LATERAL HETEROGENEITY, DEFORMATION PARTITIONING, AND THE EVOLUTION OF CONTINENTAL CRUST FROM INTEGRATED FIELD STUDIES OF EXPOSED ISOBARIC SECTIONS

Depth-dependent variations in the structure and composition of continental crust can be studied via integrated field-based investigations of isobaric sections, exhumed from specific crustal levels. We summarize three isobaric terranes in Archean to Proterozoic crust of North America. In western Canada, 35-45 km deep lower crust is exposed over a lateral area of more than 20,000 km2. The Granite Gorges of Grand Canyon, Arizona, provide a 100-km-long transect of 20-25 km-deep middle crust. The Yavapai and Mazatzal provinces in central Arizona represent a >20,000 km2 isobaric exposure of 10-15 km-deep middle crust. Isobaric sections 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. This comparison leads to several general observations. 1) The crust is heterogeneous at all levels and can not be accurately modeled as a simple vertical progression from quartz-rich to feldspar-rich lithologies. 2) The crust is segmented into foliation domains that alternate between steeply dipping and shallowly dipping. 3) Magmatism is expressed differently at different depths and may be the most important control on architecture, rheology, and heterogeneity. Hence the strength of continental crust (and potential for lateral flow) is not simply a function of temperature, depth, and compositional layering, but instead is controlled by the size and spatial distribution of cold and/or strong domains relative to hot and/or weak domains. The rheological character, especially the scale and geometry of strain partitioning, can vary in space and time at any crustal level. However, isobaric sections provide insight into processes involved with the development, stabilization, and reactivation of continental crust, and as such, provide more robust boundary conditions for tectonic models and seismic interpretations.