SPECULATIONS ON MOUNTAIN BUILDING AND THE LOST PACIFICA CONTINENT

抄録

Extensive geological and geophysical evidence suggests that numerous fragments of continents, miniplates, and so called "island arcs" have been incorporated into the Circum Pacific continents. The old rocks exposed on these bodies bear strong evidence for continental origins. This leads to the speculation that a large continental mass existed once in what is now the Pacific Ocean. This mass-which we call the Pacifica continent-could have been part of the Pangea Super Continent, adjacent to Australia and Antarctica. When this continent broke into fragments, they drifted toward continental collision in South America, North America, Alaska, Kamchatka, Japan and East Asia. Submerged platforms in the Pacific Ocean, such as the Ontong Java area, the Shatsky rise, and the Manihiki plateau, may also be remnants of Pacifica. The thick crusts of these plateaus, with velocities typical of continents, are thus predicted to be continental crusts.
Although the details of the breakup and collision of Pacifica cannot be resolved very well at present, the postulated existence of this continent supports a large generalization: We suggest that all spreading centers on earth may originate underneath continental masses. Without Pacifica of course, the present day east Pacific rise is without associated continents. If continents account for all spreading, it may be because the continental crust acts as thermal blanket, warming the lower lithosphere and upper asthenosphere.
Adding to this the further hypothesis and that subducted ridges are responsible for back arc rifting and spreading, we obtain the typical trench-continent-ridge sequences, containing volcanism, uplifted blocks, metamorphism, and rifting. Multiple collisions involving several continental slivers and ridges may result in several consecutive sequences juxtaposed on one another. Such complexities with geological record are common in belts such as Western North America.
On the basis of the similarities of (a) the geophysical aspects-seismicity and crustal thickness, (b) morphology, and (c) geological complexities, we propose in this paper that the circum Pacific mountain belts may be at least in part the result of past continental collisions, quite similar to those associated with the Alpine belts.
The notion of Pacifica and its breakup may provide an explanation for the similarities of flora, fauna, and rock sequences in widely separated locations in the mountain belts across the Pacific, and may tie in divergent paleomagnetic data.