Plate convergence, transcurrent faults, and internal deformation adjacent to Southeast Asia and the western Pacific


  • Thomas J. Fitch


A model for oblique convergence between plates of lithosphere is proposed in which at least a fraction of slip parallel to the plate margin results in transcurrent movements on a nearly vertical fault located on the continental side of a zone of plate consumption. In the extreme case of complete decoupling, only the component of slip normal to the plate margin can be inferred from underthrusting. Recent movements in the western Sunda region provide the most convincing, evidence for decoupling of slip, which in this region is thought to be oblique to the plate margin. A speculative model for convergence along the margins of the Philippine Sea is constructed from, an inferred direction of oblique slip in the Philippine region. This model requires that the triple point formed by the junction of the Japanese and Izu-Bonin trenches and the Nankai trough migrate along the Sagami trough. Absence of an offset between the trenches can be explained by rifting within the Izu-Bonin arc. Transcurrent movements in southwest Japan and the Taiwan region suggest that similar movements within well-developed island arcs are initiated during a nascent stage in the development of such plate margins and are often imposed on a pre-existing zone of weakness. The bulgelike configuration of the western margin of the Andaman basin is explained as a result of interarc extension within the western part of the basin. In the Indian Ocean plate south and west of this basin, strike-slip movements inferred from four mechanism solutions, two of which are reported in this work, suggest a distribution of stress in which axes of both maximum and minimum compression are nearly horizontal, the stress maximum trending west of north. The larger recent earthquakes in the eastern Sunda region have resulted in deformation within the arc rather than shallow underthrusting at the inferred plate margin. A narrow region between the Celebes basin and the Philippine trench, including the Talaud Islands and a northern extension of the Molucca basin, contains several zones of shallow thrusting. Mechanism solutions show that these movements trend E-W consistent with shortening normal to the arc-related structures in the region. Recent movements near the east coast of Luzon reveal a zone of underthrusting along the inner wall of a trough, marginal to the coast. There is an apparent hiatus in underthrusting between this zone and the one that follows the inner wall of the Philippine trench.