Deformation microstructures of rocks and the rheology of lithosphere

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The main task of the present research project has been to establish a comprehensive research laboratory for studying defomation microstructures and the rheology of the lithosphere by introducing a transmission electron microscopy (TEM) and by adjusting rock deformation apparatuses recently installed to Earthquake Research Institute, University of Tokyo. The TEM and an ion-thinning machine will be useful for studying the ultrafine fault rocks constituting the central part of faults. The major results from the present three-year project are as follows.(1) Two-phase instability model for the earthquake generation has been proposed based on the results of high-velocity frictional properties of faults, and the model may account for the seismic quiescence of the second kind. Studies of natural fault rocks have revealed evidence of high velocity fault motion.(2) Complete transition from brittle, through intermediate, to fully plastic deformation under extreme shear has been studied for halite … More shear zones at temperatures increasing linearly with an increase in the normal stress to the fault (as in the case of geothermal gradient) using a high-temperature biaxial machine. These experiments have revealed for the first time the strength profile of lithosphere, conditions for mylonitic deformation, and the rheological meaning of the lower bound of the seismogenic zone. Previous fault models were denied and a new fault and plate-boundary model was proposed. Experimental results also revealed that rheological properties of a fault is controlled by weaker constituent minerals whose content is as small as 5%, implying the significance of phylosilicates in the fault rheology.(3) A theory of solution-precipitation creep was proposed, and the diffusivity of solutes in pore water in rocks was measured. These results have successfully evaluated the effect of pressure solution on rock deformation. Diverse experimental data on the frictional properties of rocks under hydorthermal conditions have been summarized.(4) The relatioship between the dislocation density and the grain size in natural mylonites is different from that for experimentally deformed rocks, suggesting that deformation microstructures of mylonites have been altered after the completion of deformation. TEM observations of low-temperature mylonites revealed dynamic recrystallization of plagioclase. Less