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Perspectives on the architecture of continental crust from integrated field studies of exposed isobaric sections

By
Michael L. Williams
Michael L. Williams
Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA
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Karl E. Karlstrom
Karl E. Karlstrom
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
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Gregory Dumond
Gregory Dumond
Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA
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Kevin H. Mahan
Kevin H. Mahan
Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
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Published:
January 01, 2009

Depth-dependent variations in the structure and composition of continental crust can be studied via integrated investigations of isobaric terranes. In this contribution, we summarize three isobaric terranes in Archean to Proterozoic crust. In western Canada, 35–45-km-deep lower crust is exposed over an area of more than 20,000 km2. The Upper Granite Gorge of Grand Canyon, Arizona, provides a transect of 20–25-km-deep middle crust. The Proterozoic basement of central Arizona represents an isobaric exposure of 10–15-km-deep middle crust. Isobaric terranes yield a conceptual image of continental crust that can be compared to seismic images, xenolith data, and drill core data to clarify rheology, coupling/decoupling of crustal levels, and the interplay between deformation, metamorphism, and plutonism. General observations include: (1) The crust is heterogeneous at all levels and cannot be accurately modeled as a simple progression from quartz-rich to feldspar-rich lithologies or from felsic to mafic bulk compositions. (2) The crust is segmented into foliation domains that alternate between steeply dipping and shallowly dipping. (3) Magmatism is expressed differently at different depths due to different background temperatures and a general upward distillation from mafic to felsic composition, and may be the most important control on crustal architecture and rheology. The strength of continental crust (and its potential for low-viscosity flow) is not simply a function of temperature, depth, and compositional layering, but is controlled by the size and distribution of rheological domains. The rheological character of a particular layer can vary in space and time at any crustal level.

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GSA Special Papers

Crustal Cross Sections from the Western North American Cordillera and Elsewhere: Implications for Tectonic and Petrologic Processes

Robert B. Miller
Robert B. Miller
San José State University, San José, California, USA
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Arthur W. Snoke
Arthur W. Snoke
University of Wyoming, Laramie, Wyoming, USA
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Geological Society of America
Volume
456
ISBN print:
9780813724560
Publication date:
January 01, 2009

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