The origin of West Pacific-type geosynclines by zonal spreading

Abstract

Terrains of West Pacific-type in the modern oceans consist largely of island arcs, protocontinents and marginal seas, together with some continental fragments. These terrains are contemporary West Pacific-type geosynclines (Crook, 1974). The widths of these contemporary geosynclines, when summed along parallels of latitude around the Earth at 5° intervals, are, to a first approximation, an harmonic function of latitude, with minima at 0°, 45° N and S and 60° N and S, and maxima at 15° N and S and 55° N and S. The function is substantially symmetrical about the equator if the Pacific sector is considered alone. Depending upon the areas included in the measurement, the global pattern may also be symmetrical. This function is unlikely to be accidental; it requires an explanation. If it results from some global process, it should be discernable on global maps of past epochs. This would imply that the “plates” comprising these geosynclinal terrains expand and contract in width as they move across parallels of latitude. The model, termed zonal spreading, is tested by examining, as far as the data permit, the evolution of West Pacific-type geosynclines in the Southern Hemisphere during the last 90 m.y. Zonal spreading provides a rationale for the creation and destruction of marginal seas that stands without reference to interactions between the great lithospheric plates which bear the mid-ocean ridges and continents. The mechanism underlying zonal spreading must therefore be distinct from that which underlies sea-floor spreading. If the two mechanisms are not intimately physically interconnected, change in the Earth's radius is possible. Speculation about mechanisms for zonal spreading is warranted. Zonal rotational flow in the asthenosphere is one possibility. Zonal spreading may have important implications for the evolution of the Tasman Geosyncline, particularly in view of the 90° change in orientation of paleolatitudes that it experienced during the mid-Paleozoic. It may also explain the pattern of Paleozoic orogeny in the Tasman Geosyncline.