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Present Addresses: (Lowrie) U.S. Naval Oceanographic Office, Code 7221, NSTL Station, Mississippi 39529; (Hey) Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093.

There are several types of geologic and geophysical variations along the western margin of Chile from lat 33° to 36° S. The elevation of the Andean mountain peaks averages about 6 km north of lat 33° S, but 300 km farther south, the average elevation of the Andean peaks has decreased to 3 km. The width of the Andes also decreases abruptly from 425 to 260 km in this region. A similar elevation change is noted f or the Coastal Range mountain peaks whose average elevation decreases abruptly from about 1.8 to 0.8 km between lat 33° and 34° S. The Central Valley, which lies between the Andes and the Coastal Range, decreases about 300 m in elevation near lat 34° S but maintains a constant elevation to the south.

Volcanism is discontinuous. North of lat 27.5° S, the Chilean volcanoes have been active during Quaternary time. Between lat 27.5° and 33.5° S Quaternary volcanism is absent, but reappears at lat 33.5° S to form the southern Chilean volcanic belt.

Other variations include a major gravitational change near lat 34° S—which implies a change in crustal thickness along the center of the Andean Cordillera—and the termination of the main porphyry copper belt of northern Chile near lat 34° S.

The occurrence of these changes and discontinuities between lat 33° and 34° S suggests that they result from some type of interaction between the Nazca and South American plates.

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