Coeval formation of cataclasite and pseudotachylyte in a Miocene forearc granodiorite, southern Kyushu, Japan

Cataclastic rocks and pseudotachylytes are exposed along the Uchinoura shea r zone, a normal fault zone cutting the middle Miocene (14 Ma) Osumi granod iorite in southern Kyushu, Japan. Cataclastic rocks include non-foliated cl ast-supported to matrix-supported cataclasite and foliated clast-supported cataclastic granodiorite. In these rocks, fracturing and comminution played a major role, but dissolution and recrystallization of quartz, and plastic deformation of quartz and biotite were also active processes, especially i n foliated granodiorite. Two types of pseudotachylyte are distinguished: a foliated-type characterized by a planar arrangement of clasts and microlite s, and a spherulitic-type characterized by clasts surrounded by microlite o vergrowths. Both types are of melt origin, as attested by the presence of m icrolites and rounded or embayed clasts, and by the scarcity of biotite cla sts. Unlike spherulitic-type pseudotachylyte, which solidified without bein g deformed, the foliated-type pseudotachylyte underwent flow before complet e solidification. This deformation is thought to reflect post-seismic strai n accommodation immediately following the main slip episode. Kinematic indicators, which consist of Riedel-type secondary fractures bran ching on primary fractures, shear bands offsetting the foliation of foliate d granodiorite, or asymmetrical porphyroclast systems within pseudotachylyt e veins, show that all fault rocks were generated during N-S- to NW-SE-dire cted extensional deformation. Pseudotachylyte is closely associated both in time and space with cataclastic rocks, thus indicating that the behaviour of the Uchinoura fault zone alternated between comminution and frictional m elting. Given the slow strain rates which characterize dissolution and recr ystallization processes detected in cataclasites, the juxtaposition of pseu dotachylytes and foliated cataclasites provides an example of aseismic and seismic displacements within the same shear zone. (C) 2000 Elsevier Science Ltd. All rights reserved.