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

Abstract

Cataclastic rocks and pseudotachylytes are exposed along the Uchinoura shear zone, a normal fault zone cutting the middle Miocene (14 Ma) Osumi granodiorite in southern Kyushu, Japan. Cataclastic rocks include non-foliated clast-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 in foliated granodiorite. Two types of pseudotachylyte are distinguished: a foliated-type characterized by a planar arrangement of clasts and microlites, and a spherulitic-type characterized by clasts surrounded by microlite overgrowths. Both types are of melt origin, as attested by the presence of microlites and rounded or embayed clasts, and by the scarcity of biotite clasts. Unlike spherulitic-type pseudotachylyte, which solidified without being deformed, the foliated-type pseudotachylyte underwent flow before complete solidification. This deformation is thought to reflect post-seismic strain accommodation immediately following the main slip episode. Kinematic indicators, which consist of Riedel-type secondary fractures branching on primary fractures, shear bands offsetting the foliation of foliated granodiorite, or asymmetrical porphyroclast systems within pseudotachylyte veins, show that all fault rocks were generated during N–S- to NW–SE-directed 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 melting. Given the slow strain rates which characterize dissolution and recrystallization processes detected in cataclasites, the juxtaposition of pseudotachylytes and foliated cataclasites provides an example of aseismic and seismic displacements within the same shear zone.