Stress in the Indo-Australian plate

Abstract

We modelled the state of stress in the Indo-Australian plate in order to investigate quantitatively variations observed in tectonic style. The numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere. Estimates are presented for the average net resistive forces at the Himalayan collision zone, the suction force acting on the overriding Indo-Australian plate segment at the Tonga-Kermadec trench and the drag at the base of the lithosphere.

Our modelling shows a concentration of compressive stresses of the order of 3–5 kbar in the Ninetyeast Ridge area; the effects of the compressive resistance associated with Himalayan collision and subduction of young lithosphere off the northern part of the Sunda arc are focused in this region. The stress field as calculated gives a consistent explanation for the observed concentration of seismic activity (Stein and Okal, 1978) and significant deformation in the oceanic crust (Weissel et al., 1980; McAdoo and Sandwell, 1985) in the area.

The calculated stress field in the area adjacent to the Southeast and Central India ridges is characterized by tension parallel to the spreading axis. This explains the concentration of near-ridge normal faulting seismicity (with T-axes subparallel to the spreading ridge) in the Indian Ocean as recently observed by Bergman et al. (1984) and Wiens and Stein (1984).

The regional stress field along the strike of the Sunda arc varies from compression seaward of and parallel to the Sumatra trench segment, to tension perpendicular to the Java-Flores segment. This explains the selective occurrence of well developed grabens seaward off the Java-Flores segment of the trench, observed by Hilde (1983).

Our modelling shows that the observed rotation of the stress field (Denham et al., 1979) in the Australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down-going slab in each of these. The state of compression in west and central Australia is induced by the action of resistive forces at the Himalayan and Banda arc collision zones.

The joint occurrence of high levels of compression in the plate's interior and normal faulting seismicity in the near-ridge areas, is a transient feature unique to the present dynamic situation of the Indo-Australian plate.