6:15 PM - 7:30 PM
[SCG57-P34] Interseismic plastic deformations at ancient crustal seismogenic zones in the Hidaka metamorphic belt and Napier Complex
Keywords:pseudotachylyte, pressure solution, ultramylonite, plastic deformation, interseismic deformation, time-dependent strength recovery
Modes of occurrence of the Hidaka pseudotachylytes indicate that seimic slip with pseudotachylyte generation and slow plastic deformation (pressure solution with precipitation) occurred alternatively and repeatedly in the same fault zones and fault surfaces (Wada and Toyoshima, 2006, 2007). Pressure solution-precipitation is likely one of the principal deformation mechanisms for interseismic plastic deformation and time-dependent strength recovery of fault zones (Wada and Toyoshima, 2006, 2007). The pressure solution-precipitation processes lead to increase in number of grain-to-grain contact and in real contact areas of the fault surfaces, resulting in healing of the fault zones (Wada and Toyoshima, 2007). Very thin and sharp shear zones filled with very fine-grained materials cut the pressure solution-precipitation structures and are cut by pseudotachylytes-producing faults. The shear zones are deformation structures formed immediately before seismic faulting in the upper Hidaka crust.
Granulite-facies ultramylonites are characteristically abundant in and along the Tonagh pseudotachylyte-producing fault zones. There are two different types of the granulite-facies ultramylonites in microstructures of recrystallized plagioclase grains: type 1 and 2. Type 1 ultramylonites have polygonal medium grains of plagioclase with smooth grain boundaries and very weakly undulose extinction. Type 2 ultramylonites include very fine grains and elongated fine grains of plagioclase with strongly undulose extinction and irregular grain boundaries. Type 2 ultramylonites occur along granulite-facies pseudotachylytes-generating fault surfaces and have been cut by fault veins of pseudotachylyte. Some of the granulite-facies pseudotachylytes became type 1 and 2 ultramylonites, which have also been cut by other granulite-facies pseudotachylytes. These may also be conspicuous difference between dynamic recrystallization mechanisms of plagioclase immediately before and after seismic faulting.