This review discusses results from recent seismic experiments at continental margins conducted by U.S. investigators as well as those by non-U.S. investigators necessary to place the results of U.S. workers into context. Continental margins may be characterized as either rifted, transform, or convergent. Rifted (or passive) margins form where continents are torn apart during continental drift. Transform margins accommodate transform plate motions along strike-slip faults such as the San Andreas fault. At convergent margins oceanic crust is being thrust beneath (or subducted under) the continental crust. All three types of margins are found in North America.
This review focuses on deep crustal studies that have led to dramatic improvements in our understanding of the structure and evolution of continental margins, and results stemming from the application of new technologies. These technical advances include (1) the deployment of large numbers of matched seismic recorders in both passive and active seismic experiments [ Levander and Lafond, 1993; Nabelek et al., 1993], (2) the use of large-volume, multi-element marine air gun arrays and modern seismic streamers in two-dimensional and three-dimensional studies [ Rosendahl et al., 1991; 1993; Shipley et al., 1992; 1994; Stoffa et al., 1992], (3) the inversion of picked travel times for velocity structures in two- and three-dimensions providing quantitative error estimates [ Lafond, 1991; Hole et al., 1992; Zelt and Smith, 1992; Lutter et al., 1993], and (4) the imaging of subsurface crustal structures using migration of wide-angle seismic data eliminating the need for subjective picking or interpretation of data [ Holbrook et al., 1992; Levander and Lafond, 1993]. Wide-angle seismic data are those data for which the source and receiver offsets are large, resulting in raypaths which are refracted or reflected at angles at or near the critical angle of incidence. Air gun sources of seismic waves are particularly ideal from the standpoint of imaging the crust because they are highly repeatable and can be repeated over short distances (usually about 50 meters). This repeatability and high spatial density allows complexities in the signals to be separated from noise artifacts. These technical advances have greatly enhanced our ability to map the structure of continental margins and greatly increased the objectivity in our interpretation of seismic data.