Thermal implications for the evolution of the spitsbergen transform fault

Heat flow taken between Svalbard and Greenland reveal three thermal provinces:

1. (1) the Molloy Ridge within the Spitsbergen Transform,

2. (2) the Yermak Plateau

3. (3) the northeastern margin of Svalbard (Nordaustlandet).

The Molloy Ridge is a short spreading segment and the average heat flow is much above the Sclater et al. (1971), cooling curve but agrees with values from the Norwegian-Greenland Sea. An additional zone of intrusion identified by heat flow lies to the northwest of the Molloy Ridge. It straddles both the visible fracture zone and part of the Yermak Plateau. A thermal boundary lies between the warm western segment of the Yermak Plateau and the shelf off Nordaustlandet. If the thermal subsidence of the western Yermak Plateau can be traced to the latest heating episode then it is likely that the crust is similar to oceanic in composition and not older than 13 m.y. (approximately 20 m.y. younger than the northeastern segment of the plateau). Plate rotation shows that there was no room for the western segment of the plateau prior to anomaly 7. We postulate that the original transform is associated with the Hornsund Fault zone. In response to deviatoric stress across the oblique ridge-transform system, the Nansen Ridge propagated southwestward aborting the old transform trace, and shifted to its present position.It is suggested that this propagation and migration of the ridge-transform system across a zone of extensional deviatoric stress allowed the massive intrusion of basalt forming the Western Yermak Plateau. The propagation phenomenon coincides with large-scale Tertiary volcanic activity on Svalbard.Readjustment and migration of the oblique transform is still taking place. As the transform-ridge system is liberated from continental constraints, the migration rate will diminish as orthogonality is approached.     

Monotonicity for MPFA methods on triangular grids

Flow in a porous medium can be described by a set of non-linear partial differential equations. The pressure variable satisfies a maximum principle, which guarantees that the solution will have no oscillations. A discretisation of the pressure equation should preserve this monotonicity property. Whether a numerical method is monotone will depend both on the medium and on the grid. We study monotonicity of Multi-point Flux Approximation methods on triangular grids. We derive necessary conditions for monotonicity on uniform grids. Further, we study the robustness of the methods on rough grids, and quantify the violations of the maximum principle. These investigations are done for single phase flow, however, they are supported by two-phase simulations.     

Non-hydrostatic pressure in σ-coordinate ocean models

Numerical ocean modelling is computationally very demanding. Traditionally, the hydrostatic approximation has been applied to reduce the computational burden. This approximation is valid in large scale studies with coarse grid resolution. With faster computers and gradually smaller grid sizes, we may expect that more studies will be performed with non-hydrostatic ocean models. In recent papers several methods for including non-hydrostatic pressure in σ-coordinate models have been suggested. In this paper the sensitivity of the non-hydrostatic pressure field, the velocity fields, and the density fields to changes in the method for computing non-hydrostatic pressure in σ-coordinate ocean models is addressed.The first test case used involves the propagation and breaking of an internal wave at an incline in a tank. The other test case concerns tidally driven flow over a sill in a stratified fjord. The results from our numerical exercises suggest that the velocity and density fields are very robust to the model choices investigated here. The differences between the model results are of the same order as the uncertainty due to the internal pressure gradient error, and they are smaller than an estimate of the uncertainty due to subgrid scale closure.     

The role of the Spitsbergen shear zone in determining morphology,segmentation and evolution of the Knipovich Ridge

In 1989–1990 the SeaMARC II side-looking sonar and swath bathymetric system imaged more than 80 000 km² of the seafloor in the Norwegian-Greenland Sea and southern Arctic Ocean. One of our main goals was to investigate the morphotectonic evolution of the ultra-slow spreading Knipovich Ridge from its oblique (115° ) intersection with the Mohns Ridge in the south to its boundary with the Molloy Transform Fault in the north, and to determine whether or not the ancient Spitsbergen Shear Zone continued to play any involvement in the rise axis evolution and segmentation. Structural evidence for ongoing northward rift propagation of the Mohns Ridge into the ancient Spitsbergen Shear Zone (forming the Knipovich Ridge in the process) includes ancient deactivated and migrated transforms, subtle V-shaped-oriented flank faults which have their apex at the present day Molloy Transform, and rift related faults that extend north of the present Molloy Transform Fault. The Knipovich Ridge is segmented into distinct elongate basins; the bathymetric inverse of the very-slow spreading Reykjanes Ridge to the south. Three major fault directions are detected: the N-S oriented rift walls, the highly oblique en-echelon faults, which reside in the rift valley, and the structures, defining the orientation of many of the axial highs, which are oblique to both the rift walls and the faults in the axial rift valley. The segmentation of this slow spreading center is dominated by quasi stationary, focused magma centers creating (axial highs) located between long oblique rift basins. Present day segment discontinuities on the Knipovich Ridge are aligned along highly oblique, probably strike-slip faults, which could have been created in response to rotating shear couples within zones of transtension across the multiple faults of the Spitsbergen Shear Zone. Fault interaction between major strike slip shears may have lead to the formation of en-echelon pull apart basins. The curved stress trajectories create arcuate faults and subsiding elongate basins while focusing most of the volcanism through the boundary faults. As a result, the Knipovich Ridge is characterized by Underlapping magma centers, with long oblique rifts. This style of basin-dominated segmentation probably evolved in a simple shear detachment fault environment which led to the extreme morphotectonic and geophysical asymmetries across the rise axis. The influence of the Spitsbergen Shear Zone on the evolution of the Knipovich Ridge is the primary reason that the segment discontinuities are predominantly volcanic. Fault orientation data suggest that different extension directions along the Knipovich Ridge and Mohns Ridge (280° vs. 330°, respectively) cause the crust on the western side of the intersection of these two ridges to buckle and uplift via compression as is evidenced by the uplifted western wall province and the large 60 mGal free air gravity anomalies in this area. In addition, the structural data suggest that the northwards propagation of the spreading center is ongoing and that a `normal' pure shear spreading regime has not evolved along this ridge. This revised version was published online in November 2006 with corrections to the Cover Date.     

U–Th–Pb–REE systematics of organic-rich shales from the ca. 2.15 Ga Sengoma Argillite Formation, Botswana: Evidence for oxidative continental weathering during the Great Oxidation Event

The redox-sensitive geochemical behavior of uranium permits the use of Th/U ratios as a geochemical proxy for the oxidation state of the atmosphere during deposition. Due to the effects of post-depositional uranium mobility on Th/U ratios during events involving oxygenated fluids, direct measurements of Th/U ratios are often misleading even for drill core samples. Because both of these elements radioactively decay and produce lead isotopes, the Pb isotope composition may reflect the depositional Th/U ratio, although the Th/U ratios induced by changes shortly after deposition may not be distinguished from the true depositional Th/U ratios. In order to effectively evaluate the time-integrated Th/U ratio (κ_a), values for the initial depositional Pb isotope composition must be determined or accepted from the models for the whole Earth.While the timing for the rise of atmospheric oxygen is reasonably well constrained now, its effect on continental weathering and ocean redox state remains poorly constrained and debated. The ca. 2.15 Ga Sengoma Argillite Formation of Botswana contains organic-rich shales deposited during the Great Oxidation Event. The slope of the ²⁰⁷Pb/²⁰⁴Pb–²⁰⁶Pb/²⁰⁴Pb array of shales from the Sengoma Argillite Formation corresponds to a Pb–Pb age that is within analytical error of the depositional age and is, therefore, inferred to be the time by which the time-integrated thorogenic and uranogenic lead growth started. The time-integrated lead growth corresponds to an average κ_a of 2.63 (± 0.62, 1σ) for the organic-rich shales of the Sengoma Argillite Formation. This is lower than Th/U ratios measured in Archean shale suites or estimated for the Archean–Proterozoic average upper continental crust [Taylor, S.R. and McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, Oxford, 312 pp.], which indicates that these samples were enriched in uranium with respect to thorium (and perhaps lead) at the time of deposition. In the modern ocean, sediments are enriched in uranium under reducing conditions by reduction of the water-soluble uranyl ion, which is delivered to the ocean by oxidative weathering of continental crust. To evaluate the potential mobility of Th, U, and Pb during post-depositional processes, the concentrations of the rare earth elements (REE) were also determined. Interelement ratios of the largely immobile REE (in this study, La/Nd and Gd/Er) can be used as a proxy for the Th/U ratio, as the geochemical behavior of the lanthanide and actinide elements is similar under a variety of conditions. Furthermore, close similarity in the chondrite-normalized REE patterns and small range in La/Nd and Gd/Er ratios in studied samples indicate that variations in κ_a values are not likely to have been controlled by mixing of one or more REE-, Th-, and U-rich heavy minerals from the multiple detrital sources. Our study of shales from the ca. 2.15 Ga Sengoma Argillite Formation indicates that decoupling of U from Th, most likely related to the oxidative continental weathering, began by 2.15 Ga, at the latest.     

The heat goes on—changes in indices of hot extremes in Poland

Theoretical and Applied Climatology - On the basis of temperature observations at 60 meteorological stations in Poland, changes in the indices associated with the presence of extremely high air...     

Inversion of inline and broadside marine controlled‐source electromagnetic data with constraints derived from seismic data

We present a structural smoothing regularization scheme in the context of inversion of marine controlled‐source electromagnetic data. The regularizing hypothesis is that the electrical parameters have a structure similar to that of the elastic parameters observed from seismic data. The regularization is split into three steps. First, we ensure that our inversion grid conforms with the geometry derived from seismic. Second, we use a seismic stratigraphic attribute to define a spatially varying regularization strength. Third, we use an indexing strategy on the inversion grid to define smoothing along the seismic geometry. Enforcing such regularization in the inversion will encourage an inversion result that is more intuitive for the interpreter to deal with. However, the interpreter should also be aware of the bias introduced by using seismic data for regularization. We illustrate the method using one synthetic example and one field data example. The results show how the regularization works and that it clearly enforces the structure derived from seismic data. From the field data example we find that the inversion result improves when the structural smoothing regularization is employed. Including the broadside data improves the inversion results even more, due to a better balancing between the sensitivities for the horizontal and vertical resistivities.     

Geological events during the early formation of a passive margin

The concept of plate tectonics implies that the normal sea floor spreading stage is preceded by a sequence of events associated with the break-up of continental crust. Thus, evidence of the early development of “non-failed” rifts is to be found at passive continental margins. Of special interest is the question of the extent of the continental crust and the structural and compositional changes associated with the change in crustal type. In addressing these topics, we have focused attention on the Norwegian margin between the Jan Mayen and Senja fracture zones (66°–70°N) in an attempt to understand its history of rifting and early sea floor spreading. p ]The southern part of this rifted margin is characterized by a wide shelf and the marginal Vøring Plateau interrupts a gentle slope at a level of about 1500 m. However, the margin becomes progressively narrower towards the north and a typical narrow shelf and steep slope emerge off the Lofo—tenVesterålen Islands (Fig. 1). In a reconstructed pre-opening configuration (Talwani and Eldholm, 1977) the narrowest part of the juxtaposed EastGreenland margin is found in the south and a wide shelf and slope corresponds to the Lofoten-Vesterålen margin.The most prominent structural element is a buried basement high underneath the Vøring Plateau. The high is bounded landward by the Vøring Plateau Escarpment, a major structural boundary which defines typical changes in the geophysical parameters. These are: (1) a sudden increase of depth to acoustic basement; (2) changes in the velocity-depth function; (3) a gravity gradient; and (4) a magnetic edge anomaly separating sea-floor spreading type anomalies from a quiet zone on the landward side (Talwani and Eldholm, 1972). These observations were interpreted in terms of a sharp ocea—ncontinent crustal transition along the escarpment with sea-floor spreading commencing between anomaly 24 and 25 time (56–58 m.y. B.P.). Alternatively, the concept of ancient oceanic crust landward of this escarpment and the possible existence of continental crust under the outer basement high have been argued and we refer to Eldholm et al. (1979) for a detailed discussion.