Plate tectonics, the hypothesis of multiple crustal plates floating on a viscous layer called the asthenosphere, provides rationale for viewing the earth's outer shell as a system of shifting continents and growing ocean basins. The idea of diverging plates (or sea-floor spreading) implies that plates converge elsewhere at compatible rates. Estimated convergence rates range up to 4 in. per year, or 140 mi in the 2 m.y. since the beginning of Pleistocene time.

Convergence between oceanic crust and continental crust may result in thermal generation of oil and gas in sediments as young as Pleistocene age because of rapid deep burial associated with subduction. Mountainous source areas for sediment and steep continental slopes favor rapid burial of organic material with turbidites. Rapid subduction of oceanic crust under continental margins may carry sediments to depths which provide requisite thermal environments for generation of oil and gas from organic matter disseminated in the sediment. Continued subduction of oceanic crust under continental slopes may cause reverse faulting such that oil and gas accumulations are uplifted toward the ocean bottom.

Core samples obtained adjacent to the Aleutian Trench in the western Gulf of Alaska apparently show effects of subduction and reverse-fault uplift on a section of Pleistocene sediment. Although this Pleistocene sediment is only a few hundred feet below the ocean bottom, organic matter carbonization suggests previous burial of at least 8,500 ft and late pregeneration stage of organic carbonization. In contrast, noncommercial oil production from uplifted deep-water sediment of early Tertiary age at Katalla, Alaska, suggests formerly significant accumulations have been dissipated by faulting, uplift, and erosion. Late Tertiary rocks beneath outer continental shelves and/or upper continental slopes at convergent margins may be in the

End_Page 1459------------------------------

optimum stage of current petroleum expulsion but still buried deeply enough for entrapment of giant oil accumulations.

Regions for analogous exploration application of this hypothesis, in addition to the western Gulf of Alaska, include continental or island margins adjacent to other deep oceanic trenches such as the Japan, Mindanao, Java, Solomon Sea, Peru-Chile, and Central American trenches, and the southern end of the Puerto Rico Trench northeast of Trinidad.

End_of_Article - Last_Page 1460------------