Laramide orogeny


Laramide orogeny

The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute, as is the cause. 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, with the easternmost extent of the mountain-building represented by the Black Hills of South Dakota. The phenomenon is named for the Laramie Mountains of eastern Wyoming.

The orogeny is commonly attributed to events off the west coast of North America, where the Farallon Plate was sliding under the North American plate. Most hypotheses propose that the angle of subduction became shallow, and as a consequence, no magmatism occurred in the central west of the continent, and the underlying oceanic lithosphere actually caused drag on the root of the overlying continental lithosphere. One cause for shallow subduction may have been an increased rate of plate convergence. Another proposed cause was subduction of thickened oceanic crust.

Magmatism associated with subduction occurred not near the plate edges (as in the volcanic arc of 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 may have occurred because the subducted slab was in contact with relatively cool continental lithosphere, not hotter asthenosphere. One result of shallow angle of subduction and the drag that it caused was a broad belt of mountains, some of which 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 Jurassic — early Cretaceous.

ee also

* Geology of the Rocky Mountains

References

* Joseph M. English and Stephen T. Johnston, "The Laramide Orogeny: What Were the Driving Forces?" International Geology Review 46, p. 833-838, 2004.

* Jason Saleeby, "Segmentation of the Laramide Slab -- Evidence from the southern Sierra Nevada region." Geological Society of America Bulletin 115, p. 655-668, 2003.

External links

* [http://peterbird.name/publications/1998_Laramide/1998_Laramide.htm Maps, animation, detailed information (UCLA)]
* [http://tlacaelel.igeofcu.unam.mx/~GeoD/colision/figs/orogeny/laramide.html Laramide orogeny]


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