New Late Vendian palaeogeography of Baltica and the TESZ
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Abstract:
New palaeomagnetic poles obtained from the Vendian tuffs and basalts of western Ukraine indicate the necessity of a substantial revision of the Late Vendian-Early Cambrian palaeogeography of the Baltic plate. The palaeopole calculated for the most stable component isolated from the Vendian tuffs and basalts is far away from the Vendian-Cambrian apparent polar wander path (APWP), constructed on the basis of Scandinavian poles but is very close to the pole recently isolated from the Vendian sediments of the White Sea Region. Depending on the polarity of the newly-determined Late Vendian pole, two palaeogeographic models of the Baltic plate in the Late Vendian-Early Cambrian are possible. In our preferred model the Baltic plate moved at that time from the moderate southern latitudes to the equator rotating anticlockwise of ca. 120o . This reconstruction explains the geological structures of the marginal zones of Baltica better than the previously proposed stationary model of the Late Vendian-Cambrian Baltica. According to the new late Vendian palaeogeographic scenario, the European, passive margin of Baltica was separated from an active, Avalonian margin of Gondwana. The Late Neoproterozoic tectonic structures of the Brunovistulian Terrane and the Malopolska Block were developed near the present day southwestern corner of Baltica that was tectonically active at that time. Alternative reconstruction shows the Baltic plate moving from the moderate northern latitudes in the Vendian, crossing palaeoequator in the latest Vendian, and reaching moderate southern palaeolatitudes in the Late Cambrian. This model, however, would have required exceptionally high plate velocity (ca. 33 cm/year).Keywords:
Baltica
Palaeogeography
Apparent polar wander
Baltic Shield
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The Vendian/Cambrian segment of the Lauretian apparent polar wander path (APWP) has been poorly constrained and the subject of some controversy. The Catoctin volcanic province in central Virginia is well‐dated at 570±35 Ma (Rb‐Sr) and 597±18 Ma (U‐Pb) and therefore presented an excellent paleomagnetic target for resolving the Laurentian Vendian‐Cambrian APWP. A total of 206 samples from 32 sites were collected from the Catoctin basalts, feeder dikes and sills. The study revealed three ancient directions of magnetization. The youngest, C component, fails the fold test and yielded a characteristic in situ direction of D = 147°, I = +44° (k = 21, α95 = 9°). The corresponding paleopole falls along the Middle Ordovician segment of the Laurentian APWP and we consider this component to be the result of a Taconic remagnetization. The second component, the B component, is carried by hematite, exhibits dual‐polarities and passes a fold test. The tilt‐corrected B component characteristic direction is D = 92°, I = +17° (k = 16, α95 = 13°). The corresponding paleopole at 4°S, 193°E falls near a well‐established Late Cambrian (505 Ma) pole for Laurentia, and we consider this component to be a remagnetization during a Late Cambrian tectonic event in the central Appalachians. The third component isolated in the Catoctin basalts, the A component, yields a tilt‐corrected mean of D = 68°, I = +84° (k = 59, α95 = 9°). This component passes a fold and reversal test. A suite of samples was collected from two Catoctin feeder dikes and surrounding country rocks that yield a positive baked contact test. The A pole at 43°S, 128°E falls significantly away from previously proposed Vendian poles for Laurentia. A reevaluation of previous paleomagnetic studies from coeval rock units reveals similarly steep directions and leads us to propose a new APWP. This new APW track indicates that Laurentia was located near the pole during the interval 615–580 Ma and drifted rapidly (16 cm yr −1 ) toward its Late Cambrian equatorial position.
Apparent polar wander
Dike
Baltica
Laurentia
Sill
Polar wander
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Analysis of Vendian to Cambrian paleomagnetic data shows anomalously fast rotations and latitudinal drift for all of the major continents. These motions are consistent with an Early to Middle Cambrian inertial interchange true polar wander event, during which Earth's lithosphere and mantle rotated about 90 degrees in response to an unstable distribution of the planet's moment of inertia. The proposed event produces a longitudinally constrained Cambrian paleogeography and accounts for rapid rates of continental motion during that time.
Palaeogeography
Continental drift
Moment of inertia
Polar wander
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A palaeomagnetic study and age determinations have been performed on Ediacaran basalts from the northwestern Ukraine. Whole-rock 40 Ar/ 39 Ar age determination revealed plateau ages at 590–560 Ma and 393 Ma, the latter probably reflecting a resetting of the radiometric system. Palaeomagnetic poles have been calculated from five basalt flows, two of which (A poles) are considered reliable with ages that range from 580 to 560 Ma. Tentative poles (B poles), calculated from most probably primary magnetizations, have ages estimated at 580–545 Ma. Secondary magnetizations, possibly of late Ediacaran or Devonian age, have also been isolated (C poles). Based on the new poles, Baltica drifted together with Laurentia from an equatorial position at c . 750 Ma to occupy high southern latitude positions at c . 580 Ma. Baltica during that time period was joined to Laurentia in a similar relative position to that at 750 Ma. The two shields then split up from each other and from c . 550 Ma Baltica drifted at moderately high latitudes and rotated some 180° during the final opening of the Iapetus ocean. This reconstruction suggests that during the Ediacaran glaciation Baltica occupied high-latitude positions, which contradicts the high-obliquity model to explain low-latitude Neoproterozoic glaciations.
Baltica
Laurentia
Rodinia
Radiometric dating
Devonian
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The classic, multivariate technique of principal component analysis can be used to find and estimate the directions of lines and planes of best least-squares fit along the demagnetization path of a palaeomagnetic specimen, thereby replacing vector subtraction, remagnetization circles and difference vector paths with one procedure. Eigenvalues from the analysis are the variance of the data along each principal axis, and provide a relative measure of collinearity or coplanarity which may be used to define a general palaeomagnetic precision index. Demagnetization planes found with principal component analysis may be used in place of difference vector paths for locating Hoffman—Day directions, avoiding unnecessary vector subtraction and intensity truncation steps. Two methods are discussed for jointly estimating an average remanence direction from demagnetization lines and planes.
Line (geometry)
Subtraction
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Paleomagnetic results from an Upper Vendian sedimentary sequence exposed along the White Sea shoreline, NW Russia are described. These classical exposures have been the subject of intense paleontological investigations due to their well‐preserved Ediacara fauna, but no paleomagnetic results have as yet been published. A total of 337 hand samples and 210 oriented drill cores (35 sites) along three profiles have been collected at the locality (65.5°, 40.0°E) where a 555 ± 3 Ma U–Pb age of comagmatic zircons from volcanic ash layers has been recently obtained. Standard paleomagnetic procedures yield two main natural remanent magnetization (NRM) components: an intermediate‐temperature (150°–350°C), single‐polarity component (D = 121°, I = 72°, n = 232 samples, k = 46.0, α 95 = 1.3°, pole position at 40.0°N, 79.0°E, dp = 2.0°, dm = 2.3°) and a high‐temperature (550°–680°C) dual‐polarity component (normal polarity: D = 278°, I = 43°, n = 54 samples, k = 25.2, α 95 = 3.9°, reversed polarity: D = 101°, I = −39° n = 40, K = 23.3, α 95 = 4.8°, south pole position at 24°S, 132°E, dp = 2.3°, dm = 3.8°). This latter component, termed Z, passes reversal, stratigraphic, and consistency tests and is interpreted to reflect the direction of the Earth's magnetic field during Late Vendian times. These results put Baltica into low northern latitudes (between 10° and 35°) and the resulting pole position requires modification of the most recent Apparent Polar Wander Paths (APWP) for Baltica.
Baltica
Apparent polar wander
Polarity (international relations)
Magnetostratigraphy
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U–Pb age determinations by ion microprobe on zircons from two tuff samples within the Neoproterozoic–early Cambrian successions in Poland are presented. One sample, from the Kaplonosy borehole is within, or conformably below, rocks that contain the Sabellidites–Vendotaenia fossil assemblage of the Upper Vendian. The second sample is from the Ksi¹¿ Wielki IG-l borehole, from rocks that were referred alternatively to the early Cambrian or to the Vendian on lithostratigraphic evidence. The Kaplonosy zircons are euhedral and free of visible zircon cores, both optically and as back-scattered electron images, but they exhibit a range in 206 Pb/ 238 U ages that exceeds analytical error. The combined data-set can be resolved into three age-components in different proportions, which overlap in apparent age due to measurement errors. There is a well-defined principal age component at 551 ± 4 Ma (95% limits) and two minor detrital or inherited components at 588 ±8 Ma and 635 ± 10 Ma. The age of the Kaplonosy tuff is interpreted as equal to that of the youngest and principal component, 551 Ma. This age allows a maximum time difference of 17 ± 4 Ma between the top of the Stawatycze Formation and Lower Cambrian strata of the Heliosphaeridium dissimilare-Skiagia ciliosa acritarch Zone, the latter being time-equivalent to Middle Tommotian strata in northeastern Siberia, recently dated as 534.6 ± 0.5 Ma. The tuff from the Ksi¹¿ Wielki IG-l borehole has a similar spectrum of zircon ages, but also contains several detrital grains that have concordant and separate early Proterozoic and Archaean ages. The age of the major component is 549 ± 3 Ma (72% of the totak), and there are two older components at 578 ± 9 Ma (13%) and 619 ±s8 Ma (15%). The deposition of the tuff from the Ksig Wielki Formation therefore occurred at 549 Ma, indicating that the Ksi¹¿ Wielki Formation is not early Cambrian but Upper Vendian, unless all zircons in the tuff are detrital.
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Position (finance)
Unit sphere
Unit vector
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Laurentia
Baltica
Rodinia
Supercontinent
Orogeny
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The ever-changing distribution of continents and ocean basins on Earth is fundamental to the environment of the planet. Recent ideas regarding pre-Pangea geography and tectonics offer fresh opportunities to examine possible causative relations between tectonics and environmental and biologic changes during the Neoproterozoic and Paleozoic eras. The starting point is an appreciation that Laurentia, the rift-bounded Precambrian core of North America, could have been juxtaposed with the cratonic cores of some present-day southern continents. This has led to reconstructions of Rodinia and Pannotia, supercontinents that may have existed in early and latest Neoproterozoic time, respectively, before and after the opening of the Pacific Ocean basin. Recognition that the Precordillera of northwest Argentina constitutes a terrane derived from Laurentia may provide critical longitudinal control on the relations of that craton to Gondwana during the Precambrian-Cambrian boundary transition, and in the early Paleozoic. The Precordillera was most likely derived from the general area of the Ouachita embayment, and may have been part of a hypothetical promontory of Laurentia, the “Texas plateau,” which was detached from the Cape of Good Hope embayment within Gondwana between the present-day Falkland-Malvinas Plateau and Transantarctic Mountains margins. Thus the American continents may represent geometric “twins” detached from the Pannotian and Pangean supercontinents in Early Cambrian and Early Cretaceous time, respectively—the new mid-ocean ridge crests of those times initiating the two environmental supercycles of Phanerozoic history 400 m.y. apart. In this scenario, the extremity of the Texas plateau was detached from Laurentia during the Caradocian Epoch, in a rift event ca. 455 Ma that followed Middle Ordovician collision with the proto-Andean margin of Gondwana as part of the complex Indonesian-style Taconic-Famatinian orogeny, which involved several island arc-continent collisions between the two major continental entities. Laurentia then continued its clockwise relative motion around the proto-Andean margin, colliding with other arc terranes, Avalonia, and Baltica en route to the Ouachita-Alleghanian-Hercynian-Uralian collision that completed the amalgamation of Pangea. The important change in single-celled organisms at the Mesoproterozoic-Neoproterozoic boundary (1000 Ma) accompanied assembly of Rodinia along Grenvillian sutures. Possible divergence of metazoan phyla, the appearance and disappearance of the Ediacaran fauna (ca. 650–545 Ma), and the Cambrian “explosion” of skeletalized metazoans (ca. 545–500 Ma) also appear to have taken place within the framework of tectonic change of truly global proportions. These are the opening of the Pacific Ocean basin; uplift and erosion of orogens within the newly assembled Gondwana portion of Pannotia, including a collisional mountain range extending ≈7500 km from Arabia to the Pacific margin of Antarctica; the development of a Pannotia-splitting oceanic spreading ridge system nearly 10 000 km long as Laurentia broke away from Gondwana, Baltica, and Siberia; and initiation of subduction zones along thousands of kilometres of the South American and Antarctic-Australian continental margins. The Middle Ordovician sea-level changes and biologic radiation broadly coincided with initiation of the Appalachian-Andean mountain system along >7000 km of the Taconic and Famatinian belts. These correlations, based on testable paleogeographic reconstructions, invite further speculation about possible causative relations between the internally driven long-term tectonic evolution of the planet, its surface environment, and life.
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A reliable Early Cambrian ( c. 535 Ma) and a preliminary Late Cambrian ( c. 500 Ma) palaeomagnetic pole from Baltica (Sweden) overlap within uncertainty, and they are also broadly compatible with Vendian ( c. 583 Ma) palaeomagnetic data. Apparent polar wander for Baltica amounts to less than 25° between 583 and 500 Ma and, therefore, negates recent speculations that the Earth tipped 90° during the Early Cambrian (true polar wander). Throughout Vendian and Cambrian times, Baltica lay at southerly latitudes ( c. 30–60°S). Baltica was geographically inverted, and present-day northern Baltica faced the NW margin of Gondwana which covered the south pole. Laurentia-Eastern Baltica and Laurentia–West Gondwana were separated by the Iapetus Ocean, while the Ægir Sea separated Western Baltica from the Taimyr region of Siberia. During the Cambrian Baltica probably moved eastward along the Gondwana margin, and by c. 515–520 Ma subduction in the Ægir Sea was initiated. A major event is recognized in Late Cambrian or Early Ordovician times ( c. 500–478 Ma) when Baltica must have undergone a 55° counter-clockwise rotation in c. 22 million years (3°/Ma). We relate this to the early Caledonian Finnmarkian Orogeny which involved arc–continent collision following subduction.
Baltica
Laurentia
Apparent polar wander
Orogeny
Palaeogeography
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