Gulf of California rift and reflects transition into the San Andreas transform system (Crowell and Sylvester, 1979; Crowell, 1981; Johnson et al., 1982). Within this region several complex fault zones (Elsinore, San Jacinto, and San Andreas), characterized by braided slices, extend northwestward from spreading centers within the head of the Gulf. Crustal blocks and slices within and between these fault zones rise and sink (“porpoise structure”) as lithospheric displacements continue; some mountain blocks move upward and now stand high as the result of squeezing and shortening, whereas, not far away, other sectors are depressed to form basinal receptacles for sediments washed in from adjacent highlands. Even alluvial surfaces formed but a few millenia ago are warped and disturbed in this tectonically active region. Such a pattern of deformation is shown by earthquakes, geodetic measurements, and the interpretation of landforms (Sharp et al., 1972; Keller et al., 1982). The crustal deformation to be expected beneath a local site the size of a few city blocks—an area appropriate for most engineering structures—will therefore depend critically on exactly where it is sited in such a region.

Baja California Peninsula and Peninsular Ranges

West of the Gulf of California lies the peninsula of Baja California and its northwestern extension into southern California constituting the Peninsular Ranges (Gastil et al., 1981). This is primarily a region of deeply eroded basement rocks, broken into only a few long slices that are tipped westward. Compared with most of California the region is relatively intact and stable, except for belts along the major fault zones such as the Elsinore and San Jacinto. Its western margin is transitional into the California Borderland province along splays of the Newport-Inglewood Fault zone, which parallels the present shoreline just offshore. The northwestern margin of the Peninsular Ranges borders one of California’s most complex tectonic regions, a region abutting the Transverse Ranges. Here, bordering the Los Angeles Basin, Miocene and more recent crustal extensions and rotation of blocks have not yet been completely investigated. It is a region where at places folds are currently forming and faults are actively displacing alluvial surfaces (Harding, 1976). In rough terms, the northwestern margin of the Peninsular Ranges block is being forced into the Transverse Ranges as it is carried northwestward during the opening of the Gulf of California.

California Borderland

The topography of the seafloor to the west of southern California and the northern part of the Baja California Peninsula is characterized by basins and ranges (Moore, 1969; Howell and Vedder, 1981). Some of the ranges are surmounted by islands, others reach up to near sea level to form shoals. Many of the ranges are bordered by long, straight, discontinuous fault scarps as shown by subsea topography and the linear arrangement of earthquake epicenters. The region is one where the topography reflects the structure closely: the subsea mountains, formed mainly in post-mid-Miocene times, have been only slightly eroded. Basins, on the other hand, are being infilled by turbidity currents that are bringing sediment down submarine canyons from source areas on land well to the east. The flat floors of basins have been smoothed by sedimentation processes.

Transverse Ranges

One of the most active regions tectonically in western North America lies athwart the northwestern trend of mountain ranges fringing the continent and is appropriately named the Transverse Ranges. To the southeast the Peninsular Ranges parallel the trend of the continental margin, and to the northwest so do the Coast Ranges, Great Valley of California, and the Sierra Nevada.

In this east-west trending province, marine terraces near the city of Ventura, for example, are moving upward at rates of as much as 7.5 mm per year (Lajoie et al., 1982). In the same region, strata laid down at marine depths of several thousand feet only a half-million years ago are now warped and uplifted well above sea level and are deeply eroded (Yeats, 1977). The region, which extends both east and west of the Big Bend in the San Andreas Fault system, is one where different parts have had different tectonic histories during the past million years or so (Jahns, 1973; Allen, 1981; Crowell, 1982; Yeats, 1983). Some faults root at depths where earthquakes originate, others are related to sliding of beds across each other during folding and are relatively shallow and not so likely to generate dangerous earthquakes. The region also includes the area of the enigmatic Palmdale Bulge, where geodetic measurements show either general uplift during the last few decades or episodic uplift, although the interpretation of the measurements is controversial (Castle et al., 1976; Jackson and Lee, 1979; Mark et al., 1981; Strange, 1981, 1984; Stein, 1984). Moreover, the Transverse Ranges and parts of bordering realms include tectonic blocks that have been rotated clockwise during Miocene

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