Comparisons of the 2005 Geologic Map of North America with the 1965 Map, Areas 1-4
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This page shows areas of map comparisons in boxes 1-4. (Either scroll through pages or click on the boxes in the map to the left.)
- Alaska and parts of northwestern Canada
- Northwestern Canadian Shield
- Iceland and parts of eastern Greenland
- Eastern and Central United States and parts of southern Canada.
View comparisons of map areas shown in boxes 5-7.
- The western Caribbean, Cuba, and parts of Central America
- Cordillera of the northwestern United States and southwestern Canada
- High Plains, Laramide Rocky Mountains, Colorado Plateau and parts of the Basin and Range Province.
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| 1965 Map |
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2005 Map. Click on map for larger view (265 kb file). |
Principal differences between the 1965 map and the 2005 map are:
- Refinement of detail especially in the sedimentary rocks of the Brooks Range (A) and the Yukon Territory (B), and the depiction of the complex fold and thrust belts in these regions
- The addition of major terrane-bounding strike-slip and thrust fault such as the Kobuk-Malemute (C), Tintina (D), Border Ranges (E), Contact (F), Fairweather (G) and Aleutian megathrust (H). These structures, plus the Denali fault (I), which is shown on the 1965 map, comprise much of the framework for modern interpretations of the assembly of terranes in this part of the continent.
- Subdivision by age and composition of the plutonic rocks of the Coast batholith (J) and neighboring plutons, all of which are shown as Cretaceous intrusive rocks on the 1965 map, but which are now known to range in age from Triassic to Paleogene, and in composition from gabbro to true granite.
- Depiction of seafloor features, including the Aleutian megathrust (H), seamount chains (K), oceanic crustal isochrons (L), and transform faults, all of which are critical to understanding the history of terrane assembly.
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| 1965 Map |
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2005 Map. Click on map for larger view (259 kb file). |
Extensive systematic mapping supported by geophysical surveys and a carefully coordinated program of radiometric dating by the Geological Survey of Canada has led to a vast increase in understanding of the assembly of the shield during the Archean and Proterozoic. On the 1965 map most of the rocks of the shield were lumped into 5 units: Precambrian granite and gneiss, lower Precambrian metamorphic rocks, middle Precambrian metamorphic rocks, Precambrian basic (mafic) rocks, and upper Precambrian sedimentary rocks. On the new map the shield rocks are subdivided into dozens of units that are distinguished by age, lithology and origin. In this part of the shield this increased knowledge has led to:
- Recognition of major provinces characterized by Archean and Proterozoic rocks with different tectonic histories. These include the Slave Province (A), the Rae Province (B), and the Hearne Province (C). The provinces are bounded by orogenic zones such as the Wopmay Orogen (D), the Thelon-Taltson Magmatic Zone (E), and the Snowbird Tectonic Zone (F). The provinces are believed to be disparate blocks of Archean continental crust amalgamated about 2 billion years ago along sutures marked by the intervening orogenic zones.
- The Athabasca Basin (G), the Thelon Basin (H), and the Baker Basin (I) are filled with unmetamorphosed sedimentary rocks, chiefly quartz-rich sandstone and conglomerate that unconformably overlie Archean and Proterozoic metamorphic and plutonic rocks. The shape of these basins is essentially the same as that portrayed on the 1965 map, but the beginning of deposition of these is now established as between 1.75 and 1.73 billion years ago, and deposition of these rocks probably continued until at least 1.6 billion years ago. Note also the addition of the Early Cretaceous Carswell impact structure in the western part of the Athabasca Basin.
- The widespread MacKenzie Dike Swarm dated at about 1.3 billion years consists of north-northwest trending mafic dikes that cut the Archean and Proterozoic rocks, including those in the Athabasca, Thelon, and Baker Basins, showing that the unmetamorphosed basin-fill was deposited before the dikes were intruded. The MacKenzie is one of more than three dozen dike swarms shown in various parts of the Canadian Shield on the new map. None were shown on the 1965 map.
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| 1965 Map |
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2005 Map. Click on map for larger view (237 kb file). |
- Although the general distribution of rock types in Greenland (A) on the new map is similar to that on the 1965 map, the undivided Precambrian rocks shown on the old map are subdivided to distinguish Archean gneiss, sedimentary and volcanic rocks, and Middle Proterozoic gneiss, sedimentary and volcanic rocks, and mafic plutonic rocks. Rocks formerly shown simply as Tertiary volcanic rocks are now identified as Paleogene mafic volcanic rocks.
- The geology of Iceland (B) is also rather similar on the two maps, but more precise ages for the volcanic rocks are given on the new map, and volcanoes, calderas, and faults, none of which were depicted on the old map, are shown on the new.
- By far the most significant change in this part of the new map is the addition of seafloor features including the spreading centers along the Kolbeinsey (C) and Reykjanes (D) Ridges, both parts of the Mid-Atlantic Ridge System, and the oceanic crustal isochrons (E) that record the history spreading across these ridges as Greenland separated from Eurasia during the Tertiary and Quaternary. The connection between the spreading centers and the faults and volcanic edifices in Iceland also is obvious.
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| 1965 Map |
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2005 Map. Click on map for larger view (231 kb file). |
- In the northern part of this view the refinements in the geology of the Precambrian rocks of the Canadian Shield (A), the Adirondacks (B), and eastern Wisconsin (C) on the new map are conspicuous.
- In the central interior region there is relatively little difference between the old and new maps. Mississippian and Pennsylvanian strata have been further subdivided in the Michigan (D) and Appalachian (E) Basins, and Jurassic rocks in the Michigan Basin have been shown. Special features such as impact structures, alkaline igneous complexes, and limits of continental glaciers have been added and some contacts more accurately placed, but the changes and additions are relatively minor.
- In the Valley and Ridge province of the southern Appalachians (F) the general distribution of rock units is very similar, but the new map shows many more thrust faults. In the Taconic Province in western New England and eastern New York (G) several extensive and tectonically important thrust faults have been added.
- In the Piedmont of the Southern Appalachians (H) and in central and eastern New England (I) granitic plutons, all shown as Paleozoic on the 1965 map, are subdivided into Late Proterozoic and Cambrian plutons, Cambrian and Ordovician plutons, Silurian and Devonian plutons, and Mississippian and Pennsylvanian plutons. Granite plutons of the White Mountains in New Hampshire, shown as Paleozoic on the 1965 map, are now known to be Triassic and are shown as such on the new map. The extensive swarm of Mesozoic diabase dikes in the Piedmont has also been added to the new map.
- Cretaceous and Tertiary sedimentary rocks of the Atlantic Coastal Plain (J) have been more fully subdivided and their relation to rocks of similar ages on the Atlantic Continental Shelf (K) and Rise (L) are portrayed. Slump structures, submarine canyons, and other marine features on the shelf and rise are also shown, as are major impact structures in Chesapeake Bay and on the shelf southeast of New York City.
View comparisons of map areas 5-7.
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