Comparisons of the 2005 Geologic Map of North America with the 1965 Map, Areas 5-7
|
|
This page shows areas of map comparisons in boxes 5-7. (Either scroll through pages or click on the boxes in the map to the left.)
- 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.
View comparisons of map areas shown in boxes 1-4.
- 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
|
 |
|
 |
| 1965 Map |
|
2005 Map. Click on map for larger view (209 kb file). |
The 1965 map shows the on-land geology in this view in considerable detail, but without the concept of plate tectonics and the availability of sea-floor geology it was virtually impossible to make any sense of the complex geology of the Caribbean region. Modern tectonic interpretations are still being debated, but some general features are reasonably well established:
- The Maya block (A) is a sediment-draped continental fragment that reached its present position as North America separated from South America in the Early Jurassic during the breakup of Pangea.
- The Caribbean plate (B) is largely composed of thick and buoyant oceanic crust that originated in the Pacific as part of the Farallon plate in the Late Cretaceous and began moving northeastward into the widening gap between North and South America shortly thereafter.
- Cuba, Jamaica, Hispaniola, and Puerto Rico are parts of the Greater Antilles Arc (C), a subduction-related magmatic arc that formed along the northern boundary of the Caribbean Plate in the Late Cretaceous and moved northeastward with the plate. The Chortis block (D) is a continental fragment perhaps derived from western Mexico that was also carried along with the Caribbean plate. It is now juxtaposed with the Maya block along the Motagua shear zone (E) which marks the suture between the North American and Caribbean plates.
- Cuba was separated from the Greater Antilles arc by back-arc spreading during the Late Cretaceous and Paleocene and became part of the North American plate. A new strike-slip plate boundary between the North American and Caribbean plates developed along the complex group of strike slip-faults that extends from the Motagua shear zone northeastward past the eastern tip of Cuba. The Cayman spreading center (F) developed in the Early to Middle Eocene as a connection between two of the major transform faults in this zone.
Northeastward movement of the Caribbean plate relative to North America since development of the spreading center is thought to have been as much as 1000 km.
- The impact that formed the 170-km-wide Chicxulub structure (G) at the northern tip of the Yucatan Peninsula took place as Cuba was being transferred from the Caribbean plate to the North American plate.
 |
|
 |
| 1965 Map |
|
2005 Map. Click on map for larger view (235 kb file). |
Like the view of the Caribbean, this view illustrates the importance of the seafloor geology in understanding the on-land geology.
- Refinements of the on-land geology include more detailed subdivision of the sedimentary rocks of the California Coast Ranges (A), the metamorphic and igneous rocks of the Klamath Mountains (B) and northwest Sierra Nevada (C), and the Tertiary volcanic rocks of the Columbia River (D) and Oregon Plateaus (E). Especially notable is the subdivision of rocks shown on the old map simply as "Cretaceous granitic intrusive" in the Sierra Nevada (C) and in northern Washington and southern British Columbia (F). On the new map they are subdivided into more than a dozen units based on age and lithology. In addition, dozens of faults including those in the San Andreas fault system, the Sierra Nevada, and the Basin and Range province have been added on the new map.
- The most important change is the addition of the seafloor geology, including portrayal of the Juan de Fuca (G) and Gorda (H) Ridges, the complex of thrust faults in the accretionary wedge (I) off Washington and Oregon and the array of normal and strike slip faults (J) in the California Borderland. These additions help illustrate the relations between the Gorda and Juan de Fuca spreading centers and recent and on-going volcanic activity in the Cascades, as well as the connection between spreading centers in the East Pacific Rise and Gulf of California (in parts of the map south of this view), the San Andreas fault system (K), and the Mendocino fracture zone (L).
 |
|
 |
| 1965 Map |
|
2005 Map. Click on map for larger view (223 kb file). |
In spite of more than three decades of geologic mapping and detailed studies, parts of the map in this view changed very little from the 1965 map. Understanding of the geology of this region has greatly improved, but the new map information when generalized to this scale results in surprisingly few changes. Locally the positions of some contacts have been slightly adjusted, but the general distribution and age assignments of most of the geologic units are little changed. The most significant changes are:
- Subdivision of Precambrian rocks in the cores of the Laramide uplifts, shown on the 1965 map simply as Precambrian, to reflect the results of extensive radiometric dating. The basement rocks in the uplifts in Wyoming (such as the Laramie Range, A) are now known to be largely Archean (2.5 billion years and older), while those to the south in Arizona, New Mexico, and Colorado (such as the Front Range, B) are all Proterozoic (1.8 to 1 billion years or younger). These two Precambrian provinces are juxtaposed along the Cheyenne Belt (C), a major suture zone in southern Wyoming that marks the southern edge of the continent during the Archean. The Archean rocks are subdivided into Late Archean granititic rocks and Late Archean gneiss; the Proterozoic rocks are chiefly Early Proterozoic metamorphic rocks and granites of three different ages, Early Proterozoic 1.8-1.6 billion years, Middle Proterozoic (about 1.4 billion years) and Middle Proterozoic (about 1.0 billion years). The Uinta Mountains (D) are cored by a great thickness of little-metamorphosed sedimentary rocks of Middle and Late Proterozoic age.
- Subdivision of the Tertiary rocks in the volcanic fields surrounding the Colorado Plateau, including the San Juan (E), Mogollon-Datil (F), and Marysville (G) fields, on the basis of age and lithology. All of these rocks were simply shown as "Tertiary volcanic rocks" on the 1965 map. Calderas are not shown on the 1965 map, but have been added on the new map.
View comparisons of map areas 1-4.
Return to the Geologic Map of North America home page.
ESP Team Research Activities | ESP Team home page
U.S. Department of the Interior | U.S. Geological Survey
