Late-Phase Thrusting

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

Thrust faulting continued into middle to late Eocene time. In the northern Sevier thrust belt, the late-phase faults are mostly in western Wyoming. In many areas near the front edge of the thrust belt in central and southern Utah, duplicated strata are present only within individual formations, making the deformation more difficult to recognize and map in the field. In addition, the frontal thrust faults are typically covered by younger undeformed deposits, and extend well east of the easternmost major thrust faults that break the surface.

The shortening in this frontal zone is taken up by folds that decrease in amplitude to the east. The Sanpete-Sevier Valley anticline (the white hills along I-70 near Salina) and the Virgin anticline (the tilted rock east of I-15 near St. George) are two examples of large thrust-cored folds in the frontal part of the thrust belt.

The End of Thrusting

The most recent evidence of thrust faulting is about 50 million years old in northern Utah, and about 40 million years old in central and southern Utah. However, the end of thrust faulting is not clearly defined in the rock record because compression declined gradually as the rate of convergence between oceanic and continental crust decreased.

As the compressional forces declined, the Cordilleran thrust system (including the Sevier thrust belt) was left unsupported. Many of the original thrust faults “relaxed” and slid backwards (to the west). In general, this backsliding was not extensive, but it was enough to complicate the evidence that geologists have had to unravel.