Subduction of large bathymetric ridges causes compression in the upper plate, especially Wharton Ridge subduction underneath Sumatra over the period 15–0 Ma. ian slab along the Sumatra-Java trench, an accretionary wedge, the outer arc ridge forming the backstop Pubellier et al., 1992 Samuel and Harbury, 1996, the Bengkulu-Mantawai forearc basins off Sumatra and the Java forearc basin in front of the volcanic arc. Extension of the Sundaland back-arc region correlates with two patterns of upper plate motion, (a) retreat of the upper plate, and (b) advance of the upper plate combined with more rapid advance of the Sundaland margin due to hinge rollback. Below northern Sumatra, we find that the slab is folded at depth, exhibiting geometry similar to that of the volcanic arc and the trench at the surface. Compression in the Sundaland back-arc region can be linked to advance of the upper plate. Plate Tectonic and Geologic Setting 5 The Sumatra-Java area is part of the Sunda Arc that stretches from the Andaman Sea in the northwest to the Banda Sea in the east. We find that upper plate advance and retreat is the main influence on upper plate strain, but subduction of large bathymetric ridges, and slab-window effects, also play a significant, and at times dominant, role. In this paper we present structural features and a geologic-tectonic model for the Sumatra-Java forearc. Combining plate motions and slab geometries enables us to reconstruct a time-dependent slab window beneath Sundaland, formed through Wharton spreading ridge subduction. Here we combine absolute and relative plate motions with reconstructions of now subducted ocean floor to analyse subduction kinematics and upper plate strain from geological observations since 80 Ma along the 3200 km long Sunda-Java trench, one of the largest subduction systems on Earth.
The kinematics and time-dependence of back-arc extension or compression is one of the most poorly understood aspects of plate tectonics, and has nearly exclusively been studied from snapshots of present-day observations.
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