Physiographic Features of Faulting in Southern California

The abundance and variety of faults in southern California provide good opportunity for study of landforms created directly by faulting or indirectly by other processes acting upon faulted materials. High-angle gravity faults, high- and low-angle thrusts, and faults with large strike-slip displacement are present (see Chapter IV). Furthermore, all degrees and dates of activity are represented. Landforms created by faulting can be classed as primary and secondary, or as original and subsequent (Lahee, 1952, p. 248). Primary features are those formed by actual fault displacement. They are nearly always modified by erosion, but should be classed as primary until completely effaced. Secondary or fault-line features are those formed solely by other processes acting upon faulted materials. Further subdivision into initial and modified primary forms and into erosional and depositional secondary forms would be possible, but it is not urged

Geology of the Tehachapi Mountains, California

The San Joaquin-Sacramento Valley, also known as the Great Valley of California, separates the Coast Ranges on the west from the Sierra Nevada on the east. The southern part of this major physiographic and structural province is about 50 miles in average width, and is terminated abruptly at its southeastern end by the Tehachapi Mountains, a range that trends roughly northeast. Uplifted principally by faulting, this mountain mass rises boldly (fig. 2) from the floor of the San Joaquin Valley-a floor so smooth and so extensive that in early days it was referred to as the San Joaquin Plains. The range also presents a rather straight and imposing, though somewhat less formidable, front toward the Mojave Desert to the southeast

Relationship between seismicity and geologic structure in the Southern California region

Data from 10,126 earthquakes that occurred in the southern California region between 1934 and 1963 have been synthesized in the attempt to understand better their relationship to regional geologic structure, which is here dominated by a system of faults related mainly to the San Andreas system. Most of these faults have been considered “active” from physiographic evidence, but both geologic and short-term seismic criteria for “active” versus “inactive” faults are generally inadequate. Of the large historic earthquakes that have been associated with surficial fault displacements, most and perhaps all were on major throughgoing faults having a previous history of extensive Quaternary displacements. The same relationship holds for most earthquakes down to magnitude 6.0, but smaller shocks are much more randomly spread throughout the region, and most are not clearly associated with any mappable surficial faults. Virtually all areas of high seismicity in this region fall within areas having numerous Quaternary fault scarps, but not all intensely faulted areas have been active during this particular 29-year period. Strain-release maps show high activity in the Salton trough, the Agua Blanca-San Miguel fault region of Baja California, most of the Transverse Ranges, the central Mojave Desert, and the Owens Valley-southern Sierra Nevada region. Areas of low activity include the San Diego region, the western and easternmost Mojave Desert, and the southern San Joaquin Valley. Because these areas also generally lack Quaternary faults, they probably represent truly stable blocks. In contrast, regions of low seismicity during this period that show widespread Quaternary faulting include the San Andreas fault within and north of the Transverse Ranges, the Garlock fault, and several quiescent zones along major faults within otherwise very active regions. We suspect that seismic quiescence in large areas may be temporary and that they represent likely candidates for future large earthquakes. Without more adequate geodetic control, however, it is not known that strain is necessarily accumulating in all of these areas. Even in areas of demonstrated regional shearing, the relative importance of elastic strain accumulation versus fault slippage is unknown, although slippage is clearly not taking place everywhere along major “active” faults of the region. Recurrence curves of earthquake magnitude versus frequency are presented for six tectonically distinct 8500-km^2 areas within the region. They suggest either that an area of this small size or that a sample period of only 29 years is insufficient for establishing valid recurrence expectancies; on this basis the San Andreas fault would be the least hazardous zone of the region, because only a few small earthquakes have occurred here during this particular period. Although recurrence expectancies apparently break down for these smaller areas, historic records suggest that the calculated recurrence rate of 52 years for M = 8.0 earthquakes for the entire region may well be valid. Neither a fault map nor the 29-year seismic record provides sufficient information for detailed seismic zoning maps; not only are many other geologic factors important in determining seismic risk, but the strain-release or epicenter map by itself may give a partially reversed picture of future seismic expectance. Seismic and structural relationships suggest that the fault theory still provides the most satisfactory explanation of earthquakes in this region

Physiographic Features of Faulting in Southern California

The abundance and variety of faults in southern California provide good opportunity for study of landforms created directly by faulting or indirectly by other processes acting upon faulted materials. High-angle gravity faults, high- and low-angle thrusts, and faults with large strike-slip displacement are present (see Chapter IV). Furthermore, all degrees and dates of activity are represented. Landforms created by faulting can be classed as primary and secondary, or as original and subsequent (Lahee, 1952, p. 248). Primary features are those formed by actual fault displacement. They are nearly always modified by erosion, but should be classed as primary until completely effaced. Secondary or fault-line features are those formed solely by other processes acting upon faulted materials. Further subdivision into initial and modified primary forms and into erosional and depositional secondary forms would be possible, but it is not urged

Late Quaternary slip rate on the Kern Canyon fault at Soda Spring, Tulare County, California

The Kern Canyon fault represents a major tectonic and physiographic boundary in the southern Sierra Nevada of east-central California. Previous investigations of the Kern Canyon fault underscore its importance as a Late Cretaceous and Neogene shear zone in the tectonic development of the southern Sierra Nevada. Study of the late Quaternary history of activity, however, has been confounded by the remote nature of the Kern Canyon fault and deep along-strike exhumation within the northern Kern River drainage, driven by focused fluvial and glacial erosion. Recent acquisition of airborne lidar (light detection and ranging) topography along the ∼140 km length of the Kern Canyon fault provides a comprehensive view of the active surface trace. High-resolution, lidar-derived digital elevation models (DEMs) for the northern Kern Canyon fault enable identification of previously unrecognized offsets of late Quaternary moraines near Soda Spring (36.345°N, 118.408°W). Predominately north-striking fault scarps developed on the Soda Spring moraines display west-side-up displacement and lack a significant sense of strike-slip separation, consistent with detailed mapping and trenching along the entire Kern Canyon fault. Scarp-normal topographic profiling derived from the lidar DEMs suggests normal displacement of at least 2.8 +0.6/–0.5 m of the Tioga terminal moraine crest. Cosmogenic 10Be exposure dating of Tioga moraine boulders yields a tight age cluster centered around 18.1 ± 0.5 ka (n = 6), indicating a minimum normal-sense fault slip rate of ∼0.1–0.2 mm/yr over this period. Taken together, these results provide the first clear documentation of late Quaternary activity on the Kern Canyon fault and highlight its role in accommodating internal deformation of the southern Sierra Nevada

Granitic Domes of the Mohave Desert, California

Several granitic areas in the Mohave Desert region of southeastern California have been degraded to smooth dome-like forms, to which Lawson has given the name, panfans. They have diameters of from 3 to 6 or 8 miles and heights of from 500 to 2,000 feet over the adjacent lower land. One of the best examples is shown in Plate 12. The well graded convexity of these masses, the steepest declivity of which seldom measures more than 4° or 5°, is flanked by the long, aggraded, concave slopes of their detritus. In some instances the domes are elongated into arches, 10 or 15 miles in length. Many other areas, granitic and non-granitic, less completely and less symmetrically degraded, exhibit bold or subdued residual forms surmounting their smoothly degraded flanks. The most perfect domes or arches result from the undisturbed degradation of upheaved granitic masses which have been worked upon, according to their original form, 1 chiefly by one or the other of two somewhat unlike erosional processes, both of which are merely modifications of ordinary erosional processes appropriate to the dry climate where their action takes place

Geology of the Tehachapi Mountains, California

The San Joaquin-Sacramento Valley, also known as the Great Valley of California, separates the Coast Ranges on the west from the Sierra Nevada on the east. The southern part of this major physiographic and structural province is about 50 miles in average width, and is terminated abruptly at its southeastern end by the Tehachapi Mountains, a range that trends roughly northeast. Uplifted principally by faulting, this mountain mass rises boldly (fig. 2) from the floor of the San Joaquin Valley-a floor so smooth and so extensive that in early days it was referred to as the San Joaquin Plains. The range also presents a rather straight and imposing, though somewhat less formidable, front toward the Mojave Desert to the southeast

Brachiopod community

19 p. : ill., map ; 26 cm.Includes bibliographical references (p. 15-18).A brachiopod community from the Fossiliferous Limestone Member (Upper Anisian-Lower Ladinian) of the Triassic Saharonim Formation at Har Gevanim, Makhtesh Ramon, southern Israel, is dominated by the terebratulid Coenothyris oweni Feldman. The community shows evidence of time-averaging and is largely composed of a single cohort of juvenile mortality of one spatfall. The Saharonim Formation was deposited under normal, calm, relatively shallow marine conditions as part of the global Anisian-Ladinian transgression. One horizon, varying in thickness between 1 and 1.5 cm, represents an autochthonous obrution deposit of juvenile Coenothyris brachiopods and 10 bivalve genera that were rapidly buried by pulses of clay in the form of flocculated mud. Other faunal constituents of the Saharonim Formation include conodonts, ostracodes, foraminiferans, bivalves, cephalopods, gastropods, echinoderms, and vertebrate remains that belong to the Sephardic Province and are diagnostic of the Middle Triassic series of Israel. The faunal composition and shallow depositional environment of the strata studied are useful in correlating the Triassic rocks in the Negev with those in Europe and help to differentiate the Sephardic Province from the Germanic Muschelkalk and the Alpine Tethyan faunas to the north