Orogeny and relative plate motions: Example of the Japanese Islands

The paleogeography of the eastern margin of Asia, near Japan, for the past 250 Ma has been reconstructed on the basis of the relative motions between the oceanic plates and the Eurasian plate (Engebretson et al., 1982) and on-land geology. The relationships between the plate kinematics and orogeny are as follows:. Calc-alkaline magmatism is related to the presence of subduction. When a plate boundary is a transform fault, there is no volcanism. Very slow convergence rates do not give rise to magmatism. Migration of RFT-type triple junctions or ridge subduction produces forearc volcanism near the trench, and also a wider volcanic zone. Uplift of low-P/T-type regional metamorphic belts is related to collision episodes. The uplift of the high-P/T blueschist belts, on the contrary, is not contemporaneous with them. They appear to be related to the start of the subduction of a young plate after the migration of an RFT-type triple junction. Because the formation of a voluminous accretionary complex is contemporaneous with this, and thrusting of this young accretionary complex beneath the uplifted metamorphic belt is often observed, the underplating of the young accretionary complex beneath the older accretionary complex would be the cause of the uplift of the high P/T-type metamorphic belt. The subduction of a young plate would thus be an important factor causing these orogenic events. Highly oblique convergence also results in formation of a serpentinite mélange zone with fragments of blueschists, presumably along a mechanical plate boundary in the forearc.