid-Atlantic Ridge remains an active spreading centre today.

By contrast, the Neogene and younger Pacific margin is complex and variable. A convergent margin now exists west of Vancouver Island where the Juan de Fuca and Explorer plates, bounded on the west by actively spreading ridges, are being subducted beneath the North American Plate. Subduction is manifested by the deformation front offshore at the leading edge of an accretionary prism whose southern part emerges onto the Olympic Peninsula south of Vancouver Island. Subduction is also reflected in the chains of Neogene and younger calc-alkaline volcanic centres in the Coast Belt and by extensive, more easterly, correlative back-arc mafic lavas (Wheeler and McFeely, 1991).

A mainly transform margin extends from west of the Queen Charlotte Islands to the submergent Yakutat Terrane in the Gulf of Alaska. Along this segment the Pacific Plate continues to move northward relative to North America along dextral strike-slip faults.

North of the Yakutat Terrane the Pacific margin becomes convergent again where the Cordillera swings westward around the Gulf of Alaska. During the Neogene and Quaternary the Yakutat Terrane was carried northward by the Pacific Plate and was partially subducted beneath southern Alaska. This led to rapid uplift of the mountain ranges bounding the Gulf of Alaska, accompanied there by calc-alkalic magmatism and volcanism (Plafker et al., 1994).

Patterns and Symbols

Patterns are also used to display the distribution of evaporites within Meso- to Neoproterozoic strata within Minto Inlier on Victoria Island and in the southern part of Brock Inlier north of Great Bear Lake. Patterns also show the relationships of Devonian evaporites, carbonate, and shale near Great Slave Lake.

Elsewhere letters only are used to designate special types of sedimentary rocks such as Mesoproterozoic marble in the southwestern Grenville Province north and south of Ottawa, Paleoproterozoic conglomerate and sandstone west and north of Hudson Bay, and mélange in the Appalachian and Cordilleran orogens.

Volcanic rocks are generally shown by darker shades of colours of the equivalent sedimentary assemblages. Their composition is designated by letters. The composition of intrusive rocks is similarly noted by letters. Ultramafic mélange units are coloured the same as ultramafic intrusive rocks and are included with them in the legend because they represent proximal erosional products of oceanic ultramafic rocks and are therefore important in identifying the leading edge of oceanic accreted terranes.

Metamorphic rocks are most widespread in the Canadian Shield. There, medium grade metamorphic rocks (upper amphibolite facies), chiefly of Archean age, are designated by the letter "n" and for supracrustal rocks by letters denoting related compositions. Areas of granulite facies are shown by a "peck" pattern. Phanerozoic metamorphic rocks of upper amphibolite facies are shown in black stipple. They are most prominent on the west coast of Vancouver Island, in the Coast and Omineca belts of the Cordillera and in the Gander Zone of east-central Newfoundland. Metamorphic rocks in the Coast Belt whose protoliths are of uncertain age are shown in violet and designated by the letters "n" and "gn".

Attention is drawn to certain aspects of the following onshore geological features:

1. Volcanic centres represent volcanoes, vents, and plugs;

2. Clusters of Mesoproterozoic mega-breccias or diatremes cut Paleoproterozoic units north and east of Dawson, Yukon. A few Ordovician-Silurian diatremes occur in Mackenzie Mountains southwest of Norman Wells, another lies just west of the Rocky Mountain Trench in northern British Columbia, and one is associated with a Mississippian carbonatite in the Rocky Mountains north of Peace River. Additional clusters of diatremes mostly of Ordovician-Silurian age but including one of Devonian- Mississippian age are scattered over the southernmost Rocky Mountains in easternmost British Columbia.

3. Alaskan-type ultramafic bodies are roughly circular, comprising a dunite or wehrlite core surrounded by clinopyroxenite, hornblendite, and hornblende gabbro or diorite. Orthopyroxene is typically absent and plagioclase rare (Woodsworth et al., 1991). Those of Late Triassic age occur east and northeast of Bowser Basin and in southernmost Intermontane belt. Mid-Cretaceous bodies extend from near Prince Rupert northwest through southeastern Alaska.

4. Although few of the kimberlites have been well dated they range considerably in age. Kimberlites around Lac de Gras in central Slave Province have yielded Eocene and Late Cretaceous ages (Pell, 1995). Kimberlites in the Fort à la Corne area are late Early Cretaceous (Lehnert-Thiel et al., 1992) whereas the Cross kimberlite near latitude 50^oN in the Rocky Mountains is Permian-Triassic (Smith et al., 1988). A cluster in the Superior Province close to the Ontario-Quebec border yield Late Jurassic ages (Brummer et al., 1992).

5. A special symbol shows the leading or inboard edge of terranes accreted to ancestral North America during the Phanerozoic. In the Appalachian Orogen this boundary is the Baie Verte-Brompton Line (Williams and St-Julien, 1982; Malo et al., 1992) separating the lower Paleozoic passive continental margin sequence of the Humber Zone in the northwest from the Cambrian-Ordovician oceanic, ophiolite-bearing assemblage of the Dunnage Zone to the southeast. Accretion took place during the Early and Middle Ordovician Taconic Orogeny accompanied by emplacement of ophiolitic oceanic thrust slices onto the Humber Zone (Williams, 1995). Some parts of the accreted terrane boundary have been engulfed by plutons and overlapped by Silurian, Devonian, and upper Paleozoic cover rocks.

Pearya Terrane on northernmost Ellesmere Island in the Arctic is a fault-bounded composite continental fragment probably derived from the Caledonides. It is made up of Mesoproterozoic to Silurian rocks and contains an early Middle Ordovician suture coeval with the Taconic Orogeny. Pearya docked against the Franklinian deep water basin along sinistral faults, probably during Late Silurian-Early Devonian (Trettin, 1991).

The leading edge of accreted terranes in the Cordillera generally forms the boundary between the Devonian to Triassic oceanic Slide Mountain Terrane and the Neoproterozoic to Triassic pericratonic Kootenay Terrane. Locally, where these terranes are thin or missing the neighbouring Quesnel Terrane in the west is juxtaposed against the Neoproterozoic and younger continental margin assemblages in the east.

In the Yukon the leading edge of the accreted terranes is truncated by the through-going, northwest-trending Tintina Fault. Restoration of its dextral displacement of between 425 to 500 km (Gabrielse, 1991) permits the leading edge southwest of the fault to merge with the leading edge of the accreted terranes in a salient northeast of the fault.

The Slide Mountain and Quesnel terranes accreted to ancestral North America and its proximal pericratonic Kootenay Terrane in early Middle Jurassic time (Gabrielse and Yorath, 1991). At that time, thrust sheets now lying east of the leading edge of the accreted terranes, were emplaced tens of kilometres onto the carbonate continental shelf in a manner similar to that in the Appalachian Orogen.