Abstract

A detailed magnetic survey in the southern Gulf of Mexico shows subdued irregular magnetic anomalies that are similar in wave length to those attributed to sea-floor spreading on present-day oceanic rises. The small amplitude of these anomalies, about 75γ, would be compatible with an oceanic basalt source at a depth of 10 km, and previous seismic refraction studies in this area have shown that layer 2 of the oceanic crust (presumed to be basalt) does indeed lie at approximately that depth.

Palynomorphs in Deep Sea Drilling Project samples of cap-rock material from the Sigsbee Knolls have shown the associated salt to be Jurassic. A crustal model, based on seismic refraction evidence and on new gravity data, suggests that, whereas the salt-dome belt of the southern gulf is underlain by a thick layer with a density of 2.2 g per cm3 (presumed to be halite rock), adjacent deeper areas of the basin seem to lack this low density layer.

In northeastern Mexico, Triassic red beds fill grabens that are correlative with the Newark Group of the Atlantic Coast and suggest that the Gulf of Mexico originated at the time of the initial rifting of the North Atlantic. When the gulf was about half-opened during the Jurassic, oceanic circulation was restricted; and thick deep-basin evaporite deposits, analogous to those found in the Mediterranean Sea by the Deep Sea Drilling Project, were laid down. Further opening established normal salinity and led to the development of salt-free areas of oceanic crust that separated the Sigsbee Escarpment (together with adjacent ridges of offshore Mexico) from the Sigsbee Knolls and salt domes of Cuba.

The subsequent structural evolution of the Gulf of Mexico basin is believed to have been mainly a result of interaction between it and tectonic plates of the Pacific area. After opening of the gulf, subduction began along the Cuban arc, where Atlantic rifting had created a nearly uninterrupted tract of oceanic crust extending from the Pacific between Yucatán and Colombia. Then, folding and gravity sliding associated with Laramide deformation on the west side of the gulf led to salt anticlines that underlie the ridges offshore from Mexico. Except for continued diapirism and subsidence associated with sediment loading, present tectonic activity is confined to the southwestern corner of the gulf, where volcanism and intermediate-focus earthquakes are a distant manifestation of subduction along the Middle America Trench on the Pacific side of Mexico.

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