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

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Character of the Alleghanian orogeny in the southern Appalachians: Part I. Alleghanian deformation in the eastern Piedmont of South Carolina

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 KENNETH W. BRAMLETT Shell Western E and P, Inc., Box 831, Houston, Texas 77001 OLIVER P. COSTELLO Samson Resources, 801 Travis Street, Suite 1630, Houston, Texas 77002 OLLIE P. KIMBRELL Soil & Material Engineers, Inc., 3025 McNaughton Drive, Columbia, South Carolina 29223

ABSTRACT ward, 1957) is a fundamental problem. This report summarizes results of new field work and outlines interpretations concerning late Paleozoic de- The eastern Piedmont Province in South Carolina contains a formation along the Fall Line in west-central South Carolina. This infor- sequence of Cambrian volcanic and sedimentary rocks that was pene- mation is essential to development of a more general analysis of the tratively deformed (Dj) and regionally metamorphosed (Mi) to the Alleghanian orogeny in the southern Appalachians (Secor and others, greenschist facies during the early and/or middle Paleozoic. The east- 1986). era Piedmont was subsequently affected by late Paleozoic (Allegha-

nian) polyphase deformation (D2-D4) and regional metamorphism. The OVERVIEW earliest Alleghanian event (D2) is associated with amphibolite facies regional metamorphism and felsic plutonism in a mid-crustal infra- In the southeastern Piedmont, late Paleozoic deformation events af- structure at ca. 295-315 Ma. The gradational interface between infra- fected a region that had previously been strongly deformed in the early structure and overlying suprastructure contained a steep M2 and/or middle Paleozoic. Some aspects of the pre-late Paleozoic geologic metamorphic gradient (between amphibolite facies below and green- history are important in this discussion and are included in the following schist facies above), numerous sheets of felsic orthogneiss, and a de- overview. formation front marking the upper limit of intense D2 penetrative Our geological studies have been conducted along the Fall Line in deformation. D3 is represented by northwestward-vergent folding of west-central South Carolina in an area that includes parts of the Piedmont St and S2 foliations and M2 isothermal surfaces. The Kiokee belt in and Atlantic Coastal Plain Provinces (Figs. 1 and 2; for Fig. 2, see folded South Carolina is interpreted as a D2 infrastructure exposed within insert accompanying this issue). Here, crystalline rocks of the Piedmont are the core of a D3 antiform. This interpretation suggests that an Alle- unconformably overlain by semi-consolidated Upper Cretaceous and Ter- ghanian infrastructure may be present in the subsurface beneath much tiary strata of the Coastal Plain. The unconformity at the base of the of the Piedmont and Coastal Plain. The polygenetic Modoc zone Coastal Plain sequence dips gently southeast at ~2 m/km. The outcrop forms the northwest boundary of the Kiokee belt. It is interpreted to trace of the contact between the Piedmont and Coastal Plain is extremely represent an interface between D2 infrastructure and suprastructure irregular because it is maturely dissected and has a local topographic relief which was rotated into a steep, northwest-dipping attitude during of -75 m. Coastal Plain sequences cap hills and ridges in the central part development of the Kiokee belt antiform. Between ca. 267 and 290 of the study area. In the southeast, Piedmont crystalline rocks are exposed Ma, portions of the Kiokee belt, Modoz zone, and Carolina slate belt in numerous erosional inhere through the Coastal Plain. Additional infor- were overprinted by ductile deformation (D4) along steeply dipping, mation on the Coastal Plain is available in Siple (1967), Smith (1980), northeast-trending dextral shear zones. Bramlett and others (1982), Nystrom and Willoughby (1982), and Colqu- houn and others (1983). INTRODUCTION Previous investigators have subdivided the Piedmont Province into several northeast-trending lithotectonic belts (Crickmay, 1952; King, Geological and geochronological studies have recently indicated that 1955; Hatcher, 1972). In South Carolina, belts characterized by low- to an important belt of late Paleozoic (Alleghanian) penetrative deformation medium-grade regional metamorphism (Belair, Carolina slate, Kings and amphibolite facies metamorphism is present along the southeastern Mountain, Chauga; see Fig. 3 of Secor and others, 1986) alternate with edge of the Piedmont Province and beneath the adjacent Atlantic Coastal medium- to high-grade belts (Kiokee, Charlotte, Inner Piedmont). Our Plain (Secor and Snoke, 1978; Durrant and others, 1980; Snoke and study area includes parts of the Carolina slate and Kiokee belts. others, 1980a; Pavlides and others, 1982; Glover and others, 1983; Farrar, In west-central South Carolina, metavolcanic and metasedimentary 1985; Russell and others, 1985; see Fig. 1 of Secor and others, 1986). The map units trend obliquely across the Carolina slate belt and intersect the uncertain relationship of this deformed belt in the eastern Piedmont to the bordering Charlotte and Kiokee belts. classical Alleghanian deformation in the western Appalachians (Wood- The oldest stratigraphic unit recognized in the South Carolina slate

Geological Society of America Bulletin, v. 97, p. 1319-1328, 8 figs., November 1986.

1319

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belt is the Persimmon Fork Formation (Secor and Wagener, 1968), Carolina slate belt, the Asbill Pond contains layers and lenses of quartz which is exposed in the core of the F! Emory anticlinorium (Fig. 2). The siltstone and quartz sandstone displaying sedimentary structures suggestive Persimmon Fork exceeds ~3 km in thickness and is predominantly an of deposition on a tidal shelf (Secor and Snoke, 1978). The upper part of intermediate to felsic ashflow tuff. Geochronological studies of rocks that the Asbill Pond contains a mudstone sequence which has yielded several are considered equivalent to the Persimmon Fork Formation indicate genera of late Middle Cambrian trilobites, some of which are diagnostic of deposition during ca. 530-580 Ma (Butler and Fullagar, 1975; Carpenter the Atlantic faunal province (Secor and others, 1983; Samson, 1984). In and others, 1982; Dallmeyer and others, 1986). the Kiokee belt, mudstone sequences of the Asbill Pond are metamor- A sequence of thin-bedded to massive mudstone and siltstone, with phosed to biotite schist and biotite paragneiss. subordinate greenstone, occurs in the core of the Fi Saluda synclinorium The oldest recognizable deformation phase (Dj) in the study area was in the northwestern corner of the study area (Fig. 2). Sedimentary struc- an episode of tight to isoclinal folding with attendant development of slaty tures in these rocks suggest turbidite deposition below wave base (Brown, cleavage under conditions of greenschist facies regional metamorphism. F| 1971; Kearns and others, 1981). These rocks are here correlated with the folds occur at a variety of scales, and the S] slaty cleavage is generally Richtex Formation of Secor and Wagener (1968) because of lithological penetrative. Major F] fold structures occur which have wavelengths of as similarity. Structural data and meager sedimentary younging criteria sug- much as several kilometres (Saluda synclinorium, Emory anticlinorium, gest that the Richtex overlies the Persimmon Fork, although it is uncertain Delmar synclinorium) and are primarily responsible for the outcrop pat- whether the contact is depositional or tectonic. The thickness of the Rich- tern of stratigraphic units in the Carolina slate belt. Stratigraphic and tex probably exceeds 3 km; however, precise measurement is precluded by structural evidence (Costello and others, 1981; Fig. 2) indicates that a postdepositional penetrative strain. major south westward-plunging Fj synclinal fold (the Delmar synclino- The Asbill Pond formation (informal usage) conformably overlies the rium) occurs in the Carolina slate belt between Lake Murray and Bates- Persimmon Fork Formation in the southeastern part of the study area. It burg. Southwest of Batesburg (Figs. 2 and 3), the axis of the Delmar has a thickness in excess of 5 km and is represented by a sequence of synclinorium can be traced into the Kiokee belt. In this region, mudstone metasedimentary rocks locally interbedded with mafic to felsic metavolcanic rocks. The Asbill Pond is exposed in the northeast-trending F[ Delmar synclinorium, which crosses from the Kiokee belt to the Carolina 34° 30' slate belt between Johnston and Batesburg, South Carolina (Fig. 2). In the

kilometers

Figure 1. Generalized geologic map of west-central South Carolina showing some geographic and geologic features referred to in the text. Modified from Figure 2 and from Wagener (1977), Secor and others (1982), Halik (1983), Hauck (1984), and Kirk (1985).

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and quartz-rich siltstone and sandstone units in the Asbill Pond formation Di fabric (Butler and Fullagar, 1978; Fullagar, 1981). Recent studies by can be traced into the Kiokee belt where they are represented by para- Halik (1983) and Kirk (1985) suggest that the Charlotte belt represents a gneiss and quartzite units, respectively. A prominent band of biotite tectonic infrastructure that evolved beneath the Dj suprastructure of the paragneiss flanked by quartzite extends southwestward across the Kiokee Carolina slate belt (Fig. 5b of Secor and others, 1986) and that present belt in the Emory, Ridge Springs, Johnston, and Edgefield Quadrangles. juxtaposition of the Di infrastructure and suprastructure is a result of This band is interpreted to represent a continuation of the Delmar syncli- post-Di folding. norium axis into the Kiokee belt. This suggests that lower Paleozoic meta- Numerous upper Paleozoic plutons were emplaced along a belt ex- volcanic and metasedimentary rocks in both the Carolina slate and Kiokee tending through the Piedmont from Maryland to Georgia (Fullagar and belts experienced D] and Mj. Butler, 1979; Speer and others, 1980; Sinha and Zietz, 1982). Most of Dj is herein named the "Delmar deformation," and the region be- these are not penetratively deformed, although those near the Fall Line in tween the northeast end of the Clouds Creek igneous complex and Lake west-central South Carolina contain evidence of moderate to strong duc- Murray (Fig. 2) is designated as its type area. Because Dj affects a large tile deformation (Snoke and others, 1980a). Local evidence of deforma- area, there is no single outcrop or closely spaced group of outcrops in tion has also been observed along the east side of the belt in Georgia (Speer which the evidence for the D( Delmar synclinorium can be viewed in its and others, 1980), North Carolina and Virginia (Kish and Fullagar, 1978: entirety. An extended type area was therefore selected to provide a repre- Bobyarchick and Glover, 1979; Pavlides and others, 1982; Kish, 1983 sentative sample of both the structural and stratigraphic elements of the Russell and others, 1985), and Maryland (Hopson, 1964; Grauert, 1973 synclinorium. Reconnaissance studies suggest that the effects of the Dj deforma- ATLANTIC tional phase are correlative with widespread folding and regional green- CO schist to amphibolite facies metamorphism in the Carolina slate belt and COASTAL PLAIN Charlotte belt from Virginia to Georgia. The regional D[ event must have ÜL ROCKS postdated the deposition of Cambrian strata in the Carolina slate belt (ca. p 530-580 Ma) and must have occurred prior to emplacement of a suite of KTu 385- to 415-Ma plutonic rocks in the Charlotte belt which do not record OC LU undifferentiated Ol- kaolinitic sand era EXPLANATION FOR FIGURE 1

BELAIR BELT KIOKEE BELT CAROLINA SLATE BELT CHARLOTTE BELT ROCKS ROCKS ROCKS ROCKS co =) + + + + +H O + Car + H cc + -rM+ + LU LL undeformed granitic rocks §1 m Caa r + Cdgb Cgr

Î* 1 ¿I « SDgr, •SDgr .'^SDgb'.V ¡¡1 undeformed undeformed undeformed undeformed granitic rocks gabbroic rocks COLLI granitic rocks gabbroic rocks o €a €r €ms

Asbill Pond Richtex biotite paragneiss formation : quartz- Formation : meta- (metasedimentary) •Gvs ite, metamudstone mudstone and and metawacke metawacke

Z < J €u undifferentiated undifferentiated ? o i y CC metamudstone and paragneiss, schist, m intermediate to quartzite and Persimmon Fork intermediate to < felsic metavolcanic amphibolite Formation : inter- felsic metavolcanic rocks mediate to felsic (€fv) and meta- O metavolcanic and igneous (€dgr) rocks volcaniclastic rocks

€mv fault or ductile deformation zone overturned folds stratigraphic or undifferentiated dotted where concealed anticline syncline intrusive contact metaigneous, metavolcanic and meta- mafic to inter- sedimentary rocks mediate metavolcanic and metaigneous rocks

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unconsolidated to weakly consolidated Kaolinitic sand and gravel (Cretaceous and Tertiary)

••v igneous and metaigneous rocks Ci • » (Carboniferous)

upper metamudstone member of the Figure 3. Geologic Asbill Pond formation (Cambrian) sketch map of the Batesburg- Edgefield area showing medium to coarse grained quartzofeldspathic the inferred continuity of the metasandstone member of the Asbill Pond Delmar synclinorium from formation (Cambrian) the Carolina slaite belt into undifferentiated metasedimentary and the Kiokee belt (generalized metavolcanic rocks in the lower from Fig. 2). part of the Asbill Pond and Persimmon Fork Formations „. 33" 52' 30 (Cambrian) JBtt'

fault, stratigraphie or intrusive contact

Modoc zone

inferred position of the axial trace of the Delmar synclinorium

A. K. Sinha, 1985, personal commun.). The deformed plutons near the (Snoke and others, 1980a; Kish, 1983; Dallmeyer and others, 1986), Fall Line, together with geochronological evidence (Durrant and others, however, indicate that the amphibolite facies regional metamorphism and 1980), are the primary evidence for an important belt of late Paleozoic associated deformation in the Kiokee belt are associated with a later (Alleghanian) penetrative deformation and metamorphism largely con- deformational phase (D2). Locally in the Kiokee belt, mesoscopic fold- cealed beneath the Atlantic Coastal Plain. interference patterns have been observed within paragneiss units. These may reflect refolding of Fj by F2. In most places, however, evidence for LATE PALEOZOIC DEFORMATIONAL CHRONOLOGY transposition of an earlier fabric or complex modification of pre-existing structures is not apparent, and it is not possible to unequivocally distin- Several upper Paleozoic plutons occur in the present study area (Fig. guish F2 folds from transposed Fj folds. The character of the D2 deforma- 2). Most of those in the Carolina slate belt record only brittle deformation, tion, however, can be evaluated by examining relations with granitoid whereas most in the Kiokee belt show evidence of variably penetrative orthogneisses, for which initial late Paleozoic igneous crystallization post- ductile deformation (Snoke and others, 1980a). Results of detailed geo- dated D^ logic mapping and collaborative geochronological studies suggest that The Lake Murray gneiss and the effects of the D2 deformation phase three distinct late Paleozoic deformational phases are recorded in the study are spectacularly exposed in the Lake Murray spillway around the Dreher area (D2-D4).1 Shoals Dam (Kiff, 1963; Tewhey, 1968, 1977; Carr, 1978; Secor and Snoke, 1978). The D2 deformation phase is herein named the "Lake D2: Lake Murray Deformation Murray deformation," and the Lake Murray spillway (Fig. 2) is designated as the type locality for the Lake Murray deformation. At the western end The Kiokee belt and adjacent parts of the Carolina slate belt have of the spillway, the Lake Murray gneiss is an augen gneiss with a strong S2 been affected by a late Paleozoic (D2) deformational phase involving tight fabric resulting from alignment of inequidimensional minerals and mineral to isoclinal folding and amphibolite facies regional metamorphism. As aggregates. The S2 foliation is oriented ~N60°E 70°SE and parallel to the previously outlined, the apparent continuity of the Delmar synclinorium boundary with the Carolina slate belt ~ 1 km to the southeast. Locally, the axis between the Carolina slate belt and Kiokee belt suggests that the gneiss contains aplite dikes which have been passively deformed into Kiokee belt also experienced D[ deformation. Geochronological studies mesoscopic, tight to isoclinal F2 folds plunging steeply to the northeast. The S2 foliation in the Lake Murray gneiss is parallel to F2 axial surfaces 'See Figure 4 for a comparison of the numerical deformation chronology used and cuts across compositional layering in F2 fold hinges (see Fig. 11 of in the present study with that used previously. Secor and Snoke, 1978).

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Delmar Lake Murray Clarks Hill Irmo reference deformation deformation deformation deformation

Secor and Snoke,1977 F,. S,. L, F2 , s2, F2

D2, F2 , s2, Carr, 1978 D1, F„ ST D S 4- 4. L4 Figure 4. Correlation Kish and others,1978 F of some previously used F,, S, 2 deformation chronologies with those of the present Secor and others, D1 >* Qj, Fj, s3' L1X3> sf • s4

D4, F4, S4. L4, L L Secor and others, 0X4' 1X4- D F M S L D2, F2, M? S2 D3, F3, s3. L3 1984; and this study 1' V 1- 1- 0X1 normal slip and reverse slip crenulation cleavage

Near the western end of the spillway, the Lake Murray gneiss is 3. The Modoc zone coincides with a steep metamorphic gradient divided by a large enclave or screen of pelitic schist containing the am- between greenschist facies in the Carolina slate belt and middle or upper phibolite facies mineral assemblage quartz-plagioclase-muscovite-biotite- amphibolite facies in the Kiokee belt. kyanite-staurolite-almandine. This assemblage indicates that pressure and 4. The Modoc zone contains a deformation front marking the

temperature representative of mid-crustal depth (at least -4.5 kb and at northwestward limit of intense penetrative D2 deformation. least 530 °C; Turner, 1981, Fig. 4.4) were attained during M2. Intrusive The timing of D2 is constrained by fabrics and intrusive ages of contacts between the Lake Murray gneiss and the pelitic schist are de- several upper Paleozoic granitoids emplaced in the Kiokee belt. The Lake formed by several mesoscopic, tight to isoclinal F2 folds. The composi- Murray gneiss (313 ± 24 Ma, Rb-Sr whole-rock isochron crystallization tional layering in the pelitic schist has been deformed by an exceptional age; Snoke and others, 1980a) and the Edgefield Granite (317 ± 4 Ma,

array of passive, mesoscopic to macroscopic isoclinal folds (see Figs. 21b U-Pb zircon age; Dallmeyer and others, 1986) both contain D2 fabric. The and 21c of Secor and Snoke, 1978). These plunge to the northeast and are contacts between the Lake Murray gneiss and host metasedimentary rocks

probably F2 folds, although because the metasedimentary rocks are prob- are folded by numerous tight to isoclinal F2 folds, and the Lake Murray ably Cambrian, it may be argued that the folds are Ft greatly intensified by gneiss appears to contain D2 fabric elements in an orientation similar to D2 strain. that in the country rocks. Along the south shore of Lake Murray, both the The Modoc zone is the gradational boundary between the Kiokee Lexington metagranite (292 ± 15 Ma, Rb-Sr whole-rock isochron crystal- and Carolina slate belts in western South Carolina and eastern Georgia lization age; Fullagar, 1981) and the Batesburg augen gneiss (291 ± 4 Ma, (Snoke and others, 1980b). The Modoc zone is primarily the result of the Rb-Sr whole-rock isochron crystallization age; Snoke and others, 1980a) 40 39 D2 Lake Murray deformation, although in many places, it has been do not contain any fabrics attributable to D2. Concordant Ar/ Ar strongly overprinted by D4 (discussed subsequently). In regions in which hornblende cooling ages in the Kiokee belt indicate that M2 was on the

the effects of D2 and D4 are separable, the following characteristics of the wane by 295 Ma (Dallmeyer and others, 1986). Together, the available Modoc zone are interpreted to have developed during the D2 deformation. geochronological data indicate that the Lake Murray deformation oc- 1. As the Modoc zone is approached from the Carolina slate belt, the curred during ca. 295-315 Ma.

S( slaty cleavage is overprinted by upper greenschist and amphibolite Geologic relations suggest that the D2 (Lake Murray) deformational facies mineral assemblages which compose a locally mylonitic S2 foliation event was closely related to late Paleozoic magmatic activity. In the Kio- in the Kiokee belt. kee belt, this plutonic activity is represented by numerous intrusions which 2. Within the Modoc zone, numerous sheets of felsic orthogneiss, range in size from mesoscopic to -100 km2. These appear to have been

from a few centimetres to several metres thick, are oriented approximately emplaced synkinematically during D2, and M2 regional metamorphism parallel to the S2 foliation and themselves have a strong S2 foliation. may, in part, have been a result of this magmatic activity. Mineral assem-

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to 72 S2 foliation planes measured along the Savannah River in the Kiokee belt. These data indicate that the axis of the F3 Kiokee antiform is approximately horizontal and trends N75°E in this region. Contours: l%-4%-7%-10% (data courtesy of Harmon Maher). (b)

Data from the south side of Lake Murray indicating parallelism between LQ * l and F3. Dashed line is best-fit girdle through poles to S^ 7r is the posi tion of the pole to this girdle, (c) Poles to 262 S4 foliation planes measured in the Batesburg augen and lineated gneisses, the

Clouds Creek igneous complex, and the Lexington metagranite. Contours: 0%-3%-6%-9%-12%. (d) 45 L4 elongation lineations measured in the Batesburg augen and lineated gneisses. Contours: 0%-3%-6%-9%-12%. (e) Poles to 143 S4 reverse-slip crenulation cleavage planes

measured iri the Carolina slate belt in the study area. Contours: 0%-3%-6%-9%-12%. (f) 165 L4 lineations (including LQ * 4, Lj x 4, and F4 fold axes) measured in the Carolina slate belt of the study area. Contours: 0%-2%-4%-6%-8%. (g) Poles to 73 Si planes from the Carolina

slate belt in the Irmo antiform, Irmo Quadrangle. Contours: l%-3%-5%-7% (data from Tewhey, 1977). (h) Poles to 214 S2 foliation planes from the Kiokee belt in the Irmo antiform, Irmo Quadrangle. Contours: l%-3%-5%-7% (data from Tewhey, 1977). (i) Poles to 28 normal-slip crenulation cleavage planes from the study area. Contours: 3%-10%-21%-32%.

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blages in the Kiokee belt indicate that pressures representative of interpreted to be F3 folds. In the Carolina slate belt, mesoscopic F3 folds mid-crustal depths were maintained during M2 metamorphism. The inti- are approximately coaxial with F] and Lq x 1 (Fig. 5b). F3 folds range from mate relationship between deformation, metamorphism, and plutonic ac- open to tight, and there is considerable variation in the orientations of F3

tivity suggests that the Kiokee belt represents a D2 infrastructure. In the axial surfaces. In most places, S3 crenulation cleavage is only weakly Carolina slate belt, late Paleozoic plutonic activity is represented by a few developed or is absent, and folded layers are irregularly thickened in the

cross-cutting stocks or small batholiths which were not significantly de- hinge area. Mesoscopic F3 folds are particularly abundant along the axis of formed during These impose aureoles of static contact metamorphism the Delmar synclinorium southwest of Lake Murray (Fig. 2). on surrounding rocks. Discordant 40Ar/39Ar whole-rock age spectra in

northwestern portions of the Carolina slate belt suggest only mild reheat- D4: Irmo Deformation ing during D2 (Dallmeyer and others, 1986). These characteristics suggest that the Carolina slate belt functioned as an upper-crustal D2 suprastruc- D4 is manifested as late metamorphic ductile deformation in steeply ture. The continuity of both stratigraphic units and Dj structures across the dipping, northeast-trending shear zones in the southeastern part of the Modoc zone in west-central South Carolina (Figs. 2 and 3) indicates that Carolina slate belt and in the Kiokee belt. Several lines of evidence indi- there are no major fault discontinuities along the boundary between the cate a subhorizontal dextral displacement sense for these D4 shear zones. Carolina slate belt and Kiokee belt in this region. The Modoc zone is Between Lake Murray and Edgefield, orthogneiss in and adjacent to

therefore a gradational boundary between suprastructure (Carolina slate the Modoc zone is strongly overprinted by a mylonitic L4-S4 fabric. S4 belt) and D2 infrastructure (Kiokee belt). During the early stages of the strikes approximately parallel to the Modoc zone and dips steeply to the

Alleghanian orogeny, thus, a situation reminiscent of "stockwerk" tecton- northwest (Figs. 2 and 5c). L4, an elongation lineation in S4, commonly ics (Wegmann, 1935; see Fig. 5d of Secor and others, 1986) developed in plunges gently northeast (Fig. 5d). Here, units of the Asbill Pond forma- the eastern Piedmont of South Carolina. tion and the axis of the Delmar synclinorium can be traced from the Carolina slate belt through the Modoc zone and into the Kiokee belt (Figs. D3: Clarks Hill Deformation 2 and 3). In the slate belt northwest of the Modoc zone, So and Si strike N55°E, whereas adjacent to the Modoc zone, they have been rotated into a Previous geological studies along the Savannah River in South Caro- N70°E strike by D4. This clockwise rotation of SQ and SJ fabric elements lina and Georgia (Crickmay, 1952; Howell and Pirkle, 1976; Maher, adjacent to the Modoc zone, as well as the subhorizontal L4 elongation 1978,1979) have indicated that the Kiokee belt occupies the axial region lineation, suggests a dextral displacement sense for D4 shear strain in this of a major northwest-vergent antiform (herein referred to as the "Kiokee region.

antiform" with limbs in the adjacent Carolina slate and Belair belts (Figs. 1 In the eastern part of the Emory quadrangle, a D4 shear zone that and 5a). The folded foliation surfaces that define the Kiokee antiform are was developed along the boundary between the Kiokee belt and the Si in the Carolina slate and Belair belts and S2 in the Kiokee belt. This D3 Carolina slate belt bifurcates, and a splay trends N60°E across the Clouds deformation is herein designated as the "Clarks Hill deformation," and the Creek igneous complex and into the Carolina slate belt in the axial region region along the Savannah River between Clarks Hill Dam and Augusta of the Delmar synclinorium (Fig. 2). Microscopic and mesoscopic D4 (Fig. 1) is designated as its type area. fabrics developed in the Clouds Creek igneous complex also indicate a Geologic relations. suggest that the interface represented by the dextral displacement sense for this branch of the shear zone (Snoke and Modoc zone was originally flat lying but was subsequently folded by the Secor, 1983).

Kiokee antiform during D3. The Modoz zone is in the steeply dipping S4 reverse-slip crenulation cleavage and mesoscopic F4 folds are northwest limb of the Kiokee antiform between the Kiokee and Carolina widely developed in the Carolina slate belt within ~ 10 km of the Modoc slate belts, and the Augusta fault is in the gently dipping southeast limb zone (Fig. 2). The S4 cleavage is locally penetrative and has an average

between the Kiokee and Belair belts. Although the Modoc zone and the orientation of N25°E 80°NW (Fig. 5e). F4 folds plunge steeply (Fig. 5f), Augusta fault have been modified by post-D3 tectonism, they share many parallel to the L[ * 4 intersection lineation. These are dextral in plan view common characteristics, and they are interpreted to have originated as a (see Fig. 13 of Secor and Snoke, 1978) and are interpreted to indicate

gradational subhorizontal boundary between suprastructure and D2 infra- dextral slip along Sj foliation during D4 in the Carolina slate belt (Dennis structure that was folded into the northwest-vergent Kiokee antiform and Secor, 1987).

during D3. The metamorphic mineral assemblages in the Lake Murray At the east end of Lake Murray, a 10-km-wide D4 shear zone crosses spillway indicate that the Kiokee belt infrastructure formed at a mid- from the Kiokee belt into the Carolina slate belt. A dextral shear sense is crustal depth, whereas the Carolina slate belt is interpreted as an upper- suggested for D4 by major fold structures associated with this shear zone. crustal suprastructure. Exposure of infrastructure and suprastructure at the Detailed geologic mapping in the Irmo and Lexington 7 "¿-minute quad- same level across the Modoc zone strongly suggests post-D2 differential rangles (Heron and Johnson, 1958; Tewhey, 1968,1977; Kimbrell, 1984) uplift of the Kiokee belt relative to the slate belt. This uplift is here suggests that the Modoc zone is folded into a major antiform (the Irmo attributed to the development of the D3 Kiokee antiform. A steep thermal antiform) near Irmo and a major synform (the Lexington synform) to the gradient between the Kiokee and Carolina slate belts during the late Pa- south near Lexington (Fig. 2). The axis of the Irmo antiform is clearly 40 39 leozoic is also implied by internally discordant Ar/ Ar mineral age outlined by the geologic map pattern in the vicinity of Irmo. Analysis of spectra (Dallmeyer and others, 1986) and by differences in regional met- the orientations of Si and S2 in this region indicates that the Irmo antiform amorphic grade. A possible explanation for development of the large plunges ~30°N 60°E (Figs. 5g and 5h). The axial region of the Lexington horizontal M2 temperature gradient across the Modoc zone is that initially, synform is partly concealed by Atlantic Coastal Plain deposits and has subhorizontal M2 isothermal surfaces were rotated into a steeply been disrupted by brittle faulting. The approximate location of the synform northwest-dipping orientation during development of the D3 Kiokee anti- axis is inferred from the position of the northeastward continuation of the form (see Fig. 5g of Secor and others, 1986). Modoc zone between Lexington and Columbia (Fig. 2). The Irmo anti- The Si foliation in the Carolina slate belt and the S2 foliation in the form and the Lexington synform are interpreted to have originated as Kiokee belt are deformed by numerous mesoscopic F3 folds having hori- northeast-plunging F3 folds (perhaps parasitic to the Kiokee antiform) that zontal or gently plunging axes trending N50°E to N70°E. These are were subsequently strongly overprinted by D4 dextral shear strain (Fig. 6).

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A. PRESENT Ridge Road Modoc — „Crossroads TP #) Irmo

/ / y ^ Lexington =#= Columbia ? 5 10 / °—I'I I 1 1 1 Km

C. 290 MA

Ridge Road Modoc .

.Crossroads — —^ I / / v. .• V, Modoc zone

"Lexington Columbia

Figure 6. Schematic diagram illustrating possible development of the D4 shear zone, which crosses from the Kiokee belt to the Carolina slate belt near Irmo. (A) Present configuration of the Modoc zone. (B) Configuration of the Modoc zone (at the present erosion level) -268 Ma prior to brittle faulting. (C) Configuration of the Modoc zone (at the present erosion level) -290 Ma prior to the development of the D4 shear zone.

Mesoscopic structures associated with the Irmo antiform also indicate The Lake Murray gneiss has a strong S2 fabric which has been folded by a subhorizontal dextral displacement sense for the shear zone. In the mesoscopic F4. The Edgefield granite carries a weak S-L fabric (here

exposures of orthogneiss and paragneiss in the spillway, S2 foliation is interpreted to be a D2 fabric) that is folded by mesoscopic F4 (Metzgar, folded by numerous mesoscopic F4 folds which are dextral in plan view 1977).

and which plunge steeply (Fig. 7). In exposures of schist and paragneiss in The age of the D4 deformation phase is constrained by the results of the spillway, boudinage in thin amphibolite layers is interpreted to have recent Rb-Sr whole-rock (Fullagar, 1981; Kish, 1983) and ^Ar/^Ar developed during D4 (see Fig. 21a of Secor and Snoke, 1978). A sub- (Dallmeyer and others, 1986) geochronological studies. The Lexington horizontal dextral shear sense is indicated by the steep orientation of the metagranite (292 ± 15 Ma, Rb-Sr whole-rock isochron crystallization boudin lines and by a consistent clockwise sense of rotation of the boudins age), the Batesburg augen gneiss (291 ± 4 Ma, Rb-Sr whole-rock isochron if viewed down plunge. The D4 deformation is herein named the "Irmo crystallization age), and the Batesburg lineated gneiss (284 ±17 Ma, Rb-Sr deformation," and the region between the east end of Lake Murray and whole-rock isochron crystallization age) record an L4-S4 fabric, and, there- 40 39 Irmo is designated as its type area. fore, D4 must have occurred after ca. 290 Ma. Ar/ Ar age spectra in At numerous) locations between Edgefield and Columbia, Sj, S2, or the Kiokee belt suggest that D4 occurred at ca. 285-290 Ma, whereas

S4 foliations are deformed by normal-slip crenulation cleavage (Figs. 5i dynamically recrystallized biotites along an S4 mylonitic foliation which and 8; also see Figs. 20a and 20b of Lister and Snoke, 1984). These developed in the Clouds Creek igneous complex record ^Ar/^Ar plateau cleavages are interpreted to have formed during the D4 deformational ages of 268 ± 5 Ma (Dallmeyer and others, 1986). These geochronolog- phase as a consequence of slip along S|, S2, or S4 (Dennis and Secor, ical controls suggest that D4 was diachronously developed during ca. 268- 1987). In every instance, the orientation of normal-slip crenulation cleav- 290 Ma.

age relative to Si, S2, or S4 indicates a subhorizontal slip vector and a dextral sense for the shear strain along the pre-existing foliation. Late Brittle Faulting Overprinting relationships suggest that D4 is the youngest penetrative deformational event to affect the rocks of the Carolina slate and Kiokee Several northeast-trending, steeply dipping faults are near the bound- belts in South Carolina. In these belts, S4 cleavage and mesoscopic F4 folds ary between the Carolina slate belt and the Kiokee belt. In the area north clearly overprint mesoscopic structures associated with D^ D2, and D3. of Batesburg, a series of late brittle faults which developed along the

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younger than ca. 268 Ma. North of the study area, near Chapin, South Carolina, east-trending silicified fault zones are older than ca. 195-Ma diabase dikes (Simpson, 1981; Dooley and Smith, 1982; Secor and others, 1982). Late brittle faults in the study area thus probably formed between ca. 268 and 195 Ma. They are similar in orientation and character to the late faults containing "ultramylonite" (Conley and Drummond, 1965) or "silicified mylonite" (Butler and Dunn, 1968) that are widespread in the Piedmont and Blue Ridge of Georgia and the Carolinas. These late brittle faults may be associated with Mesozoic rifting associated with initial opening of the Atlantic Ocean.

SUMMARY OF GEOLOGICAL HISTORY AND CONCLUSIONS

1. In west-central South Carolina, both the Kiokee belt and the Carolina slate belt contain a Cambrian stratigraphic sequence consisting of intermediate to felsic metavolcanic rocks (Persimmon Fork Formation) Figure 7. Thin amphibolite layer in pelitic schist, in the Lake overlain by quartzite, mudstone, and wacke (Asbill Pond formation). The Murray spillway, showing a steeply plunging dextral F4 fold (scale upper part of the Asbill Pond has yielded several genera of late Middle indicated by pocket knife). Cambrian trilobites characteristic of the Atlantic faunal province. 2. During the early and/or middle Paleozoic, this stratigraphic sequence was penetratively deformed (Dj) and metamorphosed (Mi) in the greenschist facies. This deformation is herein named the "Delmar deformation." 3. Numerous plutons of a late Paleozoic magmatic arc were emplaced in the Piedmont between ca. 285 and 325 Ma. 4. A deep-crustal infrastructure developed in the eastern Piedmont in association with the evolution of the late Paleozoic magmatic arc, and this infrastructure was penetratively deformed at temperatures and pressures representative of mid-crustal depth during ca. 295-315 Ma. This tectonothermal event is herein named the "Lake Murray deformation" (D2),

and the associated regional metamorphism is distinguished as "M2."

5. During the Lake Murray deformation, the D2 infrastructure was separated from overlying suprastructure by a gradational subhorizontal interface containing sheets of felsic orthogneiss, a steep metamorphic gra-

dient, and a D2 deformation front.

6. The Si and S2 foliations and M2 isothermal surfaces have been folded into the northwest-vergent Kiokee antiform. This folding event is herein named the "Clarks Hill deformation" (D3).

7. The Kiokee belt is interpreted to represent D2 infrastructure ex- Figure 8. Si foliation deformed by D4 normal-slip crenulation posed in the core of the D3 Kiokee antiform. The Carolina slate and Belair cleavage. Sketch by Allen Dennis from an outcrop in the Carolina belts are interpreted to represent an originally continuous suprastructure slate belt at the east end of Lake Murray. Scale is indicated by pencil now exposed on the northwestern and southeastern flanks of the Kiokee which points toward the north. antiform. 8. Although the Modoc zone and the Augusta fault were modified by post-D3 tectonism, they are interpreted to have originated as a grada-

Modoc zone juxtapose the Carolina slate belt and Clouds Creek igneous tional subhorizontal interface between D2 infrastructure and suprastruc- complex (on the northwest) against Kiokee belt gneisses (on the southeast; ture that was later folded by the D3 Kiokee antiform. Fig. 2). South and southeast of Lake Murray, a series of late brittle faults 9. During ca. 268-290 Ma, an -10-km-wide dextral ductile shear juxtaposes Kiokee belt gneisses and schists in the axial region and south- zone developed in the steeply dipping northwest limb of the Kiokee anti- east limb of the D4 Irmo antiform against Carolina slate belt rocks of the form. This event is named the "Irmo deformation" (D4). Lexington synform (Fig. 2). In most places where late brittle fault zones 10. The crystalline rocks of the southeastern Piedmont are cut by are exposed, the fault breccia is silicified, and rocks adjacent to the fault several northeast-trending, silicified fault zones of small displacement. zone are cut by irregular extension veins containing comb quartz. No These may be related to Mesozoic rifting associated with the opening of definitive net-slip criteria have been observed for any of the faults. The the Atlantic Ocean. general continuity of rock units and folds between the Carolina slate and 11. During Late Triassic or Early Jurassic time, the study area was Kiokee belts, as well as the discontinuous nature of the brittle faults, intruded by north- or northwest-trending diabase dikes, and then in the suggests, however, that most have displacements less than a few kilome- Late Cretaceous, the crystalline rocks were unconformably overlain by tres. In the study area, the faults cut D4 structures and thus must be kaolinitic sands of the Atlantic Coastal Plain.

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