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141.
Aaron Zimmerman Holly J. Stein Judith L. Hannah Dejan Koželj Kamen Bogdanov Tudor Berza 《Mineralium Deposita》2008,43(1):1-21
The Apuseni–Banat–Timok–Srednogorie magmatic–metallogenic belt (ABTS belt), forms a substantial metallogenic province in the
Balkan-South Carpathian system in southeastern Europe. The belt hosts porphyry, skarn, and epithermal deposits mined since
pre-Roman times. Generally, the deposits, prospects, and occurrences within the belt are linked to magmatic centers of calc-alkaline
affinity. Fifty-one rhenium-osmium (Re–Os) ages and Re concentration data for molybdenites define systematic geochronologic
trends and constrain the geochemical-metallogenic evolution of the belt in space and time. From these data and additional
existing geologic-geochemical data, a general tectonic history for the belt is proposed. Mineralization ages in Apuseni-Banat,
Timok, and Panagyurishte (the central district of the larger E–W Srednogorie Zone) range from 72–83, 81–88, and 87–92 Ma,
respectively, and clearly document increasing age from the northwestern districts to the southeastern districts. Further,
Re–Os ages suggest rapidly migrating pulses of Late Cretaceous magmatic–hydrothermal activity with construction of deposits
in ~1 m.y., districts in ~10 m.y., and the entire 1,500 km belt in ~20 m.y. Ages in both Timok and Panagyurishte show systematic
younging, while deposit ages in Banat and Apuseni are less systematic reflecting a restricted evolution of the tectonic system.
Systematic differences are also observed for molybdenite Re concentrations on the belt scale. Re concentrations generally
range from hundreds to thousands of parts per million, typical of subduction-related Cu–Au–Mo–(PGE) porphyry systems associated
with the generation of juvenile crust. The geochronologic and geochemical trends are compatible with proposed steepening of
subducting oceanic slab and relaxation of upper continental plate compression. Resulting influx of sub-continental mantle
lithosphere (SCML) and asthenosphere provide a fertile metal source and heat, while the subducting slab contributes connate
and mineral dehydration fluids, which facilitate partial melting and metal leaching of SCML and asthenosphere. Cu–Au–Mo–(PGE)
porphyry deposits may develop where melts are trapped at shallow crustal levels, often with associated volcanism and epithermal-style
deposits (South Banat, Timok, and Panagyurishte). Mo–Fe–Pb–Zn skarn deposits may develop where felsic melts are trapped adjacent
to Mesozoic limestones at moderate crustal levels (North Banat and Apuseni). Systematic spatial variations in deposit style,
commodity enrichment, Re–Os ages, and Re concentrations support specific tectonic processes that led to ore formation. In
a post-collisional setting, subduction of Vardar oceanic crust may have stalled, causing slab steepening and rollback. The
slab rollback relaxes compression, facilitating and enhancing orogenic collapse of previously thickened Balkan-South Carpathian
crust. The progression of coupled rollback-orogenic collapse is evidenced by the width of Late Cretaceous extensional basins
and northward younging of Re–Os ages, from Panagyurishte (~60 km; 92–87 Ma) to Timok (~20 km; 88–81 Ma) to Apuseni-Banat (~5 km;
83–72 Ma). Generation of a well-endowed mineral belt, such as the ABTS, requires a temporally and spatially restricted window
of magmatic–hydrothermal activity. This window is quickly opened as upper plate compression relaxes, thereby inducing melt
generation and ingress of melt to higher crustal levels. The window is just as quickly closed as upper plate compression is
reinstated. The transient tectonic state responsible for economic mineralization in the ABTS belt may be a paleo-analogue
to transient intervals in the present subduction tectonics of SE Asia where much mineral wealth has been created in the last
few million years. 相似文献
142.
Marie Méheust Ruediger Stein Kirsten Fahl Lars Max Jan-Rainer Riethdorf 《Geo-Marine Letters》2016,36(2):101-111
Due to its strong influence on heat and moisture exchange between the ocean and the atmosphere, sea ice is an essential component of the global climate system. In the context of its alarming decrease in terms of concentration, thickness and duration, understanding the processes controlling sea-ice variability and reconstructing paleo-sea-ice extent in polar regions have become of great interest for the scientific community. In this study, for the first time, IP25, a recently developed biomarker sea-ice proxy, was used for a high-resolution reconstruction of the sea-ice extent and its variability in the western North Pacific and western Bering Sea during the past 18,000 years. To identify mechanisms controlling the sea-ice variability, IP25 data were associated with published sea-surface temperature as well as diatom and biogenic opal data. The results indicate that a seasonal sea-ice cover existed during cold periods (Heinrich Stadial 1 and Younger Dryas), whereas during warmer intervals (Bølling-Allerød and Holocene) reduced sea ice or ice-free conditions prevailed in the study area. The variability in sea-ice extent seems to be linked to climate anomalies and sea-level changes controlling the oceanographic circulation between the subarctic Pacific and the Bering Sea, especially the Alaskan Stream injection though the Aleutian passes. 相似文献
143.
The outer coast of Finnmark in northern Norway is where the former Fennoscandian and Barents Sea ice sheets coalesced. This key area for isostatic modelling and deglaciation history of the ice sheets has abundant raised shorelines, but only a few existing radiocarbon dates constrain their chronology. Here we present three Holocene sea level curves based on radiocarbon dated deposits from isolation basins at the outermost coast of Finnmark; located at the islands Sørøya and Rolvsøya and at the Nordkinn peninsula. We analysed animal and plant remains in the basin deposits to identify the transitions between marine and lacustrine sediments. Terrestrial plant fragments from these transitions were then radiocarbon dated. Radiocarbon dated mollusk shells and marine macroalgae from the lowermost deposits in several basins suggest that the first land at the outer coast became ice free around 14,600 cal yr BP. We find that the gradients of the shorelines are much lower than elsewhere along the Norwegian coast because of substantial uplift of the Barents Sea. Also, the anomalously high elevation of the marine limit in the region can be attributed to uplift of the adjacent seafloor. After the Younger Dryas the coast emerged 1.6–1.0 cm per year until about 9500–9000 cal yr BP. Between 9000 and 7000 cal yr BP relative sea level rose 2–4 m and several of the studied lakes became submerged. At the outermost locality Rolvsøya, relative sea level was stable at the transgression highstand for more than 3000 years, between ca 8000 and 5000 cal yr BP. Deposits in five of the studied lakes were disturbed by the Storegga tsunami ca 8200–8100 cal yr BP. 相似文献
144.
In a previous publication we developed a new mixed least‐squares method for poro‐elasticity. The approximate solution was obtained via a minimization of a least‐squares functional, based upon the equations of equilibrium, the equations of continuity and weak forms of the constitutive relationships for elasticity and Darcy flow. The formulation involved four independent types of variables: displacements, stresses, pore pressures and velocities. All of them were approximated by linear continuous triangles. Encouraged by the computational results, obtained from the two‐dimensional implementation of the method, we extended our formulation to three dimensions. In this paper we present numerical examples for the performance of continuous linear tetrahedra within the context of the mixed least‐squares method. The initial results suggest that the method works well in the nearly and entirely incompressible limits for elasticity. For poro‐elasticity, the obtained pore pressures are stable without exhibiting the oscillations, which are observed when the standard Galerkin formulation is used. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
145.
Adjoint-based gradient computations for oil reservoirs have been increasingly used in closed-loop reservoir management optimizations.
Most constraints in the optimizations are for the control input, which may either be bound constraints or equality constraints.
This paper addresses output constraints for both state and control variables. We propose to use a (interior) barrier function
approach, where the output constraints are added as a barrier term to the objective function. As we assume there always exist
feasible initial control inputs, the method maintains the feasibility of the constraints. Three case examples are presented.
The results show that the proposed method is able to preserve the computational efficiency of the adjoint methods. 相似文献
146.
Hans-Jürgen F?rster Dieter Rhede Holly J. Stein Rolf L. Romer Gerhard Tischendorf 《International Journal of Earth Sciences》2012,101(1):57-67
We present geochronological data for late-Variscan magmatism in the Lausitz Block of the Saxo-Thuringian Zone, Germany. The
Th–U–total Pb age of uraninite and the Re–Os age of molybdenite from the composite biotite–monzogranite pluton of K?nigshain
overlap at the 2σ confidence limit: 328.6 ± 1.9 Ma (uraninite), and 327.0 ± 1.3 Ma and 327.6 ± 1.3 Ma (molybdenite), indicating
that crystallization of magmatic uraninite and deposition of molybdenite were nearly contemporaneous. These data imply that
magmatic processes in this part of the Variscan orogen already started in latest Visean time, about 10 Ma earlier than previously
assumed (315–320 Ma). The new ages correspond to ages for plutonic rocks in the Elbe Zone immediately west of the Lausitz
(around 335–325 Ma) and the bulk of late-Variscan igneous rocks in the Saxo-Thuringian Zone (335–320 Ma). 相似文献
147.
Knut-Andreas Lie Jostein R. Natvig Stein Krogstad Yahan Yang Xiao-Hui Wu 《Computational Geosciences》2014,18(3-4):357-372
A Dirichlet–Neumann representation method was recently proposed for upscaling and simulating flow in reservoirs. The DNR method expresses coarse fluxes as linear functions of multiple pressure values along the boundary and at the center of each coarse block. The number of flux and pressure values at the boundary can be adjusted to improve the accuracy of simulation results and, in particular, to resolve important fine-scale details. Improvement over existing approaches is substantial especially for reservoirs that contain high-permeability streaks or channels. As an alternative, the multiscale mixed finite-element (MsMFE) method was designed to obtain fine-scale fluxes at the cost of solving a coarsened problem, but can also be used as upscaling methods that are flexible with respect to geometry and topology of the coarsened grid. Both methods can be expressed in mixed-hybrid form, with local stiffness matrices obtained as “inner products” of numerically computed basis functions with fine-scale sub-resolution. These basis functions are determined by solving local flow problems with piecewise linear Dirichlet boundary conditions for the DNR method and piecewise constant Neumann conditions for MsMFE. Adding discrete pressure points in the DNR method corresponds to subdividing faces in the coarse grid and hence increasing the number of basis functions in the MsMFE method. The methods show similar accuracy for 2D Cartesian cases, but the MsMFE method is more straightforward to formulate in 3D and implement for general grids. 相似文献
148.
R. Samadi M. -J. Goupil E. Alecian F. Baudin D. Georgobiani R. Trampedach R. Stein å. Nordlund 《Journal of Astrophysics and Astronomy》2005,26(2-3):171-184
The amplitude of solar-like oscillations results from a balance between excitation and damping. As in the sun, the excitation
is attributed to turbulent motions that stochastically excite thep modes in the uppermost part of the convective zone. We present here a model for the excitation mechanism. Comparisons between
modeled amplitudes and helio and stellar seismic constraints are presented and the discrepancies discussed. Finally the possibility
and the interest of detecting such stochastically excited modes in pre-main sequence stars are also discussed. 相似文献
149.
R. Stein B. Boucsein K. Fahl T. Garcia de Oteyza J. Knies F. Niessen 《Global and Planetary Change》2001,31(1-4)
Data on the amount and composition of organic carbon were determined in sediment cores from the Kara and Laptev Sea continental margin, representing oxygen isotope stages 1–6. The characterization of organic matter is based on hydrogen index (HI) values, n-alkanes and maceral composition, indicating the predominance of terrigenous organic matter through space and time. The variations in the amount and composition of organic carbon are mainly influenced by changes in fluvial sediment supply, Atlantic water inflow, and continental ice sheets. During oxygen isotope stage (OIS) 6, high organic carbon contents in sediments from the Laptev Sea and western East Siberian Sea continental margin were probably caused by the increased glacial erosion and further transport in the eastward-flowing boundary current along the continental margin. During OIS 5 and early OIS 3, some increased amounts of marine organic matter were preserved in sediments east of the Lomonosov Ridge, suggesting an influence of nutrient-rich Pacific waters. During OIS 2, terrigenous organic carbon supply was increased along the Barents and western Kara Sea continental margin caused by extended continental ice sheets in the Barents Sea (Svalbard to Franz Josef Land) area and increased glacial erosion. Along the Laptev Sea continental margin, on the other hand, the supply of terrigenous (organic) matter was significantly reduced due to the lack of major ice sheets and reduced river discharge. Towards the Holocene, the amount of total organic carbon (TOC) increased along the Kara and Laptev Sea continental margin, reaching average values of up to 0.5 g C cm−2 ky−1. Between about 8 and 10 ka (9 and 11 Cal ka), i.e., during times when the inner shallow Kara and Laptev seas became largely flooded for the first time after the Last Glacial Maximum, maximum supply of terrigenous organic carbon occurred, which is related to an increase in coastal erosion and Siberian river discharge. During the last 8000 years, the increased amount of marine organic carbon preserved in the sediments from the Kara and Laptev Sea continental margin is interpreted as a result of the intensification of Atlantic water inflow along the Eurasian continental margin. 相似文献
150.
Extinct isotope heterogeneities in the mantles of Earth and Mars: Implications for mantle stirring rates 下载免费PDF全文
Heterogeneities in terrestrial samples for 182W/183W and 142Nd/144Nd are only preserved in Hadean and Archean rocks while heterogeneities in 129Xe/130Xe and 136Xe/130Xe persist to very young mantle‐derived rocks. In contrast, meteorites from Mars show that the Martian mantle preserves heterogeneities in 182W/183W and 142Nd/144Nd up to the present. As a consequence of the probable “deep magma ocean” core formation process, we assume that the Earth and Mars both had a very early two‐mantle‐reservoir structure with different initial extinct nuclide isotopic compositions (different 182W/183W, 142Nd/144Nd, 129Xe/130Xe, 136Xe/130Xe ratios). Based on this assumption, we developed a simple stochastic model to trace the evolution of a mantle with two initially distinct layers for the extinct isotopes and its development into a heterogeneous mantle by convective mixing and stretching of these two layers. Using the extinct isotope system 182Hf‐182W, we find that the mantles of Earth and Mars exhibit substantially different mixing or stirring rates. This is consistent with Mars having cooled faster than the Earth due to its smaller size, resulting in less efficient mantle mixing for Mars. Moreover, the mantle stirring rate obtained for Earth using 182Hf‐182W is consistent with the mantle stirring rate of ~500 Myr constrained by the long‐lived isotope system, 87Rb‐87Sr and 147Sm‐143Nd. The apparent absence of 182W/183W isotopic heterogeneity in modern terrestrial rocks is attributed to very active mantle stirring which reduced the 182W/183W isotopic heterogeneity to a relatively small scale (~83 m for a mantle stirring rate of 500 Myr) compared to the common sampling scale of terrestrial basalts (~30 or 100 km). Our results also support the “deep magma ocean” core formation model as being applicable to both Mars and Earth. 相似文献