GEOLOGIC AND PETROLOGIC SYNTHESES
At present there are many ambiguities involved in arriving at reasonable models for crustal and upper-mantle compositions. We review several approaches previously employed by geologists and geophysicists to estimate bulk chemical and mineralogic compositions. Recent studies focusing on the petrology of xenolith suites and geologic mapping of high-grade metamorphic massifs, such as the Ivrea zone, support the thesis of heterogeneity in crustal composition. Even though crustal composition varies laterally, there is strong evidence pointing to the importance of metamorphic grade, which generally increases with depth, in controlling crustal petrology. Seismic refraction and reflection methods show the most promise for understanding the lateral variability in petrology. Laboratory studies of the seismic properties of rocks at crustal and upper-mantle pressure and temperature conditions show that seismic data can provide valuable constraints on crustal and upper-mantle composition. Seismic anisotropy is likely an important property of the continental lithosphere at all levels. Field and laboratory experiments carefully designed to investigate this directional dependence of seismic velocities will provide valuable constraints on the fabric and composition of the continental crust and upper mantle. Within the upper crust, physical properties are likely to be strongly influenced by the presence of fractures containing fluids at high pore pressures. A model for the continental crust and upper mantle, emphasizing probable extreme lateral variability, is constructed from information available on exposed deep crustal sections, xenoliths, and laboratory and field seismic studies.