Abstract The Banning fault site is located at the mouth of Cottonwood Canyon, at the eastern end of San Gorgonio Pass. San Gorgonio Pass is the low divide separating the high steep terrain of the Peninsular Ranges Province to the south, from the higher but less steep Transverse Ranges Province to the north. The San Gorgonio Pass area is reached by taking 1–10 east from the Los Angeles-San Bernardino-Riverside region toward Indio. Most places with spectacular exposures of the Banning fault are, unfortunately, on property with restricted access, but one location is readily accessible to the public in Cottonwood Canyon near the eastern end of the Banning fault. Cottonwood Canyon is reached by taking the Verbenia exit from 1–10 about 45 mi (72 km) east of Riverside and San Bernardino. Cross north over 1–10 and turn left on the first road, Tamarack. Proceed west 0.3 mi (0.5 km) on Tamarack to Cottonwood Canyon Road. Go north 1.8 mi (2.9 km) on Cottonwood Canyon Road to a sharp right turn. Park in a parking area to the north of the turn, which is at the mouth of Cottonwood Canyon. Part of Cottonwood Canyon Road is paved; the remainder is an improved dirt road easily passable by passenger cars and buses.

The structure of the northern Cottonwood Mountains, located in the Death Valley region of southeastern California, is dominated by a faulted, east-facing, monoclinal flexure developed in Paleozoic strata of the Cordilleran miogeocline. We interpret this flexure as a rollover or fault-bend fold above a listric normal fault, probably the north-ward continuation of the Tucki Mountain normal fault system exposed about 45 km south of the study area. Synorogenic Tertiary sediments in the Ubehebe basin, overlying the rollover at the northern end of the Cottonwood Mountains, record the inception of normal faulting and eastward tilting around a horizontal axis trending about N25°W. Paleozoic strata in the Dry Mountain block to the west are subhorizontal. Racetrack Valley, a north-trending topographic depression between the Dry Mountain block and the northern Cottonwood Mountains, is interpreted as a major graben that accommodates differential stratal rotations within the rollover. The juxtaposition of Mesozoic thrust plates by Tertiary normal faults has obscured the structural simplicity of the rollover. Structural and stratigraphic correlations indicate that strata exposed in the Dry Mountain block and northern Cottonwood Mountains are parts of a single Mesozoic thrust plate. This new evidence suggests that the Racetrack thrust of McAllister (1952) does not root in northern Racetrack Valley, contrary to previous interpretations. A previously unrecognized klippe and other parts of the Ubehebe thrust plate lie structurally above the northern Cottonwood Mountains. We suggest that normal faulting has downdropped a portion of the Ubehebe thrust plate into Racetrack Valley between strata of the Dry Mountain block and northern Cottonwood Mountains. Thus, the development of a major graben within the rollover has produced an apparent thrust relation along both sides of the Racetrack Valley block. We correlate the Racetrack Valley block and Ubehebe thrust plate with the Last Chance thrust plate of Stewart and others (1966). Sequentially deformed, palinspastic reconstructions of the rollover indicate that conjugate faults formed symmetrical grabens during development of the rollover. The grabens accommodated bending within the rollover and probably nucleated above areas of maximum curvature on the basal listric fault. Thus, the nucleation of progressively younger grabens during extension apparently migrated toward the breakaway within the hanging-wall block while remaining fixed relative to the bend in the basal fault plane. The pre-middle Pliocene(?) age and structural position of strata in the Ubehebe basin suggest that they are partially correlative with upper Miocene to lower Pliocene strata in the Nova basin, also located on the eastern margin of the Cottonwood Mountains structural block but adjacent to Tucki Mountain. The location of these basins in the hanging wall of the Tucki Mountain normal fault system suggests that they may be allochthonous parts of an earlier basin. Distinctive cobbles with no known local source are abundant in a relatively thin stratigraphic zone within the Ubehebe basin. We interpret these cobbles as reworked clasts derived from the Oligocene Titus Canyon Formation. This is consistent with structural and stratigraphic correlations that indicate about 70 km northwest-directed transport of the Cottonwood Mountains block from a position adjacent to the Funeral Mountains (Snow and Wernicke, 1988,1989; Snow, 1989; Snow and others, 1989).

Abstract Base- and precious-metal deposits in the central Wasatch Mountains southeast of Salt Lake City were mined for more than 100 years beginning in 1868. Deposits present in the Park City, Little Cottonwood, and Big Cottonwood mining districts include Ag-Pb-Zn ± Cu ± Au replacements and veins, a low-grade porphyry Cu-Au deposit, Cu-bearing skarns, a quartz monzonite-type (low F) porphyry Mo deposit, and high sulfidation (quartz-alunite) Au deposits. Most production came from polymetallic replacement and vein deposits in the Park City mining district, which has a recorded production of more than 1.4 million oz Au, 253 million oz Ag, 2.7 billion lbs Pb, 1.5 billion lbs Zn, and 129 million lbs Cu from 1872 to 1978. Production in the Little and Big Cottonwood districts, mostly from Pb-Ag replacement deposits, was much smaller. Most mineral deposits in the central Wasatch Mountains are genetically related to the Wasatch igneous belt, a series of high-K calc-alkaline stocks and cogenetic volcanic rocks that formed about 41(?) to 30 Ma. The mineral deposits mostly formed near the end of magmatic activity between about 36 to 31.4 Ma. A subeconomic porphyry Mo deposit in the Little Cottonwood stock is notably younger having formed about 26 to 23.5 Ma. The intrusive rocks were emplaced mostly along the westward extension of the west-trending Uinta arch during a period of NW-SE-directed extension, and much of the mineralization in the Park City district was controlled by ENE-striking normal faults. About 15 degrees of eastward tilting of the central Wasatch Mountains during Late Cenozoic Basin and Range extension has resulted in progressively deeper levels of exposure from <1 km on the east to about 11 km on the west and in profound variations in the types of mineral deposits exposed in different parts of the range. Most deposits formed at paleodepths ≤5 km, and the most productive deposits in the Park City district formed at depths of 1 to 2 km. The porphyry Mo deposit in the Little Cottonwood stock formed at greater depths of about 6 km.