Timing of thrusting in the southern Appalachians, USA: model for orogeny?

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.

Blueschists and Eclogites of the World and Their Exhumation

“…Such a pattern is consistent with north-tosouth continental growth of the European continent. These high-P/T belts were formed by consuming plate processes between the BalticRussian Platform and Greenland-North America during the Late Proterozoic to Early Paleozoic (e.g., Hatcher and Odom, 1980), between North America (Laurasia) and Gondwana (Africa + Iberian Peninsula) (Lefort et al, 1982) during the Late Paleozoic, and between Europe and Africa (Apulia) since the Jurassic (Dewey et al, 1973). Blueschists in Europe were formed either before or during continentcontinent collisions.…”

Tectonic significance of paired gravity anomalies in the southern and central Appalachians

“…Nevertheless, some differential uplift, that would probably offset any horizontal feature, is indicated by the variation in the metamorphic grades across these regions. The vertical movements would have been effected by the long history (Ordovician-Carboniferous) of faulting in the region (Hatcher and Odom, 1980). Griffin (1974b) also appealed to vertical en bloc uplift of the Inner Piedmont mobile thermal core by isostatic adjustment to better explain cataclastized portions of adjacent deformed belts.…”

The Giles County, Virginia, seismic zone– Seismological results and geological interpretations

“…The sequence of deformation in the sedimentary parts of the central Appalachians and adjacent parts of the southern Appalachians is known to have been long and complex on both map and outcrop scales (Geiser, 1977(Geiser, , 1981Perry and deWitt, 1977, p. 39-40;Perry, 1978;Bartholomew, 1979;Nickelsen, 1963Nickelsen, , 1979Nickelsen, , 1980Van der Voo, 1979a, b;Berger and others, 1979;Hatcher and Odom, 1980;Roeder and Boyer, 1981;Wright, 1981;Bartholomew and others, 1982;Bick, 1982;Gray, 1982;Henika and others, 1982;Webb, 1982;Wheeler, 1982Wheeler, , 1986. Dahlstrom (1970) documented type examples in the foothills of the Canadian Rockies where the typical sequence of relative ages of thrusts, becoming younger toward the craton, is locally reversed.…”