Geologic map of the east-central Meadow Valley Mountains, and implications for reconstruction of the Mormon Peak detachment, Nevada

Abstract

The role of low-angle faults in accommodating extension within the upper crust remains controversial because the existence of these faults markedly defies extant continuum theories of how crustal faults form, and once initiated, how they continue to slip. Accordingly, for many proposed examples, basic kinematic problems like slip direction, dip angle while active, and magnitude of offset are keenly debated. A well-known example is the Miocene Mormon Peak detachment and overlying Mormon Peak allochthon of southern Nevada (USA), whose origin and evolution have been debated for several decades. Here, we use geologic mapping in the Meadow Valley Mountains to help define the geometry and kinematics of emplacement of the Mormon Peak allochthon, the hanging wall of the Mormon Peak detachment. Pre-extension structural markers, inherited from the east-vergent Sevier thrust belt of Meso zoic age, are well suited to constrain the geometry and kine matics of the detachment. In this study, we add to these markers a newly mapped Sevier-age monoclinal flexure preserved in the hanging wall of the detachment. The bounding axial surfaces of the flexure can be readily matched to the base and top of the frontal Sevier thrust ramp, which is exposed in the footwall of the detachment to the east in the Mormon Mountains and Tule Springs Hills. Multiple proxies for the slip direction of the detachment, including the mean tilt direction of hanging wall fault blocks, the trend of striations measured on the fault plane, and other structural features, indicate that it is approximately S77°W (257°). Given the observed structural separation lines between the hanging wall and footwall, this slip direction indicates 12-13 km of horizontal displacement on the detachment (14-15 km net slip), lower than a previous estimate of 20-22 km, which was based on erroneous assumptions in regard to the geometry of the thrust system. Based on a new detailed map compilation of the region and recently published low-temperature thermochronologic data, palinspastic constraints also preclude earlier suggestions that the Mormon Peak allochthon is a composite of diachronously emplaced, surficial landslide deposits. Although earlier suggestions that the initiation angle of the detachment in the central Mormon Mountains is ~20°-25° remain valid, the geometry of the Sevier-age monocline in the Meadow Valley Mountains and other structural data suggest that the initial dip of the detachment steepens toward the north beneath the southernmost Clover Mountains, where the hanging wall includes kilometer-scale accumulations of volcanic and volcaniclastic strata.

Fluid Flow, Brecciation, and Shear Heating on Faults: Insights From Carbonate Clumped‐Isotope Thermometry

“…The hanging wall of the detachment (Figure c) contains Cambrian through Permian carbonates derived from the thrust sheet, locally overlain by late Oligocene to middle Miocene (c. 23–14 Ma) volcanic and sedimentary strata, all displaced westward >10 km relative to footwall rocks (Anderson et al, ; Swanson & Wernicke, ; Wernicke et al, ). Where present, Tertiary strata are approximately concordant with the underlying Paleozoic rocks and include interstratified scarp breccias and Miocene ash flow tuffs.…”

“…The recorded c. 200 °C temperature difference thus occurs over a length scale of less than 1 cm, in a sample directly on the detachment surface. The 215 ± 30 °C temperature is hotter than ambient conditions the fault rock would have experienced, with the footwall at that location predicted to have been no deeper than ~4–5 km (Axen et al, ; Swanson & Wernicke, ). With a surface temperature of 25 °C and a geothermal gradient of 25 °C/km, ambient temperatures would be expected to be 150 °C or colder (e.g., Figure 10a in Bidgoli et al, ).…”

“…The Mormon Peak detachment in Nevada, USA, is an extensively exposed low‐angle slip surface, active during middle Miocene extension in the Basin and Range province (Axen et al, ; Bidgoli et al, ; Swanson & Wernicke, ; Wernicke et al, ; Figure a). The detachment cuts a preexisting thrust fault, part of the frontal décollement zone of the Mesozoic‐age Sevier fold‐and‐thrust belt (Figures b and c).…”

“…In contrast, the Mormon Peak detachment and allochthon developed more distant from magmatism, about 30–40 km SE of the Kane Springs Wash caldera. The detachment is a normal fault that has exhumed structural depths in its footwall that are in excess of 6,000 m (Axen et al, ; Bidgoli et al, ; Swanson et al, ; Swanson & Wernicke, ). Thus, these two faults provide excellent opportunities to look for evidence of heating processes that might facilitate slip along low‐angle slip surfaces and distinguish whether frictional heating and/or magmatic heating was involved.…”

“…Structural map and schematic cross section of the Mormon Mountains, Nevada, modified fromAxen et al (1990) andSwanson and Wernicke (2017). (a) Map showing the sample locations.…”