The POLONAISE'97 (POlish Lithospheric ONset—An International Seismic Experiment, 1997) seismic experiment in Poland targeted the deep structure of the Trans-European Suture Zone (TESZ) and the complex series of upper crustal features around the Polish Basin. One of the seismic profiles was the 300-km-long profile P2 in northwestern Poland across the TESZ. Results of 2D modelling show that the crustal thickness varies considerably along the profile: 29 km below the Palaeozoic Platform; 35–47 km at the crustal keel at the Teisseyre–Tornquist Zone (TTZ), slightly displaced to the northeast of the geologic inversion zone; and 42 km below the Precambrian Craton. In the Polish Basin and further to the south, the depth down to the consolidated basement is 6–14 km, as characterised by a velocity of 5.8–5.9 km/s. The low basement velocities, less than 6.0 km/s, extend to a depth of 16–22 km. In the middle crust, with a thickness of ca. 4–14 km, the velocity changes from 6.2 km/s in the southwestern to 6.8 km/s in the northeastern parts of the profile. The lower crust also differs between the southwestern and northeastern parts of the profile: from 8 km thickness, with a velocity of 6.8–7.0 km/s at a depth of 22 km, to ca.12 km thickness with a velocity of 7.0–7.2 km/s at a depth of 30 km. In the lowermost crust, a body with a velocity of 7.20–7.25 km/s was found above Moho at a depth of 33–45 km in the central part of the profile. Sub-Moho velocities are 8.2–8.3 km/s beneath the Palaeozoic Platform and TTZ, and about 8.1 km/s beneath the Precambrian Platform. Seismic reflectors in the upper mantle were interpreted at 45-km depth beneath the Palaeozoic Platform and 55-km depth beneath the TTZ.
The Polish Basin is an up to 14-km-thick asymmetric graben feature. The basement beneath the Palaeozoic Platform in the southwest is similar to other areas that were subject to Caledonian deformation (Avalonia) such that the Variscan basement has only been imaged at a shallow depth along the profile. At northeastern end of the profile, the velocity structure is comparable to the crustal structure found in other portions of the East European Craton (EEC). The crustal keel may be related to the geologic inversion processes or to magmatic underplating during the Carboniferous–Permian extension and volcanic activity. 相似文献
A 3D interpretation of the newly compiled Bouguer anomaly in the area of the “Dead Sea Rift” is presented. A high-resolution
3D model constrained with the seismic results reveals the crustal thickness and density distribution beneath the Arava/Araba
Valley (AV), the region between the Dead Sea and the Gulf of Aqaba/Elat. The Bouguer anomalies along the axial portion of
the AV, as deduced from the modelling results, are mainly caused by deep-seated sedimentary basins (D > 10 km). An inferred zone of intrusion coincides with the maximum gravity anomaly on the eastern flank of the AV. The intrusion
is displaced at different sectors along the NNW–SSE direction. The zone of maximum crustal thinning (depth 30 km) is attained
in the western sector at the Mediterranean. The southeastern plateau, on the other hand, shows by far the largest crustal
thickness of the region (38–42 km). Linked to the left lateral movement of approx. 105 km at the boundary between the African
and Arabian plate, and constrained with recent seismic data, a small asymmetric topography of the Moho beneath the Dead Sea
Transform (DST) was modelled. The thickness and density of the crust suggest that the AV is underlain by continental crust.
The deep basins, the relatively large intrusion and the asymmetric topography of the Moho lead to the conclusion that a small-scale
asthenospheric upwelling could be responsible for the thinning of the crust and subsequent creation of the Dead Sea basin
during the left lateral movement. A clear segmentation along the strike of the DST was obtained by curvature analysis: the
northern part in the neighbourhood of the Dead Sea is characterised by high curvature of the residual gravity field. Flexural
rigidity calculations result in very low values of effective elastic lithospheric thickness (te < 5 km). This points to decoupling of crust in the Dead Sea area. In the central, AV the curvature is less pronounced and
te increases to approximately 10 km. Curvature is high again in the southernmost part near the Aqaba region. Solutions of Euler
deconvolution were visualised together with modelled density bodies and fit very well into the density model structures.
An erratum to this article can be found at 相似文献
The BEAR array of simultaneous electromagnetic (EM) observations probes the deep crustal and upper mantle conductivity structure of the Baltic Shield searching for the lithosphere–asthenosphere boundary beneath. The adequate interpretation of the results of this unique high latitude natural field EM sounding requires proper understanding of the actual external excitation conditions because conventionally used plane wave model assumptions may be substantially violated in the vicinity of inhomogeneous polar sources. The paper presents an overview of the morphology and statistics of source distortions in the BEAR EM field transfer functions (TF) and the ways of their suppression. The stability of the final TF estimates obtained with the exclusion of intensive non-stationary auroral effects is further justified. The external excitation model effective for the whole BEAR observation period is inferred from the array distribution of the inter-station geomagnetic transfer functions. The model is supported by the results of polar ionosphere–magnetosphere current system studies, based on the simultaneous ground and satellite geomagnetic observations, and sets bounds for the “plane wave” approach in the BEAR data interpretation to avoid unfounded inferences on the upper mantle electrical properties. The signatures of the lithosphere–asthenospere boundary under Fennoscandia derived from the BEAR data are summarized and its resolution within the traditional plane wave interpretational paradigm is analysed assuming the presented external source pattern and estimated TF uncertainties caused by the source inhomogeneity. 相似文献
Trace elements serve important roles as regulators of ocean processes including marine ecosystem dynamics and carbon cycling. The role of iron, for instance, is well known as a limiting micronutrient in the surface ocean. Several other trace elements also play crucial roles in ecosystem function and their supply therefore controls the structure, and possibly the productivity, of marine ecosystems. Understanding the biogeochemical cycling of these micronutrients requires knowledge of their diverse sources and sinks, as well as their transport and chemical form in the ocean.Much of what is known about past ocean conditions, and therefore about the processes driving global climate change, is derived from trace-element and isotope patterns recorded in marine deposits. Reading the geochemical information archived in marine sediments informs us about past changes in fundamental ocean conditions such as temperature, salinity, pH, carbon chemistry, ocean circulation and biological productivity. These records provide our principal source of information about the ocean's role in past climate change. Understanding this role offers unique insights into the future consequences of global change.The cycle of many trace elements and isotopes has been significantly impacted by human activity. Some of these are harmful to the natural and human environment due to their toxicity and/or radioactivity. Understanding the processes that control the transport and fate of these contaminants is an important aspect of protecting the ocean environment. Such understanding requires accurate knowledge of the natural biogeochemical cycling of these elements so that changes due to human activity can be put in context.Despite the recognised importance of understanding the geochemical cycles of trace elements and isotopes, limited knowledge of their sources and sinks in the ocean and the rates and mechanisms governing their internal cycling, constrains their application to illuminating the problems outlined above. Marine geochemists are poised to make significant progress in trace-element biogeochemistry. Advances in clean sampling protocols and analytical techniques provide unprecedented capability for high-density sampling and measurement of a wide range of trace elements and isotopes which can be combined with new modelling strategies that have evolved from the World Ocean Circulation Experiment (WOCE) and Joint Global Ocean Flux Study (JGOFS) programmes. A major new international research programme, GEOTRACES, has now been developed as a result of community input to study the global marine biogeochemical cycles of trace elements and their isotopes. Here, we describe this programme and its rationale. 相似文献