Regional stratigraphic units and structural features of the Lake Mead region are presented as a 1:250,000 scale map, and as a Geographic Information System database. The map, which was compiled from existing geologic maps of various scales, depicts geologic units, bedding and foliation attitudes, faults and folds. Units and structural features were generalized to highlight the regional stratigraphic and tectonic aspects of the geology of the Lake Mead region. This map was prepared in support of the papers presented in this volume, Special Paper 463, as well as to facilitate future investigations in the region. Stratigraphic units exposed within the area record 1800 million years of geologic history and include Proterozoic crystalline rocks, Paleozoic and Mesozoic sedimentary rocks, Mesozoic plutonic rocks, Cenozoic volcanic and intrusive rocks, sedimentary rocks and surficial deposits. Following passive margin sedimentation in the Paleozoic and Mesozoic, late Mesozoic (Sevier) thrusting and Late Cretaceous and early Tertiary compression produced major folding, reverse faulting, and thrust faulting in the Basin and Range, and resulted in regional uplift and monoclinal folding in the Colorado Plateau. Cenozoic extensional deformation, accompanied by sedimentation and volcanism, resulted in large-magnitude high- and low-angle normal faulting and strike-slip faulting in the Basin and Range; on the Colorado Plateau, extension produced north-trending high-angle normal faults. The latest history includes integration of the Colorado River system, dissection, development of alluvial fans, extensive pediment surfaces, and young faulting.

In northwestern Arizona, the high-standing, relatively unextended Colorado Plateau abruptly gives way across a system of major west-dipping normal faults to a highly extended part of the Basin and Range province known as the northern Colorado River extensional corridor. The transition from unextended to highly extended upper crust is unusually sharp within this region, contrasting with a broad transition zone elsewhere. The southern White Hills lie near the eastern margin of the extensional corridor in northwestern Arizona and contain a large east-tilted half graben that chronicles Miocene extension and constrains the timing of structural demarcation between the Colorado Plateau and Basin and Range province during Neogene time. This growth-fault basin is bounded on the east by the west-dipping Cyclopic and Cerbat Mountains fault zones. Greater tilts in the hanging walls suggest that these faults have listric geometries. The stratigraphy in the half graben consists of Miocene vol canic rocks intercalated with an eastward-thickening wedge of synextensional fanglomerates. Tilts in the Miocene units decrease up section from ~75° to 5°. Recent 40 Ar/ 39 Ar dating (11 new dates) of variably tilted volcanic rocks in the growth-fault basin and regional relations constrain the timing of east-west extension between ca. 16.6 and <9 Ma, with peak extension from ca. 16.6 to 15.2 Ma. Capping 8.7 Ma basalts are tilted 5°–10° and record the waning stages of extension. Thus, the sharp boundary between the Colorado Plateau and Basin and Range began developing by ca. 16.5 Ma and has changed little since ca. 9 Ma. Major extension and basin development significantly lowered base level within the extensional corridor and induced headward erosion into the western margin of the Colorado Plateau, which ultimately facilitated development of the western Grand Canyon. Abundant clasts of 1.7 Ga megacrystic granite in the eastward-thickening fanglomerates within the growth-fault basin suggest a partial provenance from the Garnet Mountain area along or near the western margin of the Colorado Plateau beginning as early as ca. 16 Ma and continuing to ca. 9 Ma.

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.

The eastern Lake Mead region, to the north of the belt of metamorphic core complexes that define the Colorado River extensional corridor, underwent large-magnitude extension in the middle to late Miocene. We present two speculative new models for extension in this area that resolve several puzzling and paradoxical relations. These models are based on new field mapping and structural, geochronologic, and thermochronologic data from the northern White Hills, Lost Basin Range, and south Wheeler Ridge. The Meadview fault, a previously underappreciated structure, is an east-side-down normal fault that separates the northern Lost Basin Range to the west from south Wheeler Ridge to the east. Proterozoic crystalline rocks of the northern Lost Basin Range yielded an apatite fission-track (AFT) age of 15 Ma, whereas 2 km to the east, across the Meadview fault, crystalline rocks of south Wheeler Ridge yielded a 127 Ma AFT age. Similarly, at the south end of the Lost Basin Range, crystalline rocks with ca. 15 Ma AFT ages lie within 5 km of crystalline rocks of Garnet Mountain that yielded a 68 Ma AFT age across the Grand Wash fault. Neither of these relations can be explained by existing tilted crustal section or tilt-block models. In our “classic” metamorphic core complex model, the Grand Wash fault (breakaway), the Meadview fault, and the South Virgin–White Hills detachment represent different structural levels of a single, regional detachment that was active between ca. 16 and 11 Ma. The hanging wall of the detachment consists of rocks at south Wheeler Ridge, the Paleozoic ridges, and possibly part of the crystalline basement of the Gold Butte block, sedimentary and volcanic rocks in the hanging walls of the Salt Spring and Cyclopic Mine faults, and possibly stranded tilt blocks beneath the Grand Wash Trough supradetachment basin. The footwall, exhumed by subvertical simple shear and characterized by middle Miocene AFT ages, includes the central and western Gold Butte block, Hiller Mountains, and crystalline rocks of the White Hills and the Lost Basin Range. The east-dipping Meadview fault bounds the crystalline core on the east; the west-dipping South Virgin–White Hills detachment bounds the core on the west. Therefore, the Grand Wash fault represents the structurally highest part of the detachment, and the South Virgin–White Hills detachment represents the structurally deepest exposed part of the detachment. In the modified core complex model, the Grand Wash, Meadview, and South Virgin–White Hills detachment faults are separate structures, and the Grand Wash Trough is a “trailing-edge” basin bound on the east by the Grand Wash fault and on the west by the Meadview fault. The South Virgin–White Hills detachment is the main detachment along which extension was accommodated, and the Meadview fault is a major antithetic normal fault that facilitated exhumation of the core at the trailing edge of the detachment system.

Extensional accommodation zones, or tilt-block domain boundaries, facilitate reversals in the dominant tilt direction of fault blocks and possibly inversions in the dip of regional detachment systems in rifted continental crust. The amount and direction of movement of the footwall (lower plate) and hanging wall (upper plate) of the detachment terrane dictate the deformational style along accommodation zones. Various models of extension can potentially be evaluated by defining modes of deformation along accommodation zones. A 40-km-long, east-west-trending, middle Miocene accommodation zone bisects the central Black Mountains, northwestern Arizona, and southern Eldorado Mountains, southern Nevada. The Black and Eldorado Mountains lie within the northern Colorado River extensional corridor, a 50- to 100-km-wide region of severely extended crust. The generally sublinear, 5- to 10-km-wide accommodation zone separates more than 5,000 km 2 of east-tilted fault blocks to the north from 25,000 km 2 of dominantly west-tilted fault blocks to the south. The zone may also mark the join between regionally extensive, oppositely dipping detachment systems. Transversely oriented segments (i.e., perpendicular to strike of tilted blocks) of the accommodation zone in the upper-plate rocks correspond to areas of intermeshing conjugate normal faults. East- and west-dipping normal faults dominate the west- and east-tilted domains, respectively, whereas east- and west-dipping faults are equally common in the axial part of the zone. Some of the major normal faults in the west- and east-tilted domains terminate in drag folds within the axial part of the zone. Fault-block tilting on either side of the accommodation zone commonly exceeds 60°. Tilting decreases progressively toward the axis of the zone, where transversely oriented, obliqueslip normal faults accommodate scissors-like torsional offset between gently tilted (10 to 35°) individual fault blocks of opposing polarity. Concomitant with the decrease in tilting, fault spacing decreases, and average fault dip increases. Fault blocks within the zone were periodically tilted in opposite directions during the same episode of extension. Minor amounts of open to tight folding characterize along-strike segments (i.e., parallel to strike of tilted blocks) of the accommodation zone.

Abstract The Virgin River Gorge and one of its tributaries are traversed by I-15 between milepost 12, about 13 mi (21 km) east of Mesquite, Nevada, and Milepost 28, about 12 mi (19 km) west of St. George, Utah (Fig. 1). Geologic features are readily observable from the road and from a scenic viewpoint in the Virgin River Canyon Recreation Area, which is entered from an interchange between mileposts 18 and 19. The site is in the northern half of the Purgatory Canyon, Arizona, 7½-minute Quadrangle and in the northeastern quarter of the Littlefield, Arizona, 15-Minute Quadrangle.