A p o s t -e a r l i e s t Miocene displacement of about 160 miles (260 km) on the San Andreas fault system in southern California is suggested by the occurrence of similar rocks and geologic histories in three terranes-the Tejon, Soledad, and Orocopia. These three terranes are interpreted as parts of an original east-west trending belt now displaced with right slip of about 130 miles (210 km) on the San Andreas fault and 30 miles (50 km) on the San Gabriel fault. Augen gneiss and blue-quartz gneiss of the amphibolite facies, in truded by a complex of gabbro, diorite, anorthosite, and syenite, and all intruded again by granitic rocks, constitute basement terranes of similar compositions and histories that are offset about 130 miles along the San Andreas fault. Other distinctive rocks on both sides of the fault include basic dikes and mafic bodies rich in ilmenite, magnetite, and apatite in the anorthosite and blue-quartz granite, quartz-bearing syenite, granophyre, and pegmatite in the syenite. Greenschist, marine Eocene strata, and Oligocene nonmarine beds and included volcanic rocks are associated. Similar terranes, but lacking the anorthosite-syenite complex, are apparently separated by about 30 miles on the San Gabriel fault, a displacement com patible with evidence from Upper Tertiary sedimentary units that suggest later right slip of as much as 2 0 miles. Post early Miocene displacement on the San Andreas fault in central California is probably about 175 miles (Hill and Dibblee, 1953). This displacement is based on the offset of unusual associations of lower Miocene volcanic works (with remarkable similarities in petrography), red beds, and marine lower Miocene and Oligocene strata. Younger and smaller displacements, although still in completely described, appear acceptable, because they are geomet rically sound and are concerned with slip and not with separation alone. Displacements of older features in California, which form the basis for statements that perhaps the San Andreas has a right slip of more than 300 miles, rest on arguments of a different order of accept ability. Available information suggests that the fault system origi nated in the earliest Tertiary. Progress in the study of the major California faults, although in separable from the complex geology along the continental margin, can be accelerated through the geometric analysis of geological ter-rains. Trace slip has displaced some low-dipping, sedimentary, volcanic, and metamorphic units; intensive studies of these and other units are required to discover gross linear features, such as basin-margin lines and lines of facies change that form the basis for finding net slip. within this zone are older than the presently active fault, but there is widespread controversy as to definition and significance. T h e term, rift, is applied to the belt o f topographic features along the fault zone (Lawson and others, 1908, p. 2 5 ); it is generally a shallow trench up to several miles wide that has resulted both from erosion in shattered and weathered ...
