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

Pseudotachylytes from the Iida-Matsukawa Fault, Central Japan (Iida pseudotachylytes), are described down to the nanometer scale using high-resolution electron microscopy, scanning electron microscopy, analyses of chemical compositions, and mercury intrusion porosimetry. The pseudotachylyte matrix chiefly consists of nanoscale particles and elongated submicron fragments with biotite composition; amorphous material of several tens of nanometers in size is also observed, 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. The presence of these submicrostructures indicates that the amorphous material formed by mechanical stress during the comminution process; that is, via the “mechanochemical effect”. 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.