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Geologická stavba Kanady (Geology of Canada)

07.03.2010
Geologická stavba Kanady (Geology of Canada)

The geological assembly of Canada has resulted from the convergence, collision, and separation of distinctive continental and oceanic fragments at various times over the last four billion years. Canada's geological architecture is dominated by its central foundation - the Precambrian shield -- the largest area of Archean rocks (> 2.5 billion years) in the world and containing its oldest rocks, dated at 4.0 billion years. The shield consists of several Archean fragments comprising granitic rocks and gneiss laced with sinuous greenstone volcanic belts and broader tracts of sedimentary rocks. Orogenic belts between the Archean fragments, such as those flanking the Superior Province or Slave Province, contain younger, Paleoproterozoic (2.5 to 1.6 billion years) rocks representing continental, oceanic, and collisional deposits and foreign or exotic fragments. Although most of the Precambrian shield was consolidated by the end of Paleoproterozoic time (known as Laurentia), its southeast portion (the Grenville Province) was stabilized about one billion years ago. The addition of Mesoproterozoic (1.6 to 1.0 billion years) and older gneisses and granitic rocks to Laurentia in the Grenville orogenic belt thus completed the assembly of the Precambrian shield.

Three younger deformed belts, mainly of Phanerozoic rocks (> 545 million years), surround the shield. The Appalachian belt, in the southeast, contains large tracts of continental and oceanic fragments that were attached to ancestral North America in early- and mid-Paleozoic times, about 475 and 375 million years ago, respectively. Similarly, in the Cordillera of western Canada, large areas of continental and oceanic fragments were added to ancestral North America in the Mesozoic era at various times during the last 180 million years. By contrast, in the Innuitian belt in the Arctic Islands, only a small foreign fragment was attached in the mid-Paleozoic about 400 million years ago. After the belts were deformed they were superimposed by extensive less deformed basins dominated by sedimentary rocks in the Appalachian and Innuitian belts, but by volcanic rocks in the Cordillera.

Large parts of the Precambrian shield are covered by a thin veneer of undeformed sedimentary rocks which, except for Western Canada Basin east of the Cordillera, are mainly of early Paleozoic age. Locally, where the Paleozoic rocks are overlain by Cretaceous strata, they form basins, as in Hudson Bay, and fault-bounded troughs in and near Hudson Strait and north of Baffin Island, related to the rifting and opening of Labrador Sea and Baffin Bay. Crustal spreading and ocean opening, east of the passive eastern continental margin, ceased west of Greenland in the early Cenozoic, nearly 40 million years ago, but still continues in the mid-Atlantic Ocean.

A westward-thickening wedge of Mesozoic and Cenozoic sediments was deposited in Western Canada Basin. Its sediments were derived from elevated sources in the mountains of the deforming ancestral eastern Cordillera.

The Pacific margin of Canada, by contrast with the passive Atlantic margin, is active. The North American continent is overriding and sliding past the Pacific Ocean crust, thereby generating earthquakes (largely offshore) and volcanoes in the western Cordillera.

Geoogical maps of Canada at: http://gsc.nrcan.gc.ca/map/1860a/ex_e.phpžba

ZDROJ: Geological Survey of Canada

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