Pseudotachylytes of crushing origin from the Shimotsuburai fault of the Itoigawa-Shizuoka Tectonic Line active fault system, central Japan

Abstract

Pseudotachylytes are found along the Shimotsuburai fault, the southern segment of the Itoigawa-Shizuoka Tectonic Line active fault system, central Japan. They occur along the shallowly-dipping fault surface between the quartz-dioritic cataclasite of the hanging wall and the Upper Pleistocene alluvial deposits of the footwall, and also in the cataclasite zone of the hanging wall about -m wide. Some of them in the cataclasite zone are fault veins associated with small faults and others are injection veins forming complex networks into extension fractures. These veins vary from a few mm to cm in thickness, and are black to dark-brown in color and are aphanitic in appearance. They consist mostly of fine-grained angular fragments of quartz and feldspar, and some biotite crystals derived from the cataclasite. X-ray diffraction analyses suggest that the veins have the same mineral assemblage as that of the quartz-dioritic cataclasite. These textural and X-ray analytical results suggest that the pseudotachylytes were mainly formed by crushing but not by melting. On the basis of structural relationships between the boundary faults and veins, we interpret that some veins along and near the boundary fault were produced by the last faulting event occurred between and y. B.P. with dip-slip displacement of about . m at a depth of less than a few tens of meters. AbstractCThe Geological Society of Japan 2004 779

Repeated seismic slips recorded in ultracataclastic veins along active faults of the Arima–Takatsuki Tectonic Line, southwest Japan

“…Japan (e.g., Lin, 1996Lin, , 2001Lin, , 2008aKano et al, 2004). This also suggests that the fine-8 grained materials are injected rapidly into the network fractures without sorting of fine-9 grained materials.…”

Ultramafic pseudotachylytes in high-pressure metamorphogenic peridotite from Luobusha, Tibet: a record of crustal paleo-earthquakes

“…It is clear that glass or glassy material within pseudotachylyte indicates the generation of melt during its formation, but the converse is not true: the absence of glass or glassy material is not a diagnostic test for the melting origin for pseudotachylyte because primary glass or glassy material can be devitrified during subsequent alteration and/or metamorphism (Lin, 2008). In addition to the melting-origin pseudotachylyte, crushingorigin pseudotachylytes, which are generated by rapid comminution and injection during seismic faulting, are also considered to represent fossil earthquakes, as with melt-origin pseudotachylytes (Kano et al, 2004;Lin, 2008). Such veins are mainly composed of fine-grained fragments of the wall rock, with little or no evidence of melting (Lin, 2008).…”

“…Tectonogenetic pseudotachylytes that are related to seismic activities have generally been considered as products formed under the condition of high strain rates within shear zones and/or seismic fault zones (McKenzie and Brune, 1972;Sibson, 1975;Spray, 1987;Magloughlin, 1992;McNulty, 1995;Lin and Shimamoto, 1998). Frictional heating and strong abrasion that are generated during rapid seismic faulting are sufficient to melt and/or crush rock within the fault zone and fluidize ultrafine-grained materials (Kano et al, 2004), which eventually give rise to pseudotachylytes. More pseudotachylytes recognized from crustal sialic protoliths, such as crystalline rocks of intermediate and granitoid composition, have been reported in the literature, but cases of pseudotachylytes derived from peridotite are less common.…”