THE MOJAVE-SONORA MEGASHEAR AND TECTONICS OF THE CORDILLERAS OF WESTERN NORTH AMERICA

The megashear concept is based upon work conducted in northwestern Sonora, Mexico. U/Pb isotopic analyses of 33 Proterozoic rock samples and structural and stratigraphic measurements reveal the character of rocks that crop out within a region known as the Caborca block. The northeastern limit of the Caborca block (terrane) coincides with the Mojave-Sonora megashear (MSM). The MSM was conceived in an effort to explain: 1) the unexpected presence of the 1.8-1.7 Ga Caborca block rocks opposite1.7 –1.6 Ga Pinal province rocks; 2) the differences among cover rocks overlying these basement provinces, and 3) the presence of mid-Jurassic (mainly175-160 Ma) volcanic and plutonic rocks that intervene between the provinces of Proterozoic crust. The MSM lies among basins containing mainly Late Jurassic and Cretaceous clastic strata that distinguish a belt about 8000 km long along the western and southwestern margin of North America. The zone delineated by the basins is a few hundred km wide and extends from Alaska to southern California where it swings gently southeastward across northern Mexico to the Gulf of Mexico. The basins began to form as early as Bathonian (~164 Ma) during the waning stages of slightly older (Toarcian to Callovian) convergence of an oceanic plate beneath the margin of western North America. Normal and lateral faults locally preserved along the basin margins suggest that the basins are structural pull-aparts that formed within the magmatic arc as well as in adjacent continental and oceanic crust. Ophiolitic rocks of Bathonian (166-161 Ma) age (e.g. Great Valley, Coast Range, Josephine, Ingalls, Fidalgo) that are characterized by diverse geochemical signatures and tectonic settings probably record MSM related rifting. In places, the principal lateral faults obliquely transect the belt of middle Jurassic arc rocks resulting in overlap (southern British Columbia; northwestern Mexico) or offset (northern Mexico) of the belt of several hundreds of kilometers. We believe that the principal lateral fault linked an axis of spreading in the Gulf of Mexico with a westward facing subduction zone along which the Kobuk Sea was consumed beneath the Togiok-Koyukuk arc, formerly in western Alaska.