Introduction and General History
The White-Inyo Range (Fig. 3.1), representing the westernmost range of the Basin and Range structural province, extends for 110 mi (175 km) from Montgomery Pass south-southeastward to Malpais Mesa opposite Owens Lake. Its maximum width, east of Bishop, is approximately 22 mi (35 km). The terminological separation of White from Inyo mountains is placed along the Westgard-Cedar Flat-Deep Springs Valley Road, a division that has no particular topographic or geologic significance. As is typical of the ranges of the province, it is bounded, generally on both sides, by normal faults of large-magnitude slip. The northern part of the range is mainly an easterly tilted block marked by an impressive escarpment from the Owens-Chalfant-Hamill valleys on the west, at 4,300 ft (1,310 m), to the crest of the range at White Mountain Peak, 14,246 ft (4,342 m). The southern, Inyo Mountains, part of the range has been tilted slightly to the west, with its maximum relief from Saline Valley at 1,100 ft (353 m) on the east to the range crest at Mt. Inyo, 11,107 ft (3,385 m).
Rocks in the White-Inyo Range span the time from the late Precambrian (700 Ma [Ma, millions of years ago]) to the Holocene, or Recent (i.e., last 10,000 years). Figure 3.2 is a simplified geologic timetable showing major time units and millions of years before the present (Ma) for the beginning of each, and the time spans of mountain building episodes (orogenies) referred to in the text. All periods of the Paleozoic (570–225 Ma), Mesozoic (225–65 Ma) and Cenozoic (65 Ma to the present) are represented, some incompletely. Rocks from the late Precambrian to the end of the Devonian Period (345 Ma) are entirely of sedimentary origin, having been deposited as sand, shale, dolomite, and limestone along the western edge of the North American continent by stream systems flowing westward into a marine basin called the Cordilleran geosyncline (Fig. 3.3). The total accumulation of uppermost Precambrian through Lower Jurassic surficial deposits in the geosyncline exceeded 4.5 mi (7 km) in thickness in the White-Inyo region. Beginning in Mississippian time, about 345 million years ago, the sedimentation pattern changed in response to elevated lands lying to the north and possibly west, resulting in the accumulation of coarse-grained sands and conglomerate, reflecting higher-energy stream systems. This pulse of uplift has been considered by some geologists as the result of the first of several collisions of the North American lithospheric plate and an ancient Pacific plate; the inferred plate collision is held responsible for the Antler orogeny, or time
of mountain building. The Pennsylvanian and Permian (320–225 Ma) were times of renewed deposition of limestone, reflecting a return to conditions of quiet carbonate bank formation.
This lull was again succeeded by extensive tectonic activity, apparently resulting from a second plate collision, the Sonoma orogeny (230–220 Ma). This event, as in
the Sierra Nevada to the west, resulted in extensive volcanism in a marine environment, the products of which are exposed as interlayered lava flows, ash beds, and continental sedimentary rocks in the southern Inyo Mountains and in the northern White Mountains.
In both the White-Inyo Range and the Sierra Nevada, surficial volcanism and its deep-seated counterpart, igneous intrusion, continued intermittently until approximately 155 million years ago as a consequence of the most intense orogeny to affect the region, the Nevadan (see Fig. 3.2). This complex episode, the result of terrane
accretion (plate collision) and amalgamation of far-traveled subcontinental fragments outboard of the North American plate and large-scale subduction of an ancient Pacific plate beneath the North American plate, produced major compressional deformation, metamorphism, and the emplacement of numerous coarse-grained intrusive bodies, termed plutons. The latest Mesozoic (90–75 Ma) was a time of renewed plutonic activity, represented by cross-cutting granitic intrusive rocks that transect the earlier-formed deformational structures. The physical conditions inferred to have attended crystallization of the metamorphic minerals associated with the intrusive rocks document the presence of a volcanic-plutonic arc marking the western margin of the North American continent throughout Mesozoic time (Fig. 3.4).
The early and middle Cenozoic Era was a time of large-scale uplift and extensive erosion; no sedimentary record of this time has been left in the White-Inyo Range. A major erosion surface truncates all previously formed rocks, including the deep-seated granitic plutons, which had solidified at depths of perhaps 6 mi (10 km) or more. Beginning approximately 10 million years ago, the range experienced the outpourings of extensive fragmental volcanic ejecta (volcanic tuff) and basaltic lava flows. Remnants of this episode can be seen in the volcanic rocks that mantle the northeast corner of the White Mountains at Montgomery Pass and east, in the table-like flows extending from the Cottonwood Basin (SE of Mt. Barcroft) to the north end of Deep Springs Valley, in the large expanse of volcanic rocks covering the saddle area between Eureka and Saline valleys on the east flank of the Inyo Mountains, and in the Malpais Mesa at the south end of the Inyo Mountains.
The latest Cenozoic was a time of renewed uplift along Basin and Range normal faults flanking the range, and transcurrent motion along the Furnace Creek fault zone at the margin of the White Mountains and Fish Lake Valley. Both styles of faulting reflect a major episode of crustal extension that was initiated in the Basin and Range Province approximately 15 million years ago. That this style of deformation continues to the present in the White-Inyo region is attested to by such events as the Chalfant earthquakes of July 1986, which measured up to 5.5 on the Richter scale.
An additional result of the uplift in latest Cenozoic time was the onset of Quaternary glaciation, which affected the area east and north of Mt. Barcroft and White Mountain Peak. This was followed by further uplift and the development of extensive aprons of alluvial deposits at the western and eastern margins of the range.