STRUCTURE OF ISLAND ARCS BOUNDING THE AUSTRALIAN CONTINENTAL PLATE

Abstract

The New Global Tectonic theory is proving exceedingly valuable to understanding the geological history of large parts of the world's surface. However, in island arc areas, such as those bounding the Australian Continental Plate, the theory is meeting with certain difficulties. These might be due to (a) the theory itself not being valid here, or (b) it needs modification.

Certain spectacular aspects of the geology of Timor, New Guinea, New Caledonia and Northland New Zealand suggest that a good case could be made for modification. Large areas of each of these islands are covered by thick chaotic masses of layered ultrabasic igneous rock, radiolarian chert, shale and other sediments of a pelagic nature normally associated with oceanic regimes. Moreover, this sequence of rocks appears to have been tectonically emplaced from an oceanward direction on to sediments of a continental shelf environment. Emplacement seems to have occurred at approximately the same time and to have involved rocks of approximately the same age in all areas.

A logical way to account for the observations above in terms of the New Global Tectonic theory is as follows:

In Late Cretaceous to Early Tertiary time the northern and eastern edge of the Australian continental plate was dragged down and began to underthrust the oceanic mantle along a north to north-east dipping subduction zone (the Tethian subduction zone, which is still active along the western edge of the Indonesian Arc).

From Early to Middle Tertiary time, light continental crust could no longer be subducted under heavier mantle and subduction ceased. Isostatic re-adjustment occurred, raising the northern edge of the Australian plate which still carried large remnants of oceanic crust that had been emplaced above it.

At about the same time, and continuing to the present, a new pattern of south to east dipping subduction formed. Edges of the Australian plate were torn away and migrated eastward, leaving areas of new oceanic crust behind.

If the sequence of events above is essentially a true picture, it leads not only to some interesting modifications of the New Global Tectonic theory, but also to certain implications as to mineral and petroleum prospects within island arc areas.