Development of interlaced mylonites, cataclasites and breccias: example from the Towaliga fault, south central Appalachians

Abstract

A variety of cataclastic rocks from crush breccias to cataclasites and silicified breccias are associated with retrograde mylonites along the Towaliga fault zone of south central Appalachians in Georgia. A zone of alternating breccias and quartz ultra-mylonites, bordered by quartz mylonites that are roughly laminated with mm-scale mica-rich bands of cataclasites, occur subparallel to the mylonitic foliation. Elsewhere along the fault zone, evidence for plastically deformed microfractures is found. In discussing the mechanical evolution of these rocks two possibilities are considered: (a) the cataclasis is an overprint representing deformation in an entirely brittle regime; and (b) cataclasis and mylonitization occurred at T > 300° C in a predominantly plastic regime. Within the framework of the latter model it is suggested that the cataclasite bands are either due to the relaxation of dilational stresses following downward propagation of seismic ruptures or represent strain-induced seismic instabilities during plastic shearing.