Laramian orogeny

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The Laramian orogeny , also known as the Laramic orogeny or the laramic folding phase , was a major stage in the orogeny process of western North America . As the Laramide Revolution , the term was first introduced into specialist literature in 1901 by George Mercer Dawson and adopted by Hans Stille in 1924 . Their name-giving type locality is in the Laramie Mountains in the US state of Wyoming .

Mountain formation began in the late Upper Cretaceous around 80  Mya and ended in the Eocene around 40 Mya. The exact duration is just as controversial as the actual cause.

The laramic orogenesis was not continuous, but took place in spurts with intervening breaks. The Rocky Mountains were mainly affected by it, but its effects were felt even in Alaska and northern Mexico ; in the Black Hills of South Dakota it reached its easternmost area of ​​activity.

Mountain formation is usually associated with events that took place off the North American west coast - here the oceanic Farallon Plate gradually slipped below the continental North American plate . Most of the explanatory hypotheses assume that the angle of the subducted Farallon plate was quite shallow and that there was therefore no magmatism in the western central part of the North American plate. In addition, the submerged oceanic lithosphere induced shear stresses in the lower section of the overlying continental lithosphere. One reason for the shallow plunge angle is possibly an accelerated convergence of the two plates; but a thickened oceanic crust could just as well be used for this.

The magmatism associated with subduction usually occurs near the plate edges (the volcanic belt of the Andes serves as a good example ), but in the case of the laramic orogeny it was offset far to the east. One explanation for the non-existent magmatism at subduction zones (referred to in English as magmatic zero ) is possibly due to the fact that the submerging oceanic crust in this particular case encountered relatively cool continental lithosphere instead of the warmer asthenosphere as usual .

The flat immersion angle together with the induced shear stresses created a very broad mountain belt - forerunners of the actual Rocky Mountains (Proto-Rocky Mountains). When the Farallon plate was completely melted apart from fragments, the lateral pressure was removed and the rocks relaxed. As a countermovement, expansion tectonics followed , giving rise to the basin-and-range province .

In this context, it makes sense to compare the previous Sevier mountain formation and the Nevadian mountain formation that took place even earlier .

literature

Individual evidence

  1. ^ A b Robert B. Smith, Lee J. Siegel: Windows into the Earth . Oxford University Press, 2000, ISBN 0-19-510596-6 , p. 102 f.