Palaeomagnetic study of the (Late Precambrian) West Greenland kimberlite-lamprophyre suite: definition of the Hadrynian Track

Kimberlite and potassic lamprophyre dykes were intensively intruded into the early Proterozoic Nagssugtoqidian mobile belt of West Greenland during an important phase of brittle reactivation in Late Precambrian-Early Cambrian times (ca. 580-570 Ma) and during at least one other minor phase. Thermal and alternating field demagnetisation studies of 52 of these dykes identify primary components residing in the critical blocking temperature range distributed between shallow westerly and steep positive directions. Near the axis of the Proterozoic shear belt the dykes (predominantly lamprophyres) have closely grouped shallow directions with a reversal; near the margins of the shear belt dykes (predominantly kimberlites) have steeper and distributed directions. The cleaned components of magnetisation appear to be single, and the distribution of directions is interpreted to record a migration of the palaeofield axis which intersecting relationships show to have been from shallow to steep. The dyke directions are grouped to define representative mean palaeopoles of 215°E 3°N (LK1, A₉₅ = 3.9°), 213°E 18°N (LK2, A₉₅ = 6.1°), 203°E 46°N (LK3, A₉₅ = 10.4°) and 259°E 54°N (LK4, A₉₅ = 11.0°); a subsidiary direction recorded in five dykes near the southern margin of the shear belt (LK5, palaeopole at 297°E 16°S (A₉₅ = 12.5°)) is derived entirely from lamprophyres and is possibly Silurian in age. An RbSr isochron on three lamprophyres of 1227 Ma and agreement of the remanence direction with ca. 1220 Ma rocks from elsewhere in Greenland suggests that the LK1 component is wholly or partly of that age.The remaining sequence of palaeopoles falls along the Hadrynian Polar Track and the age data relating to this track are re-evaluated. Evidence for a pre-800 Ma age is no longer valid and the new data from West Greenland confirm that the track is latest Precambrian to Early Cambrian in age. It is shown to connect poles of Late Precambrian and Lower Cambrian age and to embrace other data from the Laurentian shield. The rapid passage of the shield across the South Pole is consistent with the sedimentation sequences, and suggests a high-latitude origin for the tillite horizon of this age. The Hadrynian Track is also compared with the contemporaneous record from Gondwanaland and it is shown that the two shields were in juxtaposition in the identical reconstruction to the Proterozoic Supercontinent until earliest Cambrian times. This discovery links the Lower Cambrian marine transgression and the widespread ca. 580-560 Ma alkaline province in the Gondwanaland, Laurentian and Fennoscandian shields to major continental break up, and it conforms with evidence that the Iapetus Ocean did not open until Cambrian times.     

Palaeomagnetism of the Svecofennian Loftahammar gabbro and some Jotnian dolerites in the Swedish part of the Baltic Shield

The Svecofennian Loftahammar gabbro (RbSr isochron age 1,694 · 10⁶ year) and Jotnian dolerites (KAr isochron age 1,245 · 10⁶ year) in Sweden were found to have palaeomagnetic pole positions in close agreement with poles from other Svecofennian rocks in Sweden and Jotnian dolerites in Finland, respectively. These results support Neuvonen's apparent polar wandering path during the period from 1,900 · 10⁶ to 1,300 · 10⁶ year.A reconstruction of the position of the Baltic Shield during the (1,900-1,200) · 10⁶-year interval, based on available pole positions, indicates that the Baltic shield was close to Greenland and the North American Shield and reveals a contiguity of North Atlantic geologic-geochronological zones until 1,200 · 10⁶ year ago.     

Collision tectonics in the Canadian Shield?

Major late Aphebian faults cutting the margin of the western Superior craton and exhibiting large dextral offsets are considered to be a direct result of collision between suturing Superior and Churchill plates. Following initial collision in northern Quebec (Cape Smith belt) and in Ontario (west of James Bay), the Winisk River fault developed and suturing continued along the Fox River belt of the circum-Superior suture. Development of the Kenyon structure, another major shear zone, and subsequent faults, permitted further plate convergence to accommodate the stepped form of the edge of the Churchill plate. In this way the Superior craton was progressively sliced as suturing proceeded from east to southwest. The fault directions were controlled by pre-existing transform faults linking segments of a consuming trench system at the edge of the Churchill plate. Final stages of convergence and collision resulted in thickening of the Churchill plate and major sinistral motion along the Thompson-Owl River shear zone.     

Palaeomagnetic study of some charnockites of the Palni Hills

The intensity of natural remanence magnetisation (NRM) is measured along the three mutually-perpendicular directions using an astatic magnetometer. The intensity of induced magnetisation is measured using an apparatus fabricated in the laboratory. The Koenigsberger ratio has been calculated for all the samples and the ratio has been used to test the stability of NRM in the samples. The mean direction of magnetisation is determined from the three components of the NRM intensity. The palaeomagnetic pole position of the samples is determined using the direction of magnetisation and the site location. An attempt has been made to fix the geological age of the charnockites using palaeomagnetic methods.     

A three-dimensional model of the Baltic Shield crust from data of deep seismic studies

A 3-D velocity structure of the crust has been constructed for almost the entire Baltic Shield area from data of extensive deep seismic studies on the shield. The construction involved a revision of all primary data (record sections and observed traveltime curves) obtained in this region over 50 years of research. Comparative analysis of wave fields revealed that three reference reflectors traceable throughout the shield area are K1 (a boundary velocity of 6.4–6.5 km/s), K2 (～6.8 km/s), and the mantle surface M (8.0–8.2 km/s). The resulting 3-D velocity structure is represented in the form of structural maps of these surfaces and a velocity distribution scheme in the upper crust. Using this general basic model, seismic cross sections are constructed by means of mathematical modeling along all profiles. They showed that, in addition to the main layers and reflectors above the K1 boundary, a lower velocity layer is traceable almost everywhere and the majority of deep faults flatten out toward this layer. On the whole, lateral variations in the velocity structure of the crust are small up to a depth of 40 km. The variations are most significant in the M topography: its average depth being 40–45 km, two deep (down to 50–60 km) depressions exist in southern Finland and the Baltic region. The origin of this depression filled with high velocity (7.2–7.4 km/s) rocks remains unclear.     

The structure of the Baltic Shield region on the basis of DSS and earthquake data

The lithospheric models obtained for the Baltic Shield by using deep seismic soundings are discussed and results from different parts of the shield are compared with models achieved by the investigation of surface waves and of P to S converted waves. The results are found to agree rather well with each other particularly with regard to the first interface (at a depth of 10–15 km) and the whole thickness of the crust (c. 40 km). The macroseismic focal depth determinations of regional earthquakes are correlated with lithospheric structure. The main maximum in the focal depth distributions of Finnish earthquakes occur at a depth of 10–12 km. The geographical distribution of the earthquake epicentres suggests several seismo-active zones in the northern Baltic Shield. One new finding in this field concerns the Lapland zone, which runs in a north-westerly direction.     

Bundelkhand mafic dykes, Central Indian Shield: Implications for the role of sediment subduction in Proterozoic crustal evolution

Abstract The Archean to Paleo–Proterozoic Bundelkhand massif basement of the central Indian shield has been dissected by numerous mafic dykes of Proterozoic age. These dykes are low‐Ti tholeiites, ranging in composition from subalkaline basalt through basaltic‐andesite to dacite. They are enriched in light rare earth elements (LREE), large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE: Nb, P and Ti). Negative Sr anomaly is conspicuous. Nb/La ratios of the dykes are much lower compared with the primitive mantle, not much different from the average crustal values, but quite similar to those of continental and subduction related basaltic rocks. Bulk contamination of the mantle derived magma by crustal material is inadequate to explain the observed geochemical characteristics; instead contamination of the mantle/lithospheric source(s) via subduction of sediment is a better proposition. Thus, in addition to generating juvenile crust along the former island arcs, subduction processes appear to have influence on the development of enriched mantle/lithospheric source(s). The Bundelkhand massif basement is inferred to represent subduction related juvenile crust, that experienced lithospheric extension and rifting possibly in response to mantle plume activities. The latter probably supplied the required heat, material (fluids) and extensional environment to trigger melting in the refractory lithospheric source(s) and emplacement of the mafic dykes. Proterozoic mafic magmatic rocks from Bundelkhand, Aravalli, Singhbhum and Bastar regions of the Indian shield and those from the Garhwal region of the Lesser Himalaya display remarkably similar enriched incompatible trace elements characteristics, although limited chemical variations are observed in all these rocks. This may indicate the existence of a large magmatic province, different parts of which might have experienced similar petrogenetic processes and were probably derived from mantle/lithospheric source(s) with similar trace element characteristics. The minor, less enriched to depleted components of the Jharol Group of the Aravalli terrane and those from the Singhbhum terrane may represent protracted phases of rifting, that probably caused thinning and mobilization of the lithosphere, facilitating the eruption/emplacement of the asthenospheric melts (with N‐ to T‐types mid‐oceanic ridge basalts signatures) and deposition of deep water facies sediments in the younger developing oceanic basins. In contrast, Bundelkhand region did not experience such protracted rifting, although dyke swarms were emplaced and shallow water Bijawar Group and Vindhyan Supergroup sediments were deposited in continental rift basins. All these discrete Proterozoic terranes appear to have experienced similar petrogenetic processes, tectonomagmatic and possibly temporal evolution involving subduction processes, influencing the lithospheric source characteristics, followed by probably mantle plume induced ensialic rifting through to the development of oceanic basins in the Indian shield regions and their extension in the Lesser Himalaya.     

Receiver-Function Velocity Structure of the Lithosphere beneath the Khibiny and Lovozero Massifs,Northeastern Baltic Shield

Izvestiya, Physics of the Solid Earth - Abstract—The velocity structure of the lithosphere is studied beneath the Khibiny and Lovozero massifs in the northeastern part of the Baltic shield....     

Surface-wave study of the Canadian Shield

Phase and group velocities of surface waves with travel paths confined to the Canadian Shield and its surrounding areas were determined in a period range between 5 and 40 sec. The Canadian Seismic Network and three earthquakes of which source parameters are already known were used for this study. Mean phase velocities from source to receiver were determined from phase differences between the focal phases and the observed phase spectra by the single station method. Group velocities were determined by the algorithm of Dziewonski et al. They were put into a program for relocation of epicenters and regional surface-wave velocities by the least-squares method. The area under investigation was divided into six geologically significant regions. Finally, regional surface-wave velocities were inverted to obtain regional earth structures. Taking resolving power into consideration, the earth structures were assumed to have four layers, namely, uppermost crust, granitic layer, basaltic layer, and upper mantle. It was found that the shear velocities in the granitic and basaltic layers of the Canadian Shield are rather uniform laterally, differences for most of the regions are less than 0.015 km/sec. These facts may indicate the uniformness of its formation and the later crustal movements.     

Plate-plume-accretion tectonics in Proterozoic terrain of northeastern Rajasthan, India: Evidence from mafic volcanic rocks of north Delhi fold belt

Abstract   The northern part of the Aravalli mountain belt of northwestern Indian shield is broadly composed of three Proterozoic volcano-sedimentary domains, i.e. the Bayana, the Alwar and the Khetri basins, comprising collectively the north Delhi fold belt. Major, trace and rare earth element concentrations of mafic volcanic rocks of the three basins exhibit considerable diversity. Bayana and Alwar volcanics are typical tholeiites showing close similarity with low Ti–continental flood basalts (CFB) with the difference that the former shows enriched and the latter flat incompatible trace element and rare earth element (REE) patterns. However, the Khetri volcanics exhibit a transitional composition between tholeiite and calc-alkaline basalts. It appears that the melts of Bayana and Alwar tholeiites were generated by partial melting of a common source within the spinel stability field possibly in the presence of mantle plume. During ascent to the surface the Bayana tholeiites suffered crustal contamination but the Alwar tholeiites erupted unaffected. Geochemically, the Khetri volcanics are arc-like basalts which were generated in a segment of mantle overlying a Proterozoic subduction zone. It is suggested that at about 1800 Ma the continental lithosphere in northeastern Rajasthan stretched, attenuated and fractured in response to a rising plume. The produced rifts have undergone variable degrees of crustal extension. The extension and attenuation of the crust facilitated shallowing of the asthenosphere which suffered variable degree of melting to produce tholeiitic melts – different batches of which underwent different degrees of lithospheric contamination depending upon the thickness of the crust in different rifted basins. The occurrence of subduction-related basaltic rocks of Khetri Belt suggests that a basin on the western margin of the craton developed into a mature oceanic basin.     

Palaeomagnetic study of late precambrian rocks of the midland craton of England and Wales

The Precambrian basement of the British region south of the Caledonian orogenic belt is only observed in a few small inliers; this paper reports a detailed paleomagnetic study of four of these inliers. The Stanner-Hanter amphibolitised gabbro-dolerite complex of uncertain age yields a mean direction of magnetisation D = 282°, I = 51° (15 sites,α₉₅ = 11.4°) after AF and thermal cleaning. Uriconian lavas and tuffs (～700-600 m.y.) of the Pontesford and Wrekin inliers require both thermal and AF cleaning for complete analysis of NRM. The former region (Western Uriconian) yields a mean of D = 136°, I = ?25° (6 sites,α₉₅ = 15.3°) and the latter region (Eastern Uriconian) a mean of D = 78°, I = 17° (9 sites, α₉₅ = 12.8°); the Eastern Uriconian shows a marked improvement in precision after a two-stage fold test, and the palaeomagnetic data suggest that some apparent polar movement took place between eruption of the two sequences. The Uriconian rocks in both areas were intruded by dolerites which yield a mean direction of magnetisation D = 72°, I = 54° (11 sites,α₉₅ = 13.2°).The collective data give palaeomagnetic poles related to Upper Proterozoic metamorphic episodes (Stanner-Hanter Complex and Rushton Schist) which are in close agreement with earlier studies of the Malvernian metamorphic rocks, and to the late Precambrian Uriconian volcanic/hypabyssal igneous episode. All of these magnetisations are probably confined to the interval 700-600 m.y., and are indicative of appreciable polar movement during this interval. The palaeomagnetic poles define an apparent polar wander path for this crustal block between Late Precambrian and Lower Cambrian times and show that cratonic Britain south of the Caledonian suture is unrelated to the Baltic Shield.     

Rotation of the Iberian arc: Palaeomagnetic results from North Spain

NRM directions measured from 32 sites in Middle Cambrian, Upper Silurian/Lower Devonian and Lower Carboniferous redbeds follow the trend of the Variscan arc in North Spain. Thermal demagnetisation does not significantly alter this pattern. Fold tests show that the NRM is earlier than the ?₁ folds which form the arc; consistency of angle between bedding and the tilt-corrected NRM inclination (22–28°), similarity of the corresponding palaeolatitudes to Carboniferous values and microscopic evidence of Variscan redistribution of hematite indicates that the magnetisation is post-Lower Carboniferous. A statistical plot of the orientation of ?₁ fold traces against angle between ?₁ fold traces and declination of NRM shows that where these folds curve through 165° the NRM has been rotated through 110°: the arc is an orocline. Restoration of this rotation, and that needed to close the Bay of Biscay, brings the calculated mean palaeomagnetic pole reasonably close to the Upper Carboniferous part of the apparent polar wander path for Europe.     

Crustal and upper-mantle structure of the Baltic Shield investigated by a combined interpretation of deep-seismic-sounding data and surface-wave analysis

P- and S-wave velocity distributions obtained from DSS measurement have been used as a constraint in the inversion of surface-wave dispersion data. The combined interpretation was made as an attempt to enhance the resulting velocity models and to test the possibility to draw conclusions about the density distribution. The result indicates a potential value of a combined interpretation but it is obvious that very accurate velocity distributions are needed. The achieved density distribution is in good agreement with reported densities derived from gravimetric studies.     

Palaeomagnetic evidence for the origin of Madeira

The remanent magnetization of ‘basement’ volcanics from Madeira define three different axes of magnetization, each having a dual-polarity build-up. The suggested oldest of these components, with declination 302 and inclination +4, is assigned to the late Lower Cretaceous and is thought to reflect the age of the early volcanism of the island. Subsequent magnetization overprints seem to have occurred in the Late Cretaceous/Early Tertiary (minor) and in Neogene times, respectively. The latter magnetization, which is strongly developed, was most likely impressed during the extensive volcanism that swept the island in post-Late Miocene. The palaeomagnetic evidence for a Cretaceous origin of Madeira is supported by the finding of Lower-Middle Cretaceous tuff layers in DSDP site 136 which is located only 160 km north of the island. The inferred palaeomagnetic structure of the ‘basement’ rocks of Madeira is similar to that found in the old volcanic complexes of other east central Atlantic islands.     

The palaeoposition of Madagascar: Palaeomagnetic evidence from the Isalo group

Palaeomagnetic results are reported from the continental facies of the Triassic-Jurassic Isalo Group of Madagascar. Stability of the magnetic remanence was tested using the alternating field and progressive thermal demagnetization techniques. Results from 8 sites, 6 located in northwestern Madagascar and 2 from the southwestern region, yield a palaeomagnetic pole at 74.2°S, 97.1°E (N = 8, A₉₅ = 6.3°). Three models previously proposed to describe the drift history of Madagascar relative to Africa are considered and the relevant geological and geophysical information is reviewed. The palaeomagnetic data are only consistent with the pre-drift model which places Madagascar off the east coast of Africa adjacent to Kenya and Tanzania. This is also the continental drift fit favoured on geological grounds.     

Palaeomagnetic studies of the Kapiti phonolite of Kenya

Specimens of Kapiti phonolite from sixteen sites were treated in an alternating field up to 900 Oe peak. Four sites were discarded on the basis of Watson's Criterion for randomness. Variations in various magnetic parameters along a vertical section of the rock indicate that a hard secondary component of magnetization of high coercive force is present in the surface samples of the rock — a possible cause for the poor grouping of directions of the remaining sites. Weathering may have introduced this component in the surface samples.     

Tomographic study of the deep tectonics in the Yecheng-Shiquanhe area of the Tibetan Plateau

The profile crossing the west Kunlun region cov-ers the area between the Tarim Basin and the Kara-korum Fault, which is the highest (~5000 m a.s.l.) andthe most narrow part (400—500 km from north tosouth) of the Tibetan Plateau, thus attracting great at-tention of the geological community because of itsunique tectonic features. The Geological Bureau of Xinjiang Uygur Auton-omous Region and Chinese Academy of Sciences(CAS) carried out mineral exploration and geologicalmapping in th…     

Developmental disorders in larvae of eelpout (Zoarces viviparus) from German and Swedish Baltic coastal waters

Pregnant eelpout were collected for several years at German and Swedish Baltic coastal stations with different impact of pollutants and the prevalence of fry abnormalities compared between sites. Mortality of apparently normal developed larvae was observed with high variability in both German and Swedish sites. Malformation of larvae was a prominent disorder at all German stations with a prevalence ranging from about 50% up to a maximum of 90% in almost all analysed samples. At the Swedish stations malformation rates were significantly lower (range: 0-6%). Growth retardation of fry was clearly more prominent in eelpout from German coastal waters as well. Taken together the field data suggests that impaired larval development was associated with the state of environmental pollution at the sampling locations.