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EXTENT AND PATTERNS OF FAULTING

OVERVIEW OF THE PACIFIC MARGIN OF NORTH AMERICA [c1, p4]

The San Andreas fault system is part of a complex system of faults, isolated segments of the East Pacific Rise, and scraps of plates lying east of the East Pacific Rise that collectively separate the North American plate from the Pacific plate ( Figure 1.2 ). On a more generalized or global scale, the North American plate can be thought of as lying across and partly covering the northern part of the Pacific system of plates. In simplified terms, the Pacific system of plates includes three elements: a westward expanding plate (the Pacific plate), an eastward-expanding plate (the Juan de Fuca plate), and a spreading center (the East Pacific Rise) from which the plates expand as new material is added. To the north, the Pacific plate is underriding, or being subducted under, the North American plate along the Aleutian thrust.

Some investigators (Atwater, 1970; Atwater and Molnar, 1973) suggested that the North American plate has converged with and, indeed, slid over the Pacific system of plates, leaving only segments of the East Pacific Rise exposed, to which such names as "Juan de Fuca and Gorda Ridges" are applied. Similarly, related scraps of the eastward-expanding plate are the Juan de Fuca and Gorda plates ( Figure 1.2). Absolute plate motions derived by Minster and Jordan (1980), and Jordan and Minster (1988) are shown in Figure 1.2, along with the plate motion vectors derived from the relative migration of mantle plumes or hotspots responsible for volcanic activity in Yellowstone National Park and the Hawaiian Islands.

At its north end, the San Andreas fault joins the Mendocino Fracture Zone at a high angle, and there three plates are juxtaposed: one moving relatively northwestward, the second southeastward, and the third eastward and northeastward, to form a triple junction. At its southeast end, the San Andreas fault system merges more gradually with the set of transform faults underlying the Gulf of California. Just northwest of the area of merging, however, the trend of the San Andreas fault system changes to much more westerly, whereas a set of echelon faults accompanied by volcanism less than a million years old form a north-south-trending zone that extends northward across the Mojave Desert into Owens Valley of eastern California (Hill and others, 1985). This zone may be considered the East Pacific Rise overridden, and thus modified in pattern, by the North American plate.

The San Andreas fault system may be viewed as forming the hypotenuse of a right triangle of which the northward extension of the East Pacific Rise and the eastward extension of the Mendocino Fracture Zone are the legs. The model of an overridden, subducted oceanic plate within this triangle and underlying the North American plate (Dickinson and Snyder, 1979) presents significant tectonic problems (see chap. 3).

The San Andreas fault system has rearranged an assemblage of microplates, or terranes, some of which originated tens of degrees of longitude apart. During the fault's approximately 29-m.y. existence, an extremely complex pattern of rock distribution, has thus has been created (see chap. 3).