The Confusion Range, west-central Utah: Fold-thrust deformation and a western Utah thrust belt in the Sevier hinterland

The Confusion Range in west-central Utah has been considered a broad structural trough or synclinorium with little overall shortening. However, new structural studies indicate that the Confusion Range is more accurately characterized as an east-vergent, fold-thrust system with similar to 10 km of horizontal shortening during Late Jurassic to Eocene Cordilleran contractional deformation. For this study, four balanced and retrodeformable cross sections across the Confusion Range and adjacent Tule Valley were constructed using existing mapping and new field data, and these were tied with a fifth strike-parallel section. Ramp anticlines and anticlinal duplexes characteristic of strong Lower Paleozoic carbonate units are balanced by faulted and rotated detachment folds in more ductile Upper Paleozoic strata. The apparently synclinal aspect results from two different sets of thrust structures that uplift and expose Lower Paleozoic rocks on the flanks of the range. Fold-thrust structures are continuous southward for more than 130 km, forming a thrust belt of regional extent herein named the western Utah thrust belt. This thrust belt is comparable in length and magnitude of shortening to the central Nevada thrust belt, and both merge southward with the Sevier frontal thrust belt. Together, these and related fold-thrust zones in eastern Nevada indicate significant, broadly distributed Mesozoic shortening in the Sevier hinterland. The Cordilleran thrust belt in eastern Nevada and western Utah thus consists of a frontal zone-the Sevier frontal thrust belt-where major thrust faults with 50-100 km of displacement breach the surface, and a hinterland zone, characterized by a number of distributed fold-thrust belts, each accommodating on the order of 10 km of shortening.