Laramide orogeny

From Academic Kids

The Laramide orogeny was a 30 million year period of mountain building in western North America that started in the Late Cretaceous, 70 million years ago, and ended in the Late Paleogene 40 million years ago. The Laramide orogeny occurred in a series of pulses, with quiescent phases intervening. The major feature that was created by this orogeny was the Rocky Mountains, but evidence of this orogeny can be found from Alaska to northern Mexico. The phenomenon is named for the Laramie Mountains of eastern Wyoming.

This period started off the west coast of North America, where the Farallon Plate was sliding under the North American plate. During a period of rapid seafloor spreading, the angle of subduction became so shallow that no volcanics occurred in the central west of the continent, and the underlying oceanic lithosphere actually caused drag on the root of the overlying continental lithosphere. The volcanic arc associated with a subduction occurred not near the plate edges (as in the Andes, for example), but far to the east. Geologists call such a lack of volcanic activity near a subduction zone a magmatic null. This particular null was created because the 300-kilometer depth that is needed to turn a subducting oceanic plate into a magma-generating machine occurred well under an already very thick part of North America's continental crust, and this area was also spread out over a long distance. Therefore there was never enough magma generated at any one place to make it possible for it to travel anywhere near the surface and feed volcanos. The Farallon plate “dragged along the bottom of the continental crust of the North American plate, causing significant folding, and faulting of rocks, especially in Utah and Colorado” according to geologist Vicki Pedone (Earth History Happens). But the main result of shallow angle of subduction and the drag which it caused was a broad belt of mountains that were the progenitors of the Rocky Mountains.

Part of the proto-Rocky Mountains would be later modified by extension to become the Basin and Range Province.

Compare the earlier Sevier orogeny and the still-earlier Nevadan orogeny of the late Jurassicearly Cretaceous.

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