COMPRESSIONAL DEFORMATION OF ISLAND-ARC LITHOSPHERE IN NORTHEASTERN JAPAN RESULTING FROM LONG-TERM SUBDUCTION-RELATED TECTONIC FORCES - FINITE-ELEMENT MODELING

Northeastern Japan experienced an approximately constant, compressiona l deformation during the last 5 million years resulting from the stead y subduction of the Pacific plate. Because the direction of the maximu m compression axis is approximately perpendicular to the strike of the island are, 2-D finite-element modeling can be used to examine the de formation over time of the island-are lithosphere, which, in turn, all ows a test of the hypothesis that the large-scale features of topograp hy, gravity and seismicity in northeastern Japan result from tectonic compression. The model geometry is based on heat flow data and laborat ory-derived flow low, and each model run requires an assumed interplat e coupling. Novel in our modeling is the ability to include erosion/de position loading and the creation of strike-slip faults, based on a dy namically applied fracture criterion. The criterion for acceptability of a model is how well it matches observed present-day topography, gra vity, and seismicity patterns. Models with the following viscosity str ucture are consistent with present-day observations: The long-term eff ective elastic thickness is 10 km in the inner are, increasing to abou t 50 km near the trench. The effective elastic thickness in the inner are is therefore much smaller than the about 30 km short-term elastic thickness estimated from seismological data. The viscosity of the lowe r crust is on the order of 10(22) Pa s or less. Our model explains the observed positive gravity anomaly of the Kitakami range as a conseque nce of uplift resulting from a coupling which increases with depth alo ng the interface between the subducting and overriding plates. The mod el also predicts the slip on the faults adjacent to the Backbone range , as well as the crustal shortening. (C) 1998 Elsevier Science B.V. Al l rights reserved.