Abstract

Detailed geological mapping of the southern and central Tule Springs Hills and northern East Mormon Mountains, Nevada, reveals details of the interaction between a Mesozoic thrust system and Tertiary detachment faults. The main structure in the area is the subhorizontal Tule Springs detachment, along which a highly deformed hanging wall of Paleozoic and sparse Tertiary strata overlies a much less deformed footwall of Permian to Jurassic beds. The hanging wall was emplaced in Mesozoic time along a décollement-style thrust with a ramp-flat footwall geometry, then extended in Neogene time synchronous with normal reactivation of the flat and upper part of the ramp. Below that point the detachment diverged from the thrust ramp, cutting more steeply into the thrust footwall, forming a similar ramp-flat detachment geometry. The Middle Member of the Jurassic Kayenta Formation forms the flat, for ∼14 km in the Tertiary transport direction. This part of the detachment dipped 3°-15° toward the transport direction and was shallower than 5 km at the onset of normal slip. Much slip may have occurred in weak subjacent siltstone. The detachment ramp is also now subhorizontal, having been rotated from an initial moderate west dip, probably due to isostatic rebound during tectonic unloading along the ramp. Permian to Jurassic strata beneath the detachment ramp in the southwestern Tule Springs Hills now dip moderately east-northeast, having been tilted from subhorizontal while the detachment ramp was flattened. Those beds now form the west limb of a range-scale asymmetric syncline, and footwall beds below the detachment flat form the east limb. In the northern East Mormon Mountains, the detachment is represented by the Toquop Chaos fault zone, which cuts downsection through lower Paleozoic strata that also probably belong in the west limb of the syncline, beneath the detachment ramp. The basal nonconformity there dips east also, so the syncline deformed the entire stratified sequence and the underlying crystalline basement.

A large klippe of the detachment (∼185 km2) is characterized by north- trending ridges and valleys ∼2 km wide. These are bounded by moderately dipping or listric normal faults, many of which demonstrably end at, or merge with, the detachment. The topography of the klippe mimics the linear basins and ranges of the Great Basin at a smaller scale. If the structural style is also analogous, then many Great Basin range-valley pairs may be underlain by detachments at relatively shallow depths, rather than being bounded by deeply penetrating faults.

Large north-striking faults that cut the Tule Springs detachment control range- scale topography, are each several kilometers long, and cut all other structures in the map area. They postdate detachment slip. One of them cuts Pliocene(?) calcrete-cemented alluvium that is younger than the detachment, and another has ∼8 km of left-lateral offset.

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