All petroleum provinces on the American rim of the Pacific lie on or near present or past intersections of continents and spreading centers. Pacific coast production in the conterminous United States is restricted to the region between the East Pacific Rise in the Gulf of California and the Gorda Rise off northern California; the coastal basins may have started generating petroleum when North America drifted westward over the East Pacific Rise. The petroleum provinces of coastal and offshore Ecuador and Peru are near an east-west spreading center near the Galapagos Islands. Cook Inlet, Alaska, is not now on an oceanic rise, but magnetic anomalies in the Gulf of Alaska suggest that a rise intersected southern Alaska in Tertiary time. In contrast, no commercial oil has been found on the Pacific slope of any American island arc, including the Aleutian, Cascade, Mexican-Central American, and Andean arcs and volcanic chains.

Intersections of rises and continents are favorable regions for petroleum accumulation. Block faulting characteristic of rises tends to produce stagnant basins

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with sapropelic shales; adjacent fault-block mountains shed coarse detritus into the basins so that reservoir rocks are interbedded with petroleum source rocks. Faulting and folding contemporaneous with deposition may result in early-formed, time-persistent structures, High heat flow may cause petroleum generation at relatively shallow depths.

Using this hypothesis as an exploration tool, regions that warrant further prospecting include the head of the Gulf of California, Magdalena Bay in Baja California Sur, and southern Chile.

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