Similarities between crystalline crust and overlying pre-Jurassic cover in northwestern Sonora, Mexico, and in the Inyo Mountains-Death Valley region, California, are attributed to left-lateral displacement along the Mojave-Sonora megashear, a Late Jurassic transform fault related to the opening of the Gulf of Mexico. Three exposures of mylonitic rocks extending 25 km along the postulated fault occur within Sierra de Los Tanques, Sonora, Mexico. The exposures 13 km southwest of Sonoita reveal a tripartite assemblage of (1) Jurassic volcanic and volcaniclastic strata to the northeast, (2) an axial zone of mylonitic rocks, and (3) masses of granitic rocks and compositionally banded quartzo-feldspathic gneiss of Proterozoic (?) and Late Triassic age to the southwest. Mylonitic foliation in rocks within the axial zone strikes northwesterly, and lineations plunge shallowly. Microstructues record left-lateral shear. Shallow thrusts, folded mylonite, and steeply dipping mylonitic foliation with downdip mineral lineation record transpression. Triassic and Precambrian crystalline rocks are emplaced above shallow thrusts onto ductilly deformed Jurassic rocks northeast of the principal fault. In the central and northwestern exposures, pervasively foliated and lineated rocks are cut by weakly foliated Cretaceous granite that also may contain inclusions of mylonite. The northwesterly strike of steep foliation, left-lateral kinematics, and relationships with Cretaceous intrusive rocks preclude correlation with episodes of Cretaceous contraction and Tertiary extension that are recognized in the southwestern United States. The fault is interpreted to have formed in the Late Jurassic and records a change in tectonic setting from arc magmatism to rifting and basin development.