Polar motion of a triaxial earth and dynamic plate tectonics

A theoretical and quantitative analysis of the earth's polar motion, the Chandler wobble and the polar wandering was made under a triaxial, quasi-rigid and rotationally imbalanced earth model and the assumption that the polar excitation was due to the episodic energy perturbation in the earth's upper layers. The Chandler wobble was found to have two frequency components and was quasi-permanent; whereas the polar wandering linked dynamically with the secular tectonic movements in the earth's upper layers. The attempt of the earth to damp its products of inertia for rotation stability maintained the polar motion, while the polar wandering would produce a system of Coriolis torques that provided driving mechanisms to the continental drift, sea-floor spreading and related phenomena, as well as inducing viscous flows in the interior. The secondary deformation due to the earth's non-rigidity was not analyzed in the paper, but the probable connections between the dynamics of polar wandering and the thermal convection in the interior were briefly discussed. The analysis presents the attempt for an integral interpretation of the earth's dynamic evolution or an interpretation of the polar motion, plate tectonics, and the earth's generation and dissipation of excess energy under a unified dynamic theory.     

Highly depleted oceanic lithosphere in the Rheic Ocean: Implications for Paleozoic plate reconstructions

The Rheic Ocean formed at ca. 500 Ma, when several peri-Gondwanan terranes (e.g. Avalonia and Carolinia) drifted from the northern margin of Gondwana, and were consumed during the Late Carboniferous collision between Laurussia and Gondwana, a key event in the formation of Pangea. Several mafic complexes ranging in age from ca. 400–330 Ma preserve many of the lithotectonic and/or chemical characteristics of ophiolites. They are characterized by anomalously high εNd values that are typically either between or above the widely accepted model depleted mantle curves. These data indicate derivation from a highly depleted (HD) mantle and imply that (i) the mantle source of these complexes displays time-integrated depletion in Nd relative to Sm, and (ii) depletion is the result of an earlier melting event in the mantle from which basalt was extracted. The extent of mantle depletion indicates that this melting event occurred in the Neoproterozoic, possibly up to 500 million years before the Rheic Ocean formed. If so, the mantle lithosphere that gave rise to the Rheic Ocean mafic complexes must have been captured from an adjacent, older oceanic tract. The transfer of this captured lithosphere to the upper plate enabled it to become preferentially preserved. Possible Mesozoic–Cenozoic analogues include the capture of the Caribbean plate or the Scotia plate from the Pacific to the Atlantic oceanic realm. Our model implies that virtually all of the oceanic lithosphere generated during the opening phase of the Rheic Ocean was consumed by subduction during Laurentia–Gondwana convergence.     

Tectonic plate motion and deformation inferred from very long baseline interferometry

We inverted 76 rates of change of baseline lengths measured with very long baseline interferometry (VLBI) during the period 1979–1989 to estimate the parameters of motions of the North American (noam) and Eurasian (eura) plates relative to the Pacific (pcfc) plate. We considered two types of plate motion models, namely, rigid and non-rigid models. In the non-rigid models, we simultaneously determined the non-rigid motions of several stations near plate boundaries due to intraplate deformation. Intraplate deformation in the regions far away from plate boundaries is assumed to be negligible.Among several models considered, a non-rigid model called M2 is found to fit most closely to the observed data. In this model, six stations are assumed to be capable of the non-rigid motion; those are goldvenu, hatcreek, mojave12, ovro 130 and vndnberg, in the southwestern United States and kashima, in Japan. M2 gives parameter sets of 0.827 ± 0.035°/m.y., about 50.5 ± 1.2°N, 78.5 ± 5.3°W and 0.889 ± 0.049°/m.y., about 59.7 ± 1.9°N, 85.1 ± 7.4 °W, representing the noam-pcfc and eura-pcfc relative motions. The plate motion parameters of M2 are nearly identical to those of the newest global-scale plate motion model nuvel-1. The noam-pcfc and eura-pcfc rotation rates of M2 respectively deviate only 0.044°/m.y. and 0.010°/m.y. from those of nuvel-1 (these deviations are only about 6% and 1%, respectively, of the rotation rates themselves). The noam-pcfc and eura-pcfc poles of M2 both lie only 2° from those of nuvel-1 (within a 2σ error ellipse of each pole). nuvel-1 is determined from spreading rates at mid-ocean ridges, azimuths of transform faults and earthquake slip vectors. Since the spreading rates are estimated from marine magnetic anomalies integrated over a geological timescale, nuvel-1 gives the plate motions averaged over this timescale. Thus, we may conclude that there is no appreciable difference between the plate motions averaged over a geological timescale (millions of years) and those in a recent short period ( ~ 10 yr).M2 also gives the horizontal non-rigid motions of VLBI stations in the southwestern United States at rates of 6–9 mm/yr and roughly in opposite direction to the rigid motion of each station associated with plate motion. hatcreek, located near the northern part of the Basin and Range Province (B&R), also shows additional westward motion of about 9 mm/yr, suggesting crustal stretching in the northern B&R. The US VLBI stations show subsidence at rates of about 5–7 mm/yr, except for goldvenu and ovro 130, whose subsidence is negligible. The Japanese VLBI station, kashima, has a horizontal non-rigid motion of about 20 mm/yr in the west-northwest direction, roughly opposite to the direction of the rigid motion. kashima also shows subsidence at a rate of about 12 mm/yr, which is larger than that deduced from geodetic data but consistent with the result from GPS.     

Late Eocene and Maastrichtian paleomagnetic poles for the Pacific plate: implications for the validity of seamount paleomagnetic data

Mean paleomagnetic poles for the Pacific plate have been calculated for the Late Eocene (39 Ma) and the Maastrichtian (69 Ma). The former is located at 77.6°N, 7.6°E, the latter at 69.9°N, 0.9°E. Although these pole positions are little changed from previous calculations they are better constrained with additional data. Slightly less than half of the data are derived from the inversion of seamount magnetic fields providing an excellent opportunity to compare such data with other paleomagnetic data of the same age. As no significant systematic difference between the two types of data is evident, it is inferred that most seamount paleomagnetic data are probably useful indicators of the paleomagnetic field direction.     

Nature and origin of the J‐magnetic anomaly offshore Iberia–Newfoundland: implications for plate reconstructions

The nature and origin of the J‐magnetic anomaly along the Iberia–Newfoundland margins are controversial and its validity for plate kinematic reconstructions questioned. At present, it is interpreted as either an oceanic isochron or an edge effect of oceanic crust corresponding to lithosphere breakup. Both interpretations result in restorations that are in conflict with the current knowledge from Pyrenean and North Atlantic geology. We combine seismic interpretations and dating of magmatic additions with magnetic data to examine the nature and formation process of this anomaly and discuss its value for plate restorations. We show that the J‐anomaly is the result of polygenic and multiple magmatic events occurring during and after the formation of the first oceanic crust. Therefore, we conclude that the J‐anomaly cannot be used for plate kinematic studies and, more generally, we question the validity of using ill‐defined magnetic anomalies outside unequivocal oceanic domains for plate reconstructions.     

Intercontinental structural ties and mobilistic reconstructions

The authors discuss the geological data contradicting basic mobilistic reconstructions of the positions of continents in the past. Analysis is given of geological consequences of the opening of the Atlantic Ocean, of paleotectonic relationships between Africa and America, and of possible movements of the Indian plate. In these cases the geological facts show that mobilistic reconstructions, now considered classical, are unjustified.

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Zusammenfassung Es gibt geologische Daten, die gegen eine mobiltische Rekonstruktion alter Kontinentpositionen sprechen. Dies wird diskutiert an der Öffnung des Atlantiks, an der paläotektonischen Beziehung von Afrika und Amerika, an den Bewegungen, die der indische Kontinent mitgemacht hat. In all den genannten Fällen kann gezeigt werden, daß eine mobilistische Rekonstruktion nicht gerechtfertigt ist.

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Résumé Il a des données géologiques qui parlent contre reconstructions mobilistiques de vieilles positions continentales. Ceci sera discuté pour l'ouverture de l'Atlantique, pour la liaison paléotectonique de l'Afrique et de l'Amérique, pour les mouvements ausquels le continent indien a participé. Dans tous ces cas mentionnés, il sera démontré que reconstructions mobilistiques ne sont pas justifiée.

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古地理和古地形三维再造的发展趋势

“模拟技术”是当今科学研究的主要手段之一,把它应用于地质学的某些方面,能取得很好的效果,如盆地模拟已经发展到了比较成熟的阶段。但在古地理和古地形再造方面,国内外在这方面的研究甚少,尤其是在三维模拟方面。根据Hay等(1989)提出的物质平衡理论,在给定时间间隔内,作用在研究区表面的构造、侵蚀和沉积过程所造成的沉积物的侵蚀总量与沉积总量之间物质守恒,与古地形再造和古地理重建相结合,用三维数值模型来模拟研究区域的变化过程,用GIS技术把这个变化过程的动态显示出来,是一项具有理论意义和实践意义的工作。     

Mesozoic and Cenozoic reconstructions of the South Atlantic

The movement of Antarctica with respect to South America has a number of implications for paleocirculation as well as for the reconstructions of Gondwanaland. Recent papers on the Southwest Indian Ridge have published new or revised poles of opening for Africa and Antarctica which can be combined with the poles of opening between South America and Africa to give resultant motions between South America and Antarctica.The first indication of a complete closure between South America and the Antarctic Peninsula is at anomaly 28 time (64 Ma) as the two continents are now configured. Between anomaly 28 time (64 Ma) and anomaly M0 time (119 Ma) the amount of closure does not change greatly, and the small computed overlap can be explained by minor uncertainties in the rotation poles used for the reconstructions or some slight extension between East and West Antarctica. By 135 Ma some rotation or translation of the Antarctic Peninsula with respect to East Antarctica must be postulated in addition to any presumed extension between East and West Antarctica in order to avoid an overlap of South America with the Antarctic Peninsula.Having determined what we feel to be a viable reconstruction of Western Gondwanaland and holding South America fixed, we rotated Africa and Antarctica, with respect to South America, for eight different times during the past. Africa moved away from South America in a more or less consistent manner throughout the time period, closure to present, while Antarctica moved away from Africa in a consistent manner only between 160 Ma and 64 Ma. At 64 Ma its motion changed abruptly: it slowed its north-south motion with respect to Africa and began slow east-west extension with respect to South America. This change supports the hypothesis that a major reorganization of the triple junction between Africa, Antarctica and South America occurred between 60 and 65 Ma. The triple junction changed from ridge-ridge-ridge to ridge-fault-fault at the time of the major westward jump of the Mid-Atlantic Ridge just south of the Falkland-Agulhas Fracture Zone.The Mesozoic opening of the Somali Basin moved Madagascar from its presumed original position with Africa in Gondwanaland. The closure of Sri Lanka with India produces a unique fit for India and Sri Lanka with respect to Africa, Madagascar and Antarctica. This fit juxtaposes geological localities in Southeast India against similar localities in Enderhy Land. East Antarctica. The late Jurassic opening in the Somali Basin is tied to opening of the same age in the Mozambique Basin. Since this late Jurassic movement represents the initial break-up of Gondwanaland, it is assumed that similar movement must have occurred in what is now the western Weddell Sea and may also explain the opening evidenced by the Rocas Verdes region of southern South America.     

Neogene patterns of relative plate motion for Africa-Europe: some implications for recent central Mediterranean tectonics

A detailed relative motion picture for the Neogene Africa-Europe plate kinematics is presented. The kinematic reconstruction was carried out using the finite difference solution between the rotation parameters determined for Anomalies 7 to 2 in the Africa-North America-Europe plate motion circuit. The analysis shows a motion of Africa with respect to Europe which is NNE directed during Late Oligocene to Burdigalian times, becoming NNW trending from the Langhian to the early Tortonian; from upper Tortonian times onward, the motion changes to a clear north-west directed convergence. Major Late Neogene tectonic features of the central Mediterranean region can, to a large extent, be explained within the context of the reconstructed major plate motions. Late Tortonian to Recent Africa-Europe slip vectors are compatible with a variety of geological phenomenoa such as north-west directed subduction beneath Calabria, south-east translation of Calabria and extension in the Tyrrhenian Sea, north-west trending slip vectors from thrust earthquakes between Gibraltar and Sicily, and dextral strike-slip across the North African margin.     

Analysis of kames for palaeogeographical reconstructions

A well-pronounced order in the vertical and horizontal arrangement of sediments was observed in the kames ridges occurring within the compass of the Middle Polish (Riss) glaciation on the Lódź Upland in Central Poland. The order was also noticed in the distribution of the kamc sediments and in the directions of dips of layers. Palaeogeographical conclusions could be drawn concerning the mechanism of waning of the dead-ice cover, the directions of outflow of the ablation waters, the relief of surface underlying the glacier and its influence upon the present-day relief. Analysis of the Pleistocene kames brings new elements into the discussion on the course and extension of postglacial, mainly periglacial, denudation of the kamc landscape.     

Weighted Pivot Coordinates for Compositional Data and Their Application to Geochemical Mapping

The log ratio methodology converts compositional data, such as concentrations of chemical elements in a rock, from their original Aitchison geometry to interpretable real orthonormal coordinates, thereby allowing meaningful statistical processing and visualization. However, it must be taken into account that the original concentrations can be flawed by detection limit or imprecision problems that can severely affect the resulting coordinates. This paper aims to construct such orthonormal log ratio coordinates, called weighted pivot coordinates, that capture the relevant relative information about an original component and treat the redundant information in a controlled manner. Theoretical developments are supported by a thorough simulation study. Weighted pivot coordinates are then applied to the geochemical mapping of catchment outlet sediments from the National Geochemical Survey of Australia illustrating their advantage over possible alternatives.     

Tectonostratigraphy as a basis for paleotectonic reconstructions

Tectonostratigraphy deals with distinguishing megasequences and their interpretation in terms of tectonic settings. It is closely related to sequence stratigraphy and regional tectonics. Tectonosratigraphic units are distinctly displayed on seismic profiles. Examination of megasequences is a reliable tool for regional-scale research. We discuss, using several specific examples, the process of identifying tectonostratigraphic units in various types of sedimentary basins, as well as their relations to chronostratigraphic units.     

Resolving slip vector azimuths and plate motion along the southern boundary of the South Bismarck Plate,Papua New Guinea

The interaction of the Australian, South Bismarck and Solomon Sea Plates in Papua New Guinea is the source of frequent earthquakes that occur as a result of subduction and arc continent collision. Previous investigators have drawn attention to a discontinuity in the horizontal azimuth of slip vectors along the southern boundary of the South Bismarck Plate, with those to the west of 148°E being systematically rotated 20ndash;30° clockwise compared to those located east of 148°E. This has led to the suggestion that relative motion may be occurring between the Huon Peninsula and New Britain or that more than two plates are acting south of the South Bismarck Plate. Global positioning system (GPS) measurements since 1991 indicate that there is no internal deformation occurring within the South Bismark Plate and that at least two distinct plates are in contact with the southern edge of the South Bismarck Plate. We show from a study of a recent earthquake dataset that the change in slip vector azimuth can be modelled by the interaction of the overriding South Bismarck Plate with the underthrusting Australian and Solomon Sea Plates, consistent with the GPS observations, while maintaining the South Bismarck Plate as a rigid entity. We found that a transition zone exists between 147°E and 148°E where the underlying plate changes from the Australian Plate to the Solomon Sea Plate. There are insufficient data at present to indicate whether or not a third plate, the Woodlark Plate, is also interacting directly with the South Bismarck Plate in this transition zone. Slip vector azimuths were used to estimate an Euler pole (6.74°S, 144.64°E), which describes the relative motion of the South Bismarck and Solomon Sea Plates along the New Britain Trench.     

Identification of paleosol types and their applicability for paleoclimatic reconstructions

Genetic diagnostics of paleosols are based on the morphology of soil profiles, on the combination of the stable properties of microstructure and on the mineral composition of soils (taking into account diagenetic transformations). As a result of spatial paleopedological reconstructions performed both at a level of the structure of soil cover (study of paleosols in the catena) and at the zonal-provincial level allowed to compile soil maps of Eastern Europe for the Mikulino Interglacial (125 ka BP), the Bryansk Interstadial (32 to 24 ka) and the Holocene climatic optimum (5.5 ka). These data can be used for paleoclimatic reconstructions.     

Seismic activity associated with the subducting motion of the Philippine Sea plate beneath the Kanto district, Japan

The Pacific plate and the Philippine Sea plate overlap and subduct underneath the Kanto region, central Japan, causing complex seismic activities in the upper mantle. In this research, we used a map selection tool with a graphic display to create a data set for earthquakes caused by the subducting motion of the Philippine Sea plate that are easily determined. As a result, we determined that there are at least four earthquake groups present in the upper mantle above the Pacific plate. Major seismic activity (Group 1) has been observed throughout the Kanto region and is considered to originate in the uppermost part of mantle in the subducted Philippine Sea plate, judging from the formation of the focal region and comparison with the 3D structure of seismic velocity. The focal mechanism of these earthquakes is characterized by the down-dip compression. A second earthquake layer characterized by down-dip extension (Group 2), below the earthquakes in this group, is also noted. The focal region for those earthquakes is considered to be located at the lower part of the slab mantle, and the Pacific plate located directly below is considered to influence the activity. Earthquakes located at the shallowest part (Group 3) form a few clusters distributed directly above the Group 1 focal region. Judging from the characteristics of later phases in these earthquakes and comparing against the 3D structure of seismic velocity, the focal regions for the earthquakes are considered to be located near the upper surface of the slab. Another earthquake group (Group 4) originates further below Group 2; it is difficult to consider these earthquakes within a single slab. The seismic activities representing the upper area of the Philippine Sea plate are Group 3. This paper proposes a slab geometry model that is substantially different from conventional models by strictly differentiating the groups.     

Gabbro-granite intrusive series and their indicator importance for geodynamic reconstructions

One of the problems faced by researchers when paleogeodynamic reconstructions are carried out for deeply eroded orogenic terranes is the limited usage of geological and isotopic geochemical data on volcanic associations. The utilization of information on gabbroids and granitoids considered separately also fails to resolve this problem. The convergence of features of arc, collisional, and within-plate magmatic processes leads the researcher to search for indicator plutonic associations, such as paired gabbrogranite intrusive series. The latter were distinguished using geoinformation databases (including those compiled by the authors of this paper), which were composed for the Early Caledonides in the Altai-Sayan folded area and adjacent territories. This makes it possible to characterize mantle-crustal magmatism in suprasubductional and collisional environments and the conditions under which these geodynamic regimes (plate- and plume-tectonic factors) interact. This paper presents estimates of the composition of the parental basic magmas, distinctive features of their differentiation, and the compositional specifics of the accompanying extensive granite-forming processes. The example of the Altai-Sayan folded area and adjacent territories is employed to correlate the composition of the basic-ultrabasic and granitoid magmas and, on this basis, distinguish (a) differentiated gabbro-tonalite-plagiogranite intrusive series corresponding to accretionary-collisional geodynamic environments, and (b) gabbro-monzonite-granosyenite-potassic granite intrusive series, which were produced when the accretionary-collisional system was affected by a plume.     

Environmental risk management of urban growth poles regarding national importance

Urban settlements of regional importance from Romania impose within the national settlements system by their ability to converge material and human flows, leading to important unbalances in the relationships between the components of territorial systems. The status they held, both in the communist period and at present, made the economic activities developed in a short time exceed the adapting ability of the other components of the territorial system, the natural environment strongly feeling the industrialisation pressure. Identifying the specificities of the environmental risk management at the level of development poles of regional importance was done by detailed analyses accomplished in Craiova, a representative town for the high concentration of economic activities strongly pressing upon the natural environment. The construction of the model of environmental risk management supposes the classifying of risks by a series of criteria, a model which constitutes a useful instrument for the decision factors. The management of unbalances from the territorial systems determined by industrialisation is one of the major preoccupations of the decision factors from Romania, in order to be aligned to the European environment standards.