Title: Geologic map of the south half of the Boulder Canyon quadrangle, Clark County, Nevada

Author: Michael H. Darin and Barbaros Demircan

Year: 2022

Series: Open-File Report 2022-01

Version: first edition, May 2022; superseded by Open-File Report 2023-03

Format: sheet: 35 x 28 inches, color; text: 5 pages, b/w

Scale: 1:24,000

The Boulder Canyon 7.5-minute quadrangle is located east of Las Vegas, Nevada and is situated almost entirely within the boundary of the Lake Mead National Recreation Area. The south half of the quadrangle overlaps the Nevada-Arizona state line along the north shore of Lake Mead and includes rugged topography at the northern end of the Black Mountains. The only access to this area is via Northshore Road which intersects the north half of the quadrangle, or by boat. Building on two University of Nevada, Las Vegas M.S. theses (Naumann, 1987; Eschner, 1989) and detailed geologic mapping in adjacent quadrangles (Anderson, 2003; Duebendorfer, 2003), this map provides useful insights into the geologic framework of the region, its complex deformation and magmatic histories, and the potential for recent Quaternary faulting and seismic hazards in close proximity to the rapidly expanding Las Vegas urban area and within the Lake Mead National Recreation Area.

The structural architecture consists of an integrated network of N-striking normal faults, ~NE-striking left-lateral faults, ~NW-striking right-lateral and right-oblique normal faults, and few E- to NE-trending folds. The dominant structure in the area is the ENE-striking, left-lateral Hamblin Bay fault, which cuts through the northwest corner of the south half of the quadrangle. The Hamblin Bay fault accommodated at least ~15–20 km of left-lateral slip mostly between ca. 12 and 6 Ma; it is the dominant structure within the left-stepping, transtensional Lake Mead fault system for which cumulative slip likely exceeds 40 km (Anderson, 1973; Weber and Smith, 1987). Miocene volcanic and sedimentary strata are generally west-tilted in the immediate hanging wall of the ENE-dipping Boulder Wash fault, but stratal dips roll over to become east-tilted toward the eastern map boundary. Total normal displacement on the Boulder Wash fault is at least ~1,700 m, and normal slip on the west-dipping Ransome fault is at least 500 m.

Map units in the south half of the quadrangle consist of middle to upper Miocene (ca. 17–10 Ma) volcanic and sedimentary rocks and hypabyssal plutonic rocks that contain xenolithic megablocks and screens composed of Precambrian crystalline basement, Cambrian dolomite, and Miocene sedimentary and volcanic strata. The only exceptions are intensely faulted Triassic and Jurassic strata and the unconformably overlying Oligocene-Miocene Rainbow Gardens formation northwest of and astride the Hamblin Bay fault, which are only exposed in the northwesternmost part of the south half of the quadrangle. Southeast of this fault, most of the map consists of the northernmost and structurally shallowest part of the ca. 13.5–12.5-Ma Wilson Ridge pluton, which was emplaced during rapid extension across the Lake Mead region. Two relatively distinct, kilometer-scale screens of sedimentary and volcanic rock at Bearing Peak and Guardian Peak show pervasive hornfels-facies metamorphism and appear to be at least partially suspended within the Wilson Ridge pluton. The Bearing Peak screen (~1.6 km2) consists of a >1-km-thick, gently- to steeply-dipping sequence of bedded conglomerate, sandstone, tuff, and angular megabreccia composed entirely of clasts of diverse Precambrian metamorphic rocks that are not exposed in or near the quadrangle; these strata are tentatively correlated with the Thumb Member of the Horse Spring Formation. The Guardian Peak screen (~2.0 km2) is composed of a >500-m-thick sequence of similar Thumb Member strata underlain by Cambrian(?) dolomite and Precambrian gneiss and overlain by mafic Miocene lavas. East of the Boulder Wash fault is a >600-m-thick sequence of dacitic lavas, breccias, and tuffs referred to as the volcanic rocks of Boulder Wash (ca. 14.2–12.5 Ma). Geochemical correlations indicate that the volcanic rocks of Boulder Wash are the extrusive equivalents of the Wilson Ridge pluton (Naumann, 1987). These volcanic rocks are unconformably overlain by mixed gypsum, volcaniclastic sandstone and conglomerate, and tuffs of the red sandstone unit (ca. 11.7–10.0 Ma). Both of these units are interbedded with distal alkaline mafic to intermediate lava flows derived from the Hamblin-Cleopatra stratovolcano (ca. 13.0–10.7 Ma).

South of the Hamblin Bay fault, significant E-W extension occurred during and after emplacement of the Wilson Ridge pluton and Boulder Wash volcano, probably between ca. 13.5 and 10 Ma. Extension was at least partly synchronous with late Miocene left-lateral slip on the Hamblin Bay fault. More regionally, sinistral transtension along the Lake Mead fault system was coeval and kinematically linked with the NW-striking dextral faults of Las Vegas Valley shear zone and southern Death Valley fault system to the northwest, and with E-W extension in the Lower Colorado River Extensional Corridor to the southwest.

Quaternary surficial deposits are sparse within the south half of the quadrangle and consist only of unconsolidated alluvium in active washes and locally preserved and abandoned alluvial terraces and colluvium. No evidence of Quaternary fault activity is recognized within the south half of the quadrangle.

Darin, M.H., and Demircan, B., 2022, Geologic map of the south half of the Boulder Canyon quadrangle, Clark County, Nevada: Nevada Bureau of Mines and Geology Open-File Report 2022-01, scale 1:24,000, 5 p.

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