More than 8,000 mi of ship tract was surveyed on the continental shelf and slope east of Mexico by Texas A&M University and Lamont Geological Observatory. This survey has shown that structurally the Mexican shelf and slope are composed of a series of anticlines that start from the continental rise and continue under the continental shelf. The general trend of these features is in the direction of the shelf break.

The Mexican continental shelf and slope from 26° to 20° N. lat. can be divided into four distinct physiographic provinces (zones).

The northernmost (zone 1) consists of a series of steplike features with surface and subbottom characteristics similar to those of the Texas and Louisiana continental slope. This zone contains the westernmost extension of the Sigsbee scarp.

Zone 2 is a series of long, linear anticlines with average relief of 1,500 ft and a wavelength of between 5 and 7 mi. This area is 120 mi long and approximately 110 mi wide.

Zone 3 is typified by a series of large east-west-trending scarps with maximum relief of more than 5,500 ft. This zone may be the seaward continuation of the Zacatecas fracture zone.

Zone 4 is very similar in structure and form to zone 2, but has a slight arcuate trend, convex toward the Gulf basin.

The structural features observed on the eastern Mexican shelf and slope are postulated to have originated by the possible mechanism of (1) compressional folding, resulting from gravity sliding along a decollement surface or other tectonic stresses; or (2) vertical movement of shale or salt masses related to static loading.