INTERPRETING THE SEVIER DESERT DETACHMENT AS A LOW-ANGLE NORMAL FAULT: INSIGHTS FROM NEW APATITE (U-TH)/HE DATA IN THE PAHVANT RANGE, CENTRAL UTAH

In the mid-Cenozoic, cessation of subduction along the western margin of North America and the initation of strike-slip faulting resulted in a switch from compression to extension in central Utah. The gently west-dipping Sevier Desert reflector in central Utah has long been at the center of a controversy about its nature. Is it an unconformity or a detachment surface? The lack of surface exposure has fueled the debate but various recent studies, (including apatite fission-track thermochronology of the Canyon Range, surficial mapping of the range-bounding Tertiary Oak City Formation, interpretation of industry seismic lines, and reprocessing of the COCORP Utah line 1), have given more weight to the interpretation of the Sevier Desert reflector as a low-angle normal fault rather than an unconformity.

The Sevier Desert reflector is projected to surface along the western flanks of both the Canyon and Pahvant Ranges located on the eastern margin of the Sevier Desert basin. Previous mapping and thermochronologic studies have focused on the Canyon Range, while the Pahvant Range has been relatively understudied. Apatite fission-track cooling ages from the Canyon Range display a Miocene cooling event (~19 – 15 Ma), which has been interpreted as the result of rapid unroofing along a low-angle normal fault. The age distribution shows a spatial pattern of increasing age with increasing distance from the western range flank, suggesting a down-to-the-west normal fault, accommodating extensional unroofing of the Canyon Range.

In this study, we present new apatite (U-Th)/He data from the Pahvant Range. Samples collected from Neoproterozoic quartzites and Mesozoic sandstones in three across-strike transects have cooling ages that range from 25.6 – 8.5 Ma (Oligocene – Miocene) and display a similar spatial pattern of age distribution observed in the Canyon Range apatite fission-track study. This pattern of younging ages toward the western flank of the Pahvant Range is consistent with extensional unroofing via the rolling hinge model of low-angle normal faulting and leads us to interpret a continuation of the Sevier Desert detachment southward along strike.