Timing of Thrusting in the Southern Appalachians, USA 323

J. geol. Soc. London, Vol. 137, 1980, pp. 321-327, 4 figs. Printed in Northern Ireland.

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

R. D. Hatcher, Jr. & A. L. Odom

SUMMARY: 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 thrustsof the Valley and Ridgdumberland Plateau are of the dkcollement type, resultingfrom 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 inthe Blue Ridge, Chauga belt and Inner Piedmont, related to Taconic (450480Ma) metamorphism. Several later thrusts in the Blue Ridge and the GoldHill-Silver Hill fault in the Piedmonthave 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 be- haviour. Mylonites near the baseof 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 deformationcaused it to ramp over the rear of the Blue Ridge thrust sheet. There is a space-time transgressionof thrusting inthe 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 arerestricted to theflanks of theorogen. 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.

Thesouthern and central Appalachian Valley and and the nature of the rocks composing each segment Ridge has promoted the growth of several ideas and (Fig. 1). controversies on thrust faulting. Someof the first ideas The Valley andRidge and Cumberland Plateau on the nature of thrust faulting originated here (e.g. were deformed latein the Palaeozoic. Foreland thrust- Willis 1893). Until recently itwas assumed, with a few ing of the classic thin-skinned dCcollement style occur- exceptions, that much of the faulting in the crystalline redhere during the Permian Alleghanian orogeny. Appalachians occurred in the Alleghanian orogeny at Thrusts are westward-directed, toward the craton. Age the same time as that in the Valley and Ridge. With control on the timing of thrusting is contained in the the advent of geochrqnology and detailed geological largely unmetamorphosedfossiliferous sequences in- mapping of small areas in the crystallines, a multiple volved in deformation (e.g. Hardeman1966; Milici history of faulting has emerged. 1970; Harris & Milici 1977). The purpose of this paper is to present some of the The southern Appalachian Blue Ridgeconsists of an recentdata on timing of thrusting in thesouthern assemblage of continentalslope and shelf deposits, Appalachians and to place this into the context of an predominantly clastic sedimentary, metasedimentary, orogen which had a historyof separable deformational and metavolcanic rocks, with granitic plutons and ul- events. tramafic rocks. The granitic plutons, ultramafic rocks and metavolcanic sequences are confined mainlyto the Majorsubdivisions and major eastern Blue Ridge, while largely non-volcanic sequ- faults of the southern ences resting on a *1100Ma basement make up the Appalachians westernBlue Ridge (Rankin et al. 1973;Hatcher 1978). The principal metamorphic peak occurred at The southern Appalachians consist of several subdivi- least 450-480 Ma ago (Bulter 1972; Dallmeyer 1975). sionsbased upon tectonic style, metamorphic grade Thismajor event reached upper amphibolite and 0016-7644/80/0500-0321$02.00 @ 1980 The Geological Society

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GEORGIA ALABAMA

50 0 50 MILES . .. 500 5: KILOMETRES

FIG. 1. Map of the southern Appalachians showing the various subdivisions and tectonic features.

possibly granulite facies conditionsin parts of the Blue boundary extending from Virginia to Alabama (Fig. Ridge. A later greenschist overprint (possibly Aca- 1). It has been suggested that this is a cryptic suture dian) also affected these rocks. (Hatcher 1978). However, portions of this boundary A family of pre-metamorphic faults, collectively cal- near the Georgia-North Carolina border experienced led the Greenbrier Fault in the western Blue Ridge considerable movement priorto metamorphism. Other (Hadley & Goldsmith 1963; King 1964), is thought to segments of theboundary are obviouslypost- be Ordovician or older. Other large faults in the Blue metamorphic. Ridge have until recently been assumedto be Allegha- The Piedmont of the Carolinashas been divided nian, particularly those in the Blue Ridge in the vicin- into 5 or 6 subdivisions by King (1955) and Hatcher ity of the Grandfather Mountain window and to the (1972,1978) (Fig. l). The rocktypes present range NW (Bryant & Reed 1970). All these faults-are post- from clean to impuremetasandstones, and meta- metamorphic, and several are Alleghanian, but there greywackes, pure toimpure carbonates, metaconglom- isgroupa of faultshere whichmust bepre- erates,metapelites, and felsic and mafic volcanic Alleghanian (Hatcher 1978). rocks. Plutons ranging from ultramafic to granitic also Severalpre-metamorphic faults have been recog- occur. General descriptions of the rocks of the differ- nized in the eastern Blue Ridgeof North Carolina and ent subdivisionswere given by Hatcher (1972). NE Georgia (Hatcher 1976; Acker, in prep.).All may Metamorphicgrades range from sub-greenschist in be part of the more westerly Greenbrier Fault family, parts of the Carolinaslate belt tothe sillimanite- or theymay be separate. Other pre-metamorphic muscovite sub-facies of the amphibolite facies in the faults will undoubtedlybe discoveredas additional Inner Piedmont and Charlotte belt. Metamorphism is detailed mapping is completed in the Blue Ridge. interpreted to have occurred at least 430-450 Ma ago The proposed Hayesville-Fries Fault separating the across the westernand central Piedmont of the volcanic, ultramafic-, granite-bearing terrain from the Carolinas (Butler 1972). To the S, the western Pied- non-volcanic, abundant basement-bearing terrain is a mont mayhave experienced amajor progressive

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metamorphicepisode c. 370Ma (Acadian)(Russell 1978), and much of the eastern Piedmont was over- 86c Fort Mountaln Gnelss /- printed by a c. 260 Ma (Hercynian) amphibolite facies event (Kish et al. 1978). Majorfaults of thePiedmont (Fig. 1) generally record one or more pre- or syn-metamorphic episodes of ductilemovement followed by episodes of post- metamorphicductile movement. Additionally, the Brevardzone records a still laterbrittle movement event.Several faults of theeastern Piedmont fault system (Hatcher et al. 1977) may also record a brittle movement history.

Ages of faults Some of the important informationwhich exists on the actual timing of movements on faults in the southern Appalachians comes from Rb-Sr, whole-rock isochron ages of recrystallized mylonites, blastomylonites, and :0 2 30

phyllonites. It must be admitted that the techniques R- ba7 are yet to be firmly established. Homogenization of Sr S, 86 isotopes within ductile fault zones (a necessary condi- tion for the dating method) has occurred in somecases FIG. 2. Rb-Sr whole-rock isochron for samples of (e.g. Abbott 1972; Odom & Fullagar 1973) but not in Fort Mountain gneiss in N Georgia. others. Late Devonian ages have been obtained with apparent success in several cases where such studies indicate a ‘Grenville’ age for the orthogneiss (Russell have been made in the southern Appalachians. 1976). A 368~t9Ma Rb-Sr whole-rock isochron for Odom & Fullagar(1970, 1973) obtained a Rb-Sr the mylonites (Fig. 2) is thought to date a Sr isotopic whole-rockisochron age of 356*20 Ma for blasto- homogenizationassociated with ductile deformation. mylonites from the Brevard zone of North Carolina. The isotopic data and argumentswill be presented in a They obtained Cambrian whole-rock and zircon ages later paper. for theforHenderson Gneiss protolith. Because Butler & Fullagar (1978) showed that, based on the metamorphism occurred prior to the formation of the ages of plutons and cross-cutting relations, the Gold blastomylonites and did not disturb the Rb-Sr whole- Hill-Silver Hill fault zone has a similar Devonian age rock systems of the Henderson Gneiss, Odom & Ful- of movement. lagarassociated the isotopichomogenization event with the recrystallization and recovery of strained and Discussion crushed material in the ductile fault zone. A slightly older age (but within the range of the age uncertainty) Major thrusting began in the southern Appalachians at was reported for incompletely recrystallized mylonites leastas early as the Ordovicianand was probably from the Brevard Fault at the samelocality by Bond & coevalwith oneor more of thepre- to syn- Fullagar (1973). metamorphicepisodes of foldingdiscernable in the An isochron age of 375 f 11 Ma for phyllonite from metamorphic core. Faults of this episode have been the Bartlett Ferry-Goat Rock fault zone of the south- largelyobscured by subsequent deforrnational and ern Georgia Piedmontwas reported by Russell(l976). metamorphic events. Mostof these faults have littleor It hasbeen suggested on the basis of regional interpre- no mylonite along them and must therefore be recog- tation that this fault zone is coeval with the Brevard nized by careful study of the field relationships. (Clarke1952; Bentley & Neathery1970). The two The immediate late to post-metamorphic thrusts of dated localities are agreat distance apart, but the thesouthern Appalachians fall intoatleast 2 geochronological data are certainly consistentwith the categories:those which formed during the waning earlier suggestion. stages of the principal (Taconic) metamorphic event, Along the western edge of the Georgia Blue Ridge, and those produced during the Devonian (Acadian?) in the vicinity of Fort Mountain, Georgia, slices of the deformation. The late metamorphic thrusts in many Fort Mountain gneiss (Needham 1972) are exposed in instances were not extensively mylonitized along their faults just above the Great Smoky fault (base of the contacts,yet there is juxtaposition of metamorphic BlueRidge thrust sheet). Here the gneisshas been zones along them. The Alto allochthon in the western converted to a mylonite. Geochronological investiga- Inner Piedmont and Chauga belt of NE Georgia and tions of theFort Mountain gneiss and mylonites NW SouthCarolina is sucha thrust. Some of the

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CLOSE OF GRENVILLE OROGENY

LATE P-€ EXTENSION

LATE P-€ SPREADING EBR-IP CB-CS0

LATE P€- E€

E€-EOrd

Ord -Si I CHB IP KM6

Dev

Carb - Pe

FIG. 4. Sequential sections showing a possible mode of development of the southern Appalachian orogen. EBR: eastern Blue Ridge. IP: Inner Piedmont. CB: Charlotte Belt. CSB: Carolina Slate Belt. CHB: Chauga Belt. KMB: Kings Mountain Belt. BR: Blue Ridge. V & R: Valley and Ridge. BZ: Brevard zone. MF: Modoc Fault. AF: Augusta Fault.

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nappes recognizedby Gran (1974)in the South Fault (Russell 1976) (and thus probably for the Goat Carolina Inner Piedmont probably also fall into that Rock and Towaliga faults) and the Brevard Fault. This category. The Brevardzone and Towaliga-Bartlett activity formedmany of the large crystalline thrust Ferry-GoatRock faults contain an early history of sheets of the Blue Ridge. The Gold Hill-Silver Hill movement that probably also extends into the Taconic faultsystem, which lies along theCharlotte belt/ metamorphic event. Carolina slate belt boundary, was formed at this time. Post-metamorphicthrusts of the secondtype are Its sense of movement has not been determined with widespread in the southern Appalachian Blue Ridge certaintybut its location is on theedge of the and Piedmont, and several have already been men- metamorphic core. tioned. Practically all containmylonite of varying The final events of the space-time transgression are thicknesses. The Linville Falls fault in the Grandfather theemplacement of the Valley andRidge thrusts, Mountain window is an excellent but undated candi- reactivation of the Blue Ridge thrust and Brevard, and date to be another fault of this generation. formation of the eastern Piedmont fault system. All A distinctionmay bemade between mylonites constitute the last majorcompressional events to affect formed during the Devonian event and those formed the Appalachians and occur on the outermost flanks. earlier. The Devonian mylonites are clearly retrogres- It would be a simple matter to dismiss the transgres- sive, involving the formation of greenschist facies as- sion described above as a series of events taking place semblages and textures which indicate an overall re- inan orogen which remained intact throughout the ductionin grain size with minor formation of new Palaeozoic.However, depositional environments, minerals. Recovery textures are evident in quartz, but locations of ultramafic and other igneous bodies, and deformation textures (ribbon quartz)may be well pre- volcanicassociations requirethat this deformational served. plan be related to the closure of one or more small Earlier mylonites, where present, appear mylonitic oceans and eventual collision with Africa (Fig. 4). in hand specimen but the microtexture is one of total A complexseries of events whichresults in an recovery. The temperature conditions of metamorph- inside-out deformation plan might have seemed for- ism were still sufficiently elevated to permit the rock to tuitous upon initial inspection. Yet other orogens,such recover almost completely from the strain to which it as the North American Cordillera (Burchfiel & Davis had been subjected. 1975), have had a similar complex history, with com- A space-time transgression of thrusting exists in the parable closures of small oceans resulting in the for- southernAppalachians (Fig. 3). Thistransgression mation of a composite mountain chain. began in the Inner Piedmont and Blue Ridge before andduring Taconic metamorphism in what became the high grade portions of the orogen. The Brevard ACKNOWLEDGEMENTS.We gratefully acknowledge support by the Geological Surveys of North and South Carolina and Fault, large nappes in the Inner Piedmont, and several Georgia, and US. National Science Foundation grants GA- large faults of the Blue Ridge, probably began west- 1409, GA-20321, EAR 76-15564and EAR 78-26316 to ward transport at this time. Geochronological studies R.D.Hand grant DE72-01453-A01 toA.L.0; also point to major faulting again during the late Devonian Rosemarie Raymond and Dennis Cassidy for preparing the (Russell 1976). This is indicated for the Bartlett Ferry figures.

References

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Received 16 March 1979. R.D. HATCHER, JR. & A. L. ODOM,Department of Geology,Florida State University, Tallahassee, Florida 32306, USA.

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