Abstract

The Indonesian region has developed as the result of the Cretaceous and Cenozoic convergence of three great lithosphere plate systems. Plates from the Indian Ocean region have been moving northward, and from the Pacific region westward, relative to Southeast Asia. Plate boundaries have changed complexly with time as subduction zones, spreading centers, and strike-slip faults have broken through, evolved for periods ranging from one to scores of millions of years, and gone extinct. Small plates, subject to much internal deformation, have formed between huge plates which have behaved in generally more rigid fashion. No boundaries have been fixed in shape or position: all have changed length and curvature, and have migrated relative to one another.

Plate convergence is accommodated by the subduction of one lithosphere plate directly or obliquely beneath another. Reflection profiles, supplemented by refraction and gravity, define much of the geometry and processes at these subduction zones. Where an oceanic plate bearing little sediment is subducted beneath an oceanic island arc, the trench marking the trace of the subduction zone is steep-sided. The subducting plate inflects sharply downward at the outer side of the trench, and planes downward beneath the arc with steep to moderate dip, as marked by its seismic Benioff zone.

Where there is, on the other hand, a large supply of sediment, that low-density material is scraped off as a sort of standing wave above the actual convergent plate contact. The trench sides slope gently −7 degrees, for example. The inner side of the trench is the top of a wedge, thickening toward the arc, of melange and contorted sediments with imbricated dips which typically dip moderately toward the arc and are strongly disharmonic to the gentle dip of the subducting oceanic plate beneath. The outer edge of the trench is the hinge at the gentle flexture, which marks the oceanward limit of the depression of the subducting plate by the weight of the melange wedge; the more fundamental and sharper tectonic hinge lies beneath the wedge. The topographic top of the melange wedge stands as a submarine outer-arc ridge, marked by islands where the wedge is particularly thick.

Behind the ridge is an outer-arc basin, within which sediments may be 6 km or more thick. These strata can include pre-subduction continental-shelf formations, bowed into a syncline, as well as units deposited synchronously with subduction. The strata lap onto the landward basement of the basin, but become increasingly deformed downward and outward toward the melange wedge, with which they merge tectonically and to which they contribute material. The Andaman, Nicobar, and Mentawi Islands, and Timor and the other islands of the arcuate ridge around to Seram and Buru, are exposed to segments of the outer-arc ridge of the Java Trench system. The Vening-Meinesz gravity low along an outer-arc ridge is an expression of the thickness of the low-density wedge of melange, and does not indicate gross isostatic disequilibrium.

The disharmonically imbricated character of melange wedges, as seen in reflection profiles, the geology of the islands atop them, and the geology of older similar complexes (such as the Franciscan of California) around the world, appear to require that the wedges undergo great internal deformation. Growth is achieved only in part by the scraping off of packets of oceanic sediments at the toe of the wedge. The thickness of the wedge and the slope of its top, and hence the dip of the underlying oceanic plate, represent a dynamic equilibrium between the thickening of the wedge as it is pushed backward by the undersliding plate, and the thinning of it as its weak materials slide gravitationally forward. The internal motion in the wedge can be likened to that of a continental ice sheet: imbrication results because the top slopes downward, even though the base and the planes of imbrication slope upward. Materials deposited on top of the wedge become progressively imbricated into it. In the tectonic boundary zone between melange wedge and outer-arc basin strata, older strata are more severely imbricated than are younger ones. Within the wedge, materials from the base, including slabs of the oceanic plate sliding beneath it, are carried upward along imbrication planes and mixed with surficial sediments: this mechanism may account for the presence of such high-pressure metamorphic rocks as eclogite and glaucophane schist as exotic clasts in melanges.

The Indonesian region displays a number of instances of collision between two island arcs, or between an island arc and a continent, after intervening oceanic lithosphere has vanished beneath them. Two of the collisions are now underway (see Figure 1). One collision is taking place near the north coast of New Guinea, where two Benioff zones of mantle earthquakes intersect. One zone dips gently south beneath the continent, and the other steeply north beneath the Schouten-New Britain island arc. Apparently the south–dipping subduction system has recently overridden the north-dipping one, and the island arc is being added to the edge of the continent. Similarly, in the Molucca Sea, a Benioff zone dipping moderately west beneath the Sangihe arc and the Celebes Sea intersects near the surface another zone dipping gently east beneath Halmahera. The east-dipping subduction system appears on reflection profiles to have overridden the West-dipping one. The broad medial submarine rise of the Molucca Sea is the top of a wedge, at least 10 km thick, of highly deformed low-density sediments conveyor-belted from north of New Guinea and caught between the converging arcs. The opposed Sangihe and Halmahera arcs continue north into Mindanao, where the collision is complete and the suture closed; a new west-dipping subduction system, that of the Mindanao Trench, has broken through on the east side of the aggregate. The Philippines represent the accretion of a number of arcs; another among them is the northwest-facing Dent-Sulu-Zamboanga arc, active in Miocene and Pliocene time.

The collision of the Banda Island arc with the continent of Australia and New Guinea is now underway. The Banda arc appears to be expanding eastward, opening the Banda Sea in its wake, while simultaneously Vogelkop is rotating clockwise away from it. Major plate reorganizations are now occurring here, but are understood only in fragements. Another arc-continent collision is recorded in the geology of medial New Guinea. A south-facing island arc ramped up onto what had previously been a stable continental shelf of Atlantic type, which itself had developed following Early Jurassic rifting of Sumatra and Malaya away from southern New Guinea. Following this collision, a south-dipping Benioff zone developed at the north edge of the continent as enlarged by the collision. A small-scale arc-continent collision is displayed in northeastern Sulawesi, where the small Sula-Banggai sliver of Paleozoic basement rocks, torn from Vogelkop on a strand of the Sorong fault, drove into the west-dipping Sulawesi subduction system in late(?) Miocene time.

The evolving patterns of plate motions in the Indonesian region defy analysis in terms of thermal-convection models of drive for either spreading centers or subduction zones. Rather, spreading results where plates move apart, and subduction where they converge. Body forces controlled the earth’s rotation may drive the plates.