TERTIARY METAMORPHIC CORE COMPLEXES IN SONORA, NORTHWESTERN MEXICO

Several ranges encompassing more than 35,000 km(2) of Sonora, Mexico, contain distinctly lineated and foliated granitic and metamorphic rock s that constitute the lower plates of metamorphic core complexes. Pene trative deformation is characterized by gently dipping mylonitic folia tion across which northeast trending stretching lineation is everywher e developed. Prominent northwest trending fractures, dikes, and normal faults are orthogonal to the lineation. Most kinematic indicators in lower plate mylonitic rocks record top-to-the-southwest sense of shear . Upper plate stratigraphic sequences include Mesozoic supracrustal ro cks, Tertiary volcanic and sedimentary rocks, and allochthonous Precam brian basement. Tilted blocks of upper plate strata generally overlie the mylonites along gently dipping detachment faults. Previously publi shed U-Pb and K-Ar ages from lower plate granitic orthogneisses, upper plate volcanic sequences, and crosscutting dikes constrain the time o f mylonitic deformation and detachment faulting in several of these ar eas to late Oligocene-early Miocene. Partitioning of extensional strai n in Sonora was influenced by pre-Tertiary crustal structure. The belt of core complexes developed across two contrasting blocks of continen tal crust separated by the N60 degrees W striking Mojave-Sonora megash ear. Portions of the southern Papago block (northeast of the megashear ) consisting of Jurassic magmatic are rocks and Upper Jurassic-Cretace ous siliciclastic and carbonate strata resting upon a concealed, tecto nically fragmented Precambrian basement were especially susceptible to crustal attenuation. Some core complexes of the southern Papago block occur within zones trending northwest that may coincide with Late Jur assic lineaments. In the Caborca block (southwest of the megashear), c ore complex-related rocks and structures have not been identified wher e surface exposures of Middle Proterozoic basement and overlying Upper Proterozoic-Paleozoic platform strata are common. However, extensiona l mylonitic fabrics are locally developed along the margins of a Terti ary two-mica granite batholith. Core complexes on both sides of the me gashear appear to be preferentially developed where Tertiary granites have intruded regions of crust with basement disrupted by pre-Tertiary structures. Sonoran core complexes preserve an extensional tectonic h istory comparable with that described from core complexes farther nort h in the United States and Canadian Cordillera. The timing of mid crus tal extension in Sonora (25-18 Ma) is contemporaneous with the timing of core complex development in Arizona, Nevada, and Utah. Extension oc curred later in these areas than in the Pacific Northwest-British Colu mbia region but earlier than in the Mojave Desert-Death Valley region. Middle Tertiary mylonitic fabrics of similar style and orientation ha ve not been recognized farther south in Mexico. The southern terminus of the mid-Tertiary Cordilleran core complex belt appears to be in Son ora.