The movements of island arcs

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Abstract
Summary: Through geological time successive orogenic belts have shifted outward from central nuclei. Island arcs of the geosynclinal stage have often moved large distances in a horizontal direction. In the active zones of today movements continue, especially in association with the arcuate structure of the Pacific region, where the highest seismicity is reached. There has been difference of opinion as to whether faulting or folding is the dominant structural feature in island arcs. Both are very important. The age of the faults may be considerable. Periods of thrusting have been genetically connected with tear-faulting and may have been followed by normal faulting. Relative horizontal displacements, which were not simultaneous or in the same direction, are supposed to have changed the position of areas of higher stability. Older strikes are often cut obliquely by the present coast-lines. There is continuity of structural control but the locus of tectonic activity has shifted. Definite conclusions as to the possible extension and maximum depth of the Tertiary seas cannot be derived from geological evidence. The recent and earlier phases of deformation show different stages of growth. A number of non-homologous types can be distinguished. The future development of island arcs will be a complex process. Some arcs front toward the oceans and others toward continental areas, and the Pacific arcs do not all front with their convex sides toward the Pacific basin. Growth-stages of the present island arcs are discussed.

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