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

Anderson, Thomas H. and Silver, Leon T. (2005) The Mojave-Sonora megashear — Field and analytical studies leading to the conception and evolution of the hypothesis. In: The Mojave-Sonora Megashear Hypothesis: Development, Assessment, and Alternatives. Special papers (Geological Society of America). No.393. Geological Society of America , Boulder, CO, pp. 1-50. ISBN 9780813723938.

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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.

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Additional Information:© 2005 Geological Society of America. Accepted 24 March 2005. We benefited from cooperation offered generously by Guillermo Salas, Diego Cordoba, Jose Guererro, and Fernando Ortega-Gutierrez, each of whom served as Director of the Instituto de Geología of Universidad Nacional Autónoma de México. In Sonora, we had constructive interactions with former chiefs of the northwest regional office of the Instituto, including Jesus Najera, Jaime Roldán-Quintana, César Jacques Ayala, and Carlos González-León. Claude Rangin, who conducted extensive mapping principally in northeastern Sonora, generously shared knowledge of stratigraphy and structure gained from study of fossiliferous strata. Anderson was aided in the field by Jaime Alvarez, Arturo Cisneros, Clay Conway, Tom Early, Ken Ludwig, Jay Murray, Robert Powell, David Wenner, and especially Jose Luis Rodríguez-Castañeda. Patti Campbell, Joe Chepega, Karen Cohen, Chris Connors, Franco Corona, John Dembosky, Mary Beth McKee, Jose Luis Rodríguez-Castañeda, and William Stephens, each of whom pursued graduate degrees under Anderson's direction, undertook daunting mapping projects that contributed greatly to our understanding of geological relationships in Sonora. Gordon Haxel and Dick Tosdal aided Anderson in numerous ways, providing logistical support as well as a backboard for ideas. After a field trip to Sonora in 1979, Jim McKee and Norris Jones initiated cooperative work with Anderson in eastern Mexico. For two decades Anderson benefited from knowledge, logistical support, and patient shepherding provided by these two long-suffering gentlemen. Anderson, who began the project with no first-hand knowledge of isotope geochronology, owes a great debt of gratitude to Silver's laboratory staff at Caltech, where the U-Pb isotopic analyses of zircon were performed. The instruction and patient mentoring provided by Jaime Alvarez, chief rock processor, and Gerri Baenteli Silver, lab technician, were essential to any success attributed to Anderson's role in this project. Ken Ludwig and Ed Lidiak were instrumental in the interpretation and presentation of the U-Pb isotopic results. Carlos Aiken, Peter Coney, John Marzolf, Bill Muehlberger, Grover Murray, Jon Nourse, Pete Palmer, Norm Silberling, and Jack Stewart contributed good thinking and good data that they generously shared with Anderson. S.T. Kitinyrere, Bill Muehlberger, Cal Stevens, and Jack Stewart read early versions of the manuscript and offered helpful comments and constructive criticisms. Scott Davidson prepared the illustrations with skill and patient forbearance; Paul Coyle collated the data tables. W.R. Van Schmus and Wayne Premo provided useful reviews of the final draft. Special thanks to Jon Nourse who reviewed the manuscript multiple times with great care and substantially improved it.
Subject Keywords:megashear, Sonora, Proterozoic, Jurassic
Series Name:Special papers (Geological Society of America)
Issue or Number:393
Record Number:CaltechAUTHORS:20150204-135028772
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Official Citation:Thomas H. Anderson and Leon T. Silver The Mojave-Sonora megashear—Field and analytical studies leading to the conception and evolution of the hypothesis Geological Society of America Special Papers, 2005, 393, p. 1-50, doi:10.1130/0-8137-2393-0.1
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:54379
Deposited By: Tony Diaz
Deposited On:04 Feb 2015 22:22
Last Modified:10 Nov 2021 20:32

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