Bradley -Taconic Plate Kinematics-1989

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TECTONICS, VOL. 8, NO.5, PAGES 1037-1049, OCTOBER 1989

TACONIC PLATE KINEMATICS AS REVEALED BYFOREDEEP STRATIGRAPHY .APP ALACmAN OROGEND. C. Bradleyl

Lamont-Doherty Geological Observatory , ColumbiaUniversity, Palisades, New York

Abstract. Destruction of the Ordovician passivemargin ofeasternNorth America is recordedby an upward deepeningsuccessionof carbonates,shales,and flysch. A compilation ofthe age of shelf drowning (carbonate-to-shaleransition)reveals he degree o which orogeny was diachronousbothacrossand along strike. Shelf drowning occurred IrSt at thenorthern end of the orogen n Newfoundland, then at thesouthern end of the orogen n Georgia, and fmally in Quebec.Diachronism is attributed to oblique collision between anirregular passive margin, that had a deep embayment nQuebec, and at least one east dipping subduction complex.The rate of plate convergenceduring collision is estimatedat 1to 2 cm/yr, and the minimum width of the ocean hat closed sestimatedat 500 to 900 kin. Far-traveleddeepwatersequencesin the thrust belt contain anomalously old Taconic flysch,related to early arrival of the continental slope/riseat a westadvancing rench then ocated ar to the east The drowningisochron map provides a new basis or estimating tectonictransport distancesof four of theseallochthons (about 165 to ,450 kin), results not readily obtained by conventionalstructural analysis.IN1RODUCflON

Reconstruction ofpre-Jurassic plate motions is hinderedboth by the fragmentary nature of the rock record and by thecomplexity of plate tectonics. One approach o this vastproblem is the construction of global continental drift maps,which are based argely on paleomagnetism,biogeography,

1 Now at U.S. Geological Survey. Anchorage. Alaska

Copyright 1989by the American Geophysical Union.Paper number 89TC009580278-7407/89!00958$10.00

and paleoclimatology [e.g., Van der Voo, 1988]. To thosespecializeddisciplines should be added egional geology,without which continental reconstructionswould be asimpossible as they would be pointless. Field-based egionalgeology provides the fundamentalbasis or recognizing ancientcontinents, oceans,magmatic arcs, and the former plateboundariesbetween hem. On the other hand, regionalgeology has contributed little to the quantification of ancientplate motions. The primary objective of this paper s todemonstratea new, quantitative method of plate kinematicanalysis based on stratigraphy. The method is not subject tothe longitudinal uncertainty that will inevitably limitpaleomagnetism o the detection of about half of all platemotions, and it is applicable to a common class of orogeny.Arc-passive margin collisions are one orogenic settingwhere the geologic corollaries of plate tectonics are not overlycomplex. Such collisions are readily recognized n the ancientrecord, and many relatedprocesses re orderly and wellunderstood. In particular, arc-passivemargin collisionsproduce lexural foredeeps hat migrate through time inresponse o plate convergence. Cisne et al. [1982] and Bradleyand Kusky [1986] used his property to estimate he rate ofplate convergenceduring the Taconic Orogeny n New York.Thesestudies elateddiachronous oredeepstratigraphy o plateconvergence nd concluded hat the strike-normal ate of plateconvergencewas about 2 cm/yr. The presentpaper extends heconcept rom two to three dimensions (latitude, longitude,time), using data rom the entire foredeep rom Newfoundlandto Alabama rather than a single transect.The results bear onthe l-ateof plate convergence, he collisional plate geometrythe width of the Taconic ocean,and the transport distanceoffar-traveled slope/rise allochthons. Future applications willsubstantially enhanceour understandingof past plate motionsin many orogenic belts.nIE TACONlC OROGENY AND FOREDEEP

During Cambrian and Early Ordovician the easternmarginof North America (presentcoordinates)was the site of acarbonateplatform [e.g., Rodgers, 1968; Bird and Dewey,


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1038 Bradley: Taconic Plate Kinematics, Appalachian Orogen

"--;..,a murchisonJ !'," bicornis0

\,;;.~ jS "'.:SCISd',

"qiU"(.,"--"u~ , ~';1

"'~'"~'b

-.j'~~- ~ ".."". 7. 'bQ 6'. '6'. "II>

Fig. 1. Maps of (a) southern and central Appalachians and (b) northern Appalachians, showing the distribution ofOrdovician shalesand graywackes, ar-traveled slope/riseallochthons,numbered ocations (seeTables 1 and 2), andcorresponding palinspastic positions. Isochron ines (graptolite zone boundaries) rack shelf drowning through time;at a particular time represented y an sochron, shale was being deposited o the southeastof the line and carbonatesto the northwest. Where drowning isochronshave been extrapolatedoutboard of the former carbonateplatform theydo not correspond o a particular geologic event but still provide a basis for estimating the position of the plateboundary

1970] which faced an ocean, apetus. The platform isrepresented y an eastward hickening, shallow-water, mainlycarbonatesequencen both the autochthonand parautochthon.The platform was flanked to the east by a continental slopeand rise. Strata of the slope/rise sequence onsist of"dominantly deepwater argillaceousand arenaceous ediments,with lessercarbonates, arbonateconglomerates, herts,andminor volcanics" [Rowley and Kidd, 1981, p. 201], preservedin far-traveled thrust sheetssuch as he Taconic Allochthon.The irregular trace of the Appalachian old-thrust belt(Figure I) probably evolved from original irregularities alongthe passive margin [Bird and Dewey, 1970; Rankin, 1976;Thomas, 1977]. Major promontories along the continentalmargin were located n the Gulf of St. Lawrence and Alabama;lesserpromontories probably existed n Virginia and near NewYork City. A deep embaymentoccupied western NewEngland and southernQuebec; esserembaymentsmay haveexisted n western Pennsylvaniaand Tennessee.The Taconic Orogeny resulted n drowning of the carbonateplatform, followed by obduction of slope/riseallochthons andlarge ophiolite sheets. Theseeventsare widely interpreted as aresult of collision with a convergent plate boundary followingeasterly subduction of an oceanic ract. Whereas he arc-passive margin collision model is the most elegant rationalefor the Ordovician geology of the Appalachians, t is only oneof several nterpretationsproposed or the Taconic Orogenyduring the fIrst years of plate tectonics. The presently acceptedmodel was fIrSt developed n Newfoundland by Stevens 1970]and has subsequentlybeen applied, with modifications anddifferent emphases, long the entire length of the Appalachians[Hiscott, 1978; Chapple, 1973; Stanley and Ratcliffe, 1985;Rowley and Kidd, 1981; Read, 1980; Shanmugamand

Walker.1980]. In Pennsylvania,Lash and Drake [1984]interpreted he colliding object to have been a microcontinentwith Grenville basement ather than an allochthonous arc, butfor presentpurposes his difference of opinion is a minor one.Understandingof Taconic eventshasgreatly benefited romstudies of analogousarc-passivemargin collisions, includingTimor [Veevers et al.. 1978]. Papua [Pigram et al.. 1989].Oman [Gealey, 1977], Ouachita [Houseknecht, 1986], andseveral n the CanadianShield [Hoffman. 1987].Interaction betweenpassivemargin and convergentplateboundary began n Newfoundland. Arrival of the continentalmargin at the convergent boundary. which is presumed o havebeen he site of a trench analogous o Timor Trough, isrecordedby an influx of orogen-derived lysch. The flyschoverlies older slope/rise acies thought o have been derivedfrom North America [Stevens,1970]. Orogen-derived lyschalso comprises he youngest acies preserved n the Gaspe(Quebec),Taconic (New York), and Hamburg (pennsylvania)allochthons (Table 1). In each of theseplaces the flysch isrepresented y a single graptolite zone. Sedimentationendedwhen strata of the slope/rise egion were accreted o theoverriding plate by footwall imbrication, then thrust onto andacross he former shelf.On the carbonateplatform the first sign of an impendingTaconic Orogeny was uplift and erosion, an event that manyworkers attribute to flexural loading (Figure 2) [Rowley andKidd, 1981; Jacobi, 1981]. Passage f a carbonateplatformthrough a forebulge s ideally recordedby shoaling upwardsedimentation. hen erosion, hen deepeningupwardsedimentation.Ordovician unconformities which have beenattributed to Taconic lithospheric flexure occur near he top ofthe carbonatesuccession rom Newfoundland to Alabama. In


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Bradley: Taconic Plate Kinematics, Appalachian Orogen 1039

Fig. 1. (Continued)

Newfoundland, regionally discontinuousunconforrnitiesoccurat or near the St George-Table Head contact [Jacobi, 1981;Knight and James, 1987]. In New York [Rowley and Kidd,1981; Bradley and Kusky, 1986] and Pennsylvania[Shanmugamand Lash, 1982], a comparablebut youngerinterval of erosion followed deposition of the BeekmantownGroup. In Virginia [Mussman and Read, 1986] and Tennessee[Shanmugamand Lash, 1982], the post-Knox unconforrnityoccupies he analogousstratigraphic position. Theunconformities of purported orebulge origin are complex,with multiple erosion surfacesof variable extent rather than asingle surface (Figure 3). This might be expected rom thegeographic complexity of the present-day lexural arch n thesubducting Australian shelf at Kepulauan Aru (an archipelagobreachedby antecedent rainages) nd the Sahul Rise (adrowned submarinearch [Veeversand Van Andel, 1967]).A diachronousshelf drowning sequence verlies the

unconformity. Typically, shallow-water carbonates resucceeded y deepwater arbonates,hen hemipelagicshales.Superimposedon this general pattern are ocal complicationsand variations. For example, he transition from shallow-watercarbonates o deepwatershalesvaries rom abrupt togradational,perhaps eflecting the presence r absence f apronouncedshelf slope break. Local carbonatebuildupsformed along the shelf edge n Tennessee nd Virginia[Walker, 1977; Read, 1982]; these are probably analogous oAshmore Reef along the Australian flank of Timor Trough.Figure 3 illustrates the diachronousshelf drowning sequencenthe Taconic foredeepof New Yark. Comparabledrowningsequences lso exist in Newfoundland [Klappa et al., 1980],Quebec Belt et al., 1979], Pennsylvania Lash and Drake,1984], Virginia [Read, 1980], and Tennessee ShanmugamandWalker,1980]. Table 2 summarizes he age of the carbonate-to-shale transition at 40 locations throughout the Appalachians.

TABLE I. Age of Allochthonous Flysch in the Taconic Orogen

A Humber Arm,NewfoundlandExternalDomain, QuebecTaconic,New YorkHamburg,Pennsylvania

Lobster CovefkalGaspePeninsulaGranville

Lower He2d basal P. tentaculatus 270B Tourelle >450

James et alo [1987.po 1203]Barnes et alo [1981]. victoriae

c Pawlet early C. bicornis 225arly C. bicornis


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Fig. 2. Schematic block diagram of a collisional foredeepshowing the distribution of facies belts. During plateconvergence, ollisional foredeeps re characterized y asubmarine hrust belt, flysch sedimentationalong the basinaxis, and flexural extension on the outer slope. Facies beltsmigrate cratonward n response o continuedplate convergence.

siliciclastic turbidites (flysch) deposited n coalescingsubmarine ans along the foredeep axis. Petrographic studieshave evealedorogenic sourceareas or flysch in Newfoundland(Mainland Sandstoneand GooseTickle Formation [Stevens,1970; Quinn, 1988]), Quebec (Cloridorme Formation[Hiscott, 1978, 1984]), New York (Austin Glen andSchenectady ormations [Rowley and Kidd, 1981]),Pennsylvania Martinsburg Formation [Lash and Drake,1984]), and Tennessee Sevier Formation [ShanmugamandWalker, 1980]). Some ocal conglomeratic facies within theflysch (e.g., Fincastle ConglomerateMember, Virginia [Raderand Gathright, 1986]) contain clasts of platform carbonates,vein quartz, and granitic gneiss, suggesting hat rocks thatoriginated along the North American margin were exposed oerosion in the advancing thrust belt to the east.Autochthonous flysch overlying the former shelf issedimentologically indistinguishable from slightly older flyschin the allochthonous slope/rise succession Rowley and Kidd,1981].The foredeepwas flanked on the eastby the Taconic thrustbelt. The frontal thrust zone s marked by wildflyschconglomerates, ontaining blocks shed rom advancingsubmarine hrusts, set n a shale matrix. Clasts ncludeslope/rise acies and shelf carbonates. Ordovician submarineemplacementof thrusts along the Taconic front (Logan's Line)in Quebecand New York is constrainedby graptolites nmatrix shales. Subsequent verthrusting transformed someolistostromes and turbidites into melanges e.g., Bosworth andVollmer, 1981]. Melanges are also presentbeneathTaconicthrusts within the mountain belt, where they are nterpreted tomark earlier positions of the thrust front [e.g., St. Julien andHubert, 1975].

Shelf drowning was n pan accommodated y eastwardregional tilting and in pan by motion on closely spacednormal growth faults (Figure 2). Faulting was induced byextensionon the convex outer arc of the flexed plate [Bradleyand Kusky, 1986]. Normal faults commonly have down-to-trench displacements f severalhundredmeters Bradley andKusky, 1986], and some are known to correspond osignificant Middle Ordovician facies, hickness,andbathymetric changes Cisne et al., 1982].Shalesof the outer slope gradeeastwardand upward nto

Fig. 3. Middle Ordovician stratigraphy of the Taconic foredeepand allochthonous slope/risesequence n New York(Figure 1, ocations 18 to 24), adapted rom Fisher [1977, 1979] and Rowley [1983]. Foredeep ransects n thesouthern,central, and CanadianAppalachiansexperiencedsimilar stratigraphic evolution. Brick pattern, shallow-water carbonate;brick and dashpattern, deepwater arbonatewith interbeddedshale;dashpattern, shale;dash-dotpattern, siltstone; stipple, siliciclastic turbidites; conglomeratepattern, wildflysch. Graptolite zones are after Riva[1974], as modified by Finney [1986a]: C. pyg., C.pygmaeus Zone; C. spin., C. spiniferus Zone; 0. rued., 0.ruedemanni Zone; C. bic., C. bicornis Subzone; N. grac., N. gracilis Subzone.


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TABLE 2. Age of Ordovician Carbonate-to-Shale Transitions in the Taconic Foredeep.

Number Locationin Fig. 1 Shale-BearingUnit Age of Onset ofShale Deposition Reference

12345678910111213

Bames et a1. 1981]Bames et a1. 1981]Riva [1969. p. 540]Riva [1969. p. 539]Riva [1969. p. 534]Bames et a1. 1981]Riva [1969, p. 542]Belt et al. [1979, p. 1470]Belt et a1. 1979, p. 1470]Belt et a1. 1979, p. 1470]Riva [1969, p. 523-5]Riva [1969, p. 517]Fisher [1977]

Hare Bay, Nfld.Port au Port Peninsula, Nfld.LGCP Well, A.I.NACP Well, A.I.LGPL Well, A.I.GaspeN. shore, Que.Chute-aux-Galets,Que.Cap Martin, Que.Montmorency Falls, Que.Lotbiniere, Que.Bald Mtn., Que.L. Joseph 2 Well, Que.Rouses Point Quad., N .y .

1415161718192021

Port Henry Quad., N. Y.Ticonderoga Quad., N.Y.Hoosick, N.Y.Albany Quad., N.Y.Amsterdam Quad.,N.Y.Fonda Quad., N. Y.Canajoharie Quad., N.Y.Little Falls Quad., N. Y .

Fisher [1977]Fisher [1977]Rickard & Fisher [1973, p. 588]Fisher [1977]Fisher [1977]Fisher [1977]Fisher [1977]Fisher [1977]Fisher [1977]Fisher [1977]Fisher [1977]Ross et al. [1982]Ross et al. [1982]Ross et al. [1982]Ross et al. [1982]Ross et al. [1982]Ross et al. [1982]Ross et al. [1982]Ross et al. [1982]Ross et al. [1982]Bergstrom [1973, p. 278-279]Bergstrom [1973, po281]Bergstrom [1973, p. 282]Ross et al. [1982]Ross et al. [1982]Finney [1980, po 1186]Finney [1980, po 1186]

GooseTickleBlack CoveMacastyMacastyMacastyCloridonneMacastySt. IreneeUticaUticaUticaIbervilleCumberIand~bHortonvilIeSnake HillWalloomsacSnake HillUticaUticaUticaDolgevilIebUticaMartinsburgMartinsburgMartinsburgAntesMartinsburgLiberty HallbcRich ValleybcPapervilIeReedsvilleMartinsburgBlockhouseBlockhouseBlockhouseBlockhouseAthensRockmartAthensAthens

22232425262728293031323334353637383940

UticaQuad., N.Y.Ellenville Quad., N. Y .Pon Jervis Quad., N.Y.Leespon, Pa.State College, Pa.Williwnspon, Md.Catawba Valley, Va.Saltville, Va.Abingdon, Va.Hagan, Va.Thorn Hill, Tenn.St. Clair, Tenn.Mosheim, Tenn.Greeneville, Tenn.Blockhouse, Tenn.Athens, Tenn.Polk County, Ga.Calera, Ala.Pratt Ferry, Ala.

basa1 D. murchisoniD. murchisoni0. ruedemanni0. ruedemanniC. americanusbase C. americanu.saC. pygmaeusbase 0. ruedemanni0. ruedemanni0. ruedemannibase C. pygmaeusC. americanusC. americanus (E)0. ruedemanni (W)C. americanusC. americanuslate C. bicornisbase C. americanus aC. americanusC. americanusC. americanusC. americanus (E)C. spiniferus (W)c. pygmaeusc. bicornisC. bicornisC. americanusC. spiniferusc. bicornisN. gracilisN. gracilisearly N. gracilis0. ruedemanniC. bicornislateN. gracilisG. teretiuscu[usD. murchisonilate D. murchisoniG. teretiuscu[usD. murchisoniG. teretiuscu[usN. gracilis

NOd, Newfoundland; A.I., Anticosti Island; Que, Quebec;N. y ., New York; Pa, Pennsylvania; Md, Maryland;Va, Virginia; Tenn, Tennessee;Ga, Georgia; Ala., Albama.a. Could be older becausebase of unit is not exposed.b. Transitional facies ncluding deep-water imestones.c. Ignoring local carbonatebuildups.

In the Northern Appalachian orogenic hinterland,Ordovician volcanic rocks are nferred to belong to one ormore magmatic arcs hat developedabove one or more eastdipping (presentcoordinates)subduction zones. Volcanismlasted rom Tremadocian o Llandeilian in Newfoundland,

LIanvimian to Caradocian in New Brunswick, and Arenigianto Ashgillian in Maine [Neuman, 1984]. Siluro-Devonianmagmatism along the same trends cannot be readily attributedto the Taconic Orogeny and is probably related to pre-Acadianplate convergence [Bradley, 1983].


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Bradley: Taconic Plate Kinematics, Appalachian Orogen

SHELF-DROWNING ISOCHRON MAP diachronous along strike. Figures Sb-Seshow various arc andpassivemargin configurations that can ead to diachronouscollisions. Factors nclude straightnessof both arc and passivemargin, angle between arc and passivemargin, and trajectoryof relative motion. In Figures Sd and Se,convergence ateabruptly decreases uring collision. Figure Sf shows ahypothetical collision proximal to a rotation pole; platetrajectories are small circles about the rotation pole, andconvergence ates ncreaseaway from the pole.

Foredeep acies sochron analysis s basedon two precepts:(1) foredeepsubsidence nd migration are driven by plateconvergence;and (2) at a given time, major facies transitionswithin a foredeep occur at set distances rom the plateboundary The primary data are the ocation and age of a givenfacies transition; contour lines connecting data of equivalentage are sochrons. In the Taconic foredeep, he transition fromcarbonate o shale s interpreted o mark the boundary betweenshelf and foredeep, renchwardof the flexural arch. By analogywith the Timor Trough [Veevers et al., 1978], the shelf breakis interpreted to have been situated about 120 km from theplate boundary The transition from shale o turbiditesapproximates he boundary between he outer slope and axis ofthe foredeep. The sediment-filled axial zone of the TimorTrough is about 15 km wide, that is, the turbidite front isabout 15 km from the thrust front. Using the Timor Troughas a model foredeep, he position of the plate boundary at agiven time can be predicted from the position of major faciesbelts n the Taconic foredeep.Figure 4 shows a sequence f paleogeographicmapsof ahypothetical foredeepand a facies sochron map tracking themigration of the carbonate-to-shale ransition. The gradient nFigure 4d approximates he rate of relative plate motion.

bTHRUST BELT Fig. 5. Some combinations of variables that can lead tosynchronousand diachronouscollisions.THRUST BELT THRUST BELT Isochron

gradient Isconvergencerete

~T1~

OUTERSLOPEOUTERSLOPEROWNINGSHELF T2OUTERSLOPEOREBULGE DROWNINGSHELF

T3-EMERGINGSHELF DROWNINGSHELF

T2 T3 DROWNINGISOCHRON100 KILOMETERS

Fig.4. (a)-(c) Sequentialpaleogeographicmapsof ahypothetical foredeep,showing cratonward migration of faciesboundaries hrough time. (d) Facies sochron map of thecarbonate-to-shaleransition. Gradient corresponds o the rateof migration of the facies front. If foredeep migration isdriven primarily by plate convergence, hen the gradientapproximates he rate of relative plate motion.

Some actors that might influence an sochron pattern areshown in Figure 5. Figure 5a shows a highly unlikely,hypothetical collision between an irregular passivemargin andan exactly matching magmatic arc, showing successivepositions of the plate boundary. Overriding plate trajectory isperpendicular o the mean strike of both passivemargin andarc, and the rotation pole is sufficiently distant that platerotations can be approximated as translations. These dealconditions result in synchronouscollision at time 3. All realexamplesof arc-continent collision can be expected o be

ProceduresF ollowed in I sochronM ap ConstructionThe shelf-drowning isochron ines in Figure 1 wereconstructed rom stratigraphic data n Table 2. Paleontologicageswere keyed to graptolite and conodont zones, ollowingthe International Union of Geological Sciences orrelationchart for the United States Ross et al., 1982]. The drowningisochrons were labeled according to Riva's [1974] graptolitezonation scheme Figure 6), subject to two minormodifications: (1) the C. bicornis Zone replaces he D.Multidens Zone, with a slightly older lower boundary definedat the fIrst appearance f the name-giving species Finney,1986a]; and (2) for greater esolution the British D.murchisoni Zone was distinguished rom the lower part of theNorth American P. tentaculatusZone.In caseswhere the carbonate-to-shaleransition isgradational hrough an nterval of interbedded eepwatercarbonatesand shales e.g., Dolgeville Facies of New York;Figure 3) the transition was placed at the baseof thegradational nterval, basedon the assumption hat the shalesrepresentambient hemipelagic sedimentation,whereas hecarbonates epresentepisodic, rapid influx of material fromupslope. Isochrons n Figure 1 were drawn as smoothly as thedata would allow, but isochron ines show considerablepinching and swelling. This could be due to variation in plateconvergence ate, but more likely it is an artifact due to acombination of such actors as (1) eustatic sea evel changessuperimposed n flexurally induced subsidence f the shelf; (2)local water depth complications related to tilting of normal


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Fig. 6. Correlation chart relating Ordovician graptolite zones [Riva, 1974], British Series, and numerical timescalesaccording to Harland et ai. [1982], Carter et ai. [1980], and Ross et ai. [1982]. Ages in boldface wereexplicitly assigned n the original source;others were nterpolated.

nonnal faulting in a zone extending as ar as 150 km from thethrust front. Accordingly, stratigraphic data rom the Taconicforeland n the northern Appalachiansdid not requirepalinspastic correction in Figure 1. On the other hand, n theValley and Ridge province of the central and southernAppalachians, ate Paleozoicdefonnation extended arcratonward of the Taconic thrust front, displacing fonnerlyautochthonousOrdovician foredeepstrata. Theseparautochthonous ocks were restored as shown in Figure 1,basedon published sourcesas ollows. Locations 39 and 40were repositionedaccording o restoredsectionsby Thomas[1985]; locations 28 through 38 according o restored sectionsby Woodward and Gray [1985]; and ocations 26 and 27according to restored sectionsby Shumakeret al. [1985].Locations 23 through 25 were restored using Pedlow's [1976]palinspasticbasemap (a distorted grid of 15 minute quadrangleoutlines); however, 23 and 24 were slightly off Pedlow's[1976] map and had o be projected a short distancealongstrike. Despite uncertainties n all restorations,any errorsintroduced are unlikely to be as severeas errors that wouldhave resulted from ignoring the shortening.

fault blocks; (3) erroneous ossil age assignments; 4) faultybiostratigraphic correlation schemes; nd (5) erroneousstructural restorations. Nonetheless, he drowning isochronlines never cross, suggesting hat the overall pattern isqualitatively correct.Contouring the sparsedata or Newfoundland presentedseveral problems. Both shelf drowning points (Figure I,locations 1 and 2) occur in the D. murchisoni Zone, one at thevery base and one somewhat higher. The N. gracilislC.bicornis isochron was positioned 120 km cratonward of thethrust front, using the Timor Trough model foredeep. The endof the N. gracilis Zone is about when the Humber Armailochthon reached ts final position on the Port au PortPeninsula (near ocation 2), basedon the age ofpostemplacementstrata of the Long Point Group that overlieboth ailochthonous and authochtonous ocks [e.g., Lindholmand Casey, 1989]. The lower boundariesof theN. gracilis andG. teretiusculusZones were interpolated. The lower boundaryof the P. tentaculatusZone was not directly constrainedbyshelf drowning data, but its location was estimatedbyassuming hat the lower P. tentaculatusZone had twice theduration of theD murchisoni Zone, following Ross et al.[1982].Parautochthonous Data Restoration

The Taconic foredeep s divisible into northern andsouthern sectors on the basis of the relative positions of theOrdovician and late Paleozoic defonnation fronts. In thenorthern Appalachians (as far south as Albany, New York,location 17 n Figure I) the Taconic thrust front correspondsto the cratonward imit of significant contractionaldefonnation. Strata which lie cratonward of the Taconic thrustfront are essentially autochthonous,subject only to minorfolding within 10-20 km of the thrust front and high-angle

Implications/or Plate GeometryShelf drowning was diachronousboth acrossand along

strike [e.g., Rodgers, 1971; Bergstrom, 1973; Finney,1986b]. At any given foredeep ransect,drowning occurredfIrst at outboard ocations and progressed ratonwardwithtime. Across-strike diachronism s interpreted as the result ofthe normal component of plate convergence. The along-strikecomponent of diachronism is more complex. The earliestdrowning was n Newfoundland; curiously, the next well-datedshelf drowning occurred at three outboard ocations n thesouthernAppalachians. In other words, shelf drowningjumped from the northern end of the continental margin to thesouthern,while the middle remained unscathed. gnoring


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representedby the Roben's Arm Group, lay outboard at about30S. In summary, while the paleontologic andpaleomagneticdata suppon the concept of multiple Ordovicianarc terranes n Iapetus, t remains unclear whether one or moreinteracteddirectly with Nonh America.Time Scale and Plate ConvergenceRates

To obtain rates of plate motion from the drowningisochron map, it is necessary o calibrate fossil ages withisotopic ages Figure 6). In our study of Caradocianplatemotions n New York [Bmdley and Kusky, 1986] we adoptedthe time scale of Harland et al. [1982] and concluded hat thenormal component of plate convergencewas about 2 cm/yrHowever, considemtionof a single Ordovician seriesobscureda more fundarnentalproblem. The Harland et al. [1982] timescaleascribed 10 Ma to eachof the five post-Tremadocianstagesof the Ordovician. This seems mprobable.Alternatively, series n the Ordovician time scale of Carter etal. [1980] are quite variable in length, a conclusion basedon

Fig. 7. Two possible plate geometries hat could account forthe drowning isochron pattern. Letters identify irregularitiesalong the ancient passive margin of North America (stippledon landward side): A, Alabama Promontory; T, TennesseeReentrant; V, Virginia Promontory; P, PennsylvaniaReentrant;N, New York Promontory; Q, QuebecReentrant;An, Anticosti Reentrant; S, Saint Lawrence Promontory.Numbers along the continental margin indicate age of shelfdrowning; where across-strikediachronism s evident for agiven sector of the margin, only the time of earliest drowningis shown. Numbers correspond o graptolite zones n figure I,beginning with 1 as . victoriae Zone and ending with 8 as Oruedemanni Zone. Bold isochron ines are positions about120 km in advanceof the plate boundary (a) Model for theTaconic Orogeny nvolving collision betweenNorth Americaand two inboard arcs, a northern arc colliding with theNewfoundland sectorof the passivemargin and a southernarccolliding with the rest of the margin. The configuration of theimplied plate boundary between he two postulated arcs cannotbe specified. (b) Model for the Taconic Orogeny nvolving asingle inboard arc, which must be concave o account or earlycollision in the extreme north and south.

Newfoundland, shelf drowning generally progressed rom southto north, with the youngest drowning at the deepestembayment n the former margin in Quebec.Becauseexotic terranes end to be molded into parallelismwith passive margins during collision, the original shapeandorientation of the Taconic convergentboundary or boundariesis unknown. At a given sector of the orogen it is not obviousfrom presentstructure whether he convergentboundaryoriginally paralleled the passive margin or was oriented atsome angle to itIt is unclear from the foredeepstratigraphy whether hepassivemargin interacted directly with one or more convergentboundaries. Figure 7 summarizes he timing of collision andillustrates two of many possible interpretations of the plategeometry. The shelfdrowning isochron pattern betweenAlabama and Quebecseems o track the migration of a singleflexural foredeep, mplying that this sector of the passivemargin probably interacted with a single convergentboundary("arc", below). However, earlier shelf drowning inNewfoundland can either be nterpreted n terms of the sameora different arc. If collision involved two inboard arcs, aconfiguration like that in Figure 7a might have existed ustprior to impact, with both arcs convex toward the continentAlternatively, if a single arc collided directly with NorthAmerica, the configuration might have resembledFigure 7b.The concave arc in Figure 7b would be reasonable f theTaconic Orogeny involved closure of a back arc basin ike theBering Sea.Geologic, paleontologic, and paleomagneticdata romOrdovician volcanic rocks in the Northern Appalachiansallsupport the notion that more than one Ordovician magmaticarc collided with North America. However, none of thesedatabear conclusively on the question raised by the drowningisochron pattern: How many arcs directly collided with thecontinent, as opposed o colliding with each other? On thebasis of forearc basement ype, Rowley [1983] suggested hattwo distinct arcs above east dipping subduction zonescollideddirectly with the North American passivemargin. Accordingto Rowley's [1983] interpretation, a northerly arc (Rowley'sDunnagearc) collided with the Canadiansector of the passivemargin and was characterizedby an ophiolitic forearc; asoutherlyarc (Rowley's Grand Pitch arc) extended rom nearthe U.S.-Canadaborder southward or someunspecifieddistanceand was characterized y continentalbasementwhichhad been deformed during the PenobscoUian rogeny ofNeuman and Max [1988]. This interpretation of two inboardarcs s debatable,since the forearc basementof some activemagmatic arcs (e.g., Aleutian and Banda) varies along strikefrom continental to oceanic. Both Rowley [1983] andWilliams et ai. [1988] suggested hat additional Ordovician arcterranesoccur arther outboard n Newfoundlandand NewEngland; however, none of theseoutboard erraneshave beeninterpreted o have collided directly with North America.Paleontologic evidence suggests hat Ordovician volcanicislands n Iapetus were distributed in at least wo belts[Neuman, 1984]. Neuman's 1984] paleogeographicmap didnot show the locations of inferred plate boundaries,but at leasttwo subduction zonesof unspecified polarity seem o berequired for the eventual closure of Iapetus.Similarly, recentpaleomagneticstudiesof Ordovician volcanic rocks inNewfoundland have evealed wo paleomagneticallydistincttracts [Johnson et al., 1988]. North America's Appalachianmargin lay at about 100-15S. Within Iapetus Ocean aninboard volcanic tract represented y the Moreton's HarbourGroup was ocated at about 15S; a secondvolcanic tract,


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Bradley: Taconic Plate Kinematics, Appalachian Orogen 1045the assumption hat thicknessesofpelagites from widelydistributed deep marine settingsare proportional to theduration of deposition. The time scale adoptedby Ross et al.[1982] also attributes unequal duration to the variousOrdovician seriesbut differs substantially rom that of Carteret al. [1980].Despite unresolved ime scaledisputes hat are beyond hescopeof this paper, he drowning isochron map (Figure 1)provides the basis for some conclusions about Taconic plateconvergence ates. The migration of the shelf drowning linethrough time is assumed o have been a direct response oconvergentplate motion [Bradley and Kusky, 1986].Accordingly, the contour gradient should approximate he rateof plate convergence. Convergence ateswere calculated orthree oredeep ransects, erpendicular o isochron inesthrough locations A, 22, and 31 (in Newfoundland, New York,and Tennessee,espectively). Gradientswere calibratedaccording o the various proposed ime scales o determine arange of convergence ates Figure 8). In Tennessee nd NewYork, calculated convergence ates were 1 to 2 cm/yr. InNewfoundland, a convergence ate of about 1 cm/yr wasobtained using the Harland et al. [1982] and Carter et al.[1980] time scales,but a rate between4 and 5 cm/yr wasobtained from the Ross et al. [1982] time scale. This mightmean either (1) that the Ross et ai. [1982] time scale saccurateand convergence ate was significantly faster nNewfoundland than n New York or Tennessee, r (2) that theHarland et al. [1982] and Carter et al. [1980] time scalesareaccurate,and convergence ate was about he sameat all threetransects. Whichever the case,Taconic plate convergence atesappear o have been of the sameorder as present-day latemotions. While it is likely that convergence ates n arc-passive margin collisions decreasewith time (as collisiongrinds to a halt), the facies isochron maps provide nocompelling evidence of this.Width of the Closed Ocean Basin

Taconic arc-passivemargin collision is believed to havebeenprecededby subductionof an unknown amount of oceaniclithosphere. The width of the closed oceancan be estimatedfrom the rate and duration of subduction. Convergence ateduring collision is extrapolated back in time to the interval ofoceanic subduction, he duration of which can be infeITed romthe duration of arc magmatism. The most appropriate ransectfor this analysis s in Maine, where the age of Ordovicianvolcanism is well constrainedby fossils and where a morecomplete record of volcanism is present han n NewBrunswick or Newfoundland. In Newfoundland,Ordovicianvolcanic rocks are separated rom the foredeepby largeCarboniferous strike-slip faults such as the Cabot, whereas nMaine the major Carboniferous wrench faults lie outboard ofthe Ordovician arc teITanes.As noted earlier, magmatism hatcan be reasonablyattributed o Taconic plate convergencelasted from late Arenigian to Ashgillian time in Maine.Applying the Carter et al. [1980] time scale, arc magmatismlasted about 44 Ma. At a convergence ate of about 1.1 cm/yrthis suggests hat about 480 km of ocean loor was consumedprior to collision along the New England sector of the orogen.Applying the Harland et al. [1982] time scale, magmatismlasted about 39 Ma at a convergence ate of about 1.9 cm/yryielding an oceanic width of about 740 km. Applying theRoss et al. [1982] time scale, magmatism asted about 41 Maat a convergence ate of about 0.8 cm/yr, yielding an oceanicwidth of about 330 km. It is probably appropriate o add 150

c. Tennessee~ 50~

EE 40~

430 440 450 460Time interval (Ma)

470

Fig. 8. Plots of plate convergence ate versus age or threetransects: a) Newfoundland, through location A in Figure I;(b) New York, through location 22; and (c) Tennessee,through location 31. The bars represent he mean convergencerate for the time interval between he oldest and youngestisochronscrossedby each ransect,according o the time scalesof Harland et al. [1982], Carter et al. [1980], and Ross et al.[1982].

GI~GIut:GICIGi>t:O

U

30

20

10

0


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1046 Bradley: Taconic Plate Kinematics, Appalachian OrogenkIn to each result, corresponding to the minimum amount ofocean floor that must have been subducted before anymagmatism could have occurred; this yields widths of 630,890, and 480 km for the Carter, Harland, and Ross timescales, respectively. If convergence rates were decreasingduring collision, the calculated ocean widths would beunderestimations. It should be emphasized that these widths donot correspond to the width of the entire Iapetus Ocean butonly to the oceanic part of the Laurentian plate that wassubducted prior to the Taconic Orogeny. Pre- Taconic closureof a relatively narrow ocean is consistent with biogeographicdata from carbonate-bearing conglomerates associated withvolcanic rocks in the Buchans Group in central Newfoundland[Nowlan and Thurlow, 1984]. A late Arenigian to Llanvirnianconodont fauna belonging to the Toquima- Table Head Realmsuggests that the Buchans magmatic arc was not far fromNorth America.Transport Distance of Slope/Rise Allochthons

Early influx of orogen-derived urbidites n transportedslope/risesuccessions an be explained as a consequence fpostdepositional tectonic transport. However, the amount ofdisplacementcannot be readily obtainedusing standard ectionrestoration echniques;hence hesedata are not useful inestablishing acies sochron patternsor deriving plateconvergence ates. Bradley and Kusky [1986] inverted heproblem to calculate transport distance. In the autochthonousTaconic foreland of New York we suggested hat the shale-to-turbidite transition migrated westward at a rate of about 3cm/yr; while in the Taconic allochthon, the shale-to-turbiditetransition is several million years older than would be expectedfor autochthonous ocks in that location. We concluded hattransport of about 120 km would correct the age anomaly. Inlight of two new considerationsdiscussedbelow this appearsto have been an underestimation.The shelf drowning isochron map (Figure 1) allowsestimation of transport distancesof slope/rise ailochthons thatdo not happen o lie adjacent o well-exposed sectorsof theforedeep Table 1). Transport distanceswere estimated romthe drowning isochron map, as follows. Using the TimorTrough as a model foredeep, he shelf drowning isochron ineat a given time was considered o lie approximately 120 kmcratonward of the thrust front and 105 km cratonwardof theturbidite front (baseof outer trench slope). Arrivalofturbidites at the slope/risewas nterpreted o have occurredabout 15 km cratonward of the thrust front The positions ofthe thrust front and turbidite front were then estimated or thetime of the first flysch in a given ailochthon. Finally, the mapdistancebetweenpresent ocation and nferred initial positionwas measuredperpendicular o strike. Results so obtained aresubject to many possible sourcesof error, which includevariations n widths of foredeep acies belts, age uncertainties,undetectedplate kinematic complexities, and arbitrarycontouring decisions. Nonetheless, he present ransportestimatesare basedon quite different considerations hanpalinspastic structure sections e.g., Stanley and Ratcliffe,1985], which have yielded broadly comparable esults.Taconic Allochthon. New York. Transport distance of theGiddings Brook Slice of the Taconic Allochthon is estimatedat a maximum of 165 km. It now appears hat PawletFormation flysch in the Taconic Allochthon is slightly olderthan we previously indicated; this revision is minor in light ofFinney's [1986a] modified graptolite zonation. A seconderrorin our previous analysis was one of interpretation; the shale-

to-turbidite transitions used o estimateconvergence ate[Bradley and Kusky, 1986] ncluded both trench fill in the east,and postconvergence oredeep ill in the west. The newtransport estimate s basedon extrapolation from Pennsylvaniaof the isochron representingdrowning at the N. gracilislC .bicornis Zone boundary. Since the baseof the PawletFormation is probably slightly younger than the zonalboundary, 165 km is a maximum estimate. The allochthonwas palinspastically restoredperpendicular o strike, along anazimuth of 105 which was the direction of tectonic transportduring late-stageTaconic convergence Bosworth and Vollmer,1981].Hamburg Klippe, Pennsylvania. Taconic flysch occurs nthe Greenwich Slice, which is the lower of two thrust slices ofthe Hamburg Klippe of Pennsylvania Lash and Drake, 1984].Graywacke of the Windsor Township Formation can beassigned o the C. bicornis Zone, basedon occurrenceof thename-giving species Wright et al., 1979]. However, theGreenwich slice contains no pre-flysch slope/rise strata (suchrocks do occur in the Richmond Slice, but flysch does not).Therefore t is uncertain whether flysch sedimentationbegan nthe slope/rise before or during C. bicornis Zone time. Thelatter interpretation s preferred here on the grounds hat flyschsedimentation asted no more than a whole graptolite zone nthe other Taconic allochthons. As in the caseof the TaconicAIlochthon, tectonic transport was estimated rom the positionof the isochron line representing shelf drowning at the N.gracilislC. bicornis Zone boundary. The aIlochthon waspalinspastically restoredperpendicular o strike, along anazimuth of 162 that is, the direction of tectonic transportduring Taconic convergence Lash and Drake, 1984,p. 26](note the discrepancybetween his restoration direction andthat inherent n Pedlow's [1976] palinspastic map). Transportof at least 225 km is inferred. Since the Windsor TownshipFormation is somewhatolder than the zonal boundary this isa minimum estimate.Number Arm Allochthon, Newfoundland. AIlochthonousslope/rise acies n central westernNewfoundland comprise heHumber Arm AIlochthon, which structurally overlies the shelfand foredeepsequence.Late Arenigian arrival of the slope/riseregion at the trench s recordedby turbidites of the Lower HeadFormation [Jameset al., 1987]. Stevens 1970] considered heBlow-Me-Down Brook Formation to be part of theallochthonous lysch succession, ut newly discovered ossilsshow that this unit is no younger than Early Cambrian[Lindholm and Casey, 1989]. The sourceof ophiolitic debrisin the Ordovician flysch was the Bay of Islands ophiolitecomplex, which structurally overlies the transportedslope/risedeposits. The Lower Head Formation represents ne of theoldest obducted rench ill sequenceshat was depositedatopthe former slope/rise of the Appalachian passive margin. Itsgreat age s not surprising, nsofar as Newfoundland is locatednear a promontory and the Humber Arm Allochthon hasclearly been ransporteda long distance rom its depositionalsite. Transport distance of the Humber Arm Allochthon atLobster Cove Head is estimatedat 270 km, basedon thepoorly constrainedposition of the isochron ine representingshelf drowning at the P. tentaculatuslI. victoriae Zoneboundary. The allochthon was palinspastically restoredperpendicular o strike.External Domain, GaspePeninsula. The TourelleFormation occurs at the top of a far-traveled slope/riseallochthon of St Julien and Hubert's [1975] External Domain.Tourelle Formation flysch conformably overlies slope aciesof the Cap des Rosiers Group [Hiscott, 1978, p. 1580].


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1047Bradley: Taconic Plate Kinematics, Appalachian OrogenInflux of ophiolitic flysch is interpreted o mark arrival at thetrench. Transport distanceappears o exceed hat of the otherslope-rise allochthons, but is impossible to estimate withconfidence. The problem is that the Tourelle flysch iscomparatively old, while shelf drowning along this sector ofthe continental margin was comparatively young. Fivedrowning isochron lines had to be located with very little togo on other than their mean spacing elsewherealong theorogen (Figure 1). The allochthon was palinspasticallyrestored perpendicular to strike. On this basis he bestestimate of transport distance s a minimum of 450 km.CONCLUSIONSDecadesof regional tectonic studieshave ed to thedefinition, testing, and refmement of the arc-passivemargincollision model for the Taconic Orogeny. This qualitativemodel has provided a unifying rationale for existingobservationsand has servedadmirably as a predictive basis ornew observations, eading, for example, o the rejection of thedominant role of gravity sliding in the evolution of the orogen[e.g., Rowley and Kidd, 1981]. Considering the usualdisagreements mong geologists on interpretations of regionaltectonics, he consensus egarding Taconic plate tectonics sindeed emarkable.The present study representsa next logical step n regionaltectonic analysis, toward the quantification of plate motions.While this attempt has not been entirely successful,someimportant new relationships and conceptshave been evealed.Shelf-drowning isochron maps are ntroduced here as a newtool with a variety of applications n tectonic studiesof arc-passive margin collisions. The pattern of Taconic shelfdrowning clearly reveals he diachronousnature of the TaconicOrogeny, both along and acrossstrike. The shelf drowningpattern s not diagnostic of the number of arcs that collideddirectly with North America. The rate of plate convergence sinterpreted to have been I to 2 cm/yr, and the minimum widthof the ocean hat closed was probably 500 to 900 km. Theseresults are in no way subject to the latitudinal uncertaintywhich precludespaleomagneticdetection of east-westmotions.An unexpectedbenefit of the analysis s a new technique orestimating tectonic transport distancesof slope/riseallochthons. Such distancesare not readily obtained byconventional structural analysis, but appear o have been nthe range of 165 to 450 km for four Taconic allochthons.Foredeep acies sochron analysis shows enough promise thatapplications are warranted n analogoussettings n the Urals,Ouachitas, Oman, Papua,and Brooks Range.

Acknowledgments. This study was only possible becauseof the careful paleontological work of W. B. Berry, S.Bergslrom, J. Riva, J. Cis"'. "",, ~ Finney. Reviews by R.Neuman, A. Harris, and H. Williams substantially improvedthe paper.REFERENCESBarnes, C. R., B. S. Norford, and D. Skevington, TheOrdovician System n Canada,Publ. 8, 27 pp., 2 charts,Int. Union. Geol. Sci.. 1981.Belt, E.S., I. Riva, and L. Bussieres,Revision and correlationof late Middle Ordovician stratigraphynortheastof QuebecCity, Can. I. Earth Sci.. 16. 1467-1483, 1979.Bergstrom, S. M., Biostratigraphy and facies relations in thelower Middle Ordovician of easternmostTennessee, m. I.Sci.. 273-A. 261-293, 1973.


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Ordovician carbonatebuildups, Virginia Appalachians,Am. Assoc. Pet. Geol. Bull., 66. 189-209, 1982.Rickard, L. V., and D. W. Fisher, Middle OrdovicianNonnanskill Fonnation, easternNew York: Stratigraphicand structural position, Am. J. Sci.. 273. 580-590, 1973.Riva, I., Middle and Upper Ordovician graptolite faunasof St.Lawrence Lowlands of Quebec,and of Anticosti Island,Mem. Am. Assoc. Pet. Geol. 12, 513-556, 1969.Riva, I., A revision of some Ordovician graptolites of easternNorth America, Paleontology, 17. 1-40, 1974.Rodgers, ., The easternedge of North America during theCambrian and Early Ordovician, in Studies n AppalachianGeology, Northern and Maritime, edited by E. Zen et al.,pp. 95-113, Wiley Interscience,New York, 1968.Rodgers, I., The Taconic Orogeny, Geol. Soc. Am. Bull.. 82.1141-1178,1971.Ross, R. I.)r., et al., The Ordovician System n the UnitedStates, Publ. 12, 71 p., 3 charts, Int. Union. of Geol.Sci., Paris, 1982.Rowley, D. B., Operation of the Wilson Cycle in westernNew England during the early Paleozoic: With emphasison

the stratigraphy, structure, and emplacementof the TaconicAllochthon, Ph.D. dissertation, 602 pp., State Univ. ofNew York at Albany, 1983.Rowley, D. B., and W. S. F. Kidd, Stratigraphicrelationships and detrital composition of the medialOrdovician flysch of westernNew England: Implicationsfor the tectonic evolution of the Taconic Orogeny, JGeol..89, 199-218, 1981.St. Iulien, P., and C. Hubert, Evolution of the TaconicOrogen in the Quebec Appalachians, Am. J. Sci., 275A.337-362, 1975.Shanmugam,G., and G. G. Lash, Analogous tectonicevolution of the Ordovician foredeeps,southernand centralAppalachians, Geology, 10, 562-566, 1982.Shanmugam,G., and K. R. Walker, Sedimentation,subsidence, nd evolution of a foredeepbasin n the MiddleOrdovician, southern Appalachians, Am. J. Sci.. 280.479-496, 1980.Shumaker, R. C., T. H. Wilson, W. M. Dunne, I. Knotts,and R. Buckley, Pennsylvania, Virginia, and West Virginiasections, n Valley and Ridge Thrust Belt: BalancedStructural Sections,Pennsylvania to 4.labama,Stud.Geol., vol. 12, edited by N. Woodward, pp. 6-35, Univ. ofTennessee,Knoxville, 1985.Stanley, R., and N. Ratcliffe, Tectonic synthesisof theTaconic Orogeny in western New England, Geol. Soc. Am.Bull., 96, 1227-1250, 1985.Stevens,R. K., Cambro-Ordovician flysch sedimentationandtectonics n westernNewfoundland and their possiblebearing on a proto-Atlantic, Spec. Pap. Geol. Assoc. Can.,7, 165-178, 1970.Thomas, W. A., Evolution of Appalachian-Ouachita salientsand recesses rom reentrantsand promontories n thecontinental margin, Am. J. Sci.. 277. 1233-1278, 1977.Thomas, W. A., Northern Alabama sections, n Valley andRidge Thrust Belt: Balanced Structural Sections,Pennsylvania to Alabama, Stud. Geol., vol. 12, edited byN. Woodward, pp. 54-61, Univ. of Tennessee,Knoxville,1985.Van der Voo, R., Paleozoic paleogeographyof North America,Gondwana,and ntervening displaced erranes:Comparisonsof paleomagnetismwith paleoclimatology andbiogeographical patterns, Geol. Soc. Am. Bull., 100, 311-324, 1988.

lames, N. P., I. W. Bo~ford, and S. H. Williams,Allochthonous slope sequence t Lobster Cove Head:Evidence or a complex Middle Ordovician platform marginin western Newfoundland, Can. J. Earth Sci.. 24, 1199-1211,1987.Johnson, R. I. E., B. A. van der Pluim, and R. Van der Voo,Paleoreconstruction f Early Ordovician terranes n theCentral Newfoundland Mobile Belt, Geol. Soc. Am. Abstr.Programs, 20, A63, 1988.Klappa, C. F., P. R. Opalinski, and N. P. lames, MiddleOrdovician Table Head Group of westernNewfoundland: Arevised stratigraphy, Can. J. Earth Sci., 17, 1007-1019,1980.Knight, I., and N. P. lames, The stratigraphy of the LowerOrdovician St GeorgeGroup, westernNewfoundland: Theinteraction betweeneustacyand tectonics,Can. J. EarthSci., 24, 1927-1951, 1987.Lash, G. G., and A. A. Drake, Ir., The Richmond andGreenwich Slices of the Hamburg Klippe in easternPennsylvania Stratigraphy, sedimentology, structure, andplate tectonic implications, U.S. Geol. Surv. Prof Pap.1312. 40 pp., 1984.Lindholm, R. M., and I. F. Casey, Regional significance ofthe Blow Me Down Brook Formation, westernNewfoundland: New fossil evidence or an Early Cambrianage, Geol. Soc. Am. Bull.. 101, 1-13, 1989.Mussman, W. I., and I. F. Read, Sedimentologyanddevelopmentof a passive- o convergent-marginunconformity: Middle ordovician Knox unconformity,Virginia Appalachians, Geol. Soc. Am. Bull., 97. 282-295, 1986. .

Neuman, R. B., Geology and paleobiology of islands n theOrdovician Iapetus Ocean: Review and mplications, Geol.Soc. Am. Bull.. 95, 1188-1201, 1984.Neuman, R. B., and M. D. Max, Penobscottian-Grampian-Finnmarkian orogeniesas ndicators of terrane inkages,Spec. Pap. Geol. Soc. Am. 230, 1988.Now Ian, G. S , and I. G. Thurlow, Middle Ordovicianconodon~ from the BuchansGroup, central Newfoundland,and their significance for regional stratigraphy of theCentral Volcanic Belt, Can. J. Earth Sci., 21, 2824-296,1984.Pedlow, G. W.,I11, Palinspastic basemap: central andsouthern Appalachians, Geol. Soc. Am. Bull., 87, 133-136, 1976.Pigram, C. I., P. I. Davies, D. A~ Feary, and P. A. Symonds,Tectonic controls on carbonateplatform evolution insouthern PapuaNew Guinea: Passivemargin to forelandbasin, Geology, 17, 199-202, 1989.Quinn, L., Distribution and significance of Ordovician flyschuni~ in western Newfoundland, Geol. Surv. Can.. Pap. 88-1B, 119-126, 1988.Rader, E. K., and T. M. Gathright II, Stratigraphic andstructural featuresof Fincastle Valley and Eagle Rock

Gorge, Botetourt County, Virginia, Centennial Field Guide-Southeastern Section, pp. 105-108, Geological Societyof America, Boulder, Colo., 1986.Rankin, D. W ., Appalachian salien~ and recesses: atePrecambriancontinental breakup and the opening of theIapetus Ocean, J. Geophys.Res.. 81. 5605-5619, 1976.Read, . F., Carbonate amp-to-basin ransitions and forelandbasin evolution, Middle Ordovician, VirginiaAppalachians, Am. Assoc. Pet. Geol. Bull., 64, 1575-1612, 1980.Read, .F., Geometry, facies, and developmentof Middle


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Bradley: Taconic Plate Kinematics, Appalachian Orogen 1049Woodward, N. B., and D. R. Gray, Southwest Virginia,Tennessee, nd northern Georgia sections, n Thomas, W.A., Northern Alabama sections, n Valley and Ridge ThrustBelt: Balanced Structural Sections,Pennsylvania oAlabama, Stud. Geol., vol. 12, edited by N. Woodward,pp. 40-53, Univ. of Tennessee,Knoxville, 1985.Wright, T. 0., G. C. Stephens,and E. K. Wright, A revisedstratigraphy of the Martinsburg Formation of easternPennsylvaniaand paleogeographic onsequences, m. I.Sci., 279. 1176-1186, 1979.

Dwight c. Bradley, U.S. Geological Survey, 4200University Drive, Anchorage, AK 99508.

(ReceivedDecember8, 1988;revised April 17, 1989;acceptedApri128, 1989.)

Veevers, . I., and T. H. Van Andel, Morphology andbasementof the Sahul Shelf, Mar. Geol.. 5, 293-298,1967.Veevers, IJ., D. A. Falvey, and S. Robins, Tirnor Troughand Australia: Facies show topographic wave migrated 80kIn during past 3 M.Y.,Tectonophysics, 45, 217-227,1978.Walker, K. R., Holston and ChapmanRidge Formations;shelf edge skeletal sandbanks, organic buildups andquaI1zose-sand-wavenvironments, in The Ecostratigraphyof the M ddle Ordovician of the SouthernAppalachians(Kentucky, Tennessee, nd Virginia), U.S.A.: A FieldExcursion, Stud. Geol., vol. 77-1, edited by S. Ruppeland K. Walker, pp. 68-73, Univ. of Tennessee,Knoxville,1977.Williams, H. S., S. P. Colman-Sadd, and H. S. Swinden,Tectonic-stratigraphicsubdivisions of centralNewfoundland, Geol. Surv. Can. Pap. 88-1B, 91-98,1988.

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Bradley -Taconic Plate Kinematics-1989