The Mojave-Sonora megashear—Field and analytical studies leading to the conception and evolution of the hypothesis

Abstract

The megashear hypothesis is based upon reconnaissance geologic and geochronologic studies conducted principally from 1968 until 1974 in northwestern Sonora, Mexico. Our research incorporated U-Pb isotopic analyses of more than 70 zircon populations separated from 33 Precambrian rock samples with field relations and maps based upon structural and stratigraphic measurements. The results delineate a region known as the Caborca block and further reveal that the block is a principal element of an unexpected, discordant pattern of Proterozoic basement provinces. The Mojave-Sonora megashear was conceived in an effort to explain: (1) the unexpected pattern of two Proterozoic crystalline provinces with distinct chronologic histories of crust formation (1.8–1.7 Ga, Caborca block versus 1.7–1.6 Ga, Pinal Province); (2) the distribution of contrasting cover rocks overlying these basement blocks, (3) the abrupt northeastern limit of the Caborca block (terrane) against which volcanic and plutonic rocks of mid-Jurassic (mainly 180–160 Ma) age are juxtaposed, and (4) the distribution of Jurassic magmatic units that intervene between the provinces of Proterozoic crust. The similarities that exist between crystalline crust and overlying pre-Jurassic cover in northwestern Sonora, Mexico, and units in the Inyo Mountains–Death Valley region are attributed to the offset of correlative units along a Late Jurassic left-lateral strike-slip fault postulated to extend from the Gulf of Mexico to California and beyond. This large fault or megashear is a principal structure that accommodated 800–1000 km of left-lateral displacement among a set of transforms related to the opening of the Gulf of Mexico. The fault is compatible with Late Jurassic plate motion. The inferred trace of the Mojave-Sonora megashear is obscured by contractional and extensional deformation and extensive plutonism. These processes, concentrated along the fault, commonly obfuscate and displace fault zone rocks along the inferred trace as well as the rocks adjacent to it. However, the fault zone is exposed in Sierra de Los Tanques near the international boundary between Mexico and the United States, where mylonitic rocks that comprise three aligned, discontinuous, segments crop out 1 for ∼25 km. The zone of mylonitic rocks, which crosses Route 8, 13 km SW of Sonoita, is locally almost 5 km wide and separates Triassic granitoids and Precambrian gneiss from Jurassic volcanic and clastic rocks. The limited exposure of the fault zone is a principal concern of those who object to the Mojave-Sonora megashear hypothesis. Studies of paleomagnetism, structure, stratigraphy, crustal geochemistry, and detrital zircons do not refute the megashear concept; commonly they reinforce existing evidence in support of the hypothesis.