OCEANIC CRUST IN THE DYNAMICS OF PLATE MOTION AND BACK-ARC SPREADING

Some of the fundamental features of plate tectonics are interpreted in connection with the behavior of oceanic crust. It is shown to be likely that the oceanic crust which is produced at the mid-ocean ridge by chemical differentiation may be removed from the downgoing slab by melting at the depth of asthenosphere behind the deep-sea trench. The melting of crustal material after the subduction is made possible by an efficient supply of heat through the well-developed asthenosphere with a low-velocity and high-attenuation of seismic waves. The removal of subducted oceanic crust from the slab is consistent with the positive gravity anomaly behind trenches and the double Benioff zone recently discovered. We propose new type of driving forces of plate motion, which arises from the density contrast between the crust and mantle when the oceanic crust is either created or destructed. The proposed driving mechanism is consistent with the non-uniform size and shape of individual plates, the migration of mid-ocean ridges and compressional intraplate stress, while these facts are difficult to understand in the framework of conventional models. A continuous accumulation of basaltic magma beneath the trench-arc system results in a catastrophic overflow of material, which corresponds to back-arc spreading. The picture presented in this paper explains the evolution of marginal basins that is characterized by the presence of remnant arcs, the changes in stress field and the dip angle of the slab, and the anomalous depth-age relationships.