The concept of upper and lower plate margins formed on opposite sides of a low-angle detachment during the early rift phase was proposed over three decades ago Wernicke and Burchfiel (1983), Lister et al. (1986) and others. This conceptual model remains controversial to the point that the upper and lower plate terminology seldom appears in recent studies on conjugate margins. In this talk I will show that many conjugate margins can be classified into the upper and lower plate types using the basic, crustal elements and geometries outlined by the early asymmetrical rifting proponents that are summarized in the diagram below from Davison (2012). Less extended, upper plates in cross section have narrow and steep crustal profiles while the more extended lower plates produce tapered profiles with lower-relief belts of rifts where the sag phase can localize giant salt basins. Examples of these two different margin profiles was noted by Marton and Buffler in the 1990's for the less extended Yucatan GOM margin (upper plate) and the more extended US GOM with an extensive salt basin (lower plate). Similarly, Davison (1997) noted the alternation of the two margin types along the Brazilian margin.

Within the framework of these concepts of upper and lower plates a comparison from geophysical seismic reflection and refraction data are used to characterize upper and lower plates, symmetrical or neutral conjugates — in a much more systematic approach that was not available to the early proponents of the upper and lower plate concept. These geophysical data can also be used to compile the locations of volcanic margins that form late in the rifting process and are characterized by voluminous, volcanic flows expressed as "seaward-dipping reflectors". The regional map of the locations of proposed upper and lower plates along with volcanic margins is then overlain on a map of present hydrocarbon production to identify the most promising areas for future exploration.