Abstract
The dislocation density and the subgrain size of olivine in peridotite xenoliths in southwest Japan were investigated in order to draw out the lateral variation of the differential stress in the upper mantle of the island arc. Alkali basaltic and andesitic dykes including peridotite xenoliths of Dogo, Kikuma, and Shingu are situated about 200 km behind the Nankai Trough, and those of Oki-Dogo and Takashima located at the portions 400–500 km apart from the trough. The mean dislocation densities of olivine are 2 × 10 6 cm −2 for Oki-Dogo, 8 × 10 6 cm −2 for Takashima, 1 × 10 7 cm −2 for Hamada, 5 × 10 7 cm −2 for Aratoyama, 4 × 10 7 cm −2 for Kikuma, 3 × 10 7 cm −2 for Dogo, and 5 × 10 6 cm −2 for Shingu peridotites. It is concluded that the differential stress is high in the uppermost mantle beneath the island arc and low in the back-arc and the mantle wedge behind the plate boundary. The lateral variation of stress may be due to the diapiric upwelling of upper mantle materials under the island arc. The size of the diapir is suggested to be 200 km in width and 60–150 km in depth.
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