The idealized island-arc system includes a deep-sea trench on the ocean side, a chain of islands (at least partly volcanic) landward of the trench, and a wide saltwater basin between the islands and the continental mainland. There may be a large negative gravity anomaly along either the trench or the chain of islands, and there is typically an active seismic zone which dips landward from a line beneath the trench. The Japanese Islands and the Marianna Islands have been cited as good examples of island-arc systems.

The southern half of Mexico exhibits the characteristics of an island-arc system except for the fact that it is composed of no islands: neither large like Japan nor small like the Mariannas. The Mexican trench, the volcanic chain, and the northward-dipping active seismic zone are present; furthermore, the Gulf of Mexico has the same approximate size and characteristics as the middle-sized basins along the eastern edge of Asia.

In general, the trench-volcano-basin transect looks very much like cross sections taken through recognized mature island-arc systems. The gravity anomaly, although present, is small, but this is not unusual along other trenches and island arcs.

The entire system is associated with well-developed north-south tension; that is, the Mexican “island” arc lies along the narrow trailing tensional edge of a roughly triangular continent. The absence of islands is the result of two facts: (1) the Mexican system is mature in the sense that the Japanese system is mature, rather than immature like the system in the Mariannas; and (2) the development near the tip of a roughly triangular continent makes fragmentation into islands unlikely. The Mexican system is approximately the length of the main Japanese island (Honshu) which likewise is not cut by straits, and is smaller than the main Indonesian island.

Knowledge of the other island-arc systems of the world can be applied in a useful way to the Mexican system; knowledge of the petroleum production continentward from the Mexican arc (on both Mexican and U.S. sides of the Gulf of Mexico) can be extrapolated to the middle-sized basins in other parts of the world.

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