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SUMMARY [c4, p107-109]

Right-lateral strike slip on fewer than 10 major faults dominates the Quaternary record of deformation within the San Andreas fault system. Of these faults, the San Andreas is the longest and most active, but the fault system includes other structural elements, such as shorter or discontinuous strike-slip faults, reverse and thrust faults, regional fold systems, minor folds genetically related to strike slip along wrench faults, and tilted uplifted, or depressed crustal blocks. All of these structures result from relative, predominantly horizontal motion at the boundary separating the Pacific and North American plates. Though chiefly strike slip, the detailed pattern of Quaternary deformation differs from place to place within the fault system. Although the ultimate causes of these different structural patterns are ill defined and the subjects of continuing research, some geologic controls are evident: (1) the lithology and structure of basement rocks (see chap. 3), (2) the thickness of upper Mesozoic and Cenozoic strata overlying geologic basement, (3) the geometric orientation of major strike-slip faults, especially the San Andreas, and (4) the slip rates along structures controlling deformation.

Plutonic basement includes the granitic and associated metamorphic rocks of the Salinian block west of the San Andreas fault in the central Coast Ranges, similar rocks of the southern California batholith in the Peninsular Ranges, and a more complex suite of metamorphic and crystalline rocks in the central and eastern Transverse Ranges. Elsewhere in the fault system, the dominant basement-rock unit is the intensely sheared and broken Franciscan complex, which locally contains ophiolite bodies and remobilized, cold intrusive masses of serpentinite. The simplest patterns of Quaternary deformation are within regions of exposed or near-surface plutonic basement, where block-bounding strike-slip faults and block uplift or tilting prevail. Deformation is greater and more complex in regions of exposed or near-surface Franciscan basement, where broad anticlinoria, local basins, and distributed shearing are more evident. It is most intense and complex where basement rocks are covered by a kilometer or more of stratified upper Mesozoic or Cenozoic rocks.

Control of Quaternary deformation processes by fault geometry, or the interdependence of process and geometry, is best shown at the Big Bend in the San Andreas fault near the north boundary of the Transverse Ranges. There, west-northwest-trending folds and northwesttrending strike-slip faults of the northern and central sections of the fault system abut against a compressional domain characterized by east-west-trending folds, active thrust and reverse faults, and accelerated rates of vertical uplift. On much smaller scales, similar changes in structural trend and pattern appear at other changes in strike of the San Andreas or major branch faults.

The effect of different slip rates is demonstrated by the contrast in structural style along the San Andreas fault in central California and along the Newport-Inglewood fault zone in southern California. At a slip rate of 3.5 cm/yr, the San Andreas has displaced middle Pleistocene (1 Ma) outcrop belts and major structures by about 35 km, effectively creating -on opposite sides of the fault - two Independent structural domains, each of which responds differently to its new structural setting. In the same period of time, the much lower slip rates on the Newport-Inglewood fault zone have separated Quaternary structures by less than 500 m, leaving the initial structural patterns at least partly intact and connected.

Our current knowledge of Quaternary deformation within the San Andreas fault system can be expressed in terms of estimated rates for deformation processes (Figure. 4.22), even though reliable quantitative measurements of slip, folding, and uplift are few. Other lines of evidence from geologic mapping and geophysical investigations permit reasonable inferences regarding regional rates of deformation and the history of the fault system. Although some of these interpretations may change as new data are acquired, several major characteristics of the fault system and its Quaternary history are documented by the available evidence:

1. Quaternary tectonism within the San Andreas fault system has deformed an area of 160,000 km2, extending from Punta Gorda to the Salton Sea, a system length of about 1,100 km, and from the Great Valley and the Mojave Desert to fault zones off shore in the Pacific Ocean, an average system width of about 145 km.

2. Throughout the fault system, the rate of right-lateral strike slip on northwesterly-trending faults typically exceeds the geologically determined rates for other deformation processes by an order of magnitude. Despite some important exceptions, horizontal slip approximately parallel to the plate boundary domInates the Quaternary history of the fault system.

3. In the central section of the fault system, between lat 35�45' and 36�30' N., most of this slip has followed the main San Andreas fault; slip rates on this section of the fault are the highest observed to date within the system, averaging about 3.5 cm/yr.

4. Strike slip is distributed more broadly in both the northern and southern sections of the fault system, where the main San Andreas splits into several active branches; slip rates on some of these branches may equal or exceed the rate on nearby parts of the main fault.

5. Observed rates of Quaternary faulting, summed across the fault system, are insufficient to account for all of the relative movement (5.1 cm/yr) attributed to the Pacific-North American plate boundary, but neither the magnitude of the difference nor its cause is well constrained.

6. East-westerly- to northwesterly-trending reverse and thrust faults occur near many strike-slip faults and elsewhere in the system; they are longer, more active, and best defined in and near the Transverse Ranges (between lat 33�45' and 35�00' N.), where they accompany rapid Quaternary uplift and intense folding on east-west-trending axes.

7. Major west-northwest-trending fold belts deform Quaternary and older strata overlying deeply buried (1-5 km deep) basement rocks; most regional fold belts strike obliquely to strike-slip faults and are consistent with wrench-fault structural patterns, but the age and intensity of folding differ from place to place.

8. Several fault-bounded basement blocks are internally little deformed: the Sebastopol block in northern California (Franciscan basement between the San Andreas and Rodgers Creek faults), the Salinian block in central California (granitic basement between the Rinconada and San Andreas faults), and the Perris block in southern California (granitic basement between the Elsinore and San Jacinto faults).

9. Throughout the Quaternary, widespread uplift has characterized most of the fault system; locally downwarped, fault-controlled basins that reverse this general pattern are the San Francisco Bay and Santa Clara Valley (northern California), the Santa Clara River valley-Ventura Basin (southern California), the Los Angeles Basin, and basins near the southern and southwestern margin of the San Joaquin Valley.

10. Maximum observed uplift rates (0.5-1.0 cm/yr) for the late Quaternary occur near Punta Gorda and in the Transverse Ranges; elsewhere, measured uplift rates average about 0.05 cm/yr, but in large areas of the fault system no reliable measurements of uplift rates are available.

11. Major strike-slip faults bound the west edge of several uplifted blocks in northern California: the Santa Rosa block, bounded by the Rodgers Creek and Maacama faults; the Diablo Range and Temblor Range blocks, bounded by the Calaveras and San Andreas faults; and the Santa Cruz Mountains and Santa Lucia Range blocks, bounded by the San Gregorio, San Simeon, and Hosgri faults.

12. In the Transverse and Peninsular Ranges of southern California, Quaternary uplift near strike-slip faults has produced two nearly orthogonal mountain chains: (1) the west-northwesterly-trending ranges that separate the Mojave Desert from coastal lowlands and that lie along the trend of Transverse Ranges thrust faults and the southern branch of the San Andreas fault, and (2) the northsouth-trending mountain system composed of discrete, northwest-trending ranges that are separated and possibly offset by the Elsinore and San Jacinto faults west of the Salton Sea.

13. Rates of Quaternary faulting, folding, and uplift resemble those for the pre-Quaternary fault system; significantly higher rates, evident near Punta Gorda and in the Transverse Ranges, signify late pulses of accelerated tectonic activity that are still underway.

Despite many still-unsolved problems and unanswered questions, the San Andreas fault system has proved a productive laboratory in which to pursue three tasks: to quantitatively evaluate earthquake hazards in the densely populated, high-risk regions of California; to develop new investigative and analytical methods that can be applied in other regions of high earthquake risk; and to better understand how geologic processes deform the Earth's crust at plate margins. None of these tasks is finished, but the progress so far has improved our knowledge of crustal-deformation processes and aided our efforts to reduce the hazards from damaging earthquakes.