Abstract

Over the last two decades, study of the geometry, tectonic forces, and dating of thrust systems has resulted in some of the most exciting advances in geology. Utah’s Sevier thrust system has been at the center of some of this work. Probably the biggest advance has come through dating and correlating the thick piles of synorogenic deposits shed off the thrust plates.

The Sevier thrust system consists of, from west to east, a thrust belt, a foredeep basin, a forebulge, and a back-bulge basin. Each of these parts migrated eastward over time. During the Middle to early Late Jurassic, most of Utah was a broad back-bulge basin. By the Late Jurassic, the back-bulge basin had migrated east of the state, and Utah was mostly a forebulge high. Thrust faulting began in northernwestern Utah in the latest Jurassic or earliest Cretaceous, and reached its zenith during the Late Cretaceous. Single thrust plates are up to 50,000 feet (15,000 m) thick and were pushed eastward 25 to 30 miles (40-48 km), and in some cases more than 50 miles (80 km). A few of the thrust-generated folds became some of the best oil and gas fields in the state. Foredeep basin deposits in Utah locally exceed 10,000 feet ((3,000 m) in thickness. As the “thrust front” migrated eastward into thinner sediments, it abandoned one thrust fault as the “wedge” of thrusted rock became too thick (exceeded critical taper), and “stepped” forward to a new fault. The shortening in the frontal zone is taken up by thrust-cored folds that decrease in amplitude to the east. The youngest evidence of thrust faulting is about 50 million years old in northern Utah, and 40 million years old in central and southern Utah. The Sevier orogeny is often confused with the Laramide orogeny because they overlap in time and location. They were produced by the same crustal shortening, but are distinguished by style of deformation. The Laramide orogeny produced basement-cored uplifts whereas the Sevier orogeny produced thin-skinned deformation in its eastern part.