Structure and tectonic evolution of the Southern Eurasia Basin, Arctic Ocean

Multichannel seismic reflection data acquired by Marine Arctic Geological Expedition (MAGE) of Murmansk, Russia in 1990 provide the first view of the geological structure of the Arctic region between 77–80°N and 115–133°E, where the Eurasia Basin of the Arctic Ocean adjoins the passive-transform continental margin of the Laptev Sea. South of 80°N, the oceanic basement of the Eurasia Basin and continental basement of the Laptev Sea outer margin are covered by 1.5 to 8 km of sediments. Two structural sequences are distinguished in the sedimentary cover within the Laptev Sea outer margin and at the continent/ocean crust transition: the lower rift sequence, including mostly Upper Cretaceous to Lower Paleocene deposits, and the upper post-rift sequence, consisting of Cenozoic sediments. In the adjoining Eurasia Basin of the Arctic Ocean, the Cenozoic post-rift sequence consists of a few sedimentary successions deposited by several submarine fans. Based on the multichannel seismic reflection data, the structural pattern was determined and an isopach map of the sedimentary cover and tectonic zoning map were constructed. A location of the continent/ocean crust transition is tentatively defined. A buried continuation of the mid-ocean Gakkel Ridge is also detected. This study suggests that south of 78.5°N there was the cessation in the tectonic activity of the Gakkel Ridge Rift from 33–30 until 3–1 Ma and there was no sea-floor spreading in the southernmost part of the Eurasia Basin during the last 30–33 m.y. South of 78.5°N all oceanic crust of the Eurasia Basin near the continental margin of the Laptev Sea was formed from 56 to 33–30 Ma.     

The western and northern margin off svalbard

Recent geophysical measurements, including multi-channel seismic reflection, on the Svalbard passive margin have revealed that it has undergone a complex geological history which largely reflects the plate tectonic evolution of the Greenland Sea and the Arctic Ocean. The western margin (75–80°N) is of a sheared-rifted type, along which the rifted margin developed subsequent to a change in the pole of plate rotation about 36 m.y. B.P. The north-trending Hornsund Fault on the central shelf and the eastern escarpment of the Knipovich Ridge naturally divide the margin into three structural units. These main marginal structures strike north, paralleling the regional onshore fault trends. This trend also parallels the direction of Early Tertiary plate motion between Svalbard and Greenland. Thus, the western Svalbard margin was initially a zone of shear, and the shear movements have affected the adjacent continental crust. Although, the nature and location of the continent—ocean crustal transition is somewhat uncertain, it is unlikely to lie east of the Hornsund Fault. The northern margin, including the Yermak marginal plateau, is terminated to the west by the Spitsbergen Fracture Zone system. This margin is of a rifted type and the preliminary analysis indicates that the main part of the investigated area is underlain by continental crust.     

Tectonics and magmatism of ultraslow spreading ridges

The tectonics, structure-forming processes, and magmatism in rift zones of ultraslow spreading ridges are exemplified in the Reykjanes, Kolbeinsey, Mohns, Knipovich, Gakkel, and Southwest Indian ridges. The thermal state of the mantle, the thickness of the brittle lithospheric layer, and spreading obliquety are the most important factors that control the structural pattern of rift zones. For the Reykjanes and Kolbeinsey ridges, the following are crucial factors: variations in the crust thickness; relationships between the thicknesses of its brittle and ductile layers; width of the rift zone; increase in intensity of magma supply approaching the Iceland thermal anomaly; and spreading obliquety. For the Knipovich Ridge, these are its localization in the transitional zone between the Gakkel and Mohns ridges under conditions of shear and tensile stresses and multiple rearrangements of spreading; nonorthogonal spreading; and structural and compositional barrier of thick continental lithosphere at the Barents Sea shelf and Spitsbergen. The Mohns Ridge is characterized by oblique spreading under conditions of a thick cold lithosphere and narrow stable rift zone. The Gakkel and the Southwest Indian ridges are distinguished by the lowest spreading rate under the settings of the along-strike variations in heating of the mantle and of a variable spreading geometry. The intensity of endogenic structure-forming varies along the strike of the ridges. In addition to the prevalence of tectonic factors in the formation of the topography, magmatism and metamorphism locally play an important role.     

New data on the structure of the Knipovich Ridge,North Atlantic

The Knipovich Ridge extends for 550–600 km between the Mohns Ridge and the demarcation Spitsbergen Fracture Zone. The structural features of this ridge are repeatedly mentioned in the literature; however, substantial discrepancies remain in the treatment of its tectonics. New data on the structure of this ridge presented in this paper are based on the results of continuous seismic profiling in the area studied by the expedition of the Geological Institute, Russian Academy of Sciences and the Norwegian Petroleum Directorate on the R/V Akademik Nikolaj Strakhov in 2006; 56 seismic lines allow us to depict zones differing in seismic records that provide insights into their internal tectonic structure. Interpretation of the seismic data makes it possible to compile maps of the acoustic basement surface and sedimentary cover thickness in the studied area. These maps expand our knowledge of the geological history and geodynamics of the Knipovich Ridge at the neotectonic stage of its evolution.     

Strain measurements and tectonics of New Zealand

Measurements of shear strain from triangulation data have been made at 30 locations in New Zealand. The standard error of measurement in terms of strain rate is about ±1 · 10⁻⁷ y⁻¹ and values of up to 7 · 10⁻⁷ y⁻¹ are observed. Together with 22 fault-plane solutions for crustal earthquakes the measurements indicate broad-scale patterns of deformation. Between the Hikurangi and Flordland active margins is a 100-km-wide belt, the axial tectonic belt, with shear strain rate averaging 5 ± 1 · 10⁻⁷y⁻¹ and an azimuth of the principal axis of compression of 114 ± 8°. The rate of movement (45 mm y⁻¹) and direction (085°) between the Pacific and Indian plates from the Minster et al. pole can be accounted for by the measured strain in the axial tectonic belt through simple shear parallel to, and compression normal to, the belt. The similarity in the rates determined from triangulation data averaged over 20–100 years and from plate movement averaged over 5 m.y. indicates plate movement to be uniform in time. West of the axial tectonic belt in Nelson and Fiordland are two zones in which movement is highly oblique to plate movement, and can be explained by slip line deformation analogous to the deformation of Asia. The azimuth of the principal axis of compression in the Taupo rift and East Cape region is NE—SW, perpendicular to its direction in the axial tectonic belt, suggesting extension in the rift and East Cape region normal to the subduction zone.     

Segmentation and seismicity of the ultraslow Knipovich and Gakkel mid-ocean ridges

Comprehensive analysis of detailed bathymetric data obtained during legs 24–27 of the R/V Akademik Nikolai Strakhov has been carried out on the Knipovich Ridge. The revealed variations of magmatic activity along the axis supplement the available information on segmentation of this ridge [7, 19, 33]. The new statistical data characterize seismic activity under settings of ultraslow oblique extension. As follows from the seismic data, the Knipovich Ridge belongs to structural units with intermediate geodynamics between the spreading ridge and transform fault. Magmatic and amagmatic segments of the Arctic ultraslow Knipovich and Gakkel mid-ocean ridges are compared.     

Internal deformation of Sikhote Alin volcanic belt, far eastern Russia: Paleocene paleomagnetic results

We present paleomagnetic results of Paleocene welded tuffs of the 53–50 Ma Bogopol Group from the northern region (46°N, 137°E) of the Sikhote Alin volcanic belt. Characteristic paleomagnetic directions with high unblocking temperature components above 560 °C were isolated from all the sites. A tilt-corrected mean paleomagnetic direction from the northern region is D=345.8°, I=49.9°, α₉₅=14.6° (N=9). The reliability of the magnetization is ascertained through the presence of normal and reversed polarities. The mean paleomagnetic direction from the northern region of the Sikhote Alin volcanic belt reflects a counterclockwise rotation of 29° from the Paleocene mean paleomagnetic direction expected from its southern region. The counterclockwise rotation of 25° is suggested from the paleomagnetic data of the Kisin Group that underlies the Bogopol Group. These results establish that internal tectonic deformation occurred within the Sikhote Alin volcanic belt over the past 50 Ma. The northern region from 44.6° to 46.0°N in the Sikhote Alin volcanic belt was subjected to counterclockwise rotational motion through 29±17° with respect to the southern region. The tectonic rotation of the northern region is ascribable to relative motion between the Zhuravlevka terrane and the Olginsk–Taukhinsk terranes that compose the basements of the Sikhote Alin volcanic belt.     

A paleomagnetic polarity transition in the devonian columbus limestone of Ohio: A possible stratigraphic tool

A 40-m section, including the top 1 m of the Raisin River Dolomite, 31 m of the type section of the Columbus Limestone, and 7 m of the Delaware Limestone, was sampled at 15-cm intervals for paleomagnetic stratigraphy. The Raisin River Dolomite and the first meter of the Columbus Limestone were normally magnetized (position of the paleomagnetic pole: 24°N164°E, dp = 9.3°, dm = 11.3°). The next 16 m of the Columbus Limestone shows what the author interprets to be a transition zone from normal to reversely magnetized sediments. This is followed by 22 m of reversely magnetized sediments of the Columbus Limestone (position of the paleomagnetic pole: 45°N120°E, dp = 2.9°, dm = 1.6°) and 7 m of the Delaware Limestone (position of the paleomagnetic pole: 48°N118°E, dp = 4.0°, dm = 2.0°). Since the section might not represent sufficient time to average out secular variation, these pole positions may not represent the axial geocentric dipole. The reversal should be useful as a stratigraphic marker horizon. The transition zone should be useful for a detailed study of an Early Paleozoic reversal of the earth's magnetic field. Due to the low inclination values the reversal is best seen in the declinations.     

Delineation of active faulting and some tectonic interpretations in the Munich-Milan section of the eastern Alps — use of LANDSAT-1 and 2 imagery

Based on studies of images obtained from LANDSAT-1 and 2, several seemingly active movement zones have been delineated in a section of the eastern Alps and are being reported in the present paper for the first time. These zones, trending W—E to NW—SE, cut across all earlier Alpine boundaries and contacts and on either side along their length, are marked with drag effects, indicating their post-Alpine neotectonic nature. Their relation with the present-day central European stress field, as determined from fault-plane studies and in-situ stress measurements, has been sought. In conjunction with the evidence from neighbouring areas, a dextral shear tendency of the present-day Mediterranean is indicated. Further, a number of extensive lineaments have been observed in the Alpine section. Statistically, there are three major lineation sets trending N45°, N15°, N345°. They appear to have developed cogenetically as a result of shear and tensile failures due to a stress field with maximum principal stress oriented averagely at N15°. This direction of the maximum principal stress, deduced from the above lineation analysis of the eastern Alps, is in striking conformity with the one believed to have been in existence for the development of the Rhinegraben (N20°). It appears that the Rhinegraben and the Alpide belt have evolved cogenetically and concurrently under the same dominant stress field (P₁ = N—NNE, P₂ = vertical and P₃ = E—ESE) and hence the two geotectonic features are really not antagonistic and mutually incompatible as usually believed on the grounds that one involves tension (taphrogenesis — Rhinegraben) and the other compression (orogenesis — Alpide belt) but are different manifestations of the same stress field. Besides, some additional light has been thrown on the possible controls of development of the Giudicaria Line and cause of predominance of NE—SW trending sinistral faults.     

Conditions of formations of slightly enriched tholeiites in the northern Knipovich Ridge

New petrological and geochemical data were obtained for basalts recovered during cruise 24 of the R/V “Akademik Nikolay Strakhov” in 2006. These results significantly contributed to the understanding of the formation of tholeiitic magmatism at the northern end of the Knipovich Ridge of the Polar Atlantic. Dredging was performed for the first time both in the rift valley and on the flanks of the ridge. It showed that the conditions of magmatism have not changed since at least 10 Ma. The basalts correspond to slightly enriched tholeiites, whose primary melts were derived at the shallowest levels and were enriched in Na and depleted in Fe (Na-TOR type). The most enriched basalts are typical of the earlier stages of the opening and were found on the flanks of the ridge in its northernmost part. Variations in the ratios of Sr, Nd, and Pb isotopes and lithophile elements allowed us to conclude that the primary melts generated beneath the spreading zone of the Knipovich Ridge were modified by the addition of the enriched component that was present both in the Neogene and Quaternary basalts of Spitsbergen Island. Compared with the primitive mantle, the extruding magmas were characterized by positive Nb and Zr anomalies and a negative Th anomaly. The formation of primary melts involved melting of the metasomatized depleted mantle reservoir that appeared during the early stages of opening of the Norwegian-Greenland Basin and transformation of the paleo-Spitsbergen Fault into the Knipovich spreading ridge, which was accompanied by magmatism in western Spitsbergen during its separation from the northern part of Greenland.     

Depth-to-magnetic source analysis of the Arctic Ocean region

Approximately 400,000 line kilometers of high quality, low level Arctic aeromagnetic data collected by the Naval Research Laboratory, the Naval Oceanographic Office and the Naval Ocean Reseach and Development Activity from 1972 through 1978 have been analyzed for depth to magnetic source. This data set covers much of the Canada Basin, the Alpha Ridge, the central part of the Makarov Basin, the Lincoln Sea, the Eurasia Basin west and south of the 55°E meridian and the Norwegian-Greenland Sea north of the Jan Mayen Fracture Zone. The analysis uses the autocorrelation algorithm developed by Phillips (1975, 1978) and based on the maximum entropy method of Burg (1967, 1968, 1975). The method is outlined, examples of various error analysis techniques shown and final results presented. Where possible, magnetic source depth estimates are compared with basement depths derived from seismic and bathymetric data.All major known bathymetric features, including Vesteris Bank and the Greenland, Molloy and Spitsbergen fracture zones, as well as the Mohns, Knipovich and Nansen spreading ridges and the Alpha Cordillera appear as regional highs in the calculated magnetic basement topography. Shallow basement was also found under the northeastern Yermak Plateau, the Morris Jesup Rise and under the southern (Greenland-Ellesmere Island) end of the Lomonsosov Ridge. Regional magnetic source deeps are associated with such bathymetric depressions as the Canada, Makarov, Amundsen, Nansen, Greenland and Lofoten basins; more localized magnetic basement deeps are found over the Molloy F.Z. deep and over the Mohns, Knipovich and Nansen rift valleys. A linear magnetic basement deep follows the extension of Nares Strait through the Lincoln Sea toward the Morris Jesup Rise, suggesting the continuation of the Nares Strait or Wegener F.Z. into the Lincoln Sea. A sharp drop in the regional magnetic source depths to the southeast of the Alpha Ridge suggests the Alpha Ridge is not connected to structures in northwest Ellesmere Island as previously postulated from high altitude aeromagnetic collected by Canadian workers. A regional deep under the east Greenland shelf west of the Greenland Escarpment suggests the presence of 5–10 km of post-Paleozoic sediments.     

Seismotectonics of the Nepal Himalaya from a local seismic network

The National Seismological Network of Nepal consists of 17 short period seismic stations operated since 1994. It provides an exceptional view of the microseismic activity over nearly one third of the Himalayan arc, including the only segment, between longitudes 78°E and 85°E, that has not produced any M>8 earthquakes over the last century. It shows a belt of seismicity that follows approximately the front of the Higher Himalaya with most of the seismic moment being released at depths between 10 and 20 km. This belt of seismicity is interpreted to reflect interseismic stress accumulation in the upper crust associated with creep in the lower crust beneath the Higher Himalaya. The seismic activity is more intense around 82°E in Far-Western Nepal and around 87°E in Eastern Nepal. Western Nepal, between 82.5 and 85°E, is characterized by a particularly low level of seismic activity. We propose that these lateral variations are related to segmentation of the Main Himalayan Thrust Fault. The major junctions between the different segments would thus lie at about 87°E and 82°E with possibly an intermediate one at about 85°E. These junctions seem to coincide with some of the active normal faults in Southern Tibet. Lateral variation of seismic activity is also found to correlate with lateral variations of geological structures suggesting that segmentation is a long-lived feature. We infer four 250–400 km long segments that could produce earthquakes comparable to the M=8.4 Bihar–Nepal earthquake that struck eastern Nepal in 1934. Assuming the model of the characteristic earthquake, the recurrence interval between two such earthquakes on a given segment is between 130 and 260 years.     

Sediments in the Gakkel Ridge rift zone (Arctic Ocean): structure and history

The available seismic and magnetic data show the Gakkel Ridge rift zone consisting of the Atlantic and Siberian segments divided by a tectonic suture at 70° E. The two segments have had different histories recorded in their sedimentary cover. Apart from the difference in its morphology, the Siberian segment differs from the Atlantic one in the existence of a series of deposition centers, which might represent a vast Paleogenic basin that formed prior to the Gakkel Ridge. The simple model of North Atlantic spreading fails to explain the long and complex history of the Gakkel Ridge rift and the existence of the depocenters. The particular structure of this zone might have resulted from the growth of rift mountains by accretion of magmatic material during the Paleogene, without significant sea floor spreading.     

The Okavango giant mafic dyke swarm (NE Botswana): its structural significance within the Karoo Large Igneous Province

The structural organization of a giant mafic dyke swarm, the Okavango complex, in the northern Karoo Large Igneous Province (LIP) of NE Botswana is detailed. This N110°E-oriented dyke swarm extends for 1500 km with a maximum width of 100 km through Archaean basement terranes and Permo-Jurassic sedimentary sequences. The cornerstone of the study is the quantitative analysis of N>170 (exposed) and N>420 (detected by ground magnetics) dykes evidenced on a ca. 80-km-long section lying in crystalline host-rocks, at high-angle to the densest zone of the swarm (Shashe area). Individual dykes are generally sub-vertical and parallel to the entire swarm. Statistical analysis of width data indicates anomalous dyke frequency (few data <5.0 m) and mean dyke thickness (high value of 17 m) with respect to values classically obtained from other giant swarms. Variations of mean dyke thicknesses from 17 (N110°E swarm) to 27 m (adjoining and coeval N70°E giant swarm) are assigned to the conditions hosting fracture networks dilated as either shear or pure extensional structures, respectively, in response to an inferred NNW–SSE extension. Both fracture patterns are regarded as inherited brittle basement fabrics associated with a previous (Proterozoic) dyking event. The Okavango N110°E dyke swarm is thus a polyphase intrusive system in which total dilation caused by Karoo dykes (estimated frequency of 87%) is 12.2% (6315 m of cumulative dyke width) throughout the 52-km-long projected Shashe section. Assuming that Karoo mafic dyke swarms in NE Botswana follow inherited Proterozoic fractures, as similarly applied for most of the nearly synchronous giant dyke complexes converging towards the Nuanetsi area, leads us to consider that the resulting triple junction-like dyke/fracture pattern is not a definitive proof for a deep mantle plume in the Karoo LIP.     

Large earthquakes in the New Guinea-Solomon Islands area, 1873–1972

A catalogue of 1873–1972 earthquakes with M > 6.9 for the New Guinea—Solomon Islands region (130–165° E) is compiled. There are 152 events listed. Duda's (1965) results for 1900–1968 are improved for the Papua New Guinea area (141–156° E) because of the availability of historical data for that area.Although there is evidence of rapid Holocene uplift in the main seismic zones, there is little historical evidence for visible uplift or subsidence resulting directly from modern major earthquakes. Coastal subsidences commonly reported as a result of earthquakes are of smaller extent and appear to be due to settlement. However, the occurrence of tsunamigenic earthquakes does suggest that surface deformations do take place off-shore.Using Davies and Brune's (1971) method, regional fault slip rates over 5° -segments of the shallow seismic zone are determined from the seismicity catalogue. The slip rate for the island of New Guinea (Gutenberg and Richter's Region 16) is found to be at least 4.4 cm/y which is almost double the very anomalously low rate of 2.3 cm/y found by Davies and Brune (1971). If allowance is made for shear movement without seismicity and for the approximately ratio of dip-slip versus strike-slip faulting indicated by fault plane solutions, the agreement with Le Pichon's (1970) approach value of 10.7 cm/y for the Pacific—India (Australia) plates is reasonable. The fault slip rate in the area between east New Britain and Bougainville at the Pacific—Bismarck—Solomon triple junction is extremely high (20.6 cm/y at least). The smallest slip rate (1.5 cm/y) is found for westernmost New Guinea (130–135° E).Temporal cumulative summation of moments curves show a periodicity of approximately 25 years in the seismic activity at the triple junction (150–155° E). Elsewhere the rate of seismic activity is aperiodic.     

Paleomagnetic dating of Alleghanian orogenesis and mineralisation in the Mascot–Jefferson City zinc district of East Tennessee, USA

The Mascot–Jefferson City (M-JC) Mississippi Valley-type (MVT) deposits are in the Valley and Ridge province of the Appalachian orogen in East Tennessee. They have been a major source of zinc for the USA but their age is uncertain and thus their genesis controversial. About 10 specimens from each of 37 sites have been analysed paleomagnetically using alternating field and thermal step demagnetisation methods and saturation isothermal remanence methods. The sites sample limestones, dolostones, breccia clasts and sphalerite–dolomite MVT mineralisation from mines in the Lower Ordovician Kingsport and Mascot formations of the Knox Group. The characteristic remanent magnetisation (ChRM) is carried by magnetite in the limestones, by both magnetite and pyrrhotite in the dolostones and by pyrrhotite preferentially to magnetite in the mineralisation. Mineralized sites have a more intense ChRM than non-mineralised, indicating that the mineralising and magnetisation event are coeval. Paleomagnetic breccia tests on clasts at the three sites are negative, indicating that their ChRM is post-depositional remagnetisation, and a paleomagnetic fold test is negative, indicating that the ChRM is a remagnetisation, and a post-dates peak Alleghanian deformation. The unit mean ChRM direction for the: (a) limestones gives a paleopole at 129°E, 12°N (d_p=18°, d_m=26°, N=3), indicating diagenesis formed a secondary chemical remanent magnetisation during the Late Ordovician–Early Silurian; (b) dolomitic limestones and dolostone host rocks gives a paleopole at 125.3°E, 31.9°N (d_p=5.3°, d_m=9.4°, N=7), recording regional dolomitisation at 334±14 Ma (1σ); and (c) MVT mineralisation gives a paleopole at 128.7°E, 34.0°N (d_p=2.4°, d_m=4.4°, N=25), showing that it acquired its primary chemical remanence at 316±8 Ma (1σ). The mineralisation is interpreted to have formed from hydrothermal fluid flow, either gravity or tectonically driven, after peak Alleghanian deformation in eastern Tennessee with regional dolomitisation of the host rocks occurring as part of a continuum during the 20 Ma prior to and during peak deformation.     

Present-day strain rates at the northern end of the ethiopian rift valley

Five successive geodimeter surveys during a five-year period yield hints as to the manner and rate of crustal extension in the Ethiopian rift valley. The northern geodimeter network traverses an en-echelon offset in the Wonji fault belt at latitude 8°30′N; this belt comprises the youngest volcanism and faulting of the rift floor. The northern network surveys reveal progressive rift extension at mean rates of 3–5 mm/year and strain rates of 6–16 · 10⁻⁷/yr, essentially confined within the Wonji fault belt segments. Small longitudinal motions of persistent dextral sense have occurred in the intervening zone between the offset segments. It is too early to say whether these deformations are local, regional or plate-tectonic phenomena, but the present aseismicity of the rift suggests the buildup of regional strain.     

Spatial relationships between natural seismicity and faults, southeastern Ontario and north-central New York state

A statistical analysis was carried out to investigate spatial associations between natural seismicity and faults in southeastern Ontario and north-central New York State (between 73°18′ and 77°00′W and 43°30′ and 45°18′N). The study area is situated to the west of the seismically active St. Lawrence fault zone, and to the east of the Lake Ontario basin where recently documented geological and geophysical evidence points to possible neotectonic faulting. The weights of evidence method was used to judge the spatial associations between seismic events and populations of faults in eight arbitrarily defined orientation groups. Spatial analysis of data sets for seismic events in the periods 1930–1970 and post-1970 suggest stronger spatial associations between earthquake epicentres and faults with strikes that lie in the NW–SE quadrants, and weaker spatial associations of epicentres with faults that have strikes in the NE–SW quadrants. The strongest spatial associations were determined for groups of faults with strikes between 101° and 146°. The results suggest that faults striking broadly NW–SE, at high angles to the regional maximum horizontal compressive stress, are statistically more likely to be spatially associated with seismic events than faults striking broadly NE–SW. If the positive spatial associations can be interpreted as indicating genetic relationships between earthquakes and mapped faults, then the results may suggest that, as a population, NW–SE trending faults are more likely to be seismically active than NE–SW striking faults. Detailed geological studies of faults in the study area would be required to determine possible neotectonic displacements and the kinematics of the displacements.     

Conditions of Quaternary magmatism at Spitsbergen Island

Petrological and geochemical data obtained on the Quaternary lavas of volcanoes at Spitsbergen Island indicate that the rocks were produced via the deep-seated crystallization of parental alkaline magmas at 8–10 kbar. The character of clinopyroxene enrichment in incompatible elements indicates that the mineral crystallized from more enriched melts than those inferred from the composition of the host lavas. These melts were close to the parental melts previously found as veinlets in mantle hyperbasite xenoliths in the lavas. According to the character of their enrichment in Pb and Sr radiogenic isotopes and depletion in Nd, the basalts from Spitsbergen Island define a single trend with the weakly enriched tholeiites of the Knipovich Ridge, a fact suggesting the closeness of the enriched sources beneath the continental margin of Spitsbergen and beneath the spreading zone. Magmatic activity at Spitsbergen was related to the evolution of the Norwegian-Greenland basin, which evolved in pulses according to the shift of the spreading axes. The most significant of the latter events took place in the Neogene, when the Knipovich Ridge obtained its modern position near the western boundary of Spitsbergen. Early in the course of the evolution, the emplacement of alkaline melts generated at Spitsbergen into the oceanic mantle could form the enriched mantle, which was later involved in the melting process beneath the spreading zone.     

Reconnaissance paleomagnetism of Late Triassic blocks, Kuyul region, Northern Kamchatka Peninsula, Russia

The northernmost Kamchatka Peninsula is located along the northwestern margin of the Bering Sea and consists of complexly deformed accreted terranes. Progressing inland from the northwestern Bering Sea, the Olyutorskiy, Ukelayat and Koryak superterranes (OLY, UKL and KOR) are crossed. These terranes were accreted to the backstop Okhotsk-Chukotsk volcanic-plutonic belt (OChVB) in northernmost Kamchatka. A sedimentary sequence of Albian to Maastrichtian age overlaps the terranes and units of the Koryak superterrane, and constrains their accretion time. A paleomagnetic study of blocks within the Kuyul (KUY) terrane of the Koryak superterrane was completed at two localities (Camp 2: λ=61.83°N, φ=165.83°E and Camp 3: λ=61.67°N, φ=164.75°E). At both localities, paleomagnetic samples were collected from Late Triassic (225–208 Ma) limestone blocks (2–10 m in outcrop height) within a melange zone. Although weak in remanent magnetization, two components of remanent magnetization were observed during stepwise thermal demagnetization at 32 sites. The A component of magnetization was observed between room temperature and approximately 250 °C. This magnetic component is always of downward directed inclination and shows the best grouping at relatively low degrees of unfolding. Using McFadden–Reid inclination-only statistics and averaging all site means, the resulting A component mean is I_opt=60.3°, I₉₅=5.0° and n=36 (sites). The B magnetic component is observed up to 565 °C, at which temperature, most samples have no measurable remanent magnetization, or growth of magnetic minerals has disrupted the thermal demagnetization process. Combining sites with Fisher estimates of kappa (k-value)≥13 and n (sites)≥3, where bedding orientation differs within a block, most of these sites show the best grouping of B component directions at 100% unfolding, and two of the blocks display remanent magnetizations of both upward and downward directed magnetic inclination. Combining sites with Fisher estimates of kappa (k-value)≥13 and n (sites)≥3, the resulting overall B component paleolatitude and associated uncertainty are λ_obs=30.4°N or S, λ₉₅=8.9° and n=19 (sites). When compared with the expected North America paleolatitude of λ_{APWP expected}=57.9°N, our data support a model in which blocks within the Koryak superterrane are allochthonous and far travelled.