Download The Sevier Orogeny The Sevier orogeny took place at about the

The Sevier Orogeny
The Sevier orogeny took place at about the same time as the Laramide orogeny
towards the end of the Cretacious and into the Tertiary from ~70-35 Ma. The Sevier
orogeny happened largely due to the same regional stresses that created the Laramide.
What distinguishes the Sevier from the Laramide, however, is the style of deformation
and uplift. The Laramide orogeny in its various manifestations involves deeply
penetrating, steeply dipping reverse faults that deform and fault crystalline basement
rocks. The Sevier orogeny was a more ‘thin-skinned’ deformation event. It is
characterized by shallow thrust faults affecting only Paleozoic and younger sediments.
The thrust faults bottom out into a décollement that is only a few kilometers deep
and is near the top of the crystalline basement. There are no mylonites or
metamorphosed rocks in the fold/thrust belt, as it is sometimes called. This is because
none of the faulting took place at deep enough depths to cause any metamorphism or
partial melting of rocks. There is, however, extensive folding in the rocks above thrusts.
The faults preferentially developed in weaker shale and evaporate layers along which the
thrust sheets could be more easily transported. The faults run parallel to bedding except
when they ‘ramp’ up to higher bedding planes. There are many thrusts in the fold-thrust
belt and they young to the east; i.e. the faults that outcrop furthest east are the youngest.
(Miller et al. 1992)
Miller, D.M., Nilsen, T.H., and Bilodeau, W.L., 1992, Late Cretaceous to early Eocene
geologic evolution of the U.S. Cordillera, in Burchfiel, B.C., Lipman, P.W., and Zoback,
M.L., eds., The Cordilleran Orogen: Conterminous U.S.: Boulder, Colorado, Geological
Society of America, The Geology of North America, v. G-3.