The studies reviewed here represent significant contributions to our understanding of the evolution and variability of continental margins. They have provided evidence for the thickening of continental crust via the processes of tectonic and magmatic underplating and/or intrusion at convergent and divergent margins, and suggested that amagmatic continental extension may result in continental loss. Nonetheless, a number of important issues remain unresolved: (1) What is the degree of symmetry or asymmetry of rifted passive margins? Knowledge of the degree of asymmetry is needed to more fully understand the rifting process, and it can only be obtained by additional study of conjugate margins. (2) What fraction of all rifted margins have truly experienced no magmatism during continental rifting? The studies reviewed here indicate that this question can only be addressed by deep crustal refraction/reflection profiles since even margins having extensive magmatic underplated or intruded lower crusts may lack volcanic evidence for magmatism. (3) What is the nature of the lower crust at transform margins? Does it represent tectonically under thrust slabs of oceanic crust or magmatically underplated/intruded crust resulting from migration of triple junctions and slab gaps? What methodology is required to distinguish between magmatically and tectonically underplated crust? (4) What is the geometric and kinematic relationships between strike-slip and thrust faults at transpressive transform margins? (5) What is the relative role of tectonic and magmatic underplating/intrusion at convergent margins? (6) Can we recognize asperities on the subduction megathrust and determine where subduction zones are segmented?
It is worth noting that at least two of the studies reviewed here represent cooperative projects between academic and commercial partners. The work by Rosendahl et al. [1991, 1993] in the Gulf of Guinea was funded entirely by commercial interests and the images of the sedimentary section were presumably of interest for hydrocarbon exploration. The Morro Bay, California, experiment reported by several investigators was undertaken as an evaluation of seismic hazard in the area of the Diablo Canyon nuclear power plant, owned and operated by the Pacific Gas & Electric Company. This California experiment was the first large-scale survey on the west coast by U.S. researchers, and its success has spawned several subsequent west coast surveys. In addition, many of the surveys reviewed here used commercial seismic vessels, rather than vessels of the U.S. research fleet. Using commercial vessels for these studies reflects the application of technologies developed by the hydrocarbon industry to academic problems; marine seismic reflection profiling remains an important if not the primary tool of the industry for locating and mapping geological structures having economic potential.
Finally, I note that the last four years have witnessed two significant technological advances of great importance for U.S. scientists. The first is the availability, since late 1990, of a modern multichannel seismic reflection vessel, the Research Vessel Maurice Ewing, currently operating with a 4-km, 160-channel streamer, and about 137.7 liter (8400 cubic inches) of air gun capacity, both necessities for offshore deep-crustal exploration. The completed quadrennium is also the period in which large numbers of 3-component Incorporated Research Institutions for Seismology/PASSCAL Refteks became available, allowing, for the first time, continuous digital recording of offshore marine air guns by large land arrays. Both these technical advances and instrumentation programs have and will continue to drive continental margin science.
Acknowledgments. I thank my colleagues who supplied reprints and references to their work. I thank the National Earthquake Hazards Reduction and the Deep Continental Studies Programs of the United States Geological Survey for support. S. Holbrook, J. McCarthy, W. Mooney, U. ten Brink and A. Tréhu and two anonymous reviewers kindly reviewed earlier drafts of this manuscript.