POSITIVE FEEDBACK OF DEFORMATION CAUSED BY PRIMARY MUSCOVITE AND SYN-TECTONIC MUSCOVITIZATION

Formation of shear zones is one of the important processes in the earthfs crust in which strain softening has occurred. In this study, a strain localization process was examined in the Sunchang Shear Zone in South Korea. The Sunchang Shear Zone is a 16 km-width granitic mylonite zone. There are high strain zones with ca. 2 km-width where the mylonite is characterized by higher contents of max. 39 vol% of Ti-rich primary muscovite and syn-tectonic secondary muscovite and a fine grain size of recrystallized quartz (0.25-0.05 mm). In these zones, quartz c-axis fabrics pattern and feldspar microstructures indicate lower-temperature deformation (greenschist facies).

Both primary muscovite and secondary muscovite flakes are well oriented parallel to the S- and C-foliations. These muscovites within the matrix indicate higher linear mica continuity values of 77% (its defined as the percentage of mica intersected along a single, arbitrary straight-line traverse across a sample). This suggests that original heterogeneity of muscovite contents in the protolith results in an initial stage of shear localization.

Increasing the secondary muscovite indicates a reaction accompanied by fluid during the ductile shearing. Strong preferred orientations in foliation (S-C) formed by secondary muscovite indicates a strain localization caused by the formation of secondary muscovite and its high continuities. These together suggest that the reaction which forms muscovite along foliation during the deformation, leads to more enhanced ductile shearing i.e., strain softening.

In the Sunchang Shear Zone, muscovite played an important role on the shear localization during progressive ductile shearing. Pre-existing heterogeneity (the presence and contents of muscovite) in the protolith results in an initial stage of shear localization. The reaction which forms syn-tectonic muscovite, leads to localization of the deformation in the high strain zone. Such a positive feedback of deformation may be responsible for strain localization along the Sunchang Shear Zone.