Polar Wander Path Research Articles

Towards Columbia: Paleomagnetism of 1980–1960 Ma Surumu volcanic rocks, Northern Amazonian Craton

We present a paleomagnetic study on the well-dated 1980–1960Ma (U-Pb) volcanic rocks from the Surumu Group, northern Roraima State (Amazonian Craton). After thermal and AF demagnetization, characteristic northwestern remanent magnetization (ChRM) directions with downward inclinations were isolated (mean: Dm=298.6°, Im=39.4°, α95=10.1°, N=20, K=11.4), which yielded a paleomagnetic pole (SG) located at 234.8°E, 27.4°N (A95=9.8°). Magnetic mineralogy experiments show that ChRM is carried by high-Hc, and high-unblocking temperature Ti-poor titanomagnetite and/or hematite. The well-dated pole SG contributes to better define the apparent polar wander (APW) path for the Guiana Shield during the Paleoproterozoic (2070–1960Ma). Comparison of the Amazonian and West African Craton's APW paths for the same time interval suggests that these cratonic blocks were linked together at about 1980–1960Ma ago in a paleogeography along which the Guri (Guiana Shield) and Sassandra (West Africa Craton) shear zones were aligned as previously proposed.

Testing Iberian kinematics at Jurassic‐Cretaceous times

AbstractPaleogeographic reconstructions of Iberia at the Mesozoic are still a matter of debate. The incompatibility between kinematic models and paleomagnetic data older than 120 Ma is a major problem for which no cause has yet been determined. Here we use a new method to investigate the origin of this misfit. We solve the inverse problem of finding the Euler poles that fit paleomagnetic poles with the Global Apparent Polar Wander Path (GAPWP) and then test their implications on Iberian reconstructions. We show that Iberian poles from the Early Cretaceous (mean poles for 123 and 130 Ma) are incompatible with the GAPWP, bringing into question their validity. Contrarily, Late Jurassic data (mean pole at 151 Ma) are compatible with the GAPWP and, thus, can be considered reliable. Based on these results, we propose a new magnetic reconstruction of Iberia and surrounding plates at ~150 Ma (M22 anomaly). This work provides new constraints for the kinematic evolution of Iberia during Jurassic‐Cretaceous. However, the development of a detailed and consensual model for the kinematic evolution of Iberia is dependent on the acquisition of new, high‐quality paleomagnetic data and a reevaluation of seafloor magnetic anomalies.

Paleomagnetism of Paleozoic sedimentary rocks from the Karatau Range, Southern Kazakhstan: Multiple remagnetization events correlate with phases of deformation

The paleogeography of the Altaids and its kinematic and tectonic evolution during the final collision and amalgamation of Eurasia is still poorly known. Addressing this problem, a paleomagnetic study has been undertaken on Paleozoic sedimentary rocks from the Karatau, Southern Kazakhstan. Stepwise thermal demagnetization reveals the presence of a high‐temperature component of magnetization in most samples. Fold tests indicate a syn‐folding age of magnetic remanence acquisition at three of the five areas studied. Directional data of Devonian and Permian rocks yield a positive fold test, implying a primary magnetization. Resulting prefolding paleolatitudes for Permian and Devonian rocks show the proximity of the Karatau to Baltica during those times. Syn‐ and post‐folding magnetizations result in paleolatitudes for Karatau, which intersect the paleolatitude curve based on the Baltica apparent polar wander path (APWP), at times, which can be correlated to major deformational events at ~280 Ma, ~260 Ma, and ~230 Ma, respectively. We interpret this with complicated pattern of remagnetization events accompanying deformation, which can include syn‐folding remagnetization events and areas of primary magnetic signals. Additionally, the differences between reference declinations based on the APWP for Baltica and observed declinations suggest successive counterclockwise rotational reorganization of the Karatau during the late Paleozoic to Early Mesozoic, with maximal rotation values of ~65° with respect to Baltica. The remagnetization events are correlated with latest intracontinental stages of orogenic evolution in the Ural mountains and thus the Paleozoic amalgamation of the Eurasian continent and suggest synchronous and coherent tectonic evolution in the Urals and Karatau mountains.

The drift history of Adria and Africa from 280 Ma to Present, Jurassic true polar wander, and zonal climate control on Tethyan sedimentary facies

The orogenic belts surrounding the undeformed Adriatic Sea represent the margins of an area known as Adria, the African promontory. We have undertaken a critical appraisal of paleomagnetic data from regions of Adria considered parautochthonous relative to Africa, obtained either from biostratigraphically dated sedimentary rocks, corrected for inclination shallowing, or from igneous rocks that are regarded as free from any inclination shallowing bias. Paleomagnetic directions were used to calculate paleomagnetic poles for comparison with coeval, and inclination flattening-free, paleomagnetic poles from stable Africa. Visual coherence of paleopoles for several time slices from the Early Permian to the Eocene supports the construction of a composite apparent polar wander path (APWP) valid for parautochthonous Adria and stable Africa. This composite APWP is compared to previous APWPs, finding good agreement with the global APWP of Kent and Irving (2010). Both APWPs show a remarkable and rapid polar shift of ~40° in the Jurassic that other APWPs tend to underestimate. We interpret this shift to represent a major episode of true polar wander (TPW), from ~183Ma in the Early Jurassic to ~151Ma in the Late Jurassic. Using a simple zonal climate model, the drift motion of Adria attached to Africa appears to be consistent with the distribution of Permian–Cretaceous sedimentary facies on Adria.

Palaeogeography of the Siberian platform during middle Palaeozoic Times (∼450–400 Ma): new palaeomagnetic evidence from the Lena and Nyuya rivers

New reliable palaeomagnetic data from the Siberian platform help in deciphering its palaeogeography during ∼450–400 Ma. Geochronology of late Devonian mafic sills provides time constraints for tectonic deformation along the southern margin of the Siberian Platform and thus a minimum age for the regional magnetic overprint. From a late Ordovician–Silurian sedimentary section of the Nyuya syncline in the southern part of the Siberian platform with the Devonian sills hosted nearby in early Palaeozoic sediments, pre-folding presumably primary magnetization was isolated from the sediments during stepwise thermal cleaning. High unblocking temperatures imply that haematite is the main carrier of magnetization. The sample-mean direction for 37 Ordovician samples from nine sites in stratigraphic coordinates is Ds = 168.5, Is = −5, ks = 22.3 and α 95 = 5.1 and for 77 Silurian samples from six sites is Ds = 193.9, Is = 20.9, ks = 16 and α 95 = 4.2. Another component recorded in both the Silurian and Ordovician samples is pre-folding, with a sample-mean direction of Ds = 204.4, Is = 37.9, ks = 27.2 and α 95 = 2.7 for 104 samples from eight sites. This component was probably formed during a regional remagnetization event, which took place in post-early Silurian time. Putting this secondary component into a framework with available Palaeozoic data and geochronology further constrains its age to be early Devonian.The isolated components yield new late Ordovician, early Silurian and early-middle Devonian palaeomagnetic poles. The revised middle Palaeozoic segment of the apparent polar wander path (APWP) for the Siberian platform provides new palaeogeographic constraints. Our data suggest that in late Ordovician the platform was situated in equatorial latitudes and was rotated 180° with respect to its present position. During middle-late Ordovician time, the platform did not experience any notable latitudinal drift. It started drifting to the north in the late Ordovician, and by the late Silurian it had travelled ∼1500 km northwards and had rotated ∼30° counter-clockwise (CCW). During late Silurian time, the platform continued northward drift and CCW rotation, and by the early Devonian it had drifted ∼1100 km northwards and rotated 10° CCW relative to its Silurian position. After that, the rotation of the platform changed to clockwise (CW), and by the late Devonian had drifted another 1500 km to the north and had rotated ∼60° CW.We evaluated palaeomagnetically viable positions from 450–400 Ma of the three largest Laurasian cratons, Siberia, Baltica and Laurentia, based on the new data and previously published APWPs. Contrary to several published reconstructions, the Siberian platform could not have been situated to the north of the Caledonian suture in mid-Silurian time, but was probably located either at the eastern or the western side of Laurussia. The new data are compatible with an early Devonian position of Siberia similar to the modern Eurasian configuration. They also support the post- early or middle Devonian relative rotation between the Aldan and Angara blocks of the Siberian platform.

Evidence for Late Eocene emplacement of the Malaita Terrane, Solomon Islands: Implications for an even larger Ontong Java Nui oceanic plateau

[1] Most tectonic models for the Solomon Islands Arc invoke a Miocene collision with the Ontong Java Plateau (OJP) to halt cessation of Pacific Plate subduction, initiate Australian Plate subduction, and emplace the Malaita Terrane, which shares the characteristic basement age and geochemistry of OJP. Existing paleomagnetic evidence, however, required the Malaita Terrane to have been fixed to the arc from at least the Late Eocene. New sampling has yielded a paleomagnetic pole from Aptian–Albian limestones and mudstones that falls between the apparent polar wander paths for the Australian Plate and OJP, confirming the extended period of residence of the Malaita Terrane on the arc. Arc-derived turbidities within Late Eocene through Miocene limestones on Malaita and Santa Isabel, and related clasts in broadly contemporary sandstones and conglomerates on Santa Isabel, also attest to early emplacement. Modeling the emplacement at 35 Ma satisfies both the paleomagnetic data and the sediment provenance. Continuing the reconstruction to 125 Ma leaves the Malaita Terrane far from OJP at the time of plateau formation. OJP is now understood to have formed as part of a larger Ontong Java Nui, also comprising the Hikurangi and Manihiki plateaus, separated by spreading during the Cretaceous. Restoring the separation of the known elements, and invoking an additional triple junction, unites the (now largely subducted) Malaita Terrane with the rest of Ontong Java Nui. Subduction of substantial areas of the Ontong Java Nui plateau, with little geological signal other than a reduction in arc volcanism, is a corollary.

New paleomagnetic and paleointensity data from Pliocene lava flows from the Lesser Caucasus

A paleomagnetic, rock-magnetic and paleointensity study has been carried out on 14 basaltic lava flows from two Pliocene (K–Ar age between 3.09±0.10Ma and 4.00±0.15Ma) sequences (Apnia and Korxi) from the eastern Djhavakheti Highland in southern Georgia (Caucasus).Measurement of strong-field magnetisation versus temperature curves yielded three types of thermomagnetic curves: (i) Reversible curves with magnetite as only remanence carrier (type H); (ii) irreversible curves with magnetite as only carrier of remanence (type H−) and (iii) irreversible curves showing a low Curie-temperature phase and magnetite (type L). Analysis of hysteresis curves showed that samples were characterised by a mixture of single-domain and multi-domain grains.Paleomagnetic experiments allowed determining characteristic components for all flows and normal polarities (6 flows), reversed polarities (7 flows) and intermediate polarities (1 flow) were observed.. Paleomagnetic poles were calculated using only those sites unequivocally showing normal or reversed polarities. The paleomagnetic pole obtained from flows of both combined sequences (latitude λ=77.9°N, longitude ϕ=152.1°E, n=13, A95=11.8°, k=13.4) showed a good agreement with the 5Ma window of the European synthetic apparent polar wander path of Besse and Courtillot (2002). The paleomagnetic direction of the combined Apnia-Korxi flows agrees well with the expected one, showing no significant tectonic rotation. The latter cannot be however, completely excluded in the Korxi section. In that section, analysis of the angular dispersion of virtual geomagnetic poles yields a much higher value than expected.Paleointensity experiments using the Coe method were performed on 31 specimens from 10 flows. After application of specific selection criteria, 19 samples from 8 flows were observed to provide successful determinations, with mean flow values showing a wide scatter. If only flows with more than one successful paleointensity determination are taken into account, virtual dipole moments (VDMs) vary between 3.5×1022Am2 and 8.3×1022Am2. In intermediate polarity site AP2 no weak transitional paleostrength values were observed.

Reconnaissance paleomagnetic studies of Mesoproterozoic alkaline igneous complexes in the Kaapvaal craton, South Africa

We report paleomagnetic data from three different parts of a diffuse Mesoproterozoic alkaline igneous province in the eastern part of the Kaapvaal craton in South Africa. Except for the ∼1.3Ga Pilanesberg dikes, these rocks have not previously been studied paleomagnetically. Isotopic dates from igneous rocks in the province generally indicate emplacement between 1.4 and 1.35Ga, which falls in a time frame where the apparent polar wander path (APWP) for the Kaapvaal craton is poorly known. In order to help address this problem, we report paleomagnetic data for syenitic rocks in the large Pilanesberg Complex near the western limit of the alkaline province, two separate syenitic intrusions in the Pienaars River Complexes near Pretoria, and the Spitskop ijolite–nepheline syenite–carbonatite complex near the eastern limit of the province. These intrusive units yield poles that allow a tentative extension of the Mesoproterozoic Kaapvaal APWP back to 1.4Ga. The data suggest a pronounced bend or cusp in the APWP at 1.1Ga, which may record a major change in plate motion occurring at the same time as emplacement of the Umkondo large igneous province in southern Africa.

Analysis of Ontong Java Plateau palaeolatitudes: evidence for large-scale rotation since 123 Ma?

SUMMARY We have discovered evidence of a previously unrecognized, large-scale rotation of the Ontong Java Plateau (OJP) recorded in its basement palaeolatitudes. When palaeolatitude differences computed among Ocean Drilling Program Sites 807 and 1183–1187 are plotted versus their present-day site latitude differences, a systematic 2:1 slope bias is evident. While it is possible to resolve this bias by introducing ad hoc tilt corrections at all six sites, drilling records indicate relatively undisturbed conditions at Sites 1183 and 1185–1187. Of the possible causes of the bias, only whole plateau rotation resolves it while honouring the majority of published palaeolatitudes. This implies that only Sites 807 and 1184 palaeolatitudes, both questioned in the literature, are erroneous. A 9 ◦ northward dip previously reported at Site 1184 appears to stem from inclined deposition rather than post-emplacement deformation. We also estimate an 8 ◦ southward tilt correction at Site 807 to make the data set self-consistent. Based on the six sites analysed, we find that OJP may have experienced ∼40 ◦ of clockwise rotation since its formation at ∼123 Ma. In contrast, available Pacific absolute plate motion (APM) models predict less than 10 ◦ of rotation. If our analysis is correct, it suggests that the plateau moved independently of the Pacific Plate early in its history or that Pacific APM models for the Lower Cretaceous are unreliable. While our corrections to Sites 807 and 1184 combined with ∼40 ◦ rotation resolve the internal inconsistencies, the mean palaeolatitude value of Ontong Java remains largely unchanged and is still anomalous with respect to the Pacific apparent polar wander path at ∼123 Ma.

The Continental Drift Controversy

The resolution of the sixty-year debate over continental drift, culminating in the triumph of plate tectonics, changed the very fabric of Earth science. This four-volume treatise on the continental drift controversy is the first complete history of the origin, debate and gradual acceptance of this revolutionary theory. Based on extensive interviews, archival papers and original works, Frankel weaves together the lives and work of the scientists involved, producing an accessible narrative for scientists and non-scientists alike. This second volume provides the first extensive account of the growing paleomagnetic case for continental drift in the 1950s and the development of apparent polar wander paths that showed how the continents had changed their positions relative to one another, more or less as Wegener had proposed. Paleomagnetism offered the first physical measure that continental drift had occurred and helped determine the changing latitudes of the continents through geologic time.

Age and genesis of the White Pine stratiform copper mineralization, northern Michigan, USA, from paleomagnetism

AbstractAt White Pine, the stratiform sedimentary copper (SSC) mineralization occurs mostly as chalcocite in gray‐black shales in the basal approximately 6 m of the 1081 ± 9 Ma Nonesuch Formation of the Keweenawan Supergroup. Paleomagnetic analysis of 176 specimens (19 sites) isolated collinear characteristic remanent magnetization (ChRM) directions in both magnetite and hematite in both the oxidized SSC zones and overlying pyritic and upper zones of the Nonesuch Formation. A paleomagnetic fold test shows that the normal‐polarity ChRM is prefolding in origin. Combining the ChRM directions from this study (18 sites) and from (Canadian Journal of Earth Sciences, 14, 1977, 1128; 11 sites), the Nonesuch mineralized and nonmineralized sites give a paleopole at 7.3°N, 174.7°E (N = 29, A95 = 3.0°), yielding an age of 1063 ±8 Ma on the Keweenawan apparent polar wander path. The observed 1063 ± 8 Ma age is interpreted to date both the average age of oxidation and mineralization of the SSC zone. Thus, ore genesis occurred after burial by the overlying Freda Formation and after conversion by oxidation of the underlying original clastic sediments of the Copper Harbor Formation to red beds, most likely by gravity‐driven recharge of subsurface brines by meteoric water from adjacent highlands on the southeast side of the Keweenawan Rift.

Counterclockwise block rotation linked to southward propagation and overlap of sub-aerial Red Sea Rift segments, Afar Depression: Insight from paleomagnetism

Thirty-four cooling units from the Pleistocene extrusive volcanic rocks exposed in regions bordered by the active Red Sea magmatic segments, in the northwestern central Afar Depression, were sampled for paleomagnetic study. Six to 12 samples were collected from each paleomagnetic site. Samples were demagnetized using Alternating Field (AF) and Thermal (TH) demagnetization techniques. The samples were then measured using the JR–6A spinner magnetometer available at the department of Earth Sciences of Addis Ababa University. The Natural Remanent Magnetization (NRM) direction reveals mostly one or two simple and straightforward components of magnetizations; the first, low-stability component is isolated by heating to 100°C–300°C or by AF of 10–30mT. The magnetization directions after these steps generally define straight lines that are directed towards the origin, and interpreted as primary NRM or ChRM (Characteristic Remanent Magnetization) and the direction of magnetization was determined. Results of the magnetization decay curve plots and rock magnetic analyses using a Variable Field Translation Balance (VFTB) indicate that magnetic mineralogy is dominated by Ti-poor titano-magnetite with magnetic grain sizes in the pseudo-single domain (PSD) range, with minor presence of goethite and maghemite in a few cases. An overall mean direction calculated for the 26 sites located within the overlap zone is obtained (D=354.4°, I=13.2°, N=26, K=43.5, α95=4.3°), and compared to the expected Geomagnetic Axial Dipole (GAD) field, based on the Apparent Polar Wander Path (APWP) Curve of the African plate (Besse and Courtillot, 1991, 2003), a difference in declination ΔD=−6.5°±4.0° is obtained. This declination difference is interpreted as counterclockwise rotation about a vertical axis, in agreement with rift propagation and right stepping overlap geometry of the Alayta–Dabbahu magmatic segments. The difference in inclination ΔI=7.8°±3.9° is believed to be related to the long-standing non-dipole field in Afar (Courtillot et al., 1984; Kidane et al., 2003).

Paleomagnetism of ca. 1.35 Ga sills in northern North China Craton and implications for paleogeographic reconstruction of the Mesoproterozoic supercontinent

We report paleomagnetic data on precisely dated Mesoproterozoic sills, in zircon and baddeleyite 207Pb/206Pb ages of 1345±12 and 1353±14Ma respectively, intruding the Xiamaling and Wumishan formations in North China with an aim of evaluating how the North China Craton (NCC) was involved in the evolution of the Columbia supercontinent from ca. 1.78 to 1.35Ga. After systematic thermal demagnetization we isolate a high temperature characteristic remanence from 18 sites with a tilt-corrected site-mean direction of D=294.4°, I=−31.7° and α95=4.3°, corresponding to a mean paleomagnetic pole at λ=5.9°N, φ=359.6°E (N=18) with angular standard deviation of 10.0° and A95=4.3°. Rock magnetic experiments, light microscopy and scanning electron microscopy all indicate that the main magnetic minerals in the sampled sill rocks are medium sized titanomagnetites. The positive fold test and fresh titanomagnetite grains suggest that the characteristic remanence is likely to be a primary acquired at ca. 1.35Ga. The preliminary paleogeography reconstruction based upon the well-matched apparent polar wander paths between the Laurentia–Baltica–Siberia united block and the NCC supports the interpretation that the NCC may have drifted together with this united block before ca. 1.35Ga during the late history of the Columbia supercontinent. We argue that the NCC was located in low latitudes and kept tight connections with the Siberian and Indian cratons during the interval between ca. 1.8 and 1.35Ga, and that the ca. 1.35Ga sills widely developed in the Mesoproterozoic Xiamaling and Wumishan formations, as well as the ca. 1.3–1.2Ga magma events well-developed in Siberia and northern margin of the NCC, may represent the complete fragmentation of the NCC with the neighboring cratons.

A planetary perspective on Earth evolution: Lid Tectonics before Plate Tectonics

Plate Tectonics requires a specific range of thermal, fluid and compositional conditions before it will operate to mobilise planetary lithospheres. The response to interior heat dispersion ranges from mobile lids in constant motion able to generate zones of subduction and spreading (Plate Tectonics), through styles of Lid Tectonics expressed by stagnant lids punctured by volcanism, to lids alternating between static and mobile. The palaeomagnetic record through Earth history provides a test for tectonic style because a mobile Earth of multiple continents is recorded by diverse apparent polar wander paths, whilst Lid Tectonics is recorded by conformity to a single position. The former is difficult to isolate without extreme selection whereas the latter is a demanding requirement and easily recognised. In the event, the Precambrian palaeomagnetic database closely conforms to this latter property over very long periods of time (~2.7–2.2Ga, 1.5–1.3Ga and 0.75–0.6Ga); intervening intervals are characterised by focussed loops compatible with episodes of true polar wander stimulated by disturbances to the planetary figure. Because of this singular property, the Precambrian palaeomagnetic record is highly effective in showing that a dominant Lid Tectonics operated throughout most of Earth history. A continental lid comprising at least 60% of the present continental area and volume had achieved quasi-integrity by 2.7Ga. Reconfiguration of mantle and continental lid at ~2.2Ga correlates with isotopic signatures and the Great Oxygenation Event and is the closest analogy in Earth history to the resurfacing of Venus. Change from Lid Tectonics to Plate Tectonics is transitional and the geological record identifies incipient development of Plate Tectonics on an orogenic scale especially after 1.1Ga, but only following break-up of the continental lid (Palaeopangaea) in Ediacaran times beginning at ~0.6Ga has it become comprehensive in the style evident during the Phanerozoic Eon (<0.54Ga).

A new Late Cretaceous to Present APWP for Asia and its implications for paleomagnetic shallow inclinations in Central Asia and Cenozoic Eurasian plate deformation

Based on a compilation of 533 Cretaceous to present-day palaeomagnetic poles obtained from both sedimentary and igneous rocks, we present a new analysis of the so-called 'Asian inclination anomaly' and demonstrated the anomaly to be twofold: a 2nd-order anomaly, characterized by high palaeolatitudes in Indochina and low palaeolatitudes over Tibet and Central Asia, is superimposed on a 1st-order anomaly, characterized by Cenozoic low palaeolatitudes found all over northeastern Asian stable blocks. The analysis herein convincingly shows that the Europe Apparent Polar Wandering Path (APWP) can no longer be used to interpret palaeomagnetic data East of the Urals, including interpretation of Asian Tertiary deformation related to the India-Asia Collision. We, thus, construct a new APWP for East Asia, based on palaeopoles from blocks assumed to be stable. This new APWP is consistent with and reinforces previous analyses of Asian tectonics, such as the age (∼55 Ma) and locus (∼5-10°N) of the Indo-Asian collision, the lateral extrusion of SE Asian continental blocks and the intracontinental shortening in Central Asia. Possible origins of the 1st-order palaeolatitude anomaly are: (1) a geomagnetic origin, due to long-lasting non-dipolar contribution to the magnetic field and (2) a tectonic hypothesis, in which a newly defined East Asia Plate was located ∼10° farther south than expected from the current Europe APWP. Based on a set of six new reconstructions from 90 Ma to present, we show that our tectonic model reconciles geophysical, geological and tectonic observations throughout Eurasia, from Siberia to Europe, including kinematics in the Arctic Ocean, up to northwestern Arctic Alaska. Beyond possible occurrences of non-dipolar field contribution and/or local inclination flattening in the sedimentary data, our model leads us to conclude that Cenozoic tectonics is the dominant contributor to the observed 1st-order ∼10° low palaeolatitude anomaly over Asia during the Tertiary.

Paleomagnetism of the Ulkan trough (Southeastern Siberian Craton)

The first results of the paleomagnetic study of one of the key Paleoproterozoic objects of the Aldan-Stanovoy Shield (the Ulkan trough) in the Bilyakchan-Ulkan volcanoplutonic belt are presented. The volcanosedimentary rocks of the Elgetei Formation and the granites of the Ulkan Complex were studied. According to these data and their comparison with the apparent Paleoproterozoic polar wandering path in the Angara-Anabar province, the Ulkan trough was (1) located during the timing of the studied rocks at 18°–26° S and (2) subjected to rotation (relative to the Angara-Anabar block) at 70° ± 8° in the time interval of 1732–1720 Ma ago. Based on the combined interpretation of the paleomagnetic, geochronological, and geochemical data published previously, a paleogeodynamic model is proposed. According to this model, the Aldan-Stanovoy and Angara-Anabar provinces of the Siberian Craton became a single rigid block about 1720 Ma ago.

Chapter 2 New global palaeogeographical reconstructions for the Early Palaeozoic and their generation

Abstract New palaeogeographical reconstructions are presented at 10 myr intervals from the Lower Cambrian at 540 Ma to the Lower Devonian at 400 Ma, showing continental crustal fragments and oceans (not lands and seas), with appropriate kinematic continuity between successive maps. The maps were chiefly generated by revised and selected palaeomagnetic data and revised Apparent Polar Wandering paths linked to present-day polygons from the main continents. These have been reinforced by analysis of the distributions of some fossils and sediments. Gondwana was the dominating supercontinent from its final assembly in the Latest Neoproterozoic at about 550 Ma until the Carboniferous, and covered much of the Southern Hemisphere. The Northern Hemisphere was largely occupied by the vast Panthalassic Ocean. The relative positions of the major continents and the latitudes and rotation histories of Gondwana, Baltica, Siberia and Laurentia (Laurussia from the mid-Silurian) are now well known. Although Laurentia was oriented in a similar direction to the present, Siberia was inverted throughout the Lower Palaeozoic, and Baltica too was initially inverted, but rotated through 120° between the Late Cambrian and Late Ordovician before collision with Laurentia in the mid-Silurian Caledonide Orogeny. Through reconstructions of the Caledonide and some other orogenies, the progressive history of the Iapetus Ocean between Laurentia and Baltica/Gondwana is well constrained. Less major continents whose positions are also well known include Avalonia (initially peri-Gondwanan but migrating in the Early Ordovician to join Baltica by the end of the Ordovician), Sibumasu (now considered an integral part of Gondwana) and Mongolia (adjacent to Siberia). A large number of other terranes are reviewed and plotted on the reconstructions with varying degrees of certainty. However, significant continents with less well constrained or controversial positions are South China, North China (Sinokorea), Annamia (Indochina) and Arctic Alaska–Chukotka. The European areas of France, Iberia and southern Italy, previously considered by some as a separate Armorican Terrane Assemblage, remained parts of core Gondwana until the opening of the Palaeotethys Ocean near the end of the Silurian, but it is uncertain whether Perunica (Bohemia) was one of that group or whether it left Gondwana during the Middle Ordovician.

Paleomagnetic dating of Phanerozoic kimberlites in Siberia

article i nfo Diamondbearing kimberlite pipesare exposedacross the north-central part of theSiberianplatform.Three main time intervals are considered to be the age of emplacement: the Devonian-Early Carboniferous, Triassic, and Cretaceous. However, isotopic age data from of the pipes are scattered and provide a very broad age interval for the magmatic activity. New paleomagnetic poles from four kimberlite pipes (Eastern Udachnaya, Western Udachnaya, International and Obnazhennaya) are obtained to estimate their paleomagnetic age. The mean primary magnetization directions for the pipes are as follows: D=4.3°, I= −44.5° (k=29.4, α95=7.4°, N= 14); D=340.5°, I= −65.6° (k=12.9, α95=19.4°, N=6); D=291.1°, I= −78.1° (k=27.5, α95=14.9°, N= 5); and D=306.7°, I=−82.6° (k=38.4, α95=5.8°, N=17), respectively. On the basis of a comparison with the Siberian apparent polar wander path (APWP) we estimate the age of kimberlite magmatism, assuming pri- marymagnetizationsinthese rocks.The paleomagneticagesare asfollows: 428±13 Ma for Eastern Udachnaya; 251±30 Ma for International pipe; and 168±11 Ma for Obnazhennaya pipe. The Western Udachnaya pipe was remagnetized and no clearpaleomagneticage could bedetermined. The agesof magmaticactivity spanthe Early SiluriantoMiddleLate Jurassic.EarlySilurianmagmatismcouldbeassociatedwiththeformation of theViluyrift. MiddletoLateJurassicmagmaticactivityismostlikelyrelatedtosubductionrelatedtotheaccretionofsurround-

Paleomagnetism of the Late Cretaceous volcanic rocks of the Shimaoshan Group in Yongtai County, Fujian Province

It has been debated whether there was southward movement of the South China Block (SCB) during the Cretaceous. In this study, a paleomagnetic investigation was carried out on the Late Cretaceous volcanic rocks (~88 Ma) of the Shimaoshan Group in Yongtai County, Fujian Province. Rock magnetic experiments showed that magnetite in pseudo-single-domain and multi-domain grain and hematite were predominant magnetic phases. Stepwise thermal demagnetization successfully isolated characteristic directional components at high-temperature interval (> 500°C) from 383 specimens in 19 sites, which yielded a paleomagnetic pole for the studied section at 83.1°N, 152.6°E (N = 19, A95 = 3.9°), and the scatter SB = 9.0. The Fisher distribution of virtual geomagnetic poles (VGPs) and the consistence of SB with the expected value at the 95% confidence level indicate that the yielded paleomagnetic pole is free of paleomagnetic secular variation influence. The new pole, which is well consistent with that from the Eurasian apparent polar wander path (APWP) curve, suggests no obvious southward movement of the sampling site during the Cretaceous. South China Block, Late Cretaceous, volcanic rock, paleomagnetic secular variation, paleomagnetic pole

New interpretations on palaeomagnetic data from the Nagssugtoqidian mobile belt in Greenland

The principles governing directions of magnetisation in a metamorphic terrain, which has experienced slow cooling during uplift, are reiterated. These principles are illustrated in areas affected by the Nagssugtoqidian metamorphism in West and East Greenland. Directions of magnetisation in the Kangâmiut dykes in the Nagssugtoqidian block and in the craton to the south are similar to each other. The directions in the dykes are close to those in the surrounding gneisses, implying magnetisation within a relatively short time. Their palaeomagnetic poles are close to those from Nagssugtoqidian localities elsewhere in West and East Greenland.Between-site and within-site directions in the dykes are asymmetrical, which is attributed to Apparent Polar Wandering. A statistical method of plotting Apparent Polar Wander Paths is presented. Such paths from dykes in the Archaean and Nagssugtoqidian regions are similar, from which a Composite path is drawn, thus establishing that the Apparent Polar Wander Path for Greenland lies between 0°N, 310°E, and 22°N, 260°E, in the Palaeoproterozoic at about 1750Ma, upon cooling after the Nagssugtoqidian metamorphism. Similar Apparent Polar Wander Paths have been drawn from other localities in West Greenland. Numerous Poles from East and West Greenland fall close to this path, and are thus of Nagssugtoqidian age. The mean Nagssugtoqidian pole from eight representative studies lies at 20.3°N, 278.3°E (dp/dm=4.8°/6.4°).