INTRODUCTION [c3, p61-62]

The San Andreas fault is a transform fault along the boundary between the Pacific and North American plates. Bedrock along the fault includes various lithologic units that range in age from Precambrian to Tertiary and younger. Some bedrock units that can be matched across the fault suggest strike-slip displacement of as much as 560 km.

The great scar across the land of California, extending from the Gulf of California to Point Arena on the north coast, was recognized as a major fault during early geologic study of the San Francisco peninsula; it was named for San Andreas Valley, which lies a few kilometers south of San Francisco. Interest in the San Andreas fault was heightened as a result of movement on the fault, in some places as much as 5 m of strike-slip displacement, that occurred during the great San Francisco earthquake of 1906. Because of this earthquake, a concerted study was carried out by several leading geologists of that time (Lawson, 1908) that dramatically increased our knowledge of the regional extent and general features of the fault. Much controversy ensued during the following decades as to the time of origin of the fault, the magnitude of cumulative displacement along it, and, indeed, even whether the displacement might be principally dip slip rather than strike slip. An early proponent of substantial lateral movement on the San Andreas fault was Levi Noble (1927) of the U.S. Geological Survey, who suggested a 38-km right-lateral displacement based on the similarity of Tertiary strata on opposite sides of the San Andreas fault at Cajon Pass and Rock Creek (lat 34°26' N., long 117°50' W.). Substantial lateral offset of Quaternary terraces along the Mojave segment of the fault was recognized by R.E. Wallace (1949) while working on his Ph. D. thesis at the California Institute of Technology. He estimated a slip rate of 0.4 cm/yr, which he extrapolated to 120 km of right-lateral slip since mid-Tertiary time (approx 30 Ma). In 1953, a benchmark paper by M. L. Hill and T.W. Dibblee, Jr., of the Richfield Oil Corp. cited various evidence for great right-lateral offset along the San Andreas fault and speculated that the total offset amounts to 560 km or more since Jurassic time.

During middle and late 1960's, a time of great ferment of concepts regarding the plate-tectonic development of the planet Earth, the foundation was laid for much of the present view of the tectonics of California and the San Andreas fault. A highly significant breakthrough to our understanding of the development of the fault system was the brilliantly simple construction by J. Tuzo Wilson (1965, fig. 9), who showed the San Andreas fault as a transform fault connecting two spreading oceanic ridges (Figure. 3.1, and Figure 3.2). This view was soon modified by McKenzie and Morgan (1969) and Atwater (1970) to account for the effects of migrating triple junctions and for the timing, rates, and vectors of plate movement. Their plate-tectonic analysis of the San Andreas fault was based on calculations of plate motions between North America, Africa, India, Antarctica, and the Pacific, and many land-based geologists of the time would have agreed that it seemed "outrageous" to be "studying the San Andreas fault by using data that is no closer to California than 7,000 km" (Atwater and Molnar, 1973).