TECTONIC REGIONS OF NORTH AMERICA



The distribution of tectonic activity around continents (including North America) usually provides a clear contrast between continental interiors and margins; and between leading margins (converging or active) and passive margins (diverging or trailing). This division provides us with distinct large-scale structural regions:
a) Shields: old, exposed precambrian metamorphic and igneous rocks at the "core" of continents. The metamorphic rocks are the "roots" of ancient mountain belts, long since eroded away.
b) Platforms: adjacent to shields, similar rocks, but covered by newer sedimentary rocks (shield + platform = craton).
c) Orogenic belts - the currently or formerly active edge of continents, severely deformed due to earth movements (orogenic = "mountain-building").
d) Passive margins - the trailing edge of continents; little or no tectonic activity.

Craton: The North American craton is exposed mainly in the Canadian Shield. Cratonic rocks are also called "basement rocks" because they underlie other more recent rocks in Platform regions. The craton tends to be very stable, but has undergone broad warping in the past - especially in platform areas, creating basins, domes and tilted strata.

Platforms Platforms are parts of the craton which are covered by relatively undeformed layers of sedimentary rock, formed when seas occupied these areas (due to tilting/sinking of the craton or rise in sea level). A large part of the interior of North America consists of platform areas. Structure within the platform rocks is usually quite subdued: however, many parts of platforms are not completely flat; instead broad warping of the underlying craton has produced gently tilted beds (< 5o), domes (e.g. the Black Hills of Dakota; Ozark Mountains of Missouri) and basins (e.g. Michigan Basin, Permian Basin).

An exception to the generally subdued structure of platforms, resulting from plate movements, is intracratonic rifting which forms rift valleys, seas and aulacogens in platform regions.

A relatively new idea in plate tectonics suggests that rifting may actually be initiated by hot spots which cause the continental crust to dome up and split into a triple rift.

Commonly, one of the rift arms fails, while the other two become active and contribute to an opening linear ocean basin. So the convection cell that drives sea-floor spreading may actually be a line of "hot spots".

The rift valley forms a large linear trough, bounded by fault scarps and floored by volcanic rocks (e.g. the East African Rift Valley). If the valley extends below sea level, it becomes the site of sedimentation.
The failed arm, represented by a deep linear trough into the adjacent continent, may become a natural location for a major river valley within platform areas - probable examples include the Mississippi and the Amazon.
It is also possible for a failed arm of a rift to become "squeezed shut" by later plate movements - forming an aulacogen. The Arbuckle Mountains lie on the northeastern margin of the southern Oklahoma aulacogen. Closure of the aulacogen resulted in severe folding and faulting of marine sedimentary rocks which had accumulated in the trough - one of the few places that platform strata are so deformed.


FOLDED ROCKS IN THE ARBUCKLES

Orogenic Belts In the case of North America there are two major orogenic belts - the older Appalachian chain in the east, and the much younger Rockies in the west. Both were formed by plate convergence and have similar structural features. Both areas underwent uplift, metamorphism, volcanism, folding, thrust faulting and later erosion; some crustal extension was also experienced in the west.

The present-day relief of these mountains is a result of erosion - the Appalachians being much older are more subdued and lower and consist mainly of eroded folds.

The western Cordillera has a number of other features related to orogenesis:
Erosion has exposed the granitic batholiths emplaced during orogenesis, forming the Sierra Nevada Mountains.
Another part of the crust in the west underwent stretching during the orogeny - forming the Basin and Range Province. These ridges and valleys are formed by normal faulting caused by crustal stretching.

Another large region in the west experienced uplift, but with little deformation, forming the large Colorado plateau - rivers cut down through the uplifted strata forming many canyons, including the Grand Canyon.

The Atlantic and Gulf Coast margins of the U.S. are PASSIVE MARGINS, characterized by low-lying flat coastal plains extending below sea-level into a wide continental shelf.

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