MOJAVE-SONORA MEGASHEAR: EVIDENCE FROM NEOPROTEROZOIC TO LOWER JURASSIC STRATA

Major successions as well as individual units of Neoproterozoic to Lower Jurassic strata appear to be systematically offset left laterally from eastern California and western Nevada in the western United States to Sonora, Mexico. This pattern is most evident in units such as the 1- to-2 m thick, Neoproterozoic, "Johnnie oolite" of the United States and of Sonora, Mexico, and in the Lower Cambrian Zabriskie Quartzite of the western United States and the correlative Proveedora Quartzite of Sonora. Matching of isopach lines of the Zabriskie Quartzite and Proveedora Quartzite suggests 700-800 km of left-lateral offset. The apparent offset pattern is also suggested by the distribution of 1) Neoproterozoic diamictite (tillite), 2) Early Cambrian archeocyathids, 3) Middle Ordovician quartzite, 4) an unconformity below Devonian strata that overlaps Silurian and Ordovician strata, 5) Devonian to Ordovician bedded barite deposits in the Roberts Mountains allochthon and in older strata of the Sonora allochthon, 6) Permian McCloud or McCloud-like faunas, and 7) the Early Jurassic bivalve Weyla.

In the western United States, the distribution of facies in Neoproterozoic and Paleozoic strata indicates that the Cordilleran miogeocline trends about north-south . A north-south trend is also suggested in Sonora, and if so is compatible with offset of the miogeocline but not with the ideas that the miogeocline wrapped around the continental margin and trends east-west or east-southeast in Sonora. An imperfect stratigraphic match of supposed offset segments of the megashear is apparent. Some units are almost perfectly matched, but others show considerable differences. The mismatch is attributed to uncertainties concerning the original relative positions of the strata and/or to the possibility that indigenous succession of the western United States and offset segments in Mexico were not precisely side by side before offset but were separated by an area, now buried, eroded, or destroyed by igneous intrusion that contained strata of intermediate facies.