INTRODUCTION [c4, p83-85]
deformed rock units and landforms record the past 2 m.y. of faulting, folding,
uplift, and subsidence in California. Properly interpreted, such evidence provides
a quantitative basis for predicting future earthquake activity and for relating
many diverse structures and landforms to the 5 cm/yr of horizontal motion at
the boundary between the North American and Pacific plates.
Modern techniques of geologic dating and expanded research on earthquake hazards have greatly improved our knowledge of the San Andreas fault system. Much of this new knowledge has been gained since 1965, and that part which concerns crustal deformation during the past 2 m.y. is briefly summarized here.
In emphasizing recent work, I cite only a few of those earlier investigators who first recorded the extent and timing of deformation in upper Cenozoic deposits. Their contributions deserve a more complete accounting, and in several works cited herein such credit is given, most thoroughly by Wahrhaftig and Birman (1965). Also especially important in understanding the framework of the fault system are the 1:250,000-scale, 10 by 20 sheets of the "Geologic Atlas of California" published by the California Division of Mines and Geology, the 1:750,000-scale "Geologic Map of California" (Jennings, 1977), and the 1:750,000-scale "Fault Map of California" (Jennings, 1975).
The Quaternary deformation processes - chiefly faulting, folding, and uplift - represent crustal changes that in many places continue today but at barely perceptible rates. The perspective of 2 m.y. of geologic time permits us to detect and measure these processes, and where historical deformation is evident - as shown by seismology, geodetic studies, or geologic investigations of recent earthquakes - the Quaternary record provides an independent check on the reliability of our observations and measurements. More importantly, we also use the Quaternary viewpoint to sketch the outlines of the currently active fault system, its intact crustal elements and major active faults, and how these faults propagate and change over time. From such evidence we can better understand the pre-Quaternary history of the system and build the predictive kinematic models needed for earthquake hazard assessment.
Evidence of Quaternary deformation comes chiefly from the observed displacement of strata or geomorphic features ( Figure. 4.1 ). Where this displacement can be dated or, at least, bracketed in time, the age of deformation can be established, and its rate -generally an average - can be determined. In this report, most deformation rates are converted to units of centimeters per year, in part to facilitate comparison of different data sets but also because the dominant process, strike slip on major faults, is conveniently expressed in such units. Both the amount of displacement and the materials used to determine the age of displaced geologic markers require careful geologic analysis to ensure that field relations are unequivocal and correctly interpreted; the most reliable deformation rates depend on highly detailed analyses of local stratigraphy, which provide multiple measures of displacement over broad timespans, typically thousands to tens of thousands of years.
Deposits of Quaternary age can be dated by several methods, most of which ultimately depend on geochemical analyses that require highly sophisticated laboratory practices and expertise, as well as carefully chosen samples. The principal methods of dating Pleistocene and Holocene deposits within the San Andreas fault system ( Figure. 4.2) include radiocarbon, soil chronostratigraphy, correlation with standard sea-level stages (chiefly used for uplifted marine terraces), tephrochronology, and mammalian and invertebrate paleontology; supplementary techniques include paleomagnetism, uranium-series analysis, and amino-acid racemization. The underlying theory and limitations of these methods (for example, Pierce, 1986) are beyond the scope of this review. Each method, however, differs in resolving power, the time-span over which it is effective, and applicability to different rock types.
Difficulties in dating deformed Quaternary deposits are not the only deterrent to reconstructing the history of faulting. Large areas within the fault system are nearly devoid of Quaternary deposits; others are masked by landslide deposits, which conceal Quaternary folds and faults. Few published geologic maps differentiate between faults with Quaternary movement and those that have long been inactive, and for even the best known faults our knowledge of Quaternary faulting is still incomplete. Quaternary reverse and thrust faults, more difficult to identify and map than their strike-slip counterparts, are surely underrepresented on published geologic maps and in the literature.
Our knowledge of Quaternary deformation is thus incomplete and biased. We have learned much, and are rapidly learning more, about the Holocene and latest Pleistocene, but the view much beyond the past 100 ka is still poorly resolved. Despite the problems, the results of the past 2 decades of Quaternary research have brought new insights into the mechanics of the fault system and the promise of more discoveries yet to come.