The heterogeneity of the continental crust: Results from deep crustal seismic reflection profiling using the Vibroseis technique

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Abstract
Deep reflection surveys carried out by the Consortium for Continental Reflection Profiling (COCORP) are revealing major structures within the continental crust in a variety of tectonic settings across the U.S. In many cases, these structures bear directly upon such geologic problems as the state of stress during large scale crustal deformations, the migration of magma, and the nature of the crust‐mantle transition zone. COCORP, formed during the U.S. Geodynamics Project and funded by the National Science Foundation, has surveyed eight different localities along lines totalling over 800 km in length. The VIBROSEIS technique has been used in all cases and has proven to be a flexible and effective tool for deep crustal exploration. Among the more significant observations made thus far are: (1) the details of the extensional structure of the Rio Grande Rift; (2) evidence for at least 21 km horizontal and 13 km vertical displacement along the Wind River Thrust, which dips uniformly at about 30 degrees and extends to a depth of at least 25 km; (3) evidence for at least 225 km displacement along a major subhorizontal thrust system beneath the southern Appalachians, in which the Brevard Zone appears to be rooted; and (4) the apparently discontinuous nature of the Moho or crust‐mantle transition zone, which has been observed to some degree at most of the sites studied. Taken together, the results of all surveys analyzed thus far demonstrate a much larger degree of heterogeneity in crustal structure than is indicated by more conventional geophysical techniques for exploration of the basement.