Abstract

In the internal southern Appalachians, orogen-parallel lineation in flat-lying amphibolite facies mylonites occurs over more than 700 km, from North Carolina to Alabama, along the Inner Piedmont western boundary. In several areas distributed along this zone, evidence of orogen-parallel tangential movement is consistently associated with the lineation, suggesting the amphibolite facies mylonites mark an orogen-scale movement zone, probably a décollement zone that formed in the middle crust.Amphibolite facies mylonites occur in the Clairmont Formation north of Atlanta, Georgia. Abundant mesoscopic and microscopic kinematic indicators together with a dominant southwest-trending lineation on a subhorizontal mylonitic foliation reveal a ductile southwestward movement of the Inner Piedmont on subhorizontal shear planes during the mylonitization. Synkinematic hornblende and biotite crystals extracted from a specimen of amphibolite suggest minimum K–Ar cooling ages of, respectively, 349 and 327 Ma. A minimum age of 350 Ma is thus suggested for the main activity of the western Piedmont décollement zone.Orogen-parallel tangential movement along the western Piedmont décollement zone is coeval with orogen-parallel strike-slip faulting in the high-angle Ocmulgee fault in central Piedmont. These faults shape a shear zone system that probably formed at the boundary of the North American craton, in relation with a transpressional regime of deformation.

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