[SGL31-P08] Classification and Deformation Conditions of Cataclastic Rocks in Volcanic Rocks of the Anamizu Formation in Western Noto Peninsula, Japan
Keywords:cataclastic rocks, fault, volcanic rocks, microstructure, deformation temperature
The cataclastic rocks along the basal part of an andesitic lava flow show reddish-brown color due to high-temperature oxidation. The cataclastic rocks consist of rock fragments and matrix, and the rock fragment is composed of euhedral plagioclase phenocryst and matrix. The euhedral plagioclase phenocryst occasionally exhibits subgrains, microfracture with subgrains, deformation band and undulose extinction, and the groundmass contains undeformed acicular plagioclase. The matrix of the cataclastic rocks consists of plagioclase, pyroxene and alteration minerals, and deformation band, kink band and undulose extinction present in the plagioclase fragments.
The low-angled flow structures which are defined by flat and elongated breccia occur in part of tuff breccia, and it indicates that the tuff breccia is deformed relatively weak. The matrix consists of plagioclase, pyroxene and alteration minerals. The plagioclase exhibits deformation band and undulose extinction.
The steeply dipped cataclastic rocks are across a flow unit of andesite and tuff breccia. The cataclastic rocks show grayish color due to hydrothermal alteration. In further detail, some cataclastic rocks accompanied with fractures, containing a gouge, mineral vein and clastic dike. Dragged rock fragments along the fractures suggest normal faulting. The cataclastic rocks consist of rock fragments and matrix. The matrix is a fine grained aggregate of plagioclase, pyroxene and alteration minerals. Plagioclase in the matrix is various grain size and angler shape, and showing deformation band, kink band, deformation twinning and undulose extinction.
From the above, we supposed that the cataclastic rocks along the basal part of a andesitic lava flow represent flow-foot-breccia. The flow structures in a part of tuff breccia are caused by compaction. The steeply dipped cataclastic rocks are a fault rocks which were collapsed at the extension tectonics at Early Miocene. Furthermore, the microstructures of plagioclase in these cataclastic rocks imply that dominant deformation temperature is about 300 to 400°C (Passchier and Trouw, 1996) . The flow-foot-breccia and the flow structure in the tuff breccia are formed during extrusion and sedimentation of the Anamizu formation, therefore, the steeply dipped fault also inferred to deform at the similar tectonic environment to the others. The cataclastic rocks can be formed in shallow involving high geothermal gradient and hydrothermal process, if they deformed at volcanism period relate to expanding the Sea of Japan at Early Miocene.