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Character of the Alleghanian Orogeny in the Southern Appalachians: Part III

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Character of the Alleghanian Orogeny in the Southern Appalachians: Part III Character of the Alleghanian orogeny in the southern Appalachians: Part III. Regional tectonic relations DONALD T. SECOR, JR. Department of Geology, University of South Carolina, Columbia, South Carolina 29208 ARTHUR W. SNOKE Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071 R. DAVID DALLMEYER Department of Geology, University of Georgia, Athens, Georgia 30601 ABSTRACT THE ALLEGHANIAN OROGEN IN sultant strata." The Dunkard Group in Ohio, THE WESTERN APPALACHIANS Pennsylvania, and West Virginia is the youngest Geological and geochronological studies in extant stratigraphic unit affected by Alleghanian the eastern Piedmont of the southern Appala- Introduction deformation (Rodgers, 1970, p. 64); however, chians have documented a polyphase late the age of the Dunkard is uncertain. Modern Paleozoic deformational chronology (D2-D4) Careful reading of Woodward's (1957) paper estimates range from Late Pennsylvanian (286 ± that is contemporaneous with the Allegha- on the chronology of Appalachian folding re- 12 Ma; Bode, 1975; Clendening, 1975) to Early nian orogeny recorded in the western Appa- veals that he intended the Alleghanian orogeny Permian (266 ±17 Ma; Berman and Berman, lachian foreland. The scale and vergence of to include (1) the deformation that was respon- 1975; Durden, 1975; Havlena, 1975; Lund, D3 anticlinoria in the eastern Piedmont are sible for the northwestward-prograding Carbon- 1975; Remy, 1975; Tasch, 1975). Stratigraphic similar to those of anticlinoria in the western iferous and Permian (?) clastic wedges in the evidence from the Valley and Ridge and Cum- Appalachians which are related to ramping of Valley and Ridge and adjacent Cumberland Pla- berland Plateau thus indicates that Alleghanian underlying thrust surfaces. We suggest that teau and (2) the deformation manifested as the deformation began along the southeastern edge the Appalachian décollement extends south- northwest-vergent fold and thrust belt, of the of the North American plate ca. 327 ± 21 Ma eastward beneath the entire Piedmont Prov- Valley and Ridge and adjacent Cumberland Pla- (as evidenced by the development of clastic ince and that all of the Piedmont rocks were teau, which affects all exposed Paleozoic rocks. wedges) and ended, at the earliest, in the Appa- displaced at least 175 km northwestward rel- lachian foreland shortly after the deposition of ative to North America during the Allegha- The Clastic Wedge the Dunkard Group ca. 286 ± 12 Ma (if the nian orogeny. Late Paleozoic deformational Dunkard is Late Pennsylvanian) or ca. 266 ± 17 effects in the eastern Piedmont thus are con- From southwestern Pennsylvania to Ala- Ma (if the Dunkard is Early Permian; Fig. 2). sidered cogenetic with Alleghanian foreland bama, the base of the Permian-Carboniferous deformation. We interpret the Alleghanian clastic wedge is marked by an upward transition Structural Development orogeny to be the result of oblique, dextral from marine limestone (Loyalhanna, Maxville, convergence and collision of Gondwana and Greenbrier, Newman, Bangor) to marine or During the past two decades, drilling and Laurentia. brackish shale (Pennington, Mauch Chunk; seismic studies (Gwinn, 1964, 1970; Jacobeen Ferm, 1974a). Although this stratigraphic and Kanes, 1975; Harris and Milici, 1977; INTRODUCTION boundary is time-transgressive (Ferm, 1974b; Perry, 1978; Tegland, 1978) have confirmed the Home and others, 1974; Haney, 1979), it occurs thin-skinned character of the Alleghanian de- This paper presents evidence that late Paleo- within rocks assigned to the Late Mississippian formation in the Valley and Ridge and Cumber- 1 zoic deformation recorded in the southeastern (321 ± 21 Ma). Petrologic studies and facies land Plateau first suggested by Rich (1934) and Piedmont developed synchronously with the Al- analysis in the Carboniferous Pocahontas basin supported by Rodgers (1949, 1964). Major de- leghanian orogeny (Woodward, 1957) in the in Kentucky, Virginia, and West Virginia (Fig. tachment horizons occur within the Rome- Appalachian foreland and that late Paleozoic de- 1) suggest the progressive unroofing of a "batho- Waynesboro Formation (Lower Cambrian), formation in the southern Appalachians is re- lithic" source terrane located in the Piedmont of Chickamauga Group (Middle Ordovician), lated to oblique, dextral convergence and the Carolinas during the Namurian and West- Reedsville Shale-Martinsburg Formation (Mid- collision of Laurentia with Gondwana during phalian (Davis and Ehrlich, 1974; Ferm, 1974a; dle and Upper Ordovician), Salina Group the late Paleozoic. Essential to this correlation is J. C. Ferm, 1983, personal commun.). Padgett (Upper Silurian), Chattanooga Shale (Devo- a comparison of the character and timing of the and Ehrlich (1978, p. 789) concluded that pa- nian-Mississippian), and Gizzard Group (Penn- Alleghanian orogeny in the Valley and Ridge leodrainage networks in the Pocahontas basin sylvanian). In most places, the sole faults of with the late Paleozoic deformation in the "indicate structural control by the same tectonic thrust sheets ramp up section northwestward, southeastern Piedmont. It is therefore necessary framework that uplifted and deformed the re- and major surface folds locally have developed to outline background information, both on the over buried ramps. Recent attempts at palinspas- current interpretation of the Alleghanian orog- tic restoration of the Valley and Ridge Province in Tennessee indicate at least 140 km of north- eny and on the geologic history of the southeast- 1 Ages based on the 1983 Decade of North Ameri- ern Piedmont. can Geology calibration time scale (Palmer, 1983). westward transport along the basal Alleghanian Geological Society of America Bulletin, v. 97, p. 1345 1353, 6 figs., November 1986. 1345 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/11/1345/3445165/i0016-7606-97-11-1345.pdf by guest on 23 September 2021 1346 SECOR AND OTHERS Figure 1. Index map of a part of eastern North America showing some of the geographic and geologic features referred to in the text. Location 1 is the Green Mountain anticlinorium, and location 2 is the Blue Ridge anticlinorium. The shaded areas in the southeastern Piedmont and in coastal Mew England are places where late Paleozoic penetrative deformation and amphibolite facies metamorphism are well documented. décollement (for example, Pedlow, 1976; King, 1964; Newman and Nelson, 1965; the Blue Ridge Province increases from lower Roeder and Witherspoon, 1978). Hardeman, 1966; Rankin and others, 1972; greenschist facies in the northwestern part to Upper Proteroroic and Lower Cambrian Espenshade and others, 1975; Hatcher, 1983; upper amphibolite facies (Carpenter, 1970) and sedimentary, metisedimentary, and metavol- L. S. Wiener, unpub. data). These thrust faults are locally to granulite facies in the central and canic rocks and middle Proterozoic crystalline interpreted, at least in part, to record Allegha- southeastern parts (Force, 1976, Fig. 1; Hatcher basement rocks have been brought to the surface nian movements because some (for example, and Butler, 1979, p. 74; Absher and McSween, in the northwestern Blue Ridge along a series of the Cartersville fault) override Carboniferous 1985). Geochronological studies (Butler, 1973; northwest-vergent thrust faults (Fig. 3; King and rocks in the footwall (Cressler, 1970). The in- Dallmeyer, 1975) indicate that a major episode Ferguson, 1960; Hadley and Goldsmith, 1963; tensity of Paleozoic regional metamorphism in of penetrative deformation and regional meta- morphism occurred in the Blue Ridge in the (development of late early to middle Paleozoic. Geologic studies Paleozoic infrastructure) (Hadley and Goldsmith, 1963; Hatcher, 1978, hornblende *%•/ cooling ages in 1979) indicate that some of the major thrusts in the Kiokee belt the Blue Ridge predate or are synchronous with late Paleozoic o D3 (northwestward - this regional metamorphism (and, hence, must tectonothermal ' vergent folding) be early to middle Paleozoic in age), whereas events in the < other thrusts displace mineral isograds and must eastern D4 (dextral movement along northeast - trending postdate regional metamorphism. Exposures of Piedmont ductile shear zones) low-grade upper Proterozoic and lower Paleo- biotite wAr/ 38Ar cooling ages zoic metasedimentary rocks in the Grandfather in the Kiokee belt Mountain window along the southeastern side 350 300 250 of the Blue Ridge Province indicate that the Blue -I 1 1 1 1 time in Ma Ridge allochthon was transported at least 56 km northwestward along late-metamorphic to postmetamorphic thrusts (Bryant and Reed, — Mississippian H-"— Pennsylvanian H-> Permian 1970). Along the Brevard zone in northwestern South Carolina, Hatcher (1971, 1978) and timing of events „ clastic wedge Hatcher and others (1973) have described defining the i depoSjtjon weakly metamorphosed slices of carbonate rock Alleghanian * which are chemically similar to the Cambrian- orogeny in the foreland f folding and Ordovician Knox Group exposed in the Valley western thrusting Appalachians and Ridge Province in eastern Tennessee. These carbonate slices are interpreted to have been stripped off the footwall during a late-metamor- Figure 2. A time-line comparison of late Paleozoic tectonothermal events in the eastern phic to postmetamorphic episode of northwest- Piedmont with events defining the Alleghanian orogeny in the western Appalachians. Dashes vergent thrusting along the Brevard zone. The
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