The surface expression generated by convergence of two oceanic plates and sinking of the older lithosphere into the mantle is one of the most spectacular features of our planet, and yet very little is understood about the onset of this dynamic process. This is largely due to the lack of modern-day analogues where causal relationships among various subduction parameters can be investigated. Here we report a rare example in the western Pacific where variations in style of deformation and state of stress are observed on a young trench-arc system as a result of subduction of the Oligocene Caroline plate beneath the Philippine Sea plate along the Yap Trench. The unique features of this system include a short trench-arc distance (similar to50 km), narrow trench (<50 km), and the presence of the Caroline Ridge, a region of massive volcanic outflow formed on the subducting Caroline plate, which is divided into two by the Sorol Trough trending WNW-ESE. While the timing of collision is uncertain and the possible rejuvenation of an old subduction zone can not be ruled out, the convergence apparently changed to subduction when the Caroline plate reached the mechanically weak part of the overriding plate. The style of deformation changes at approximately 9degreesN, which corresponds to the southern border of the Sorol Trough. To the north, the area is generally characterized by extensional features on the arc and slab with a large half graben (100 km x 50 km) on the plate entering the subduction zone. To the south, compression is the dominant mode of deformation on the arc and slab: The average depth of the subducting plate to the north lies 500 m higher than to the south, and this height appears to be maintained by broad flexural bending applied by the Yap and southern Mariana :Trenches. Although the convergence rate is higher to the north of Sorol Trough than the south, the gravity anomalies show that the slab has advanced deeper into the asthenosphere to the south, which is interpreted as evidence that subduction started later to the north than south. The study also reveals that during the initial early stage of subduction a broad flexure may develop on the slab, but with further convergence, the deformation is localized and the slab is deflected at the maximum bending point. (C) 2003 Elsevier Ltd. All rights reserved.
