New Reflections on the Structure and Evolution of the Makkovikian- Ketilidianorogen in Labrador and Southern Greenland

TECTONICS, VOL. 16, NO. 6, PAGES 942-965, DECEMBER 1997

New reflections on the structure and evolution of the Makkovikian- KetilidianOrogen in Labrador and southern Greenland

Andrew Kelt 1 GeologicalSurvey, Newfoundland Department ofMines and Energy, St. John's, Newfoundland, Canada

JeremyHall DepartmentofEarth Sciences, Memorial University ofNewfoundland, St.$ohn's, Newfoundland, Canada RichardJ. Wardle,Charles F. Gower,and Bruce Ryan GeologicalSurvey, Newfoundland Department ofMines and Energy, St. John's, Newfoundland, Canada

Abstract.Marine seismic reflection profiles across the Early "eak •zonesbetween rigid bounding blocks. A speculative ProterozoicMakko•- KefilidianOrogen in theLabrador multistagemodel for thebelt incorporates initial northward Searegion suggest that it is a doublyvergent, asymmetric subductionbeneath the craton,shif•g to later southward orogenicbelt, comparablein widthto youngercollisional subcluction,followed by oblique accretion of a compositeare orogens.A southeastdipping reflector package is correlated terraneand juvenile (?) continentalblock. The accreted, with on-land shear zones that mark the southeasternlimit of hotter,juvenile, Proterozoic crust behaved differently than the exposedreworked Archcan crustand is also associatedwith stable,cool, Archcan crust and experiencedsubhorizontal a crypticisotopic boundary in granites,which documents a shearingin lowercrustal and Moho regiom, associated with transitionfrom •ancient" to •juvenile" basement.This southeastdirected imbrication of the middleand upper crust boundaryis interpretedas a suture,along which juvenile by thmsfing.However, the present reflectivity pattern of the Proterozoiccrust has been juxtaposed over (thrust over ?) the orogenmay alsoinclude elements related to postcollisional Areheartcraton. A major synorogenicto postorogenicextemional collapse. plutonitterrane to the southeast has a poorlyreflective upper crestbut shows strong subhorizontal reflectivity in thelower crustand Moho regions. The southeastern part of theprofile 1. introduction correlateswith metasedimentaryterranes of the Ketilidian Beltin Greenlandand comains low-angle, northwest dipping, Reflection seismologyassists in understandingcrustal reflectorpackages suggestive of large-scalecrustal imbri- anatomy by providing vertical sectionsthat augmentthe cationby thatsting. At leasttwo broad zones of reflectivityat largely two-dimemionalplan view affordedby bedrock manfiedepths (up to 16 s) are alsorecognized. One dips geology.Profiles acquired under the CanadianLithoprobe northwestbelow the Archeart eraton, but the most widespread project provideimportant constraints for multidisciplinary mantlereflectivity is southeastdipping in oppositionto the modelingof thetectonic evolution of orogenicbelts in North dominantfabric in the overlyingcrust. These contrasting America. The LithoprobeEastern Canadian Shield Orebore crustaland subcrustal reflectivity patterns define a geometric - Offshore Transect (ECSOOT) is directed toward the Nfocus•beneath the orogenic belt and may provide informa- Precambriancrest of Labradorand adjacentQuebec (Figure tionabout subduction polarity during its development. The 1), where approximately1250 km of marineseismic reflec- doublyvetgent reflectivity pattern resembles images from tionprofries were acquired in 1992. Technicaldata, acquisi- possiblycorrelative Precambrian orogenic belts such as the tion parameters,processed seismic sections,and initial Penokeanand Svecofennianand also younger"small col- interpretationsof large-scalecrustal anatomy are presented lisionalorogens" such as the Appalachian and Pyrerican belts. elsewhere[Hall et al., 1995]. It alsobroadly resembles some results of geodynamiccrustal This paperis a more detailedexamination of part of the deformationmodels based on detachmentand underthrusting ECSOOT profile acrossthe southernNorth Atlantic Craton of mantlelithosphere following collision and the squeezing of and Proterozoicorogenic belts that boundit to the south (Figure 1). It focuseson a ~ 400-kin crosssection of the MakkovikianOrogen, an Early Proterozoic(2.1 to 1.7 Ga) Departmentof Earth Sciences, Memorial University of Newfoundland,St. John's,Newfoundland, Canada. mobilebelt, whichwas contiguous with theKetilidian Orogen of southwestGreenland prior to Mesozoicopening of the Copyright1997 by theAmerican Geophysical Union. LabradorSea (Figures 1 and2). The seismicreflection data permitassessmere and development of plate-tectonicmodels Papernumber 97TCO2286. for this belt and for Proterozoicorogens in general.An 0278-7407/97/97TC-02286512.00 associatedpaper [Crower et al., 1997] discussesthe adjoining

942 KE• ET AL.' MAKKOVIKIAN - KETILIDIAN OROGEN 943

Archean eratons <':"•'•"EarlyProterozoic Belts BALTI,C, •".... ::...f..•'...;'-- •t ::-..:'::ii• N.ATLANTIC / SLAVEAI IItI1•? • '

...... •:..,..•••..,,...... • '•;,,,,• Svecofennian '¾-' ,•-----•,,q Makkovikian- Penokean •'• "'...%_0 Ketilidian

...... -,:..-, ...... Frotits.'.-...': ::.

Figure1. Tectonicprovinces and orogenic belts of theNorth Atlantic region in mid-Proterozoictimes [after Goweret al., 1990], showinglocations of Makkovikian-Ketilidian Orogen and the LithoprobeEastern CanadianShield Onshore-OffshoreTransect (ECSOOT) seismicreflection profiles. Inset map showsthe locationsof potentiallycorrelative belts along the reconstructed southern margin of Laurentia- Baltica. portionof theECSOOT profile across the Early and Middle Labrador - Greenlandcorrelations have been extensively ProterozoicLabradorian and Grenvillian orogenic belts discussed [e.g., Kranck, 1939; Sutton et al., 1972; (Figures1 and2). Bridgwateret al., 1973; Cowerand Ryan, 1986;Kerr et al., 1996], and the former continuityof the two Precambrian 2. RegionalSetting and Pre-MesozoicPlate shields is widely accepted. On a larger scale, the Reconstruction Makkovikianand Ketilidian Orogens are partsof an extensive belt of Proterozoiccrust on the southernmargin of Lauremia The Makkovikian- Ketilidian orogenicbelt is located - Baltica (Figure 1), probably includingthe Penokeanof betweenthe Archcan North Atlantic Craton (termed Nain central North America and the Svecofennian of Scandinavia Provincein Labrador)and the Grenville Province (Figure 1). [Cower et al., 1990]. 944 KERR ET AL.: MAKKOVIKIAN- KETILIDIAN OROGEN

The bathymetricfit of Bullardet al. [1965]was the first 3.2. KaipokokDomain (Labrador) - Border Zone pre-Mesozoiereconstruction of the Labrador Sea area and has (Greenland) beenwidely used in continentalreconstructions. This paper Thissection ofthe orogenic belt (Figure 2) comistslargely usesa reconstruction(Figure 2) thatis specificto theLabra- of reworkedArcheart gneisses. The "reworkingfront" is a 2- dor Sea,which shifts Greenland southward by approximately lorn-widevertical mylonitezone in Labrador(Kanakiktok 100 lcm[Srivastava and Roest,1989; Roest and Srivastava, Shear Zone), juxtaposinggreenschist and amphibolite 1989]. Note that the fit doesnot accountfor any stretching Proterozoicmetamorphic facies. The 2.23-Ga Kikkertavak associatedwith rifting, which may have increased the widths dykesare undeformed to thenorth but are stronglyattenuated of outershelf regions by up to 50% [Srivastavaand Verhoef, and deformedto the south[Ryan et al., 1983]. There is no 1992]. Stretchingmay be an importantfactor, as the recon- well-defined"front" in Greenland,where metamorphism and structedcontinental shelves reveal an axial positivegravity deformation insteadincrease over a distanceof some75 km anomalyup to 150 km wide [Srivastavaand Roest, 1989] [Berthelsenand Hemiksen, 1975]. As in Labrador,Protero- (Figure3b), whichmay in part representcrustal extension. zoic mafic dykesrecord a progressiveincrease in structural However,the seismic reflection profile is locatedlandward of reworkingtoward the south. These differences suggest a more the axial positivegravity anomaly,except near lcm 400 gradualtransition in theProterozoic crustal level in Greenland (Figure3b) andthus provides images of crustthat should not comparedto Labrador.Recent U-Pb datingin Labrador havebeen strongly affected by Mesozoicextension. Sedimen- indicatesmetamorphism of the dykesat 1.9 Ma [Ketchurnet tarycover is thin (generally< 1 lcm)in thenorthern part of al., 1995], consistentwith the pre-1895 Ma reworking theprofile but thickens to around3 lcmnear the turning point inferredby Kerr et al. [1992]. The latestactivity along the at lcm430. Small sedimentarybasins in the northernpart of Kanairiktok Shear Zone, however, is younger than ~ 1.81 the profilerepresem local extensional features related to the Ga, as it affectsgranites of that age [Kerr et al., 1996]. birth of the Labrador Sea. Proterozoicsupracrustal sequences rest unconformably uponArchcan crust in the northbut are progressivelydis- raptedto the southand east.In Labrador,the MoranLake 3. GeologicalFramework of the Makkovikian- Group [Ryan, 1984] comprisesshallow-water sedimentary Ketilidian OrogenicBelt rocks,passing upward into greywacke and shale, overlain by submarine marie volcanic rocks. In Greenland, the Vallen The recomtmctedKetilidian- MakkovikianOrogenic Belt Group (mostly sedimentary)and Sortis Group (mostly (Figure2) measuressome 600 km by 350lcm and is compara- volcanic)define a similarstratigraphy [Aliaart, 1976; Crower ble in width to Paleozoic orogenic belts such as the and Ryan, 1986]. In Labrador,deformation of the Moran Applachians.In Labrador,its westwardcontinuation has Lake Grouppredates 1.89 Ga granites [Kerr eta!., 1992]. apparentlybeen removedby rifting and/ortranscurrent To the southand east, the basalunconformity is obliterated, displacementalong the line of thepresem Grenville Front andbasement-cover contacts become mylonitic zones (termed [e.g., Wardleet al., 1990]. The belt comprisesseveral domainsor tectonostratigraphiczones, which are summarized "gneissicschism" in Greenland).Northwest directed thrusting below.For details,see review papers for Labrador[Ryan et and foldingare recognizedin Greenland,but thereis also evidence for westward and southeastward transport al., 1983; Crowerand Ryan, 1986;Kerr et al., 1996] and southernGreenland [Allaart, 1976; Kalsbeeket al., 1990; [Berthelsenand Henriksen,1975]. Northwestverging iso- Chadwicket al., 1994; Chadwickand Garde, 1996], andtheir clinalfolding is alsorecognized in Labrador[Ryan, 1984], containedreferences. A generalizedtime-stratigraphic chart but there have also been suggestionsof early eastward (Table 1) summarizesthe availablegeochronological con- trampon,possibly associated with eventsin the adjoining straintson the developmentof the bek. TomgatOrogen [Ketchurn et al., 1995]. In thesouth, near to the KaipokokBay andKobberminebugt Zones (see below), 3.1. Archeart Foreland high-gradesupracrustal rocks are entirelyallochthonous and are rotatedinto parallelism with "straightened"(i.e., tram- Archeart crust of the North Atlantic Craton forms the posed),mylonitized, gneisses and dyke remnants [Ryan et al., forelandto the belt (Figures1 and2). In Labrador,Archcan 1983; Allaart, 1976]. gneissesof thenorthern Nain Provinceresemble those of west Proterozoicplutonic suites are presentin bothareas. In Greenland[e.g., Bridgwaterand Schiotte,1990] and date Labrador,the IslandHarbour Bay plutonicsuite includes backto ~ 3.9 Ga; however,the southernpart of the craton early (undated) tonalitic-trondhjemiticrocks and younger mostly formed ~ 3.2 to 2.8 Ga ago [e.g., Loveridgeet al., fluorite-bearinggranites dated at 1.8 Ga [Loveridgeet al., 1987; Kalsbeeket al., 1990]. The Archcan cratonconsists of 1987; Ermanovics,1992]. In Greenland,"early" (foliated) greytonalitic to granodioriticgneisses, with early supracrustal andlate (unfoliated) granites cut the gneisses [Ka!sbeek et al., remnants, and small greenstonebelts (Florence Lake and 1990]. Potassicgranites in the KaipokokDomain, dated at Hunt River beltsin Labradorand Tartoq Groupin Green- 1.89 to 1.87 Ga [Kerr et al., 1992;Ketchurn et al., 1995]are land). On both sidesof the LabradorSea, Early Proterozoic olderthan most other Malckovikian and Ketilidian plutonic dyke swarms(2.23-Ga Kikkertavakdykes in Labradorand rocks(see below); similar ages were obtained from offshore undatedIggavik dykesin Greenland)constrain Proterozoic wells ELS-62 and ELS-90 (Figure2) [Wasteneyset al., deformationbecause they are folded, attenuated,and meta- 1992]. Field evidencein Labradorindicates that granite morphosedin reworkedareas (see below). eraplacementand migmatization(s) preceded interleaving of KERR ET AL.' MAKKOVIKIAN - KETILIDIAN OROGEN 945

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JJJJ J j,,.I jJ.IJJ ,,.i J JJ JJJJJ jJJJ .I , ..I ,J .I JJJJJj JJjJJJ I JJJ JJJ J jjj j jjJ J jj J J J J, ,:,.,..-,.,-....,.,,. ,,.i N::IDOI•O J,'J'aJ J J '•:':':':':':':':':':':':'Z' ::::::::::::::::::::::',•t-" 946 KERR ET AL.: MAKKOVIKIAN- KETILIDIAN OROGEN

Archeartbasement and Proterozoic cover (Iggiuk Event [Ryan et al., 1983;Sc•rer et al., 1988]), indicatingan unresolved earlythermal history throughout this region. Although details arescanty, it seemsclear that the northwestern margin of the orogenicbelt, at leastin Labrador,has a complexearly tectonichistory. Detailed geochronological dataare lacking from equivalentregions in Greenland. 3.3. KaipokokBay Zone (Labrador) - Kobberminebugt Zone (Greenland) On both sidesof the Labrador Sea, the reworked Arcbean regionis separatedfrom the internidesof the orogenby a majordiscontinuity. In Labrador,the KaipokokBay Zoneis a zone of intenseductile deformation, where supracrustal rocks(Lower Ai!lik Group) are juxtaposedagainst reworked Archeangneisses across kilometer-scale mylonite zones. The LowerAillik Groupis consideredto be a high-gradeequiva- lent of the Moran Lake Group[Wardle and Bailey, 1981]. The shearzones are subvertical,but someprobably originated as thrusts,although kinematic indicators are largely over- printedby pervasivelate dextraldeformation [Ryan et al., 1983; Culshawand Ketchum,1994]. Foliated to gneissic granites(locally recording aratexis) within the zoneare dated at ~ 1.8 Ga. U-Pb titaniteages of 1.76 - 1.74 Ga suggesta long historyof thermalactivity [Kerr et al., 1992; Ketchurn et al., 1996]. In Greenland,the KobbermiaebugtZone is a 15-kin-wide subverticalto steeply south dipping zone of ductiledeformation including high-grade supracrustal rocks, gneisses,and deformed (locally anatectic)granites, with subhorizontallineations suggesting transcurrent motion. As in Labrador, the senseof earlier motionsand original attitudes of structures are difficult to establish.

3.4. Aillik Domain (Labrador) In Labrador,a 20-kin-widezone southeast of the Kaipokok Bay Zone is dominatedby sedimentaryrocks and 1.86 and 1.81 Ga felsic volcanicrocks of the Upper Aillik Group [Gower and Ryan, 1987; Schi•reret al., 1988]. The latter have potassic,high-SiO 2 compositionsand local alkaline tendencies.The stratigraphicrelationship between the Lower and Upper Aillik Groupsis unknown,as their contactis a zone of intense deformation. Felsic volcanic rocks are less abundantin Greenland,but metasedimentary and intermediate volcanicrocks of theIlordleq Group [Aliaart, 1976; Kalsbeek et al., 1990]occupy an equivalentspatial location. These are generallyconsidered to overliethe high-gradesupracrustal rocksof the KobberminebugtZone, but, as in Labrador,the relationshipis equivocal.Intermediate and felsic volcanic rocksalso occur in inlandareas (Figure 2) associatedwith plutonicrocks [Aliaart, 1976; Kalsbeeket al., 1990]. Syntectonicand posttectonic plutonic rocks of 1.80, 1.72, and 1.65 Ga age alsooccur as scatteredplutons within the Aillik Domain;the olderexamples were likely comagmatic with youngerparts of the UpperAillik Group[Gower and Ryan, 1987; Kerr et al., ! 992].

3.5. Cape Harrison Domain (Labrador) - Julianeh•b Batholith (Greenland) The CapeHarrison Domain in Labradoris dominatedby plutonicsuites and is equivalentto the "GraniteZone" KERR ET AL.' MAKKOVIKIAN - KETILIDIAN OROGEN 947

Table1. A SummaryTime-Stratigraphic Chart for theMakkovikian- Ketilidian Orogen in Labradorand Southern Greenland

Makkovik Province of Labrador

KaipokokDomain Aillik Domain CapeHarrison Domain

1.65Ga Minor maficplutonic rocks 1.65 Ga minormafic and granitic 1.65 Ga emplacementof layered plutonicrocks gabbro-monzonite- syeniteintrusions

1.72 Ga posttectonic"A type" 1.72 Ga posttectonicgranitoidsuites; "A type"possible granitoidsuites 1.77to Thermalactivity, resetting coeval felsic volcanics 1.74Ga of titaniteages 1.78 Ga latest movements on 1.80 Ga Latest movements on KaipokokBay shear Kanairiktok Shear Zone zones

1.81to Granitoidmagmatism of 1.81 to granitoidmagmatism of 1.81 to voluminous and varied 1.79Ga syntectonicand posttectonic 1.79 Ga syntectonicand 1.79 Ga granitoidmagmatism of setting posttectonicsetting syntectonicand post- 1.81 Ga YoungestUpper Aillik tectonicsetting Groupfelsic volcanism 1.83 to NW-directedthrusting, fol- 1.82 Ga lowedby uprightfolding, andwidespread dextral shearing ???? deformation

1.835 Ga Developmentof shearzones 1.84 Ga earliestdated granitoid plutonism 1.86 Ga earliestUpper Aillik Groupfelsic volcanism ???? supracrustalrocks equiva- lent to Upper Aillik 1.87 Ga Granitoidplutonic suites 1.87 Ga granitoidplutonic suites Group?

1.88 Ga Earliestrecognized thrusting 1.88 Ga juxtapositionof Lower in supracrustalrocks, trans- Aillik Groupand lation of unconformity Archean basement

1.89Ga Emplaceme•ntof potassic granitoidrocks ???? deformation and metamor- phismof CapeHarrison 1.90 Ga Metamorphosedand rework- MetamorphicSuite ing of diabasedyke swarm

> 1.90Ga Depositionof supracrustal > 1.90Ga depositionof LowerAil!ik <2.1Ga formationofprotoliths rocks,mafic volcanism Groupsupracrustal rocks to CapeHarrison MetamorphicSuite 2.23 Ga Emplacementof Kikkertavak ???? basement enclaves of orthogneisses diabasedyke swarm uncertainage and affinity 2.84Ga Tonaliticplutonic suites

3.10Ga Oldestrecognized Arcbean 3.10 Ga basement rocks that resem- gneisses ble Archeangneisses of KaipokokDomain 948 KERR ET AL.: MAKKOVIKIAN- KETILIDIAN OROGEN

Table 1. (continued)

Ketilidian Mobile Belt of South Greenland

Border Zone lulianeh•b Batholith Psammite-Pelite Zone

1.77 to Emp!acementof granitoid 1.72 Ga plutonsinto basementrocks 1.76 to emp!acementof late, 1.76 to emplacementof late, 1.74 Ga rapakivitype granitoid 1.74 Ga rapakivitype granitoid plutons plutons

1.79 to emplacementof 1.81 to 1.78 Ga "syntectonic"and anatectic 1.79 Ga granitoidrocks

1.79 Ga latestdetrital zircon grain (Detailedhistory recognizedin psammites unknown) 1.82to granitoidplutonism in the 1.81 Ga centralpart of the 1.82 Ga age of granitoidcobble batholith, sinistra! in Psammite Zone transpression conglomerate

1.84 Ga earliestdated granitoid 1.84 to main populationof detrital plutonism,southern part 1.80 Ga zircongrains in psammites of batholith

???? formationof sedimentary ???? Later uprightfolding and felsic volcanic rocks (derailedhistory of theIlordleq Group unknown) ???? NW directedthrusting and translation of basement- coverunconformity (derailedhistory unknown) (Derailedhistory unknown

???? Depositionof Vallen andSortis 1.96 Ga oldestdetrital zircon grain Groupsupracrustal rocks recognizedin psammites

2.13 Ga Emplacementof Iggavik diabasedyke swarm (Rb-Sr, approximateage only)

3.2 to Formation and stabilization 2.8 of Arcbeanbasement complex of southern Greenland

For detailsof sources,see text.

KERRET AL.' MAKKOVIKIAN- KETILIDIAN OROGEN 951

o o o o _.. 100km 62NL' o • • o T • I

54N mmmmmmm

Seismic : Profile o- (a) Total Magnetic Field

100 km 62 i

Gravity (mGal

60N

Seismic

Profile ...."•:•:•:•:'"*•:•:• ß-.:.:::::'..:..:....: ...... (b) Bouguer Gravity Anomaly Figure3. (a) Totalfield magnetic anomaly map for the reconstructedLabrador Sea region, showing reconstructedcoastlines and major geologicalfeatures. (b) Bouguergravity anomalymap for the reconstructedLabrador Sea region, showing reconstructed coastlines and major geological features. Both mapsare simplified from Srivastava and Roest [1989]. 952 KE• ET AL.: MAKKOVIKIAN - KETILIDIAN OROGEN

[Allaan, 1976] or JulianehfibBatholith [Chadwick et al., to 2-kin-wideSardloq Zone (Figure 2), whereultramylonites 1994]in the Ketilidianbelt (Figure2). Thismajor plutonic derivedfrom granitoidrocks show subhorizontal lineafions terraneis characterizedby a prominentaeromagnetic high and evidenceof sinistraltranscurrent motion [Chadwicket thatextends across both continental shelves and appears to be al., 1994]. Mylonitizedgranites south of the shearzone tnmcatedalong the GrenvilleFrom (Figure3a). appearto havebeen sites of anatecticmelting [Chadwick et In Labrador,the oldestidentified rocks are gneissesof the al., 1994]. Chadwickand Garde [1996] note that the shear CapeHarrison Metamorphic Suite (Figure 2) which,on the zone doesnot definethe southernlimit of the batholithat the basisof Nd studies,are unlikelyto be older than - 2.1 Ga surface,as previouslysuggested [Bridgwater et aI., 1973; [Kerr and Fryer, 1993, 1994]. Foliated1.84- to 1.80-Ga Windley,1991], but it mustbe important,as it coincideswith granitoidrocks, and massive suites of 1.80-, 1.72-and 1.65- a prominentbreak in the aeromagneticpattern, perhaps Ga ageare also recognized [Kerr et al., 1992];the 1.80-Ga indicativeof contrastinggranitoid associations (Figure 3a). groupis alsopresent offshore (Figure 2) in wellsELS-51, The continuityof the magnetichigh associatedwith the ELS-53,and ELS-68 [Wasteneyset al., 1992].The youngest batholithsuggests that the SardloqZone shouldcross the (1.65 Ga) suitesare probablyrelated to subsequentevents continental shelf some 50 km off Labrador, east of the withinthe LabradorianOrogen to the south[e.g., Goweret onshoreportion of theMakkovikian Orogen (Figure 3a). al., 1990]. The older suitesare potassic,compositionally evolvedmonzonites, syenites, and granites which are transi- 3.7. Psammite and Pelite Zones (Greenland) tional betweenso-called "I type" and "A type" granites. SouthernmostGreenland comprises the "foldedmigmatite" These are dissimilarto younger arc-typebatholiths but and "fiat-lyingmigaxmtite" zones of Aliaart [ 1976], for which resembleAppalachian postorogenic suites [Kerr, 1989a;Kerr Chadwick and Garde [1996] use the names Psammiteand et aI., 1996].Nd isotopicstudies of theserocks, which are Pelite Zones, respectively(Figure 2). This terminology dominatedby 1.8-Ga intrusions[Kerr and Fryer, 1994] conveysthe dominance of metasedimentarygneisses in strong indicate that Arcbean crust of the North Ariantic Craton does contrastto the adjacentJuliaherb Batholith.Southernmost not extendfar beyondthe easternlimit of theAillik Domain Greenlandcoincides with a prominentaeromagnetic low, (Figure2). Nd isotopicdata from offshorewell intersections which continues across both continental shelves but does not [Kerrand Wardle, 1997] suggestthat the equivalentisotopic extendinto the onshore(Figure 3a). Orthogneissesintersected boundarylies between km 160 andkm 200 nearthe seismic by offshorewells ELS-51 and ELS-68 yielded ~ 1.80 Ga profilebetween wells ELS-69/90 and ELS-62/53 (Figure 2). U-Pb ages[Wasteheys et al., 1992], confinaxingthe existence In Greenland,the JulianehfibBatholith increases from ~ 40 of Makkovikian- Kefilidiancrust in the offshoreregion, but lcm wide in the east to over 100 lcm wide on the west coast no metasedimentaryrocks were intersected. andincludes several subparallel zones of foliatedgranites and The PsammiteZone is dominatedby amphiboliteto more massivesuites. Largely unfoliated("late") granites granulite-faciespsammific paragneiss, with lesseramphibo- composeabout 70% of thebatholith, particularly in the north lites and orthogneissunits [Kalsbeeket al., 1990; Chadwick [Aliaart, 1976;Kalsbeek et al., 1990], but recentdescriptions and Garde, 1996]. The natureof any basementto the psam- [Chadwicket al., 1994] suggestthat penetrative fabrics may mite zoneis unclear,and banded gneisses reported by Aliaart be morewidespread in thesouth. Chadwick and Garde [1996] [ 1976]may be stronglymigmatized metasediments [Chadwick suggestthat much of thebatholith was eraplaced in a regime and Garde, 1996]. On the basis of granitoid clastsin of sinistraltranspression, which contrastswith the largely metaconglomerates,it has been suggested that many of these dextraltrampression inferred for the KaipokokBay Zonein rocks are younger than (and perhapsderived from) the Labrador (Culshawand Ketchum, 1994). The region also granitoidbatholith to the north [Chadwicket aI., 1994]. An includesmafic intrusive complexes, characterized by hydrous ageof 1.82 Ga from a granitecobble [Hamilton et al., 1995] gabbrosand diorites, which have ambiguousintrusive supportsthis idea, and well-preserved metasediments contain relationshipswith the granitoid rocks, suggesting to Kalsbeek detrital zircons dated at 1.96 to 1.79 Ga [Hamilton et aI., et al. [1990]that they were in part coeval.U-Pb agesof 1.85 1996]. The 1.79-Ga detritusprovides a maximum age for Ga, 1.80 Ga and 1.75 Ga [van Breemenet al., 1974; Gulson deposition,but it is not certainthat theseages apply to all andKrogh, 1975] resemble the agedistribution noted by Kerr metasedimentaryrocks in the region. The Pelite Zone is et al. [1992]but are lessprecise. More recentdating suggests dominatedby pervasivelymigmatized pelific gneisses, with an importantmagnintic pulse at ~ 1.82 Ga [Hamiltonet al., gently dippingto subhorizontallayering and foliation.The 1995]. Southwardshifts in the Pb, Sr, and Nd isotopic pelific gneissesare interlayeredwith minor marble, calc- compositionsof Ketilidiangranites indicate that Archean crust silicate rocks, and amphibolites [Aliaart, 1976] and doesnot occurvery far southof the KobberminebugtZone subconcordantsheets of gamefiferousgranite, presumably of [Patchettand Bridgwater, 1984; Kalsbeek and Taylor, 1985; anatecticorigin. The Psammiteand Pelite Zones were also Hamiltonet al., 1995]. Althoughthe JulianehfibBatholith is affected by extensivelate-stage magmatism, typified by widelydescribed as "Andean"[e.g., Bridgwateret al., 1973; fayaliteand orthopyroxene-bearing "rapakivi suites," includ- Allaart, 1976; Windley, 1991; Chadwickand Garde, 1996], ing riorite,monzonite, syenite, and granite (s.s.). Thesehave there are few geochemicaldata availablefor comparison been interpreted as diapiric mushroom-like bodies eitherwith Labradoror modemanalogues. [Bridgwateret al., 1973] or as sheet-likebodies associated 3.6. SardloqShear Zone (Greenland) with extensionalductile shear zones [Hutton et al., 1990]. U-Pb agesof 1.76-1.74 Ga [Gulsonand Krogh, 1975] from The JulianebatbBatholith includes several ductile shear theseintrusions indicate that they are youngerthan much of zones[Chadwick et al., 1994].The mostprominent is the 1- theJulianeMb Batholith, and they have largely "juvenile" Nd KERR ET AL.: MAKKOVIKIAN - KETILIDIAN OROGEN 953 isotopicsignatures [Patchett and Btqdgwater, 1984]. Syntec- nismoccurred ~ 1.86 Ga ago,but somewas probably coeval tonicgranites examined by Hamiltonet al. [1996]gave ages with 1.81- to 1.80-Gagranitoid plutonism, prevalent to the of 1.79to 1.78 Ga, implyingthat deformation is relatively southeast. young. 4. Thereis a zonedominated by complexplutonic suites, The Psammite and Pelite Zones exhibit a distinctstructural with few remnantsof supracrustalrocks. In Labrador,this pattern[Allaart, 1976]. Steeply dipping to subverticalincludes 1.84-Ga, 1.80-Ga, and 1.72-Ga assemblages,with structures,with complexinterference patterns, predominate affinitiesto Paleozoicand youngerpostorogenic magmatic in the north but become flatter toward the south. There is suites.However, early membersand gneissicenclaves may evidencefor earlysoutheast verging recumbent fold structures representearlier arc-type rocks. In Greenland,1.82- to 1.80- onboth the eastand west coastsof Greenlandand for later Ga plutonicrocks are viewedas arc related,although geo- uprightfolding [Aliaart, 1976; Chadwick and Garde, 1996]. chemicaldata are lackingand 1.74-Ga plutonicassemblages Thesouthern tip of Greenlandhas been interpreted as a gentle of "rapakivi"type are alsorecognized. The 1.65-Gamafic domalstructure [Btidgwater et al., 1973],possibly represent- andgranitoid suites are recognizedin Labradorbut apparently iaga large-scalenappe [Windley, 1991]. Metamorphism in not in Greenland.Isotopic signatures on both sidesof the the Psammiteand Pelite Zones representslow-pressure, Labrador Sea indicate that there is no Arcbean basement high-temperatureconditions, with abundantsi!limanite and beneath this zone. andalusite;estimates of 2-4 Kbars and 650ø-850øC indicate 5. Thereis a zonedominated by metasedimentarygneisses veryhigh geothermal gradients [Dempster et al., 1991].A andgranitoid rocks, preserved only in southGreenland. At 1.78-Gaage from an anatecticgranite indicatesthat this leastsome psammitic rocks probably had sourceswithin the metamorphismis significantly younger than metamorphism in magmaticbelt to thenorth and were deformedand metamor- thenorth of the orogen,interpreted to be largelysynchronous phosedafter 1.78 Ga, but theregion may includemore than with1.82-Ga granitoidrocks [Hamilton et al., 1995]. onegroup of supracrustalrocks. Major structuresin thiszone are southverging, in contrastto thosedocumented in the 3.8. YoungerIgneous and SedimentaryRocks north. Extensive1.75- to 1.74-Ga granitoidplutonism of postorogenic,"rapakivi" type impliesan extendedthermal In theNain Province(Figure 2), thereare abundant Middle history throughoutthis zone. As far as is known, the Proterozoic(1.46- 1.23 Ga) mafic,anorthositic, and granitic metasedimentaryrocks had largely "juvenile"sources, and plumnspresumed tobe associated with crustal extension and there is little direct or indirect evidence for older continental rifting[e.g., Emslie,1978]. Plutonic rocks of similarage, basement. includingalkaline granites, also occur within the Ketilidian In summary,the Makkovikian- Ketilidianprovince may 0rogen(Figure 2), wherethey form the Gardar Province. A not preservea completecross section through an orogenic ~ 1.27-Gaage from offshore well ELS-56[Wasteheys et al., belt, but it includescrustal regions with very different 1992]indicates that these intrusions may alsobe presentnear historiesand a first-ordercrustal boundary between Archcan theseismic profile (Figure2). andjuvenile Proterozoic crust. This tectonic zonation across theorogen should be documentedby progressivechanges in 3.9. Summary of Tectonic Zonation crustalreflectivity patterns, as revealedby the seismic The Makkovikian- Ketilidian Orogen is divisibleinto reflectionprofile, discussed below. severaldiscrete tectonic zones. Working outwardfrom the NorthAtlantic Craton, these are as follows: 1. There is a zone of reworked Archcan basement rocks, 4. Seismic Reflection Profile variablydeformed Early Proterozoicsupracrustal sequences, andsubordinate plutonic rocks. In Labrador,this zone has a 4.1. Locationand Representation complexearly history, including 1.89- to 1.87-Gagranitoid plutonismand early (pre-l.87 Ga)juxtaposition of basement TheECSOOT profile lies 40 to 120km fromthe coastline andcover, with superimposed1.80-Ga deformationand of Labradorand 100 to 150 km from the restoredGreenland magmatism.The latest deformations,at least, involved coast(Figure 2); if the effectsof preriftingstretching are northwestdirected thrusting.The equivalentregion in removed[cf. Srivastavaand Verhoef,1992], the coastlines Greenlandmay share this early history, but as it hasreceived wouldbe approximately equidistant from the line. The profile limitedattention since the 1970s,geochronological data are lieslandward of significantcrustal extension associated with sparse. the formationof the LabradorSea [Hall et al., 1995] (Figure 2. There is a shear zone in which basementand supra- 3b).This paper discusses theprofile from lcm 0, a pointwell crustalrocks akin to thoseof the above zone are strongly withinthe Archcan eraton to beyondkm 450, a locationthat deformedand juxtaposed with younger supracrustal rocks (see may representthe GrenvilleFront [Hall et al., 1995]. below).In Labrador,there was early, pre-l.86-Ga deforma- Regionalstructural patterns in bothLabrador and Greenland tion, but most of the deformationoccurred ~ 1.8 Ga ago. areoriented roughly at 90ø to the seismic line, suggesting that Thiszone apparently delimits the surface extent of Archcan dippingstructures should show approximate true dips assure- basementin both Labradorand Greenlandand may represent hagthat they strike parallel to regionaltrends. In thispaper, a suture. we use the phrases"northwest dipping" and "southeast 3. Thereis a zonedominated by felsicvolcanic rocks and dipping"to referto reflectororientations, but we recognize youngerplutons, which is well preservedin Labradorbut thatin somecases these may be apparentdip directionsand moredeeply eroded in Greenland.Some of thefelsic volca- magnitudes. 954 KE• ET AL.' MAKKOVIKIAN- KETILIDIAN OROGEN

NAINPROV. --->i:! < ...... MAKKOVIK PROVINCE I

Makkovik front Kaipokokdomain Cape Harrisondomain (Kanairiktoks.z.) • Aillikdomain 25O 300 0- 100 D 150 Kalpokol

-- " - - - _ ------_-?.-.--_-.. "' .'-- %'_- •- - ..,"---•.-'" ..• -- =..,_-.. -,..,,..,, .,. _ C..2 - ..-.-. 10- === --' - - - " - F '=•-- . ---; •-"_ _'"_--_--- .... f_+------• ------. z lO ------:[•u.zs•- E --- •".'•-.-Z%------.. '- '-=--.=-'- _--..---:-_------;.-- -

15s-

• Mohoposition (from Reid, 1996) MAKKOVIKPROVINCE ...... > ili<-'GRENVILLE PROV.

Cape Harrisondomain I ?Grenville front? 30O 350 400 NW S 450 5OO PsammiteandPelite Zones? L• • 0

s-- -- -' =':P'I'------_R:- - -..... • -_ -2__ _-- -"--_ - . __ - _., c --C :-. _ -.';•- 2,--•.""•-' P2.';-'-:':; •-:-:'--It:)3 -:'--- -:-'•-'• -'--:.•': -'-'..: "--:'"-"'" "-:'"' - - . •'•" .•..•_ -_-':--._=._-_:..- -- ; ': •-•-• •,nz .... '---•. -'-_-•'-- ' --' -• • - -"-•- ..-"- -" ' - -W2---' ";--•- 10- ½_-,?_-L:-.•-- :--_.:_•:_-::_•-- :--_'_;t•;•_•= "_- :- --•--_&_.•':,,_- •.' -:: , .---'--_--•- ' .w½; :'J; :: '_; .-: _--:_-: '- :_--.---_':__- - :-L ------'- 15S- : { .:::_.T__ --_-'_ .... - - --..- -;-; _.;- -'-- -:: -15S Figure4. Line drawingsof southernECSOOT seismic reflection profile, adaptedfrom Hall et al. [1995]. For detailsof acquisitionand processing, see Hall et al. [1995].Moho position is afterReid [1996].Note that thetwo halvesof thediagram incorporate some 30 km of overlapto aid in linldngthem. Featuresdesignated by lettersare alsoindicated on Plate 1.

Full details of line positioning,data acquisitionand aboutkm 125(Figure 2). However,there is no clearbreak in processing,original seismic profries, and a regionalanalysis reflectorpatterns at thispoint (label A) andno suggestionof of data from the entire ECSOOT profile (Figure 1) are a majordipping structure. The profile through the Kaipokok presentedby Hall et al. [1995].Line-segment diagrams after Domain(lcm 130 to km 190)is similarto the adjacentNain Hall et al. [1995]are presented at a reducedscale as a basic Province.Reflector patterm are generally weak and inconsis- frameworkfor discussion.The mostimportant section of the tent,although there is a northwestdipping domain around km profile,from km 180to km 380, hasbeen reprocessed with 150 (labelB) andpossibly more widespread subhorizontal muchstronger coherehey filters and a highergain than used reflectivityin thelower crest from km 150to 190(label C). by Hall et al. [1995], in orderto betterillustrate reflection In the caseof B, theremay be a mimeticlink to a small fabricscritical to this paper (Plate 1). A "brighter"plot sedimentarybasin visible in theupper few seconds(label D). results,with the minor disadvantage that it is moredifficult to The baseof the crustin this region lies at 9.5 - 10.2 s, as seereflections in low-amplitudeparts of the section.The line determinedfrom coincidentwide-angle seismic experiments drawingsand profile are drawn without vertical exaggeration (indicatedon Figure 4) [Reid,1996]. Sporadic subhorizontal for a seismicvelocity of 6 kin/s, consistentwith estimatesof to northwestdipping reflectivity is presentat 10 - 15 s, C/n• andLouden [1992]. Reflector groups are designatedby particularlyapproaching theinferred location of theKaipokok lettersA-Z, qualifiedby numberswhere necessary, on the BayZone (labels E andF). As notedby Hall et al. [1995], line drawings(Figure 4) andoriginal profile (Plate 1). thishas a similarorientation to reflectivityin themiddle crest beyondkm 200 (labelsK1 andK2). 4.2. Nain Provinceand KaipokokDomain 4.3. Kaipokok Bay Zone, AllIlk Domain, and Cape From km 0 to km 130, the prof'fielies within the Nain Harrison Domain Province(Figure 2). This generallyunreflective region is describedand discussedelsewhere [Hall et al., 1995]. The Extrapolationof the KaipokokBay Zone to meet the Kanairiktok Shear Zone should intersect the seismic line at KobberminebugtZone suggests that this structure should cross KERR ET AL.: MAKKOVIKIAN- KETILIDIAN OROGEN 955 theline aroundlffrt 180 to 190 (Figure 2, seelater discus- 4.4. Psammite and Pelite Zones of Southernmost sion).This corresponds with southeast dipping upper crustal Greenland reflectors(label G) apparentlybounding and below an overlyingsedimentary basin. At deeperlevels, there are Regionalgeological and geophysical correlations imply that scatteredsoutheast dipping reflectors (between labels G and theprofile beyond km 320 shouldimage crest corresponding H),'forming a zonethat may extenddownward to link with to southernmostGreenland (Figures 2 and 3a). This portion thedeep reflectors N1. Thediscrete bright reflections at label of the line is well offshore, as indicated by the thicker H maybe partof thiszone but could partly represent multi- Mesozoic- Tertiary sedimentarysection, but is landwardof plesof overlyingbasins and/or migration smiles. There is no the axialpositive gravity anomalyof the plate reconstruction clearramcation relationship between the southeastdipping (Figure 3b). The seismicprofile from km 300 to km 400 is reflectivityand the wider zone of northwestdipping reflec- probablythe most stronglyreflective segmentof the cross tionsthat dominate the profileto the southeast(see below). section(Plate 1). Northwestdipping reflective zones (labels Refraction data indicate that the base of the crest lies at P1, P2, and P3) appearto "sole"into the stronglyreflective lower crustnoted above (labels L andM). Northwestdipping around9.5 - 10.2 s in thispart of the profile(indicated on zonesintersect the Mesozoicunconformity at km 340, lcm Figures4 and 5) [Reid,1996], and it is likelyrepresented by 370, andkm 400 (labelsQ1, Q2, and Q3), wherethey are subhorizontalreflectivity (labels C and F). Below this, spatiallyassociated with smallMesozoic - Cenozoicbasins. northwestdipping reflectivity extends to depthsbelow 15 s There is alsoa lessobvious southeast dipping reflectivity in (labelJ) andis assumedto be at mantledepths. the uppercrust (label R). At depth,these northwest dipping Thecharacter ofthe seismic section changes significantly zones coalesce with a strongzone of subhorizontalreflectivity beyondkm 190 (nearsurface) and km 215 (at 10 s). The at around10 s (labelsS 1 and S2), which here showsmostly diffusesoutheast dipping reflector (labels G andH) merges subhorizontal orientation. The base of the crest is inferred to intoa morediffuse zone of reflectivityat aroundkm 220 at lie around10 s, basedon resultsof On'an and Louden[1992] depthsof 7 s. Beyondthis point, the middlecrest shows a and is underlainby an opposing,southeast dipping, reflec- northwestdipping reflectivity (labels K1, K2, andK3), and tivity (label T). This mantle reflectivity is continuouswith thereis a stronglyreflective lower crestfrom about7 s to 10 similar reflectorsnoted previously (labels N1 and N2) but s. Deeperparts of the sectionare characterizedby southeast appearsstronger, particularly around km 330. dippingreflectivity, extending to 15 s (labelsN1, N2, and The final sectionof the profile extendsfrom km 400 to N3).The baseof thecrust beneath the CapeHarrison Domain about km 500; note that there is a changein directionof hasnot been locatedfrom coincidentwide-angle seismic approximately30 ø at aboutkm 430 (Figure 2). This portion experiments,but its positionmay be estimatedfrom other of theprofile is over 120 lcmoffshore from Labradorand the data.Modeling of datarecorded at a nearbycoastal land site restored Greenland coast. The Mesozoic - Cenozoic sedimen- suggestsa Moho reflectiontime of 12 s midwaybetween the tary sectionis thicker and more continuoushere than else- profileand the coast[Louden and Fan, 1997],greater than the whereon theprofile, and its thicknessis inverselycorrelated 9.5-to 10.2-sestimate near the MakkovikFrom [Reid,1996]. with the apparentthickness of the crest [Hall et al., 1995]. Coincidentnormal incidence and wide-angle data perpendicu- Thus this region may have been affected by extension lar to the ECSOOT profile, and linked to it near km 150, associated with the formation of the Labrador Sea. Northwest indicatethat crustalthinning is largely restrictedto areas dippingreflectivity discussed above is less obvioushere, outboardof theprofile [Chian and Louden, 1992; Keen et al., althoughstill visible (e.g., label U). There is also strong 1994],although it mayhave affected the crestin areaswhere subhorizontalreflectivity in the lower crest(label V). Discon- sedimentarycover is thicker(e.g., km 350-475).The laterally tinuoussoutheast dipping structures that project to the surface extensive,but opposed,orientations of reflectionfabrics aroundkm 430 -km 450 (labelsW), associatedwith sedimen- above10 s and from 11 s to 15 s (Figure 4) stronglysuggest tary basins(labels X andY), were interpretedby Hall et al. thatthe intervening depth range includes a significantbound- [1995] as a potentiallocation for the GrenvilleFrom and ary, and interpretationas the Mohois consistentwith the mark the southernlimit of interpretationin thispaper. Note, wide-angledam. This indicatesa crustalthickness slightly however,that northwest dipping reflectivity is visiblesouth of greaterthan in thearea of theMakkovik From [Reid, 1996]. thesestructures (label Z). In contrastto the middle and lower crest, the upper crest throughoutthis region (above 6 s) is in generalfairly trans- 5. Interpretationof SeismicReflection Profile parentbut showssome sporadicnorthwest and southeast 5.1. Age and Significanceof ReflectorPackages dippingreflectors (labels O 1 and02). The lowercrustal region from aboutkm 220 to km 250 The reflectorpackages discussed in thispaper are inter- appearstobe a "focalpoint" for divergent crustal and mantle pretedto be related to collisionalaria/or accretionmy orogenic reflectivitypatterns. To the northwest,southeast dipping processesof Proterozoic age, but an alternative interpretation, crustalreflectivity opposes a deeper northwest dipping mantle that is, that these fabrics relate to Mesozoicor younger reflectivity;to thesoutheast, the pattern is reversed,with extensionassociated with formation of the Labrador Sea must northwestdipping structure in the crest and southeast dipping alsobe consideredand discussed.However, we considerthis reflectorsin subjacentmantle. unlikelyfor severalreasons. 956 KERR ET AL.: MAKKOVIKIAN - KETILIDIAN OROGEN

The dippingreflections are of crustalscale and, although extensionis laterallydisplaced from moreobvious upper theyare most obvious in themiddle and lower crust, they can crustaldevelopment of majorsedimentary basins. Thus, in be tracedthrough the upper crust to the sub-Mesozoic conjunctionwith the evidencefrom wide-angleseismic unconformity,where they die out. There are isolatedexam- surveys[Ctdan and Louden, 1992; Reid, 1996]and the trans- ples of major basementstructures that appearto control Labrador Sea line [Keen et al., 1994], we believe that sedimentarybasins (e.g., at the KaipokokBay Zone and Mesozoicextensional effects here are minor and that the bulk proposedGrenville Front), suggesting their local reactivation, of thecrustal architecture is of Proterozoicage. However,we but mostbasement reflectivity appears to be truncatedby the doaccept that some of themore prominent reflector packages rift-driftunconformity. The positionof the ECSOOTprofile may havebeen reactivated as extensionalstructures, either was carefully selected(using abundant petroleum industry during Proterozoicpostorogenic collapse or during much data for the Labrador shelf) to lie landward of major youngerrifting associatedwith the LabradorSea. extensionalfaults in the continentalmargin basins,except at the southernmostend of the sectiondiscussed in this paper, 5.2. General Features andthe sedimentaryveneer is generallythin, whichlimits the The profile can be subdividedinto two halves, shown magnitudeof any extension.There are a few areas where schematicallyin Figure5. The northwesternportion, from km sedimentthicknesses approach 3 s, but thisis still lessthan 0 tokm 190(approximately) has a weaklyand inconsistently half of the sedimentthickness in deeper marginal basins. reflectivecrust. This type of structure,or rather a lack of it, Aside from the distal sectionfrom km 400 to km 510, there is alsotypical of the Nain Provinceas imagedby the northern is no apparentinverse correlation between crustal thickness ECSOOT profile [Hall et al., 1995] and appearsto be a and the depthof sedimentarybasins, and thereis no correla- generalcharacteristic of thisregion. The KaipokokDomaM tion betweenthe intensityof crustalreflection fabrics and the and Border Zone in Greenlandare affected by pervasive thicknessof the overlyingsedimentary basins, where present. Proterozoicstructural reworking, but there is little seismic Finally, there is no indicationfrom the trans-LabradorSea contrastbetween pristine and reworkedArchcan crust. This survey of Keen et al. [1994] that the pattern of basement contrastswith partsof the northernECSOOT profile across

Kilometres along Profile

0 0 CiD

NW SE Reworked•$ ArchcanCrust• ReactivationofBasement Structures ShearKanairiktokZone ,,•'•- Kaipokok CrustalMajorS-vergingStructures Sedimentary CoverRocks / Grenville 0 t7:.',..' ..... ".'/,..:.,..:.•.•-...... :-..,--'-,."--•7•~.•,.•.••... '-:-•.....:••":'•-.:::...;:: ....•'.r::" 7.:---.::::•:-• '.-.. '.'". l,.: .. :::• Front?

(secs)

:,',...... ''. ,• ...... ,. ,•.•..,,,•. • , ...... •,.. _ ',',.z .•.,...... •. • ...... ,. , n. . • ...... 16 "' -"":'"":".....""< "' *"," '•:'" ...... : .: '"" '• ...... '...'...- ...... /:::.,,,...t .' '

• Man;eReflectors• ReactivationofBasement Structures ..... ,, ReflectorGroups

' o .•...:x. , . NOTE:Thickness of Mesozoic- ß Sedimentary.sozo,c-Oenozo,oCover i.'½%'•½:11•.:.•Mantle Cenozoic section isexaggerated. Archcan Crust •'•11'111111':11"]...... "Ketilidia"ProterozoicJuvenileCrust [-• I[-• BoundaryNdIsotopic Figure5. Summaryofreflectivity patterns and crustal domains within the Makkovikian - Ketilidian Orogen, asshown in detailby Figure 4 andPlate 1. Notethat there is slightvertical exaggeration toadequately representthe post-Mesozoicsedimentary section. KERRET AL.: MAKKOVIKIAN- KETILIDIAN OROGEN 957 theRae Province,which showpervasive reorientation of explicableif thecrustal contributions in the granitoidrocks reflectors[Hall et al., 1995].The southeastern portion, from were derivedfrom relativelydeep crustal levels, where lan200 to lcm430, displays much stronger and more system- Archcanbasement would predominate. Pb isotope(galena) aticreflectivity and crustal-scale structures mostly dipping data[l•Iton, 1991]define an ancient-to-juveniletransition northwest.This is accompaniedby strong,largely subhori- similarto that of Kerr and Fryer [1994] but closerto the zontalreflectivity in the lowercrest at 8 - 10 s andby KaipokokBay Zone. As thegalena data likely record shallow- consistentlydipping reflectivity in the underlyingmantle, levelhydrothermal systems, they would be morelikely to commonlywith an opposingsense. samplethe upper (Proterozoic) block. The southeastdip of the In conjunctionwith geologicalarguments, these two km 190 structure and the evidence for northwest directed sectionsof the profileare interpretedto representcrest of thrustingand translation in Labradorand Greenland [Aliaart, contrastingage and origin. The first 200 km or sorepresent 1976;Ryan et al., 1983]suggest that Ketilidia has been thrust partof theNorth Atlantic Craton, but the remainder of the northwestwardover the ArcheanBlock at leastduring the profileis a Proterozoiccrustal block, for whichthe name later stages of orogenesis.If the Kaipokok Bay - Ketilidiais usedbelow. The two regions are juxtaposed in the KobberminebugtZone correlationis valid, the seismicdata vicinityof km 200, whichis by definitionsome type of suture alsosupport interpretation of the latter as an importantsuture zone. zone [cf. Windley,1991].

5.3. Kaipokok Domain- Border Zone 5.5. Ai!!ikDomain - CapeHarrison Domain - JulianchUb Batholith There is no doubtthat this regioncomprises reworked Archeartcrest [Ryan et al., 1983]. However, there is little At its northwesternend, this sectionof the profile is contrastin reflection characterbetween this region and boundedby the suggestedKaipokok Bay Zone and at its unreworkedArcheart crust of the foreland(Figure 4 andPlate southeasternend is boundedby a prominent,northwest 1).The Kanairilaok Shear Zone also has no clearexpression dipping,structure near km 340 (labelP1). The lattercorre- onthe profile, which suggeststhat it is not preservedas a spondsgenerally with the southern edge of an aeromagnetic majorlow-angle, dipping structure. The muchwider Border high ttmt definesthe magmaticbelt on both continental Zonein Greenlandand the absenceof an equivalentstructure shelves(Figure 3a). There is no onlandstructure in Labrador characterizedby a jump in metamorphicfacies imply that this thatcan be correlatedwith this feature, and the Sardloq Zone shearzone may be a later, discontinuousstructure. However, in Greenland is now considered to lie within the Julianeh,Sb anabsence of seismicexpression in suchpoorly reflective Batholith[Chadwick and Garde, 1996] rather than to separate Archcancrest doesnot precludethe existenceof a major it from metasedimentaryterranes, as previouslythought steeplydipping or verticalcrustal-scale structure at thispoint [W/ndley,1991]. Nevertheless, it is a potentialcandidate for inthe profile; thereare simplyno clear,traceable reflectors the km 340 structure. to define offsets. The Cape HarrisonDomain and JulianebAbBatholith are dominatedby syntectordcand posttectonicplutonic rocks, 5.4. KaipokokBay Zone - KobberminebugtZone whichmay have obscuredand disruptedthe earlier crustal The narrow, southeastdipping reflector packagethat fabrics,and (as they are relativelyisotropic) would likely not ascendsto the Mesozoicunconformity (Figure 4b, labelG) is provide a good seismicresponse. This may explainthe probablythe offshoreextension of the KaipokokBay Zone. relativelytransparent upper crust through much of thisregion. A simplelinear extrapolation from Labrador would suggest a Isotopicand geochemicalstudies [Patchett and Bridgwater, more northerly intersection,but correlationwith the 1984; Kalsbeekand Taylor, 1985; Kerr and Fryer, 1993; KobberminebugtZone in Greenlandrequires an eastward 1994]indicate that many of thesegranites contain significant swingof 10ø-20ø (Figure2), whichwould bring the structure amountsof juvenile,probably subcrustal material, indicating to km 180-190. On land, the Kaipokok Bay and thattheir effects are not restrictedto the uppercrust. Genetic KobberminebugtZones mark the southeastern limits of clearly models[e.g., Kerr and Fryer, 1994] imply ttmt mantle- definedsurface Archean rocks, and they also have a general derived, mafic magmaswere eraplaced(underplated or correspondencewithisotopic contrasts in Proterozoic granites "intraplated")and interacted themally and chemically with the fromnegative end (northwest) topositive end (southeas0. This lower crest.The stronglyreflective lower crustbetween km isotopicshift is interpretedto markthe southeastern'edge of 240 and km 330 may owe its origin to such a process. theNorth Atlantic Craton [Kalsbeek and Taylor, 1985;Kerr However, there are other possibleexplanations, such as andFryer, 1993;1994] and occurs some 20-30 km southeast shearingat the crust-mantleboundary or withinthe lowermost ofthe Kaipokok Bay Zone. Nd isotopicdata from offshore crest (seelater discussion). wells confirm that there is a similar contrastin isotopic 5.6. Psammite and Pelite Zones of Greenland signaturesbetween km 160and km 200 adjacentto theprofile [Kerrand Wardle,1997]. This provides further support for This sectionof the profile showslow-angle, crustal-scale interpretationof the km 190structure as the Kaipokok Bay structuresthat dip gently to the northwestand sole into a Zone. highlyreflective zone in the lower crest. Much of southern- Theprofiles suggest that the Kaipokok Bay Zone represents most Greenland is characterizedby high-gradeparagneisses a suture,along which Ketilidia has been juxtaposed against displayingflat-lying and low-angle structural trends that have theArchean Craton 0•igure 5). Theeastward displacement of been modified by later folding and intrusionof rapakivi theisotopic boundary relative to surfacemylonite zones is granitemassifs [e.g., Bridgwateret al., 1973;Aliaart, 1976; 958 KERR ET AL.: MAKKOVIKIAN - KETILIDIAN OROGEN

Chadwickand Garde, 1996].This patternhas been inter- ECORS Team, 1989] and also from Archeanterranes in pretedin termsof domingand lateral extension associated Canada [Calvert et al., 1995].All areinterpreted as remnan• withgranite eraplacement [e.g., Bridgwater et al., 1973]or of subductedoceanic crest or evidencefor attemptedsub- regionalthrusting and imbrication of the crest(Windley, duction of continental crust. 1991).At firstsight, the huge northwest dipping structures Giventhese recurrent observations from otherPrecambrian visibleon the seismic profile support the latter concept and andPhanerozoic Orogens, it seemsunlikely that the spatial are comistentwith reports of recumbentisoclinal folds and associationof manfiereflectivity with divergentcrustal fabrics possiblenappe structures in Greenland[Aliaart, 1976; and a potentialsuture zone in the Makkovikian- Ketilidian Chadwickand Garde, 1996].The lessprominent southeast belt is coincidental.These reflectorsmay indicate the loca- dippingreflectors in theupper crest (label R, Figure4 and tion(s)and polarity(ies) of suture(subduction) zones and have Plate 1) couldalso be portionsof large-scale,"mushroom- an importantbearing on evolutionarymodels discussed below. like" nappes;alternatively, they could be earlierstructures subsequentlydisrupted by south verging thrusts, as suggested for the LithoprobeEast on-land profiles in Newfoundland 6. Plate-Tectonicand GeodynamicModels [Quinlanet al., 1992]. However, we cannotrole out an alternativeinterpretation, that is, thatsome of thisstructure 6.1. Plate-Tectonic Models is relatedto postorogenicextensional collapse in thispart of Modelsproposed for theMakkovikian - KetilidianOrogen thebelt [e.g., Huttonet al., 1990]. maybe convenientlydivided into "Andean"or "Himalayan" options. Andean models, suggestedmostly for southern 5.7. Mantle-Depth Reflectors:Evidence for Plate Greenland[Bridgwater et al., 1973; Chadwickand Garde, Tectonics ? 1996], envisagenorth directedsubduction of oceaniccrest The profile displaysat least two sets of reflectorsat beneaththe North Atlantic Craton, the Julianel•tb batholithas inferred mantle depths.The most obviousdip southeast a continentalmagnintic arc, and metasedimentaiy terranes of southernmostGreenland as a forearcbasin (Figure 6a). This beneathKetilidia from km 250 to km 290 (labelsN) andfrom km 300 to km 350 (label T). There is also a northwest subductionpolarity was impliedalso by Ryan [1984] for the MakkovildanOrogen. A variantof thismodel [Brownet at., dippingreflective zone from aboutlcm 220 to km 180 below theKaipokok Domain crust (label J). In thenorthwestern part 1992]envisaged south directed subduction, with rapakivi-type granitesin southernmostGreenland formed in a back arc of theprofile, these divergent zones of mantlereflectors meet setting. The Himalayan option was proposedby Windtey aroundkm 250, whichis thedepth extension of thesuggested [1991], who interpretedthe JulianehttbBatholith as a mag- KaipokokBay Zone structures and a focalpoint of asymmetry matic arc sandwiched between the North Atlantic Craton and amongstcrustal reflectors. The orientationsof crustal and a southerncontinental block of unknownheritage. The mture mantlereflectivity are opposedthrough much of the profile of the crustal substrate to the batholith or the subduction and, southeastof km 200 (beyondthe North AtlanticCraton polaritywas not explicitlystated, but the latterwas implied to - Keti!idiaboundary), appear to be separatedby a lowercrest be southward. The Psammite and Pelite Zones of southern- - Mohoregion with strongsubhorizontal reflectivity. Similar,but more focused,mantle-depth reflective zones mostGreenland were interpretedas a backarc basinthat later havebeen reported from ProterozoicOrogenic belts that are underwenttectonic thickeningand postorogeniccollapse coeval and possiblycorrelative with the Makkovikian- (Figure 6b). The presumed,but largely undocumemed,arc KetilidianOrogen. The Penokeanbelt in theLake Michigan affinityof the JulianebAbBatholith is a crucialelement in all arearevealed dipping reflectors descending to 18 s, spatially three models. Kerr [1989b] proposeda model for the associatedwith a zone interpretedas an arc-cratonsuture. Makkovikianbelt with elementsof both,suggesting accretion Thesereflectors were interpretedto representattempted of a compositearc terrane of juvenile affinities through subductionof Archcancrest beneath the arc duringcollision largely southdirected subduction, with possibleearly north [Cannonet al., 1991].A profileacross the Svecofennianbelt directedsubduction, suggested by early magmatismin the in theGulf of Bothniaarea showed reflectors dipping to 20 s, KaipokokDomain (Figure 6c). The abundantplutonic suites spatia!lyassociated with divergentreflectivity in the crest. of the Cape Harrison Domain were interpretedas late Thepattern is s'mailarto theMakkovik Province profile at km orogenicto postorogerdcand (asidefrom early members) unrelated to subduction of oceanic crest. 250 (Figure4 andPlate 1), andmantle-depth reflectors were interpretedas a "fossil"subduction zone containing remnant 6.2. Relevance of Seismic Reflection Data oceaniccrust [BABEL Working Group, 1990]. In bothcases, mantle reflectorsdip north toward the Archcan cratons. The overall reflectivitypattern of the Makkovikian- Mantle reflectors offshore from northern Britain and Ireland Ketilidianbelt (Figure5) suggestsa 350-kin-wide,highly havebeen interpreted as eclogiticslabs representing relict asymmetric,orogenic belt, in which many reflectorsets subductionzones [Warner et al., 1996]and may originally transectthe entirecrest. The geometryis suggestedto be havebeen positioned along strike to the eastof the Ketilidian doublyvetgent but characterized largely by northwest dipping belt, dippinggenerally toward the south [McBrideet al., crustalstructures. The generalpattern resembles those from 1995; Snyderet al., 1996] (Figure 1). Analogousmantle- the PhanerozoicAppalachian - Caledonian [Quinlan et al., depthreflectors, associated with divergentcrustal reflection 1992;Hall andQuinlan, 1994; Beaurnont and Quinlan, 1994] patterns,have been reported from youngerbelts such as the andPyrenean collisional orogens [Frei et al., 1989;Quinlan Alps [Frei et al., 1989]and the Pyrenees[Choukrone and et al., 1993; Beaumontand Quinlan, 1994]. It doesnot KERR ET AL.: MAKKOVIKIAN- KETILIDIAN OROGEN 959

NorthAtlantic Julianehab Batholith Psammite/PeliteZones Craton•, • (ForearcBasin)

JulianehabPsammite/Pelite Zones NorthAtlantic Shelf Batholith (Back-ArcBasin) Craton• Sequence• ,• /

• ...... ii!'i:::,i::•!•!ii!"••....""' ' ...... •,,.... \ , , '•!:i!iiii!i:i:i'iii....•::::ii?:iiiii::::•:' :.. UncertainCrustalType UnknownCratonicBlockSouthern ?

EarlyMagmatism in Eady CHD / JB NorthAtlantic Shelf KaipokokDomain ArcMagmatism Protoliths to Craton• Seq•nce / PeliteZone? (c) '•-•'.... ii•'•,,::..i•? ? ?

Stage 1 ß ..•:•i!i:i?i?.... :: iili::•:..Ketilidia JuvenileBlock ca. 1.85 Ga Earlier(pre-"•7 Ga)"' "::•::' orComposite ArcComplex Subduction Zone ?

Posttectonic Protolithsto NorthAtlantic Granites PsammiteZone ? Craton , • . Protoliths to Pelite Zone ? ;"'"--...•'v_•-_•_••"'"

Stage 2: Ketilidia Block ' ' ' ca. 1.80 Ga ,.?_?_,,.?_,..:_._.;_,...... Continued PostcollisionalConvergence Figure6. Schematicsummary ofplate-tectonic models proposed for the Makkovikian - Kedlidian Orogen. (a)Andean option, with subduction tonorth, and the Julianeb3b Batholith, interpreted asa continentalmargin are, andthe Pelite/Psammite Zones as a forearcbasin [after Chadwick and Garde, !996]. (b) Himalayan option,with subducfion tosouth, Julianeh,Sb Batholith asan arc of unspecified affinity, and pelite/psammite zonesas a backarc basin [after Windley, !991]. (c) Modifiedcollisional model [after Kerr, 1989b].Initial subduction(pre-l.86 Ga) is northward under the North Atlantic Craton but gives way to southward subduedon (stage1), followedby collisionof NorthAriantic Craton and Kefilidia (stage 2) andgeneration of postorogenicgranitoid suites. The setting ofthe Pelite and Psammite Zones is unclear, but they are implied to includesedimentary sequences of different age, for example, early "flysh" and later "molasse" sequences. obviouslyresemble patterns from modemsubduction- and persistentmantle reflectivity is southeastdipping, accretionzones such as VancouverIsland [Cloweset at., associatedwith the northwest dipping features in theoverlying 1987]or New Zealand[e.g., Quinlanet al., 1993].This crust.If themantle-depth reflectors are relatedto subduction empiricalsimilarity suggests that the Makkovikian- Ketilidian processes[cf. BABEL Working Group, 1990; Warner et aI., Orogendeveloped its ultimate configuration through either 1996], this suggestspredominantly southward subduction. continentalor arc-continentcollision processes, but it doesnot However, less extensivenorthwest dipping mantle-depth rule out the "Andean"analogy at an earlierstage in its reflectorsspatially associated with the KaipokokBay Zone development. (Figure5) raisethe possibility also of northwardsubduction Reflective zones at inferred manfie depthsdip both beneaththe NorthAtlantic Craton. Although it is possiblethat northwestand southeast (Figure 5), butthe most widespread all thesemantle features are of the sameage, we considerit 960 KERRET AL.: MAKKOVIKIAN-KETILIDIAN OROGEN more likelythey indicate shifts in subductionpolarity over The most enigmaticpart of the entire Makkovikian• time, whichhave also been suggested in otherProterozoic KetilidianOrogen is southernmost Greenland, represented by orogens[Snyder et al., 1996].The growingevidence for a the Psammiteand PelJleZones. This regionhas beeninter- complexearly tectonic history on the northern margin of the pretedas eithera forearc[Chadwick and Garde, 1996]or Makkovildanbelt [Kerr et al., 1992;Ketchurn et al., 1995] backarc [W•ndley, 1991] sedimentary basin, depending on the impliesthat convergence may have started as early as 1.9 Ga. preferredsubducfion polarity. The presenceof 1.82-Ga Thiscomplexity emphasizes that models based on a single granitoidcobbles in a conglomerateand detrital zircon ages tectonicconfiguration are inadequateand that the present rangingfrom 1.97 to 1.79Ga [Hamiltonet al., 1995,1996] architectureof the belt probablydeveloped in two or more implythat at leastsome of thepsammites had sources in the stages,as discussed in section6.6 of thepaper. JulianebfbBatholith, which could favor either interpretation. The presenceof arnphiboliteand mariegranulite [Aliaart, 6.3. GeologicalArguments 1976; Chadwickand Garde, 1996] impliescontemporaneous mariemagmatism, which is more consistentwith a backare In ourview, other types of dataargue for predominantlysetting, as is the evidencefor high geothermalgradients southwardsubduction, at leastin later stagesof evolution. throughoutthe region [Dempster et al., 1991].However, both Mostgranitoid rocks of theeastern Makkovik Province and modelsare difficult to reconcilewith growingevidence that the Julianel•bBatholith were developedin juvenile crust at leastsome of the historyof the Psammiteand Pelite Zones [Kerr and Fryer, 1994; Patchettand Bridgwater,1984; postdatesevents recorded by terranesto the northwest.The Kalsbeekand Taylor, 1985; Hamilton et al., 1995],and the 1.79-Gadetrital zircons [Hamilton et al., 1996] imply thatat crustalsuture apparently lies northwest of themain magmatic leastsome of the psammiteswere deposited20-30 Ga after belt rather than southeastof it. This is the reverse of the the formation of most of the Julianehfib Batholith and could expectedpattern for a continentalarc batholith developed over thereforealso include detritus from younger(postcollisional?) a northdipping subduction zone. New evidenceshows that plutonicsuites. "Syntectonic"intrusions and concordant supracrustalrocks of theKaipokok Domain must be older gametiferousgranites of probable anatecticorigin were than1.88 Ga [Ketchurnet al., 1995],and they are unlikely to formed at 1.79 to 1.78 Ga, suggestingthat deformationand representa back arc setting synchronous with the batholith, as metamorphismquickly followed sedimentation [Hamilton et impliedby Chadwickand Garde [1996] for their Greenlandic al., 1995, 1996]but postdatesthe 1.82-Gametamorphism and equivalents.Northwest of the Kaipokok Bay - deformationinferred for the JulianehfibBatholith. Finally, KobberminebugtZone, the Archcan eraton contains relatively widespread"rapaldvi-type" plutohie suites of 1.76- to !.74-Ga few Proterozoiccalc-alkaline granitoid rocks, and in Labrador age in southernmostGreenland indicate that this region theseare in partof 1.89- to 1.87-Gaage [Kerr et al., 1992; remainedthermally active for a muchlonger period of time Ketchurnet al., 1995]. These,and possibly the tonalires and thanareas to the north. Thusit seemslikely that at leastsome trondhjemitesof the Island Harbour Bay Intrusive Suite [Ryan of the sedimentaryprotoliths represent a younger(molasse- et al., 1983;Kerr et al., 1996], mustrecord a differentevent type?)sequence, which underwentsubsequent deformation from the dominant 1.82- to 1.80-Ga suitesof the Cape andmetamorphism. HarrisonDomain- JulianehttbBatholith. Many of these1.80- It is thus critically important to know if the 1.79-Ga Ga granites,especially in Labrador, probablypostdate alepositionalconstraint applies to all of the metasedimentary subduction,like the voluminouspostcollisional suites of rocks in southernGreenland or if the region includesmore Paleozoicbelts such as the Appalachianand Hercynian than onesedimentary sequence. Geochronological work has Orogens.The presence of suchyounger phtonic suites north namra!lyemphasized the bestpreserved conglomeratic and of the proposedsuture has no significancein terms of psammificmaterial, but these may notbe fully representative. subductionpolarity. The evidencethat somemetasedimentary rocks are intruded by granitoidrocks of theJulianel:fib Batholith [Chadwick and 6.4. GeologicalUncertainties Garde,1996] certainly indicates a widerrange of alepositional The spatialand temporalrelatiomhips between earlier ages.The more pelitic assemblagesof southernmostGreen- (arc-like?)and later (postcollisional?) plutonic suites within landare a criticalpart of thispuzzle, as these may providea the CapeHarrison Domain and Julianeh,•ib Batholith remain record of earlier sedimentation or sedimentation linked to an largelyunresolved and impededetailed modeling of these opposingcontinen• margin. The evidence for later, terranes.Relatively young (1.8 Ga) suiteswith distinctly post-l.79-Ga,deformation is difficultto interpret,but coupled nonarcsignatures are most abundant in Labrador[Kerr et al., withthe high geothermal gradients and extensive plutonism of 1996], but some older rocks such as the Cape Harrison thearea, it couldbe a manifestationof postcollisionalcollapse MetamorphicSuite and 1.84-Gagranodiorites [Kerr et al., and extension[cf. Dewey, 1988]. 1992] could representolder arc-type suites. Published The availableisotopic data from southernGreenland show accountsof theJulianehfib Batholith imply a largerproportion no evidenceof ancient(pre- 2.0 Ga) continentalcrest in this of tonaliresand granodioritesthan in Labrador,and recent region, which arguesagainst continent-continent collision datingsuggests older ages of 1.84to 1.82Ga, withthe oldest modelsof the type proposedby Windley[1991], unlessthe rocksnear the southernmargin of the batholith[Hamilton et tinidentifiedsouthern block was of largelyjuvenile heritage or al., 1995].There is a needfor moredating in bothareas and, lay beyondthe presentconfines of the belt. However, particularlyin Greenland,for systematicgeochemical studies althoughcollisional orogens are generally thought of as to complementany improved geochronological data. involvingtwo ancienteratonic blocks, this does not have to be KE• ET AL.- MAKKOVIKIAN - KETILIDIAN OROGEN 961 thecase. For example,the northernAppalachian Orogen crustmay have responded differently because it wascooler recordsthe Ordovician to Siluriancollision of Laurentiawith andmore stronglyattached to its underlyingmanfie, com- Avalonia,a Late Proterozoic block of generallyjuvenile, arc- paredto thejuvenile Ketilidia crust, which had been heated likeaffinity which is only some0.2 Ga olderthan postoro- andweakened through magmatism. genicAppalachian granites. In our view, the Appalachian- Thisfinal point emphasizes a fundamental shortcoming of Caledonianand Hercynian Orogens may ultimately provide geodynamicmodels of thisgeneral type; that is, collisional farmore suitable modem analogues for the Makkovikian- orogenswill almostinevitably involve the interactionof two Ketilidianthan either the Himalayan or Andeanbelts. or morecrustal blocks with differentproperties, because of thethermal and materialinfluence of arc-relatedmagmatism 6.5. Relevanceto GeodynamicModels of Compressional thatpreceded collision. The modeling of thesemore complex 0rogens situationsis still in its infancy[e.g., Ellis and Beaurnont, The Makkovikian- Ketilidianreflectivity pattern (Figure 1996],but it appearsthat deformation is largelyconfined 5)belongs toa growingfamily of seismicimages interpreted within (and particularly at theedges of) any "weaker"crustal asdoubly vetgent collisional orogens [e.g., Quinlanet al., block that is sandwichedbetween two strongerblocks more 1993],although it is moreasymmetric than most of these. firmly attachedto their underlyingmantle. Because the Computermodels have been developed in attempt to under- weakerzone becomes detached from its underlying mantle, it standthe origins of thisrecurrent pattern [Willett et al., 1993; is squeezedand shortened and, with continued convergence, Beaumontand Quinlan,1994; Beaumont et al. , 1994].The migratesretroward (i.e., in the directionof subduction) basicmodel assumes that deformationin the overlyingcrust relativeto thedetachment point, as in Figure7c [e.g., Ellis is linked to detachmentand continuedunderthrusting of and Beaumont,1996]. The pointof detachmentthus shifts lithosphericmantle originally attached tothe downgoing plate prowardrelative to the zone of most active deformation tolie (Figure7). Theresults vary according toboundary conditions beneath or adjacent to thestronger crust on the proside. This (seelater discussion) butcommonly produce divergem high- retrowardmotion of theweaker zone is apparentlyindepend- strainzones ("retrowedges" and "prowedges"),which are entof theinitial position of thedetachment point beneath it rootedat thedetachment point (Figure 7a). In simplemodels andmay thus provide an indirectmeasure of subduction whereno crustis subductedand the detachment point remains polarity.Such a processduring ternfinal collision may explain fixed,the retrowedge is a narrow,focused zone (i.e., a shear theasymmetric reflectivity pattern of the belt and is consistent zone),and the prowedge is a morewidely distributed thrust withthe location of mostmagmatism and deformation south of the crustalsuture and southof the focal region where belt(Figure 7a). At facevalue, the Makkovikian- Ketilidian pattern (Figure crustaland mantle fabrics diverge (Figure 5). It mightalso 5) resemblesthis basic configuration (Figure 7a), with the explainthe apparentlylater timing of deformationand KaipokokBay - KobberminebugtZone representing the metamorphismin the south, which could involve material retrowedgeanda widerprowedge insouthernmost Greenland. eroded from the developing orogen. Although this qualitative However,such an interpretationis inconsistentwith the amlogyseems capable of explainingthe overall geometry of southeastdipping fabric at mantle depths below much of the thereflection pattern, we stress that a multistagemodel is still orogen,which implies subduction in the opposing direction, required to satisfyall of thegeological constraints. supportedalso by geologicalarguments (see above). The greatestproblem inapplying geodynamic models of thistype 6.6. MultistageGeodynamic or Plate Tectonic Models is that crustalstructures with oppositepolarities may be relatedtovery different stages in development,assuggested Althoughdivergent tectonic patterns can be modeled in for divergentreflectivity in the northernAppalachians terms of a singleprocess, as above, many workers prefer [Quinlanet al., 1992].As discussedabove, the limited multistagemodels that invoke subducfion polarity reversals or informationontiming of deformationin the Makkovikian- late-stage crustal detachments [e.g., CoIman-Sadd,1982; Culottaet al., 1990;Quinlan et al., 1992].These models are KetilidianOrogen now indicates that it isdiachronous from moreconsistent with thecomplexity of modemconvergent north to south. Thethermal structure of the crustinfluences the type of marginsand with the geological evidence for temporally strainpattern developed insuch models, ashigher geothermal discrete deformation events in ancientorogenic belts. The gradientsdecrease coupling between crust and mantle or totalreflectivity pattern in the Makkovikian- Ketilidian betweencrustal layers [Beaumont and Quinlan,1994; Orogenisdifficult toexplain bysimple one-stage geodymmic Beaumontetal., 1994].Under such circumstance.s, deep-level models, but a speculativetwo-stage model, similar to that subhorizontalshearing develops via detachment of the crust developedforthe Appalachian - Caledonian Belt [Quinlan et fromthe underlying mantle and/or lower crustal detachments, al., 1992;Hall and Quinlan, 1994], may be applicable. andthe width of both prowedges andretrowedges isincreased It is suggestedthat convergence began with an early (Figure7b). The lower crust and Moho region throughout episode of north directed subduction beneath the Archcan mostof thesoutheastern ("Ketilidia") section of theprofile craton,possibly represented byparts of the Island Harbour exhibitsstrong subhorizontal reflectivity, which may indicate Bay Intrusive Suite, 1.89-Ga granites, and early (1.86 Ga) theeffects of the relatively high geothermal gradients inthis componentsofthe Upper Aillik Group. Northwest dipping region[e.g., Dempster etal., 1991].This strong subhori- mantle-depth reflectors northwest of the km 190 region are zontalreflectivity islargely absent from the North Ariantic interpreted toprovide arecord ofthis event. The relationships Eratonsection ofthe profile, where the Moho is very poorly betweenthese events and supracrustal sequences and early defined(Figure 4 and Plate 1). This suggests thatthe Archean deformation within the Kaipokok Domain and Kaipokok Bay 962 KERR ET AL.' MAKKOVIKIAN - KETILIDIAN OROGEN

"PRO" SIDE - >< ß "RETRO" SIDE > "Pro-Wed g e" "Retro-We d g e"

(a) ß• /, _shear zones MANTLE • - •'S• •Detachment Point :••or"Singularity"

(b)

• RetrowardMigration of Weak CrustalZone ...... ':"%'":":i•i';.':•":'•"•.%"I':::"::'wEAKZONE"::::I"...... ":' ,'• STRONG (c) I'•U,,• '.:..-';'--.:'.:':'. ß • ::'::'if:':'....:...... : ....:...... :::..::::'. '• ß'- '.'."::: ' " "' : ..• """'••'/•-• 'Z'.•.'" '• "' : MohoorLowerCrusta' MANTLE•. •. •Or,g,nalS,teofS.DuctileShearZones "• •owardMigrationofOetachment

JUVENILE N.CRATONATLANTIC -• KETILIDIA BLOCK ? ? ? .-.•.•.:.:'•"...... "•,• "':;•:'.•.:.(•:.i::•:!i•:;'i. "WEAKZONE" !?';'i•i::::{;:'i:"?JJ-,•ff• •'.'•_•'•'•'• CrustalScale (d) CRUST •::•:."'':S 2 .•.;.:...::..•,?•:•::'..;f.'::..•:;.•/"• ,..'• ThrustZones

,, ß. ,,•, MANTLE •/Sl %%•• ••••• Moho orLower Crustal --/ • X • '• DuctileShear Zones .• ,. . N •N • SE-dipping Earher- NW d•pp•ng R,.to,s Mantle Reflectorsctors • • OriginalSiteof S2? Collgional Detachment Point

ofSense crustalofvergence faults • Convergence

Figure7. (a) Simplegeodynamic model for crustal deformation caused by detachmere and underthrusting atpoint S of lithosphericmantle formerly attached tothe downgoing plate [after Beaumont etal., 1994].Co) Variantof abovemodel derived for hotter crust with weaker coupling to underlyinglithospheric mantle, showingthe widening of deformationzone and developmere of deep crustal or Mohoductile deformation [afterBeaumont etal., 1994].(c) Variantof modelderived for a weaker(hotter) block sandwiched between stronger,cooler crust, showing concentration of deformationat marginsof weakblock, and retroward migrationofweak zone relative to detachment [after Ellis and Beaumont, 1996]. (d) Illustration ofreflectivity patternin Makkovikian- Ketilidian Orogen and its possible interpretation in the light of thesemodels. S1 representsoriginal detachmere point of pre-1.85-Ga subduction; S2is final position of collisionaldetachment, whichmay originally have been located farther from S 1. Notethat all of thesediagrams are schematic. KERR ET AL.: MAKKOVIKIAN - KETILIDIAN OROGEN 963

Zoneare presently unclear, as are the reasons for itstermina- inference, someof the structuresimaged by the seismic tion.The preservation of northwestdipping manfie reflectors profilemay have developed or havebeen, accentuated during fromthis earlier event implies that later convergence involved postcollisionalextensional collapse of the orogenrather than consumptionof oceaniccrust that lay betweenthe North throughcontinued convergence. Many of the featuresof AtlanticCraton and Ketilidiaby predominantlysouthward southernmostGreenland (e.g., highgeothermal gradients and subduction.Ketilidia is envisagedas a (possiblycomposite) posttectonicmagmatism) correspond to those outlinedby arc terrane, representingjuvenile crust of intermediate Dewey [1988] for suchzones, as noted also by Winalley thickness,and it may be analogousto the Late Precambrian [1991]. Our mode!has some similarities to the proposalsof AvalonBlock in the Appalachians,which is apparentlya Windley[1991] but implies a "subductionflip" of theopposite collageof arc-typeterranes that are onlyslightly older than senseto thatproposed by Snyderet al. [1996] as part of a theearliest Paleozoic arc terranesof Iapetus.It may have generalizedarc-continent collision model for Proterozoic beenbounded to the southby an older, but as yet undocu- orogenicbelts. However, we stressthat our proposal in no mented,cratonic block. It is suggestedthat parts of the wayimplies that the latter is incorrectbut instead emphasizes Julianeh/tbBatholith and some early Makkovik Province thatthe development of variousbelts and lateral segments of granitesrepresent an arcdeveloped on the leading edge of the samebelt can be complexand varied. In the caseof the Ketilidia, but the main episodeof 1.82- to 1.80-Ga mag- Maklcovikian- KetilidianOrogen, the reflectionfabrics matismaccompanied and postdated terminal oblique collision relatedto theearlier event were preserved, whereas they were ofKetilidia and the North AtlanticCraton along the Kaipokok eradicatedelsewhere because subduction polarity reversals Bay-Kobberminebugt Zone. This event juxtaposed relatively postelated,rather than predated, arc accretion and because the cool,stable Archcan crust against thinner, hotter, Proterozoic youngersubduction zone "crossed"the olderrelict zone crust,and thesetwo blocksbehaved very differently,as [Snyderet al., 1996]. indicatedgenerally by the modelsof Beaurnontet al. [1994] Althoughspeculative, our model is consistentwith many and Ellis and Beaurnont[1996]. Following collision,the featuresof the belt and the seismicreflection profiles and hottercrust along the leadingedge of Ketilidiaexperienced providesa startingpoint for more sophisticatedanalyses. detachmentand shearing at or nearthe crest-mantle boundary Althoughwe believe the basic premises of simplegeodyrmmic andwas squeezed and shortened, leading to southeastverging models[e.g., Beaumontet al., 1994]to be correct,we stress thrustsand nappes, some of whichwere rooted in the deep thatsuch analyses must ultimately incorporate the superposi- crustalshear zone. It may also have movedsouthward, tion of crustalfabrics related to temporallydiscrete events, relativeto thedetachment point, which we considerto be now thejuxtaposition of crustal blocks that have differing thermal representedby the focal region around km 250 (Figure5). and/orcompositional properties, and the effects of oblique Thepreservation of mantle-depth reflectors potentially related convergence.All of thesefactors represent "general cases" in to an earlier (northward)subducfon implies that the two orogenicdevelopment and will governthe response of an detachmentpoints (S1 and$2 in Figure7d) wereseparated orogenicsystem to continuedconvergence following col- laterallyand have been brought together by subductionof lisional or accretionaryevents. interveningmantle lithosphere.. The origin of thePsammite and Pelite Zones of southernmost Greenland remains Acknowledgments.This paperrepresents Lithoprobe con- shroudedin uncertainty,as discussedearlier, but theymust tribution998, and seismicreflection work forms part of the includeat least some material erodedfrom the developing CanadianLithoprobe project, supported by theNational Sciences orogen,which was thenquickly affected by continuedand Engineering Research Council of Canada(NSERC) and the deformationand metamorphism. However, they could also GeologicalSurvey of Canada.Geological field work in Labrador includeolder components representing an earlierback arc relevantto this paperwas largelysupported by cost-shared environmentor a sedimentaryprism developed adjacent to an agreementsbetween the governments of Canada and Newfound- outboardcratonic block. The dataare presently inadequate to land-Labrador.Reviewers James Knapp and Steve Lucasare explorethese alternatives further. Finally, some of thelater thankedfor pointingout shortcomings and suggesting improve- deformationalhistory in thesouthern part of thebelt and, by mentsto the originalmanuscript.

References

Berthelsen,A., andN. Henriksen,Geological map sional tectonicsand magmaticunder-plating. Aliaart, J. H., Ketilidianmobile belt of West Tram. R. Soc. EdinburghEarth Sci., 83, Greenland,in Geologyof Greenland,edited by of Greenland(descriptive text), Rep. 186, A. Escher and W.S. Watt, pp. 120-150, Ivigtut sheet,1:100,000 scale, 169 pp., 173-178, 1992. BullaM,E. C., J. E. Everett,and A. G. Smith,The Grm•landsGeol. Unders., Copenhagen, 1976. GronlandsGeol. Unders., Copenhagen, 1975. Bridgwater,D., andL. Schiotte,The Archcan fit of the continentsaround the Atlantic, Philos. BABELWorking Group, Evidence for EarlyPro- Tram. R. Soc., Ser. A 258, 41-51, 1965. terozoicplate tectonics from seismic reflection gneisscomplex ofnorthern Labrador: A review of currentresults, ideas and problems,Bull. Calvert, A. J., E. W. Sawer,W. J. Davis, andJ. profilesin the Baltic Shield, Nature, 348, 34-38, N. Ladden,Archaean subduction inferred from 1990. Geol.Soc. Den., 39, 153-166, 1991. Bridgwater,D., A. Escher,and I. Watterson, seismicimages of a mantlesuture in theSupe- Beaumont,C., and G. Quinlan,A geodynamic rior Province,Nature, 375, 670-674, 1995. frameworkfor interpreting crustal scale seismic Tectonicdisplacments and thermal activity in two contrastingProterozoic mobile belts from Cannon,W. F., M. W. Lee, W. I. I-Iinze,K. J. reflectivitypatterns, Geophys. J. lnt., 116,754- Schulz,and A. G. Green.Deep crustalstructure 783, 1994. Greenland,Philos. Trans. R. Soc.London, $er. A, 273, 513-533, 1973. of the Precambrian basement beneath northern Beaumont,C., P. Fullsack,and I. Hamilton,Styles Lake Michigan,midcontinent North America, of crustaldeformation in compressionalorogens Brown,P. E., T. J.Dempster, T. N. Harrison,and D. H. Hutton,The rapakivigranites of south Geology,19,207-210, 1991. causedby subductionof the underlying litho- Chadwick,B., andA. A. Garde,Paleoproterozoic sphere,Tectonophysics, 232,119-132, 1994. Greenland- crustalmelting in responseto exten- 964 KERR ET AL.: MAKKOVIKIAN - KETILIDIAN OROGEN

obliqueconvergence in southGreenland: A southeast Labrador: evidence from marine, Province,Labrador: U-Pb geochronological data reappraisalof theKetilidian Orogen, in Crustal deep-seismicreflection date, Tectonics, 16, 795- andgeological implications, Can. J. EarthSci., Evolutionin theNorth Atlantic Region, edited by 809, 1997. 29, 1166-1179, 1992. T.S. Brewerand B.P. Atkin, Spec.Pap. Geol. Gulson, B. L., and T. E. Krogh, Evidence of Kerr, A., B. Ryan, C. F. Gower, and R. J. Soc. Am., 112, 179-197, 1996. multipleintrusion, possible resetting of U-Pb Wardie,The MakkovikProvince: Extension of Chadwick,B., P. Erfurt, T. Friseh,R. A. Frith, A. ages,and new crystallization of zirconsin the theKetilidian Mobile Belt intomainland North A. Garde,H. K. Schonwandt,H. Stendal,and post-tectonicintrusions (Rapakivi Granites) and America, in CrustalEvolution in the North B. Thomassen,Re-interpretation of aspectsof gneisses from S. Greenland, Geochim. AtlanticRegion, edited by T.S. Brewerand B.?. Ketilidiangeology, Rep. 163, 31 pp., Gramlands Cosmochim.Acta, 39, 65-82, 1975. Atkin, Spec.Pap. Geol. Soc. Am., II2, Geol. Unders.,Copenhagen, 1994. Hall, J., andG. Quinlan,A collisionalcrustal fabric 155-179, 1996. Chian,D., andJ. Louden,The structureof Archcan recogn•ed from seismicreflection profiles of Ketchum,L W. F., N. G. Culshaw,and G. R. - Ketilidiancrust along the continentalshelf of the Appalachian - Caledonide Orogcn, Dunning,Tectonic history and U-Pb geochron- southwest Greenland from a seismic refraction Tectonophysics,232, 3142, 1994. ology of the Early Proterozoic Makkovik- profile,Can. J. EarthSci., 29, 301-313,1992. Hall, L, R. J. Wardie, C. F. Gower, A. Kerr, K. KetilidianOrogen near Kaipokok Bay, Labra- Choukrone,?., and ECORS Team, The ECORS Coffin, C. E. Keen, and P. Carroll, Proterozoic dor: Preliminaryresults, paper presentedat Pyreneandeep seismicreflection profile data orogensof the northeasternCanadian Shield: International Conference on Tectonics and and the overall structureof an orogenicbelt,. New information from crustal reflection seismic Metallogeny of Early/Mid Precambrian Tectonics,8, 23-39, 1989. surveys, Can. J. Eanh Sci., 32, 1119-1131, OrogenicBelts, Universityof Quebec,Mon- Clowes,R. M., M. T. Brandon,A. G. Green, C. 1995. treal, Quebec,Canada, August 1995. J. Yorath, A. Sutherland-Brown,E. R. Hamilton, M. A., A. A. Garde, B. Chadwick, and Kranck, E. N., The rock-groundof the coastof Kanasewich,and C. Spencer,IXrHOPROBE- C. Swager,Kefilidian Orogen, south Greenland: Labrador and the connection between the southernVancouver Island: Cenozoic subducfion Paleoproterozoicare accretion,sedimentation Pre-Cambrianof Greenland and north America, compleximaged by deepseismic reflections, andmetamorphism recorded by U-Pb geochron- Bull. Comm.Geol. Finl., I25, 39-65, 1939. Can. J. Earth Sci., 24, 31-51, 1987. ology,paper presented at InternationalConfer- Louden, K. E., and Fan, J., Crustal structuresof Colman-Sadd,S. P., Two-stagecontinental colli- ence on Tectonics and Metallogeny of GrenvilleMakkovik and southernNain prov- sionand plate driving forces, Tectonophysics, Early/Mid PrecambrianOrogenic Belts, Univer- incesalong the LithoprobeECSOOT transect: 90, 263-282, 1982. sity of Quebec,Montreal, Quebec, Canada, Preliminary resultsof seismic refractionand Culotta, R. C., T. Pratt, and I. Olivier, A tale of August1995. gravitymodels and their tectonic implications, twosutures: COCORP's deep seismic surveys of Hamilton,M. A., A. A. Garde, B. Chadwick,and Can.J. Earth $ci., in press,1997. the Grenville Province in the eastern U.S. C. Swager, Observationson paleoproterozoic Loveridge, W. D., I. F. Ermanovics,and It. W. midcontinent,Geologyø 18, 6464549,1990. fore-arc sedimentation and deformation: Prelim- Sullivan,U-Pb ages on zircon from the Maggo Culshaw, N. G., and 1. W. F. Ketchurn, The inaryU-Pb resultsfrom theKetilidian Orogen, Gneiss, the Kanairiktok Plutonit Suite and the KaipokokZone of the Makkovik Orogen- An south Greenland, Rep. 57, pp. 112-123, Island Harbour Plutonit Suite, coastof Labra- earlyProterozoic terrane boundary ?, Rep. 45, LithoprobeSecr., Universityof BritishColum- dor, Newfoundland,in RadiogenicAge and pp. 7-22, LithoprobeSeer., University of bia, Vancouver,B.C., 1996. IsotopicStudies: Report 1, Geol. Surv. Can., BritishColumbia, Vancouver, B.C., !994. Hutton, D. H., T. L Dempster,P. E. Brown, and Pap. 87-2, 59-65, 1987. Dempster,P. I., T. N. Harrison,P. E. Brown, and S. M. Becker, A new mechanismof granite McBride,1. H., D. B. Snyder,M.P. Tate, R. W. D. H. Hutton,Low-pressure gramlites from the emplacement:Rapakivi intrusionsin active England, and R. W. Hobbs, Upper mantle Ketilidian Mobile Belt of south Greenland, J. extensionalshear zones, Nature, 343, 452-454, reflectorstructure and origin beneath the Scot- Petrol., 32, 979-1004, 1991. 1990. fish Caledonides,Tectonics, 14, 135!-1367, Dewey, J. F., Extensionalcollapse of orogens, Kalsbeek,F., andP. N. Taylor,Isotopic and chem- 1995. Teaonics, 7, 1!23-!139, 1988. ical variationin granitesacross a Proterozoic Patchett,P. J., andD. Bridgwater,Origin of conti- Ellis, S., andC. Beaumont,Models of convergent continentalmargin - The Ketilidianmobile belt nentalcrust of 1.9-1.7 Ga age definedby bid bounda•tectonics: implications for theinterpre- of South Greenland, Earth Planet. Sci. Lett., isotopesin theKetilidian terrain of SouthGreen- tationof Lithoprobedata, Rep. 57, pp. 59-109, 73, 65-80, 1985. land, Contrib.Mineral. Petrol., 87, 311-318, LithoprobeSeer., Universityof BritishColum- Kalsbeek,F., L. M. Larsen, and J. Bondam,Geo- 1984. bia, Vancouver, B.C., !996. logicalmap of Greenland,(descriptive rex0, Quinlan,G. M., 1. Hall, H. Williams,I. Wright,$. Emslie,R. F., Anorthositemassifs, rapakivi gran- Sydgrgnland, sheet 1, 1:500,000 scale, Colman-Sadd,S. I. O'Brien, G. Stockmal,and ins andLate Proterozoicrifting of North Amer- GrgnlandsGeol. Unders., Copenhagen, 1990. F. Mari!licr, Onshore seismic reflection ica, PrecambrianRes., 7, 61-98, 1978. Keen,C. E., P. Potter,a•xi S. P. Srivastava,Deep transeetsacross the NewfoundlandAppala- Ermanovics,i. F., Geology of Hopedale Block, seismicreflection data acrossthe conjugate chians, Can. J. Earth Sci., 29, 1865-1877, southernNain Province, and the adjacentPro- margins of the Labrador Sea. Can. J. Earth 1992. terozoic terranes, Labrador, Newfoundland, Sci., 31, 192-205, 1994. Quinlan,G. M., C. Beaumont,and I. Hall, Tec- Mem.o Geol. Surv. Can., 431, 161 pp, 1992. Kerr, A., Geochemistryof the Trans-Labrador tonic model for crustal seismicreflectivity Frei, W., P. Heitzmann,P. Lehner, S. Muller, R. GranitoidBelt, Canada:A quantitativecompara- patternsin compressionalorogem, Geo/ogy, 21, Olivier, A. Pfifner, A. Steek, and P. Va!asek, tive studyof a ProterozoicBatholith and possi- 663-666, 1993. Geotraversesacross the Swiss Alps, Nature, ble Phanerozoiccounterparts, Precambrian Reid, I., Crustal structure across the Nain- 340, 544-548, 1989. Res., 45, 1-17, 1989a. Makkovikboundary on the continentalshelf off Gower,C. F., andA. B. Ryan, Proterozoicevolu- Kerr, A., EarlyProterozoic granitoid magmatism Labrador from seismicrefraction data, Can. J. tion of the Grenville Province and adjacent and crustal evolution in the Makkovik Province Earth Sci., 33, 460-471, 1996. MakkovikProvince in east-centralLabrador, in of Labrador:A geochemicaland isotopic study, Roest, W. R., and S. P. Srivastava, Sea-floor The GrenvilleProvince, editedby J.M. Moore, Ph.D. thesis,528 pp., Mere. Univ. of New- spreadingin the LabradorSea: A new recon- A.1. Baer, andA. Davidson,Spec. Pap. Geol. foundland,St. John's,Newfoundland, Canada, struction.Geology, 17, !000-1003, 1989. Assoc. Can., 3I, 281-295, !986. 1989b. Ryan,A. B. Regionalgeology of the centralpart of Gower, C. F., and A. B. Ryan, Two stage felsic Kerr, A., andB. I. Fryer,Nd isotopicevidence for the Central Mineral belt, Labrador, Mere. 3, volcanismin the Lower ProterozoicUpper crust-mantleinteraction in thegenesis of A-type 185pp., Miner. Dev. Div., Newf• Dep. AllIlk Group,Labrador, Canada: Its relationship granitoidsuites in Labrador, Canada,Chern. of Mines andEnergy, St. •Iohn's,NF, !984. to syn- andpost-kinematic plutonism, in Geo- Geol, 103, 39-60, 1993. Ryan, A. B., A. Kay, and I. Ermanovics,The chemistryand mineralizationof Proterozoic Kerr, A., andB. J. Fryer, The importanceof late- geologyof the MakkovikSubprovince between VolcanicSuites, edited by T.C. Pharoh et al., and postorogeniccrustal growth in the Early KaipokokBay and Bay of Islands,Labrador, Geol. Soc.Spec. Publ., 33, 201-210, 1987. Proterozoic: Evidence from Sm-Nd studies of Map 83-38 and83-41, 21 pp. (with descriptive Gower, C. F., A. B. Ryan, and T. Rivers, igneousrocks in the MakkovikProvince, Can- notes),Miner. Der. Div., NewfoundlandDep. Mid-Proterozoic Laurentia-Balfica: An overview ada,Earth Planet. Sci. Lett., 125, 71-88, 1994. of Mines andEnergy, St. Iohn's, NF, 1983. of its evolutionand a summaryof the contribu- Kerr, A., and R. J. Wardle, Definition of an Sch•rer, U., T. E. Krogh, R. •I. Wardle, A. B. tionsmade by thisvolume, in MM-Proterozoic Arcbean- Proterozoic crustal suture by isotopic Ryan,and S.S. •i, U-Pb agesof Earlyand Laurentia- Baltica, edited by C.F. Gower, studies of basementintersections from offshore Middle Proterozoic volcanism and metamor- A.B. Ryan, and T. Rivers, Spec. Pap. Geol. wells in the southernLabrador Sea, Can. J. phiamin theMakkovik Orogen, Labrador, Can. Assoc. Can., 38, 1-23, 1990. Earth Sci., 34, 209-214, 1997. J. F,arth Sci., 25, 1098-1107, 1988. Gower, C. F., I. Hall, G. L Kilfoil, G. M. Kerr, A. , T. E. Krogh,F. Corfu,U. Sch/irer,S. Snyder,D. B., S. B. Lucas, and 1. H. McBride, Quinlan,and R. J. Wardie, Rootsof the Labra- S. Gandhi,and Y. Y. Kwok,Episodic Early Crustal and mantle reflectors from dorian orogen in the Grenville Province in Proterozoicgranitoid plutonism in theMakkovik Paleoproterozoieorogens and their relationto KERR ET AL.: MAKKOVIKIAN - KETILIDIAN OROGEN 965

arc-continentcollisions, in CrustMEvolution in Thecase of the missing crust, paper presented at cationsof Pb isotopedata for galenaseparates the North Atlantic Region, edited by T.S. AnnualMeeting of theGeological Association of from the LabradorCentral Mineral Belt, Econ. Brewerand B.P. Akin, Spec.Pap. Geol. Soc. Canada-MineralogicalAssociation of Canada, Geol., 86, 1721-1736, 1991. Am., 112, 1-25, 1996. Vancouver,Br. Columbia,Canada, 1990. WindIcy,B. F., Early Proterozoiccollision tecton- Srivastava,S. P., and W. R. Roest, Seafloor Wartile,R. I., andD. G. Bailey,Early Proterozoic ics, and rapakivi granitesas intrusionsin an spreadinghistory, in EastCoast Basin Atlas: sequencesin Labrador, in ProterozoicBasins of extensional thrust-thickened crust: The LabradorSea, editedby I.S. Bell, Aft. Geosci. Canada,edited by F.H.A. Campbell,Spec. Ketilidian Orogen, south Greenland, Cent., Geol. Surv. Can., Ottawa,ON, 1989. Pap. Geol.Surv. Can., 81-10, pp. 331-359, Tectonophysics,195, 1-10, 1991. Srivastava,S. P., and J. Verhoef, Evolutionof 1981. Mesozoicsedimentary basins around the north Waxnet,M., J. Morgan,P. Barton,P. Morgan,C. C.F. Gower, A. Kerr, B. Ryan, and RJ. centralAriantic: A preliminaryplate kinematic Price, and K. Iones, Seismicreflections from Wardie, GeologicalSurvey of Newfoundlandand solution, in Basins on the Atlantic Seaboard: the mantlerepresent relict subduerionzones Labrador,Department of Minesand Energy, P.O. PetroleumGeblogy, Sedimentology and Basin withinthe continental lithosphere, Geology, 24, Box8700, St. Iohn's, Newfoundland,Canada A1B Evolution,edited by I. Panroll,Geol. Soc. Spec. 39-42, 1996. 416.(e-mail: [email protected]. ca;cfg@ Publ., 62, 397-420, 1992. Wasteheys,It., R. J. Wardie,and T. E. Krogh, zeppo.g•.gov.nf. ca; [email protected]. SuCh,L S., B. E. Marten, A.M. S. Clark, andI. Extrapolationof tectonicboundaries across the nf.ca; [email protected]) Knight,Correlation of Precambriansupracrustal LabradorShelf: U-Pb geochronologyof well I. Hall, Deparunentof Earth Sciences,Memo- rocks of coastal Labrador and southwestern samples,Rep. 32, pp. 145-174, Lithoprobe rial Universityof Newfoundland,St. •Iohn's,New- Greenland,Nature, 238, 122-123, 1972. Seer.,University of BritishColumbia, Vancou- foundland,Canada A1B 3X5. (e-mail: jhall@ vanBreeman, O., M. Aftalion, and I. H. Aliaart, ver, B.C., 1992. tomig.esd.mun.ca) Isotopicand geochronologic studies on granites Willett,S., C. Beaumont,and P. Fullsack,Mechan- from the Ketilidian mobile belt of South Green- ical modelfor thetectonics of doubly-vergent land,Geol. Soc. Am. Bull., 85, 403412, 1974. compressionalorogens, Geology, 21, 371-374, 0•eceived March 27, 1996; Wardie,R. •I., C. F. Gower, and A. Kerr, The 1993. revisedMarch 13, 1997; southeasternmargin of Laurentiaca. 1. 7 Ga: Wilton,D. H. C., Metallogenicand tectonic impli- acceptedJuly 21, 1997.)