A KINEMATIC MODEL FOR EVOLUTION OF ISLAND ARC-TRENCH SYSTEMS

Plate subduction zones are typically characterized by the patterns of surface topography and gravity anomalies consisting of island-arc high , trench low, and outer-rise gentle high. These patterns are stable on the time scale of 10(6)-10(7) yr. At some subduction zones, regardles s of its age, steady uplift of marine terraces formed by eustatic sea- level changes during the last 10(5) yr can be also observed. The stabl e patterns of topography and gravity anomalies and the steady uplift o f marine terraces seem to contradict each other. We constructed a kine matic model which could explain the evolution process of island arc-tr ench systems and demonstrated that this puzzle could be solved by cons idering the effect of accretion at plate boundaries. In our model the lithosphere-asthenosphere system is represented by a stratified visco- elastic half-space under gravity, which consists of a high-viscosity s urface layer and a low-viscosity substratum. The rheological propertie s of both layers are assumed to be a Maxwell fluid in shear and an ela stic solid in bulk. Interaction between oceanic and continental plates is represented by the steady increase of discontinuity in tangential displacement across the plate boundary. The other essential factors co nsidered in our model are accretion of oceanic sediments at plate boun daries, erosion on land, and sedimentation on inner trench wall. We co mputed the evolution process of island arc-trench systems by using the kinematic model and obtained the following results: the island arc-tr ench systems grow at nearly constant rates in the early stage of plate subduction. Therefore, at young subduction zones, we can generally ex pect the steady uplift of marine terraces. The stable patterns of topo graphy and gravity anomalies are formed within several million years a fter the initiation of plate subduction. When the accretion process co ntinuously proceeds at plate boundaries, these stable patterns gradual ly migrate seaward as a whole. In such a case, we can expect the stead y uplift of marine terraces even at old subduction zones. We demonstra te that our model can consistently explain the observed surface topogr aphy, gravity anomalies, and uplift rates of marine terraces at subduc tion zones.