Pseudotachylytes, related fault rocks, asperities, and crustal structures in the Hidaka metamorphic belt, Hokkaido, northern Japan

Abstract

Many pseudotachylytes and their related fault rocks are found in the Hidaka metamorphic belt representing an ancient crustal section. On the basis of field observations of the pseudotachylytes and related fault rocks, nature of seismogenic faulting in the Hidaka crust is examined. The field observations suggest the following conclusions.(1) Two structural types of pseudotachylytes are distinguished: layer-parallel and layer-oblique. The latter are scattered in the metamorphic belt, but the former occur only in the southern part of the metamorphic belt. (2) An abundance of the layer-parallel pseudotachylytes suggests that earthquakes occurred repeatedly and frequently in the southern part, where complicated and duplicated crustal structures occur with many lowtemperature thin mylonite zones. The southern part with such crustal structures was an ancient seismogenic area containing asperities and having a radius of a few tens of kilometers in the Hidaka crust. In the seismogenic area, the layer-parallel pseudotachylytes resulted from seismic slip on the mylonitic foliation within the lowtemperature mylonite zones with strong preferred orientation of micas. (3) The layer-parallel pseudotachylytes and the subsequent layer-oblique pseudotachylytes post-date the latest and very-low-temperature mylonitization in the metamorphic belt. The former pseudotachylytes formed just above the upper side of the brittle-plastic transition zone.

Architectural evolution of the Nojima fault and identification of the activated slip layer by Kobe earthquake

“…Melting and fluidization seems to be the main mechanisms of dynamic weakening of shear zones at this depth [ Tanaka et al , 2001c; Ohtsuki et al , 2003]. Similar occurrences of deep shear zones are also recognized in Hidaka metamorphic zones, northern Japan [ Toyoshima et al , 2004; Tanaka et al , 2004; Shimada et al , 2004]. Fracture zones became greatly widened at the middle depth of the crust, associated with the precipitation of zeolite (stage II shear zones).…”

Strain-induced amorphization of graphite in fault zones of the Hidaka metamorphic belt, Hokkaido, Japan

“…Subsequently, localized low-temperature mylonitization related to the intrusion of sheet like tonalites and brittle deformations took place during retrograde metamorphism. Koyasu et al (2006Koyasu et al ( , 2007 also summarized that most of the deformation events in the upper sequence and tonalite in the study area cannot be correlated with those of the lower sequence events reported by Toyoshima et al (2004).…”

“…4c, e). We consider these brittle fault rocks to be comparable to the stage III fault rocks of Toyoshima et al (1997Toyoshima et al ( , 2004, because of their similar modes of occurrence, deformation types, orientation, and metamorphic conditions.…”

“…The Hidaka metamorphic belt was then uplifted and tilted steeply eastwards through the two stages of mylonitization (Toyoshima et al, 1994). Subsequently, during Stage III, the exhumed Hidaka metamorphic rocks were locally modified by ductile and brittle deformations under very low temperature conditions (Toyoshima et al, 2004).…”

“…On the basis of structural analysis of the lower sequence, the deformation history of the Hidaka metamorphic belt can be divided into three main stages (Toyoshima et al, 1994(Toyoshima et al, , 1997(Toyoshima et al, , 2004Osanai et al, 2007). Stage I is subdivided into the D0 and D1 stages.…”