PLATE-TECTONIC DEVELOPMENT OF
THE SAN ANDREAS FAULT [c3, p73-77]

The Pacific coast of North America is a highly mobile zone of tectonic interaction between continental crust on the east and oceanic crust on the west. At some times and places, the relative movements of these contrasting crustal domains have been sufficiently convergent that the oceanic crust has underthrust the continental crust and swept island arcs and other crustal materials into the zone of interaction (Hamilton, 1969). However, much of the movement along this zone of interaction seems to have been oblique or lateral in an overall northwest-southeast direction, and so as the two types of crust moved past one another, fragments of all sizes were sliced or pulled from them and carried various distances away from their source. By these processes of convergence and lateral translation, large crustal fragments referred to as "terranes" (Irwin, 1972; Coney and others, 1980; Schermer and others, 1984) are juxtaposed against others that may differ strikingly in lithology, age, genetic environment, stratigraphy, metamorphic facies, plutonism, and mineral deposits.

The general directions and rates of movement of the major crustal domains that converged along the western margin of North America are amenable to explanation by the theory of global plate tectonics as far back in time to at least the Mesozoic (Engebretson and others, 1985). According to this theory, the crust of Earth is a mosaic of interacting rigid plates. Boundaries between the plates are spreading ridges where the plates pull apart, oceanic trenches above subduction zones where the plates converge, and transform faults where the plates slide laterally past one another. These boundaries are the loci of seismic activity. The corollaries of the theory of global plate tectonics have been enormously valuable as an aid in recognizing the genetic environments of the various terranes, whether oceanic or continental crust, volcanic island arc, or oceanic trench. The presence of multiple ophiolite and blueschist belts along parts of the western margin of North America (Irwin, 1977) indicates that some of these terranes were subduction related during Paleozoic and Mesozoic time. Although much has been learned of the relative motions between the oceanic and continental rocks for the past 180 m.y. (for example, Engebretson and others, 1985), the tectonics of the zone of allochthonous terranes is 50 complex, and the sites of origin of the various terranes generally so obscure, that much painstaking research remains to be done before the Paleozoic and Mesozoic margins of western North America can be palinspastically reassembled.

The tectonic setting of the continental margin during late Mesozoic time, before development of the San Andreas fault, is highly controversial. Some geologists (for example Dickinson, 1981) favor a model of highly convergent plate interaction to develop a continental margin of the Andean type ( Figure. 3.11). Others, however, interpret certain paleontologic and paleomagnetic evidence to indicate that some late Mesozoic rocks of the Coast Ranges were translated great distances northward from equatorial sites of origin during Late Jurassic time, before accretion to North America and deposition of the Great Valley sequence (Hopson and others, 1986). This movement is thought to have been followed in Late Cretaceous and early Tertiary time by a second episode of dextral translation during which part of the Franciscan assemblage was accreted and parts of the Coast Range ophiolite and overlying Great Valley sequence moved northward for distances as great as 1,120 km (McLaughlin and others, 1988).

The rates and relative directions of motion of the principal tectonic plates are based mainly on patterns of magnetic anomalies in the oceanic crust. These patterns indicate that a subduction-related trench lay offshore of western North America during early Tertiary time because of the convergence of the Farallon plate (see Figure. 3.1 ), and that strike-slip movement on faults of the San Andreas system began no earlier than approximately 30 Ma (late Oligocene), when the Pacific plate first impinged on the North American plate (McKenzie and Morgan, 1969; Atwater, 1970). Triple junctions formed at the point of contact of the Pacific plate with the North American plate and migrated to the northwest and southeast as subduction of the Farallon plate continued. These triple junctions are now approximately 2,500 km apart: The Mendocino triple junction is off the coast of northern California, and the Rivera triple junction is at the mouth of the Gulf of California. The relative motion along the transform fault that formed the lengthening boundary between the Pacific and North American plates was right lateral. This early transform movement probably was not along the modern trace of the San Andreas fault but must have been along other faults of the system that now lie mostly to the west and at the edge of the continent (Figure. 3.12 ). The modern San Andreas fault apparently did not come into being in southern California until the opening of the Gulf of California during Pliocene time, about 4 Ma, since which time Baja California has moved 260 km away from mainland Mexico (Larson and others, 1968). The San Andreas fault is commonly referred to as the boundary between the Pacific and North American plates, which is true in the sense that the rocks on the west side of the fault are moving somewhat in concert with the Pacific plate, although those rocks actually are displaced fragments that once were part of the North American plate (Figure. 3.13 ).

During the early development of the San Andreas fault system, the principal movement must have been along a transform fault that formed the boundary between rocks of the North American plate and newly formed oceanic crust of the Pacific plate as the triple junction migrated southward. At some point during southward migration of this triple junction, the transform apparently jumped eastward one or more times to positions within the North American plate, to become the northern section of the modern San Andreas fault. The modern trace of the San Andreas fault in central California probably had only minor slip until about 12.5-10 Ma and probably was not the strand of dominant slip before 7.5-5 Ma (Dickinson and Snyder, 1979). The southern section of the modern San Andreas fault was formed by similar eastward jumps of the transform fault, resulting in opening of the Gulf of California. Some of the major faults that lie between the San Andreas fault and the continental margin may represent these earlier positions of the transform, and some of these faults are still active. Possible candidates for such early intermediate faults include, among others, the San Gregorio-Hosgri fault in central and northern California and the Elsinore and offshore faults in southern California. With our present state of knowledge, however, it is unclear which of the many faults in the Coast Ranges are early faults of the San Andreas system and which may have formed before mid-Tertiary initiation of the San Andreas system.

The models of McKenzie and Morgan (1969) and Atwater (1970) have been widely used in relating Pacific plate interactions to the tectonics and geology of California and other parts of western North America (Figure. 3.1 ). A more actualistic developmental sequence of diagrams was drawn by Dickinson (1981), in which the position of an early San Andreas fault is shown before the opening of the Gulf of California (Figure. 3.12 ). In Figures 3.1 and 3.12, the boundary between the Pacific and North American plates is shown as a transform fault that was formed by the passage of migrating triple junctions. However, marine geologic studies offshore of California between Point Conception and Cape Mendocino indicate that this old interface between the Pacific and North American plates is an inactive east-dipping low-angle fault which is interpreted to be a fossil subduction zone (McCulloch, 1987). There, magnetic stripes of the oceanic (Pacific) plate can be traced some distance eastward-locally as much as 30 km-beneath the leading edge of the upper (North American) plate, and both plates are covered along the suture by a thin veneer of undeformed Miocene strata. The presence of a Miocene or older subduction zone rather than a transform fault at the ocean-continent interface is difficult to reconcile with a strict interpretation of McKenzie and Morgan's (1969) and related models, although it is interpreted to indicate that the transform motion was accompanied by oblique convergence (McCulloch, 1987).

The present overall rate of relative movement between the Pacific and North American plates, earlier thought to be about 6 cm/yr (Atwater, 1970) or 5.6 cm/yr (Minster and Jordan, 1978), is now thought to be more likely about 4.8 cm/yr (DeMets and others, 1987). This rate probably has varied over time (Atwater and Molnar, 1973); however, the rate of relative motion between the two plates is substantially greater than the slip rates based on measured offsets of geologic features along the San Andreas fault. For example, the 315-km offset of the lower Miocene Pinnacles and Neenach Volcanic Formations indicates a minimum overall slip rate of 1.3 to 1.4 cm/yr (Matthews, 1976), and the offset of the channel of Wallace Creek by the San Andreas fault in central California indicates a slip rate of about 3.4 cm/yr for the past 3,700 yr and of 3.6 cm/yr for the past 13,250 yr (Sieh and Jahns, 1984). Geodetic-survey measurements indicate that slip rates during the past 90 yr in central California average 2.9 cm/yr for the upper 15 km of crust but 3.7 cm/yr below 15 km (Thatcher, 1977). The discrepancy between the rate of relative motion between the Pacific and North American plates and the much smaller slip rate on the San Andreas fault has been noted by many workers (for example, Atwater, 1970; Minster and Jordan, 1978; Weldon and Humphreys, 1986). Part of the total slip along the boundary between the Pacific and North American plates probably is occurring in small increments along other faults in a broad zone of interaction that may extend from the continental margin eastward even as far as the Basin and Range province.