THE CONFUSION RANGE “SYNCLINORIUM”: A WESTERN UTAH THRUST BELT ANALOGOUS TO THE CENTRAL NEVADA THRUST BELT

The Confusion Range in west central Utah has been considered a broad structural trough or synclinorium with little overall shortening. However, new balanced cross sections indicate that the Confusion Range is more accurately characterized as an east-vergent, fold-thrust belt with ~10 km of horizontal shortening during the late Mz to Paleogene Sevier orogeny. Subsurface structure is dominated by a series of frontal and lateral ramps in lPz strata on the west side of the range. Ramp anticlines and anticlinal duplexes characteristic of lPz strata are balanced by faulted and rotated detachment folds in uPz strata, with a major detachment zone in shales of the Chainman and Pilot formations. The apparently synclinal aspect of the Confusion Range results from two different sets of thrust structures that uplift and expose lower Paleozoic strata on the flanks of the range. The east-dipping Snake Range decollement projects under the Confusion Range at a depth of 5 to 10 km or more, and may truncate deeper level structures of the fold-thrust belt.

Fold-thrust structures and structural style are continuous southward into the Burbank Hills and Mountain Home Range for more than 130 km of total strike length. Thus the “Confusion Range synclinorium” of previous authors is a fold-thrust belt of regional extent, herein informally named the western Utah thrust belt (WUTB). To the south the WUTB merges with the Wah Wah thrust and related structures of the Sevier frontal thrust zone near the south end of the Indian Peak Range. To the north, restoring 47 km of displacement on the Sevier Desert Detachment aligns the Confusion Range with uPz strata and fold-thrust structures in the Cedar Mountains. These structural correlations suggest that the WUTB is a coherent fold-thrust belt that diverges from the Sevier frontal zone in southwestern Utah and can be traced northward into west central Utah for at least 250 km.

The western Utah thrust belt is similar in size and structural style to the central Nevada thrust belt. Together these thrust belts and related structures in eastern Nevada indicate significant, broadly distributed Mz shortening. The Sevier hinterland is thus not an undeformed interior zone as originally envisioned, but instead preserves an important component of Mz fold-thrust deformation during the Sevier orogeny.