ABSTRACT

The steep inclinations and rugged topography of the continental shelf slopes of the Gulf indicate diastrophic origin but not of the primary arcuate Pacific type common in the Caribbean, which is marked by seismicity and volcanism. Internal evidence that the Gulf is now an active primary tectonic basin seems lacking. Umbgrove suggests that periods of basining (less compression) alternate with periods of mountain making (stronger compression). The viewpoint is here explored that the peripheral diastrophism of the contracting Cenozoic Gulf basin may be of a secondary adjustment type due to down-dropping of fault blocks from the fronts of inadequately supported sedimentary prisms advancing from the lands and from volcanic vents. Where this has occurred, submarine canyons seem to have been, in some cases, broken up by diastrophic disturbances.

Early Tertiary Gulf shores were discordant in the Suess-Bucher-von Engeln sense where they ran across the strike of the Appalachians and other positive rock masses. Coastal plain building now leaves the Gulf largely concordant and in strong contrast with the Caribbean, which is crossed by arcuate tectonic axes. Discordance at present Gulf shorelines is found (1) where the Mexican Neo-Volcanic belt cuts across the extreme southern lobe of the basin (Gulf of Campeche), (2) in a 25° discordance with an outpost axis (Sierra Tamaulipas) of the Sierra Madre Oriental of Mexico and (3) in a few minor, mostly gentle axes in Florida and southern Mexico.