concept of a major left-lateral fault (the Mojave-Sonora mega- shear) that has been proposed to offset rocks from eastern California to the Caborca region (Silver and Anderson, 1974; Anderson and Schmidt, 1983). The concept is based mainly on the distribution pattern of dated Precambrian crystalline base- ment rocks, on the similar distribution pattern of uncon- formably overlying Neoproterozoic and Cambrian rocks (Anderson and Silver, 1979, 1981; Anderson and Schmidt, 1983; Stewart er al., 1984, 1990, in press), on the distribution pattern of the Eureka Quartzite and rocks considered correla- tive in Sonora (Ketner, 1986; Stewart er al., 1990), and on the distribution pattern of Triassic (Stanley and Gonzalez-Leon, 1995) and of Jurassic (Jones er al., 1995) rocks. The concept of a megashear has been greatly debated (see Stewart er al., 1990; and in press, for discussion of the pros and cons of the con- cept) and no consensus has been reached as to whether such a fault is necessary to explain the geologic patterns noted in the western United States and northwestern Mexico. In addition to questions concerning the validity of the concept, different interpretations have been made as to the amount and timing of the proposed offset. Anderson and Schmidt (1983) propose 700 to 800 km of offset based on the supposed offset of Cambrian rocks from the Death Valley area in California to the Caborca region. Ketner (1986) proposed 1,000 km of offset of the Ordovician Eureka Quartzite and rocks considered correla- tive in Sonora; Stanley and Gonzalez-Leon (1995) propose as much as 1,000 km of offset of Triassic strata from western Nevada to the El Antimonio area in Sonora; and Stewart and collaborators (in press), in a discussion of several possible interpretations, suggest 600 km of offset from the San Bernardino Mountains to the Caborca region. The time of dis- placement has generally been considered to be Jurassic (Silver and Anderson, 1974; Anderson and Silver, 1979; Anderson and Schmidt, 1983; Jones er a/., 1995) although Stevens and collaborators (1992) suggested a late Paleozoic age.

The data presented in this report do not clearly support or refute the concept of the Mojave-Sonora megashear. This