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

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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 the KetilidianOrogen 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 dating in 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
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