10:45 AM - 12:15 PM
[SCG50-P04] Deformation mechanisms in shallow forearc mantle wedge serpentinites: An example from the Tomisato serpentinite body in the Sanbagawa belt, central Shikoku, Japan
Keywords:serpentinite, antigorite, forearc mantle wedge, dislocation creep, pore fluid pressure, Sanbagawa belt
In warm subduction zones such as Cascadia and Nankai, a large amount of water derived from the subducting oceanic crust infiltrates the overlying shallow serpentinized forearc mantle wedge, which may result in high pore fluid pressure possibly linked to the generation of episodic tremor and slip (Hirauchi et al., 2021, Earth Planet. Sci. Lett.). In order to understand deformation mechanisms operative in the forearc mantle wedge, we conducted structural analysis of the Tomisato serpentinite body in the Sanbagawa belt, central Shikoku, Japan. We found that the serpentinite shows a network of extensional (mode I) and extensional-shear (modeI–II) fractures, suggesting fault-fracture mesh development under near-lithostatic pore fluid pressures. After the fracturing events, antigorite grains are newly precipitated as a matrix to fill open spaces between the fractured blocks. The newly precipitated antigorite grains are randomly oriented or aligned parallel to the fractures. The aligned antigorite grains show intense undulatory extinction and irregular subgrain boudaries, with a strong B-type crystal-preferred orientation (CPO) indicative of active (001)[010] dislocation slip system. However, the relationship between misorientation axis and subgrain boundary in each grain demonstrates that the slip system cannot be achieved through dislocation ceep with both tilt and twist grain boundaries. Therefore, we suggest that the antigorite CPO develops by other deformation mechanisms such as grain boundary sliding and/or dissolution precipitation creep in the presence of H2O, a conditon being favorable for the base of the forearc mantle wedge.