During the war the Navy accumulated a vast number of soundings in the Pacific Ocean. Many sounding lines crossed areas far from normal shipping routes and in waters seldom traversed by ships. Combining these soundings with those already existing on Pacific charts it was possible to construct far more accurate and detailed bathymetric charts of the area than heretofore. The first of these charts to be printed by the Hydrographic Office U. S. Navy covers the area from Korea to New Guinea including within it the Marianas, the western Carolines, the Philippines, the Ryukyus, southern Japan, the Bonins, and the Volcanoes.

The great elongate, deep trenches form the most striking topographic and structural features on the chart. The deeps are much more continuous than was previously indicated. East of the deeps from Japan to Palau a portion of the North Pacific Basin province is portrayed which differs markedly in topography, structure, and petrology from the island-arc provinces to the west. The so-called “Andesite Line separates the North Pacific Basin province from the western island-arc provinces. The concave side of the curved deeps contains from one to three parallel geanticlinal swells on which volcanic islands are characteristic.

The vulcanism and seismic activity of the area are discussed. The geologic history of the region is outlined insofar as it may be surmised from fragmentary data. In general the eastern and southern belt of arcs and mountains is related to Late Cretaceous-Early Tertiary deformation which has continued spasmodically up to Recent time, whereas a western belt including southern Japan, the Ryukyus, Formosa, and the western Philippines is probably the product of mid-Mesozoic deformation. Both deformations have belts of periodotite intrusions associated with them.

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