Reevaluating the Structural Evolution of the Beaver Dam Mountains, Utah; Evidence for Sevier Contraction Overprinted by Basin and Range Extension

Vivian Spiess, San Diego State University
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Abstract

The Beaver Dam Mountains of southwestern Utah lie at the transition between the Basin and Range Province and the Colorado Plateau, a region that preserves evidence for both Sevier-age contraction and Miocene extension. While the Beaver Dam Mountains have long been cited as a classic example of large-magnitude Basin and Range extension accommodated by the Castle Cliff detachment system, numerous structural relationships remain difficult to reconcile with an exclusively extensional origin. This study investigates whether Mesozoic contraction established the primary structural framework of the Beaver Dam Mountains and was subsequently reactivated and overprinted during Miocene extension.
Regionally, Sevier-age contraction is recorded by the Square Top Mountain thrust, the Virgin anticline, and the Kanarra anticline, whereas Cenozoic extension is expressed by the Castle Cliff Detachment System and the Red Hollow normal fault along the western flank of the Beaver Dam Mountains. The Beaver Dam Mountain anticline occupies a key position within this structural framework because its origin remains questionable. Although it has been interpreted as a contractional anticline, its northwest-southeast–trending axis contrasts with the northeast-southwest orientation of neighboring Sevier structures, leading some geologists to attribute its development to Miocene extensional footwall doming rather than Mesozoic fold-thrust uplift.
To evaluate these competing models, this study combines 1:24,000-scale geologic mapping, structural analysis, and apatite (U-Th)/He thermochronology. Preliminary mapping documents folded and overturned Pennsylvanian-Permian strata, synclinal geometries within lower Paleozoic rocks, gently dipping Triassic-Jurassic strata, and oriented cleavage planes in Cambrian rocks that suggest a significant contractional component to deformation. Approximately 30 samples spanning Precambrian crystalline basement through Paleozoic sedimentary rocks have been collected for low-temperature thermochronology to constrain cooling and exhumation histories and distinguish Mesozoic thrusting and uplift from Miocene extensional unroofing.
By integrating detailed structural field observations with thermochronology constraints, this study seeks to refine the tectonic evolution of the Beaver Dam Mountains, evaluate the Mesozoic deformation preserved within the range, and improve regional reconstructions of the magnitude of crustal extension and fault reactivation in the eastern Basin and Range Province.

Session

Deformation in the upper crust