| Research Abstract |
We have investigated brittle-ductile transition, modes of deformation, and deformation histories under the upper to lower crustal conditions in natural examples, using integrated field observations and structural and petrological analyses. The examples are from the Hidaka, Unazuki, Ryoke, and Higo metamorphic belts, Napier Complex, Budo and Gozu granitic massifs, Jurassic Toi Formation, and Tertiary sediments at western Joetsu and eastern Nagaoka cities.
Gabbroic mylonites, pseudotachylytes, and tonalitic small mylonite zones in the northern Hidaka metamorphic belt, Hokkaido, formed under granulite facies conditions. These rocks suggest that seismic faulting and brittle failure occurred under lower crustal conditions, resulting from the effects of variations of pore fluids and/or strain rate. In the Napier Complex, Enderby Land, mylonites, mylonitized pseudotachylytes, and pseudotachylytes formed during the same deformation stage under retrograde granulite facies conditions. Complex rel
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ationships of the pseudotachylytes with the mylonites suggest that seismic faulting and plastic deformation alternated at lower crustal levels. The change in mode of deformation would have resulted from the changes in microstructures and/or strain rate. In the Budo and Gozu granites, Niigata Prefecture, foliated cataclasites, random-fabric cataclasites, and ultracataclasites formed in turn in the same fault zones. The sequential formation shows that deformation mechanisms change from breaking to grinding during the structural evolution of the faults. The foliation of the foliated cataclasites, which formed in the initial phase of brittle faulting, is defined either by the alignment of elongated biotite fragments or by the alignment of elongated lenses made of the same kinds of mineral fragments, such as quartz or feldspar. The fault zone originated from the former cataclasite widened in thickness during faulting, but the latter did not. The former cataclasite is associated with pseudotachylyte, indicating that seismic faulting occurred. Mainly temperature change produced the brittle-ductile transition in the above metamorphic belts, granitic massifs, and sedimentary basins. Less
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