field trip guidebook: blue ridge province

PALEOZOIC STRUCTURE,METAMORPHISM, AND TECTONICSOF THE BLUE RIDGE OF WESTERN

NORTH CAROLINAEDITORS:

KEVIN G. STEWARTMARK G. ADAMS

CHARLES H. TRUPE

CAROLINA GEOLOGICAL SOCIETY1997 FIELD TRIP AND ANNUAL MEETING

Trip Leaders: Kevin Stewart, Mark Adams,Chuck Trupe, Rick Abbott, and Loren Raymond

Banner Elk, North CarolinaSeptember 26-28, 1997

CAROLINA GEOLOGICAL SOCIETY1997 FIELD TRIP GUIDEBOOK

September 26-28, 1997

PALEOZOIC STRUCTURE,METAMORPHISM, AND TECTONICS OF

THE BLUE RIDGE OF WESTERNNORTH CAROLINA

Edited by:Kevin G. StewartDepartment of GeologyUniversity of North CarolinaChapel Hill, NC 27599-3315

Mark G. AdamsDepartment of GeologyAppalachian State UniversityBoone, NC 28608

Charles H. TrupeDepartment of GeologyUniversity of North CarolinaChapel Hill, NC 27599-3315

iii

TABLE OF CONTENTS

Foreword And Acknowledgements ..................................................................................... iv

Dedication .............................................................................................................................. iv

Road Log And Stop Descriptions ........................................................................................1

Paleozoic Structural Evolution Of The Blue Ridge Thrust Complex,Western North Carolina .......................................................................................................21

Kevin G. Stewart, Mark G. Adams, Charles H. Trupe

Conditions And Timing Of Metamorphism In The Blue Ridge ThrustComplex, Northwestern North Carolina And Eastern Tennessee ....................................33

Mark G. Adams And Charles H. Trupe

Structural Relationships In The Linville Falls Shear Zone, Blue RidgeThrust Complex, Northwestern North Carolina ...............................................................49

Charles H. Trupe

Petrology And Tectonic Significance Of Ultramafic Rocks Near The Grandfather Mountain Window In The Blue Ridge Belt, ToeTerrane, Western Piedmont Zone, North Carolina ...........................................................67

Loren A. Raymond And Richard N. Abbott, Jr.

Petrology Of Pelitic And Mafic Rocks In The Ashe And AlligatorBack Metamorphic Suites, Northeast Of The Grandfather Mountain Window ............87

Richard N. Abbott, Jr. And Loren A. Raymond

Walker

Walker

iv

FOREWORD ANDACKNOWLEDGEMENTS

The Blue Ridge of western North Caro-lina has served as a classic field area for thestudy of the tectonics of crystalline rocks.Beginning with Arthur Keith at the turn of thecentury, the work of innumerable Blue Ridgefield geologists has provided the sound baseupon which we now build our research. Cer-tainly one of the greatest contributors was J.Robert Butler from the University of NorthCarolina at Chapel Hill.

The papers in the guidebook covervarious aspects of the structural and metamor-phic history of the Blue Ridge thrust complex.In addition we have provided a detailed roadlogto accompany the field trip stop descriptions.We hope this additional information will pro-vide participants with an understanding of thegeology between field stops.

We thank the authors of the papersincluded in the guidebook. We should note thatthe first two papers (Stewart et al. and Adamsand Trupe) are syntheses of previously pub-lished data and did not undergo peer review.The remaining papers were reviewed and wethank the reviewers for their careful work.

We are grateful to the sponsors of the1997 CGS meeting. As of the time of thiswriting, we have received generous donationsfrom Vulcan Materials, Olson Enterprises,Turner Environmental Consultants, and Appala-chian Resources. The generosity of our spon-sors allowed us to keep the registration costsdown.

We also thank the student assistants whoprovided invaluable help in many phases of themeeting: Lauren Hewitt, Shelly Kitchens, CalebPollock, and Cheryl Waters. We are especiallyindebted to Jane Gue who handled all of theregistration.

Dedication to the memory of

J. Robert Butler

We dedicate the 1997 Carolina Geologi-cal Society Guidebook to the memory of J.Robert Butler. Bob Butler was a geologyprofessor at the University of North Carolina atChapel Hill for over thirty years and was wellknown for his work on the geology of theCarolinas. Bobs passion for field geology wasunmatched and he was a regular participant andtrip leader at past Carolina Geological Societymeetings. Our own interest in the geology ofthe Blue Ridge can be almost entirely attributedto Bob. His wealth of ideas, encyclopedicknowledge of Carolina geology, and readinessto head into the field made Bob a perfect col-league and mentor. Many of the ideas in thisguidebook originated during conversations andfield trips with Bob Butler. Bobs influence isespecially evident when one considers howmany of his former students have continued towork in the Blue Ridge and on geologic prob-lems throughout the Carolinas. We believe it isonly fitting that we dedicate this book to hismemory.

Kevin Stewart, Mark Adams, and Chuck Trupe

vBob Butler and his Blue Ridge Bullys. Pictured from left to right: MarkAdams, Rod W illard, Chuck T rupe, Bob Butler . Not pictur ed: Kevin Stewart.Photo taken during SEGSA Field Trip, Banner Elk, NC, Mar ch 21, 1992.

1INTRODUCTION

This field trip will focus on the structuraland metamorphic history of the Blue Ridgethrust complex in the vicinity of the GrandfatherMountain window in northwestern North Caro-lina. The Blue Ridge thrust complex is definedas the sequence of thrust sheets overlying theGrandfather Mountain window west of theBrevard zone. At least four thrust sheets havebeen recognized in the complex. These thrust

sheets include, from structurally lowest tohighest, the Pardee Point, the Beech Mountain,the Pumpkin Patch, and the Spruce Pine thrustsheets.

The rocks of the Blue Ridge record atectonometamorphic history that ranges fromthe Grenville orogeny (ca. 1 Ga) to the LatePaleozoic Alleghanian orogeny. The Grenvilleorogeny was a Mesoproterozoic collisionalevent associated with the assembly of a largePrecambrian supercontinent, Rodinia. Intru-

CAROLINA GEOLOGICAL SOCIETY1997 FIELD TRIP AND ANNUAL MEETING

ROAD LOG AND STOP DESCRIPTIONS

Grandfa

ther

Mountai

n

window

Linville

Gossan

-Lead f

ault

Burnsville

fault

Stone M

ountain

fault

Unaka M

ountain

fault

Iron M

ounta

in fau

lt

Holsto

n Moun

tain fau

lt

Long

Brevar

d Faul

t zone

Mountai

n City w

indow

Spruce Pine thrust sheet

Pumpkin Patch thrust sheet

Beech Mountain thrust sheet

Pardee Point thrust sheet

0 5 10

Kilometers

NCTN

N

Boone

Banner Elk

Linville Falls

Burnsville

Bakersville

Falls fault

Ridgefault

Stop 2 - 2

Stop 2 - 1

Stop 1 - 1Stop 1 - 2

Stop 1 - 3

Stop 1 - 4

Stop 1 - 5

Stop 1 - 6

Stop 2 - 3

Linville

Pineola

Spruce Pine

Area oflarge map

Generalized tectonic map of the Blue Ridge thrust complex showing locations and numbers of field trip stops.

2sions of mafic rocks and the presence of largerift basins (such as the Grandfather Mountainwindow) provide evidence for theNeoproterozoic break-up of Rodinia whichresulted in the formation of the North Americanpaleocontinent, Laurentia. Following the break-up of Rodinia was the accumulation of sedimen-tary strata along the passive margin ofLaurentia, and the formation of oceanic crustand marine sediments in the evolving basin ofthe Iapetus Ocean. Paleozoic orogenic eventscommenced with the destruction of theLaurentian margin during early stages of theOrdovician Taconic orogeny. Closure of theIapetus Ocean basin was associated with variouscompressional or transpressional events result-ing from arc-continent or continent-continentcollision. Paleozoic orogenic events in theAppalachian belt culminated with theAlleghanian orogeny. During this time, theBlue Ridge thrust complex was transported fromsoutheast to northwest (present geographiccoordinates) over rocks now exposed in theGrandfather Mountain window. Duplex faultingat structurally lower levels created doming ofthe area and subsequent erosion exposed rocksbelow the thrust sheets, thus creating the Grand-father Mountain window.

Formation of the Grandfather Mountainwindow has provided a unique opportunity toexamine the overriding thrust sheets and theirexposed boundaries. Within a relatively smallarea, there are exposures of rocks that recordevidence for the Grenville and subsequentPaleozoic orogenies. The objective of the fieldtrip is to present the participants with a geologiccross section through the Blue Ridge thrustcomplex. The field stops will range from theGrandfather Mountain window and its boundingfault, the Linville Falls fault, to the uppermostthrust sheet of the complex, the Spruce Pinethrust sheet. The trip will emphasize results ofrecent research by the authors. Day One willbegin with an examination of the shear zone andfault bounding the Grandfather Mountain

window at the base of the complex, followed bystops in the structurally highest thrust sheet toexamine ultramafic rocks, eclogite and otherhigh grade metamorphic rocks. Day two willconcentrate on the nature of deformation in theBeech Mountain thrust sheet, followed by a stopto recently discovered retrogressed eclogitenortheast of the Grandfather Mountain window.

Mi. DESCRIPTION

DAY ONE SATURDAY,SEPTEMBER 27, 1997

0.0 Leave parking lot at Holiday Inn ofBanner Elk. Turn left (south) on NC 194.The town of Banner Elk and the HolidayInn are at the northwestern edge of theGrandfather Mountain window. Theprominent mountain to the northwest isBeech Mountain, which lies outside thewindow. The prominent mountain to thesouthwest is Sugar Mountain.

1.1 Outcrop of Grandfather MountainFormation sandstone on left side of road.

2.0 Escarpment at shopping center showscontact between Montezuma Member(metabasalt) with sandstone of theGrandfather Mountain Formation.Continuing to the south along 194 theprominent mountain due south is Grand-father Mountain.

3.0 Intersection of NC 194 and NC 105 atLinville Gap. Turn right on 105 South.

3.8 Outcrops of Linville Metadiabase onright side of road.

7.0 Intersection of NC 105 and US 221.Proceed straight through intersection.

Carolina Geological Society

38.7 Intersection with NC 181 and US 221.Turn left on US 221/ NC 181 South.

10.5 Bear to the right on US 221 South.

13.5 Cross the western boundary of theGrandfather Mountain window into theLinville Falls shear zone in the hangingwall of the Linville Falls fault. For about2 miles, US 221 runs along the westernboundary of the window.

18.2 Entrance to Blue Ridge Parkway. Turnleft on Blue Ridge Parkway North.

19.3 Turn right on Blue Ridge Parkway spurto Linville Falls Visitor Center.

19.6 Cross Linville Falls fault back into theGrandfather Mountain window. Out-crops along road are Chilhowee Groupsandstone within the Table Rock thrustsheet.

20.8 National Park Service Linville FallsVisitor Center parking area.

STOP 1-1 LINVILLE FALLS FAULT ATLINVILLE FALLS, LINVILLE FALLS, NCQUADRANGLE

Location: Blue Ridge Parkway National Park,Linville Falls Overlook, approximately 0.3 mi Sof the National Park Service Linville FallsVisitors Center, Linville Falls, NC quadrangle.UTM coordinates: 416510mE, 3978520mN

Note: This stop is within the Blue Ridge Park-way National Park, and taking samples orbreaking rock is prohibited except by specialpermit. Cross the wall on the upstream side ofthe overlook to exposure of mylonite at the base

of the outcrop. Space at the outcrop is limited;we will lead groups of 25 people at a time toview this exposure.

Stop Leader: Charles H. Trupe

The Linville Falls fault frames theGrandfather Mountain window, and separatescrystalline rocks of the Blue Ridge thrust com-plex from underlying rocks of the window. AtLinville Falls, Neoproterozoic andMesoproterozoic rocks of the Beech Mountainthrust sheet overlie Cambrian Chilhowee Groupmetasedimentary rocks of the Tablerock thrustsheet. The fault was named for this exposure byBryant and Reed (1970), which they interpretedas a top-to-northwest thrust fault. Van Campand Fullagar (1982) and Schedl et al. (1992)obtained a Rb-Sr whole-rock date of 302 Ma formylonite from this location.

The Linville Falls fault dips gently westat this location. The overlook is on ChilhoweeGroup metaquartzite, which exhibits northwest-trending mineral stretching lineation, isoclinalfolds with hinges parallel to this lineation, andopen folds with northeast-trending hinges(Bryant and Reed, 1970; Hatcher and Butler,

Stop 1-1

Road Log and Stop Descriptions

41986). Crenulation cleavage is parallel to axialplanes of the folds. Hatcher and Butler (1986)stated that this cleavage (F3 cleavage of Butler,1973) is a penetrative feature that affects themylonite zone, and rocks in both the hangingwall and footwall.

Bryant and Reed (1970, p. 163) de-scribed the fault as marked by 6 to 18 inches ofwhite to green finely laminated blastomylonitewhich is parallel to bedding in the quartzite andto foliation in the gneiss. The blastomyloniteseparates Cranberry Gneiss in the hangingwall from Chilhowee Group quartzite of theTablerock thrust sheet in the footwall. Bryantand Reed (1970) noted extensive shearing inbasement rocks above the fault, and describedslices of quartzite, amphibolite, and mica schistintercalated with mylonitic gneisses along thefault at several locations around the GrandfatherMountain window.

New mapping in the vicinity of LinvilleFalls shows that the hanging wall of the faultlocally consists of tectonic blocks of alkalifeldspar granite, informally referred to as theLinville Falls granite (Trupe, this volume). Theblastomylonite of Bryant and Reed (1970)occurs between the granite and ChilhoweeGroup quartzites in the footwall. The LinvilleFalls granite is similar petrographically andchemically to the Beech Granite, and mayrepresent tectonic slices of that unit. The gran-ite is typically strongly sheared, and myloniticfabric is locally overprinted by cataclasticfabric. Cataclastic fabric in the granite is well-exposed in outcrops on the north side of theLinville River across from the National ParkService Linville Falls Campground. ChilhoweeGroup metasedimentary rocks in the footwallare strongly sheared to quartz-sericite ( feld-spar) mylonite.

The Linville Falls granite is structurallyoverlain by a thick ductile shear zone consistingof several hundred meters of basement-derivedmylonite and ultramylonite, termed the LinvilleFalls shear zone by Trupe et al. (1990). The

mylonitic rocks record top-to-northwest shearsense, and were deformed at greenschist faciesconditions (Trupe, 1997, this volume). Theshear zone contains tectonic blocks of variouslithologies, including slices of magnetite-hornblendite, metagranite, and amphibolite.The Linville Falls shear zone extends to thecontact with the Ashe Metamorphic Suite, ~ 3kilometers southwest of the Linville Falls fault.

Some previous studies have assumedthat the Linville Falls fault is a meter-scalefeature at Linville Falls, and changes to a muchthicker zone in exposures along the northernmargin of the Grandfather Mountain window(e.g., Wojtal and Mitra, 1988; Newman andMitra, 1993). These previous workers failed torecognize the significant thickness of myloniticrocks present in the Linville Falls shear zone,the occurrence of cataclasites near LinvilleFalls, and the presence of slices of granite alongthe base of the shear zone. Models that attemptto explain thickening of the fault zone are notnecessary, as there is no large variation in shearzone thickness between Linville Falls andBanner Elk. The mylonite exposed at theLinville Falls overlook did not accommodate allof the strain associated with emplacement of theBlue Ridge thrust complex. This strain isrepresented by hundreds of meters of myloniteof the Linville falls shear zone.

ReferencesBryant, B., and Reed, J. C., Jr., 1970, Geology of the

Grandfather Mountain window and vicinity, NorthCarolina and Tennessee: United States GeologicalSurvey Professional Paper 615, 190 p.

Butler, J.R., 1973, Paleozoic deformation and metamor-phism in part of the Blue Ridge thrust sheet, NorthCarolina: American Journal of Science, v. 273-A, p.72-88.

Hatcher, R. D., Jr. and Butler, J. R., 1986, Linville Fallsfault at Linville Falls, North Carolina: GeologicalSociety of America Centennial Field Guide, South-eastern Section, p. 229-230.


5Newman, J., and Mitra, G., 1993, Lateral variations inmylonite zone thickness as influenced by fluid-rockinteractions, Linville Falls fault, North Carolina:Journal of Structural Geology, v. 15, p. 849-863.

Schedl, A., McCabe, C., Montanez, I., Fullagar, P. D., andValley, J., 1992, Alleghanian regional diagenesis: Aresponse to the migration of modified metamorphicfluids derived from beneath the Blue Ridge-Piedmontthrust sheet: Journal of Geology, v. 100, p. 339-352.

Trupe, C. H., 1997, Deformation and metamorphism inpart of the Blue Ridge thrust complex, northwesternNorth Carolina: Ph.D. Dissertation, University ofNorth Carolina at Chapel Hill, 176 p.

Trupe, C. H., Butler, J. R., Mies, J. W., Adams, M. G., andGoldberg, S. A., 1990, The Linville Falls fault andrelated shear zone: Geological Society of AmericaAbstracts with Programs, v. 22, p. 66.

Van Camp, S. G., and Fullagar, P. D., 1982, Rb-Sr whole-rock ages of mylonites from the Blue Ridge andBrevard zone of North Carolina: Geological Societyof America Abstracts with Programs, v. 14, p. 92-93.

Wojtal, S., and Mitra, G., 1988, Nature of deformation insome fault rocks from Appalachian thrusts, in Mitra,G., and Wojtal, S., editors, Geometries and Mecha-nisms of Thrusting, With Special Reference to theAppalachians: Geological Society of AmericaSpecial Paper 222, p. 17-33.

COFFEE BREAK

Return to Blue Ridge Parkway.

22.3 Turn left (south) on Blue Ridge Park-way. Parkway continues within theLinville Falls shear zone (hanging wallof Linville Falls fault). The shear zone iscomposed of blocks of sheared andunsheared rocks incorporated intomylonites predominantly derived fromcrystalline basement rocks.

26.0 Outcrops on the right (mile marker 320).Buses will unload here, then continuesouth to Gillespie Gap and turn aroundat the Museum of North Carolina Miner-als.

STOP 1-2 CONTACT BETWEEN THELINVILLE FALLS SHEAR ZONE ANDTHE ASHE METAMORPHIC SUITE,BLUE RIDGE PARKWAY, LINVILLEFALLS, NC QUADRANGLE

Location: Blue Ridge Parkway National Park,Blue Ridge Parkway, mile marker 320, LinvilleFalls, NC quadrangle. UTM coordinates:413650 mE, 3977350 mN.

Note: This stop is within the Blue Ridge Park-way National Park, and taking samples orbreaking rock is prohibited except by specialpermit. There is limited space along the shoul-der of the Parkway. Participants must be ex-tremely careful of traffic on the Parkway.


The purpose of this stop is to examinethe contact between Ashe Metamorphic Suite(AMS) rocks in the Spruce Pine thrust sheet andmylonitic rocks of the Linville Falls shear zone.Bryant and Reed (1970) and Abbott andRaymond (1984) postulated that the contactbetween the AMS and underlying basementrocks is a fault. The basal contact of the Spruce

Stop 1-2

Blue Ridge Parkway


6Pine thrust sheet was defined as a northwest-directed, post-metamorphic thrust fault (Butleret al., 1987), and subsequent work has verifiedthese relationships adjacent to and northeast ofthe Grandfather Mountain window (Mies,1990). Along strike to the southwest, the con-tact between the AMS and basement is a dextralstrike-slip fault (Burnsville fault, Stop 1-4) thatpredates the thrust fault adjacent to the window(Adams et al., 1995a). Field relations indicatethat the strike slip faulting is overprinted in thevicinity of the Grandfather Mountain windowby Alleghanian shearing and thrust faulting(Adams et al., 1995a). Farther to the southwest,other workers have stated that the presumablyequivalent contact is a pre-metamorphic thrustfault, correlative with the Hayesville fault(Hatcher, 1987; Merschat and Wiener, 1988).

At this stop, garnet mica schist ( kyan-ite) and amphibolite of the AMS overlie LinvilleFalls shear zone mylonite derived from biotitegneiss and granitic gneiss of the Beech Moun-tain thrust sheet. The mylonites contain alkalifeldspar and plagioclase porphyroclasts in amatrix of recrystallized quartz and fine-grainedbiotite, with minor epidote, sphene, and opaqueminerals. Foliation and compositional layeringin the hanging wall and footwall rocks dipsouthwest. Northwest-trending mineral stretch-ing lineation is well-developed in the myloniticfootwall rocks. Pegmatites in the AMS aresheared and boundinaged; asymmetric boudinsdemonstrate top-to-northwest shear sense.

Amphibolite facies assemblages in theAMS rocks have been sheared and retrogradednear the contact. Hornblende in the amphibo-lites is partially replaced by green biotite, andepidote is abundant. Garnet in the mica schistshas been partially to totally replaced green bybiotite ( chlorite). Plagioclase is sausseritic,and fine-grained white mica is abundant. Theearlier regional metamorphic foliation is over-printed by shearing. Muscovite fish and shearbands show top-to-northwest shear sense. The

retrogressive metamorphism and shearing areinterpreted to represent late Paleozoic deforma-tion during Alleghanian thrusting.

ReferencesAbbott, R.N., Jr., and Raymond, L.A., 1984, The Ashe

Metamorphic Suite, northwest North Carolina:Metamorphism and observations on geologic history.American Journal of Science, 284, 350-375.

Adams, M.G., Stewart, K.G., Trupe, C.H., and Willard,R.A., 1995a, Tectonic significance of high-pressuremetamorphic rocks and dextral strike-slip faulting inthe southern Appalachians: in Hibbard, J., van Staal,C.R., Cawood, P. and Colman-Sadd, S., editors, NewPerspectives in the Appalachian-Caledonian orogen,Geological Association of Canada Special Paper 41,p. 21-42.

Bryant, B., and Reed, J. C., Jr., 1970, Geology of theGrandfather Mountain window and vicinity, NorthCarolina and Tennessee: United States GeologicalSurvey Professional Paper 615, 190 p.

Butler, J.R., Goldberg, S.A., and Mies, J.W., 1987,Tectonics of the Blue Ridge west of the GrandfatherMountain window, North Carolina and Tennessee:Geological Society of America Abstracts withPrograms, v. 19, p. 77.

Hatcher, R.D., Jr., 1987, Tectonics of the southern andcentral Appalachian internides: Annual Review ofEarth and Planetary Sciences, v. 15, p. 337-362.

Merschat, C.E., and Wiener, L.S., 1988, Geology of theSandymush and Canton quadrangles, North Carolina:North Carolina Geological Survey Bulletin 90, 66 p.

Mies, J.W., 1990, Structural and petrologic studies ofmylonite at the Grenville basement - Ashe Formationboundary, Grayson County, Virginia to MitchellCounty, North Carolina: Ph. D. Dissertation,University of North Carolina at Chapel Hill, 330 p.

Turn around. Head north on Blue RidgeParkway.

29.7 Turn left into Linville Falls Picnic Area.

LUNCH

30.8 Exit for US 221. Turn right (north) onUS 221.


732.0 Intersection with NC 194. Turn left(south) on NC 194.

33.0 In this vicinity, cross from the LinvilleFalls shear zone into the Ashe Metamor-phic Suite of the Spruce Pine thrustsheet.

36.0 Turn left (south) on US 19E.

43.2 Intersection with NC 226. Continuesouth on US 19E through the Town ofSpruce Pine, The Mineral City, thecenter of the Spruce Pine mineral dis-trict. This area is the leading producer ofdomestic feldspar and is also a majorproducer of scrap and sheet mica, quartz,and kaolin. The area is also known formuseum-quality specimens of beryl,tourmaline, garnet, and other preciousand semiprecious minerals. Abandonedand active mica and feldspar mines arevisible on many of the mountainsides inthis area.

49.8 Turn right on NC 80 North. Park inparking area adjacent to buildings onright, or continue ~100 yards across ToeRiver Bridge and park on gravel pull-over on right.

STOP 1-3 NEWDALE DUNITE NEARNEWDALE, NC, MICAVILLE, NC QUAD-RANGLE

Location: NC 80 ~400 meters north of SouthToe River bridge, Micaville, NC quadrangle.UTM coordinates: 392830mE, 3974520mN.

Stop leader: Loren A. Raymond

The Newdale Dunite is one of the nu-merous ultramafic rock bodies present withinthe Ashe Metamorphic Suite of the Spruce Pine

Thrust Sheet (Toe Terrane). Immediately east ofthe Highway, a quarry was operated in thedunite during the 1970s, but termination ofmining operations allowed the quarry to fill withwater by the late 1980s. Dunite is exposed bothalong the highway and along an east trending,local road that intersects the highway near thesouth edge of the quarry.

The Newdale Dunite body, mapped aspart of the Spruce Pine District mapping projectof the USGS (Brobst, 1962), is an east-northeasttrending, elliptical body. The body is about440m long and 210m wide (Vrona, 1977).Brobst (1962) shows the body to be in contactprimarily with hornblende schist and gneiss(amphibolite). Vrona (1977) made a moredetailed map of the Newdale Dunite and foundthat the dunite is surrounded by hornblendeschist and gneiss, but it seemingly cuts a bodyof anthophyllite-plagioclase gneiss enclosedwithin the hornblende-rich rocks.

In spite of the exposures of ultramaficrocks in mine workings and road cuts, Vrona(1977) was unable to locate any exposures ofthe contacts. He noted, however, no evidence ofcontact metamorphism, he observed apparentcross-cutting relations between structures in theultramafic rocks and those in surroundinggneisses, and he recognized serpentine- and

Stop 1-3NC Hwy 80


8talc-bearing rocks near the contacts. From thesedata, Vrona (1977) concluded that the contactwas tectonic and he figured it as a thrust fault.

The dominant rock of the Newdaleultramafic body is metadunite, butmetaharzburgite and metachromitite are presentlocally (Vrona, 1977). The metadunite is typi-cally somewhat serpentinized, but is character-ized by LPO (Lattice preferred orientation)fabrics (Vrona, 1977). Metaharzburgite occurswhere orthopyroxene forms layers, and chromiteoccurs both in layers and in one podiform mass(Vrona, 1977, p. 30). More commonly, however,sparse chromite and orthopyroxene are scatteredamong the dominant olivine grains within themetadunite. Tremolite is similarly distributed.Veins containing talc, anthophyllite, and magne-site, as well as locally extensive veins of serpen-tine minerals reflect local fluid-enhanced,retrograde metamorphism of the metadunite.Thin-section petrography reveals textures andmineral associations that suggest at least threesuccessive metamorphic events. An earlyolivine + chromite + pyroxene assemblage isoverprinted by an Amphibolite Facies olivine +chromite + tremolite + chlorite + pyroxeneassemblage, which in turn is overprinted by oneor more Greenschist Facies assemblages com-posed of serpentine + magnetite + chlorite + talc+ tremolite. The vein assemblage composed oftremolite + anthophyllite + chlorite + talc +magnesite may represent an Amphibolite Facies,fluid-dominated, localized metasomatic event.Textural evidence suggests that the GreenschistFacies metamorphism followed earlier Am-phibolite Facies metamorphism.

References:Brobst, D.A., 1962, Geology of the Spruce Pine district,

Avery, Mitchell, and Yancey Counties, NorthCarolina: United States Geological Survey Bulletin1122-A, 26 p.

Vrona, J.P., 1997, Structure of the Newdale ultramafite,Yancey County, North Carolina: M.S. Thesis,Southern Illinois University, 115p.

Return to intersection of NC 80 and US19E. Turn right (south) on US 19E.

55.0 Turn right (north) on NC 197.

59.0 Turn left on Clearmont School Road(NCSR 1416). Day Book ultramaficbody is on the right. Clearmont SchoolRoad traverses the Burnsville faultwhich separates the Spruce Pine thrustsheet from the Pumpkin Patch thrustsheet.

60.0 Park at gravel area adjacent toClearmont School Road. Walk back eastalong road for 0.2 miles to Stop 1-4.

STOP 1-4 SHEARED BAKERSVILLEGABBRO ALONG THE BURNSVILLEFAULT ZONE, CLEARMONT SCHOOLROAD, BURNSVILLE, NC QUADRANGLE

Location: Clearmont School Road (NCSR1416) approximately 0.3 miles east of intersec-tion with Jacks Creek Road (1336), Burnsville,NC quadrangle. UTM coordinates: 383390mE,3981720mN.

Stop Leaders: Kevin G. Stewart, Mark Adams

This outcrop contains strongly shearedBakersville Gabbro associated with theBurnsville fault zone. The rock is stronglyfoliated and lineated. Foliation strikes northeastand dips moderately to the southeast. The well-developed mineral-stretching lineation trendsnortheast and is subhorizontal. Northeast trend-ing, mineral-stretching lineations have beenrecognized along the Burnsville fault from theCarvers Gap quadrangle to at least the Mars Hillquadrangle to the southwest for a distance atleast 50 kilometers. Kinematic indicators fromrocks along the Burnsville fault are consistent


9with dextral strike-slip movement (Mallard etal., 1994; Adams et al., 1995; Burton, 1996).

Petrographic observations and P-Testimates based on mineral chemistry indicatethat deformation along the Burnsville fault zoneoccurred under amphibolite facies conditions.Ribbons of dynamically recrystallized horn-blende indicate crystal-plastic deformation ofamphibole, which is thought to occur at 600-650 C (cf., Burton, this volume; Adams et al.,1995). Additionally, equilibrium grain boundarytextures in dynamically recrystallized quartz andplagioclase are consistent with deformationunder high temperature conditions. Adams andStewart (1993) reported P-T estimates of 6 - 8kbar and 580 - 640 C for sheared pelitic schistassociated with the Burnsville fault. Theseauthors interpreted these conditions to representfinal equilibration during shearing. Burton(1996) estimated temperatures of 640 - 730 Cat assumed pressures of 6 - 8 kbar for dynami-cally recrystallized hornblende and plagioclasefrom sheared amphibolite along the Burnsvillefault zone.

Amphibolites facies conditions ofdeformation along the Burnsville fault contrastwith greenschist facies conditions reported fordeformation along other major shear zones in

the Blue Ridge thrust complex (e.g., LinvilleFalls fault, Long Ridge fault, Fries-Gossan Leadfault, Stone Mountain fault).

The Burnsville fault juxtaposes rocks ofthe Ashe Metamorphic Suite with crystallinerocks of the Laurentian margin. This contacthas been interpreted by many workers to be theTaconic suture in this part of the Blue Ridge.Most tectonic syntheses of this area correlate theBurnsville fault with the Hayesville fault to thesouth (for details of fault correlations see Adamset al., 1995). If the Burnsville fault correspondsto the Taconic suture then it was originally anOrdovician thrust. Recent geochronology ofmylonites within the Burnsville shear zone,however, suggest strike-slip faulting occurredduring the Siluro-Devonian (Goldberg andDallmeyer, 1997). The Bakersville gabbro inthis outcrop is intruded by a Spruce Pine-typepegmatite (Siluro-Devonian; Kish, 1983).Spruce Pine pegmatites are generally restrictedto the Spruce Pine thrust sheet while Bakersvilleintrusive rocks are found only in the Laurentianbasement rocks. The presence of the pegmatitein the gabbro indicates that the Spruce Pine andPumpkin Patch thrust sheets were initiallyjuxtaposed prior to the intrusion of the pegma-tite, possibly during the Ordovician. Shearingof the pegmatite indicates that strike-slip fault-ing followed intrusion. These field relationscombined with the geochronology are consistentwith the interpretation that the strike-slip motionon the Burnsville fault is a Siluro-Devonianreactivation of an original Taconic thrust.

To the southwest, in the Barnardsvillequadrangle, the Burnsville fault zone widens toseveral miles and consists of large blocks ofrelatively unsheared basement and Ashe Meta-morphic Suite bounded by thick mylonite zoneswith consistent dextral strike-slip kinematicindicators.

To the northeast, approaching the Grand-father Mountain window, the nature of the faultchanges. The lineations become more north-westerly and the conditions of shearing are


Stop 1-4

10

upper greenschist. We believe this is an over-printing of later Alleghanian thrusting along theLinville Falls shear zone, as seen at Stop 1-2.North of the window, the base of the SprucePine thrust sheet (the Fries/Gossan-Lead fault)is a greenschist facies, northwest-directed thrust.This fault has also been interpreted as anAlleghanian feature and in our model wouldrepresent late transport of rocks which wereoriginally east of the Taconic suture.

ReferencesAdams, M.G., and Stewart, K.G., 1993, Tectonic implica-

tions of the occurrence of eclogite in the Blue Ridge,northwestern North Carolina, Geological Society ofAmerica Abstracts with Programs, v.25, p. 424.

Adams, M.G., Stewart, K.G., Trupe, C.H., and Willard,R.A., 1995, Tectonic significance of high-pressuremetamorphic rocks and dextral strike-slip faulting inthe southern Appalachians: in Hibbard, J., van Staal,C.R., Cawood, P. and Colman-Sadd, S., editors,Current Perspectives in the Appalachian-Caledonianorogen, Geological Association of Canada SpecialPaper 41, p. 21-42.

Burton, F.H., 1996, Kinematic study of the Taconicsuture, west-central North Carolina: M.S. thesis,University of North Carolina at Chapel Hill, 114 p.

Goldberg, S.A., and Dallmeyer, R.D., 1997, Chronologyof Paleozoic metamorphism and deformation in theBlue Ridge thrust complex, North Carolina andTennessee: American Journal of Science, v. 297, p.488-526.

Kish, S.A., 1983, A geochronological study of deforma-tion and metamorphism in the Blue Ridge andPiedmont of the Carolinas: Ph. D dissertation,University of North Carolina at Chapel Hill, 220 p.

Mallard, L.D., Adams, M.G., and Stewart, K.G., 1994,Kinematic analysis of a possible suture in thesouthern Appalachians, northwestern North Carolina:Geological Society of America Abstracts withPrograms, v. 26, n. 4, p. 25-26.

Return east along Clearmont SchoolRoad to NC 197.

61.0 Turn left (north) on NC 197.

62.6 Cross the Burnsville fault into thePumpkin Patch thrust sheet.

67.7 On north side of Toe River Bridge,beside railroad tracks. Outcrop ofmylonitic Bakersville gabbro exhibitingwell-developed, sub-horizontal, north-east-trending mineral stretching linea-tions.

68.3 Turn right (south) on NC 226.

69.6 Outcrops of Bakersville Gabbro alongleft side of road.

72.5 Outcrops of Mesoproterozoic biotite-hornblende gneiss typical of basementrocks of the Pumpkin Patch thrust sheet.

73.1 Cross the Burnsville fault back into theSpruce Pine thrust sheet.

73.8 Enter Bakersville, NC. Turn right fol-lowing NC 226. Then turn left at stoplight onto North Mitchell Avenue(NCSR 1211).

74.1 Turn left on Redwood Road (NCSR1217).

74.6 Unload from buses for Stop 1-5.

STOP 1-5 ECLOGITE AND RETRO-GRESSED ECLOGITE ALONGHONEYCUTT BRANCH, BAKERSVILLE,NC-TN QUADRANGLE

Location: NCSR 1217 ~ 3000 feet NE ofintersection with NCSR 1211, northeast ofBakersville, NC, Bakersville, NC-TN quad-rangle. UTM coordinates: 396550mE,3986740mN

Stop Leader: Mark G. Adams

Eclogite and retrogressed eclogite areexposed in the road cut along the northwest sideof the road. The exposure is continuous for


11

approximately 300 feet and discontinuous forapproximately 1000 feet. This occurrence is theoriginal outcrop of eclogite discovered by RodWillard and described by Willard and Adams(1994). This exposure is one of several largeblocks of eclogite that occur in the area. Themajority and the largest of the blocks occuralong Lick Ridge (8000 feet, N24E from here)and are described in Adams et al. (1995).

The outcrop shows a complete progres-sion from relatively pristine eclogite to thor-oughly retrogressed eclogite (amphibolite). Theprimary assemblage in the eclogite is garnet +omphacite + quartz + rutile. Variably the rockscontain retrograde minerals that include one ormore of the following: diopside, plagioclase,hornblende, sphene, ilmenite, and epidote. In themost thoroughly retrogressed samples, theresulting mineralogy is dominated by horn-blende + plagioclase + quartz sphene il-menite epidote. The most thoroughly retro-gress eclogite is petrographically indistinguish-able from other amphibolite in the Ashe, whichled Adams et al. (1995) and Abbott andRaymond (this guidebook) to speculate thatmuch of the Ashe Metamorphic Suite wasmetamorphosed to eclogite facies conditions,

but retrogressive metamorphism has obscuredmost evidence for earlier, high pressure condi-tions.

The eclogite shows a characteristiccompositional layering defined by alternatingconcentrations of garnet-rich and pyroxene-richmaterial. This layering is generally oblique tothe regional metamorphic foliation. Locally, asretrogression becomes more pervasive, thelayering is transposed into concordance with theregional foliation (a feature more prevalent inexposures on Lick Ridge; see photograph oncover of guidebook).

The blocks of eclogite are incorporatedinto sheared pelitic schist, gneiss, and amphibo-lite of the Ashe Metamorphic Suite. An expo-sure of the contact between this block and thepelitic schist is located in the bed of HoneycuttBranch off of the road bank to the east. Theexposure in the creek bed also shows severalsmall (~ 30 cm in diameter) pods of eclogiteincorporated into the pelitic schist matrix. Inaddition to these blocks of eclogite, small bodiesof ultramafic rock are also incorporated into thepelitic schist and amphibolite. One small bodyof altered ultramafic rock (predominantlyactinolite schist) occurs approximately 2000 feetto the southwest, but is not easily accessible.

Adams et al. (1995) reported P-T esti-mates from geothermobarometry based onmineral chemistry for eclogite and adjacentrocks in the area. Garnet-omphacite pairs fromeclogite yield temperatures ranging from 626to 790 C. Omphacite-plagioclase pairs yieldpressure estimates ranging from 13 to 17 kbar.As plagioclase is a retrograde product, thesepressure estimates are considered to be mini-mum pressures.

The structural base of the Ashe Meta-morphic Suite is represented by the Burnsvillefault. This fault occurs approximately 2000 feetto the northwest. Locally, sheared rocks associ-ated with the Burnsville fault show mesoscopicand microscopic kinematic indicators (such as,shear bands, porphyroclasts with asymmetric

Stop 1-5

NCSR 12

17


12

tails, and composite planar fabric) indicating amajor component of dextral shear sense alongthe fault.

References:Adams, M.G., Stewart, K.G., Trupe, C.H., and Willard,

R.A., 1995, Tectonic significance of high-pressuremetamorphic rocks and dextral strike-slip faulting inthe southern Appalachians: in Hibbard, J., van Staal,C.R., Cawood, P. and Colman-Sadd, S., editors, NewPerspectives in the Appalachian-Caledonian orogen,Geological Association of Canada Special Paper 41,p. 21-42.

Willard, R.A., and Adams, M.G., 1994 Newly discoveredeclogite in the southern Appalachian orogen, north-western North Carolina: Earth and Planetary ScienceLetters, v. 123, p. 61-70.

Walk 0.6 miles northeast to McKinneyCove Church. Very large, well-exposedoutcrops of eclogite occur on LickRidge, which is the prominent ridgeobservable to the northeast during thiswalk.

COFFEE BREAK

Return to North Mitchell Avenue, turnright. At stop light turn right onto NC226. Go ~ 100 feet to flashing light andproceed straight (north) on NC 261.

76.9 Cross Burnsville fault from Spruce Pinethrust sheet into Pumpkin Patch thrustsheet.

79.5 Outcrop of two-pyroxene granulite onleft side of road. Granulite facies rockshave been reported from a few scatteredlocations in basement rocks of thePumpkin Patch thrust sheet. PumpkinPatch Mountain is the prominent moun-tain on the left. Meadlock Mountain isthe prominent mountain on the right.

81.5 In this vicinity, enter back into theBurnsville fault zone. NC 261 continueswithin the fault zone for approximately3.5 miles, then turns north out of thefault zone into the Pumpkin Patch thrustsheet.

85.2 Outcrop of Meadlock Mountain gneisson left side of road. Continue 0.2 milesand park on wide shoulder on left side ofroad. Walk back to outcrop of MeadlockMountain gneiss.

STOP 1-6 MEADLOCK MOUNTAINGNEISS ALONG NC HWY 226 NEARROAN VALLEY, CARVERS GAP, NC-TNQUADRANGLE

Location: Along NC - 261 ~ 1.2 mi NE of GlenAyre, Carvers Gap, NC-TN quadrangle. UTMcoordinates: 402150mE, 3993830mN


This stop highlights the MeadlockMountain gneiss, a mafic basement gneiss in thePumpkin Patch thrust sheet. The Meadlock

Stop 1-6NC Hwy 261


13

Mountain gneiss is juxtaposed with the AsheMetamorphic Suite along the Burnsville fault inthe eastern part of the Bakersville quadrangleand the western part of the Carvers Gap quad-rangle. The fault is located approximately 1000feet to the south. East of this area, the nature ofthe Ashe-basement contact changes from arelatively high grade (amphibolite facies),dextral strike-slip shear zone to a lower grade(greenschist facies), northwest directed thrustfault (Adams et al., 1995a). Adams et al.(1995a) suggested that the original juxtapositionof the Pumpkin Patch and Spruce Pine thrustsheets was a result of collision during theTaconic orogeny, the dextral movement resultedfrom reactivation of the fault during the Acadianorogeny, and the thrust fault overprinted theearlier strike-slip fault during the Alleghanianorogeny.

The stable metamorphic assemblage inthis rock includes garnet + hornblende + diop-side + biotite + plagioclase + quartz + rutile +ilmenite. Adams et al. (1995b) estimated peakP-T conditions of ~13 kbar at 725 C and finalequilibration conditions of ~9 kbar at 660 C.Thin sections from this locality show deforma-tion of primary metamorphic phases, but do notshow significant retrograde phases. Deformationis manifested by grain-size reduction by dy-namic recrystallization and the formation ofribbons of quartz, hornblende, and pyroxene.Equilibrium textures of dynamically recrystal-lized grains in pyroxene ribbons indicate defor-mation occurred under relatively high gradeconditions.

Gulley (1985) and Monrad and Gulley(1983) documented granulite facies conditions(6.5 - 8 kbar at 750 - 847 C) for the Cloudlandand Carvers Gap gneisses on Roan Mountain.These authors interpreted these granulite faciesconditions to represent metamorphism duringthe Precambrian. They also estimated upperamphibole facies conditions (10 - 12 kbar at 680- 760 C) interpreted to record Paleozoic(Taconic orogeny) metamorphism. The latter P-

T estimates are consistent with those from theMeadlock Mountain gneiss reported by Adamset al. (1995).

The Meadlock Mountain gneiss may becorrelative with parts of the Carvers Gap gneissas described by Gulley (1985) and Monrad andGulley (1983). However, these authors statedthat mafic components of the Carvers Gapgneiss constitute a minor volume of the gneiss.Geologic mapping (Adams, 1995) indicates thatthe Meadlock Mountain gneiss covers a signifi-cant area in map view. Although felsic zoneslocally occur (exposed ~ 500 feet east of thisstop) within the Meadlock Mountain gneiss, themafic component is the dominant lithologyalong the Burnsville fault from Bakersville, NCto the community of Valley in the Carvers Gapquadrangle (Adams, 1995).

References:Adams, M.G., 1995, The tectonothermal evolution of part

of the Blue Ridge thrust complex, northwesternNorth Carolina: Ph.D. dissertation, University ofNorth Carolina, 193 p.

Adams, M.G., Stewart, K.G., Trupe, C.H., and Willard,R.A., 1995a, Tectonic significance of high-pressuremetamorphic rocks and dextral strike-slip faulting inthe southern Appalachians: in Hibbard, J., van Staal,C.R., Cawood, P. and Colman-Sadd, S., editors, NewPerspectives in the Appalachian-Caledonian orogen,Geological Association of Canada Special Paper 41,p. 21-42.

Adams, M.G., Trupe, C.H., Goldberg, S.A., Stewart,K.G., and Butler, J.R., 1995b, Pressure-temperaturehistory of high-grade metamorphic rock along theeastern-western Blue Ridge boundary, northwesternNorth Carolina: Geological Society of AmericaAbstracts with Programs, v. 27, p. 33.

Gulley, G. L., Jr., 1985, A Proterozoic granulite-faciesterrane on Roan Mountain, western Blue Ridge belt,North Carolina - Tennessee: Geological Society ofAmerica Bulletin, v. 96, p. 1428-1439.

Monrad, J.R., and Gulley, G.R., Jr., 1983, Age and P-Tconditions during metamorphism of granulite-faciesgneisses, Roan Mountain, North Carolina-Tennessee,in Lewis, S.E., editor, Geologic investigations in theBlue Ridge of northwestern North Carolina: CarolinaGeological Society Guidebook, North CarolinaGeological Survey, Article IV, 29 p.


14

Walk back to buses.

Continue north on NC 261.

89.3 NC-TN state line at Carvers Gap. Op-tional stop: turn left at Carvers Gap toRoan Mountain overlook. The overlookprovides a panoramic view of the BlueRidge and Valley and Ridge Provinces.For detailed description of the geology atthis stop, refer to 1983 Carolina Geo-logical Society Field Trip Guidebook.

At the TN-NC state line, NC 261 be-comes TN 143. Continue north on TN143 to US 19E. Turn right (south) on US19E. Continue for approximately 7 milesto NC 194 in the town of Elk Park. Turnleft (north) on NC 194 toward BannerElk. Continue to NC 184 in Banner Elk.Turn right at stop light and continue 1.3miles to Holiday Inn.

DAY TWO SUNDAY,SEPTEMBER 28, 1997

0.0 Leave Holiday Inn parking lot. Turnright (north) on NC 184.

1.0 Low outcrop on left is stretched pebblemetaconglomerate of the GrandfatherMountain Formation.

1.3 Turn left at traffic light onto NC 184/NC 194.

1.6 Park in Sunrise Shopping Center lot onright.

STOP 2-1. LINVILLE FALLS FAULT ATBANNER ELK

Location: Intersection of NC highways 184 and194, Elk Park 7.5' quadrangle. UTM coordi-nates: 420990mE, 4002050mN.


The Linville Falls fault (Bryant andReed, 1970) is exposed on the west side ofBanner Elk, North Carolina, approximately 30meters northwest of the intersection of BeechMountain Parkway (NC 184) and NC 194(Figure 4). The fault juxtaposes Precambriancrystalline rocks of the Beech Mountain thrustsheet and underlying rocks of the GrandfatherMountain window. Approximately 50 meters ofhanging wall and footwall rocks are exposed ina recent excavation. Metaquartzite and sericiticquartz mylonite and ultramylonite in the hang-ing wall dip moderately northwest, concordantwith the orientation of the fault surface. Thefootwall consists of low-grade metasedimentaryrocks of the Grandfather Mountain Formation inwhich sedimentary layering dips moderatelynortheast and cleavage dips moderately east.Although the major Blue Ridge thrusts are


Stop 2-1NC Hwy 194

NC Hwy 184

15

generally recognized as hinterland dippingstructures, the northwest dip of the fault atBanner Elk is due to large amplitude folding ofthe thrust surface during formation of theGrandfather Mountain window (Bryant andReed, 1970), possibly due to the presence of anunderlying thrust duplex (Boyer and Elliott,1982).

Recent work in the area has shown thatthe Linville Falls fault lies at the base of a thickshear zone, appropriately termed the LinvilleFalls shear zone (Trupe et al., 1990; Trupe, thisvolume). Adams and Su (1996) showed theshear zone to be approximately 1 kilometerthick in the Banner Elk area. Kinematic indica-tors in the mylonites of the shear zone yield top-to-northwest sense of shear (Trupe, 1989;Adams, 1990; Adams and Su, 1996). The shearzone contains tectonic slices of metaquartziteand exotic crystalline rocks (granoblastic lay-ered gneisses and metagranite) surrounded bygreenschist-grade mylonite and ultramylonite.Contrary to assumptions by Newman and Mitra(1993), the relatively small, exotic slice ofquartz diorite gneiss (Potts Cemetery gneiss ofAdams and Su, 1996) that crops out 500 metersabove the fault zone is not a likely protolith ofthe hanging wall mylonites.

Cataclastic deformation overprintsmylonitic fabric several meters into the hangingwall rocks at Banner Elk. A progression fromultracataclasite (2-3 cm) to cataclasite (1-2 m)to protocataclasite may be observed immedi-ately above the fault. Cataclastic rocks alsooccur at an exposure of the fault at Bowers Gap,4 km east of the Banner Elk exposure, and at thetype locality of the fault at Linville Falls, NC(Trupe, this volume). The Long Ridge fault(Adams and Su, 1996) to the northeast, and theStone Mountain fault farther to the northwestalso exhibit cataclastic deformation overprintingmylonitic fabrics, and are related to the LinvilleFalls shear zone .

ReferencesAdams, M.G., 1990, The geology of the Valle Crucis area,

northwestern North Carolina: Masters Thesis,University of North Carolina at Chapel Hill, 95 p.

Adams, M. G., and Su, Q., 1996, The nature and timing ofdeformation in the Beech Mountain thrust sheetbetween the Grandfather Mountain and MountainCity windows in the Blue Ridge of northwesternNorth Carolina: Journal of Geology, p. 197-213.

Boyer, S.E., and Elliott, D., 1982, Thrust systems:American Association of Petroleum GeologistsBulletin, v. 66, p. 1196-1230.

Bryant, B., and Reed, J.C., Jr., 1970, Geology of theGrandfather Mountain window and vicinity, NorthCarolina and Tennessee: United States GeologicalSurvey Professional Paper 615, 190 p.

Newman, J., and Mitra, G., 1993, Lateral variations inmylonite zone thickness as influenced by fluid-rockinteractions, Linville Falls fault, North Carolina:Journal of Structural Geology, v. 15, p. 849-863.

Trupe, C.H., 1989, Kinematic analysis and deformationmechanisms in mylonites from the Blue Ridge thrustcomplex, northwestern North Carolina: GeologicalSociety of America Abstracts with Programs, v. 21, p.62.

Trupe, C.H., Butler, J.R., Mies, J.W., Adams, M.G., andGoldberg, S.A., 1990, The Linville Falls fault andrelated shear zone: Geological Society of AmericaAbstracts with Programs, v. 22, p. 66.

Leave Sunrise Shopping Center. Turnleft on NC 184 South/ NC 194 North.

1.9 Turn right at traffic light on NC 184South.

6.2 Turn left on NC 105 North.

14.4 Turn left on NC 105 North Truck Route.Felsic metavolcanic rocks of the Grand-father Mountain Formation are exposeon the left side of road.

15.4 Outcrops of Montezuma Member(metabasalt) and metaconglomerate ofthe Grandfather Mountain Formation.


16

15.7 Cross the Linville Falls fault along thenorthern border of the GrandfatherMountain window into the Beech Moun-tain thrust sheet. The first well-exposedrock on the left, north of the fault, is partof a slice of Broadstone Camp graniteincorporated into the Linville Falls shearzone.

17.3 Intersection of NC194 Truck Route withNC 194 in the town of Valle Crucis.Continue north on NC 194.

18.0 Cross Watauga River. In this area theflood plain of the Watauga River coin-cides with the location of the LongRidge fault zone.

18.7 Turn left on Mast Gap Road (NCSR1117). Outcrop on right is Valle Crucisgneiss (Adams, 1990) Cranberry Mine-layered gneiss of Bartholomew andLewis (1984); Cranberry Gneiss ofBryant and Reed (1970).

20.9 Turn left on US 321 North.

23.4 Cross Fork Ridge fault of Bartholomewand Lewis (1984). The Fork Ridge faultis a thrust within the Beech Mountainthrust sheet and separates Valle Crucisgneiss in the hanging wall from WataugaRiver Gneiss in the foot wall.

25.0 Park in store parking lot on right. Walk0.1 miles and turn right on Bull HarmonRoad. Walk about 700 feet to Stop 2-2.

STOP 2-2 LONG RIDGE FAULT

Location: Along Bull Harmon Road ~700 feetNW of intersection with US 321, Sherwood,NC-TN quadrangle. UTM coordinates:423500mE, 4012390mN


This exposure was designated as thetype locality of the Long Ridge fault byAdams and Su (1996). Part of the fault wasmapped as the upper branch of the StoneMountain fault by Bryant and Reed (1970).Bryant and Reed (1970) mapped this fault fromthe Stone Mountain fault along the southeasternboundary of the Mountain City window to thesoutheast where it dies out in the vicinity ofLong Ridge. Adams (1990) traced the faultfarther to the southeast where it merges with theLinville Falls fault in the vicinity of ValleCrucis. Adams and Su (1996) interpreted theLong Ridge fault to be a low angle tear faultwithin the Beech Mountain thrust sheet, result-ing from differential movement of the thrustsheet during the Alleghanian orogeny. Mineral-stretching lineations and kinematic indicators

Stop 2-2US Hwy 321


17

are consistent with top-to northwest, dextralstrike-slip movement along the Long Ridge fault(Adams and Su, 1996)

The Long Ridge fault is similar to theborder faults (Linville Falls and Stone Mountainfaults) of the Beech Mountain thrust sheet inseveral respects. Associated with the LongRidge fault is a thick (up to 400 meters), mixed-rock mylonite zone termed the Long Ridgeshear zone. This shear zone contains blocks ofsheared and unsheared rock and discontinuousslices of metasedimentary rocks (predominantlymetaquartzite, meta-arkose, andmetaconglomerate of the Chilhowee Group and/or the Grandfather Mountain Formation) incor-porated into mylonite, ultramylonite, andphyllonite derived predominantly from base-ment rocks. In addition to ductile deformation,manifested by mylonite and ultramylonite; theLong Ridge shear zone shows evidence ofbrittle deformation evidenced by cataclasite,ultracataclasite, and pseudotachylyte (?)(OHara, 1992; Adams, 1994; Adams and Su,1996). The deformed rocks show evidence ofalternating episodes of brittle and ductile defor-mation. Adams and Su (1996) documentedevidence for overprinting episodes including asequence of ductile-brittle-ductile-brittle defor-mation. Adams and Su (1996) stated that bothstyles of deformation (brittle and ductile) oc-curred essentially synchronous under the sameregional P-T conditions.

At this stop, the outcrop shows a slice ofmetasedimentary rock incorporated into theLong Ridge shear zone. Also exposed areveinlets of pseudotachylyte (or ultracataclasite)cross cutting the basement rocks. The outcropalso locally shows ductilely deformedpseudotachylyte (?), indicating alternatingepisodes of brittle-ductile deformation. Isotopicevidence indicates that both the ultramyloniteand pseudotachylyte from this locality wereformed around 300 Ma. (Adams and Su, 1996).

References:Adams, M.G., 1994, Major- and trace-element constraints

on the petrogenesis of a fault-relatedpseudotachylyte, western Blue Ridge province, NorthCarolina - Comment: Tectonophysics, v. 233, p. 145-147.

Adams, M.G., and Su, Q., 1996, The nature and timing ofdeformation in the Beech Mountain thrust sheetbetween the Grandfather Mountain and MountainCity windows in the Blue Ridge of northwesternNorth Carolina: Journal of Geology, p. 197-213.

Bryant, B., and Reed, J.C., Jr., 1970, Geology of theGrandfather Mountain window and vicinity, NorthCarolina and Tennessee: United States GeologicalSurvey Professional Paper 615, 190 p.

OHara, K., 1992, Major- and trace-element constraintson the petrogenesis of a fault-relatedpseudotachylyte, western Blue Ridge province, NorthCarolina. Tectonophysics, 204: 279-288.

Trupe, C.H., and Adams, M.G., 1991, Cataclastic defor-mation in the Linville Falls shear zone, western NorthCarolina Blue Ridge: Geological Society of AmericaAbstracts with Programs, v. 23, p.141.

COFFEE BREAK

Return to Buses.

Turn around and head south on US 321.

30.0 Intersection with US 421. The top of theprominent ridge directly in front (to theeast) is Ashe Metamorphic Suite of theSpruce Pine thrust sheet. The thrust fault(Fries/Gossan Lead fault) separating theSpruce Pine thrust sheet from the under-lying Beech Mountain thrust sheet isabout half way up the mountain. Turnright and head south on US 321/421. Thehighway parallels a relatively narrowstrip of the Beech Mountain thrust sheetbounded above by Fries/Gossan Leadfault and below by the Linville Fallsfault.


18

33.5 Intersection with US 321/421 TruckRoute. Continue straight on US 321/421South.

34.9 Enter Boone, North Carolina city limits.

37.1 Turn left (north) on NC 194. In this areaNC 194 crosses the Fries/Gossan Leadfault into the Spruce Pine thrust sheet.

38.7 Turn left on Howards Creek Road(NCSR 1306).

44.8 Unload from buses at open field on left(southeast). Walk 0.1 miles further toStop 2-3.

STOP 2-3 RETROGRESSED ECLOGITEON TATER HILL, ZIONVILLE, NC-TNQUADRANGLE

Location: NCSR 1306 ~ 300 feet E of intersec-tion with NCSR 1102, Southeast of Silverstone,NC, Zionville, NC-TN quadrangle. UTMcoordinates: 434620mE, 4014890mN.

Stop Leader: Richard N. Abbott

The purpose of this stop is to examinerecently discovered retrograded eclogite in theAshe Metamorphic Suite (AMS) north of theGrandfather Mountain window. The only otherarea of bona fide eclogite occurs in a similarstructural setting in the AMS, but southwest ofthe Grandfather Mountain window (Willard andAdams, 1994; Adams et al., 1995). The bestexposures of retrograded eclogite north of theGrandfather Mountain window (this stop) arealong the east side Howard Creek Rd, 50 to 100meters north of the junction with Tater Hill Rd.and the north end of Junaluska Rd (lat. N 3616' 44.2", long. W 81 43' 41"). While theoutcrop is clearly on a state right-of-way, pleaserespect the owners of adjacent properties.Permission to examine the outcrop should beobtained from Mr. Donald Price of Zionville.

The local distribution of retrogradedeclogite has not been mapped in detail. Bouldersof retrograded eclogite, on the south slope of thehill, immediately north of the roadside expo-sures, suggest that retrograded eclogite extendsat least 200 meters to the northeast. One otherlocation of retrograded eclogite has been discov-ered in a small outcrop approximately 2 km tothe south (lat. N 36 15' 34", long. W 81 43'22", along Junaluska Rd.).

The retrograded eclogites are close to thebase of the AMS, near the western edge of theSpruce Pine thrust sheet. The actual base of thethrust sheet, which is down hill to the west, isconcealed by colluvium. The location of theretrograded eclogites is consistent with the areaof highest metamorphic pressures in the AMS,as inferred from the general direction of increas-ing P (Abbott and Raymond, this volume). Theretrograded eclogites occur as thin (cm-scale),gray-green granoblastic layers in otherwisetypical garnet-hornblende schist. The essentialmineralogy consists of symplectic intergrowthsof diopside and plagioclase (representing formeromphacite), generally euhedral to subhedralgarnet (< 1mm), polygonal epidote, and quartz.Boundaries between layers of retrograded

Stop 2-3 NCSR 1306NC

SR 110

2


19

eclogite and hornblende schist are gradational,showing a progressive replacement of theeclogite by hornblende schist. Plagioclaseoccurs only in the symplectite and with quartz incoronas around garnet. Adams et al. (1995) havesuggested that such coronas formed in responseto a retrograde reaction between garnet andsurrounding symplectite. Hence, at its highest-grade the rock was bona fide eclogite. Thedistinctive coronas are common in thesymplectite-free hornblende schist, especiallynear the base of the AMS. Interpreted as relictfeatures inherited from an earlier, high-gradecondition of the rock, the coronas suggest that asignificant volume of the hornblende schist inthe AMS is retrograded (amphibolitized)eclogite (Adams et al., 1995). The presence ofepidote distinguishes these eclogites from thosedescribed by Willard and Adams (1994), andprobably reflects a higher fugacity of O2. Addi-tional details of the petrography are given inAbbott and Raymond (this volume).

The presence of eclogite, or retrogradedequivalents, in the AMS is significant for atleast three reasons. (1) The eclogites record theearliest recognizable and highest grade meta-morphism affecting mafic rocks during theTaconic Orogeny. (Evidence for even earlier,and higher grades of metamorphism may bepreserved in ultramafic rocks of the AMS.) (2)The eclogites strongly support an ensimaticorigin for the AMS, as part of a subduction-related accretionary melange. (3) Much of theAmphibolite Facies metamorphism in thewestern part of the AMS is retrograde.

References:Adams, M.G., Stewart, K.G., Trupe, C.H., and Willard,

R.A., 1995, Tectonic significance of high-pressuremetamorphic rocks and dextral strike-slip faulting inthe southern Appalachians: in Hibbard, J., van Staal,C.R., Cawood, P. and Colman-Sadd, S., editors,


Current Perspectives in the Appalachian-Caledonianorogen, Geological Association of Canada SpecialPaper 41, p 21-42.

Willard, R.A., and Adams, M.G., 1994, Newly discoveredeclogite in the southern Appalachian orogen, north-western North Carolina: Earth and Planetary ScienceLetters, v. 123, p. 61-70.

Return to buses.

Turn around and follow Howards CreekRoad back to NC 194. Return to BannerElk via US 421 - NC 105 - NC 184.

21

ABSTRACT

The Blue Ridge thrust complex in theregion surrounding the Grandfather Mountainwindow consists of four major thrust sheets.Structurally lowest to highest these are thePardee Point, the Beech Mountain, the Pump-kin Patch, and the Spruce Pine thrust sheets.The Spruce Pine thrust sheet is especiallynoteworthy because it contains large bodies ofeclogite near its base.

All the thrust sheets were transported tothe northwest during the Alleghanian orogenyat the end of the Paleozoic. The Pumpkin Patchand the Spruce Pine thrust sheets, however,were juxtaposed prior to the Alleghanian andwere affected by the Taconic and possiblyAcadian orogenies. Published metamorphicmineral ages from amphibolite facies rocks inthe Pumpkin Patch and Spruce Pine thrustsheets and eclogite from the Spruce Pine thrustsheet are Ordovician, indicating that they wereinitially juxtaposed during the Taconic orogeny.The eclogite bodies within the Spruce Pinethrust sheet were probably generated in aTaconic subduction zone and thrust onto theLaurentian margin as relatively coherentbodies. The contact between the PumpkinPatch and Spruce Pine thrust sheets is now an

amphibolite facies dextral strike-slip shearzone. Preliminary mineral ages from rocks inthis shear zone are Devonian, suggesting thatthe original Taconic thrust in this area wasreactivated during the Acadian orogeny as astrike-slip fault.

The faults bounding the Beech Moun-tain and Pardee Point thrust sheets areAlleghanian northwest-directed thrusts. Thefault separating the Pumpkin Patch and SprucePine thrust sheets is cut by one of theseAlleghanian faults in the vicinity of the Grand-father Mountain window, indicating that thisfault was transported to the northwest alongwith the composite Blue Ridge thrust complexat the end of the Paleozoic.

INTRODUCTION

The Blue Ridge thrust complex refers tothe stack of crystalline thrust sheets west of theBrevard fault zone which structurally overliethe Linville Falls fault (Figure 1; Goldberg etal., 1989). Individual thrust sheets are distin-guished by changes in rock type, metamorphichistory, and the presence of thick mylonitic orcataclastic fault zones. The thrust sheets, fromstructurally lowest to highest, are the PardeePoint thrust sheet, the Beech Mountain thrustsheet, the Pumpkin Patch thrust sheet, and theSpruce Pine thrust sheet.

Other authors have subdivided therocks of the North Carolina Blue Ridge into

PALEOZOIC STRUCTURAL EVOLUTION OF THE BLUE RIDGE THRUST COMPLEX,WESTERN NORTH CAROLINA

Kevin G. Stewart, Mark G. Adams1 , and Charles H.Trupe2

Department of GeologyUniversity of North CarolinaChapel Hill, NC 27599-3315

________________________________________________Present addresses:1. Dept of Geology, Appalachian State Univ., Boone, NC28608 and Appalachian Resources, 205 Providence Rd.,Chapel Hill, NC 27515.2. 2619 Mt. Gilead Church Rd., Pittsboro, NC 27312.

22

different terranes or thrust sheets, using similarcriteria. Figure 1 in Raymond and Abbott (thisvolume) summarizes the classification schemesused by other authors. We have used the classi-fication of Goldberg et al. (1989) in our pastwork and will continue this practice in thispaper.

The structural history of the area out-lined in this paper is primarily based on kine-matic studies of the fault zones bounding thethrust sheets, structural geology of the rockswithin the thrust sheets, and the grade and ageof metamorphism. Although some of the thrustsheets record Precambrian deformation, meta-

morphism, and igneous activity, we are restrict-ing our analysis to Paleozoic events. Notsurprisingly, the record of early Paleozoicevents has been obscured by later Paleozoicevents and therefore our model becomes moregeneralized for early events. Our currentmodel should be considered one in a series ofsuccessive approximations which are refinedwhen new data become available. Neverthe-less, we feel that we can provide a reasonableinterpretation of the structural evolution of theBlue Ridge of western North Carolina from theOrdovician through the Pennsylvanian.

Grandfa

ther

Mountai

n

window

Linville

Gossan

-Lead f

ault

Burnsville

fault

Stone M

ountain

fault

Unaka M

ountain

fault

Iron M

ounta

in fau

lt

Holsto

n Moun

tain fau

lt

Long

Brevar

d Faul

t zone

Mountai

n City w

indow

Spruce Pine thrust sheet

Pumpkin Patch thrust sheet



0 5 10

Kilometers

NCTN

N

Boone

Banner Elk

Linville Falls

Burnsville

Bakersville

Falls faultRidge

fault

Area oflarge map

Figure 1. Generalized geologic map of the Blue Ridge of western North Carolina. Four thrust sheets of the Blue Ridgethrust complex are shown. All faults are Alleghanian thrusts and strike-slip faults except for the Burnsville fault, whichis Ordovician - Devonian. Modified from Adams et al. (1995).

Stewart, Adams, and Trupe

23

PALEOZOIC METAMORPHISM WITHINTHE BLUE RIDGE THRUST COMPLEX

Detailed descriptions of the metamor-phic grade of the four thrust sheets in the BlueRidge thrust complex can be found in Butler(1991) and Adams and Trupe (this volume).Ages of metamorphism of the thrust sheets canbe found in Goldberg and Dallmeyer (1997)and are summarized by Adams and Trupe (thisvolume). The following summary is based onthese studies.

ORDOVICIAN (TACONIC) AMPHIBOLITEAND ECLOGITE FACIES METAMOR-PHISM

Evidence for Ordovician amphibolitefacies metamorphism is restricted to the SprucePine and Pumpkin Patch thrust sheets. Al-though Goldberg and Dallmeyer (1997) reporta metamorphic mineral age of 451 Ma for asample within the Beech Mountain thrust sheet,this sample actually belongs to the PumpkinPatch thrust sheet (Adams and Trupe, thisvolume).

Eclogite facies metamorphism has beenrecognized in two places within the SprucePine thrust sheet: near Bakersville, NorthCarolina (Willard and Adams, 1994; Adams etal., 1995), and northwest of Boone, NorthCarolina (Abbott and Raymond, this volume).Sm-Nd mineral-whole rock isochrons fromeclogite near Bakersville give a range of agesfrom about 410-450 Ma (Adams et al., 1995).

SILURO-DEVONIAN (ACADIAN) AM-PHIBOLITE FACIES METAMORPHISMAND STRIKE-SLIP FAULTING

Goldberg and Dallmeyer (1997) reportSilurian and Devonian amphibolite faciesmetamorphism in the Pumpkin Patch andSpruce Pine thrust sheets (see also Trupe andAdams, this volume). There is no evidence of

Siluro-Devonian metamorphism in the BeechMountain or Pardee Point thrust sheet.

The Burnsville fault (Figure 1), whichseparates the Spruce Pine thrust sheet from thePumpkin Patch thrust sheet, is an amphibolitefacies dextral strike-slip shear zone (Adams etal., 1995; Burton, 1996). Goldberg andDallmeyer report a Devonian cooling age forrocks within the Burnsville fault zone.

MISSISSIPPIAN-PENNSYLVANIAN(ALLEGHANIAN) GREENSCHIST FACIESMETAMORPHISM AND THRUSTING

Evidence of significant Alleghanianmetamorphism is restricted to the BeechMountain and Pardee Point thrust sheets(Butler 1991, Adams and Trupe, this volume).The Beech Mountain thrust sheet experiencedpervasive greenschist facies metamorphism andthe Pardee Point experienced greenschist tosub-greenschist facies metamorphism.

The Alleghanian thrusts throughout theBlue Ridge thrust complex were active undergreenschist facies, or lower, conditions (Abbottand Raymond, 1984; Mies, 1990; Trupe, 1989;Adams, 1990; Schedl et al., 1992).

STRUCTURAL MODELS

ORDOVICIAN CONVERGENCE ANDCOLLISION

According to Hatcher (1989), theTaconic orogeny in the southern Appalachianswas the result of the collision between thePiedmont terrane and Laurentia. Convergencebegan in the Early Ordovician as the Piedmontadvanced over an east-dipping subduction zone(Figure 2). Raymond et al. (1989) and Adamset al. (1995) interpret the Ashe MetamorphicSuite (AMS) to be the metamorphosed accre-tionary wedge formed at the leading edge ofthe Piedmont terrane. The AMS containspelitic schist, amphibolite, and ultramafic rocks

Structural Evolution of the Blue Ridge

24

and commonly has a block-in-matrix texture,which is typical of accretionary melanges(Raymond et al., 1989).

Deeply subducted basaltic crust wasmetamorphosed to eclogite, which was eventu-ally detached and incorporated into the accre-tionary wedge. The uplift mechanism of high-pressure metamorphic rocks is an enduringproblem in structural geology. Cloos (1982)proposed that eclogite blocks in the FranciscanComplex of California were entrained in anupward-flowing mud melange. In his model,only blocks which are less than about 25meters in diameter can be uplifted. The eclogiteblocks in the Blue Ridge range in size fromcentimeter-scale blocks to continuous layers upto 200 meters thick and a kilometer long(Adams et al., 1995). The size of the largestblocks seems to prohibit uplift by Cloosmechanism. Mechanisms which involve pairedthrust and normal faults (e.g. Platt, 1986) havebeen proposed to explain uplift of large

eclogite facies terranes, but as of this writingwe have no evidence for large-displacementnormal faults in the AMS. We do not know theextent of eclogite facies metamorphism withinthe AMS although Adams et al. (1995) sug-gested that high pressure metamorphism mayhave been widespread (see also Abbott andRaymond, this volume).

Collision of the Piedmont terrane withLaurentia occurred by the Middle Ordovician(Figure 3; Hatcher, 1989; Raymond andJohnson, 1994). The collision induced am-phibolite facies metamorphism in the under-thrust Laurentian margin and in the overlyingAMS. The amphibolite facies metamorphicfront propagated to the west as the collisionprogressed and affected rocks which wereultimately incorporated into the Pumpkin Patchthrust sheet. The eclogite bodies are locallyretrograded to amphibolite which probablyoccurred sometime during the Ordovician and

eclogite

AMS protolith

102030405060708090

100

Laur

entia

Piedmont

0 km

continental marginsediments

abyssal sediments

Early Ordovician

Figure 2. Ordovician convergence between Laurentia and Piedmont terrane. Subducted oceanic crust undergoes eclogitefacies metamorphism. Accretionary wedge is protolith for Ashe Metamorphic Suite (AMS).


25

Silurian as the eclogite was brought to shal-lower levels.

SILURO-DEVONIAN STRIKE-SLIP FAULT-ING

Following the collision of the Piedmontterrane with Laurentia, the Taconic sutureevolved into a dextral strike slip fault called theBurnsville fault (Adams et al., 1995; Figure 4).The presence of post-Taconic, dextral motionon the suture has been recognized at localitiesfrom Alabama to Newfoundland (Adams et al.,1995). This may have been a discrete event(Acadian orogeny?) which occurred tens ofmillions of years after the end of the Taconiccollision or it could have been the final phaseof a protracted event which began in the Or-dovician and ended in the Devonian. Existinggeochronologic data cannot discriminatebetween these two models (Goldberg andDallmeyer, 1997).

The Spruce Pine Plutonic Suite wasemplaced between 390 and 410 Ma (Kish,1983). Spruce Pine pegmatites intrudemylonites within the Burnsville fault zone andare also sheared, indicating that shearingoccurred sometime during the Siluro-Devo-nian. The part of the Burnsville fault zonecurrently exposed was active under amphibo-lite facies conditions (Adams et al., 1995;Burton, 1996).

The rocks of the Spruce Pine thrustsheet were initially juxtaposed against therocks of the Pumpkin Patch thrust sheet duringthe Ordovician. Movement between these twoterranes stopped by the end of the Devonian.

DEVONIAN-MISSISSIPPIAN UPLIFT ANDCOOLING

Goldberg and Dallmeyer (1997) report40Ar/39Ar muscovite ages which indicateregional cooling of the Blue Ridge thrust

Ashe Metamorphic Suite

102030405060708090

100

0 km

eclogite

Piedmont

amphibolitegreenschist

Laur

entia

Taconic suture

Middle to Late Ordovician

amphibolite facies metamorphism induced in AMSand Laurentian basement

Figure 3. Taconic orogeny resulting from Middle to Late Ordovician collision between Piedmont and Laurentia.Underthrust Laurentian margin undergoes amphibolite facies metamorphism. Eclogite body is incorporated into AsheMetamorphic Suite.


26

complex during the Devonian and Mississip-pian (Figure 5). Rocks metamorphosed underamphibolite and eclogite facies during the

Ordovician through Devonian were upliftedand the amphibolite metamorphic front mayhave rotated to a steeper orientation.

AMSPiedmont

102030405060708090

100

0 km

Spruce Pine pegmatites

Siluro-Devonian

amphibolitegreenschist

Burnsville fault

Figure 4. Taconic collision evolves into Siluro-Devonian dextral strike-slip shear zone (Burnsville fault). Spruce Pinepegmatites intrude fault zone, eclogite, AMS, and Laurentian basement rocks.

AMS

eclogite

Piedmont

102030405060708090

100

0 km

Devonian - Mississippian uplift and cooling

Burnsville fault

uplift and erosion of eclogite and relictOrdovician-Devonian amphibolite facies

metamorphic rocks

amphibolite

Figure 5. Uplift and cooling during Devonian - Mississippian time.


27

MISSISSIPPIAN-PENNSYLVANIAN(ALLEGHANIAN) THRUSTING

Thrusting during the Alleghanianprogressed from the southeast towards thenorthwest. The thrust below the compositeSpruce Pine/Pumpkin Patch thrust sheet cutsthrough the part of the Laurentian crust thathad experienced Ordovician - Devonian am-phibolite facies metamorphism (Figure 6). Ifthe thrust sheet had originally included lowergrade Laurentian rocks, they have since beenremoved by erosion.

Northwest transport of the compositeSpruce Pine/Pumpkin Patch thrust sheets wasprobably accompanied by movement of thePiedmont thrust sheet (Figure 7). This out-of-sequence thrust would have decapitated theTaconic suture and transported it to the north-west (see Rankin et al., 1991, for a similarinterpretation). In addition, the relict Ordovi-cian - Devonian amphibolite front would have

been overridden such that later thrusts wouldcut Laurentian crust which had not experiencedhigh grade Paleozoic metamorphism (Figure8).

In our model, the pervasive Alleghaniangreenschist metamorphism in the Beech Moun-tain thrust sheet was induced by emplacementof the composite Spruce Pine/Pumpkin Patchthrust sheet. The weak Alleghanian metamor-phism present in the Pardee Point thrust sheetwas induced by emplacement of the BeechMountain thrust sheet along the Linville Fallsfault. All of the thrust sheets were folded byduplexing of basement rocks beneath theGrandfather Mountain window (Figure 9;Rankin et al., 1991; Schedl et al., 1997).

102030405060708090

100

0 kmAMS

Piedmont

Burnsville fault

future trace of thrust below Pumpkin Patchand Spruce Pine thrust sheets

Mississippian

edge of Ordovician-Devonianamphibolite facies

metamorphic front in Laurentian margin

amphibolite

Figure 6. First Alleghanian thrust cuts through Taconic suture and into Ordovician - Devonian amphibolite grade rocksin the Laurentian basement.


28

102030405060708090

100

0 km

Taconic suture below InnerPiedmont thrust sheet

AMS Piedmont

Burnsville fault

Spruce Pine thrust sheetPumpkin Patch thrust sheet

future trace of Linville Fallsfault below Beech Mountain

thrust sheet

greenschist facies metamorphisminduced in Beech Mountain

thrust sheet

Mississippian - Pennsylvanian

Ordovician-Devonian amphibolitefacies metamorphic front

overridden by later thrusts

amphibolite

102030405060708090

100

0 km

AMS Piedmont

Burnsville fault


future trace of Linville Falls fault

future trace of thrust belowPardee Point thrust sheet

future trace of thrusts belowGrandfather Mountain window



Figure 7. Composite Spruce Pine/Pumpkin Patch thrust sheet induces greenschist facies metamorphism in adjacentBeech Mount thrust sheet rocks. Decapitated Taconic suture in footwall is beneath Piedmont thrust sheet.

Figure 8. Later Alleghanian thrusts propagate into Laurentian basement which has not been affected by Ordovi-cian - Devonian events.


29

SUMMARY

The four thrust sheets of the Blue Ridgethrust complex contain a fairly complete recordof at least two and possibly three distinctorogenic episodes. The structurally highestSpruce Pine thrust sheet contains eclogite andamphibolite facies metamorphic rocks associ-ated with the Ordovician Taconic orogeny.This event sutured the Spruce Pine thrust sheetto the Laurentian rocks of the Pumpkin Patchthrust sheet and induced amphibolite faciesmetamorphism within the Pumpkin Patchsheet. Siluro-Devonian transpression, dueeither to a separate event (Acadian orogeny?),

or a prolonged transpressional collision initi-ated during the Taconic orogeny, is recorded bythe dextral strike-slip Burnsville fault. Devo-nian amphibolite facies rocks in the SprucePine and Pumpkin Patch sheets are a record ofthis event.

Northwest thrusting during theAlleghanian orogeny transported the compositeSpruce Pine/Pumpkin Patch thrust sheet overthe Laurentian rocks of the Beech Mountainthrust sheet. This induced pervasivegreenschist facies metamorphism within theBeech Mountain sheet, which in turn was thrustover the adjacent Pardee Point thrust sheetalong the Linville Falls fault. The lack of

102030405060708090

100

0 km

AMS

Piedmont

Burnsville fault


Linville Falls fault




eclogiteBrevard fault

future location ofGrandfather Mountain

window

Figure 9. Duplexing beneath Grandfather Mountain window domes thrust sheets. Later erosion creates GrandfatherMountain window.


30

significant Paleozoic metamorphism in thePardee Point thrust sheet suggests that thethickness of the thrust stack above the PardeePoint sheet was significantly less than abovethe Beech Mountain sheet.

ACKNOWLEDGMENTS

Work presented in this study waspartially supported by NSF grant EAR9316033. Many of the ideas in this paper arosefrom discussions with J.R. Butler, R.A.Willard, F.H. Burton, and S.A. Goldberg.Additional financial assistance to CHT andMGA came from the University of NorthCarolina at Chapel Hill Department of Geol-ogy, Sigma Xi, the Geological Society ofAmerica, and the North Carolina GeologicalSurvey.

REFERENCES

Abbott, R.N., Jr., and Raymond, L.A., 1984, The AsheMetamorphic Suite, northwest North Carolina:metamorphism and observations on geologic history:American Journal of Science, v. 284, p. 350-375.

Adams, M.G., 1990, The geology of the Valle Crucis area,northwestern North Carolina: Masters Thesis,University of North Carolina at Chapel Hill, 95 p.

Adams, M.G., Stewart, K.G., Trupe, C.H., and Willard,R.A., 1995, Tectonic significance of high-pressuremetamorphic rocks and dextral strike-slip faulting inthe southern Appalachians: in Hibbard, J., van Staal,C.R., Cawood, P. and Colman-Sadd, S., editors,Current Perspectives in the Appalachian-Caledonianorogen, Geological Association of Canada SpecialPaper 41, p. 21-42.

Burton, F.H., 1996, Kinematic study of the Taconicsuture, west-central North Carolina: Masters Thesis,University of North Carolina at Chapel Hill, 114 p.

Butler, J.R., 1991, Metamorphism, in Horton, J.W.,Jr.,and Zullo, V.A., editors, The geology of the Caroli-nas: Carolina Geological Society Fiftieth Anniver-sary Volume: Knoxville, The University of Tennes-see Press, Carolina Geological Society, p. 127-141.

Cloos, M., 1982, Flow melangs: Numerical modellingand geologic constraints on their origin in theFranciscan subduction complex: Geological Societyof America Bulletin, v. 93, p. 330-345.

Goldberg, S.A., Butler, J.R., Mies, J.W., and Trupe, C.H.,1989, The southern Appalachian orogen in northwest-ern North Carolina and adjacent states: InternationalGeological Congress Field Trip T365 Guidebook,American Geophysical Union, Washington, D.C., 55p.

Goldberg, S.A., and Dallmeyer, R.D., 1997, Chronologyof Paleozoic metamorphism and deformation in theBlue Ridge thrust complex, North Carolina andTennessee: American Journal of Science, v. 297, p.488-526.

Hatcher, R. D., Jr., 1989, Tectonic synthesis of the U. S.Appalachians, p. 511-535, in Hatcher, R. D., Jr.,Thomas, W. A., and Viele, G. W., eds., The Appala-chian-Ouachita orogen in the United States: Boulder,Colorado, Geological Society of America, thegeology of North America, v. F-2, 767 p.

Kish, S.A., 1983, A geochronological study of deforma-tion and metamorphism in the Blue Ridge andPiedmont of the Carolinas: Ph. D dissertation,University of North Carolina at Chapel Hill, 220 p.

Mies, J.W., 1990, Structural and petrologic studies ofmylonite at the Grenville basement - Ashe Formationboundary, Grayson County, Virginia to MitchellCounty, North Carolina, Ph.D. dissertation, Univer-sity of North Carolina at Chapel Hill, 330 p.

Platt, J.P., 1986, Dynamics of orogenic wedges and theuplift of high-pressure metamorphic rocks: Geologi-cal Society of America Bulletin, v. 97, p. 1037-1053.

Rankin, D.W., Dillon, W.P., Black, D.F.B., Boyer, S.E.,Daniels, D.L., Goldsmith, R., Grow, J.A., Horton,J.W.,Jr., Hutchinson, D.R., Klitgord, K.D.,McKowell, R.C., Miltop, D.J., Owens, J.P., Phillips,J.D., with contributions by Bayer, K.C., Butler, J.R.,Elliott, D.W., and Milici, R.C., 1991, CentennialContinent/Ocean Transect #16, E-4, Central Ken-tucky to the Carolina Trough, The Geological Societyof America, 41 p.

Raymond, L.A., and Johnson, P.A., 1994, The Mars HillTerrane: An enigmatic southern Appalachian terrane:Geological Society of America Abstracts withPrograms, v. 26, n. 4, p. 59.

Raymond, L.A., Yurkovich, S.P., and McKinney, M.,1989, Block-in-matrix structures in the NorthCarolina Blue Ridge belt and their significance forthe tectonic history of the southern Appalachianorogen, in Horton, J.W., Jr., and Rast, N., editors,Melangs and Olistostromes of the U.S. Appala-chians: Geological Society of America Special Paper228, p. 195-215.

Schedl, A., Fullagar, P.D., and Valley, J.W., 1997, Implica-tions of geochemical and gravity data for the natureof strata beneath the Blue Ridge - Piedmont thrust


31

sheet, USA: Earth and Planetary Science Letters, v.146, p. 165-179.

Schedl, A., McCabe, C., Montanez, I., Fullagar, P.D., andValley, J., 1992, Alleghenian regional diagenesis: Aresponse to the migration of modified metamorphicfluids derived from beneath the Blue Ridge-Piedmontthrust sheet: Journal of Geology, v. 100, p. 339-352.

Trupe, C.H., 1989, Microstructural analysis of mylonitesfrom the Blue Ridge thrust complex, northwesternNorth Carolina: M.S. Thesis, University of NorthCarolina at Chapel Hill, 101 p.

Willard, R.A., and Adams, M.G., 1994, Newly discoveredeclogite in the southern Appalachian orogen, north-western North Carolina: Earth and Planetary ScienceLetters, v. 123, p. 61-70.


33

ABSTRACT

The Blue Ridge thrust complex in thevicinity of the Grandfather Mountain windowconsists of at least four thrust sheets with differ-ent metamorphic and deformational histories.The lower three thrust sheets contain Grenville-age amphibolite to granulite grade metamorphicrocks that were variably affected by subsequentPaleozoic tectonothermal events. In the lowestsheet, the Pardee Point thrust sheet, Paleozoicmetamorphism was no higher than chloritegrade, and Grenville metamorphic assemblagesare not significantly retrograded. Grenvillebasement and Neoproterozoic intrusive rocks ofthe Beech Mountain thrust sheet were perva-sively deformed during greenschist gradeAlleghanian faulting, but evidence for earlierPaleozoic metamorphism is equivocal. ThePumpkin Patch thrust sheet contains Grenvillebasement gneisses and NeoproterozoicBakersville intrusive rocks that were affected byamphibolite facies Paleozoic metamorphism.The uppermost tectonic unit, the Spruce Pinethrust sheet, consists of metasedimentary andmetavolcanic rocks of the Ashe and AlligatorBack Metamorphic Suites. The occurrence ofeclogite at the base of the Spruce Pine thrustsheet documents early high-pressure metamor-phism. Pelitic schists and amphibolites of theAshe Metamorphic Suite record kyanite grade

peak regional metamorphism followed bycooling and decompression along a clockwiseretrograde P-T path. Both Ordovician andSiluro-Devonian radiometric ages are recordedin metamorphic rocks from the Spruce Pine andPumpkin Patch thrust sheets. Dextral strike-slipfaulting along the Spruce Pine-Pumpkin Patchcontact occurred under amphibolite-gradeconditions, and was broadly contemporaneouswith intrusion of the Spruce Pine pegmatites(~390-400 Ma). Late Paleozoic greenschist-grade deformation records assembly and em-placement of the Blue Ridge thrust complexalong top-to-northwest thrust faults.

INTRODUCTION

The Blue Ridge thrust complex west andnorthwest of the Grandfather Mountain window(Fig

field trip guidebook: blue ridge province

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