Heterogeneous material distribution, an important reason for generation of strain-localized mylonite and frictional slip zones in the Hidaka metamorphic belt, Hokkaido, Japan

Abstract

Lithological heterogeneity of low P/T metamorphic rocks in southern area of Hidaka metamorphic belt (HMB) was formed through historical development of HMB while these rocks had been laid in ductile lower crust. Many strain-localized mylonite zones (<100 m in thickness) are preferentially developed within S-type tonalite and pelitic gneiss, which are characterized by a large modal amount of phyllosilicates (biotite+muscovite+chlorite) and quartz, compared to other lithofacies in HMB. Mylonitic foliations are more conspicuous with close to the center of the shear zone associated with increase in amounts of phyllosilicate minerals, indicating fluidenhanced weakening mechanisms were operated in plastic shear zones. Pseudotachylyte veins are observed exclusively in these mylonite zones, which were generated during exhumation stage of HMB. We conclude the seismic slip zones in southern HMB had been initiated in the ductile lower crust by concentration of localized plastic shear zones within the phyllosilicate- and quartz-rich lithofacies, which were heterogeneously formed by old metamorphic and magmatic events. Then these zones were further weakened by fluid-enhanced plastic deformation, and finally seismic slips occurred at the bottom of seismogenic upper crust, during exhumation of HMB.