Abstract

Field and geochronological studies in the southern Appalachians reveal a space-time relationship of thrust and other large faults to their relative positions in the orogen, and their times of formation in relation to thermal-metamorphic peaks. Alleghanian thrusts of the Valley and Ridge-Cumberland Plateau are of the décollement type, resulting from compression of the foreland during the waning stages of mountain building.

At least one pre-metamorphic thrust is known in the Blue Ridge. Other pre-, syn-and late metamorphic thrusts have been recognized in the Blue Ridge, Chauga belt and Inner Piedmont, related to Taconic (450–480 Ma) metamorphism. Several later thrusts in the Blue Ridge and the Gold Hill–Silver Hill fault in the Piedmont have been dated as Devonian. Syn- to post-metamorphic faults in the eastern Piedmont are Hercynian. These were developed during or following the Hercynian metamorphism, which overprints earlier events.

Several large faults, notably the Brevard, record episodic movement histories spanning much of the Palaeozoic. The Brevard Fault had a pre- to syn-metamorphic (Taconic) movement history. Homogenization of Sr isotopes occurred in blastomylonites in the Brevard Fault after regional metamorphism, dated at 356 ± 20Ma. Both these early events involve ductile behaviour. Mylonites near the base of the Blue Ridge thrust sheet developed by 367 ± 20 Ma, yet elsewhere the allochthon locally overrides rocks as young as Carboniferous, indicating later (Alleghanian?) transport of the allochthon. Phyllonites in the Goat Rock–Bartlett Ferry fault zone are dated at c. 380±20Ma. One or more later events (Hercynian?) occur in the brittle realm. The Brevard Fault probably served as the root zone for early-to mid-Palaeozoic thrusts in the Blue Ridge. Later brittle deformation caused it to ramp over the rear of the Blue Ridge thrust sheet.

There is a space-time transgression of thrusting in the southern Appalachians, beginning with early pre-, syn- and late metamorphic (Taconic) thrusting in the metamorphic core. Devonian (Acadian) thrusting and high angle faulting also affected the metamorphic core. Later (Hercynian–Alleghanian) faults are restricted to the flanks of the orogen. This space-time relationship of faulting to thermal peaks and position in the orogen should be observable in other well-exposed orogenic belts and other portions of the Appalachian–Caledonide system.

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