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Balticasiberia Connection Challenges Traditional Tectonics Notions

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Balticasiberia Connection Challenges Traditional Tectonics Notions Eos, Vol. 75, No. 40, October 4, 1994 At the same time, drilling can contribute its increasing vulnerability to natural haz­ curred in an ocean-continent transition zone to a number of problems related to obtaining ards, and in finding solutions to problems that was marginal to Baltica but faced north­ a better understanding of the structure, evolu­ associated with remediation of existing envi­ ern Siberia rather than Laurentia. tion, and dynamics of the lithosphere. These ronmental damage and prevention of include problems related to petrochemical additional environmental degradation. At An Alternate View of Early and structural evolution of Earth's crust in­ the same time, Earth scientists are motivated Landmass Positions cluding lithospheric rheology and strength, by an inexorable need to better understand and the distribution of plate driving forces; planet Earth. This report contains the scien­ A reconciliation of the paleomagnetic, pa­ and heat and mass transfer through the litho­ tific rationale for establishment of an leoclimatic, and biogeographic data (Figure sphere and thermal structure of the crust and International Continental Drilling Program to 1) indicates that Avalonia and the European upper mantle. enable the international geoscientific com­ Massifs (Armorica) were located close to Gondwana in high southerly latitudes during There is a broad consensus that any ICDP munity to meet the challenges they face by Early Ordovician times. Laurentia was posi­ to be established should carry out fundamen­ complementing ongoing field, laboratory tioned in equatorial latitudes during most of tal science covering as broad a spectrum of and theoretical studies with critical observa­ the Ordovician. Conversely, Baltica was first Earth science research as possible. The pro­ tions. Without such observations substantive situated at intermediate southerly latitudes gram should be proposal-driven and address scientific progress on many problems of and drifted northward while undergoing rota­ key questions at optimal geological sites great importance will be essentially impossi­ tions [Torsviketal 1992], approximately 90° from around the world and should involve ble." counterclockwise from Early Ordovician to drilling and coring at a variety of depths. That Copies of this report can be obtained Middle Silurian times. Avalonia had rifted is, the program should not be restricted to from J. Lauterjung, GeoForschungsZentrum, away from Gondwana by Llanvirn time, only deep or shallow drilling, but drilling Telegrafenberg A17, D-14473 Potsdam, Ger­ while Armorica remained in high latitudes should be used as required to address the many; fax: 331-288-1002; e-mail along with Gondwana throughout the Ordovi­ specific problem of interest. [email protected].—Mark D. Zoback, Stan­ cian (Figure lb). TheTornquist Sea, Following the Potsdam meeting, scientific ford University, Stanford, Calif; and Rolf separating Avalonia and Baltica, narrowed managers from many of the participating Emmermann, GeoForschungZentrum, gradually during the Ordovician, as indi­ countries traveled to Windischeschenbach, Potsdam, Germany cated by the increasing faunal similarity, the site of the ongoing German deep drilling whereas faunal bonds between Avalonia and project (KTB), to consider the establishment Gondwana declined. of an International Program of Continental By the end of the Ordovician, the Torn­ Scientific Drilling. The communique issued quist Sea between Avalonia and Baltica had at this meeting contained the following Baltica-Siberia closed sufficiently to form Balonia (Figure points: Connection Challenges lc). Finally, by Early-Middle Silurian times • There is an essential role for continental scien­ (-425 Ma), Balonia collided with Laurentia, tific drilling in the solid Earth sciences. Scientific causing the early stages of the Scandian Oro­ drilling should be an integral part of modern Traditional Tectonics Earth sciences to obtain critical data and test hy­ geny (Figure Id) and closing the intervening potheses in a broad spectrum of scientific disci­ Notions part of the Iapetus Ocean. plines. PAGES 461, 462 New View Counters Long-Held Ideas • Full realization of that role requires a compre­ Despite the large amount of paleomag- hensive international program. Problems to be netic, paleoclimatic, biogeographic, and The overturned paleo-orientation of Bal­ addressed are global in nature and in impor­ tance to society. The international geoscientific tectonic data available, uncertainty still sur­ tica during Early Ordovician time (Figures la community needs to use drilling to deal with rounds the paleotectonic evolution of Siberia and lb) casts doubt on the traditional con­ problems that span the full range of solid Earth and its proximity to other continents during cept of orthogonal relationships of Baltica science disciplines throughout the world. Such the Ordovician to Silurian periods. Much of and Laurentia across a single Iapetus Ocean problems could best be addressed by estab­ lishing an international continental drilling this uncertainty arises from the fact that pa- throughout the entire period of Caledonide program. leomagnetic data cannot be adequately evolution. The Scandinavian Caledonides assessed. are essentially complexes that formed in loca­ • It is time to embark on an international conti­ nental drilling program. The time is right be­ However, recent Ordovician-Early Silu­ tions other than where they were found cause of the outstanding scientific and technical rian paleomagnetic poles from the southern [Roberts and Gee, 1985]. Their current con­ achievements of the KTB project, widespread rec­ Siberian Platform (Lena River) show that Si­ figuration is the result of Mid-Late Silurian ognition of the importance of scientific drilling beria was geographically inverted at low collisional orogeny (Scandian) and sub­ in Earth science, and the recent endorsement of coordinated international scientific drilling activi­ southerly latitudes during the Early Ordovi­ sequent extensional events. ties by the Organization of Economic Coopera­ cian (Figure la). The Siberian plate then The lower nappes—sheetlike bodies of tion and Development. Also, political changes drifted slowly northward and across the equa­ solid rock that have moved long distances at in Eastern Europe offer many new opportunities tor at an average paleolatitudinal velocity of low angles over the the underlying rocks— for scientific collaboration, and developing coun­ 1 tries around the world need assistance in Earth -5-8 cm yr" . During Late Ordovician-Early Si­ consist of rock types indigenous to Baltica science training. lurian time, its velocity increased northward. and its miogeoclinal margin. The middle, or A final meeting report, "Scientific Ration­ In view of the paleomagnetic data from Si­ Seve Nappe Complex, contains essentially ale for Establishment of an International beria, coupled with recent paleomagnetic miogeoclinal rocks, but at high metamorphic Program of Continental Scientific Drilling," data and geological evidence from Baltica grade it includes Late Cambrian/Early Ordovi­ was drafted by the leaders of the thematic [Torsvik etai, 1992], we propose that Siberia cian eclogites [MorketaL, 1988]. Eclogiteis groups. The following is a quote from the re­ and Baltica formed conjugate margins dur­ granular metamorphic rock composed essen­ port: "We live in a geologically complex ing Latest Cambrian to Early-Middle tially of garnet and pyroxene, but with the world which presents the international geos­ Ordovician time (Figures la and lb). In this bulk of its composition very similar to that of cientific community with tremendous reconstruction, subduction-related changes basalt. The higher nappes are marked by challenges. Earth scientists must play a key to mineral assemblages of basaltic composi­ ophiolites—mafic and ultramafic igneous role in satisfying society's ever-increasing de­ tion in Latest Cambrian/Early Ordovician rocks—and island arc sequences of essentially pendence on natural resources, in reducing time in the Scandinavian Caledonides oc­ Ordovician age [Sturt and Roberts, 1991]. This page may be freely copied. Eos, Vol. 75, No. 40, October 4, 1994 Early Caledonian eclogitization, in Tre- madoc time, correlates with westward subduction of Baltic continental crust. This a) »=i®@[Mte b) ^Bf^WMk subduction event, with inferred arc develop­ ment locally, was closely followed by uplift and retrogression of the eclogites and plate accretion of Early Ordovician ophiolites [Sturt and Roberts, 1991]. At the highest tec- tonostratigraphic level, the heterogenous Uppermost Allochthon has been considered to be of probably Laurentian affinity. Evidence of a Paleo-Ocean Between Baltica and Siberia The paleo-orientations and the latitudinal drift-histories for Siberia and Baltica suggest the presence of a paleo-ocean between Bal­ tica and Siberia. This contrasts with earlier models that postulate Latest Cambrian to Early Ordovician subduction and Early Ordo­ vician plate accretion between Laurentia and Baltica (Iapetus Ocean). From Mid-Ordovician to Silurian times, Baltica (Balonia) gradually rotated counter­ clockwise. Such a rotation likely gave rise to a deep-seated, strike-slip fault regime in the narrowing oceanic tract between Baltica and Siberia (Figure lc), and especially between the obliquely converging plates of rotating Baltica and a southward-drifting Laurentia. This developing fault system may have influ­ enced and possibly controlled the generation
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