Abstract: Segmentation of Island Arcs and Its Application to Petroleum Geology

T. S. M. Ranneft

Island arcs, as the name implies, nearly always are depicted as curved or sinuous structures. This concept appears to be incorrect. Most island arcs, perhaps all, are composed of straight segments that may change direction suddenly. The segments range in length from less than 100 mi (160 km) to several hundred miles or more. They are separated by hinge zones or boundaries apparently composed of transverse fault systems, which may be currently active and which may have at least a Tertiary history of movement. The segments themselves also are cut by many smaller transverse faults. Though the individual segments are straight, they often are arranged in a generally arcuate pattern.

The transverse faults sometimes have been mapped on the surface. At other times they are inferred from geomorphic evidence, such as offsets of graben valleys, knicks in the coast, embayments, straights between islands and alignments of volcanoes. The faults may act as conduits for igneous rock and thus reveal their presence. Some also have been traced by geophysical means.

Examples of segmented island arcs are many. The Andaman and Nicobar Islands of India form typical aligned islands that suddenly change direction. In Indonesia the evidence of segmentation is ubiquitous, even in the Banda arc which almost invariably is shown on geologic maps as smoothly curved. The Philippines are fragmented, with a diagonal transverse-fault trend suggested by the alignment of some of the islands. The Solomon Islands and New Hebrides also are fragmented; some of the islands are positioned like partly opened trapdoors. The Tonga Islands are notably straight; offsets of some of the islands suggest major and minor transverse faulting. Similar conditions can be observed in the Kuriles, Kamchatka, the Aleutians, the Lesser Antilles, Central America, and Greece. Other workers have observed these aspects in several other island "arcs."

The segmentation of island arcs and the transverse-fault system seem to be basic aspects of geology and therefore must have an effect on petroleum accumulation. First, many petroleum provinces are in or near currently active island arcs, such as Indonesia, Taiwan, Japan, or New Zealand. Commonly, oil or gas fields trend parallel or subparallel with the main longitudinal trend; however, occasionally oil or gas fields trend parallel with the transverse trend, suggesting that this trend became dominant locally.

Second, it has become generally accepted that many basins, now parts of various continents, passed through an island-arc stage. Such basins could have had a history of segmentation. Because the transverse faults may have had a long history of movement, sedimentation may have been affected and this, in turn, should have influenced the petroleum geology.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA