The Advantage of Length in Deep Crustal Reflection Prospecting

Since its first survey in 1975, the Consortium for Continental Reflection Profiling (COCORP) has applied the continuous seismic profiling technique developed by the petroleum industry to a very wide range of fundamental geologic problems in the conterminous United States. Thrust belts at the continental margin and in the continental interior have been studied (Appalachian-Ouachita trend, Laramide province); ancient and modern rifts have been examined (Mid-Continent Geophysical anomaly, Anadarko aulacogen, Rio Grande rift, Basin and Range province); cratonic areas including shield, platform and basins have been explored (Adirondack mountains, Minnesota, Michigan basin); neotectonic earthquake and volcanic zones have been studied (San Andreas fault, Charleston, Rio Grande rift). Many of these results are discussed by Brewer and Oliver (1980) or by Oliver, Cook and Brown (1983). In general, the trend in survey design has been towards longer and longer lines. Some early surveys spanned only a few tens of kilometres. Most recent surveys span 150 km or more and are thought of as segments of still longer profiles. The ultimate goal of the COCORP project is to obtain a coherent picture of the whole of the North American basement. To achieve this goal, the long-term plans for COCORP field-work propose to link up the investigations of regional geologic problems to create continental traverses. COCORP's earliest profiles in Hardeman basin, Texas (Oliver et al. 1976), unlike earlier studies of basement reflectors in the U.S. (Junger 1951, Widess and Taylor 1959, Dix 1965), exploited the Vibroseis (TM Conoco) technique. Such work has demonstrated the ability of continuous profiling technology to study basement reflections from throughout most or all of the continental crust. (Schilt et al. 1979 review the data acquisition and processing methods used by COCORP.) Although the Hardeman basin study probed deeper into, the crust than earlier experiments and found many reflectors and diffractors at depth, these first COCORP profiles shared with earlier work the disadvantages of very short lines. Little perspective on the structures was obtained. Increase in line length has allowed COCORP to treat geologic problems of regional importance and especially to trace deep features to surface outcrop or to drillable subcrop (Oliver 1982a). Cross-lines, sometimes as ties between sub-parallel lines of the main traverse, are also useful. Closely spaced parallel lines or other forms of areal coverage would also be useful, of course, but are not normally used in modern studies of basement geology, primarily because of cost but also because of difficulty of access to off-road sites. Now that a distance equivalent to one traverse across the U.S. has been surveyed, tentative plans to link up separate COCORP sites into continental traverses seem achievable. With longer lines our ability to characterise lower crustal terranes should advance more rapidly. Surface-constrained problems are currently more readily solved than those of the very deep crust, but surface constraints can sometimes be extended to provide information on great depths. On the basis of COCORP results and associated geologic and geophysical data, it has been supposed that beneath the southern Appalachians, in the present-day lower crust, there lies former continental crust, former oceanic crust and crust of the former continental margin that is transitional between the other two types (Cook and Oliver 1981, Cook et al. 1981). Although the transition between the different lower crustal types in this area is not apparent from the exposed surface geology, indirect evidence on the changing lower crust was obtained from COCORP profiling. Major results of COCORP surveying-e.g. the tracing of faults down to the middle or lower crust in the Wind Rivers and Southern Appalachians, and the identification of a magma body in the Rio Grande rift have been reported elsewhere. This paper considers a rather different issue, the advantages of running very long profiles, by showing how our understanding of the U. S. mid-continent has developed with successive phases of reflection profiling. In this and other areas, still longer profiles may provide additional scientific insights in the near future.