Search

The magmatic source of the Late Triassic granites in the East Kunlun Orogenic Belt (EKOB) is controversial, which restricts our understanding of crustal evolution and geodynamic evolution in the EKOB. Therefore, this study conducts zircon U-Pb isotope dating, major and trace geochemical analysis, electron microprobe analysis of feldspar and amphibole, and in situ zircon Hf isotope analysisof Xiangride host granites and their mafic microgranular enclaves (MMEs) in the EKOB to reveal their magmatic source and geodynamic implications. Zircon U-Pb dating shows that the Xiangride host granites and the MMEs crystallized simultaneously at ca. 228-223 Ma. The MMEs show an igneous texture, finer grain size, higher crystallization temperature and water content, and lower oxygen fugacity than those of the host granite, suggesting that they were probably derived from two distinct primitive magmas. Therefore, the MMEs are considered the products of magma mixing between granitic and mafic magmas. The Xiangride host granites show high Sr/Y [(42.0-73.1) and (La/Yb).sub.N] (12.7-30.7) ratios, showing afnity with adakites derived from a thickened lower crust. Combined with the indistinguishable [[epsilon].sub.Hf](t) values (-3.47 to +0.08) from the coeval mafic rocks, it is concluded that they are derived from partial melting of juvenile thickened lower crust. The adakitic features of the Xiangride host granites and widespread coeval granites indicate the existence of a thickened lower crust before 228 Ma and delamination of the lower lithosphere is likely the geodynamic process resulting in the postcollisional extension regime. Key words: geochemistry, East Kunlun Orogenic Belt, MMEs, postcollision, 1. Introduction The East Kunlun Orogenic Belt (EKOB) is located in the Qinghai-Tibet Plateau, northwestern China (Fig. 1a), East Paleo-Tethys tectonic domain. Since the Neoproterozoic, the EKOB has experienced at [...]

In southern Alaska, Wrangellia-type magnetic crustal character extends from the Talkeetna Mountains southwest through the Alaska Range to the Bristol Bay region. Magnetic data analyses in the Talkeetna Mountains showed that there are mid-crustal differences in the magnetic properties of Wrangellia and the Peninsular terrane. After converting total field magnetic anomaly data to magnetic potential, we applied Fourier filtering techniques to remove magnetic responses from deep and shallow sources. The resulting mid-crustal magnetic characterization delineates the regional magnetic potential domains that correspond to the Wrangellia and Peninsular terranes throughout southern Alaska. These magnetic potential domains show that Wrangellia-type crust extends southwest to the Iliamna Lake region and that it overlaps the mapped Peninsular terrane. Upon reconsidering geologic ties between Wrangellia, Peninsular, and Alexander terranes, we conclude that Peninsular terrane is part of what we here call Western Wrangellia. Western Wrangellia contains the Lower Jurassic Talkeetna volcanic arc and is similar to Wrangellia of the Vancouver Island area, Canada (Southern Wrangellia), which contains the Lower Jurassic Bonanza volcanic arc. Others have previously made this correlation and proposed that the Talkeetna arc-bearing part of southern Alaska was displaced from the Bonanza arc-bearing part of Canada. We generally agree and propose that about 1000 km of dextral displacement along ancestral Border Ranges fault segments and other faults of south-central Alaska separated Western Wrangellia from Southern Wrangellia. We think this displacement was mostly in the Late Jurassic and earliest Cretaceous, perhaps between about 160 and 130 Ma. Key words: Wrangellia, Peninsular terrane, southern Alaska, Talkeetna arc, Bonanza arc. Dans le sud de l'Alaska, des caracteristiques magnetiques crustales typiques de la Wrangellie s'etendent des monts Talkeetna vers le sud-ouest en passant par la chaine de l'Alaska jusqu'a la region de la baie de Bristol. Des analyses de donnees magnetiques pour les monts Talkeetna montrent qu'il existe des differences, dans la croute intermediaire, entre les proprietes magnetiques de la Wrangellie et du terrane peninsulaire. Apres conversion des donnees d'anomalies magnetiques de champ total en potentiel magnetique, nous avons applique des techniques de filtrage de Fourier pour retirer les reponses magnetiques de sources profondes et peu profondes. La caracterisation magnetique de la croute intermediaire qui en resulte delimite des domaines de potentiel magnetique regional qui correspondent a la Wrangellie et au terrane peninsulaire dans tout le sud de l'Alaska. Ces domaines de potentiel magnetique montrent que de la croute typique de la Wrangellie s'etend vers le sud-ouest jusqu'a la region du lac Iliamna et qu'elle chevauche le terrane peninsulaire tel que cartographie. Un reexamen des liens geologiques entre la Wrangellie et les terranes peninsulaire et d'Alexander nous amene a conclure que le terrane peninsulaire fait partie de ce que nous denommons la Wrangellie occidentale. Cette derniere contient l'arc volcanique d'age jurassique inferieur de Talkeetna et est semblable a la Wrangellie de la region de l'ile de Vancouver (Canada--Wrangellie meridionale), qui contient l'arc volcanique d'age jurassique inferieur de Bonanza. D'autres auteurs ont deja releve cette correlation et propose que la portion de la Wrangellie contenant l'arc de Talkeetna dans le sud de l'Alaska a ete separee de la portion canadienne contenant l'arc de Bonanza. Nous sommes generalement d'accord et proposons qu'environ 1000 km de deplacement dextre le long de troncons du systeme de failles des chaines frontalieres et autres failles ancestrales du centre-sud de l'Alaska ont separe la Wrangellie occidentale de la Wrangellie meridionale. Nous croyons que ce deplacement s'est principalement effectue au Jurassique tardif et au debut du Cretace, possiblement d'environ 160 Ma a 130 Ma. [Traduit par la Redaction] Mots-cles : Wrangellie, terrane peninsulaire, sud de l'Alaska, arc de Talkeetna, arc de Bonanza., Introduction This study delineates the westward extension of Wrangellia in southern Alaska. As originally mapped (Fig. 1A; Jones et al. 1977, 1987), extensive overlap by Cretaceous and Cenozoic rocks prevented [...]

A new three-dimensional (3D) inversion strategy is applied to new compilations of gravity and magnetic data to reassess the role of crustal lineaments in the development of the western Laurentian margin, Selwyn Basin, and associated sediment-hosted zinc-lead (Zn-Pb) deposits. The region's history is obscured by multiple tectonic overprints, including terrane accretion, plutonism, and thrust faulting. Regionally continuous, broadly northeast-trending crustal lineaments, including the Liard line, Fort Norman structure, and Leith Ridge fault, were interpreted as having had long-standing influence on craton, margin, and sedimentary basin development. An east-northeast-trending lineament, Mackenzie River, traced from the Misty Creek Embayment to Great Bear Lake, is interpreted as the southern edge of a cratonic promontory. The location of the Liard line, associated with a transfer fault that bounds the Macdonald Platform promontory, is refined. New geophysical results support the continuity of the Fort Norman structure below the Selwyn Basin, but limited evidence exists for the Leith Ridge fault in this area. A northwest-trending lineament that bounds the craton is interpreted as a crustal manifestation of litho-spheric thinning of the Laurentian margin, as echoed by a change in the depth of the lithosphere-asthenosphere boundary. The structure delimits the eastern extent of mid-to Late Cretaceous granitic intrusions and is straddled by Mississippi Valley-type Zn-Pb occurrences, following their palinspastic restoration. Clastic-dominated Zn-Pb occurrences are aligned along another northwest-trending lineament interpreted to be associated with a shallowing of lower crustal rocks. Key words: Canadian Cordillera, gravity, Laurentia, Selwyn Basin, sediment-hosted Zn-Pb deposits. Une nouvelle strategie d'inversion tridimensionnelle (3D) est appliquee a de nouvelles compilations de donnees gravi-metriques et aeromagnetiques afin de reevaluer le role de lineaments crustaux dans le developpement de la marge laurentienne occidentale, du bassin de Selwyn et de la mineralisation en zinc-plomb (Zn-Pb) associee dans des sediments. L'histoire de la region est masquee par de multiples episodes tectoniques surimposes comprenant l'accretion de terranes, du plutonisme et la formation de failles de chevauchement. Des lineaments crustaux regionaux continus, generalement d'orientation nord-est, dont la ligne de Liard, la structure de Fort Norman et la faille de Leith Ridge, ont ete interpretes comme ayant eu une influence prolon-gee sur le developpement du craton, de la marge et du bassin sedimentaire connexe. Un lineament d'orientation est-nord-est, du fleuve Mackenzie, observe depuis l'echancrure de Misty Creek jusqu'au Grand lac de l'Ours, est interprete comme marquant la bordure sud d'un promontoire cratonique. L'emplacement de la ligne de Liard, associee a une faille de transfert qui delimite le promontoire de la plateforme de Macdonald, est precise. De nouveaux resultats geophysiques soutiennent l'interpretation de la continuite de la structure de Fort Norman sous le bassin de Selwyn, mais les observations concernant la faille de Leith Ridge sont limitees dans cette region. Un lineament d'orientation nord-ouest qui borde le craton est interprete comme etant une manifestation crustale de l'amincissement lithospherique de la marge laurentienne, comme en temoigne un changement de la profondeur de la limite lithosphere-asthenosphere. La structure delimite l'etendue vers l'est d'intrusions granitiques du Cretace tardif et est surmontee par des indices de Zn-Pb de type Mississippi Valley, apres leur reconstitution palinspatique. Les indices de Zn-Pb a dominante clastique sont alignes le long d'un autre lineament d'orientation nord-ouest interprete etre associe a une diminution de la profondeur des roches de la croute inferieure. [Traduit par la Redaction] Mots-cles: Cordillere canadienne, gravite, Laurentie, bassin de Selwyn, gites de Zn-Pb dans desrochessedimentaires., Introduction The tectonic history of the western margin of the North American craton (Laurentia) in northwestern Canada suggests that it formed from the progressive accretion of 2.3-1.8 Ga magmatic arcs [...]

Earth's magnetic field is recorded as oceanic crust cools, generating lineated magnetic anomalies that provide the pattern of polarity reversals for the past 160 million years.sup.1. In the lower (gabbroic) crust, polarity interval boundaries are proxies for isotherms that constrain cooling and hence crustal accretion. Seismic observations.sup.2-4, geospeedometry.sup.5-7 and thermal modelling.sup.8-10 of fast-spread crust yield conflicting interpretations of where and how heat is lost near the ridge, a sensitive indicator of processes of melt transport and crystallization within the crust. Here we show that the magnetic structure of magmatically robust fast-spread crust requires that crustal temperatures near the dike-gabbro transition remain at approximately 500 degrees Celsius for 0.1 million years. Near-bottom magnetization solutions over two areas, separated by approximately 8 kilometres, highlight subhorizontal polarity boundaries within 200 metres of the dike-gabbro transition that extend 7-8 kilometres off-axis. Oriented samples with multiple polarity components provide direct confirmation of a corresponding horizontal polarity boundary across an area approximately one kilometre wide, and indicate slow cooling over three polarity intervals. Our results are incompatible with deep hydrothermal cooling within a few kilometres of the axis.sup.2,7 and instead suggest a broad, hot axial zone that extends roughly 8 kilometres off-axis in magmatically robust fast-spread ocean crust. A record of Earth's magnetic field constructed from near-bottom magnetization observations and oriented samples provides three-dimensional imaging of magnetic stripes in fast-spread crust, and suggests slow cooling off-axis, as opposed to deep hydrothermal cooling close to the spreading ridge., Author(s): Sarah M. Maher [sup.1] , Jeffrey S. Gee [sup.1] , Michael J. Cheadle [sup.2] , Barbara E. John [sup.2] Author Affiliations: (1) Scripps Institution of Oceanography, University of California, [...]

The Snow Lake gold camp is located within amphibolite facies volcanic rocks of the ca. 1.88-1.87 Ga Flin Flon--Glennie Complex (FFGC) in the Trans-Hudson Orogen, Manitoba. During thrusting and collision with the Archean Sask craton, volcanic rocks were interleaved with turbidites of the ca. 1.855-1.84 Ga Burntwood Group and sandstone and conglomerate of the ca. 1.845-1.835 Ga Missi Group. The main cleavage in the turbidites was previously attributed to thrusting and used as a marker for correlating structures across the camp. A re-examination of this cleavage suggests that it overprints the thrust faults and formed during later collision between the FFGC and the Archean Superior craton. This has important implications as it further suggests that (1) previously unrecognized, early brittle thrust faults repeat volcanic stratigraphy and may have created the boundary conditions that enabled the formation of ductile thrust faults, fold nappes, and mega sheath folds; (2) shear sense indicators along ductile thrust faults formed during their reactivation as sinistral shear zones rather than during thrusting; and (3) peak metamorphic conditions were caused by thrusting and stacking during collision with the Sask craton but were attained later during collision with the Superior craton due to the time lag between orogenesis and the re-equilibration of regional isotherms. Results from this study may be applicable to other complexly deformed terranes where the dominant regional cleavage differs in expression in mixed volcanic and sedimentary successions and has been used as a marker for correlating structures. Key words: Trans-Hudson Orogen, Snow Lake, New Britannia Mine, Flin Flon Belt, Snow Lake arc assemblage, brittle thrust faults. Le camp aurifère de Snow Lake se trouve dans des roches volcaniques au faciès des amphibolites du complexe de Flin Flon--Glennie (CFFG), d'environ 1,88-1,87 Ga, de l'orogène transhudsonnien, au Manitoba. Durant le chevauchement et la collision avec le craton archéen de Sask, des roches volcaniques ont été intercalées avec des turbidites du Groupe de Burntwood, d'environ 1,855-1,84 Ga, et des grès et conglomérats du Groupe de Missi, d'environ 1,845-1,835 Ga. Le clivage principal dans les turbidites était auparavant attribué au chevauchement et utilisé comme marqueur pour corréler des structures à l'échelle du camp. Le réexamen de ce clivage donne à penser qu'il est superposé sur les failles de chevauchement et s'est formé durant la collision ultérieure du CFFG et du craton archéen du Supérieur. Cette nouvelle interprétation a d'importantes conséquences, puisqu'elle indiquerait que (1) des failles de chevauchement cassantes précoces non reconnues auparavant produisent une répétition de la stratigraphie volcanique et pourraient avoir créer les conditions limites qui ont permis la formation de failles de chevauchement ductiles, de nappes de recouvrement et de plis en fourreau mégascopiques, (2) des indicateurs de sens de cisaillement le long de failles de chevauchement ductiles se sont formés durant la réactivation de ces dernières en tant que zones de cisaillement senestres plutôt que durant le chevauchement et (3) les conditions du pic du métamorphisme ont été causées par le chevauchement et l'empilement durant la collision avec le craton de Sask, mais ont été atteintes plus tard durant la collision avec le craton du Supérieur en raison du décalage entre l'orogenèse et le rééquilibrage des isothermes régionaux. Les résultats de l'étude pourraient être appliqués à d'autres terranes de déformation complexe dans lesquels le clivage régional dominant s'exprime différemment dans des séquences volcaniques et sédimentaires mixtes et a été utilisé comme marqueur pour la corrélation de structures. [Traduit par la Rédaction] Mots-clés : orogène transhudsonnien, Snow Lake, mine New Britannia, ceinture de Flin Flon, assemblage d'arc de Snow Lake, failles de chevauchement cassantes., Introduction The Snow Lake gold camp contains one of the largest gold mines in the Paleoproterozoic Trans-Hudson Orogen. The New Britannia Mine in the town of Snow Lake, Manitoba, produced [...]

van Staal et al. (2022) present basement geology tests for our terrane wreck (TW) model (Keppie et al. 2021) and their preferred model (e.g., van Staal et al. 2009; van [...]

There continues to be debate regarding the timing of the collision between the Indian and Eurasian plates and the uplift of the Tibetan Plateau. This study presents zircon U-Pb geochronology, whole-rock geochemistry, and Lu-Hf isotopic data for the Saiduopugangri granite of the Qiangtang Terrane, located within the core of the Tibetan Plateau. These data provide the basis for the geodynamic setting, petrogenesis, and characteristics of its magma source. Zircons from the Saiduopugangri granite yield a weighted-mean [ (.sup.206] [)Pb/.sup.238]Uage of 62.72 [+ or -] 0.06 Ma, indicating that these rocks formed during the early Palaeocene. The rocks are members of the highly calc-alkaline to shoshonitic series, with weak peraluminous characteristics. Trace elements are characterised by high Sr (483-616 ppm), and low Y (6-10 ppm) and Yb (1 ppm) content, typical of a high Sr and low Yb granite. The [[epsilon].sub.Hf(t)] of zircon ranges from -2.14 to 2.35, with two-stage Hf model ages ([T.sub.DM2]) ranging from 1182 to 895 Ma. These data suggest that the Saiduopugangri granite magma was derived from the melting of lower-crustal clastic meta-sedimentary rocks and mantle-derived basalts. The high Sr and low Yb granite characteristics and experimental results indicate that melting occurred at >1.2 GPa and >750 [degrees]C, consistent with a crustal thickness greater than 50 km. Magmatism occurred from the Late Cretaceous to the early Palaeogene and is broadly synchronous with the collision timing between the Indian and Eurasian plates. The Saiduopugangri granite provides evidence of crustal thickening of the Tibetan Plateau and its age and petrogenesis constrain the timing of the initial uplift. Key words: Tibetan Plateau, Qiangtang Terrane, thickening and uplifting of crust, granite, petrochemistry, U-Pb chronology. Un debat persiste quant au moment de la collision des plaques indienne et eurasienne et du soulevement du plateau tibetain. Nouspresentonsdes donneesde geochronologie U-Pb sur zircon, de geochimiesur rochetotaleet dechimie desisotopesde Lu-Hf pour le granite de Saiduopugangri du terrane de Qiangtang, situe dans la partie centrale du plateau tibetain. Ces donnees permettent d'asseoir le contexte geodynamique, la petrogenese et les caracteristiques de la source du magma. Des zircons du granite de Saiduopugangri donnent un age [ (.sup.206] [)Pb/.sup.238]U moyen pondere de 62,72 [+ or -] 0,06 Ma, ce qui indique que ces roches se sont formees durant le Paleocene precoce. Les roches sont des membres de la serie tres calcoalcaline a shoshonitique, presentant des caracteristiques faiblement hyperalumineuses. Les elements en traces sont caracterises par de fortes concentrations de Sr (483-616 ppm) et de faibles concentrations de Y (6-10 ppm) et Yb (1 ppm), typiques desgranites richesen Sret pauvres en Yb. Les valeursde [[epsilon].sub.Hf(t)] des zircons vont de -2,14 a 2, 35, avec des ages de Hf selon un modele a deux etapes ([T.sub.DM2]) allant de 1182 Ma a 895 Ma. Ces donnees donnent a penser que le magma du granite de Saiduopugangri est derive de la fusion de roches metasedimentaires clastiques de la croute inferieure et de basaltes derives du manteau. Les caracteristiques d'un granite riche en Sr et pauvre en Yb et des resultats experimentaux indiquent que la fusion s'est produite a >1,2 GPa et >750 [degrees]C, ce qui concorde avec une croute de plus de 50 km d'epaisseur. Le magmatisme s'est produit du Cretace tardif au Paleogene precoce, a peu pres en meme temps que la collision des plaques indienne et eurasienne. Le granite de Saiduopugangri presente des preuves d'un epaississement crustal du plateau tibetain, et son age et sa petrogenese permettent de delimiter le moment du soulevement initial. [Traduit par la Redaction] Mots-cles: plateau tibetain, terrane de Qiangtang, epaississement et soulevement de la croute, granite, petrochimie, chronologie U-Pb., Introduction The Tibetan Plateau is the highest and largest plateau on Earth, forming part of an orogenic belt undergoing continent-continent collision. Numerous studies have investigated the timing and mechanisms of [...]

Depositional ages and provenance of metasedimentary rocks provide constraints on the architecture of the interface between the Slave and Rae cratons and processes related to the Thelon Orogen. Clastic rocks analysed from the central Thelon tectonic zone are Paleoproterozoic in age and not remnants of the Archean Yellowknife Supergroup (Slave Province), as originally considered. Two assemblages are recognized. An older clastic assemblage deposited after 2.09 Ga contains detrital zircon age modes of 2.3 and 2.17 Ga, with subordinate Neoarchean and Paleoarchean detritus. Its deposition is interpreted to predate Thelon magmatic activity given that (1) it lacks ca. 2.01-1.97 Ga detritus of Thelon magmatic origin, and (2) correlative clastic rocks occur as inclusions in Thelon plutons and contain ca. 2.0 Ga metamorphic monazite. This assemblage is correlative with both the Mary Frances and Rutledge River groups, establishing a >800 km long basin at ca. 2.1 Ga that received detritus from the western Rae and (or) Buffalo Head terrane(s). Separation from the Slave craton at this time is consistent with the absence of any Slave-affinity detritus. A younger assemblage deposited after 1.95 Ga and prior to 1.91 Ga contains mainly 2.02-1.95 Ga detrital zircon, age modes comparable with adjacent Thelon convergent-margin plutonic rocks. The younger assemblage records deposition of the uplifted and eroded Thelon magmatic arc in an intermontane or foreland basin setting during the later stages of post-collisional convergence. These U-Pb zircon data support a tectonic model for western Laurentia that reconciles differences between the Thelon and Taltson magmatic zones involving ca. 2.1 Ga rifting, ca. 2.01-1.97 Ga convergence, followed by Key words: Thelon orogen, Precambrian geology, U-Pb geochronology, detrital zircon, provenance, Buffalo Head - Chinchaga. Des ages de depot et la provenance de roches metasedimentaires permettent de delimiter l'architecture de l'interface entre les cratons des Esclaves et de Rae et des processus associes a l'orogenedeThelon. Contrairement a l'interpretation initiale, des roches clastiques analysees provenant de la partie centrale de la zone tectonique de Thelon sont d'age paleoproterozo'ique, et non des restes du Supergroupe archeen de Yellowknife (province des Esclaves). Deux assemblages sont cernes. Un assemblage clastique plus ancien depose apres 2, 09 Ga donne des modes d'ages de zircons detritiques de 2, 3 et 2, 17 Ga, ainsi que des detritus neoarcheens et paleoarcheens en quantites moins importantes. Son depot est interprete comme ayant precede l'activite magmatique de Thelon etant donne que (1) il est exempt de detritus d'environ 2, 01-1, 97 Ga d'origine magmatique de Thelon et (2) des inclusions de roches clastiques correlatives sont presentes dans des plutons de Thelon et contiennent de la monazite metamorphique d'environ 2, 0 Ga. Cet assemblage est correlatif des Groupes de Mary Frances et de Rutledge River, etablissant la presence d'un basin de plus de 800 km de longueur vers 2, 1 Ga qui recevait des detritus des terranes de Rae occidental et (ou) de Buffalo Head. La separation du craton des Esclaves a cette epoque concorde avec l'absencedetout detritus ayant une affinite avec ce craton. Un assemblage plus jeune depose apres 1, 95 Ga et avant 1, 91 Ga donne principalement des modes d'ages de zircons detritiques de 2, 02-1, 95 Ga, comparables a ceux de roches pluto-niques de marge convergente attenantes de Thelon. Cet assemblage plus jeune temoigne du depot de l'arc magmatique de Thelon souleve et erode dans un milieu de bassin intramontagneux ou d'avant-pays durant des stades tardifs de convergence post-collision. Ces donnees U-Pb sur zircons appuient un modele tectonique pour l'ouest de la Laurentie qui aplanit des differences entre les zones magmatiques de Thelon et de Talston et comprend des episodes de rifting vers 2, 1 Ga et de convergence vers 2, 01-1, 97 Ga, suivis d'un episode d'exhumation par chevauchement apres 1, 95 Ga. [Traduit par la Redaction] Mots-cles : orogene de Thelon, geologie du Precambrien, geochronologie U-Pb, zircon detritique, provenance, Buffalo Head Chinchaga., Introduction The Thelon tectonic zone is a major geological and geophysical feature within northwestern Laurentia that delineates the tectonic boundary between the Archean Slave and Rae provinces (Fig. 1; Hoffman [...]

The northwestern Avalon Terrane, Newfoundland, is underlain by Neoproterozoic rocks traditionally divided into older Love Cove Group, medial Connecting Point Group, and the unconformably overlying Musgravetown Group. New lithogeochemical, isotopic, and updated U-Pb (zircon) age data demand changes to stratigraphic nomenclature and maps and help constrain the tectonomagmatic evolution. U-Pb age constraints include 620 [+ or -] 2 Ma for the calc-alkaline Broad Island Group (former Love Cove Group); 605 [+ or -] 1.2 Ma for rhyolite from near Bull Arm (type area, Bull Arm Formation, lower Musgravetown Group); 589 [+ or -] 2.0 Ma for a schist from the Love Cove type locality; 568.7 [+ or -] 1.4 Ma for rhyolite of the Rocky Harbour Formation, upper Musgravetown Group; and 572 [+ or -] 2 Ma for Louil Hills granite. In light of these results, remnants of the main Avalonian arc are re-designated 'Broad Island Group', for the site of the dated 620 Ma tuff. The ca. 589 Ma schist from Love Cove is included in the Musgravetown Group and may be a tectonized equivalent of Bull Arm Formation. Our data outline a tectonomagmatic change from arc-dominated magmatism at ca. 620 Ma, to an extensional regime ca. 605589 Ma, culminating in alkaline magmatism by ca. 572 Ma. eNdt values (t = 620-569 Ma) for felsic rocks range from 3.7 to 5.6 and yield [T.sub.DM] ages consistent with derivation from a juvenile Neoproterozoic (878-730 Ma) basement. Maficrocks exhibit a time-progressive increase in [[epsilon]Nd.sub.t], indicating more juvenile mantle sources through time. Further delimitation of map units of the area await integrated lithostratigraphy, precise modern U-Pb geochronology, and lithogeochemistry. Key words: Avalonia, Neoproterozoic, geochronology, Sm-Nd isotope chemistry, lithogeochemistry, felsic and maficmagmatism. Le nord-ouest du terrane d'Avalon (Terre-Neuve) est sous-tendu par des roches neoproterozoiques traditionnellement divisees en trois groupes, soit leGroupe de LoveCove, plus ancien, leGroupe intermediaire de Connecting Point et, reposant en discordance, le groupe de Musgravetown au-dessus. De nouvelles donnees lithogeochimiques et isotopiques et des ages U-Pb sur zircons a jour appellent des modifications a la nomenclature stratigraphique et aux cartes et permettent de mieux definir l'evolution tectonomagmatique. Les ages U-Pb comprennent des ages de 620 [+ or -] 2 Ma pour le Groupecalcoalcalin de Broad Island (anciennement le Groupe de Love Cove), de 605 [+ or -] 1, 2 Ma pour une rhyolite des environs de Bull Arm (region type, Formation de Bull Arm, Groupe de Musgravetown inferieur), de 589 [+ or -] 2, 0 Ma pour un schiste de la localitetype de Love Cove, de 568, 7 [+ or -] 1, 4 Ma pour une rhyolite de la Formation de Rocky Harbour, du Groupe de Musgravetown superieur, et de 572 [+ or -] 2 Ma pour le granite de Louil Hills. A la lumiere de ces resultats, les restes de l'arc avalonien principal sont renommes[much less than] Groupe de Broad Island [much greater than], pour la localite du tuf de 620 Ma. Le schiste d'environ 589 Ma de Love Cove est inclus dans le Groupe de Musgravetown et pourrait etre un equivalent tectonise de la Formation de Bull Arm. Nos donnees font ressortir une transition tectonomagmatique qui passe d'un magmatisme a dominance d'arc vers 620 Ma a un regime d'extension vers 605-589 Ma, culminant en un magmatisme alcalin vers 572 Ma. Les valeurs de [[epsilon]Nd.sub.t] (t = 620-569 Ma) pour les roches felsiques vont de 3, 7 a 5, 6 et donnent des ages TDM qui concordent avec l'interpretation que ces roches seraient derivees d'un socle neoproterozoique juvenile (878-730 Ma). Les roches mafiques presentent une augmentation au fil du temps des valeurs de [[epsilon]Nd.sub.t], ce qui indique des sources mantelliques de plus en plus juveniles. Une meilleure delimitation des unites cartographiques de la region necessitera l'integration de leur lithostratigraphie, de leur geochronologie U-Pb precise et moderne et de leur lithogeochimie. [Traduit par la Redaction] Mots-cles : Avalonie, Neoproterozoique, geochronologie, chimie des isotopes de Sm-Nd, lithogeochimie, magmatisme felsique et mafique., Introduction The Avalon Terrane in Newfoundland is the type locality for the microcontinent Avalonia, one of the peri-Gondwanan terranes accreted to the Laurentian margin by the latest Silurian as the [...]

Paleomagnetic results and a U--Pb baddeleyite age from the Silurian Mavillette gabbroic sill in southwest Nova Scotia, Canada, provide new evidence about the Paleozoic tectonic evolution of the Meguma terrane. The Mavillette gabbro sill intruded ca. 440-430 Myr old bimodal rift-related metavolcanic and metasedimentary rocks of the White Rock Formation in the Silurian--Devonian Rockville Notch Group. The 426.4 [+ or -] 2.0 Ma Mavillette gabbro age is notably younger than the ca. 440 Ma magmatism, but is part of a geochemically defined suite of within-plate sills and volcanic rocks of the Rockville Notch Group with ages as young as Early Devonian. Paleomagnetic investigation of 13 sites distributed along the Neoacadian (ca. 390 Ma) synclinal limbs of the Mavillette sill reveal magnetization directions that fail a fold test and therefore postdate Silurian emplacement of the gabbro. The post-folding remanence has a mean direction of D = 153.4, I = 17.1[degrees]; [[alpha].sub.95] = 6.5[degrees] (n = 12 sites), with corresponding paleopole 31.9 [degrees]S, 325.2 E; dp = 3.5[degrees], dm = 6.7[degrees] that resembles a pervasive late Carboniferous Kiaman overprint magnetization in North America, but is rotated significantly 22.2[degrees] [+ or -] 8.1[degrees] counter-clockwise (CCW). Mavillette remanence acquisition likely occurred in concert with fluid mobilization related to Alleghanian deformation, recorded locally by ca. 320 Ma muscovite [.sup.40][Ar.sup.-39] Ar ages. Previously published paleomagnetic results from the Meguma terrane also have Carboniferous remanence directions with similar ~24[degrees] CCW discordance. The regional CCW rotation of the southwest Meguma terrane post-dates this ca. 320 Ma tectonothermal remanence acquisition event, likely recording the development of an oroclinal bend of the Meguma terrane during the Alleghanian orogeny. Key words: rift magmatism, terrane accretion, tectonic rotation, Appalachian, overprint magnetization, Meguma orocline, faunal associations.. Les resultats d'analyses paleomagnetiques et un age U-Pb sur baddeleyite du filon-couche gabbroique silurien de Mavillette, dans le sud-ouest de la Nouvelle-Ecosse, Canada, fournissent de nouveaux renseignements sur l'evolution tectonique paleozoique du terrane de Meguma. Le filon-couche gabbro'ique de Mavillette s'est mis en place dans des roches metasedimentaires et metavolcaniques bimodales associees au rifting formees vers 440-430 Ma, de la Formation de White Rock, dans le Groupe siluro-devonien de Rockville Notch. L'age de 426,4 [+ or -] 2,0 Ma du gabbro de Mavillette est sensiblement plus jeune que le magmatisme vers 440 Ma, mais fait partie d'une serie definie par la geochimie de filons-couches et de roches volcaniques intraplaques du Groupe de Rockville Notch, dont les ages vont jusqu'au Devonien precoce. L'etude paleomagnetique de 13 sites repartis le long des flancs du synclinal neoacadien (vers 390 Ma) du filon-couche de Mavillette revele des directions de magnetisation qui echouent une epreuve de plissement et doivent donc etre posterieures a la mise en place du gabbro au Silurien. La remanence post-plissement a une direction moyenne de D = 153,4, I = 17,1[degrees]; [[alpha].sub.95] = 6,5[degrees] (n = 12 sites), avec un paleopole correspondant a 31,9[degrees]S, 325,2E; dp = 3,5[degrees], dm = 6,7[degrees] qui ressemble a une magnetisation penetrante superposee de Kiaman, d'age carbonifere tardif, en Amerique du Nord, mais caracterisee par une rotation antihoraire (AH) significative de 22,2[degrees] 6 8,1[degrees]. L'acquisition de la remanence de Mavillette s'est produite de concert avec la mobilisation de fluides associee a la deformation alleghanienne, dont temoignent localement des ages [.sup.40][Ar.sup.-39]Ar sur muscovite d'environ 320 Ma. Des resultats d'analyses paleomagnetiques publies precedemment pour le terrane de Meguma montrent aussi des directions de remanence carboniferes presentant une discordance semblable de ~24[degrees] AH. La rotation AH regionale du sud-ouest du terrane de Meguma est posterieure a cet episode d'acquisition de remanence tectonothermique, temoignant vraisemblablement du developpement d'une flexion oroclinale du terrane de Meguma durant l'orogenese alleghanienne. [Traduit par la Redaction] Mots-cles : magmatisme de rifting, accretion de terranes, rotation tectonique, Appalaches, magnetisation superposee, oroclinal de Meguma, associations fauniques.., Introduction The Meguma terrane is exposed in the southern half of mainland Nova Scotia (Fig. 1), Canada, and related rocks constituting a larger 'Megumia' (Waldron et al. 2011) also underlie [...]

Oceanic transform faults are seismically and tectonically active plate boundaries.sup.1 that leave scars--known as fracture zones--on oceanic plates that can cross entire ocean basins.sup.2. Current descriptions of plate tectonics assume transform faults to be conservative two-dimensional strike-slip boundaries.sup.1,3, at which lithosphere is neither created nor destroyed and along which the lithosphere cools and deepens as a function of the age of the plate.sup.4. However, a recent compilation of high-resolution multibeam bathymetric data from 41 oceanic transform faults and their associated fracture zones that covers all possible spreading rates shows that this assumption is incorrect. Here we show that the seafloor along transform faults is systemically deeper (by up to 1.6 kilometres) than their associated fracture zones, in contrast to expectations based on plate-cooling arguments. Accretion at intersections between oceanic ridges and transform faults seems to be strongly asymmetric: the outside corners of the intersections show shallower relief and more extensive magmatism, whereas the inside corners have deep nodal basins and seem to be magmatically starved. Three-dimensional viscoplastic numerical models show that plastic-shear failure within the deformation zone around the transform fault results in the plate boundary experiencing increasingly oblique shear at increasing depths below the seafloor. This results in extension around the inside corner, which thins the crust and lithosphere at the transform fault and is linked to deepening of the seafloor along the transform fault. Bathymetric data suggest that the thinned transform-fault crust is augmented by a second stage of magmatism as the transform fault intersects the opposing ridge axis. This makes accretion at transform-fault systems a two-stage process, fundamentally different from accretion elsewhere along mid-ocean ridges. Oceanic transform faults are systemically deeper than their associated fracture zones, owing to the plate boundary experiencing increasingly oblique shear at increasing depths below the seafloor., Author(s): Ingo Grevemeyer [sup.1] , Lars H. Rüpke [sup.1] , Jason P. Morgan [sup.2] , Karthik Iyer [sup.1] [sup.3] , Colin W. Devey [sup.1] Author Affiliations: (1) GEOMAR Helmholtz Centre [...]