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

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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 metamorphism 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 and/or question marks indicate uncertain timing.

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 EXPLANATION* other symbols Cambrian Permo-Carboniferous Cretaceous attitudes of planar and linear fabric elements symbols for axial traces of folds A A a r and Tertiary (numbers indicate fold generation) ASBILL POND FORMATION: (quartzite 57 L.3-axes of crenulations A So (bedding) S2 foliation (Lake overturned M2 almandine garnet •Cam RICHTEX FORMATION: or mesoscopic folds IKS. 178 and phyllite in the Carolina slate belt, r A inclined vertical inclined vertical Murray deformation) 5 anticlinorium isograd, tic marks on metamudstone and meta (Clarks Hill deformation) Delmar quartzite, schist and paragneiss in the high grade side, this state and federal «L& deformation Kiokee belt), -€am - metamudstone and •Gr wacke (phyllite), and mafic Xdgb g* ' KTu surface contained 83 , overturned highways metawacke,-€as - medium to coarse grained metavolcanic rocks j^» So facing direction indicated So and S2 parallel synclinorium within the Modoc zone (greenstone) in the inclined vertical overturned by sedimentary structures inclined vertical quartzofeldspathic metasandstone, -€ass - fine iL4 elongation lineatjion Clarks Carolina slate belt unconsolidated to weakly homoclinal fault, dashed where grained quartzofeldspathic metasandstone, Cgr - undeformed undeformed intrusive deformed intrusive 10 (Irmo deformation) y Hiii consolidated kaolinitic flexure inferred, dotted •€av - intermediate to felsic metatuff intrusive granitic rocks dioritic rocks metadioritic and deformation Si foliation S3 foliation (Clarks where concealed PERSIMMON FORK FORMATION: (phyllite Cdgr - deformed metagabbroic rocks sand and gravel \—* »• * 1 * * , inclined > A " « « « < < , inclined vertical (Delmar deformation) vertical Hill deformation) in the Carolina slate belt, schist, amphibolite and intrusive metagranitic ! antiform A A 4 A V * vr stratigraphie or in- V rocks vergence sense of steeply Irmo «> , y <"» ^ * V'*,.-T* * paragneiss in the Kiokee belt).-6pfv - intermediate trusive contact, dashed » „ 1 „ A* 76 plunging mesoscopic A folds in deformation tlv , >T <> •GpfV> r'/r\ '4 >1 »V to felsic metatuff,-€pfs - metamudstone, meta- So and Si parallel S4 foliation where inferred, dotted plan view (Irmo deformation) synform wacke and quartzofeldspathic metasandstone inclined vertical inclined vertical (Irmo deformation) where concealed

CROSS SECTIONS

Modoc Modoc Saluda synclinorium Emory antic inorium zone Delmar synclinorium Delmar synclinorium surface covered by -€r •€pfv -Cass Cdgr |«- surface covered by 0 of KTu -Gpfv ass 0 -150' of KTu

Kiokee rf^P

* Mesozoic diabase dikes and Quaternary alluvium occur in the mapped area but their distribution is not indicated. U.S. Geological Survey quadrangle maps Geology from field work by Secor (1963-1984), Snoke (1976-1984), Costello (1977-1979), Bramlett (1978-1980),and Kimbrell (1981-1984), and modified from Overstreet and Bell (1965), Clarke (1969), Johnson (1972), Metzgar (1977), Tewhey (1977), and Pirkle (1977,1981).

SECOR AND OTHERS, FIGURE 2 Geological Society of America Bulletin, v. 97, no. 11

Figure 3. Index map of a part of the southern Appalachian orogen showing some geographic and geologic features referred to in the text. Mas- sifs of crystalline basement rock in the Blue Ridge Province are shown in black. Point B shows the probable original position of the allochthonous crytalline basement massifs near point A (modified from Williams, 1978).

carbonate slices indicate that lower Paleozoic Ridge and Inner Piedmont are allochthonous. imbricated ramp and that the Alleghanian dé- platformal rocks must extend eastward beneath Crystalline basement massifs exposed along the collement continues southeast in the deep crust the entire Blue Ridge thrust sheet, which in this northwestern edge of the Blue Ridge Province beneath the Charlotte and Carolina slate belts. region is —125 km wide. Northwestward dis- (A, Fig. 3) are herein interpreted as allochthons The relationships summarized above sug- placement of at least 125 km is therefore indi- derived from the southeast edge of the subsur- gest that all of the rocks presently exposed in the cated for the Blue Ridge thrust sheet. Geological face autochthonous Precambrian block (B, Fig. Piedmont Province of Georgia and the Caroli- and geochronological data thus suggest that the 3). At least 175 km of northwestward displace- nas may have been displaced northwestward by Blue Ridge was affected by early to middle Pa- ment of the allochthons thus is indicated. This Alleghanian thrusting. The late Paleozoic pene- leozoic polyphase deformation and regional displacement is comparable to that estimated for trative deformation and regional metamorphism metamorphism and was displaced at least the same décollement in the Valley and Ridge along the southeast edge of the Piedmont 125 km to the northwest during late-metamor- Province from the interpretations of Pedlow Province (Fig. 1) may therefore also be a mani- phic to postmetamorphic thrusting. (1976) and Roeder and Witherspoon (1978) festation of Alleghanian deformation and repre- On the recent COCORP seismic-reflection and is therefore thought mainly to reflect Al- sent the internal zone of a complex Alleghanian profile across the southern Appalachians (Cook leghanian deformation. orogen. Recent geologic and geochronologic and others, 1979), a series of strong reflecting Possible continuation of the Alleghanian dé- studies in the southeastern Piedmont (Secor and horizons are present in the subsurface from the collement southeastward beneath the eastern others, 1986; Dallmeyer and others, 1986) sup- Valley and Ridge Province (at a depth of 4-6.5 Piedmont and Atlantic Coastal Plain (Harris port this interpretation. km) to the southeastern edge of the Inner Pied- and Bayer, 1979) is not clearly resolved. On the mont belt (at a depth of 9-11 km). Beneath the basis of COCORP seismic data, Cook and oth- THE ALLEGHANIAN OROGENY IN Valley and Ridge, these reflections are inter- ers (1979, 1981) suggested that the décollement THE EASTERN APPALACHIANS preted to be associated with an autochthonous ramps downward at the southeastern edge of the sequence of lower Paleozoic rocks resting non- Inner Piedmont and then continues southeast as Introduction conformably on middle Proterozoic crystalline a flat-lying surface of deformation at a depth of basement rocks and overridden by Alleghanian 10-20 km. After reprocessing some of the CO- In the eastern Appalachian Piedmont, the ef- allochthons of Paleozoic sedimentary rocks. The CORP seismic data, Iverson and Smithson fects of the Alleghanian orogeny are superim- southeastward continuity of these reflectors be- (1982) concluded that the Alleghanian décolle- posed on the following group of lithostrati- neath the Blue Ridge and the Inner Piedmont, as ment terminates in a root zone beneath the graphic terranes (Fig. 4; see also Williams and well as regional gravity and magnetic data northwestern side of the Charlotte belt (as origi- Hatcher, 1983, Fig. 1). (Hatcher and Zietz, 1980; Cook and Oliver, nally suggested by Hatcher and Zietz, 1978, 1. Piedmont terrene. The Piedmont terrene is 1981; Dainty and Frazier, 1984), indicates that 1980; see also Cook, 1983). Alternatively, Ando predominantly composed of biotite schist and an autochthonous North American crystalline and others (1983, Fig. 7) suggested that the mul- paragneiss that are thought to be derived from basement extends at least to the southeastern tiple, southeastward-dipping seismic reflections upper Proterozoic and/or lower Paleozoic con- edge of the Inner Piedmont and that the Blue beneath the edge of the Inner Piedmont mark an tinental slope and rise sediments deposited off

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Figure 4. Generalized terrane map of the eastern Piedmont and Coastal Plain (modified from Cohee, 1962; Zietz, 1982; Chowns and Williams, 1983; Williams srnd Hatcher, 1983; Farrar, 1985). Brunswick magnetic anomaly shown by heavy shading; south Georgia Mesozoic rift basin, light shading.

the southeastern edge of the Laurentian plate. 4. Goochland terrane. The Goochland ter- meyer, 1978; Dallmeyer and Hatcher, 1985; The Piedmont teiTane was deformed and meta- rane (Farrar, 1984, 1985) is interpreted as a Dallmeyer and others, 1986) and during ca. morphosed to the amphibolite fades in the early group of middle Proterozoic metaigneous and 415-485 Ma (Kish and others, 1979; Fullagar, and/or middle Paleozoic (Glover and others, metasedimentary rocks exposed in a tectonic 1981; Harper and Fullagar, 1981; Sutter and 1983) and was then thrust northwestward onto window through the Carolina terrane. The others, 1983, 1984). The approximate corre- the Laurentian plate during the Alleghanian Goochland terrane does not contain a lower Pa- spondence of the deformation histories of the orogeny (Cook and others, 1979). leozoic miogeoclinal cover sequence. It may rep- Piedmont and Carolina terranes with that of the 2. Pine Mountain terrane. The Pine Moun- resent either a basement block that was rifted southeastern edge of the Laurentian plate is tain terrane, containing middle Proterozoic from North America during the late Protero- permissive of an interpretation wherein the ter- metaigneous gneisses (Odom and others, 1985) zoic opening of Iapetus or an exotic crystalline ranes were accreted during the Ordovician and unconformably overlying lower Paleozoic terrane that originated away from North (Figs. 5a and 5b). miogeoclinal (?) strata, is exposed in a tectonic America. Numerous granitoid plutons were emplaced window through the Piedmont terrane and is Sedimentologic or stratigraphic linkages use- at ca. 285-330 Ma in an arclike belt extending bounded on the northwest by a west-dip- ful in constraining the Paleozoic accretionary through the Piedmont from Maryland to Geor- ping Mesozoic (?) normal fault (Schamel and history of the above terranes have not been gia (Fig. 5c; Sinha and Zietz, 1982). Plutons others, 1980; Kish and others, 1985; Nelson and found in the southern Appalachians. The accre- along the west side of this belt develope contact others, 1985a; R. J. Hooper, 1986, personal tionary history is constrained, however, by geo- metamorphic aureoles and have experienced commun.). The rocks in the Pine Mountain ter- physical, geochronological, and structural data. only brittle, postcrystallization deformation. In rane are similar to demonstrably North Ameri- Paleomagnetic studies (EUwood, 1982; Barton many places along the east side of the arc (Fig. can crystalline basement and cover sequences in and Brown, 1983; Dooley, 1983) of igneous 1), however, the plutons have been penetratively the Blue Ridge Province. The Pine Mountain rocks intruding the Carolina terrane indicate ac- deformed and regionally metamorphosed in the terrane may have originated as a basement block cretion to North America before the Carbonif- greenschist or amphibolite fades (Grauert, 1973; that was rifted from North America during the erous. The deformational history of the Pied- Kish and Fullagar, 1978; Secor and Snoke, late Proterozoic opening of Iapetus, or it may be mont and Carolina terranes is similar to that of 1978; Bobyarchick and Glover, 1979; Snoke an unfaulted block exposing parautochthonous the southeastern edge of Laurentia. Studies of and others, 1980a, 1980b; Speer and others, North American crystalline basement and cover rocks thought to have been deposited on or ad- 1980; Pavlides and others, 1982; Kish, 1983; that is thought to underlie the allochthonous jacent to Laurentia suggest tectonothermal activ- Russell and others, 1985). Piedmont terrane. ity along the continental margin during the 3. Carolina terrane. The Carolina terrane is a Permian-Carboniferous (ca. 250-330 Ma; Deformational Chronology sequence of upper Proterozoic and Cambrian Rodgers, 1970; Ferm, 1974a, 1974b; Padgett metavolcanic and metasedimentary rocks that and Ehrlich, 1978), Devonian (ca. 370-410 Ma; Detailed geological and geochronological compose the Belair, Kiokee, Carolina slate, and Tull, 1982), and Ordovician (ca. 450-480 Ma; studies (Dallmeyer and others, 1986; Secor and Charlotte belts. The Carolina terrane likely was Benedict and Walker, 1978; Shanmugam and others, 1986) indicate that three distinct defor- away from North America (because it contains Walker, 1978). Geochronological studies of mational episodes constitute the Alleghanian Atlantic Province trilobites, Secor and others, rocks in the Piedmont and Carolina terranes orogeny in the eastern Piedmont of South Caro- 1983) until its accretion in the early or middle suggest tectonothermal activity during the peri- lina. The earliest Alleghanian event (the D2 Paleozoic. ods ca. 270-315 Ma and ca. 340-360 Ma (Dall- Lake Murray deformation, Fig. 5d) was asso-

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 REGIONAL TECTONIC RELATIONS, SOUTHERN APPALACHIANS 1349 a. early Devonian N.W. S.E. N.W. S.E. early Paleozoic North Piedmont Carolina American passive margin terrane terrane suprastructure •t J \ ; : ^VÇÇ,- r^i-'inffâsTructuîe-.-; ^ ^ _ + + — ' + + + +\\ ++ IVn infraeiriininfrastructurh rae ^ + ' *. ' + + + + + + 2 + + + + + + + + ++ + + ++ + + + + + + \ + + + + future position of + Piedmont - Carollna\ + + + terran+ + e+ boundar+ + +y + + + + + detachmen+ + + +t surfac+ + e+ + + + + + + + + + + f. early or middle Paleozoic Di (Delmar) deformation Alleghanian D3 (Clarks Hill) S.E. N.W. ^ _ _ s.E. deformation, 285 - 295 Ma N.W.

r - D, suprastructure^ - -7 —^ ' <-° - -' c ^ , - c- %£r®enschist {¡¿.¡g? amphibolite" —S.A .• + + + 0, infrastructure + + + + + +X+ + + + + + + + + + +\+ + + + + + + + + \ + + + + + + + + + + +\ + + + + + + \ + + + + + + + + + + + + + -C—+ + + Alleghanian sole decollement

g- Alleghanian D4 (Irmo) c. 298 - 327 Ma deformation, 268 - 290 Ma northwestward prograding N.W. N.W. clastic wedge S.E. early Paleozoic North Piedmont Carolina American passive margin terrane terrane

+ + - - • / + + + + + +\+ + + + + + / + + + + ÌN.+ + + + + \ + + + + + +/ + + -t\+ + + + +\ + + + + +/+ + + + -i\+ Alleghanian 'sole* decollement

© . ® a «c ® 3Ï £ S a> 3 a o to M 3 t 3 S Alleghanian D2 (Lake Murray) »S» „ o ® S5 2« d. deformation, 295- 315 Ma ».£ C IÜ O C= k. (0 = ~ •= li — O N.W. S.E. 2 q-2 S q.2 <0 = o 2 léo S»9 ü ?<- (S N.W..I oSu " S.E. ^ suprastructur^ i»- e C-PT-'' Z • • /h + yt%\ D, infrastructure <\SC^'- + + .1X—-+ + + + + « + + 4N. + + + —>. ..eÇ-.'wov + + + + +Y + + + + + /+ + + 4V + + + in,rastruc,ure Ni + + + +X+ + + + + +.+ + + + + -i\+ + + + + + \—^ + + v +• +. + . j ' ..+ + + +\ ^ ^ • Alleghanian sole decollement • • •>-

Figure 5. Schematic diagram illustrating structural development of the southern Appalachian orogen during the Paleozoic. Suspect terranes in the Appalachian Piedmont were deformed, metamorphosed, and accreted to North America prior to the Alleghanian orogeny. During Dj, a subhorizontal interface between infrastructure and suprastructure developed at mid-crustal depth (b). During 298-327 Ma, numerous granitoid plutons were emplaced in the eastern Piedmont (c), and heating and deformation associated with the plutonic activity led to the development of D2 infrastructure (which overprints both Dt infrastructure and Dt suprastructure in the eastern Piedmont) (d). During 285-295 Ma, a westward-vergent overthrust belt developed (D3) as a consequence of continental collision, and flat-lying paleoisothermal surfaces between infrastructure and suprastructure are folded and displaced to the northwest (e, f)- During 268-290 Ma, the steep northwest-dipping limb of the Kiokee belt was overprinted by a shear zone (D4) having dextral, horizontal displacement (g). At the present erosion level, the alternating belts of low- and medium- to high-grade metamorphic rocks in the Piedmont are a consequence of D3 folding and imbrication of paleoisothermal surfaces (h).

dated with amphibolite fades regional meta- facies below and greenschist facies above), nu- faces during ca. 285-295 Ma (Figs. 5e and 5f). morphism (M2) and felsic plutonism in a merous sheets of felsic orthogneiss, and a defor- The alternation of high- and low-grade belts of mid-crustal infrastructure at ca. 295-315 Ma. mation front marking the upper limit of intense regional metamorphism in the South Carolina The gradational interface between infrastructure D2 penetrative deformation. A D3 (Clarks Hill) Piedmont (Fig. 3) is interpreted to be a conse- and overlying suprastructure contained a steep deformation is manifested as northwestward- quence of regional D3 folding. In the Charlotte M2 metamorphic gradient (between amphibolite vergent folding of Mj and M2 isothermal sur- belt, the rocks of an early to middle Paleozoic

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mid-crustal infrastructure (formed during the Dj cogenetic with the D4 (Irmo) deformation re- basin. Nelson and others (1985b) interpreted the Delmar déformatio n) are exposed in the core of corded in South Carolina. Brunswick anomaly along its entire length to a D3 antiform. The: Kiokee belt is interpreted as mark the late Paleozoic suture between the Tal- Alleghanian infrastructure (formed during the TECTONIC SYNTHESIS lahassee/Suwannee terrane and the rocks of the D2 Lake Murray deformation) in the core of the Appalachian orogen. As pointed out by Chowns D3 Kiokee antiform. The Belair and Carolina On the basis of borehole and paleontological and Williams (1983), the pre-Mesozoic subcrop slate belts are interpreted as suprastructural information, Wilson (1966) suggested that a dis- position of the suture (as determined from bore- rocks preserved in cores of D3 synforms. junct fragment of Gondwana (now known as hole and shallow magnetic data) does not Regional geophysical data (Hatcher and oth- the Tallahassee/Suwannee terrane, Williams everywhere coincide with the Brunswick anom- ers, 1977) suggest that the Kiokee belt can be and Hatcher, 1983) lies beneath the Coastal aly. The eastward divergence between the tectonically correlated with the Raleigh belt in Plain of southern Alabama and Georgia and Brunswick anomaly and the subcrop position of eastern North Carolina and Virginia. Structural northern Florida (Fig. 4). The Tallahassee/Su- the suture (Fig. 4) may indicate that in eastern studies in the Raleigh belt (Parker, 1979; Farrar, wannee terrane, which has been penetrated by Georgia and South Carolina, the subcrop posi- 1985) indicate thai, it also is an antiformal struc- numerous wells, consists of an undeformed tion of the suture has been transported to the ture. The scale, geometry, and vergence of the lower to middle Paleozoic sedimentary sequence northwest (relative to its position in the deep Kiokee antiform rssembles that of other major which unconformably overlies a metamorphic/ crust) by overthrust faulting during the Alle- antiforms in the Appalachian orogen (such as igneous basement complex of variable grade (for ghanian orogeny. the Blue Ridge and Green Mountain anticlino- example, Applin and Applin, 1965; Wicker and The polarity of Benioff or B-subduction dur- ria). Seismic-reflection data (Cook and others, Smith, 1978; Chowns and Williams, 1983; ing the late Paleozoic convergence of Gondwana 1979; Harris and others, 1983; Ando and others, Dallmeyer, 1984). Fossils within the Paleozoic and Laurentia is a fundamental uncertainty in 1984) suggest that the Blue Ridge and Green cover are clearly exotic with respect to North regional tectonic analyses of the Alleghanian Mountain anticlin oria developed as a result of American fauna (for example, Cramer, 1971; orogeny. We feel that three contrasting scenarios ramping of underlying thrust surfaces. We sug- Pojeta and others, 1976), and the over-all suc- are possible (Fig. 6). If B-subduction was be- gest a similar origin for the Kiokee antiform. cession displays direct sedimentological and neath the North American plate (for example, as The Kiokee belt is thus interpreted to be a D2 stratigraphic correlations with sequences exposed suggested by Sinha and Zietz, 1982), we would infrastructure exposed in the core of a D3 ramp in the Bove basin of Senegal and Guinea (for interpret the D2 Lake Murray deformation (syn- antiform which developed during northwest- example, Rodgers, 1970; Villeneuve, 1984). chronous with late Paleozoic plutonism during 40 39 ward translation of the crystalline thrust sheet Ar/ Ar ages of minerals separated from cen- ca. 295-315 Ma) to have resulted from devel- above an Appalachian décollement (Fig. 5f)- tral and southeastern parts of the variably met- opment of an arc-related infrastructure that The Charlotte beilt may be a similar thrust- amorphosed basement of the Tallahassee/Su- formed in the region of plutonic activity and wannee terrane range from 500-530 Ma and are related D3 antiform that exposes the Dj infra- high-temperature metamorphism in the upper structure which underlies the Carolina slate belt identical to mineral ages recorded in similar (Laurentian) plate during consumption of oce- (Halik, 1983; Fij;. 5h). During 268-285 Ma, rocks exposed in southeastern Senegal (R. D. anic crust between Laurentia and Gondwana parts of the Kiokee and Carolina slate belts in Dallmeyer, 1984, unpub. data). ^Ar/^Ar (Fig. 6a). Conversely, if B-subduction was the steep northwest limb of the D3 Kiokee anti- mineral ages within the Tallahassee/Suwannee beneath Gondwana (for example, as suggested form were overprinted by ductile deformation in terrane, however, are systematically lowered to by Pindell and Dewey, 1982), we would inter- northeast-trending; dextral shear zones associated ca. 300-315 Ma as the northern boundary of the pret D2 deformation, metamorphism, and plu- with the D4 Irmo deformation (Fig. 5g). terrane is approached (Dallmeyer, 1984, unpub. tonic activity to be a consequence of the initial data), suggesting that significant Alleghanian The Alleghanian deformational history of stages of continental collision (Fig. 6d). Finite- tectonothermal activity was concentrated along the eastern North Carolina Piedmont is similar element model studies of collisional orogens the terrane boundary. to that described above for the South Carolina (Bird and others, 1975; Toksôs and Bird, 1977; Piedmont. Rocks within the Raleigh belt were The position of the suture between the Talla- Bird, 1978a, 1978b; England and Thompson, deformed and regionally metamorphosed in the hassee/Suwannee terrane and the Carolina ter- 1984) have shown that synorogenic granites are amphibolite facie; during intrusion of granitoid rane can be approximately located using bore- not likely to be produced by frictional and ra- plutons at ca. 285-313 Ma (Farrar, 1985; hole and geophysical information. Chowns and dioactive heating which may result from tec- equivalent to D2 in the eastern South Carolina Williams (1983) suggested that the pre-Meso- tonic thickening of continental crust. In order to Piedmont?). Diachronous Rb-Sr biotite ages zoic subcrop position of the suture extends from explain the regional metamorphism and synoro- across the Butterwood Creek pluton along the southern Alabama to southernmost South Caro- genic plutonic activity south of the Indus suture east side of the Raleigh belt (Russell and others, lina (line A-A', Fig. 4). In a recent COCORP in the Himalayas, Bird (1978a) postulated that during the initial stages of continental collision, a 1985) suggest mid-Alleghanian southeastward seismic-reflection profile in northern Florida and portion of descending lithosphere pulled away tilting of isothermal surfaces. The Wake-Warren western Georgia (Nelson and others, 1985a), the from the continental crust of the Indian plate, antiform in the Raleigh belt (Parker, 1979; Far- suture between the Tallahassee/Suwannee ter- thereby exposing it to the hot asthenosphere rar, 1985) is inteipreted to be a middle Allegha- rane and the crystalline rocks of the Appala- (a process which he termed "delamination"). nian fold (equivalent to D3 in the eastern South chian orogen was interpreted to coincide with Finite-element model studies of the Himalayas Carolina Piedmont?) which exposes an Allegha- an intense, long-wave-length magnetic low (the indicate that temperatures adequate for regional nian infrastructure in the axial region. North- Brunswick anomaly). The southeastward trend metamorphism and partial melting can be at- east-trending dextral shear zones, which transect of the Brunswick anomaly across Georgia (Fig. tained in the continental crust a few million the eastern Piedmont in North Carolina and 4) is discordant to the trend of the Mesozoic years after delamination (Bird, 1978a). If B- Virginia (Farrar, 1985; Gates and others, 1986), south Georgia rift basin, and, therefore, the subduction was beneath Gondwana during the are herein interpreted to be contemporary and anomaly is thought to be unrelated to the rift

c. LAURENTIA GONDWANA LAURENTIA GONDWANA LAURENTIA GONDWANA

LAURENTIA GONDWANA

LAURENTIA GONDWANA LAURENTIA GONDWANA LAURENTIA GONDWANA I?" rsgi ir?— W W w —- ! /\

Figure 6. Schematic diagram illustrating some of the possible plate configurations during the Alleghanian orogeny. See text for discussion.

late Paleozoic, we feel that a process of delami- zones (Figs. 6c, 6f, and 6i). Because of the con- suggest that the Appalachian décollement under- nation must have occurred (Figs. 6d and 6g) to sistent dextral shear sense of late Alleghanian lies the entire Piedmont Province and that all of explain the D2 synorogenic regional metamor- faults along the eastern margin of the Appala- the rocks presently exposed in the South Caro- phism and plutonic activity recorded in the chian orogen (Gates and others, 1986), we lina Piedmont have been displaced at least 175 North American plate. interpret D4 as an intraplate manifestation of the km northwestward relative to cratonic North As previously noted, regional geological and relative dextral motion between the African- America. The Kiokee and Charlotte belts are geophysical relationships indicate at least 175 South American plate and North America dur- interpreted to be mid-crustal rocks exposed km of crustal shortening above the décollement ing final stages of continental collision, rather within regional fold structures which developed in the Appalachians. If B-subduction were be- than as escape tectonics (Tapponnier and Molnar, as a result of ramping of underlying thrust sur- neath North America, it would be difficult to 1976) during collision of irregular continental faces. The southeastward-dipping seismic reflec- envision how a comparable shortening could margins. tions beneath the Kings Mountain belt are have occurred in the subcrustal North American Recent geological and geochronological stud- interpreted to indicate a blind duplex structure lithosphère. Either a substantial amount of ies in the eastern Piedmont (Dallmeyer and controlled by the original southeastern edge of crustal material above the décollement was others, 1986; Secor and others, 1986) indicate Laurentian basement (Ando and others, 1983). transferred from the Gondwanan plate to the that episodes of tight westward-vergent folding We interpret the late Paleozoic uplift, erosion, and rapid cooling indicated by the 40Ar/39Ar Laurentian plate (flake tectonics of Oxburgh, (D3), crustal uplift, and rapid cooling occurred 1972) during continental collision (Fig. 6b), or during ca. 285-295 Ma. Within error limits, data in the eastern Piedmont to result from an ~175-km-wide portion of subcrustal North these events were contemporaneous with the convergence-related crustal thickening followed American lithosphère was subducted (Fig. 6e; development of the foreland fold-and-thrust belt by up-dip motion along the Appalachian A-subduction of Bally, 1975; Bally and Snel- along northwestern portions of the orogen (Fig. décollement. son, 1980). If B-subduction were beneath Afri- 2). All of the above deformational effects are ca-South America (Fig. 6h), the shortening interpreted to result from a continent-continent CONCLUSIONS envisioned to have occurred during D3 would collision between Laurentia and Gondwana in have had to be the result of continued plate the late Paleozoic. It is difficult to understand In summary, we suggest that in the southern convergence subsequent to delamination, and how a late Paleozoic collision with Gondwana Appalachians, the Alleghanian orogen is a mani- A-subduction would not have been required be- somewhere southeast of the Piedmont Province festation of the late Paleozoic oblique (dextral) cause the subcrustal lithosphere would have could lead to similar structural effects in both the convergence and collision of Laurentia and been removed during the preceding delamina- eastern and western Appalachians if the Appa- Gondwana. In spite of geophysical and geo- tion. lachian décollement is rooted beneath the Kings chemical arguments to the contrary (Iverson and

During D4, the steeply dipping northwest Mountain belt (Fig. 6e) as suggested by Hatcher Smithson, 1982; Sinha and Zietz, 1982), we limb of the Kiokee antiform was overprinted by and Zietz (1980) and Iverson and Smithson suggest that the Appalachian décollement under- ductile deformation related to dextral horizontal (1982). We, thus, favor the scenarios indicated lies the entire Piedmont Province and that the displacement along northeast-trending shear in 6a, 6b, and 6c and in 6d, 6g, 6h, and 6i. We Carolina slate belt and Charlotte belt have been

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