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Department of Geology and Mineralogy, Kyoto University, Kyoto 606, Japan
Faculty of Marine Science and Technology, Tokai University, Shimizu 424, Japan
Under the forearc region off the Japanese Islands a conspicuous basal acoustic reflection plane can be traced at the surface of the acoustic basement from the oceanic plate into the bottom of the accretionary prism. Recent studies indicate this plane cannot be the upper boundary plane of the subducting oceanic plate, but must be the upper layer of the oceanic basaltic basement (or chert layer).
The tectonic activity of the Wadati-Benioff zone appears to be a thrust movement which cuts the boundary between the oceanic basement and the overlying sediments. Thrusts cut the basal reflector and extend into the accretionary prism. Large earthquakes which occur at intermediate and shallow depths under the continental shelf and slope are a direct manifestation of these thrust movements.
Thrust faulting also occurred in past geological time. Acidic and basic magma, possibly generated along the earthquake zone at intermediate depths, ascended along the thrust faults or associated normal faults. Intrusions occurred 15–16 Ma ago in the forearc region of Southwest Japan and 22 Ma ago in that of Northeast Japan; subsequently basins subsided landward of the intrusions. In some cases the forearc igneous rocks carried continental and oceanic crustal xenoliths. Good examples of such forearc igneous rocks, including xenoliths, occur in the Permian and Triassic structural belts of the Japanese Islands.
The model described here is specific to the forearc regions of the Japanese Islands. We envisage that different tectonic parameters such as convergence rate and sediment supply may engender different structural processes in other forearcs.
