MICROSTRUCTURES AND SENSE OF SHEAR IN THE BREVARD ZONE, SOUTHERN APPALACHIANS by CAROL ANNE EVANS, B.S. THESIS Presented the Graduate School of to the Faculty of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of MASTER OF ARTS THE UNIVERSITY OF TEXAS AT AUSTIN December 1986 Copyright by Carol A. Evans 1986 To Mark, for understanding and waiting. ACKNOWLEDGEMENTS First and foremost I thank Sharon, my advisor, for her encouragement and advice, and for patiently enduring the various difficulties in the field and my seem­ ingly endless stream of questions. The following persons I thank accordingly; Bob Hatcher for the generous use ofhis unpublished geologic maps and deluxe housing accomodations while in Mark and Beth Hartford for their the field August, 1986; hospitality at home and in the field; Bill Carlson for the use of his petrographic microscope for thesis photography; Suzanna Ross Moses for her generous help in thin sections and for her and John and making hospitality special friendship; Madclyn La Favc for their wonderful friendship and support; and Charles and me and their Ruby Evans, my parents, for their never-ending support and belief in financial assistance with a field vehicle. Last, but forever first in my life, I thank husband Mark Avery to whom this volume is dedicated. my Generous grants from the Geological Society of America, Sigma Xi and the University of Texas Geology Foundation made this research possible. Their receipt is gratefully acknowledged. V ABSTRACT Microstructures and Sense of Shear in the Brevard Zone, Southern Appalachians Carol Anne Evans, M.A. 1986 The University of Texas at Austin, Supervising Professor: Sharon Mosher, Ph.D. The Brevard Zone, which the Blue and Inner Piedmont separates Ridge geologic provinces in the southern Appalachians, is a major structural feature with a multiple deformation Microstructurcs in oriented thin sections from history. rocks in the Brevard Zone in Tugaloo, Whetstone and Tamasscc quadrangles, South Carolina, Rosman quadrangle, North Carolina, and in the sheared Ben Hill Granite in Atlanta, Georgia, indicate that there were at least two early ductile def­ ormations and a later, locally developed, brittle deformation. The oldest recognizable microstructurcs arc a prominent foliation (Sj), The of these features and the sense of quartz ribbons and garnets. age shear during their formation is unknown. The remainder of the observed microstructurcs are on categorized into groups A, B, C and D the basis of orientation, overprinting re- indicated sense lationships and direction of motion as by of shear criteria present. Group A is the oldest ofthese microstructurcs and group D is the youngest. VI Group A features consists of northwest-verging, tight to isoclinal F 2 folds, a weakly developed, axial planar foliation ($2), and scattered folds, coaxial with The Fj folds of Roper and Dunn (1973) arc not observed due to later defor­ mation. Group A microstructures are ductile features which formed during a west- to northwest-directed thrusting motion. Group B features include type II s-c scattered shadows. The orien­ mylonites, c-surfaces, folds and garnet pressure tation of these features indicates that they formed during a period of dextral an exten­ strike-slip shearing with a possible thrust component. Group C contains sional crenulation is than features in cleavage (ECC) which relatively younger groups A and B. The orientation of ECC is incompatible with dextral motion, thus they suggest a change in the direction of bulk motion in the Brevard Zone, the di­ rection of which is unknown. Along strike a notable change in deformation conditions occurred during the ductile deformation(s) which formed features in groups A, B and C. This change is reflected in highly recrystallized quartz textures in Tugaloo, relative to partially recrystallizcd textures in Rosman quadrangle. Retrograde metamorphism postdates the formation of features in A, B and C. groups Group D contains the youngest microstructurcs which formed during a lo­ calized brittle deformation. Brecciation is visible in thin section and outcrop, how­ ever no sense of shear direction can be determined. Drag folds and faults are present in several outcrops but their geometry is highly variable. The bulk motion during brittle deformation is unknown. Sense of shear criteria in group A are compatible with tectonic models for both the Taconic and Allcghanian orogenies in the southern Appalachians. How- VII the most intense ductile de­ ever, group A probably formed in the Taconic because formation has been reported for this time period. Microstructures in group B and C are found in the sheared, Permian Ben Hill Granite in Atlanta and thus are Rocks B and Cin the northeastern Alleghanian in age. containing group study areas cannot be radiomctrically dated with confidence, however, their orientations, deformation conditions and of shear is similar to B and C in the Ben sense group Hill Granite indicating that they are also Alleghanian in age. The dextral strike-slip motion indicated by groups B is compatible with the results of previous workers along the Brevard Zone who (Reed and Bryant, 1964; Bobyarchick, 1983) elsewhere have also demonstrated Thus the results of this study Alleghanian dextral motion. confirm an episode of ductile, dextral strike-slip motion in the Brevard Zone during the Alleghanian. D also be the result of a Group C and may be Alleghanian or they may separate and more recent deformation, possibly related to the Triassic opening of the present-day Atlantic ocean. viii TABLE OF CONTENTS Abstract vi List of Tables xi List of Figures xii Introduction 1 Nature and scope ofproject 1 Study Area 4 Methods 6 Geologic setting 8 Lithologies 12 Northeastern areas 12 Atlanta area 13 Microstructures 18 Oldest Microstructures 19 24 Folds 24 Group A Axial planar foliation 27 Summary 28 Group B 30 Type II s-c mylonites 30 Northeastern areas 30 Atlanta area 35 Other c-surfaces 39 Folds 43 Pressure shadows 47 Summary 49 51 Group C Extensional Crenulation Cleavage 51 Northeastern 52 areas Atlanta area 54 Summary 58 Changes in deformation conditions 61 63 Retrograde metamorphism 64 Group D Atlanta area 64 66 Whetstone Quadrangle Tamassee Quadrangle 70 IX Rosman Quadrangle Sega Lake 74 Quarry 78 82 Summary Summary 85 87 Timing Conclusions 93 Appendices 96 97 Appendix A Tables of microstructurcs 97 Appendix B 105 Structural Data 105 Brittle Faults 107 112 Appendix C Minor Microstructural Features 112 Crcnulations 112 Rare Pressure Shadows 113 Veins 114 Kinks and Shear Bands 114 117 Appendix D Quartz c-axis data 117 References 122 126 Vita X LIST OF TABLES Table 1. Microstructure groupings 20 Table A.I: Microstructure 98 Table for Tugaloo Quadrangle Table A.2: Microstructure Table for Whetstone Quadrangle 101 Table A.3: Microstructure Table for Tamassee Quadrangle 102 Table A.4: Microstructure Table for Rosman Quadrangle 103 Table A.5; Microstructure Table for Northwest Atlanta Quadrangle 104 xi LIST OF FIGURES I. 109 Fig. Major geologic provinces in the southern Appalachians 2. 110 Fig. Study area Fig. 3. Generalized geologic map of the greater Atlanta, Georgia area 11l .... Fig. 4. Late Precambrian-Barly Paleozoic Tectonic Setting 115 5. Structure produced during the Taconic Orogeny (400-440 Ma) .... Fig. 116 Fig. 6. Cross-section of the present structural setting of Brevard Zone 119 .... 7. 120 Fig. Geologic map of Tugaloo quadrangle, SC Fig. Geologic map of Whetstone quadrangle, SC 8. 121 9. 16 Fig. Geologic map of Tamassee quadrangle, SC 10. 17 Fig. Geologic map of Rosman quadrangle, NC Fig. 11. Foliation in the Brevard phyllite 21 . Fig. 12. Weathered garnets 22 Fig. 13. Group A folds 25 Fig. 14. Relic hinges of early isoclinal folds 26 15. Group A coaxial folds 27 Figure Fig. 16. Axial planar foliation to Group A folds 29 17. S-surfaces in Type II s-c mylonitc from the Henderson Mylonite 32 ... Fig. Fig. 18. S-surfaccs defined by muscovite folia 33 Fig. 19. S-surfaces defined by dimensional preferred orientation 34 20. C-surfaces are defined by muscovite and biotite 35 Fig. Fig. 21. Shear zone in Henderson Mylonite 36 Fig. 22. Henderson Mylonite away from shear zone 37 Fig. 23. Highly recrystallized quartz ribbons 38 XII Fig. 24. High shear strain in type II mylonitc 39 Fig. 25. No microstructure development outside a small shear zone 40 26. 41 Fig. Type II s-c mylonitc 27. Fig. Crystallographic preferred orientation in a type II s-c mylonitc 42 .... Fig. 28. Highly deformed quartz ribbons 43 Figure 29. C-surface defined by deflected muscovite 44 Figure 30. A c-surface has sheared garnets 45 31. fold 46 Fig. Group B Fig. 32. Sketch ofa possible fold interference pattern 48 33. shadows 49 Fig. Poorly developed pressure 34. 54 Fig. ECC from the Brevard Phyllite Fig. 35. ECC's deform a quartz ribbon 55 36. ECC deforms a B shadow 56 Fig. group pressure 37. ECC deform B c-surfaces in the Brevard Phyllite 57 Fig. group 38. ECC deforms B fold 58 Fig. group Figure 39. CloseupofECC 59 40. Quartz ribbon necked by an ECC from the Ben Hill Granite 60 Fig. 62 Figure 41. Partially recrystallized quartz ribbon Fig. 42. Brittle imbricate fault zone 66 Fig. 43. Field sketch ofimbricate faults in the Ben Hill Granite 67 44. 68 Fig. Late quartz-filled fractures in the Ben Hill Granite Brecciation near exotic slice 71 Fig. 45. 46. 73 Fig. Faults in Tamassee quadrangle 47. Prominent faults and fault slivers 75 Fig. XIII 48. 76 Fig. Field sketch of Sega Lake roadcut, Rosman Fig. 49. Intense deformation along the Rosman fault 79 50. 81 Fig. Folds in graphitic phyllite Fig. 51. Incipient brecciation of folded quartz ribbons 82 Fig. 52. Intensely brecciated phyllite 83 Fig. 8.1. Stereonet plots of foliation planes across faults 109 Fig. 8.2. Stereonet plots of foliation planes across faults 110 Fig. 8.3. Stereonet plots offoliation planes across faults 11l Fig. C.l. shadows 115 Rotated quartz pressure on opaque grains Fig. C.2. Sheared veins 116 D.l. 119 Fig. Quartz c-axes plots for Tu3 and Ta 142 Fig. D.2. Quartz c-axes plots for TuI37A and R05162 120 D.3. 121 Fig. Quartz c-axes plots for Atl2 XIV INTRODUCTION The Brevard Shear Zone is a narrow, linear, deformed belt of rocks which stretches approximately 375 miles (625 km) within the southern Appalachians. This 1) and zone separates the Blue Ridge and Inner Piedmont geologic provinces (Fig. is a major structural feature with a multiple deformation history. The zone trends northeast from eastern Alabama where it disappears under coastal plain deposits, to northwest North Carolina near the James River synclinorium (Roper and Justus, 1973). NATURE AND SCOPE OF PROJECT The Brevard Zone has been intensively studied over the past 20 years by numerous researchers. A wide range of theories on its origin have been proposed because of the complex deformation history of the zone. For example, the initial research of Keith (1905) suggests that the Brevard Zone is a simple fold belt. More recent workers postulate that the zone is a normal fault (White, 1950) or an alpine- root zone (Livingston, 1966). and Justus believe it has a type Roper (1973) polytcctonic origin. The latter theory is probably the most accurate one given the long and complex history of the zone. Several reasons for this diversity of theories are the quality and nature of the somewhat variable structure along the exposures, zone and the lack of offset An extensive list of length of the geologic markers. theories which further illustrates the past controversy over the origin ofthe Brevard Zone appears in Roper and Justus (1973, p.l 19). this focused on the two most At present controversy is widely accepted theories. These theories are that the Brevard Zone is: 1) a thrust fault (Jonas, 1932; 1 2 is A-A' Appalachians: 3-5). (f southern igs. the in cross-sections provinces following all geologic of Major position !. Fig. Hatcher, 1969, 1971) and/or 2) a dcxtral (Reed and Bryant, 1964; Bobyarchick, 1983, 1984) or a sinistra! strike-slip fault (Reed, Bryant and Myers, 1970; Bryant and Reed, 1970). Thus the direction and timing of motion(s) which affected the zone arc in question. This investigation was undertaken to determine: 1) whether there was more thanonedirectionofmotionand,ifso,therelativetimingofthosemotions; 2)the direction ofmotion(s); 3) the conditions of deformation; and 4) the absolute tim­ ing ofmotion, ifpossible. These objectives were accomplished by studying the orientation, over­printing relationships and direction of motion indicated by microstructurcs and sense of shear criteria present in the zone. Analysis of these features indicates that there were at least two ductile deformations, followed by retrograde metamorphism, and then a locally developed brittle deformation. There was also a major change in the direction of motion and in the deformation conditions under which micro- structures formed. For the purpose of explanation, microstructures for which the sense of shear direction could be determined are categorized into Groups A-D, on the basis of this direction and overprinting relationships. Group A features are the oldest of these microstructurcs and arc over- features, and so on. Results of this study, combined with printed by Group B known structural relationships in the southern Appalachians, suggest that Group A microstructurcs were probably formed during the Taconic orogeny, although they be Acadian Other structural and could or Alleghanian features. relationships comparison of microstructurcs with those that are datable elsewhere in the Brevard that C and D are Zone, strongly suggest Groups B, Alleghanian in age. The the geologic implications of these results help to better constrain tectonic models of Brevard Zone in the study area. STUDY AREA The area consists Northwest study of outcrops in the Atlanta, Georgia quadrangle and the Tugaloo, Whetstone, and Tamassce quadrangles in northwest­ern South Carolina, and the Rosman quadrangle in southwestern North Carolina (Fig. 2). The latter four quadrangles are referred to as the "northeastern areas" in this paper. The Atlanta study area was chosen because at that locality the Hercynian Ben Hill granite (Sinha and Zietz, 1982) dated at 280-290 Ma (Sinha, pers. comm.) is truncated by Brevard motion (Fig. 3). One particularly well-exposed outcrop (McConnell and Costello, 1980, p.253) was the focus of sampling in this area. The other four study areas were sampled because good kinematic indicators are present. Unpublished geologic maps of Tugaloo and Whetstone quadrangles at 1:24,000 same (compiled by Hatcher in 1968-1976) and the published maps, at the scale, of Tamassee (Roper and Dunn, 1970) and Rosman quadrangles (Horton, 1982) were used to locate exposures. Other detailed field studies pertainent to the present study include those by Livingston, 1966; Higgins, 1968; Hatcher, 1969; Horton, 1974; and 1972. Roper, 1971, Outcrop in the northeastern areas is not abundant, consisting predomi­ nantly of heavily vegetated, northeast-trending, creek exposures and a few useful road from cut exposures. Outcrop quality is generally poor, ranging poorly indurated to saprolitic, bank and road-cut outcrop, to well-indurated but water or lichen-covered creek In the southwest, the Ben Hill Granite outcrop is exposure. 2. The consists of five Fig. Study area: study area quadrangles in the southern Appalachians. easily accessible, unvegetated, and contains fresh exposure, unlike that in the located northeastern study areas. This granitic outcrop is on 1-285 2.9 miles (4.8 north of the 1-285 and 1-20 junction and 0.5 miles (0.8 km) south of the km) Chattahoochee River. Road cut exposures of both the granite and the adjacent Long Island Gneiss arc about 900 ft (2700 m) long, up to 30 ft (10 m) high, and Fig. 3. Generalized geologic map of the greater Atlanta, Georgia area: Illustrates areal extent of crystalline rocks adjacent to the Brevard Zone (McConnell and Abrams, 1984). both sides of the interstate highway, although sampling is confined to present on the west side of 1-285. This outcrop is the only one sampled in this quadrangle. METHODS In the northeastern study areas, surface exposure of the Brevard Zone ranges from 2500 to 6000 ft in width (830 m to 2 km), reaching a maximum width of 10,000 to 15,000 ft (3.3 km to 5 km) in the Tamassce quadrangle. Foliation in the zone has a consistent southeast dip ranging from 10° to 70° the average is 45°. In Atlanta, dip of foliation in the outcrop sampled averages 25°, also in a consistently southeastern direction. A faintly to strongly developed lineation is This is a present in the majority of samples from all study areas (sec Appendix B). Ori­ consistently northeast-trending, subhorizontal, mineral elongation lineation. ented samples were collected along traverses perpendicular to the zone during Au­ gust 1985, and January and March of 1986. For specific sample locations see Plates I to IV in the back These plates also contain arrows and strike and dip pocket. symbols which indicate the approximate sense of shear or shear plane orientation of microstructures in samples for which this direction can be determined. Samples were cut normal to the foliation and either parallel or normal to the lineation, and sometimes both. Each sample was then reoriented in a sand box recorded and and the strike and dip of the blank cut from that sample was perma­ nently marked on the blank, along with the up direction. All blanks were then impregnated with epoxy to prevent problems in the thin section process. The standard, 30 micron thin sections produced are marked with the sample number, up arrow, and two notches which represent a strike line for that section. By refer­ ring to the appropriate strike and dip recording, it is possible to accurately reorient each thin section in and thus determine the true orientation, at least in two space dimensions, of any kinematic indicators present. Using the hand sample and thin was possible to determine the orientation ofkinematic indicators section together, it in three dimensions. Quartz c-axis fabrics are also used as a tool to help determine the sense of shear. Plots of c-axis orientations and their interpretations are included in Appen­ dix D. All figure captions for photomicrographs give the thin section label in pa­ rentheses. This label consists of the quadrangle abbreviation and the thin section number which corresponds to the sample number. This number can be used to re­ fer to the sample location and lithology on the enclosed overlays and referenced geologic maps. Sense of shear direction, where appropriate, is given in captions as "ss=". This direction is for features as viewed in the photomicrographs and not, in all cases, for the zone as a whole. The long dimension, "Id =", of each A total of 133 thin sections from photomicrograph is also given in each caption. Brevard zone rocks were analyzed, 73 from Tugaloo, 18 from Whetstone, 11 from Tamassee, 24 from Rosman, and 7 from Atlanta. GEOLOGIC SETTING The Brevard Zone in the study area forms the northwestern edge of the Chauga belt synclinorium in the northeastern study areas (Fig. 1). The Chauga belt (also known as the Low Rank a belt) is narrow synclinc of sheared low grade bounded the Blue mctamorphic rocks, abruptly by high grade Ridge geologic province to the northwest. The southeastern boundary of the Chauga belt with the migmatitic Inner Piedmont is believed to be the result of a mctamorphic gradient (Hatcher, 1972, 1978). are apparently Precambrian and were de- Rocks of the Chauga belt in age The first deformation posited in a backarc basin (Fig. 4) (Roper and Justus, 1973). occurred during the Mid-Ordovician to Earlicst-Silurian Taconic orogeny (Glover et al., 1983) when the Inner Piedmont island arc collided with the North American continent. Intense shearing and the formation ofthe Chauga belt syncline occurred at this time (Fig. 5) (Hatcher, 1972). 4. Late Paleozoic Tectonic Fig. Precambrian-Early Setting: Schematic cross-section in the southern Appalachians (Roper and Justus, 1973). Roper and Dunn (1973) have identified Fj and Sj microstructures which they correlate with the Taconic deformation. Their Sj is the prominent schistosity in the Brevard Zone, the Sj foliation of this paper, and it is parallel to S strongly , transposed sedimentary bedding, except in the hinges of Fj isoclinal folds where Sj is an axial planar foliation. Their Fj folds deform S The Fj ofthese authors . Q was not observed in the present study. Further deformation presumably occurred during the Middle to Late- Devonian Acadian orogeny. Roper and Dunn (1973) correlate F 2 and M 2 features with the Acadian orogeny. of Roper Dunn (1973) are open to F 2 folds and out and rootless isoclinal, and also coplanar and coaxial with Fj. Many are sheared associated an at the microscopic scale. These F 2 folds in places have axial planar 10 Future time this Ma): at zone root (400-440 thrust Orogeny a acts Taconic indicated) the during (position produced Zone Structure Brevard 5. Fig. 1972). (Hatcher, cleavage, s2* expressed as either a schistosity, S2a, or a slip cleavage, (Roper and Dunn, 1973, p.3374). Their F 2 appears to correspond to the F 2 microfolds and axial planar foliation (S2) described in A ofthis F 2 folds deform Sj. group paper. Evidence of an Acadian age disturbance is given by Odom and Fullagar (1970, 1973). These authors found that Sr at isotopic homogenization occurred 356 + /-S Ma within mylonites in the Brevard Zone in Rosman, North Carolina. They postulate that this age may represent distributed shearing rather than major displacement during the Acadian orogeny. Furthermore, Bond and Fullagar (1974) calculated a radiometric age of 387 + /-14 Ma for six same mylonites from the Henderson Gneiss unit in North Carolina. The last major deformation that occurred was the Allcghanian orogeny in late Paleozoic time. The Blue Ridge, Chauga belt and Inner Piedmont were thrust westward as a unit along the Blue Ridge decollement over flat-lying Valley and Ridge sediments (Fig. 6). When the deeper structure of the Inner Piedmont reached a ramp in the decollement, the Brevard fault zone, proper, then formed as a subsidiary or splay thrust above this ramp (Hatcher, 1972, 1978). It was during this thrusting that a small slice of carbonate was plucked from the footwall and brought to the surface. This exotic slice is present in Whetstone quadrangle, South area. Carolina (Hatcher, 1971) within the study Further Alleghanian motion is proposed by workers who also present evidence for strike-slip faulting either syn­ chronous with, or after, major Blue Ridge thrusting (Reed and Bryant, 1964; Reed A single, undeformed diabase dike from et al, 1970; and Bobyarchick, 1983, 1984). the Upper Triassic completely crosses the zone in North Carolina (Reed and The undeformed nature ofthis dike indicates Bryant, 1964; Bryant and Reed, 1970). that all motion along the Brevard Zone had ceased by the Triassic. LITHOLOGIES Northeastern areas Lithologies exposed within the Brevard Zone arc part of the Precambrian Chauga River Formation which is correlative with other rocks of the Chauga belt further to the southeast (Hatcher, 1969, p. 131). The Chauga River Formation dips southeast parallel to the zone and consists of a basal graphitic phyllitc, and an upper and lower chlorite-muscovite phyllitc which includes a thin but continuous carbonate unit. This phyllitc is also known as the Brevard phyllitc (Matcher and Cross-section of the present structural setting of Brevard Zone: Exotic 6. Fig. footwall slice not shown due to scale (Matcher, 1978). Griffin, 1969) and the button-schist or fish-scale schist of Roper (1972). Phyllitic units are garnetiferous and quartz-rich in some areas. Within the River Formation there arc discontinuous Chauga narrow, quartzofeldspathic units known as the Henderson Gneiss. This gneiss is also pres­ ent as large body to the southeast of the Brevard Zone in all study a areas except Atlanta, Georgia. In many areas the Henderson Gneiss is more appropriately termed a mylonite and this is the terminology used here. The Henderson mylonite is believed to be derived from a Cambrian pluton (Sinha and Glover, 1978) which intruded lithologies in the Chauga belt and Inner Piedmont prior to the Taconic orogeny. The pluton was subsequently deformed resulting in the present outcrop pattern of the mylonite in the Brevard Zone, which is parallel to the strike ofthe zone (Hatcher, unpub. maps; Roper and Justus, 1973; and Horton, 1982). These long, narrow units are interpreted by Hatcher (unpub. as fault slivers ( Figs. 7 and and by Roper and Justus (1973, p. 113) as maps) 8), the cores of folds (Fig. 9). Horton (1982) lists these units as a gradational and interlayered mylonite, probably derived from the Henderson Gneiss (Fig. 10). In the Brevard Zone the Henderson mylonite was subjected to and records the same deformation(s) as recorded in the surrounding rocks. In North and South Carolina, samples were collected from both the Chauga River Formation and the Henderson mylonite in the Brevard Zone and from immediately adjacent rocks in the Blue Ridge and Chauga belt geologic provinces. Atlanta area Lithologies in the Brevard Zone in Atlanta are not stratigraphically con­ F7 Fig. 7. Geologic map of Tugaloo quadrangle, SC: See ig. 2 for location (simplified from Hatcher, 1969). Heavy line weight represents faults. 8. of Whetstone See 2 for location Fig. Geologic map quadrangle, SC: Fig. and Fig. 7 for explanation of symbols (simplified from Hatcher, 1969). 9. of Tamassee See 2 for location Fig. Geologic map quadrangle, SC: Fig. and Fig. 7 for explanation of symbols (Roper and Justus, 1973). See Fig. 2 for location and 10. Fig. Geologic map of Rosman quadrangle, NC: Fig. 7 for explanation of symbols (simplified from Horton, 1982). sistent with those in the northeastern areas. In the Atlanta greater region the Brevard Zone separates the northern Piedmont from the southern Piedmont geologic province (McConnell and Abrams, 1984). In Atlanta, lithologies within the Brevard Zone consist of the Long Island Gneiss, the Brevard Mylonite Gneiss, a phyllonite unit and a button schist unit 1968). The southeastern (Higgins, boundary of the Brevard Zone is in fault contact with the sheared Ben Hill Granite (Fig. 3). The Ben Hill Granite is a fairly extensive pluton of the Georgia Piedmont its (Higgins, 1968). The pluton becomes progressively sheared and mylonitized as contact with the Brevard Zone is approached. It is composed predominantly of quartz, abundant pink feldspar augen up to 0.4 in (1 cm) in length, minor muscovite and fairly clear, cuhedral garnets which from 0.004 in (0.1 mm) to 0.03 in (0.8 range mm) in diameter. The outcrop sampled contains Ben Hill Granite in fault contact with the Long Island Gneiss (McConnell and Costello, 1980). The granite at this particular outcrop may be within a fault sliver because this outcrop is not present on the maps by Higgins (1968) and McConnell and Abrams (1984, Plate I East). was concentrated across several imbricate fault zones localized Sampling along shear zones that are found the Ben Hill Granite. preexisting completely within These zones are referred to as fault zones in this The microstructures ob- paper. served adjacent to fault zones are described in groups B and C, whereas the fault themselves are described in group D. zones MICROSTRUCTURES The remainder of this paper describes the microstructures, associated of direction(s) of motion, and deformation conditions for each of the four groups features recognized. Groups arc discussed in their order of overprinting relation- found in each thin section. Se- ships. Not all features and their relationships arc veral for this absence reasons are 1) mesoscopic/microscopic changes in lithology which may affect the development of a particular feature; 2) strain partitioning on a mesoscopic or greater scale; 3) local perturbations in the stress field; 4) obliter­ ation of preexisting features due to later overprinting; and 5) features are not pervasively developed. Thus, microstructurcs have been correlated by similar morphologies, orientations, senses of shear and deformation conditions. Table lists the observed microstructures in the order in which they are discussed and the group to which each is assigned. Appendix A contains tables for each quadrangle which list thin sections, their orientation with to lineation and foliation in the rock, and the respect microstructuues found in each section. Appendix B contains sterconct plots of lineations, slickensidcs and fold axes by quadrangle. Features such as shadows compressional(?) crcnulations, pressure on opaque minerals, sheared veins, kinks and shear bands are several kinematic indicators that were also observed, but are not discussed because of their localized occurrence, varying geometry, unclear timing and inconsistent sense of shear. Descriptions of these microstructurcs are included in Appendix C. OLDEST MICROSTRUCTURES The oldest recognizable microstructurc is a pervasive foliation (Sj) (Fig. 11). This prominent, southeast-dipping foliation is defined by quartz ribbons, muscovite and minor chlorite in quartzofeldspathic rocks, and by sheaves (buttons) or folia of muscovite, chlorite and some graphite in phyllitic rocks, Sj cannot be Table 1. Microstructure groupings: Categorized on the basis of orientation and sense of shear. included in the other groupings because the timing and direction of motion during formation be determined all microstructures in cannot except that it predates groups A-D. Recall that the isoclinal Fj folds of Roper and Dunn (1973) which deform S were not observed in the areas studied for this project. Q Scattered garnets are present in the northeastern areas, predominantly in and lower Brevard phyllite. These once euhedral fractured, the upper garnets are cloudy and extremely altered (Fig. 12). Many garnets arc so weathered that they Fig. 11. Foliation in the Brevard phyllite: Note the presence of a recrystallizcd quartz ribbon (QR) which, in part, define Sj (Tu4, = ss unknown, xnicols, Id = 4.75 mm). have lost their high relief and isotropic character. Garnet size from 0.006 in ranges (0.15 mm) to 0.23 in (5.9 mm). The have mineral inclusions majority of garnets quartz and/or opaque relic "Snowball" which are either randomly oriented or overgrowing a foliation. garnets, although they record the rotation direction during growth, are not consid­ ered reliable kinematic indicators in this study because of their extreme rarity and Fig. 12. Weathered garnets: from the Brevard Phyllite (WHI27D, plane = light, Id 4.75 mm). their possible reorientation during later dcformation(s). "Snowball" structures arc usually poorly defined and are observed only in samples Whl27D and TalsoA. Garnet compositions were not determined in this study, however, on the basis of their similar appearance and textural relationships, all garnets in the Brevard Zone in the northeastern areas are believed to be associated with the same as early metamorphic event. Metamorphic grade at this time was at least high as the staurolite zone of epidote-amphibolite facies staurolite metamorphism because was observed in two samples of Brevard Phyllitc (Tal43 and Tal43A). Structural or petrologic evidence linking garnets to the prominent foliation (Sj) is that: 1) they are both the oldest recognizable features; 2) garnets have overgrown a discernible foliation; and 3) all later events apparently occurred at lower temperature because muscovite defines an axial planar foliation in later folds. Therefore, the relative timingofmetamorphismrepresentedby these garnets isprior toformationofgroup formation of the A features, and probably syn-or postkinematic with respect to foliation, Sj. GROUP A Microstructures in Group A are F 2 folds, a secondary foliation ($2), and F-j folds. The folds and S 2 are grouped together because: 1) S 2 is axial planar to west to northwest-directed F 2 folds, and 2) they all appear to have formed under a thrust motion. Atlanta samples do not contain group A features. Hence the following discussion pertains only to features present in Tugaloo, Whetstone, Tamassee and Rosman quadrangles. FOLDS Numerous asymmetric, open to isoclinal, intrafolial F 2 folds which verge west to northwest {Fig. 13) are best seen in sections normal to the lincation and foliation. These folds fold the preexisting foliation. In rare places folds have the lower limb sheared the direction of shear. out (not pictured) which clearly shows the reliable kinematic Although asymmetric folds arc often not most indicators, these F 2 folds arc reliable because they have a consistent sense of asymmetry, i.e. to the northwest, and no larger folds are observed in the zone. However, this evi­ dence does not rule out the possibility that F 2 folds are parasitic to a larger unob­ served fold in the Chauga belt to the southeast. In places relict hinges of isoclinal F 2 folds are found in sheaves and buttons of muscovite. marked by coarse muscovite blades that have re- These hinges are the sheared out limbs of the original isoclinal fold atan to crystallized angle (Fig. 14). These relict hinges could be Fj fold hinges, rather than F 2 hinges. 24 Fig. 13. Group A folds: Asymmetric and isoclinal F 2 folds deform Sj (quartz = ribbons and muscovite), (Tu4o, plane light, Id 6.35 mm). have folds are coaxial with F 2 folds (Fig. 15) indicating that they may formed because ofcontinued shearing in the same deformation as folds are F2. only observed in TuB9. There are also scattered F 2 folds which can be traced in cuts that are par­ allel, or approximately parallel to the lineation. These cuts are slightly oblique to F 2 and F 3 fold axes. The slight difference in orientation of these folds may be a resultofthelithology,local changesinthedirectionofmotionduringdeformation, or later shearing. Quartz-feldspar layers and quartz ribbons in F 2 and folds have been highly, but not completely, recrystallized by subgrain enhancement (Figure 11). The surrounding matrix, which has also recrystallized, is finer-grained due to dis­ seminated muscovite which inhibited Most grains exhibit smooth grain growth. grain boundaries and uniform to undulatory extinction, although discontinuous undulatory extinction and subgrains arc present. Some features in group B are also highly recrystallizcd suggesting that ifgroup A features were partially recrystallizcd during their formation, then conditions during formation of B have in- group may creasedtheirdegreeofrecrystallization. Noneofthesamplesinthisstudycontain evidence of annealing such as triple grain boundaries or pervasive uniform ex­ tinction in quartz grains. This absence and the presence of subgrains indicates that annealing did not occur. Fig. 14. Relic hinges of early isoclinal folds; Hinges found in muscovite sheaves in the Brevard Phyllite. These arc cither F. or hinges. (Tu32, xnicols, ld= 1.5 mm). AXIAL PLANAR FOLIATION In some places tight F 2 folds, discussed above, exhibit a weakly developed, southeast-dipping, axial-planar foliation, S2, of nccdle-likc blades of muscovite (Fig. 16). In thin section this axial planar foliation is oriented from 0° to 37° clockwise from Sj. In some isoclinal folds S 2 is parallel to and indistinguishable from Sj except in fold hinges. A secondary foliation is present in many sections at 20°-30° in the same clockwise orientation from Sj as S 2 but no F 2 folds are visible. This foliation is also defined by scattered muscovite grains and in sections Figure 15. Group A coaxial folds: These folds deform a preexisting Ffold in a quartz ribbon (TuB9, plane light, Id= 3.95 mm). without F 2 folds, it is equated with the axial-planar foliation (S2) because they have the same character and general orientation with respect to Sj. SUMMARY features consist Group A of asymmetric folds, folds that appear coaxial with F2, and poorly-developed secondary foliation, 82* which is axial- a planar to F 2 folds. F 2 deforms the preexisting foliation (Sj) which is the oldest recognizable feature and age origin. Asymmetric F 2 and is of unknown and microfolds indicate a bulk motion of west-to northwest-directed thrusting that oc­ curred under ductile conditions. These asymmetric folds are reliable kinematic in­ dicators because: I) all folds have a consistent asymmetry and direction ofmotion; observed 2) mesoscopic folds were not in the field thus these folds do not appear sheared to be parasitic folds; 3) in rare places folds have a out lower limb indicating the direction of motion; and 4) the thrusting motion indicated by these folds is compatible with both a Taconic and Allcghanian history whereas normal faulting, the alternative, is not compatible. This shear sense is consistent throughout the northeastern study areas. The Ben Hill Granite docs not contain any group A microstructures. Fig. 16. Axial planar foliation to Group A folds: Muscovite defines in = folds (Tu32, plane light, Id 4.95 mm). GROUP B Group B microstructures include type II s-c mylonitcs, c-surfaces, minor shadows. folds and garnet pressure These features are grouped together because they were formed under ductile deformation conditions and, more importantly, they indicate a dextral strike-slip period of shearing in the Brevard Zone with a possible thrust component. For sample locations see plates in back pocket. TYPE II S-C MYLONITES Northeastern areas In the Tugaloo, Whetstone, Tamassce and Rosman quadrangles, type II s-c mylonites (Lister and Snoke, 1984) arc present only within the Henderson a Mylonite, as this lithology is the only one appropriate for the formation of such in thin sections feature (Mosher et al., 1985). These structures arc seen cut per­ pendicular to foliation and parallel to the northeast-trending, subhorizontal mineral lineation in the zone. This lincation, although usually faint, is present in virtually all samples and is always in the same general orientation (sec Appendix B). In the Henderson Mylonite, lineation is defined by a streaking of quartz ribbons, augen tails and minor muscovite, and is undoubtedly a kinematic a lineation. In the BrevardPhyllite,thelineation isdefinedbyanelongationofmuscovite andchlorite which appears parallel to F 2 and (group A) fold axes, suggesting that this line­ation is a kinematic b lineation. and surfaces Type II s-c mylonites contain s c using the terminology of Berthe et al. (1979). In the Henderson Mylonite, s-surfaces are defined by: 1) rare 30 to abundant, round to elongate feldspar porphyroclasts and, in places, recrystallized tails of feldspar and some quartz (Fig. 17); 2) scattered muscovite folia; 3) folia which wrap porphyroclasts (Fig. 18); 4) elongate grains in the matrix; and in places a dimensional preferred by 5) orientation of obliquely recrystallized quartz that shows a strong crystallographic preferred orientation (Fig. 19). C-surfaces (zones ofhighshearstrain)aredefinedbyfairlycontinuous,planarzonesofmuscovite and rare biotite and by tails on porphyroclasts which wrap smoothly into c-surfaces, giving some indication of the direction of motion ( Figs. 17 and 20). Zones are usually 0.3 mm or less in width and arc regularly spaced in thin sections at up to 1.5 mm increments. Spacing between c-surfaces varies sections. among Rccrystallized tails offeldspar which, in part, define s and c surfaces, reflect high temperatures and/or slow strain rates during deformation. These structures are true s-c mylonites because s and c-surfaces indicated formed simultaneously as into c-surfaces. by rccrystallized augen tails (s-surfaces) that wrap Areas of more intense strain or smaller shear zones occur within the Henderson of the US64 and Mylonite. In Rosman, NC, just cast Rt 178 junction, several small shear which strike and dip parallel to the Brevard Zone, were zones, observed within the Henderson Mylonite. These zones were also noted by Bond 11 (1974) and Sinha and Glover (1978). An example, shown in Fig. 21, is in (27.5 well-indurated. Thin sections taken across the zone cm) wide, well-exposed and indicate a decrease toward the center of the zone in size and abundance and augen in muscovite content, but an increase in very thin zones of muscovite rather than disseminated grains. Progressing inward from SzA and SzE, which have a very similar appearance in thin section, to SzD in the center of the zone, there is: 1) an increase increase in extremely fine-grained recrystallized layers in the matrix; 2) an Fig. 17. S-surfaces in Type II s-c mylonite from the Henderson Mylonite: are defined by feldspar augen with long, rccrystallizcd tails (outlined). Tails wrap into c-surfaces (Tu3l, gypsum plate, ss=dextral, Id=5.55 mm). in the grain size in recrystallized quartz ribbons; and 3) an increase in the com­ pleteness of recrystallization in ribbons, i.e. grain boundaries are more distinct and grains somewhat more equidimensional (compare Fig. 22 to Fig. 23). There is also a faint type II s-c microstructure developed in SzD (not pic­ tured) where s-surfaces are defined by elongate grain and subgrain shapes and scattered muscovite grains, and c-surfaces are defined by very thin zones of finely recrystallized muscovite. RosI26A was collected from the Henderson Mylonite in an adjacent shear zone near the fault contact of the mylonite with the Brevard Phyllite (Plate IV overlay for Horton, 1982). In this shear zone high shear strain during deformation is reflected in: 1) a small c-surface spacing; 2) a long tail of recrystallized feldspar and thin section. The and quartz (Fig. 24); and 3) a lack ofmany augen in outcrop width of this particular shear zone is unknown due to lack of exposure. shear in the Henderson An example from Tugaloo quadrangle of a zone shows to II structures and Mylonite poorly-developed undeveloped type crystallographic preferred orientations of quartz (Tu92 and Tu1378) (Fig. 25), less than 30 from microstructure ft (10 m) a well-developed type II s-c (Tul37A). Tul37A also exhibits a crystallographic preferred orientation of quartz grains which is a result of partial oblique recrystallization (Fig. 19). The close proximity ofthese Fig. 18. S-surfaces defined by muscovite folia: Folia wrapping elongate also define S in the Henderson Mylonite (Whl3B, porphyroclasts = = xnicols, ss dextral, Id 4.35 mm). Fig. 19. S-surfaces defined by dimensional preferred orientation; of elongate subgrains in quartz ribbons, from the Henderson Mylonite (Tul37A, = gypsum plate, ss=dextral, Id 4.35 mm). three samples and their differences in microstructure development, suggest that Tul37A is from a small shear zone although this zone was not observed in outcrop. In summary, well-developed type II s-c mylonites are present in Tugaloo, Whetstone, Tamassee and Rosman. The orientation ofthese microstructures indi­ cates that they formed during dextral strike-slip motion. The presence of recrys­ tallized quartz indicates that deformation occurred under ductile conditions. Type II s-c microstructures be concentrated in discrete, small-scale shear zones. One may zone indicates a component of oblique-slip (thrust) motion (Fig. 21) when micro- structures in SzD are reoriented. Fig. 20. C-surfaces are defined by muscovite and biotite: Large tails wrap into C's, from the Henderson Mylonite (Whl27C, plane light, ss = dextral, Id = 12.7 mm). Atlanta area s-c Type II mylonites are also present in Atlanta samples adjacent to rela­ tively narrow fault zones in the sheared Ben Hill Granite (see for reference Fig. 43). These structures are found in sections cut perpendicular to the foliation the lineation. Recall that foliation and lineation in the Ben Hill and parallel to Granite have the same general orientation as that in the northeastern study areas. Abundant quartz ribbons and minor muscovite define the foliation. Lineation is tails and ribbons defined by recrystallized feldspar on feldspar augen of quartz (Appendix B). Type II microstructures in Atlanta samples differ only slightly from those Here, s-surfaces are previously discussed. defined by very small, elongate feldspar 21. Shear zone in Henderson Fig. Mylonite: Note abrupt absence of augen in center of zone. Sample locations, where pictured, are labeled. (Rosman quadrangle). augen and oblique subgrains in quartz ribbons. C-surfaces are marked by extremely fine-grained muscovite (Fig. 26). Partial obliquerecrystallization, by both subgrain enhancement and bulge nucleation, is responsible for the development ofa dimen­ sional preferred orientation of quartz and feldspar (Fig. 27). Thin layers in the quartzofeldspathic matrix are extremely fine-grained, a result of dynamic recrystallization during mylonitization. Type II microstructurcs arc only developed adjacent to and just within fault zones. Friable rock prohibited sampling in the center of these zones. Away from fault zones mylonitization and recrystallization are less ad­ vanced, grain size is coarser, and augen are more abundant. Extremely sheared from quartz ribbons, yet unrecrystallized quartz ribbons are well-preserved away and fault zones (Fig. 28). Quartz-feldspar tails are present on some feldspar augen recrystallization has developed around the edges of some porphyroclasts. Alter­ ation sericite is extensive in to some feldspar porphyroclasts and matrix grains. Annealed textures are not present in any ofthe samples studied. In all five study areas, type II microstructures and their associated augen tails and quartz ribbons formed under ductile deformation conditions at were a 22. Henderson Mylonite away from shear zone: (SzA, plane light, Fig. ld= 13.7 mm). relatively high temperature and slow strain rate. When these microstructures are reoriented, they indicate a dextral strike-slip sense of shear possibly with a thrust component. OTHER C-SURFACES The Brevard Phyllite in the northeastern areas does not exhibit type II structures because its lithology is inappropriate, i.e., a lack of feldspar porphyroclasts and quartz, and a predominance of muscovite. C-surfaces are re- this orientation cognized in lineation-parallel thin sections as appears to be parallel to their kinematic direction. Thus, dextral shearing is expressed in phyllite units as 23. Fig. Highly recrystallized quartz ribbons: From ultramylonite in center of shear zone in the Henderson Mylonite (SzD, xnicols, Id = 4.75 mm). c-surfaces which crosscut the preexisting foliation (S|) rather than as type II s-c microstructures. These c-surfaces are defined by muscovite (Sj) which bends smoothly but abruptly into c-surfaces. C-surfaces (Fig. 29) are of muscovite thin, planar zones and rarely biotite, 0.25-1.0 mm wide but slightly wider where poorly developed. C-surfaces are often iron-stained and fairly discontinuous in thin section. Figure 30 shows the sheared edge ofa garnet indicating offset and high shear strain along this c-surface. An extensional crenulation cleavage (FCC from group D) which deforms this c-surface is also pictured in Figure 30. Note that garnet pres­ sure shadows are wrapping into the c-surface. Muscovite has not recrystallized Fig. 24. High shear strain in type II mylonite: resulted in a long quartz-feldspar tail and small c-spacing (Rosl26A, plane light, ss = dextral, Id = 11.76 mm). Fig. 25. No microstructure development outside a small shear zone: Compare to Fig. 19 collected inside the zone. (Tu92, gypsum plate, Id=5.75 mm). along c-surfaces. Figure 29 illustrates the crosscutting nature of a c-surface with earlier, preexisting F 2 folds, indicating that s-surfaccs (i.c. Sj) and c-surfaces did not form synchronously and thus are not true s-c microstructures. The c-surfaces strike northeast and dip southeast, with an approximately horizontal trace in most thin sections. Sj ranges from 28° to 50° clockwise from this trace, but 30° to 40° is the most common orientation. C-surfaces appear to have the sense of offset same angular relationship with Sj and the same apparent as D extensional crenulations. group However, the two features are separated be­ cause extensional crenulations are at an angle to c-surfaces and they deform c­ 30 and When surfaces (see for example Figs. 37). crosscutting relationships are 26. s-c S's Fig. Type II mylonite: defined by elongate grains. C's marked by extremely fine-grained muscovite in the Ben Hill Granite (Ats, = = ss dextral, gypsum plate, Id 2.26 mm) absent, these two fabrics are difficult to distinguish, even in thin section, because their lack of ofthe lithology or development (see Fig. 38). Deformation conditions cannot be well constrained, although muscovite has not recrystallized along c-surfaces but bends into parallelism with c-surfaces. C-surfaces have been found only within the Tamassec study area adjacent to the Brevard/Bluc Ridge contact (sec Appendix A). They are not present in the majority of phyllite samples collected possibly because of strain partitioning, that is, movement occurred along discrete small-scale shear zones as was the case with s-c microstructures in the Henderson Mylonite. Alternatively, this absence type II could be a function of exposure and sampling. C-surfaces arc included in group B because they reflect the same dextral strike-slip movement history and ductile con­ ditions as other B microstructures. group FOLDS A number of microfolds folds) which support an episode of dextral shearing in the Brevard zone are present, but only in the northeastern areas. The Ben Hill Granite where sampled does not contain either mesoscopic or microscopic folds. In the northeast folds are present in northeast-trending sections, parallel to the lineation. These are ductile folds, most of which deform recrystallized quartz ribbons or quartz-feldspar layers. All folds discussed below are believed to be F^ 27. orientation in a II s-c Fig. Crystallographic preferred type mylonite: Augen tail (outlined) wraps into a c-surface from the Ben = Hill Granite (Ats, gypsum plate, ss= dextral, Id 4.75 mm). Fig. 28. Highly deformed quartz ribbons: Ribbons are not extensively recrystallized, from the Ben Hill Granite (At 12, gypsum plate, = ss dextral, Id= 1.86 mm). folds and to have formed as a result of dextral shearing under ductile conditions. Thus these folds are B features on the basis of their orientation and group overprinting relationships. Several in examples of folds which appear only lineation-parallel sections are Tals3 from the Brevard Phyllite (Fig. 31) and Rosl26A and Tu7BA (not illustrated) from the Henderson Mylonite. All three contain southwest-verging No fold interference patterns are present in these folds (southeast plunging axes). samples even though fold axes are approximately perpendicular to F 2 and fold axes discussed previously. The Brevard Phyllite (Tals3) contains tightly folded quartz-feldspar layers folds) which verge southwest and contain an axial planar Figure 29. C-surface defined by deflected muscovite: This c-surface truncates = a probable group A fold (outlined) (Tal43, xnicols, ss dextral, ld = 5.55 mm). muscovite foliation (Fig. 31). This foliation is present throughout the slide. Rosl26A contains a similar southwest-verging fold in hand sample and Tu7BA contains that deforms microstructurc. a southwest-verging microfold a type II s-c The latter crosscutting relationship suggests that folding occurred just after the formation of type II microstructures during continued dcxtral shearing. It is significant that these folds do not appear in lincation-normal and sections because this that the kinematic a (northwest-trending) cuts suggests direction of these folds lies in a lineation-parallel (southwest-trending) orientation, i.e. parallel to the thin section. Evidence for interference between in mutually F 2 and folds is present perpendicular sections Tu23 and Tu23A. Tu23, oriented normal to the lineation, the contains northwest-verging asymmetric F 2 folds. Tu23A, oriented parallel to lineation and normal to Tu23, contains a possible interference pattern which re­ sulted from shearing of the preexisting F 2 folds in Tu23 (Figs. 32). Interference patterns are not unusual or unlikely because dextral shearing in a northeast- trending shear zone, ofpreexisting, northwest-verging F 2 and folds, will not re­orient fold axes, but rather shear out fold limbs in the new shear direction. In some instances (e.g. Wh52A) folds can be traced from lineation-parallel to lincation­normal cuts. This observation suggests that locally some F 2 and fold axes are Figure 30. A c-surface has sheared garnets: and deflected pressure shadows in the Brevard Phyllite (TalsoA, plane light, ss= normal for ECC, Id 4.75 mm). = Fig. 31. Group B fold: fold with an axial planar foliation of muscovite, from the Brevard Phyllite (Tals3, plane light, ss=dextral, ld= 11.76 mm). oblique to the dominant northeast trend and/or that locally fold axes are oblique to the dominant southeast trend of other axes. Alternatively, dcxtral shearing with an oblique-slip component could be responsible for folds with oblique fold axes. Thus dextral shearing of preexisting F 2 and microfolds probably resulted in: 1) the observed interference pattern; and 2) the scattered lincaton-parallel folds which can be traced in lineation-normal cuts. One problem with the above explanation for lineation-parallel folds folds) is the presence of isoclinal folds (F 2 folds) in muscovite sheaves also found The latter folds are A features be- in lineation-parallel sections (Fig. 29). group cause they have the same morphology as F 2 and folds and they are truncated by c-surfaces (Fig. 29). A possible explanation for group A folds within sections Fig. 32. Sketch of a possible fold interference pattern: F 2 and fold interference resulted from dextral shearing in quartz-rich Brevard Phyllite (Tu23A). cut normal or at a direction of these folds is that high angle to the original vcrgcncc strike-slip motion rotated preexisting muscovite buttons, some of which contained Rotation of buttons is feasible because of their high muscovite content. F 2 folds. However, reorientation of folded quartz layers and ribbons (F2) due to later shear- less ing (discussed earlier) is likely because of the competent nature of F 2 folds. Thus, some F 2 folds are now present in cuts approximately perpendicular to their direction. original vergence In the to summary, presence of southwest-verging, open tight micro- indicates folds, with southeast-dipping axes, that some folding occurred during, or just after, formation of other group B microstructurcs as a result of continued dextral shearing. Possible interference patterns of with Fj folds arc evidence of A. dextral shearing of preexisting, northwest-verging F 2 folds from group PRESSURE SHADOWS Pressure shadows a minor feature on are present as many garnets but only on those garnets in the northeastern study areas. Pressure shadows are composed of quartz and/or muscovite and, in many places, chlorite which is mimicking muscovite. Pressure shadows are present in cuts both parallel and perpendicular to lineation but are best developed in the former orientation. In lineation-normal cuts shadows are on pressure shorter and usually present only one side of garnets indi­ not as cating that they arc well-developed as in lincation-parallcl cuts (Compare Fig. 30 and Fig. 36 with Fig. 33). These differences in development suggest that shadows A features that were largely obliterated by a more some pressure are group recent deformation (i.e. dextral shearing) and/or that the kinematic a direction for pressure shadows is lincation-parallel. Pressure shadow formation occurred after garnet growth. Thus garnets are group A features or older and pressure shadows arc group B features. Pressure because shadows shadows formed or just prior to, synchronous with, c-surfaces in the orientation to have been bend are smoothly into c's (Fig. 30), and proper formed by the same sense of shear. Furthermore, crosscutting relationships with crenulations shadows formed prior to C fea­ extensional indicate pressure group tures. SUMMARY Group B microstructures useful for determining sense of shear are type II s-c mylonites, c-surfaces, folds and garnet pressure shadows. These ductile fea­ tures indicate that a dextral strike-slip period of motion occurred in the Brevard Zone after the northwest-directed thrusting responsible for the formation of group A microstructures. Some thin sections from group B (SzD and Tal43) have microstructurcs whose orientation suggests an oblique-slip, thrust component to this dextral motion. Group B features post-date group A features because: I) c-surfaces trun­ possible group A folds in muscovite sheaves; and 2) microstructures from the cate shadows: on one side in lineation-normal sections Fig. 33. Poorly developed pressure Shadows (outlined) are usually present only of garnets = (TalsoAl, plane light, Id 4.95 mm). reflect dilTcrcnt directions ofbulk motion in the Brevard Zone. Dcxtral two groups folds in but oblique-slip motion can account for west-verging group A, not for those folds that verge northwest, further suggesting separate episodes of motion, but not necessarily distinct deformation events. GROUP C Three sets ofcrenulations are considered group C microstructures. One set of crenulations is definitely a The result of extension of the preexisting foliation. other two sets are either compressional or extcnsional crenulations which cannot be used as kinematic indicators because of their questionable origin. These two crenulations and an associated foliation arc described in Appendix C. EXTENSIONAL CRENULATION CLEAVAGE The crenulation discussed in this section is a result of extension of the called extcnsional crenulation ECC preexisting Sj foliation and is an cleavage or usingtheterminologyofPlatt(1979). ThetermECCratherthanshearbandisused to decribe these features because the of necked quartz ribbons indicate a presence definitecomponentofextensionalongSjAnECC hasthefollowingcharacteristics: 1) crenulations are very open; 2) cleavage lies at a low angle (<4s°) to the envel­ narrow zones oping surface of the older foliation; 3) cleavage is defined by of in- occurs in rocks with a tense deformation; 4) it very strong preexisting foliation; and results in a exten­ 5) the sense of displacement along cleavage zones component of states that sion parallel to the preexisting foliation (Platt, 1979). Platt (1979) also ECC are responsible for the button-schist or augcn-schist texture found in many shear zones. Refer to Platt and Visser (1980, p. 402-403, Figs. 6 and 7) for other field and thin section examples of extcnsional crenulations. ECC's are fairly uncommon in the sections studied which is possibly a ECC's, however, are significant microstructures for two rea­ function of sampling. are found in all as sons: 1) they are geographically persistent they sample areas 51 orien­ except in the Whetstone quadrangle; and 2) they all generally have the same tation and local sense of shear. The local motion on ECC's is normal with top down to the southwest. These microstructures crosscut A and B. those in groups The orientation and sense of shear of ECC's is not compatible with either primary orsecondaryshears duetothrustingand/orstrike-slipmotion,thatis,ifECC'sare related to bulk motion on the Brevard Zone. Thus ECC's appear to reflect a change in the direction ofmotion. Northeastern areas Extensional crenulation cleavages appear mainly in lincation-parallel sections. ECC's are localized in phyllites along the northwestern edge of the Brevard Zone in Tamasscc and Rosman quadrangles and are found in both the Brevard Phyllite and Henderson Mylonite in the northwestern half of the zone in Localization result of sample distrib- Tugaloo quadrangle. may be apparent and a ution in the first two areas, or it may be real and related to late, brittle faulting that (group D). .However, sample distribution in Tugaloo qudranglc suggests ECC's are, in fact, localized at the Brevard Zonc/Blue Ridge contact (see plate I) as are features in group 4. ECC's are not observed in Whetstone quadrangle pos­ sibly because of outcrop and sample distribution or their lack of development. Most ECC's are poorly-developed in three dimensions. However, on the basis of those that arc well-developed in hand sample, combined with lineation­ normal thin sections from the same samples, the general orientation of ECC's was determined. The ECC plane varies in strike from north to west-northwest and dips west to south-southwest, with a mean northwest strike and southwest dip. Dips are the approximately 25°-45°, but may be slightly steeper at Brevard/Blue Ridge contact (e.g. Rosll8A) because S| also steepens. The trace of ECC's in thin sec­ tion lies between 20° to 47° counterclockwise from Sj. This angle varies along the length of individual traces. The local sense of shear on individual ECC's is always normal with top down towards the west to south-southwest. The orientation of ECC's cannot be more precisely determined because oftheir poor development in three dimensions. All of the photomicrographs in this section are from lineation­ parallcl thin sections. Talsoß and Several examples ofthe ECC developed in the study area arc RosllSA. Talsoß illustrates a that (Fig. 34) quartz-feldspar layer is extremely extended by a crosscutting ECC. Rosl 18A (Fig. 35) contains 3 parallel ECC's that have begun to extend and neck a quartz ribbon. ECC's are demonstrably younger than the microstructures in groups A and B. ECC's deform A folds (not pictured), group B pressure shadows group (Figs. 36), preexisting c-surfaccs (Figs. 37) and group B folds (Figs. 38). In thin section, ECC's can be differentiated from c-surfaces on the basis of the extensional nature, southwest dip direction of ECC and, in places, cross-cutting relationships. The strike of the ECC plane is approximately 90° to the strike of c-surfaccs. The ductile conditions under which ECC formed arc reflected in necked muscovite sheaves, quartz ribbons (Fig. 39) and quartz-feldspar layers. Figure 39 contains a quartz ribbon which has undergone more recrystallization in the necked area of the ECC than elsewhere along the length of the ribbon. Recrystallization advance in these necks because ofthe high strain associated with extension. is more Thus these areas had a higher dislocation density than the remainder of the ribbon and a greater potential for recrystallization with appropriate temperature. Atlanta area Scattered ECC's are also present in lineation-parallcl sections from the Ben Hill Granite. These cxtensional crcnulations arc fairly rare and arc found only in samples away from fault zones where shearing was somewhat less intense. Quartz ribbons have been necked by cxtensional crcnulations (Fig. 40) and in places are a re- almost completely offset. A finer subgrain size in necks (high strain areas) is sult of dynamic recrystallization, indicating that conditions were still ductile. ECC's in Atlanta are It not well-developed or traceable in hand samples. the is reasonable to assume that thin sections containing ECC's arc parallel to kinematic a direction (northeast) of the ECC because they arc not recognized in Note the thinness of the extended 34. ECC from the Brevard Phyllite: Fig. are layer, marked by arrow. Trace of ECC and preexisting c-surface labeled. (Talsoß, plane light, ss= normal for ECC, ld= 13.7 mm). 35. ECC's deform a quartz ribbon: From the Brevard to Fig. Phyllite next Brevard Zone/Blue Ridge contact. Trace of ECC is labeled. = (Rosl 18A, plane light, ss normal, ld= 13.7 mm). other cuts. Therefore, in Atlanta, the apparent strike of the ECC plane is north­ west, perpendicular to the kinematic a direction. Thus from thin section observa­ tion only, the apparent local sense of shear on ECC's in the Ben Mill Granite is normal, with top down to the southwest, the same orientation and direction as ECC's in the northeastern areas. Because the strike is unconstrained, This orien­ tation from down to the west to down to the south. may vary The rarity and poor development of ECC's in the study areas makes it dif­ ficult to determine ifthey are related to bulk motion in the zone. However, if we assume that local motion on ECC's does reflects bulk motion in the Brevard Zone, then the orientation and sense of shear of ECC's is not compatible with either pri­ mary or secondary shears for thrusting or strike-slip motion. This incompatibility indicates that ECC's reflect a change in the direction ofbulk motion in the Brevard Zone or are unrelated to bulk motion. SUMMARY Although relatively uncommon, ECC's are present in all quadrangles sam­ pled except the Whetstone quadrangle. Conjugate ECC's are only observed in At 12. These microstructures are described separately from previous groups because ECC's crosscut microstructures in A and B and are therefore relatively groups ECC's also predate retrograde metamorphism and brittle deformation. younger. ECC deforms a B shadow: Muscovite in shadow Fig. 36. group pressure been replaced by chlorite, from the Brevard Phyllite (outlined) has = (TalsoA, plane light, ss= normal, Id 5.55 mm). 57 often for are normal s n-= c ss ’ light. Phvllir.. , 1 plane , Brevard s°B, «, a|i l the IT:,( in c-surfaccs here B illustrated group as Id=2omm) deform iron-stained ECC ECC. 37. 8 Fig. 38. ECC deforms group B fold: Trace of ECC is labeled, from the = Brevard Phyllite (Tu3B, plane light, ss normal, ld= 19.8 mm). These microstructures generally trend northwest, dip southwest and their local sense of shear is normal with top down toward the southwest. This orien­ tation and direction is not compatible with shears for either thrusting or strike-slip bulk motion in the Brevard Zone. In conclusion, ECC's appear to reflect a change in the direction of bulk motion in the Brevard Zone. Figure 39. Closeup of ECC: From Figure 35. Higher strain and more recrystallization is present in necked area of quartz ribbon, from = the Brevard Phyllite (Rosl 18A, xnicols, ss= normal, Id 5.15 mm). Fig. 40. Quartz ribbon necked by an ECC from the Ben Hill Granite: (Atl2, = plane light, ss = normal, Id 4.35 mm). CHANGES IN DEFORMATION CONDITIONS Quartz deformation textures in microstructures from groups A, B and C indicate that in the northeastern study areas there was notable change in defor­ a mation conditions along strike in the Brevard Zone. rib- For example, the quartz bon in Fig. 39 probably formed during earliest shearing (i.e. during formation of Sj), prior to formation of 1 features because ribbons are involved in Fj and group F-j folds. This ribbon and others like it in the Rosman quadrangle contain partial recrystallization textures resulting from subgrain enhancement and/or bulge nucleation These are enhanced in high strain textures areas processes (Fig. 41). where ECC's have necked ribbons. However, further south in Tugaloo and Tamassee quadrangles the same-generation ECC's arc deforming quartz-feldspar layers that have been highly, but completely recrystallizcd by subgrain not en­ hancement (Fig. 11). Note that recrystallization is not complete because some subgrains and discontinuous extinction are present. This change along strike indicates that in the Rosman quadrangle relative to areas further southwest, either temperatures were not high enough and/or strain rate was not slow enough to allow for advanced rccrystallization. This change did not necessarily occur during formation of group C ECC's (refer to Figure 39), but could have occurred at any point during the ductile deformation(s) which resulted in the microstructures described in this study. Because features in groups A, B and A C formed under ductile conditions, it is not possible to determine whether group formed them or features recrystallized during the deformation which later during either group B or C formation or both. 61 This change in conditions along strike may reflect the of deeper exposure structural levels in and Tamassee relative to Rosman Tugaloo quadrangles are quadrangle, because Rosman outcrops topographically higher than those in Tugaloo and Tamassee. ribbon: Note nucleation Figure 41. Partially recrystallized quartz bulge along deformation bands in quartz-rich BP (Rosll3B, xnicols, ld= 1.7 mm). RETROGRADE METAMORPHISM The of chlorite in most of the sections studied attests to a period presence of retrograde metamorphism in Tugaloo, Whetstone, Tamassce and Rosman quadrangles. No evidence of retrogression was observed in the Ben Hill Granite. Chlorite is most often observed in microstructurcs from Be- groups A, B, and C. cause its later growth mimics preexisting muscovite, chlorite at first glance appears 36 also deformed but its formation actually postdates that of groups A-C. Figure illustrates this relationship. Chlorite is often found replacing garnet in the north­ east or forming rims on garnet and mimicking muscovite in pressure shadows (Fig. 30). Chlorite is concentrated in areas of greatest muscovite strain, that is, along c-surfaces, ECC's and in fold hinges. Chlorite appears to predate brittle faulting (group D) because it is not found in association with D microstructurcs. group It is possible that chlorite for­ mation and brittle faulting were synchronous but that the latter feature obliterated any chlorite or enhanced its alteration such that chlorite is no longer present in sections containing brittle deformation. Thus, the relative timing ofchlorite growth as a result of retrograde grecnschist metamorphism appears to be after the forma­ groups A, B and C but prior to formation of D features. tion of group 63 GROUP D Field and thin section observations indicate that a period of brittle defor­ mation occurred in the Brevard Zone resulting in mesoscopic and microscopic faults, breccia and drag folds These features are categorized as group D fea­ tures because they reflect the most recent (youngest) deformation in the study area, and they overprint all previous groups of microstructurcs. Brittle deformation is concentrated in the Brevard Zone adjacent to the Blue Ridge contact. Mesoscopic faulting and imbrication are present in the Ben Hill Granite in Atlanta, while faulting and brccciation are present in Whetstone, Tamassee and Rosman quadrangles but not in Tugaloo quadrangle. Brittle features appear to increase in abundance and intensity northeastward, where their expression reaches a maximum just west of Rosman, NC near U.S. 64 at an abandoned quarry. This outcrop contains the best exposures of brittle deformation in the study area. I be­ lieve the increase in intensity and distribution is real, and is not a function of ex­ posure due to the lack of brittle microstructurcs observed in Tugaloo quadrangle, relative to Rosman quadrangle. The absence of faulting in Tugaloo is probably due to a lack of exposure because faulting is present in all other quadrangles in the First field, then thin section northeast at the same stratigraphic and structural level. evidence ofbrittle deformation is described beginning in the southwest with Atlanta and moving northeast to Rosman (Fig. 2). ATLANTA AREA Faults in the Ben Hill Granite are continuous, low angle to subhorizontal 2-Bin in width. Faults are marked by zones of weathered, shaley-appearing zones, 64 material which is foliated parallel to fault zones (Fig. 42). Because of the friable nature of fault material, samples could not be collected in the center of fault zones, see Fig. 43 for sample locations. In places gouge has developed along these zones. Neither breccia nor cataclasites are present in the Ben Hill but one set of dip- parallel slickensides was found (see Appendix B). Faults are also delineated by the presence of two foliations mesoscopic which decrease in spacing and become more parallel as faults arc approached. Augcn size and abundance also decrease. The above observations coupled with the of ductile microstructures from B and C that presence groups strongly suggest imbricate fault zones arc localized along preexisting ductile shear zones. Thin section evidence for brittle deformation is almost nonexistent indi­ localized in Scattered cating that motion was fault zones. quartz-filled fractures such as that in Fig. 44 formation of all described ductile postdate previously microstructures in Atlanta. These fractures evidence of ductile fol- support field lowed by brittle deformation. Fault slivers of Brevard mylonitc, imbricates within the Ben Hill Granite, a and rare dip-parallel slickensides suggest thrusting motion, however, the first two features can be accounted for by either dextral or sinistral, oblique-slip motion. It ispossiblethat thisbrittledeformation istheresult ofanincrease instrain ratelater in the same deformation that formed the preexisting shear zones and the group B and group C microstructures associated with these shear zones. This explanation would account for localization of fault zones along shear zones, but requires that motion changed from the strike-slip motion which formed group B structures, to the imbricates and fault oblique-slip or thrusting motion which formed the observed slivers. It is impossible to determine how much thrust or strike-slip displacement occurred along these various faults given the lack of exposure and offset markers. One vertical at fault which postdates thrusting is present this outcrop (Fig. 43). This fault has an apparent normal motion of 6 feet. This is the only such fault observed and due to its limited exposure and weathered nature, it is not known whether this fault is related to thrusting or to a more recent event. WHETSTONE QUADRANGLE In Whetstone, an exotic carbonate slice is present in the Brevard zone at the northwestern edge of the zone where it is bounded by the Blue Ridge geologic province (Fig. 8). This slice was mapped by Hatcher (unpub. maps; 1971) who Fig. 42. Brittle imbricate fault zone: in Atlanta, GA. Light unit is the Ben Hill Granite and the dark unit is Long Island Gneiss. 67 section This Granite: Hill Ben the in faults imbricate of sketch Field nucleated have that faults brittle 3 zones. contains shear exposure ductile roadcut preexising the along of 43. Fig. 44. Fig. Late quartz-filled fractures in the Ben Hill Granite: postdate ductile deformation (At6, plane light, Id =7.8 mm). believes that the chemical composition of the slice matches that of either the Cambrian Shady Dolomite (Hatcher, 1971) or, more recently, the Cambrian Knox Dolomite (Hatcher and Butler, 1979, p.93; and Hatcher et al., 1973). Both of these formations are found in the footwall underlying the Blue Ridge decollement. The carbonate slice is unrelated to the carbonate which is of the unit (Crc) part stratigraphic sequence in the Brevard Zone (Hatcher, 1971). The creek exposure Whetstone of the exotic slice was the focus of sampling in the quadrangle, see Hatcher and Butler (1979, p.93-94) and Plate II for specific field location. The exposed carbonate slice is the only sampling location where field evi­ dence for brittle deformation was observed in Whetstone. The contact of the carbonate with the graphitic phyllitc to the southeast is a brccciatcd zone about 3 ft (1 m) wide. The exact width ofthis zone is unknown due to dense vegetation and limited creek The contact of the slice with Blue exposure. Ridge lithologies which bound the 100 ft (33 m) to the northwest (Hatcher, unpub. slice approximately not found. shear features such maps) was Mesoscopic sense of as asymmetric folds or slickcnsides were not observed. Thin sections from the slice (CS2 and CS3) indicate that it has been dis­ rupted in a ductile manner on the basis of deformation textures in the calcite matrix and scattered, intcrstital Mg-rich chlorite (R. Folk, pers. comm.) Bent lamclli in recrystallized calcite indicate plastic deformation. Some chlorite has undergone bulge nucleation its serrate and indistinct resulting in extremely fine grain size, subgrain boundaries. Other interstitial chlorite exists as highly deformed, structures. These textures could not have formed due to unrecrystallized ribbon brecciation. Broken and disrupted calcite-filled veins which postdate chlorite tex­ tures are evidence of later brittle deformation. Field evidence ofbrittle deformation is absent in creek exposure as the slice is approached from the southeast. In thin section, however, highly recrystallized brecciation quartz textures are abruptly overprinted by minor fracturing and (Fig. 45) between sample locations Whl3lß and Whl33A. Sense of shear direction cannot be determined from these microstructurcs. This minor overprinting further indicates that brittle deformation associated with of the slice emplacement postdates an earlier period of ductile deformation. Samples Tg and Whl3s were collected at the first outcrop beyond the slice in the Late Precambrian unit Graywacke Schist of the Blue Ridge province (see Plate 11, Fig. 8 and Hatcher, unpub. maps). This outcrop is the first creek exposure northwest of the slice. In thin section, Whl3s and Tg contain highly recrystallized quartz ribbons which record an earlier ductile deformation, however, there is no evidence of brittle deformation. This observation that late, brittle defor­ suggests mation associated with emplacement of the exotic slice did not affect nearby rocks of the Blue Ridge because motion was localized along the slice contact with sur­ rounding rocks. the slice to the The incompetent graphitic phyllitc which bounds southeast, appears to have undergone more deformation during slice emplacement to the northeast. than the more rigid and competent schist In it that brecciation is mainly localized along the slice summary, appears Calcitc and chlorite deformation contact with the surrounding graphitic phyllite. in the slice indicate that preexisting ductile textures were overprinted in localized by veining and brecciation. Sense of motion during brittle deformation can- areas not be determined from the brecciation observed in Whetstone quadrangle. TAMASSEE QUADRANGLE Brittle (group D) features are found at only two locations within the Tamassee field area; I) at an outcrop in a curve in the road between station Tal47 and Tal4B; and 2) at outcrop Talso (Plate III). The former outcrop lies within the Blue Ridge geologic province about 1300 ft (430 m) northwest from the Brevard Here, changes in foliation orientation across two faults indicate Zone contact. ap­ Fig. 45. Brecciation near exotic slice: overprints preexisting ductile fabrics in = the Brevard Phyllite (Whl33A, plane light, ss indeterminant, Id 11.7 mm). = parent reverse motion, although poor exposure prohibited collecting any structural data, samples or the observation of any folding. These faults are 3-4 ft (1 m) in length and they probably experienced minor movement. The second location in Tamassee quadrangle where faulting is observed is to represent one deformation as indicated Talso (Plate III). These faults all appear shown by their similar outcrop appearance and extent. The best-exposed faults are in Fig. 46 and are labeled f and for reference. Samples Talsoß and C were collected where shown in the same sketch and closely-spaced foliation measure­ ments were taken across f Exposure is better at this outcrop than the one de­ . scribed above, revealing fairly planar, discrete faults with apparent reverse motion andf of on f and other minor faults are poorly exposed and/or the sense , a c< motion is indeterminant. Stcreonet plots for fault planes, minor structural features fold (i.e. axes, slickcnsides) and foliation measurements (for sense of shear deter­ minations) are included in Appendix B. Particularly well developed along f is a pronounced foliation, probably formed base of f during fault movement. Almost everywhere, except at the , a foliation is truncated abruptly by faults and does not curve into them. Small fault blocks have been rotated along f and and drag folds folds) have developed a in places along several faults. Thin sections could not be made of fault material because of its friable or weathered nature. Brccciation was not observed, but gouge is present in places along several faults. Northeast-trending, horizontal slickcnsides were measured on but there is no evidence to suggest whether these are a result of dextral or sinistral slip. These features indicate that late faulting occurred under brittle conditions with a fairly high confining pressure and fast strain rate because of the of presence throughgoing faults. Where measurable, faults trend cast-northeast, dip to the north and have a variable direction of reverse and dextral or sinistral strike-slip motion as indicated by drag folds and slickcnsides, respectively. Displacement cannot be determined, but given their limited extent, it is likely that these faults also There is no evidence ofthis late brittle deformation experienced minor movement. in thin sections or from samples elsewhere in from immediately adjacent samples the Tamassee quadrangle. In summary, field data from Tamassee supports an episode of localized brittle deformation in the Brevard Zone. Faults probably experienced minor dis- extent. Sense of shear placement given their limited size and data is lacking but foliation changes and slickcnsides on faults indicate a local reverse motion with a 73 at changes foliation of sketch Field lalso. at roadcut quadrangle: in Tamassee exposed in faults Faults four 46. Fig. strike-slip component. This deformation was localized at the Brevard Zone/Blue Ridge contact, and did not disrupt preexisting microstructures in immediately ad­jacent samples. ROSMAN QUADRANGLE The Brevard Zone in the Rosman quadrangle contains a narrow (200 ft wide), linear zone of brittle deformation called the Rosman fault (Horton, 1982). Two excellent exposures of along this fault arc called Sega Lake and the quarry (Fig. 10). These arc found in the only adequate exposures this quadrangle which contain group D features. Sega Lake Sega Lake exposure, located in the very northwest part of the quadrangle (see Plate IV) consists of one east-trending vertical outcrop of Brevard Phyllite and Two low graphitic phyllite which is about 50 ft (17 m) long and 12 ft (4 m) high. angle faults, numerous fault slivers, and several drag folds folds) are prominent features ( Fig. 47). Figure 48 is a field sketch of Fig. 47. Faults are labeled in lowercase letters (e.g. f and and locations of where structural data was collcted are labeled in capital letters and sample locations are labeled RosllBA-C. foliation is dis- across Faults have undulating fault planes which outcrop rupted and along which a foliation is present. The latter feature probably formed by rotation along faults of the preexisting S| foliation. Breccia was not observed, The but gouge or a thin layer of soil, sometimes graphite-rich, marks most faults. strike of f varies from northwest to northeast. The presence of fault slivers, the 47. Prominent faults and fault slivers: in Brevard at Fig. Phyllite Rosman Dark unit Brevard/Blue Ridge contact, quadrangle. in a normal sense. (outlined) is ofTset differences in slickensidc orientations within slivers, and the ofslickensides presence on foliation planes away from faults indicates that motion in the Brevard Phyllite occurred along jostling fault slivers, not along discrete, planar faults, as is the case with faulting in the Tamassee quadrangle to the southwest (sec Appendix B). The overall direction of motion along of f appears to be oblique-slip, V normal motion with top down towards the southeast, as seen in the offset of the dark layer in Fig. 47. This direction is substantiated by northeast-trending, sub- horizontal fold axes which are definitely related to drag on faults because they are only observed adjacent to faults. Drag folds do not disrupt preexisting lineations 76 46. Fig. as view Same Rosman: roadcut, Lake Sega of sketch Field 48. Fig. symbols. of explanation for text Sec because fold motion (the kinematic a direction of folds) is perpendicular to loc­ ations. Changes in foliation orientation along 3 across transects f also support c oblique-normal motion on this fault. See Appendix B for plots and a more detailed explanation. Amount of apparent normal displacement on f is about seven feet as seen V in the offset dark layer in (Fig. 47), but only inches along an adjacent, smaller fault which contains an offset quartz vein (not pictured). Other minor faults in this outcrop include a high angle reverse fault (not pictured) at the west end of the outcrop across which a graphitic lens has been offset only seven inches. Foliation is the foliation bends abruptly into this fault and is not suddenly disrupted by it as across f It is reasonable to assume that all faults in this outcrop are associated . with the same deformation given their similar even appearance and minor offset, though orientations and directions of motion vary. Thin sections of samples collected from Sega Lake contain scattered, minor related be features. fractures which may not be to faulting, but may more recent Brittle deformation has not pervasively affected these rocks as indicated by the ab­ sence of such deformation in thin section. In minor faults at Sega Lake have variable strikes and directions summary of motion, however, the most prominent fault has an oblique, normal motion with top down toward the southeast. Apparent offset is only several feet and was probably facilitated by motion on anastomosing fault slivers. Drag folds suggest that deformation conditions were at the more ductile end of the brittle field. Evi­ dence of brecciation or cataclasis is not present in either outcrop or thin section. Brittle deformation appears concentrated along small faults and fault slivers within the Rosman fault. Quarry The quarry is located at the contact of the Brevard Zone with the Blue Ridge geologic province in the southwestern corner of the Rosman quadrangle Hatcher and (Plate IV; Butler, 1979, p.96). The outcrop is in the brecciated phyllonite and ultramylonite, bpu, ofthe Rosman fault (Horton, 1982) and consists of an abandoned quarry face in a hillside and an adjacent, steep bank below the road. quarry Samples are graphitic and nongraphic phyllitc which both contain an abundance shows of quartz. Figure 49 the extent of the quarry face outcrop. The trees in the background are in the Blue Ridge geologic province which is ex­ posed just behind the quarry along old U.S route 64. The Brevard/Blue Ridge contact which lies between the quarry face and background trees is not exposed. The quarry appearance is described as tectonic melange by Horton (1982) a and Hatcher and Butler (1979). Unit descriptions by Horton (1982) describe bpu as a "broken formation derived by pervasive tectonic brccciation and mixing, at all scales. Numerous mesoscopic crosscutting faults (too small to show at map scale) and drag folds of various orientations produce a chaotic appearance at outcrop; however, southeast-dipping reverse faults arc the most pervasive." weathered rock in this The extremely soft, outcrop prohibited collecting structural data except for two fault planes and one set of dip-slip slickenside meas­ urements (see Appendix B). Most faults are discrete, planar faults which are several feet long and die out or are covered at both ends. All folds appear to have behaved ductilely because of their graphite content and/or because of somewhat ductile de­ formation conditions (Fig. 50). No consistent sense of shear can be determined in of variable orientations and material. outcrop because drag fold poorly exposed The vergence of what appear to be large folds in the quarry face suggests that a top to the west motion resulted in these folds (Fig. 49). Because of their association with brittle deformation features in outcrop, it is likely that these large folds are related to brittle deformation folds) rather than to earlier ductile F2, or folds. This fact, coupled with their poor exposure and unknown origin, makes these folds unreliable kinematic indicators. A 1 fl (0.3 m) thick pod of breccia was found, but no evidence exists to suggest whether: 1) it formed in response to an increase in strain rate during the brittle deformation responsible for faulting, or 2) it formed in a separate event after brittle deformation had ceased. Thin sections from the quarry and an adjacent outcrop exhibit varying de­ of folding, faulting and brecciation. For example, Rosll3B, 113D, 114 E and grees Fig. 49. Intense deformation along the Rosman fault: Exposure is at a quarry west of Rosman, NC. Apparent (group D?) structure is outlined. that be 114 F contain open to isoclinally folded quartz ribbons arc beginning to broken into large fragments (Fig. 51). Ribbons in these fragments and elsewhere in Rosman quadrangle most likely formed prior to group A features, but because of the chaotic nature of the structures, it is impossible to determine iffolding of ribbons also belongs to group A. Other samples (RosllC, 113 A and 113C) show a more intensely brccciated, but previously ductilcly deformed rock in which frag­ 50. Folds at the Relative Fig. in graphitic phyllite: quarry outcrop. timing of these folds is unknown. merits of various sizes have rotated along anastomosing, iron-stained zones of graphite during cataclastic flow (Fig. 52). On the other hand, sample Rosl 148, less than 150 ft (50 m) away from previous samples, is unaffected by nearby deforma­ tion. None of the microstructures found in quarry samples contain useful kinematic indicators. Samples 8R123A, 8R123C and BRI2O were collected from rocks of the Blue Ridge province just northwest ofthe 8R123A quarry on old U.S. highway 64. and 123 C were collected about 650 ft (216 m) from the in a mylonite gneiss quarry unit, while BRI2O was collected 2400 ft (800 m) away in a biotite-muscovite gneiss None of these evidence of brittle defor­ unit (Morton, 1982). samples contain any mation. 8R123A contains as indi­ quartz recrystallizcd by subgrain enhancement cated the numerous ribbons with excellent by subgrains present. Quartz an crystallographic preferred orientation and numerous muscovite buttons arc present. 8R123C and BRI2O have undergone a slightly higher degree of rccrystallization, by the same mechanism and they contain minor muscovite and very few, recogni­ zable quartz ribbons. This evidence further indicates that late brittle deformation localized along the northwestern edge of the Brevard Zone along the Rosman was fault and that deformation did not affect the surrounding rocks. In summary, folding and faulting at the quarry outcrop of the Rosman fault NC associated with a brittle deformation. near Rosman, appear late, a Brecciation may possibly be slightly later process resulting from an increase in strain rate during this event. There is no consistent sense of shear in the observed microstructures; however, apparent structure in outcrop suggests a generally west- Deformation is directed motion which supports that of Morton (1982). concen­ trated along the northwestern edge of the Brevard Zone. Fig. 51. Incipient brecciation of folded quartz ribbons: in the Brevard Phyllite (Rosll3D, plane light, bulk ss indeterminant ld= 13.7 mm). = SUMMARY Group D features consist of brittle faults, breccia, and scattered drag folds and slickensides. These features are observed in widely scattered outcrops in each Evidence for brittle study area except the Tugaloo quadrangle in South Carolina. deformation is neither abundant nor pervasive, but where present such evidence is localized along the Brevard/Blue Ridge contact in each ofthe study areas. Amount of apparent displacement on faults at Sega Lake in Rosman is several feet at most, whereas at all other fault outcrops displacement is unknown. Direction of motion is also unknown or highly variable on most faults in the northeastern study areas. At the quarry in Rosman, sense of motion on asym­ to be generally west-directed. In the Ben Hill metric, mesoscopic folds appears Granite, imbrication suggests northwest-directed thrusting, however, a bulk, oblique-slip thrusting motion can also account for these features. The overall bulk motion in the Brevard Zone during brittle deformation cannot be determined due to the lack and the variability in orientation and direction of motion of exposure on faults. Group D features in the Ben Hill Granite arc correlative with those in the northeast because of the following similarities: 1) brittle deformation in the Ben Hill Granite also postdates all preexisting ductile features; and 2) there is no evi­ dence D micro- for retrograde metamorphism occurring after formation of group structures. Fig. 52. Intensely brecciated phyllite: from the quarry, Rosman quadrangle (Rosll2C, plane light, ss = indeterminant, Id= 13.2 mm). D features indicates that a Thus the presence of group separate, more re­ cent deformation occurred after that/those which formed microstructurcs in groups A, B and C. This brittle deformation was localized along the Brevard/Blue Ridge contact, but was apparently regionally extensive because evidence for such defor- In the Rosman mation is present in 4 out of 5 study areas. quadrangle brittle de­ formation was laterally continuous resulting in the Rosman fault, a narrow zone of faulting and brecciation parallel to the Brevard Zone/Bluc Ridge contact. Bulk motion during this deformation is indeterminant in the study area. SUMMARY The results of this study indicate that there exists more than one direction ofbulk motion within the Brevard Zone as indicated by the microstructures present. The relatively oldest recognizable microstructure is a pronounced foliation, Sj, for which the sense of shear is unknown. Extremely weathered garnets overprint Sj, reflecting their growth syn-to postkinematically with Sj. Group A microstructures are folds which deform a west-to northwest-directed F 2 and Sj, reflecting ductile thrusting motion. Group B microstructurcs arc type II s-c mylonites, c- surfaces, folds and garnet pressure shadows, all of which are relatively younger than group A. The orientation of these features indicates that a period of bulk, dextral, strike-slip motion, possibly with a thrust component, was required for their formation. Group C contains only one microstructure, an cxtcnsional crenulation cleavage (ECC). ECC's postdate the microstructurcs in groups A and B, and they have a bulk sense of shear that is incompatible with cither primary or secondary riedel shears in a dextral strike-slip shear zone. Thus ECC's appear to represent a change in the direction of bulk motion in the Brevard Zone. The most recent fea­ tures arc faults and associated breccia (group D) which do not reflect a consistent direction of bulk motion in the zone. Retrograde metamorphism occurred after formation of groups A, B and C, but prior to formation of group D. Groups A, B and C formed under ductile conditions as seen in the defor­ mation textures preserved and the types of microstructurcs formed. Annealed tex­ of the samples studied. The deformation conditions tures are not present in any along strike in the northeastern study areas were difTercnt at some point in the de­ formation history of the Brevard Zone, prior to formation of group D features. 85 Group D features formed under relatively brittle deformation conditions and the presence ofbreccia suggests that a possible increase in strain rate occurred toward the end of this deformation. TIMING The following is a discussion of the timing of formation of each of the above groups. This discussion is an attempt to associate these microstructures with the known dcformational events in the southern Appalachians. Correlation is on the basis of determinations from known structural relationships and radiometric age the Brevard Zone. The timing of formation of foliation (Sj) in the Brevard Zone is unknown in the northeast. It is likely that these features were formed during the Taconic Ma orogeny, preceding or synchronous with high grade metamorphism ca. 480-435 (Glover et al, 1983), as this was the first and most intense, ductile deformation to affect the southern Appalachians. Garnet in the northeast probably formed at this time because most overprint a foliation. In both the Taconic and Alleghanian orogenies, tectonic models include west-to northwest-directed ductile thrusting which is compatible with group A microstructurcs IfA features arc Taconic in then they from this study. group age, formed during the presumed westward collision of the Inner Piedmont island arc with the North American continental margin (Fig. 5). This intense deformation would have formed the northwest-verging F 2 microfolds and axial planar foliation, S2, after the formation ofSj presumably earlier in the Taconic orogeny during ini­ tial deformation. Scattered refolded folds arc coaxial with F 2 and probably same event. formed during the The Acadian orogeny is believed to have been less intense than the Taconic orogeny. Demonstrable evidence for the sense of shear during any Acadian defor­ mation in the Brevard Zone is nonexistent, unless northwest-verging folds are 87 actually Acadian in age. Bond and Fullagar (1974) interpret a Rb-Sr whole rock age of 387 4-/-14 Ma (from the Henderson mylonite in the Rosman quadrangle), as a result of re-equilibration during Acadian metamorphism. Also, Odom and Fullagar (1973) postulated that their 356 + /-S Ma Rb-Sr whole rock age on the same rocks is a result of distributed shearing and recrystallization rather than major displacement during the Acadian orogeny. Both of these dates represent a resetting Sinha and Glover (1978) obtained a 596 Ma of the Henderson mylonite protolith. crystallization age (zircon discordia) and Odom and Fullagar (1973) obtained a 5354-/-27 Ma crystallization and cooling age (Rb-Sr whole rock isochron) for the relatively undeformed protolith. Therefore, the Acadian orogeny is not a strong of the microstructurcs possibility for the time of formation of any in this study be­ cause 1) there is a lack of information concerning the nature and extent of defor­ mation and 2) there is stronger evidence linking microstructures to either the Taconic or Alleghanian orogenies. The Alleghanian orogeny is another possibility for the timing of formation A microstructures. of group In this case Sj and garnets would remain as the only evidence ofTaconic deformation and metamorphism. West-to northwest-directed thrusting is compatible with at least part of an Alleghanian history for two reasons. and Inner Piedmont One, the Blue Ridge province, Brevard rocks, province were thrust westward as a unit along the Blue Ridge dccollemcnt during the Permian. and related Pennsylvanian and The timing of Blue Ridge thrusting is known because these thrusts deform Carboniferous rocks of the Valley and Ridge The Brevard Fault Zone formed (e.g. Rodgers, 1967; Hatcher and Odom, 1980). as a subsidiary thrust synchronous with, or just after, movement along the Blue Ridge decollement (Reed et a1.,1970; Hatcher, 1978). Two, as the Brevard fault zone formed, its movement brought to the surface an exotic slice ofcarbonate from the footwall underlying the dccollement (Hatcher, 1971). Emplacement was ac­complished by thrusting or oblique-slip motion and could have resulted in the for­ mation of group A microstructures in the Allcghanian. Although the exotic slice present in Whetstone quadrangle appears to have been thrust into place under brittle conditions, it was probably emplaced under ductile conditions. Then, later brittle deformation (group D) overprinted previous ductile features, resulting in the final position of the slice close to the surface. Ductile Allcghanian microstructures been documented in the Brevard it seems (group B) have Zone (see below), thus likely that Allcghanian thrusting was also ductile. In structural that the foliation is summary, relationships suggest Sj Taconic in Group A microstructures formed in either the Taconic were or age. Alleghanian orogenies. Recall that the Ben Hill Granite, an Allcghanian intrusive, does not contain A microstructures. This observation is the only evi­ any group dence to suggest that group A microstructurcs in the northeastern areas are Taconic rather than Alleghanian in age. B and C microstructurcs postdates that of The origin of group group group A and represents a period of ductile, dextral strike-slip motion in the Brevard Zone. Field data and thin section observations from this study attest to a striking simi­ in the morphology, deformation conditions and direction of motion of larity microstructurcs in B and C in the northeastern study areas, with those from groups the sheared Ben Hill Granite in Atlanta. Sinha (pers. comm.) obtained a U-Pb zircon date of 280-290 Ma from the Ben Hill Granite which he interprets as a This intrusive was crystallization age. subsequently deformed by movement along the Brevard Zone, thus dating some of the most recent motion in the zone. Therefore, microstructures in groups B, C and D preserved in the sheared Ben Hill Granite are no older than Alleghanian in By analogy, similar microstructures age. in Tugaloo, Whetstone, Tamassce and Rosman must also be quadrangles Alleghanian or younger in age. Dextral strike-slip motion during the Alleghanian orogeny is supported by the research of Reed and Bryant (1964) and Bobyarchick (1983, 1984) who con­ ducted field studies in the Brevard Zone in, and adjacent to, the Grandfather Mountain Window (GFMW) in North Carolina (Fig. 1). Rccd and Bryant (1964) found that lincations in the GFMW into northw'cst-trending gradually swing in the Zone the parallelism with northeast-trending lincations Brevard as zone is this in orientation dextral approached. They interpreted change as reflecting strike-slip motion in the Brevard Zone. Bobyarchick (1983, 1984) working south­ west of the GFMW interpreted the orientation of oblique crcnulation cleavages in Dextral motion in the Brevard the Brevard Zone as also reflecting dextral offset. also the movement of the Inner Zone is compatible with Alleghanian history Piedmont (Bobyarchick, 1981; Gates et al., 1984). in the Atlanta Higgins (1966) suggested dextral motion along the Brevard as did McConnell and Costello (1980, p. 253). The latter two researchers area, suggested adextraldisplacementof24mi(35-40km)onthebasisofapparentoffset in the Palmetto and Ben Hill Granites and in quartzites ofthe Sandy Springs group. Bryant (1964) estimate 135 mi (225 km) of late Paleozoic dextral offset. Reed and This estimate is calculated on the basis of exposed Henderson Gneiss stretching from Toccoa, GA to Lenoir, NC. However, Bryant and Rccd (1970) suggest that the Brevard Zone experienced greater than 135 mi (225 km) of sinistral offset. The microstructures in the present study are not quantitative strain markers and cannot be used to estimate displacement in the zone. Ductile deformation conditions indicated by features in groups B and C are also consistent with an Alleghanian deformation. For example, the change in line­ ation orientations cited above (Reed and Bryant, 1964) and the oblique crcnulation cleavages of Bobyarchick (1983) are also features which require ductile conditions for their formation. The origin of group C features postdates that of group B. Group C microstructures, i.e. ECC's, crosscut preexisting features and arc found both in the Ben Hill Granite and in the northeastern areas. Therefore, because the orientation of ECC's is not compatible with either thrusting or strike-slip motion, they must represent a change in the direction of bulk motion in the Brevard Zone. Group C features they are found in the Ben Hill can be no older than Alleghanian because Granite. However, it is possible that they arc more recent than Alleghanian and related to Mesozoic rifting. The of formation of D features can be no older than timing group Alleghanian in age because: 1) microstructurcs from group B arc demonstrably and 2) features in group Alleghanian in age; D postdate those from groups A, B and The direction of bulk motion during this late brittle deformation is unknown C. localized extent and variable geometries and movement because of poor exposure, directions on faults. The presence of stages of microscopic faulting and brecciation, and drag folds and breccia in the same outcrop, both suggest that D features group formed under an increasing strain rate. I suggest that after formation of features in groups B and C earlier in the Alleghanian, that deformation conditions became more brittle toward the end ofthe Alleghanian, possibly as the strain rate increased resulting in group D features. There are two alternatives to this theory. Rather than an increase in strain rate, cooling via unroofing of rocks in the Brevard Zone could have resulted in more brittle deformation conditions, if unroofing and defor­ mation were synchronous. Alternatively, the extreme differences in deformation conditions and the apparently random nature of brittle features, suggests that this deformation could be a separate, more recent event, possibly related to the rifting events which led to the opening ofthe present-day Atlantic Ocean. CONCLUSIONS 1. Microstructures from the Brevard Zone in the Northwest Atlanta, Georgia quadrangle, the Tugaloo, Whetstone and Tamassce quadrangles in northwest­ ern South Carolina and in the Rosman quadrangle in southwestern North Carolina indicate that there were at least two periods of ductile deformation in different direction ofbulk motion. A late the Brevard Zone each representing a brittle deformation overprints all previous features and the bulk motion at this time is unknown. 2. Observed microstructures can be separated into four on the basis of groups microstructure orientation, overprinting relationships and dircction(s) of mo­ tion. 3. are and weathered is a The oldest recognizable features Sj very garnets. Sj prominent foliation defined by muscovite, chlorite and quartz ribbons. Sj and garnets probably formed during the Taconic orogeny. Fj folds are not obsrved but these are likely Taconic in well. age as features consist of 4. Group I tight to isoclinal Vj folds, an axial planar foliation, and S2, and scattered folds. folds deform Sj. Group A microstruc­ 93 tures formed under a west-to northwest-directed ductile thrusting motion probably during the laconic orogeny. 5. Group B microstructures include type II s-c mylonites, c-surfaces, folds and garnet pressure shadows, all of which reflect a period of dcxtral strike-slip mo­ tion in the Brevard Zone. Group B features formed during the Alleghanian orogeny because the Ben Hill Granite in Atlanta, which contains these micro- structures is dated at 280-290 Ma. The Ben Hill Granite is truncated by motion on the Brevard Zone which resulted in the formation of features in B and group possibly those in groups C and D as well. 6. Group C consists of extensional crenulation orien­ cleavages (FCC's) whose tation suggests that they reflect a change in the direction ofbulk motion in the Brevard Zone. These are either Alleghanian or Mesozoic in age. 7. ductile deformation conditions occurred along strike in the north- A change in eastern study areas sometime prior to formation of group D features. 8. Retrograde metamorphism appears to postdate formation of groups A, B and C and predate formation of group D. 9. Group D contains microscopic and mcscoscopic faults, breccias and drag folds which formed under brittle conditions and an unknown direction of bulk mo­ tion. Group D features probably formed toward the end of the Alleghanian in response to an increase in strain rate after the formation of micro­ orogeny, structures in B and C. groups 10. The results ofthis study confirm an episode of dcxtral, strike-slip motion in the Brevard Zone the This motion was during Alleghanian orogeny. regional in extent, having afTected rocks from Atlanta, Georgia to the Grandfather Mountain Window in North Carolina. The amount of displacement during this deformation is unknown. APPENDICES APPENDIX A TABLES OF MICROSTRUCTURES The following data tables were compiled for each quadrangle listing each thin section made, the general orientation (Cut) of the thin section with respect to lineation and foliation, the microstructures from that groupsA, B,CandD,ifany, are observed in each section, and any pertinent comments. 97 • Saetioo Muabar ** o o • f-* O Va .* o • rs 8 g V J o 0 1 ta 5 o \ m .> c • 9 S Tul Pp Tu2 Pp • Tu3 pp • • Tu6 Pp Saapla Oran froa Chauga belt; Tu7 Pp • Tull Pp•• Seven parallel fractures Tul2 Pp • aarked by aicas Aaphlbolita froa thaTuliA p Chauga belt Tul5 Pp•• Chauga bait taapla Tul 6 • Chauga bait aaaple; Pp Highly recryst. Tu17i p Chauga bait aaapla Oldaat cran aarkad byTu18 • pp recryst Habe in sheaves Tul 9 Pp • Tu21 Pp • • Oldest cran liabsPp• • visible in sheaves Tu22 Tu23 • Pp Perpendicular to Tu23* F2/F4 fold Interference Tu23* p • pettem BlueTuZt’ Pp • Ridge aaaple; highly recryst; S2? Blue Tu26a p Ridge aaaple; Ore vein Tu27 p Blue Ridge staple Tu29 p Chauga belt staple; cren Oldest cren llebs In • Tu30 Pp • p• Tu31 OB • Tu32 Cren • P-thln auction cut psirallal to linaation Total nuaber of thin sections* 73 Pp-thin section cut perpendicular to linaation 0&»thin taction cut obliqua to linaatioo PRT*partial obliqua recrystall 1satIon textures Cran-S la Granulated, a Granulation la praaant1 racry cry at-re stellited Table A.I: Microstructure Table for Tugaloo Quadrangle: Tugaloo data continued Section Number Cut «o T3 r—4 O rv u. rw CO P 3 folds Type II 8-c microstructures Recryet, qtz-feld tails on augen C—surfaces F. folds A v h 3 S3 83 C a. to 7 -*-> o ® T3 E5 qj m O ECC Brecclatlon Comments/ other Tu3i3 P • Fi fold? Tu38 P • • Tu39 Pp • Cron TuiO Pp • Tul2 TuA3 PP Pp • Vein Blue cren Ridge sample; and fractures Tui5 Tui6 PP PP • Oldest cren limbs in sneave “hot same sample as Tu4.6A; Oldest cren limbs in sheaves Tui6A Pp • Tu47 Pp Cren Tui8 Pp ¦ Tu5i Pp Tu55 OB Tu60A PP PRT in qtz Tu60B Tuol P P? • • • Poor type II; PRT in qtz; Perpendicular to TuoOA Chauga belt sample; cren; S2? Tuo2 Tuoi ?u67 ?D PP • Chauga belt sample Folds of unknown age ; oldest crens in sheaves Chauga belt sample; Two foliations Tu63 Tu 693 TuTO 7u?1 ?p ?P p;. • • • Chauga belt sample Chauga belt sample; vein Chauga belt sample; hie'” 1 v recrystallized Chauga belt sample; frant T* ™ **» • Veins i Tugaloo data continued Section Number 9-r-0 •3i: -J vn »~9C n) O' ;*> •~3 O' 03 »-3C-J CD> *¦3C<1 NO -3C00o TuSIB -9c00M S’00 S’00 v-n 200o S’CD-J S’00 vD »-3C vDK) *-9C vO -J C vO-3 2vO00 ?vOvD ; TulOOA 2 ofO * S’ —o\>i> V-O > UJ-J 03 za 03 2 v>> Cut S’ 03 d * 03 •S’ 'd S’ o 03 O)03 S’ S’ S’ o03 •3 S’ ¦3 0303 •? ¦3* O03 o 03 03 •? o03 03 e ur t s qta-feld augen pressure true on II s o folds folds folds r s-c Recryst. tails C-surfaces Carnet shadows Brecclatlon 2 23 mi Type c ECC FSF • • • • • • • • • • Comments/other W*.— o rr H-£1 ®p r w ®p<(TP -rort pPU® ppU03»-*® PP 9 -JJ'U <® H-P <® H*PP 0303 H H*3 X) ftN ftoCK-* o«+Cro3 pPg03h-J® tl£--O ®OiH3-t -tJ-O3 -*•3 3+g ®3 <® H*-3« o*1®p Pymrt-ppB03I—*® a?2£ -• 3®P o 01 K—' M oP TJXTM-O 03o® *-> • ££ §9 o ® Prt»-o Drt h v^j oj ocft oc *->oog —* LM 03 rzJ >TO o 9O5POQ® 3 Mcl M>| PH-nft rt 5 F CO Typo F. Garnet ECC Number 2 OJ 4 Wh52> p • Wh12731 rp • Wh127B2 p • Wh127C p •• Wh127D p Whl 29 Pp '->7)130 p ’.7)1318 p • Whl32F OB '.7)133A P Whl33B OB Whl35 P 'Whl 38 VhUOA P WhUOB PP Tg CB ,-\t5 CS2 C33 -p ¦=thin section cut to lineation parallel T=‘lii section to cut perpendicular lineation v=thin. section cut oblique to lineation chi.=chlorite porph:s-porphyroblasts Total number of thin sections= 13 U 0) JZ 0 (0 c g 1 I Brecciation +* 3 Axial planar foliation (S4) Abundant chi. highly renrvst. atz nhear band Incipient Prom samnle WH.127B From sample Wh127B, qtz-filled fractures in porphs Wh127B,C,D from HK near fault? contact u/ BP Snowball garnets Throe shear bands} obliquely recryst. qtz layers Crens • Float next to creek; numerous• veins postdate brecciation Fractured • porphsj thin veins; one kink fold • Thin veins Blue Ridge sample Recrvstallized Fylonite Gneiss of the Chauga belt: numerous aueen Ditto Blue Ridge sample Highly recryst. Carbonate-filled veins are• brokeniearly ductile textures C32 t-C33 from exotic slice;• early ductile textures Table A.2: Microstructure Table for Whetstone Quadrangle: tores qta-feld augen pressure s-c 03 on X! ¦o rH II r—i oo folds 03 microstruc Hecryst. tails C-surfaces shadows — Brecciation Comments/other Section Cut Ciu™ Type Garnet Number 00 ECC TaU2 P • Type II is faint, • late, qtz-filled vein TaU3 OB • Tiny, qtz-filled recent • • fractures; staurolite present TaU3A OB Cut to Tal13 perpendicular • One staurolite e-rain seen TaU9 P • Folds of unknown age Tal50A1 OB • • From sample Ta150A Tal50A P Chlorite rims on garnets Tal50B P •••• Ditto Ditto Folds axe either F2 or FI Tal50C P • Weak axial planar foliation Tal53 P • (SI) to FI Three shear bandsTal55 PP • Fractured porohs Tal 58 ?P • vein PRT in a qtz ?=thin section cut parallel to lineation Total number of thin sections= 11 section Pp=thin cut perpendicular to lineation 3=thin section cut to lineation roblique PP.T=partial recrystallizatlon textures A.3: Table Microstructure Table for Tamassee Quadrangle: T3 • C Ve • other A S’ pressure c B B•0 T3 T3 e-> <—AH BB II folds O O >S'H »-c xylonite eJcr « C-surfaces Garnet ehadous Brecclatlon Comments/ Section S3 O Cut a."I* Type a Nuaber CM tob. cc PORTRosl12C • Qtl fragments contain Pp • ditto Soil 13A OB ribbons contain PORT Ros113B • Qtz OB • Ho*113C OB unknown Folded qtz ribbons of Ro»113D OB • w/ PORTage HoslUB OB • RoslUC OB • paint, thin veins Folds of unknown age RoslUE OB PORT • In qtz Folds of unknown age HoslUF PRT 03 • Inqtz PRT in ribbons RoslISA P • qtz crens or Open ECO? RoslIBD O PRT In ribbons qtz veins of qtz u/ PRTRosl 19 OB Sheared • Rob 120 P From the Blue Ridge province Blue Ridge province sample Ro*i23A P ECO? Open crens or Rosl23C P veins Faint, thin, recryst. Rosl24 PP • Ros126A P • • ¦Rosl 263 ? Folds of unknown sheared age, Roe 162 OB veins, PORT in qtz SZA OB • SZB OB SZCPp • SZC OB • • From center of shear zone SZE P P-thln section cut parallel to llneation Total nuaber of thin sections* 24 Pp«thln section cut perpendicular to llneation ?ORT*partlal oblique recrystallliation textures PRT-partial recrystallliation textures 0&»thln section cut oblique to and llneation Table A.4: Microsfructure Table for Rosman Quadrangle: ditto 7 porphs and sections= Gneiss garnets rotated garnets porphs ft thin Island of subhedral fractured anhedral garnets rotated ft Long number the fractured, and fractured, subhedral fractured Total Coranents/other from ditto ditto Brecciation oow •• lineation folds • shadows to lineation pressure Ga-met to 3O PpP PF FFF id tV 'Section Number Atl At2 At£ AT5 Atb At1 A Quadrangle: Atlanta Northwest for Table Microstructure 5: A. Table APPENDIX B STRUCTURAL DATA Features such as lineations, slickensidcs and mesoscopic fold axes were measured and plotted on the Schmidt (equal angle) stereonets shown below. Data is plotted by quadrangle. "N" is the number of data points plotted. Tugatoo Quadranaia WhatatoM Owdrawqla 105 TuuumQuodraneta Rotman OiMdrMfli* Atlanta Ooadran«la Nor BRITTLE FAULTS Changes in foliation orientation across brittle faults exposed in the Brevard Zone were used to substantiate the direction of motion on these faults as deter- Foliation mined by field observation of drag folds and offset beds where present. orientations were measured across two faults in the Tamassce quadrangle (Fig. 46) and three sets were measured across the main fault exposed at Sega Lake in the Rosman quadrangle (Fig. 48). The great circle for the fault (as measured at that transect) and the foliation measurements were plotted on a WulfF(equal angle) stcrconct. Then the fault plane and all foliations were rotated about the axis required to rotate the fault plane to horizontal. The original strike of the fault plane is assumed to be the kinematic b direction and 90° to bis the kinematic a movement direction. It was also assumed that the foliation measurement furthest from the fault approximates the foliation orientation prior to faulting. The rotated plot is then compared to those from 7 (b-f)) which illustrate planes in various orientations Skjernaa (1980, p. 105, Fig. during progressive deformation. By comparing the orientation of the plane which approximates the undeformed state (rotated plot) to the final, deformed orientation of each plane (unrotated plot), the general direction of motion on each fault can be determined. 8.l and 8.2 contain the unrotated and rotated stcreonet plots for Figures fault f at Lake with the kinematic directions labeled. The undeformed Sega foliation orientation for these three plots is U and all other planes plotted are those that are deformed. In general, the final foliation orientation in each of these three a top down to the southeast or transects suggests northeast motion depending on the orientation of kinematic a. When compared to the true fault plane orientation, f each transect normal motion on . suggests Figure 8.3 contains the unrotated and rotated plots for two small faults in Tamassee quadrangle (Fig. 46). The undeformcd orientation is that ofA forf and V E for f In general when A is compared to each deformed foliation adjacent to the . fault a reverse motion is required on f to rotate A into that position. The same direction applies to that for the undeformcd orientation of E on f At some point . during movement there was also a strike-slip component to motion on f as seen in slickenside orientations. B.l: Stereonet plots of foliation planes across faults: Sega Lake, Rosman Fig. quadrangle. Fig. 8.2: Stereonet plots of foliation planes across faults: Sega Lake, Rosman quadrangle. faults: 8.3: Stereonet plots of foliation planes across Tamassee quadrangle Fig. APPENDIX C MINOR MICROSTRUCTURAL FEATURES Crenulations Two sets ofcrenulations other than the ECC's of C arc observed in group many phyllites in the northeastern areas. These crenulations are observed study only in phyllites because an expresses abundance of muscovite best these features. It cannot be determined whether these crenulations are extensional or compressional features. a small The oldest crenulation recognizable has very amplitude and wave­length, the original dimensions of which cannot be determined given the recrystal­ lized nature of this crenulation. This crenulation is as coarse blades of expressed muscovite which have recrystallizcd along the original crenulation limbs. These blades are recognized only in sheaves ofmuscovite. A second set of crenulation very broad, open cleavages is also present and abundant than the former set. This set is than that described above more younger because it muscovite sheaves which contain the oldest crenulations. This warps second crenulation has broad, gentle limbs with an amplitude of 0.04 in (1.1 mm) and 0.08 in (1.95 mm), and a wavelength of0.65 in (16.2 mm) and 0.94 in (23.4 mm) in Tu32 and TuB9, respectively. The crenulations may have formed synchronous or just after the formation of ECC's because the second with set overprints both B microstructurcs. a group A and group A lack of overprinting relationships and 112 similar morphology in thin section are the only evidence to suggest that the second set of crenulations are actually poorly developed ECC's. It cannot be determined whether these two sets of crenulations are exten­ sional axes or compressional features. Furthermore, the orientation of crenulation is not discernible in hand sample because of lithology, interference with preexisting extensive For these nei­ features, weathering and/or lack of development. reasons, ther of the above two sets of crenulations were used as kinematic indicators. In several sections a weak secondary foliation defined by rotated muscovite grains, has developed parallel to the apparent western limb of the second set of crenulations and thus it is probably related to the formation of these crenulations. In thin section, this foliation lies 20° clockwise from Sj, whereas 82* the other foliation observed, lies in counterclockwise direction. This weak foliation is present in several sections in which crenulations are not observed, but it has a consistent the same weak foliation associated angular relationship to Sj suggesting that it is with the second set of crenulations. This foliation is also an unreliable kinematic indicator. Rare Pressure Shadows shadows Rarely-developed, rotated quartz pressure are present on opaque These shadows or beards have grains in Rosll9 (Fig. 4). grown on rectangular, grains 0.02-0.04 in (0.5-1.0 mm) in length. Curvature on some shadows opague indicates clockwise rotation during growth, others counterclockwise rotation, and rotation. These some have not undergone any pressure shadows were not used as sense ofshear indicators because of their ambiguity, rarity and unknown timing of formation. Veins Both deformed and undeformed veins arc Brevard Zone rocks. present in was observed in rare cases. Quartz is the predominant vein-filling although chlorite The best example of sheared veins are three cn echelon veins in Rosll9 (one is shown in Fig. 5). These veins are filled with quartz which was sheared and partially recrystallized. Numerous other scattered veins are present (sec comments in tables in Appendix A) varying from partially rccrystallizcd (R05162) to highly recrystal­ lized quartz vein-fillings (R05124) to calcitc vein-fillings (CS2). Veins arc generally less than 1.0 mm in width and arc best seen in plane light. The timing of vein for­ mation is unclear. For example, those veins that arc sheared arc no younger than C microstructurcs. However, some undeforrned veins arc cut by brecciation group (Whl33A) indicating that they formed prior to group D and that there was more than one episode of vein formation. The inconsistent orientation of veins and their as kinematic indicators unknown timing prohibited their use in this study. Kinks and Shear Bands kink bands Rare, narrow are present a high angle to Sj, some of which muscovite chlorite foliation along their length (Tul2). have developed a or The adjacent foliation bends into some kinks while it is abruptly truncated by others These were (Rosl I4C). observed only in the Brevard Phyllitc. Rotated Fig. C-1 quartz pressure shadows on opaque grains: (Rosl 19). Shear bands are used to describe very straight, thin (0.04-0.08 in, 1-2 mm) zones of ductile shearing that arc also at a high angle to Intense Sj. recrystallization has occurred along some shear bands, and Sj is disrupted and/ or porphyroclasts are ofTset (Talss). These shear bands arc best developed in the Henderson Gneiss. Note that these are not the same as C-surfaces described under B. Shear bands are not useful sense of shear criteria in this study because group their direction ofmotion varies, their timing is unclear and their orientation in three dimensions cannot be well-constrained. veins. of formation is unknown. (Rosl 19, xnicols, ld = 11.76 mm). Fig. C.2: Sheared Quartz filling has partially rccrystallized. Timing APPENDIX D QUARTZ C-AXIS DATA from Five samples (Atl2, Tul37A, R05162, Tu3 and Tal42) were chosen Sections At 12 and TuI37A widely separated areas for quartz c-axis measurements. were chosen because they contain microstructurcs which reflect a dcxtral shear di­ rection. R05162 was chosen because it contains an extremely well-developed, par­tial, oblique recrystallization ribbons. Tu3 contains of of quartz F 2 folds recrystallized quartz-feldspar layers and Tal42 exhibits a good crystallographic preferred orientation of quartz. C-axis orientations were measured on the Universal stage relative to the fixed geographic coordinates (strike and dip) of each thin section. Plots were the and the Fortran produced using Cyber computer system plotting program written by Renee Drcicr at the University ofTexas at Austin. For each sample two were on Schmidt Plot A is quartz c-axis plots produced (equal area) stcrconets. rotated so that Sj (as measured for that sample) is in a vertical orientation and L (a mineral elongation lineation, ifpresent) is approximately horizontal. The data was then rotated again to produce plot B with Sj approximately vertical and L vertical. The number of c-axes measured and plotted is given as "N=". In some cases (Tu3 and Tal42) c-axcs plots did not yield recognizable patterns (Fig. D.l). On the other hand, the patterns in Tul37A, plot B and R05162 (Fig. D.2) somewhat resembles a girdle maximum. At 12, plot A (Fig. D.3) re­ 117 sembles a weak single girdle such as that ofSimpson and Schmid (1983, p. 1287, fig. 11) except that distinct maxima have not developed. Simpson and Schmid (1983) used two criteria for inferring the sense of shear from quartz c-axis fabrics; 1) the asymmetry about the foliation (S) of outline" maxima developed in a single girdle; and 2) the asymmetry of the "skeletal in a crossed-girdle pattern. Neither of these patterns is recognizable in the plots from the present study and the sense of shear using these plots only is indetermi­ nant. The scatter in these plots is probably a result of recrystallization at high temperature which involves both basal and prism slip in quartz. Several other rea­ sons for this scatter are that: 1) more than one episode of recrystallization has probably occurred during the multiple deformation history of the zone; 2) the as­ sumption ofhomogeneous strain (Simpson and Schmid, 1983) may be incorrect; 3) deformation(s) may have involved some flattening rather than only simple shear; a and 4) samples are not quartzites but are rocks which contain significant amount of muscovite and feldspar which can act as rigid bodies and influence quartz de­ formation. In summary, plots of quartz c-axcs from a variety of samples within the Brevard Zone do not yield any useful sense of shear information. plots for Tu3 and Tal42: D.l: Quartz c axes No pattern has developed. Fig. for TuI37A and R05162: Possible Fig. D.2: Quartz c-axes plots weak girdle maximums have developed. Fig. D.3: Quartz c-axes plots for At 12: Weak single girdle pattern is present. 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