HR: 0800h
AN: T11A-0354    [Abstracts]
TI: Amorphous Material Formed by the Mechanochemical Effect in Natural Pseudotachylyte of Crushing Origin: A Case Study of the Iida-Matsukawa Fault, Nagano Prefecture, Central Japan
AU: * Ozawa, K
EM: kanaoz@geol.tsukuba.ac.jp
AF: Division of Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki, 305-8572, Japan
AU: Takizawa, S
EM: takizawa@geol.tsukuba.ac.jp
AF: Division of Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki, 305-8572, Japan
AB: Glass or amorphous material in pseudotachylyte are interpreted as melt texture; however, naturally occurring amorphous material can form via the rapid cooling of melt, alteration, and mechanochemical effects induced by mechanical energy during the comminution process. Recent experimental studies suggested that amorphization and the formation of gel via shearing and comminution under wet conditions is one of the weakening mechanisms of faults during seismic slip (Goldsby and Tullis, 2002; Di Toro et al., 2004). These studies show that the presence of amorphous material formed by comminution is an essential factor in understanding fault strength and faulting processes during earthquakes; however, the presence of amorphous material formed by comminution has yet been reported from natural fault rocks. In the present study, the crush-origin pseudotachylytes from the Iida-Matsukawa Fault (Iida pseudotachylytes) are described down to the nanometer scale using transmission electron microscopy (TEM), scanning electron microscopy (SEM), analyses of chemical compositions, and mercury intrusion porosimetry. The matrix of the Iida pseudotachylyte chiefly consists of nanoscale particles and elongated submicron fragments with biotite compositions. Amorphous materials of several tens of nanometers in size are scattered randomly among the submicron crystalline fragments within the matrix, despite the absence of melt textures. Lattice fringe images reveal that the amorphous phase coexists with lattice distortion in deformed biotite fragments that are several hundreds of nanometers in size. These submicrostructures indicates that the amorphous material formed by mechanical stress during the comminution process. The fragments in the Iida pseudotachylyte have a low degree of roundness (< 0.4), suggesting that the pseudotachylyte formed by processes other than melting. Most of the pores in the pseudotachylyte are smaller than 100 nm, and that the host rock is more porous than the pseudotachylyte, thereby suggesting that fluid is unlikely to have passed through the pseudotachylyte subsequent to its formation. It is therefore inferred that the Iida pseudotachylyte is negligibly affected by chemical dissolution in groundwater and hydrothermal alteration subsequent to fault movement. We conclude that amorphous material does not always provide evidence of the rapid cooling of melt in pseudotachylyte. The amorphous materials present in the Iida pseudotachylyte resulted from the mechanochemical effect due to both shear stress and normal stress during the comminution process that accompanied fault motion.
DE: 8100 TECTONOPHYSICS
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting