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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.

Summary It is very important to have good subsurface data in order to understand the nature and behaviour of thrust faults. Deep crustal seismic reflection profiling is the best technique currently available to make detailed subsurface studies of such important problems as the attitude and extent at depth of major faults, and hence deduce the mode of deformation and tectonic forces producing them. The Consortium for Continental Reflection Profiling (COCORP) is collecting large quantities of seismic reflection data from the deep crust and upper mantle in many parts of the U.S.A. Areas of major thrusting which have been profiled so far by COCORP include the Wind River Mountains in Wyoming and the Southern Appalachians of Georgia and Tennessee. Seismic profiles have been very successful in delineating a major thrust fault of moderate dip underlying the Wind River Mountains, thus demonstrating that compressional tectonics were dominant in their formation. In Georgia and Tennessee the seismic profiles demonstrate that the major tectonic feature of the Southern Appalachians is a relatively thin overthrust sheet, which may have moved at least 260 km. Deep crustal seismic reflection profiling thus appears to be an indispensable tool for the study of areas in which thrusting and nappe formation have occurred.

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A mineral assemblage comprised of chalcedony, drusy quartz, and very fine grained iron hydroxide (limonite), with or without blocky calcite (QFC), coats joint surfaces, fills fractures, and commonly occurs as pressure shadow encrustations on brittle fault surfaces in central New Hampshire but only occurs sporadically in other areas of New England. Locally, QFC includes silicified breccias of highly altered host and euhedral quartz crystal fragments suspended in a matrix of chalcedony and limonite. One hundred and four brittle faults from central New Hampshire, mineralized with QFC, can be separated into two families. Set N is composed of primarily north–northeast to northeast trending, steeply dipping normal faults (n=61), and set S is composed of north‐northwest to north‐northeast trending, steeply dipping, primarily right‐lateral strike‐ to normal oblique‐slip faults (n=43). These two sets of faults are thought to record two paleostress conditions. The extension direction for both tensors plunges gently east‐southeast, but the compression direction is near vertical for set N and trends roughly northeast, plunging 30° for set S. Fault‐slip inversion suggests that set N faults are best fit with σ1 most deviatoric (σ2 ≈ σ3). Set S is best fit with σ3 most deviatoric (σ1 ≈ σ2) and low values for fault plane coefficient of friction and/or high fluid pressure. QFC occurs in the 158 Ma Belknap Mountain Complex of the White Mountain Magma Series and is thought to have developed at relatively shallow crustal levels based on the assemblage, nature of fluid inclusions, and association of QFC as brittle fault and fracture filling mineralization. Occurrence of QFC in the 158 Ma Belknap Mountain complex, paleostress implications of QFC‐mineralized brittle faults, and range and character of fault fabrics in the Belknap units suggests that QFC developed around the time of Belknap intrusion and cooling and that Late Jurassic to Early Cretaceous is the approximate time at which a regional change in paleostress conditions occurred in New England. This age and nature of the paleostress change is in accord with a change postulated by de Boer et al. (1988).