Abstract

Recent conceptual models for fault-related rocks imply that mylonites with internal shear surfaces may exhibit frictional characteristics and that they potentially can form within the seismogenic zone. This paper describes a lower- greenschist-facies quartz-rich mylonite zone from the western Blue Ridge province in the southern Appalachians that displays evidence for episodic deformation by both crystal plastic and brittle mechanisms. Strain was largely accommodated by quartz, whereas feldspar behaved rigidly and passively. Plastically deformed monocrystalline quartz ribbons and the assemblage chlorite- quartz-muscovite are consistent with temperatures of deformation between 300 and 400 °C and, together with fluid-inclusion measurements, indicate that fluid pressures fluctuated between 90 and 300 Mpa. If one assumes that the maximum pressure represents lithostatic conditions, this corresponds to a depth of about 12 km. Quartz c-axis fabrics display both symmetrical and asymmetrical patterns with respect to the foliation and indicate that deformation occurred under coaxial and noncoaxial conditions. Healed microcracks and extensional quartz veins that are cut by quartz ribbons indicate that brittle and crystal plastic deformation occurred simultaneously or cyclically. By analogy with experimental studies, internal zones of localized shear in the mylonites may represent frictional behavior. These results are discussed within the context of recently developed conceptual continental fault-zone models.

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