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 rocks that constitute the lower plates of metamorphic core complexes. Penetrative deformation is characterized by gently dipping mylonitic foliation across which northeast trending stretching lineation is everywhere 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 rocks, Tertiary volcanic and sedimentary rocks, and allochthonous Precambrian basement. Tilted blocks of upper plate strata generally overlie the mylonites along gently dipping detachment faults. Previously published U-Pb and K-Ar ages from lower plate granitic orthogneisses, upper plate volcanic sequences, and crosscutting dikes constrain the time of mylonitic deformation and detachment faulting in several of these areas to late Oligocene-early Miocene. Partitioning of extensional strain in Sonora was influenced by pre-Tertiary crustal structure. The belt of core complexes developed across two contrasting blocks of continental crust separated by the N60 degrees W striking Mojave-Sonora megashear. Portions of the southern Papago block (northeast of the megashear) consisting of Jurassic magmatic are rocks and Upper Jurassic-Cretaceous siliciclastic and carbonate strata resting upon a concealed, tectonically 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 Jurassic lineaments. In the Caborca block (southwest of the megashear), core complex-related rocks and structures have not been identified where surface exposures of Middle Proterozoic basement and overlying Upper Proterozoic-Paleozoic platform strata are common. However, extensional mylonitic fabrics are locally developed along the margins of a Tertiary two-mica granite batholith. Core complexes on both sides of the megashear 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 history comparable with that described from core complexes farther north in the United States and Canadian Cordillera. The timing of mid crustal extension in Sonora (25-18 Ma) is contemporaneous with the timing of core complex development in Arizona, Nevada, and Utah. Extension occurred later in these areas than in the Pacific Northwest-British Columbia region but earlier than in the Mojave Desert-Death Valley region. Middle Tertiary mylonitic fabrics of similar style and orientation have not been recognized farther south in Mexico. The southern terminus of the mid-Tertiary Cordilleran core complex belt appears to be in Sonora.
© 1994 by the American Geophysical Union. Received April 15, 1993; revised November 16, 1993; accepted November 26, 1993. The distinct character of the Tertiary metamorphic rocks described above was initially recognized by Leon T. Silver, who during a field trip in 1970 noted the similarity in structural style between gneiss in northern Sonora and some of the forerange rocks of the Santa Catalina Mountains near Tucson. The experience and perspicuity of L. T. Silver and the insight into structural style of George Davis were of cmcial importance to T. H. A. and J. A. N. during this research. Anderson benefited from appropriate intellectual bludgeoning conducted by Dick Tosdal and Gordon Haxel during many stimulating, enjoyable, and fruitful exchanges. Discussions with Peter Coney and Doug Shakel were significant. Guillermo Salas, formerly of the Universidad de Sonora, and Jesus Najera and Jaime Roldan of the Instituto de Geologia provided logistical support and helpful advice. Franco Corona assisted during some of the field work. Much research was funded by a National Science Foundation grant EAR 76-84167 awarded to the University of Arizona as part of a cooperative project with George Davis. Gordon Haxel recognized and mapped the detachment fault at Sierra Mezquital during a cooperative field project sponsored by the United States Geological Survey and the Cosejo de Recursos Naturales de Mexico. Anderson also benefited from this program in that considerable structural information was obtained during his part of the mapping. Thesis mapping by J. A. Nourse in the Magdalena-Maderdao main and nearby areas was funded in part by two Sigma Xi grants and an NSF grant EAR 8519297 awarded to L. T. Silver. Nourse also gained insight into the geology of Sonora from discussions and field trips with geologists of the Institutod e Geologia. Use of the library and air-conditioned facilities at the Instituto is gratefully acknowledged. Reviews by Jaime Roldan, Steve Reynolds, and an anonomous person substantially improved the original manuscript.
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