Oblique plate convergence along arcuate trenches on a spherical Earth. An example from the Western Sunda Arc

Abstract

We propose a new definition of oblique plate convergence. Our model considers arc-trench curvature on a spherical Earth, and defines obliquity as the angle between the direction of plate convergence and the azimuth from the central point along the arc to the center of curvature of the arcuate trench. We also show how this model may be extended to multiple-arc trenches.We apply this definition to the Western Sunda Arc, which has been traditionally considered the prime example of oblique plate convergence, particularly the segment corresponding to Sumatra. We define eight arc-like segments along this arc-trench system. Obliquity angles on these arc-like segments vary from 92° in the northernmost (Burmese) arc to 7° offshore the Sunda strait (between Sumatra and Java). The angle of obliquity offshore Sumatra is 31°, almost within the definition of slightly oblique convergence. Given the low angle of obliquity, partitioning along the Sumatran fault, in the southern portion of the Western Sunda Arc, is difficult to explain by oblique plate convergence alone. We suggest that the entire segment (or forearc) inboard of the Western Sunda Arc be dragged from the Burmese arc, where overriding and subducting plates are completely coupled and oblique plate convergence is high.