by Grant C. Willis
Survey Notes article, v. 32 no. 1 January 2000

Map
Sevier Thrust System
Middle Jurassic Back-bulge Basin
Late Jurassic to Early Cretaceous Forebulge High
Early Cretaceous Thrust Faulting
Late Cretaceous Thrust Faulting
Late-Phase Thrusting
The End of Thrusting
I Thought that was the Laramide Orogeny!
Advances

The western or Cordilleran thrust system extends from Mexico to Alaska, and formed mostly in the Middle Jurassic to early Tertiary (170 to 40 million years ago).

It formed as dense oceanic crust beneath the Pacific Ocean (Farallon plate) converged with, and slid beneath the more buoyant continental crust of the North American plate during a mountain building episode called the Sevier orogeny (the Sevier River area of central Utah is the namesake of this event).

The Utah part of the Cordilleran thrust system is called the Sevier thrust system. Though the basic geometry and age of the Sevier thrust system in Utah have been known for more than 50 years, knowledge of the timing, method, and sequence of emplacement of individual thrust sheets has advanced slowly.

Sevier Thrust System

Typical parts of a thrust system.
The thickened thrust wedge overloads the earth’s crust, which flexes in response, similar to loading rock on a wooden raft floating on water.

The Sevier thrust system is a typical thrust system consisting of, from west to east, a thrust belt, a foredeep basin, a forebulge, and a back-bulge basin. The thrust belt is the wedge of stacked thrust plates.

In Utah, single plates are up to 50,000 feet thick and, when thrusted into thick stacks or culminations, may have formed mountains similar in magnitude to the modern Andes Mountains of South America.

The tremendous load of the stacked plates depressed the crust under and in front of the thrust belt (visualize forcing down the end of a raft floating on water by loading it with rock) forming a “foredeep” basin into which thousands of feet of coarse synorogenic sediment was shed. Foredeep-basin deposits in Utah commonly exceed 10,000 feet.

Farther east, the land bowed upward, a counter-response to the depressed foredeep basin, forming a forebulge, a relatively high area with minor or no deposition. At times, the Utah forebulge was an area of erosion.

Still farther east, a second, much shallower basin formed, the back-bulge basin. The Farallon plate, subducting beneath the continental crust in the approximate position of modern central California, was the driving force behind the Sevier thrust system. The collision produced deformation that started in the west and migrated eastward. Thus, each of these four parts of the thrust system migrated eastward over time. Back-bulge basin deposits provide the earliest evidence of thrusting in Utah.