Mesoscopic and microscopic kinematic indicators throughout ca. 690 Ma peralkaline plutonic rocks in the footwall of the Fries fault zone, Blue Ridge province, southwest Virginia, record dip-slip normal (southeast-down) movement under greenschist facies deformation conditions. Mylonites, cataclasites, and associated precursor mineralized fractures and dikes are consistent in geometry and movement history with extensional faulting. Although over 50 km of northwest-directed thrust movement was proposed by earlier workers for parts of the >300-km-long Rockfish Valley-Fries fault system, along major sections of the Fries fault the only concrete evidence for thrusting is in the form of late-stage shear bands that crosscut the earlier, extension-related mylonitic fabric. We suggest that the entire Rockfish Valley-Fries fault zone formed during the late Precambrian extension of the (now) eastern margin of the North American craton to create the Iapetus Ocean, and was locally reactivated as a thrust zone during Paleozoic time.

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