Geophysical and Geological Observations in the Pacific Area and Tectonic Hypotheses

Abstract

Since the meeting of the Pacific Science Congress in 1939, many new data concerning geophysical phenomena in the Pacific area have been published. The correlation between ocean deeps, negative gravity anomalies, and shallow earthquakes; between somewhat deeper earthquakes and extinct volcanoes; the sequence of these groups and of the very deep shocks in circum-Pacific arcs, and their relationship to the andesite or Marshall line have been studied in more detail (1). In addition, many attempts have been made-e.g., by Vening Meinesz, Escher, Griggs, DuToit, Umbgrove, Verhoogen, Sonder, Bucher, Wegener, to sketch processes which may produce the observed phenomena. While these studies resulted in a great variety of hypotheses, there seems to be fair agreement on a few conclusions. The most important result is the growing accumulation of evidence that the Pacific Basin shows unique features which are not duplicated in any other oceanic or continental area of the earth. This is true for the geophysical as well as the geological and petrographic evidence. There is no feature on the surface of the earth which compares in dimensions and importance with the Marshall line within which the younger eruptive rocks are basaltic rather than andesitic. This discontinuity in the material of the crustal layers is called here the "boundary of the Pacific Basin."