LOW-TEMPERATURE THERMOCHRONOLOGIC CONSTRAINTS ON THE KINEMATIC HISTORIES OF THE MORMON PEAK, TULE SPRINGS, AND CASTLE CLIFFS DETACHMENT FAULTS, SOUTHEASTERN NEVADA AND SOUTHWESTERN UTAH

The Basin and Range is the preeminent example of an extensional orogen and is a structural analogue for rifts around the globe. Although decades of field-based research have provided important structural and stratigraphic constraints on the tectonic evolution of the central Basin and Range, reconstructing the displacement histories of the major faults has remained problematic. A key challenge is determining robust and appropriate timing constraints. Kinematic reconstructions commonly rely on restorations of Mesozoic structures that are significantly older than the extensional and transtensional faults that overprint them, and on chronostratigraphic constraints from pre-, syn- and post-kinematic strata that lack the resolution necessary for detailed strain budget analysis. Additionally, only limited thermochronologic data (which give a signal for both timing and magnitude) exist within this region.

Here we use apatite and zircon (U-Th)/He thermochronometry to evaluate the timing, magnitude, and spatial pattern of Miocene strain within the Mormon Mountains-Tule Springs Hills-Beaver Dam Mountains region of southeastern Nevada and southwestern Utah. The region is host to three major detachment faults, thought to be responsible for >50 km of extension across this portion of the central Basin and Range. From west to east these are the Mormon Peak, Tule Springs, and Castle Cliff detachments. Samples were collected from Precambrian basement gneisses and Paleozoic to Jurassic siltstones and sandstones exposed in the footwalls of these detachments. Zircon He ages from the Beaver Dam Mountains are invariant at the structurally lowest exposures, recording the onset of footwall exhumation at ~18 Ma. In contrast, samples from the Mormon Mountains and Tule Springs Hills reside within the zircon He partial retention zone. A preliminary interpretation based on these observations is that initiation of these detachment faults may have progressed from east to west; however, additional analyses are pending. The complete data set (~50 samples), including apatite He ages and published apatite fission-track ages, will be used to map the spatial distribution of cooling ages, model thermal/ exhumation histories, and ultimately, update palinspastic reconstructions for these ranges.