New studies of selected basins in the Miocene extensional belt of the northern Lake Mead domain, southern Nevada, suggest refinements on previous models for the early extensional history of the region. Critical data come from (1) the Longwell Ridges area, west of Overton Arm and within the Lake Mead fault system; (2) the Salt Spring Wash Basin, in the hanging wall of the South Virgin Mountains–White Hills detachment fault; and (3) previously studied subbasins of the South Virgin Mountains in the Gold Butte step-over region. Our model focuses on the early history of extension and involves analysis of the lower Horse Spring Formation and correlative strata. The basins and fault patterns suggest two stages of basin development related to two distinct faulting episodes, an early period of detachment faulting, followed by a switch to faulting mainly along the Lake Mead transtensional fault system while detachment faulting waned. Apatite fission-track ages suggest that the footwall block of the detachment fault began cooling at 18–17 Ma. The 18–17 Ma time period appears to be the age of the upper limestone of the Rainbow Gardens Member of the Horse Spring Formation, which is interpreted to be a pre-extensional unit deposited only north of Gold Butte block in the Gold Butte step-over basin, where facies patterns and slow rates of sedimentation make faulting uncertain. The first definite basin stage occurred ca. 16.5–15.5 Ma, during which there was slow to moderate faulting and basin subsidence in a contiguous basin along the South Virgin Mountains–White Hills detachment fault and in the Gold Butte step-over basin; the step-over basin had complex fluvial and lacustrine facies and was synchronous with landslides and debris flows in the basin in the hanging wall of the detachment fault. At ca. 15.5–14.5 Ma, there was a dramatic increase in sedimentation rate related to formation or increased activity on the Gold Butte fault, a change from lacustrine to widespread fluvial, playa, and local landslide facies in the step-over basin, and the peak of exhumation and faulting rates on the detachment fault. The simple early Gold Butte step-over basin broke up into numerous subbasins at ca. 15.5–14.5 Ma as initial faults of the Lake Mead fault system formed. From 14.5 to 14.0 Ma, a major change occurred from dominantly detachment faulting to dominantly transtensional (strike-slip + normal) faulting in the Lake Mead fault system as detachment faulting waned. At this time, the Lake Mead fault system began to propagate to the west, and activity on faults and in subbasins north of Gold Butte slowed or ceased, accompanied by major progradation of alluvial conglomerates over the step-over basin. The geometry of the South Virgin Mountains–White Hills detachment fault that dominated the early Lake Mead extension history fundamentally controlled patterns of faulting and magmatism throughout the rest of the extensional history, even as the detachment faulting itself slowed from 14 to 11 Ma, when it ceased to be active. In a regional view, the detachment faulting in eastern Lake Mead is linked to and forms the northern end of the ca. 20–11 Ma northern Colorado River extension corridor. Similar to the rest of the corridor, faulting and exhumation peaked at 15 Ma, but at the north end of the corridor in eastern Lake Mead, detachment faulting changed rapidly to dominantly transtensional left-lateral faulting of the Lake Mead fault system. Eastern Lake Mead shows evidence for a spatial boundary between the southern and central Basin and Range that is best thought of as a northeast-southwest–trending feature located on numerous older tectonic boundaries. The area also records a temporal change from detachment to transtensional faulting characteristic of the central Basin and Range after 15 Ma.

The Muddy and North Muddy Mountains were highly deformed during the Cretaceous and early Tertiary(?) Sevier orogeny. Two major thrust plates, the Muddy Mountain and Summit-Willow Tank plates, and a well-developed belt of recumbent folds are present there. A foreland basin developed east of the thrust plates as well. Many of the Paleozoic rock units exposed in these ranges vary in thickness and character between the thrust plates and the authochthon; the thicker and more shelflike units occur in the thrust plates, and the thinner, cratonic ones occur in the autochthon. This is especially true of the lower Paleozoic formations, whereas Mississippian through Permian formations vary much less in thickness. Pre-Sevier age Mesozoic formations exposed locally in the upper plate of the Summit-Willow Tank thrust and in the autochthon are consistent in thickness, and they are stratigraphically similar to equivalent rocks on the nearby Colorado Plateau. The oldest syntectonic Albian and Cenomanian(?) rocks of the foreland basin are cut by the Summit-Willow Tank thrust, but the youngest of them overlap it. The Muddy Mountain thrust overrode all of the foreland deposits. New interpretations are presented herein regarding many of the Mesozoic structures of the Muddy and North Muddy Mountains. The Muddy Mountain, Glendale, and Arrowhead faults are interpreted to be parts of the same thrust and are collectively called the Muddy Mountain thrust. Three major faults, the Summit, the Willow Tank, and the North Buffington, form the older and structurally lower Summit-Willow Tank thrust. The North Muddy Mountain fold belt is interpreted to have developed synchronously with thrusting as it occurs between the two major thrusts. The monocline or drape fold described by early workers is reinterpreted as a truncation of one thrust plate by another. The North Muddy Mountains are now interpreted to underlie the Muddy Mountain plate and are thus autochthonous with respect to it. After thrusting and foreland sedimentation, the next major tectonic and sedimentologic events occurred during Miocene time. Several complex, nonmarine, closed basins of the Horse Spring Formation formed in taphrogenic environments in conjunction with left slip on the Lake Mead fault system and simultaneous growth of the andesitic Hamblin-Cleopatra volcano. Basin and ranges of late Miocene age are superposed over all of these earlier-formed structures. South of the Muddy Mountains, upper plate Paleozoic rocks dip southward at varying degrees and are overlapped by Tertiary rocks. The upper plate rocks are in apparent tectonic contact with autochthonous Mesozoic rocks in the Gale Hills, Bowl of Fire, northern Bitter Spring Valley, and east of the Longwell Ridges. Unfortunately, this contact is almost completely covered and it is poorly understood. Available data suggest that the contact is a fault that formed prior to the deposition of the Thumb Member of the Horse Spring Formation (pre-15 m.y.), and it might have been (1) a north-dipping normal fault with a large amount of displacement; (2) the trace of the Las Vegas Valley shear zone with a large amount of right slip; or (3) the trace of the Muddy Mountain thrust (Mississippian over Cretaceous and Jurassic rocks at that point). The interpretation that it is the trace of the Las Vegas Valley shear zone is favored herein because neither of the other interpretations is as consistent with the known local and regional geology. Post-15-m.y.-old deformation south of the Muddy Mountains includes a complex group of tectonic elements. A relatively minor amount of extensional tectonism occurred in the Gale Hills and was bounded to the north by the northwest-striking Gale Hills fault and to the south by the east-trending West Bowl of Fire fault. Uplift and relative southward displacement of the Bowl of Fire horst occurred on major oblique slip faults probably at the same time as the extension in the Gale Hills. North-south directed compression in the Gale Hills also apparently occurred at this time, as east-trending folds and reverse faults are present there. Normal faulting and graben formation occurred in White Basin in conjunction with oblique slip on the Rogers Spring fault. All of these local tectonic elements were probably active synchronously with left slip on the Lake Mead fault system and right slip on the Las Vegas Valley shear zone, but the details of interaction are poorly understood.

The Oligocene-Miocene Horse Spring Formation consists of sedimentary strata that record the onset and evolution of Miocene extensional tectonics in the Lake Mead region. The sedimentary basins of this formation hold critical clues for evaluating and testing competing models that attempt to explain the tectonic evolution of this important part of the Basin and Range. Detailed sedimentology, stratigraphy, isotope geochem istry, and new geochronology of carbonates of the Horse Spring Formation shed light on the details of middle Miocene depositional systems and provide important paleoclimatic and paleotopographic data that further our understanding of the geological evolution of this area. We investigated four carbonate sections in detail, two from the Bitter Ridge Limestone Member (Slot Canyon section, near the Gale Hills, and the West Longwell section, at the Bitter Ridge), one from the Thumb Member (East Longwell section, near the Bitter Ridge), and one from the Rainbow Gardens Member (Rainbow Gardens Recreation Area section), to understand the evolution of carbonate lake systems, to extricate paleoclimatic from tectonic signals in the sedimentary record, and to develop a more clear picture of the evolution of Horse Spring sedimentary basins. New 40 Ar/ 39 Ar dates from the Bitter Ridge Limestone, combined with dates in the published literature, suggest that the Bitter Ridge Lake may have evolved time-transgressively from the White Basin area in the east to the Rainbow Gardens area in the west. Possibly contemporaneous with this evolution, the lake gradually shifted from an open to a closed lake system, most likely due to tectonic partitioning of the basin or the creation of a tectonic sill that cut off the overflow for the lake. Stable isotope and lithofacies analyses provide one of the first detailed proxy records of paleoclimate for the Miocene of the Basin and Range and show strong evidence for an orbitally forced climate signal that represents changes in the precipitation/evaporation ratio for the Bitter Ridge Lake system. Because we can effectively show a climatic signal in the Bitter Ridge Limestone units over 100 k.y. and, likely, 40 k.y. time cycles, longer time-scale shifts in isotopic ratios are more likely due to tectonic processes. Based on a strong negative shift in oxygen isotopic ratios, previous researchers have suggested that the Lake Mead region experienced an increase in paleoelevation during Horse Spring time, while the remainder of the central Basin and Range to the north experienced a decrease in elevation for the same time period. Our data, when compared with data from the Pliocene Hualapai Limestone and those presented by previous researchers, appear to constrain the timing of this isotopic shift to between 15 and 13 Ma, coincident with the timing of the onset of rapid extension in this part of the Basin and Range. We hypothesize that this isotopic shift was due not to a change in paleoelevation due to magmatic activity alone, but was influenced by either (1) longer travel distances of air masses and the development of increased topographic corrugation as the Lake Mead region experienced accelerated rates of extension or (2) drainage reorganization of the early Colorado Plateau and the infusion of isotopically lighter waters from this emergent source.

Abstract Recent studies, including structural mapping, stratigraphic and sedimentologic studies, geothermochronology, and geodetic measurements, have improved our understanding of the kinematics of Miocene to Recent deformation in the central Basin and Range. Based on reconstructions of rocks in the extensionally dismembered foreland and leading edge of the Sevier thrust belt, offset along the Las Vegas Valley shear zone, and on the provenance of a unique clast assemblage in proximal channel facies deposits at Frenchman Mountain, the southern and northern Lake Mead extensional domains have extended ~94 km and ~46 km, respectively. A compilation of >70 cooling ages from the Gold Butte crystalline block indicates that onset of this extension occurred at ~20 Ma, with rapid, large-magnitude extension beginning at ~15 Ma. In the Death Valley extended domain, studies of the provenance, depositional environment, and age of the Eagle Mountain Formation show that middle Miocene siliciclastic strata occurring in a northwest-trending belt from Chicago Valley to the Cottonwood Mountains were all deposited in an environment proximal to the Hunter Mountain batholith of the Cottonwood Mountains. This requires ~100 km of roughly southeast-northwest extensional and strike-slip displacement since ~11 Ma. Identification of extensionally dismembered Cenozoic structures, correlative with structures in the Cottonwood Mountains, Panamint Range, Bare Mountain, the CP Hills, and the Funeral Mountains, are also consistent with ~100 km of west-northwest extension across the Death Valley region .

Abstract From the south: From Henderson, Nevada, take Lake Mead Drive (Nevada 147). At 7.1 mi (11.4 km) east of U.S. 95, turn north onto Northshore Road and go 3.2 mi (5.1 km) to Lake Mead Boulevard. The site is 3.7 mi (6 km) north on Lake Mead Boulevard. From the north: From North Las Vegas, take Lake Mead Boulevard (also Nevada 147) east from 1–15. At 11.4 mi (18.3 km) from 1–15 a paved road from the east (left) intersects Nevada 147. Continue on 147 for 1.8 mi (2.9 km) to the site. The site is marked by the high-tension power line that crosses the road. Park along the side of the road or on the dirt roads that intersect the main road in this area.

Three major low-angle normal faults in the eastern Lake Mead area, Nevada and Arizona, are segments of a regional, 55-km-long, detachment fault. This fault, the South Virgin–White Hills detachment, consists of the Lakeside Mine, Salt Spring, and Cyclopic Mine fault segments. All three segments dip gently west and record top-to-the-west displacement. Based on apatite fission-track and apatite and titanite (U-Th)/He thermochronology of footwall rocks, tilt relations, and 40 Ar/ 39 Ar dates on tuffs and basalts within hanging-wall synextensional sedimentary sequences, significant extension along the South Virgin–White Hills detachment occurred between 16.5 and 14 Ma. Minor extension continued until ca. 8 Ma. Displacement on the South Virgin–White Hills detachment decreases from a maximum of ~17 km at the Gold Butte block in the north to 5–6 km at the Cyclopic Mine in the south. The along-strike, southward decrease in displacement is accompanied by a change in type of fault rock from mylonite along the Lakeside Mine fault (northern segment), to chloritic cataclasite along the Salt Spring fault (central segment), to unconsolidated fault breccia along the Cyclopic Mine fault (southern segment). Differences in fault rock may reflect decreasing exhumation of footwall rocks as a result of decreased displacement to the south. About 40% of the displacement gradient can be accommodated along a series of left-slip faults in the upper plate of the detachment. The Golden Rule Peak lineament, an east-trending alignment of structural and topographic features, may be a transverse structure that accommodates differential displacement between the Salt Spring and Cyclopic Mine faults. The trace of the South Virgin–White Hills detachment is highly sinuous in map view and is marked by three prominent salients that define west-plunging antiformal warps in the detachment surface. We interpret the corrugations in the South Virgin–White Hills detachment to have formed by a process of linkage of originally separate en echelon fault segments followed by eastward tilting of the footwall. Depositional patterns, particularly between the Lakeside Mine and Salt Spring segments, support this interpretation. The Grand Wash fault forms the present-day physiographic boundary between the Colorado Plateau and the Basin and Range Provinces; however, based on greater amount of displacement and exhumation, we suggest that the South Virgin–White Hills detachment is the principal structure accommodating regional extension in the eastern Lake Mead extensional domain.