DOCSLIB.ORG

Crustal and Mantle Reflectors from Palaeoproterozoic Orogens and Their Relation to Arc-Continent Collisions

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Crustal and Mantle Reflectors from Palaeoproterozoic Orogens and Their Relation to Arc-Continent Collisions Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 Crustal and mantle reflectors from Palaeoproterozoic orogens and their relation to arc-continent collisions D. B. SNYDER t, S. B. LUCAS 2 & J. H. McBRIDE 1 1 BIRPS, Bullard Laboratories, Madingley Road, Cambridge CB30EZ, UK 2 Geological Survey Canada, 601 Booth Street, Ottawa, Ontario KIA OE8, Canada Abstract: Two prominent geological features characterize the Palaeoproterozoic orogenic belts of Laurentia and Baltica: (1) Archaean cratons form the stable footwall during early stages of convergence and throughout crustal collision; and (2) juvenile, predominantly arc-derived crust was accreted to Archaean cratons through arc-continent collision. Seismic reflection profiling over the Svecofennian (Baltic Shield), Lewisian (British & Irish Isles) and Trans-Hudson (Canadian Shield) segments of an arguably once continuous orogenic belt has provided geometries of reflectors throughout both Palaeoproterozoic and Archaean crustal blocks as well as laterally coherent mantle reflectors. Two BABEL deep seismic reflection profiles within the Baltic Shield revealed structures along an irregularly shaped boundary between the juvenile 2.0-1.8 Ga Svecofennian domain and an Archaean craton (Karelia Province). An important result of the survey is the approximately 100 km horizontal offset between the inferred mantle suture, or palaeo-subduction boundary, and the geochemically-mapped crustal suture between juvenile Proterozoic crust and the Archaean craton. On the other side of the Atlantic, LITHOPROBE reflection profiles across the Trans-Hudson Orogen in central Canada reveal that 1.92-1.83 Ga juvenile arc and oceanic terranes of the Reindeer Zone form an allochthonous carapace about an Archaean basement block. Unexpectedly, the juvenile allochthons dip beneath the bounding Superior and Hearne cratons, defining a crustal-scale culmination in the core of the Orogen. The British and Irish Isles form an important bridge between the Palaeoproterozoic orogens of Laurentia and those of Baltica. Twenty deep reflection profiles on the continental shelf north and west of Scotland have traced a mantle reflector for over 800 km along its reconstructed strike, which is at a high angle to younger structures. This feature dips away from the reconstructed cratonic nucleus of Laurentia and may trace the Palaeoproterozoic (1.7-1.9 Ga) suture between the Archaean Lewisian block and accreted juvenile crust represented by the c. 1.8 Ga Rhinns complex. A common theme that emerges from the integration of all these geological and seismic results is the role played by the Archaean lithosphere during arc-continent collision. In all cases studied, juvenile 1.9-1.8 Ga lithosphere was delaminated and its crustal flakes overrode the Archaean margins. This consistency reflects the relative strength and durability of Archaean crust/lithosphere, and suggests that large parts of the lithosphere underlying detached and flaked Palaeoproterozoic juvenile terranes, such as the Svecofennian or Reindeer Zone terranes, may be Archaean in age but modified during Proterozoic tectonism. Over the past decade, deep seismic reflection marine profiles (McGeary & Warner 1985; Flack profiles of Precambrian orogenic belts (e.g. Green et aL 1990; Lie & Husebye 1994; Calvert et aL et al. 1988; BABEL Working Group 1993; Nelson 1995). Many of these observations consist of single et al. 1993; Lewry et al. 1994) have revealed crust features recorded on a lone profile (e.g. Flack et al. and mantle reflectors that are geometrically similar 1990; Best 1991). However, a majority of the to structures at modem plate convergence zones mantle features were substantiated by their con- (e.g. Choukroune & ECORS Team 1989; Pfiffner sistent appearance on local grids or neighbouring et al. 1990). Whereas the pattem of cmstal reflec- parallel profiles (Flack & Warner 1990; BABEL tivity generally complements collisional archi- Working Group 1993; Baird et al. 1995). These tecture inferred from geologic studies (e.g. Ga~il & deep mantle reflectors occur predominantly Gorbatschev 1987; Lucas 1989; Lewry et al. 1990), beneath outcrops of Proterozoic or Archaean the mantle reflectors provide particularly important continental crust (e.g. Calvert et al. 1995), clues to the processes of Precambrian plate excluding those reflectors clearly associated with convergence and collision. Wadati-Benioff zones and active subduction (e.g. For over a decade unequivocal reflections from Davey & Stem 1990). Several of the best studied the uppermost mantle have been observed on deep mantle reflectors lie along the c. 1.7-2.0 Ga seismic reflection profiles, most commonly on boundaries between Archaean cratons and From Brewer, T. S. (ed.), 1996, Precambrian Crustal Evolution in the North Atlantic Region, Geological Society Special Publication No. 112, pp. 1-23. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 2 D.B. SNYDER ET AL. Palaeoproterozic terrains that occur around the data is that the crustal portions of Palaeoproterzoic present North Atlantic (Fig. 1). Palaeoproterozoic juvenile terranes appear to have been largely ages for both crust and mantle events in these areas detached from their lithospheric mantle during the are inferred from the continuity of reflectors with process of accretion to, or juxtaposition between, dated crustal structures (Lucas et al. 1994), the lack relatively rigid Archaean cratons. This result under- of younger major thermo-tectonic events (BABEL lines the importance of delamination and flake Working Group 1993; Lewry et al. 1994), or super- tectonics in crustal growth and recycling, via the position relationships in which reflectors associated mechanisms of accretion of juvenile crust and the with dated crustal deformations cut other, pre- subduction of mantle and possibly lower crust. sumably older, reflectors (Snyder & Flack 1990). Largely independent geological, geochrono- Terrain links and correlations logical and geochemical studies in the Baltic Shield (Ga~il & Gorbatschev 1987; Ohlander et al. 1993), Recent compilations of age and structural data have the British Isles (Muir et al. 1992), Greenland led to a reconstruction of Proterozoic Laurentia, (Kalsbeek et aL 1987, Bridgewater et aL 1990), and reuniting Archaean blocks and Proterozoic belts the Canadian Shield (Hoffman 1988; Bickford et al. observed in Labrador and Greenland (Van 1990; Lewry et al. 1990; St-Onge et al. 1992; Van Kranendonk et al. 1993). This reconstruction Kranendonk et al. 1993; Lucas et al. 1996; Stern requires only small translations to produce reason- et al. 1995a, b) have established that the interval able terrane fits. Attempts to make reconstructions 1.9-1.8 Ga was characterized by the generation of that include the Precambrian blocks of the British juvenile crust and the accretion of juvenile terranes Isles and the Baltic are substantially more difficult onto Archaean cratonic margins throughout and involve much larger translations that developed Laurentia and Baltica (Fig. 2). These Palaeo- over longer periods of time. At present, no general proterozoic belts formed an orogenic system on the agreement exists as to the fit between Laurentia scale of the Tethyan Alpine-Himalayan Orogen, and Baltica following the 1.8-1.7 Ga accretion/ and preserve remnants of the destroyed oceanic collision events (compare Wardle et al. 1986; realm in the form of ophiolites (Kontinen 1987; Gower 1990; Kalsbeek et al. 1993; Park 1994). Scott et al. 1991; St-Onge et al. 1992; Stern et al. Recent advances in quantifying the palaeo- 1995b), oceanic island arcs (Syme 1990; Stern location of Baltica up to 600 Ma ago using palaeo- et al. 1995a) and accretionary collages (Park 1991; magnetic data have already produced some Lucas et al. 1996). As with the modern Alpine- surprising implications concerning Vendian Himalayan Orogen, the individual components of continental assemblages (Torsvik et al. 1992; the Precambrian Orogen are better understood than Soper 1994). The implied drift history for Baltica their linkage and interrelationships along-strike, indicates that an anticlockwise rotation of nearly although recent syntheses made significant 180 ~ occurred between 525-425 Ma ago, coeval advances (e.g. Patchett & Arndt 1986; Hoffman with a more general northward drift throughout the 1988; Park 1991; Muir et al. 1992; Van Palaeozoic (Torsvik et al. 1992; Personen et al. Kranendonk et al. 1993). 1991). This orientation enables the juxtaposition of Seismic reflector geometries along segments of a combined northern Scotland--eastern Greenland this proposed Proterozoic Orogen provide import- block and the Tornquist Zone region of central ant constraints on former convergence and collision Europe across a major rift zone in order to explain processes. To date, no attempt has been made extensive rift sequence sediments in these areas to compare the structure of Palaeoproterozoic (Soper 1994). orogenic segments as interpreted from deep seismic Reconstructions showing Laurentia and Baltica reflection data, or to examine similarities and at 1.8-1.7 Ga must be extrapolated from the differences in accretion and collision events Vendian (600 Ma) configuration using much between segments. In this paper, the principal inferior palaeomagnetic constraints. Most recon- results of seismic reflection profiling of the structions have therefore attempted to link regions Svecofennian Orogen, the Laxfordian deformation of Grenvillian and Sveco-norwegian
Load more

Recommended publications
  • Assessment of Undiscovered Conventionally Recoverable Petroleum Resources of the Northwest European Region
    U.S. GEOLOGICAL SURVEY CIRCULAR 922-A Assessment of Undiscovered Conventionally Recoverable Petroleum Resources of the Northwest European Region Assessment of Undiscovered Conventionally Recoverable Petroleum Resources of the North\Nest European Region By Charles D. Masters and H. Douglas Klemme U. S. G E 0 L 0 G I CAL SURVEY CIRCULAR 9 2 2- A Prepared in cooperation with Weeks Exploration Company under contract to the U.S. Geological Survey A resource assessment and a brief description of the petroleum geology, including play distribution, that accounts for the petroleum accumulation in the North Sea and adjoining areas 1984 Department of the Interior WILLIAM P. CLARK, Secretary U.S. Geological Survey Dallas L. Peck, Director Free on application to Distribution Branch, Text Products Section, U. S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 CONTENTS Page Pref~--------------------------------------------------------------------- v Absttact-------------------------------------------------------------------- 1 Inbuduction ---------------------------------------------------------------- 1 Regional geology ------------------------------------------------------------- 2 Pettoleum geology ------------------------------------------------------------ 6 Resource assessment---------------------------------------------------------- 14 COmmen~ ------------------------------------------------------------------ 15 References cited -------------------------------------------------------------- 22 ILLUSTRATIONS Page FIGURE 1. Map
    [Show full text]
  • GY 112 Lecture Notes D
    GY 112 Lecture Notes D. Haywick (2006) 1 GY 112 Lecture Notes Archean Geology Lecture Goals: A) Time frame (the Archean and earlier) B) Rocks and tectonic elements (shield/platform/craton) C) Tectonics and paleogeography Textbook reference: Levin 7th edition (2003), Chapter 6; Levin 8th edition (2006), Chapter 8 A) Time frame Up until comparatively recently (start of the 20th century), most geologists focused their discussion of geological time on two eons; the “Precambrian” and the Phanerozoic. We now officially recognize 4 eons and the Precambrian is just used as a come-all term for all time before the Phanerozoic: Eon Time Phanerozoic 550 MA to 0 MA Proterozoic 2.5 GA to 550 MA Archean 3.96 GA to 2.5 GA Hadean 4.6 GA to 3.96 GA The “youngest” of these two new eons, the Archean, was first introduced by field geologists working in areas where very old rocks cropped out. One of these geologists was Sir William Logan (pictured at left; ess.nrcan.gc.ca ) one of the most respected geologists working with the Geological Survey of Canada. His study area was the Canadian Shield. Logan was able to identify two major types of rocks; 1) layered sedimentary and volcanic rocks and 2) highly metamorphosed granitic gneisses (see image at the top of the next page from http://www.trailcanada.com/images/canadian-shield.jpg). Logan found evidence that the gneisses mostly underlayed the layered rocks and hence, they had to be older than the layered rocks. The layered rocks were known to be Proterozoic in age.
    [Show full text]
  • Periodicity of Rayleigh Phase Velocities Observed in Scandinavia
    Originally published as: Mauerberger, A., Maupin, V., Gudmundsson, Ó., Tilmann, F. (2021): Anomalous azimuthal variations with 360° periodicity of Rayleigh phase velocities observed in Scandinavia. - Geophysical Journal International, 224, 3, 1684-1704. https://doi.org/10.1093/gji/ggaa553 Institutional Repository GFZpublic: https://gfzpublic.gfz‐potsdam.de/ Geophys. J. Int. (2021) 224, 1684–1704 doi: 10.1093/gji/ggaa553 Advance Access publication 2020 November 13 0GJI Seismology Anomalous azimuthal variations with 360◦ periodicity of Rayleigh phase velocities observed in Scandinavia Downloaded from https://academic.oup.com/gji/article/224/3/1684/6012847 by Bibliothek des Wissenschaftsparks Albert Einstein user on 12 February 2021 Alexandra Mauerberger ,1,2 Valerie´ Maupin ,3 Olafur´ Gudmundsson4 and Frederik Tilmann 1,2 1GFZ German Research Center for Geosciences, Geophysics, Telegrafenberg, 14473 Potsdam, Germany. E-mail: [email protected] 2Freie Universitat¨ Berlin, Institute of Geological Sciences, 12249 Berlin, Germany 3University of Oslo, CEED, 0371 Oslo, Norway 4 Uppsala University, Geocentrum, 752 36 Uppsala, Sweden Accepted 2020 November 13. Received 2020 November 6; in original form 2020 February 27 SUMMARY We use the recently deployed ScanArray network of broad-band stations covering most of Norway and Sweden as well as parts of Finland to analyse the propagation of Rayleigh waves in Scandinavia. Applying an array beamforming technique to teleseismic records from ScanArray and permanent stations in the study region, in total 159 stations with a typical station distance of about 70 km, we obtain phase velocities for three subregions, which collectively cover most of Scandinavia (excluding southern Norway). The average phase dispersion curves are similar for all three subregions.
    [Show full text]
  • Tectonic Regimes in the Baltic Shield During the Last 1200 Ma • a Review
    Tectonic regimes in the Baltic Shield during the last 1200 Ma • A review Sven Åke Larsson ' ', Bva-L^na Tuliborq- 1 Department of Geology Chalmers University of Technology/Göteborij U^vjrsivy 2 Terralogica AB November 1993 TECTONIC REGIMES IN THE BALTIC SHIELD DURING THE LAST 1200 Ma - A REVIEW Sven Åke Larsson12, Eva-Lena Tullborg2 1 Department of Geology, Chalmers University of Technology/Göteborg University 2 Terralogica AB November 1993 This report concerns a study which was conducted for SKB. The conclusions and viewpoints presented in the report are those of the author(s) and do not necessarily coincide with those of the client. Information on SKB technical reports from 1977-1978 (TR 121), 1979 (TR 79-28), 1980 (TR 80-26), 1981 (TR 81-17), 1982 (TR 82-28), 1983 (TR 83-77), 1984 (TR 85-01), 1985 (TR 85-20), 1986 (TR 86-31), 1987 (TR 87-33), 1988 (TR 88-32),. 1989 (TR 89-40), 1990 (TR 90-46), 1991 (TR 91-64) and 1992 (TR 92-46) is available through SKB. ) TECTONIC REGIMES IN THE BALTIC SHIELD DURING THE LAST 1200 Ma - A REVIEW by Sven Åke Larson and Eva-Lena Tullborg Department of Geology, Chalmers University of Technology / Göteborg University & Terralogica AB Gråbo, November, 1993 Keywords: Baltic shield, Tectonicregimes. Upper Protero/.oic, Phanerozoic, Mag- matism. Sedimentation. Erosion. Metamorphism, Continental drift. Stress regimes. , ABSTRACT 1 his report is a review about tectonic regimes in the Baltic (Fennoscandian) Shield from the Sveeonorwegian (1.2 Ga ago) to the present. It also covers what is known about palaeostress during this period, which was chosen to include both orogenic and anorogenic events.
    [Show full text]
  • South and North Barents Triassic-Jurassic Total Petroleum System of the Russian Offshore Arctic
    U. S. Department of the Interior U. S. Geological Survey South and North Barents Triassic-Jurassic Total Petroleum System of the Russian Offshore Arctic Paper Edition by Sandra J. Lindquist1 Open-File Report 99-50-N This report is preliminary and has not been reviewed for conformity with the U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 1999 1 Consulting Geologist, Contractor to U. S. Geological Survey, Denver, Colorado Page 1 of 16 South and North Barents Triassic-Jurassic Total Petroleum System of the Russian Offshore Arctic2 Sandra J. Lindquist, Consulting Geologist Contractor to the U.S. Geological Survey, Denver, CO October, 1999 FOREWORD This report was prepared as part of the World Energy Project of the U.S. Geological Survey. In the project, the world was divided into 8 regions and 937 geologic provinces. The provinces have been ranked according to the discovered oil and gas volumes within each (Klett and others, 1997). Then, 76 "priority" provinces (exclusive of the U.S. and chosen for their high ranking) and 26 "boutique" provinces (exclusive of the U.S. and chosen for their anticipated petroleum richness or special regional economic importance) were selected for appraisal of oil and gas resources. The petroleum geology of these priority and boutique provinces is described in this series of reports. A detailed report containing the assessment results will be available separately, if such results are not reported herein. The priority South Barents Basin Province ranks 35th in the world, exclusive of the U.S.
    [Show full text]
  • Rutile Mineral Chemistry and Zr-In-Rutile Thermometry In
    minerals Article Rutile Mineral Chemistry and Zr-in-Rutile Thermometry in Provenance Study of Albian (Uppermost Lower Cretaceous) Terrigenous Quartz Sands and Sandstones in Southern Extra-Carpathian Poland Jakub Kotowski * , Krzysztof Nejbert and Danuta Olszewska-Nejbert Faculty of Geology, University of Warsaw, Zwirki˙ i Wigury 93, 02-089 Warszawa, Poland; [email protected] (K.N.); [email protected] (D.O.-N.) * Correspondence: [email protected] Abstract: The geochemistry of detrital rutile grains, which are extremely resistant to weathering, was used in a provenance study of the transgressive Albian quartz sands in the southern part of extra-Carpathian Poland. Rutile grains were sampled from eight outcrops and four boreholes located on the Miechów, Szydłowiec, and Puławy Segments. The crystallization temperatures of the rutile grains, calculated using a Zr-in-rutile geothermometer, allowed for the division of the study area into three parts: western, central, and eastern. The western group of samples, located in the Citation: Kotowski, J.; Nejbert, K.; Miechów Segment, is characterized by a polymodal distribution of rutile crystallization temperatures ◦ ◦ ◦ Olszewska-Nejbert, D. Rutile Mineral (700–800 C; 550–600 C, and c. 900 C) with a significant predominance of high-temperature forms, Chemistry and Zr-in-Rutile and with a clear prevalence of metapelitic over metamafic rutile. The eastern group of samples, Thermometry in Provenance Study of corresponding to the Lublin Area, is monomodal and their crystallization temperatures peak at Albian (Uppermost Lower 550–600 ◦C. The contents of metapelitic to metamafic rutile in the study area are comparable. The Cretaceous) Terrigenous Quartz central group of rutile samples with bimodal distribution (550–600 ◦C and 850–950 ◦C) most likely Sands and Sandstones in Southern represents a mixing zone, with a visible influence from the western and, to a lesser extent, the eastern Extra-Carpathian Poland.
    [Show full text]
  • Precambrian Plate Tectonics: Criteria and Evidence
    VOL.. 16,16, No.No. 77 A PublicAtioN of the GeoloGicicAl Society of America JulyJuly 20062006 Precambrian Plate Tectonics: Criteria and Evidence Inside: 2006 Medal and Award Recipients, p. 12 2006 GSA Fellows Elected, p. 13 2006 GSA Research Grant Recipients, p. 18 Call for Geological Papers, 2007 Section Meetings, p. 30 Volume 16, Number 7 July 2006 cover: Magnetic anomaly map of part of Western Australia, showing crustal blocks of different age and distinct structural trends, juxtaposed against one another across major structural deformation zones. All of the features on this map are GSA TODAY publishes news and information for more than Precambrian in age and demonstrate that plate tectonics 20,000 GSA members and subscribing libraries. GSA Today was in operation in the Precambrian. Image copyright the lead science articles should present the results of exciting new government of Western Australia. Compiled by Geoscience research or summarize and synthesize important problems Australia, image processing by J. Watt, 2006, Geological or issues, and they must be understandable to all in the earth science community. Submit manuscripts to science Survey of Western Australia. See “Precambrian plate tectonics: editors Keith A. Howard, [email protected], or Gerald M. Criteria and evidence” by Peter A. Cawood, Alfred Kröner, Ross, [email protected]. and Sergei Pisarevsky, p. 4–11. GSA TODAY (ISSN 1052-5173 USPS 0456-530) is published 11 times per year, monthly, with a combined April/May issue, by The Geological Society of America, Inc., with offices at 3300 Penrose Place, Boulder, Colorado. Mailing address: P.O. Box 9140, Boulder, CO 80301-9140, USA.
    [Show full text]
  • Thermochronology and Exhumation History of The
    Thermochronology and Exhumation History of the Northeastern Fennoscandian Shield Since 1.9 Ga: Evidence From 40 Ar/ 39 Ar and Apatite Fission Track Data From the Kola Peninsula Item Type Article Authors Veselovskiy, Roman V.; Thomson, Stuart N.; Arzamastsev, Andrey A.; Botsyun, Svetlana; Travin, Aleksey V.; Yudin, Denis S.; Samsonov, Alexander V.; Stepanova, Alexandra V. Citation Veselovskiy, R. V., Thomson, S. N., Arzamastsev, A. A., Botsyun, S., Travin, A. V., Yudin, D. S., et al. (2019). Thermochronology and exhumation history of the northeastern Fennoscandian Shield since 1.9 Ga:evidence from 40Ar/39Ar and apatite fission track data from the Kola Peninsula. Tectonics, 38, 2317–2337.https:// doi.org/10.1029/2018TC005250 DOI 10.1029/2018tc005250 Publisher AMER GEOPHYSICAL UNION Journal TECTONICS Rights Copyright © 2019. American Geophysical Union. All Rights Reserved. Download date 01/10/2021 04:51:21 Item License http://rightsstatements.org/vocab/InC/1.0/ Version Final published version Link to Item http://hdl.handle.net/10150/634481 RESEARCH ARTICLE Thermochronology and Exhumation History of the 10.1029/2018TC005250 Northeastern Fennoscandian Shield Since 1.9 Ga: Key Points: 40 39 • Since 1.9 Ga, the NE Fennoscandia Evidence From Ar/ Ar and Apatite Fission was characterized by a slow exhumation (1‐2 m/Myr) Track Data From the Kola Peninsula • Total denudation of the NE Roman V. Veselovskiy1,2 , Stuart N. Thomson3 , Andrey A. Arzamastsev4,5 , Fennoscandia since 1.9 Ga did not 6 7,8 7,8 9 exceed ~3‐5km Svetlana Botsyun , Aleksey V. Travin , Denis S. Yudin , Alexander V. Samsonov , • The Kola part of Fennoscandia and Alexandra V.
    [Show full text]
  • Crustal Conductivity in Fennoscandia—A Compilation of a Database on Crustal Conductance in the Fennoscandian Shield
    Earth Planets Space, 54, 535–558, 2002 Crustal conductivity in Fennoscandia—a compilation of a database on crustal conductance in the Fennoscandian Shield Toivo Korja1, Martin Engels3, Abdoulkhay A. Zhamaletdinov2, Aida A. Kovtun4, Nikolai A. Palshin5, Maxim Yu. Smirnov4, Alexander D. Tokarev2, Vladimir E. Asming2, Leonid L. Vanyan5†, Isabella L. Vardaniants4, and the BEAR Working Group 1Academy of Finland/University of Oulu/Geological Survey of Finland, POB 96 FIN-02151, Espoo, Finland 2Russian Academy of Sciences, Kola Science Centre, Geological Institute, Apatity RUS-184200, Russia 3Uppsala University, Department of Earth Sciences, Geophysics, Uppsala SE-75236, Sweden 4Institute of Physics, St. Petersburg University, St. Petersburg RUS-198904, Russia 5Russian Academy of Sciences, Shirsov Institute of Oceanology, Moscow RUS-117218, Russia (Received February 19, 2001; Revised October 4, 2001; Accepted December 5, 2001) A priori knowledge on large-scale sub-surface conductivity structure is required in many applications investi- gating electrical properties of the lithosphere. A map on crustal conductivity for the Fennoscandian Shield and its surrounding oceans, sea basins and continental areas is presented. The map is based on a new database on crustal conductance, i.e. depth integrated conductivity, where all available information on the conductivity of the bedrock, sedimentary cover and seawater are compiled together for the first time for the Fennoscandian Shield. The final model consists of eight separate layers to allow a 3D description of conductivity structures. The first three layers, viz. water, sediments and the first bedrock layer, describe the combined conductance of the uppermost 10 km. The other five bedrock layers contain the data of the crustal conductance from the depth of 10 km to the depth of 60 km.
    [Show full text]
  • Mid-Proterozoic Magmatic Arc Evolution at the Southwest Margin of the Baltic Shield$
    Lithos 73 (2004) 289–318 www.elsevier.com/locate/lithos Mid-Proterozoic magmatic arc evolution at the southwest margin of the Baltic Shield$ T. Andersena,b,*, W.L. Griffinb, S.E. Jacksonb, T.-L. Knudsenc, N.J. Pearsonb a Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway b GEMOC Key Centre, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia c Geological Museum, P.O. Box 1172 Blindern, N-0318 Oslo, Norway Received 29 January 2003; accepted 9 December 2003 Abstract Mid-Proterozoic calc-alkaline granitoids from southern Norway, and their extrusive equivalents have been dated by LAM- ICPMS U–Pb on zircons to ages ranging from 1.61 to 1.52 Ga; there are no systematic age differences across potential Precambrian terrane boundaries in the region. U–Pb and Lu–Hf data on detrital zircons from metasedimentary gneisses belonging to the arc association show that these were mainly derived from ca. 1.6 Ga arc-related rocks. They also contain a minor but significant fraction of material derived from (at least) two distinct older (1.7–1.8 Ga) sources; one has a clear continental signature, and the other represents juvenile, depleted mantle-derived material. The former component resided in granitoids of the Transscandinavian Igneous Belt, the other in mafic rocks related to these granites or to the earliest, subduction-related magmatism in the region. Together with published data from south Norway and southwest Sweden, these findings suggest that the western margin of the Baltic Shield was the site of continuous magmatic arc evolution from at least ca 1.66 to 1.50 Ga.
    [Show full text]
  • The Fennoscandian Shield Within Fennoscandia
    SCIENTIFIC CORRESPONDENCE THE FENNOSCANDIAN SHIELD WITHIN FENNOSCANDIA JOAKIM DONNER DONNER, JOAKIM, 1996. The Fennoscandian Shield within Fennoscandia. Bull. Geol. Soc. Finland 68, Part 1, 99-103. Keywords: nomenclature, Baltic Shield, Fennoscandian Shield, Precambrian, Fennoscandia. Joakim Donner, Department of Geology, P.O. Box 11, FIN-00014 University of Helsinki, Finland The Commission for the Geological Map of the Platform (Russian Platform), as seen, for in- World, CGMW, recommended in a resolution at stance, on the maps presented by Holmes (1944, its General Assembly in Kyoto, 24-28 August 1965). It is for this area (Fig. 1), geologically 1992, "that the name Baltic Shield should be similar to the Canadian Shield, that the name replaced by the name Fennoscandian Shield". Fennoscandian Shield should, according to the This was taken as an international acceptance of above-mentioned recommendation, be used. the use of this name (Vorma 1993), earlier rec- The wish to replace the name Baltic Shield by ommended both by the unofficial Finnish work- the the name Fennoscandian Shield stems fore- ing group of bedrock stratigraphy (Aro 1986, most from a linguistic confusion, as pointed out by 1988) and by the Geological Survey of Finland Saltikoff (1992). The name Baltic Shield, as it has (Marttila 1988), and mentioned as an alternative been used, refers to the Baltic (Sea), the English name already by Simonen (1980). The area of name corresponding to Mare balticum in Latin and the shield with exposed Precambrian rocks, al- la Baltique in French, as also to the name in Rus- though largely covered by a sheet of Quaternary sian, Baltiiskoye morye.
    [Show full text]
  • Seismic Structure of the Crust and Uppermost Mantle of South America and Surrounding Oceanic Basins
    Journal of South American Earth Sciences 42 (2013) 260e276 Contents lists available at SciVerse ScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames Seismic structure of the crust and uppermost mantle of South America and surrounding oceanic basins Gary S. Chulick a, Shane Detweiler b,*, Walter D. Mooney b a Mt. Aloysius College, 7373 Admiral Peary Hwy, Cresson, PA 16630, USA b United States Geological Survey, MS 977, 345 Middlefield Road, Menlo Park, CA 94025, USA article info abstract Article history: We present a new set of contour maps of the seismic structure of South America and the surrounding Received 19 July 2011 ocean basins. These maps include new data, helping to constrain crustal thickness, whole-crustal average Accepted 5 June 2012 P-wave and S-wave velocity, and the seismic velocity of the uppermost mantle (Pn and Sn). We find that: (1) The weighted average thickness of the crust under South America is 38.17 km (standard deviation, Keywords: s.d. Æ8.7 km), which is w1 km thinner than the global average of 39.2 km (s.d. Æ8.5 km) for continental Crustal structure crust. (2) Histograms of whole-crustal P-wave velocities for the South American crust are bi-modal, with Seismic velocity the lower peak occurring for crust that appears to be missing a high-velocity (6.9e7.3 km/s) lower crustal South America layer. (3) The average P-wave velocity of the crystalline crust (Pcc) is 6.47 km/s (s.d. Æ0.25 km/s). This is essentially identical to the global average of 6.45 km/s.
    [Show full text]