SUBDUCTION HINGE MIGRATION ALONG THE IZU-BONIN MARIANA ARC

Hinge migration or "rollback" is thought to have a significant influence on the tectonics of trench-arc-backarc systems. Where the backarc region is actively opening, the hinge is usually either stationary or retreating in a mantle (hotspot) frame of reference. The Izu-Bonin-Mariana system represents a highly anomalous case in which the hinge is reportedly advancing behind the retreating eastern edge of the overriding Philippine Sea plate. We have re-evaluated the motion of the subduction hinge using the best available estimates of the motion of the Philippine Sea plate and estimated maximum rates of extension within the Izu-Bonin Arc: if the plate motions have not changed appreciably, the subduction hinge of the Izu-Bonin Trench has advanced over the last 5 Ma. The approximate average rate of hinge advance is 0-6 mm a-1 at the north end of the Izu-Bonin Arc, increasing to 24-30 mm a-1 at 27°N. The rate of advance of the Mariana hinge (at 18°N) is 10-20 mm a-1. The dip of the subducted slab is strongly correlated with the motion of the Philippine Sea plate, suggesting that the rapid retreat of the overriding plate affects mantle flow. We have found a tentative hinge-advance mechanism in a simple Newtonian-viscous corner flow model. Though the model has severe geometrical and rheological limitations, it suggests that the retreat of the overriding plate gives rise to excess uplift pressures acting on the top surface of the subducted slab. If the hinge is not assumed to be fixed, the excess pressure will tend to rotate the slab arcward, steepening the slab and advancing the subduction hinge. Thus, the westward motion of the Philippine Sea plate may explain both the advance of the subduction hinge and the observed relationship between the motion of the Philippine Sea plate and the dip of the subducted slab along the Izu-Bonin-Mariana Trench. © 1990.