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101.
Hans-Dieter Vosteen Volker Rath Andreas Schmidt-Mumm Christoph Clauser 《Tectonophysics》2004,386(1-2):81-95
The thermal regime and the distribution of heat flow at the base of sedimentary basins is fundamental to the understanding of the process of basin evolution and the associated mobilization and migration of hydrocarbon and other fluids. For the Northeastern-German sedimentary basin, available information on structure, temperature, and thermal properties along a seismic DEKORP reflection profile allow high resolution 2-D forward and inverse simulations. This approach is attractive in situations where much information is available, if only with considerable uncertainty. In particular, this allows to introduce “soft” information into the analysis. In our case, forward simulations yield initial a priori estimates of the parameters while inversion calculations yield a posteriori estimates of the parameters and their uncertainty. The a priori parameters as well as their assumed uncertainty are input for a Bayesian parameter estimation scheme. In respect to the Northeastern-German sedimentary basin, the inverse analysis postulates a significant and characteristic a posteriori variation of thermal conductivity of the Zechstein unit along the entire profile as well as a generally large a posteriori thermal conductivity of the (pre-Permian) basement in the northern part of the basin. For inverse calculations, we used two alternative scenarios: One assumes the thermal conductivity of the Zechstein unit to be homogeneous along the profile while the other allows a lateral variation. A posteriori heat flow across the base of the model varies from 40 to 60 and 50 to 65 mW m−2 for models in which values for thermal conductivity and radiogenic heat generation rate were either based on literature values or direct measurements, respectively. 相似文献
102.
103.
104.
The main driving force behind Al/Si ordering in tetrahedral framework aluminosilicates is nearest-neighbour Al/Al avoidance.
Computer simulation is used to explore the direct consequences of such Al/Al avoidance. The main result is that the order-disorder
transition temperature T
c
falls dramatically as the concentration x of Al in the structure is reduced, and if the only interactions are those associated with nearest-neighbour Al/Al avoidance,
T
c
becomes zero for x less than some critical value x
c
, where x
c
=0.31 for the feldspar framework and x
c
=0.34 for cordierite. Also a large degree of short range order is found above T
c
. Both results differ radically from the standard Bragg-Williams model. Plots of entropy and enthalpy of ordering are given
as functions of x and T, which may be used to interpret experimental data or for extrapolation into ranges of x and T inaccessible to experiment.
Received: 14 May 1997 / Revised, accepted: 2 June 1997 相似文献
105.
Pipe G3b is part of the Upper Cretaceous carbonatitic Gross Brukkaros Volcanic Field in southern Namibia. The pipe represents
the root zone of a diatreme and is located 2800 m west of the rim of Gross Brukkaros, a downsag caldera. The pipe is exposed
approximately 550 m below the original Upper Cretaceous land surface. It cuts down into its own feeder dyke, 0.3 m thick.
The pipe coalesced from two small pipes and in plan view is 19 m long and 12 m wide. It consists of fragmented Cambrian Nama
quartzites and shales of the Fish River subgroup. Despite intensive brecciation, the stratigraphic sequence of the country
rocks is almost preserved in the pipe. In addition, the feeder dyke became fragmented too and can be traced in a 2- to 3-m-wide
zone full of carbonatite blocks along the southern margin of the pipe. The void space of the breccia is 30–50% in volume.
Finally, after the disruption of country rocks and feeder dyke, a little carbonatite magma intruded some of the void space.
The breccia of pipe G3b is considered to represent a root zone at the transition from the feeder dyke into a diatreme above.
Formation of the breccia required a shock wave thought to have been associated with a last explosion of the diatreme immediately
above the present level of exposure. The explosion can be shown to have been phreatomagmatic in origin.
Received: 11 October 1996 / Accepted: 6 March 1997 相似文献
106.
In the Upper Cretaceous Gross Brukkaros Volcanic Field, southern Namibia, a radial dyke system surrounds a dome structure
and its 74 closely related carbonatite diatremes. This paper focuses on volcanological features which seem to be typical for
a low-viscosity melt in various settings such as dykes, sills and diatremes. The total or near absence of vesicles in carbonatite
ash grains and lapilli inside the diatremes is evidence against explosive exsolution of volatile phases and in favour of a
phreatomagmatic fragmentation mechanism and thus for a phreatomagmatic eruption mechanism of the carbonatite diatremes.
Received: 15 August 1996 / Accepted: 13 January 1997 相似文献
107.
Dr. Volker Wrede 《International Journal of Earth Sciences》1988,77(1):101-114
Zusammenfassung Der nördliche Harzrand stellt eine tektonische Grenze dar zwischen dem varistischen Grundgebirge und dem triassischmesozoischen Deckgebirge des süd-niedersächsischen Berglandes. Der Charakter dieser Störungslinie wurde in der Vergangenheit vielfach diskutiert. Es läßt sich zeigen, daß der nördliche Harzrand keine mehr oder weniger flach nach Süden einfallende Schaufelfläche darstellt. Durch den Vergleich mit ähnlichen tektonischen Elementen, die in den Steinkohlenlagerstätten Nordrhein-Westfalens aufgeschlossen sind, ergibt sich vielmehr das Bild eines bereits varistisch angelegten und saxonisch reaktivierten wrench-fault-Systems.
The northern margin of the Harz-Mountains is the tectonic border between the Variscan basement and the Triassic-Mesozoic sequence of southern Lower Saxony. The nature of this fault-zone has been controversial. It can be demonstrated by comparison with similar tectonic features exposed in the coal-mining areas of Northrhine-Westphaha that it is part of a wrench-fault-system rather than a shallow dipping décollement-structure. This fault system has been generated during the Variscan orogeny and has been reactivated by Saxonian movements.
Résumé Le bord du nord du Harz se présente comme une limite tectonique entre le soubassement varisque et les couches de couverture mésozoïques-triasiques du pays vallonné de la Basse-Saxe du Sud. Le caractère de cette ligne de dislocation a fait l'objet de discussions multiples dans le passé. On peut montrer que la limite nord du Harz n'est pas un décollement à pendage sud plus ou moins faible. Au contraire, par comparaison avec des structures tectoniques analogues, exposées dans le district houiller de la Rhénanie du Nord-Westphalie, le modèle s'impose d'un système de failles de décrochement (wrench-faults), engendrées pendant l'orogenèse varisque et réactivées par les mouvements saxoniens.
- . . , . , -- , , .相似文献
108.
Volker von Seckendorff Hugh St. C. O'Neill 《Contributions to Mineralogy and Petrology》1993,113(2):196-207
The partitioning of Mg and Fe2+ between coexisting olivines and orthopyroxenes in the system MgO-FeO-SiO2 has been investigated experimentally at 1173, 1273, 1423 K and 1.6 GPa over the whole range of Mg/Fe ratios. The use of barium borosilicate as a flux to promote grain growth, and the identification by back-scattered electron imaging of resulting growth rims suitable for analysis by electron microprobe, results in coexisting olivine and orthopyroxenene compositions determined to a precision of±0.003 to 0.004 in molar Fe/(Mg+Fe). Quasi-reversal experiments were performed starting with Mg-rich olivine and Fe-rich orthopyroxene (low KD) and vice versa (high KD), which produced indistinguishable results. The distribution coefficient, KD, depends on composition and on temperature, but near Fe/(Mg+Fe)=0.1 (i.e. mantle compositions) these effects cancel out, and KD is insensitive to temperature. The results agree well with previous experimental investigations, and constrain the thermodynamic mixing properties of Mg-Fe olivine solid solutions to show small near-symmetric deviations from ideality, with
between 2000 and 8000 J/mol. Multiple non-linear least squares regression of all data gave a best fit with
(implying 5450 J/mol at 1 bar) and
, but the two W
G
parameters are so highly correlated with each other that our data are almost equally well fit with
, as obtained by Wiser and Wood. This value implies
, apparently independent of temperature. Our experimental results are not compatible with the assessment of olivine-orthopyroxene equilibria of Sack and Ghiorso. 相似文献
109.
Oliver Ritter Ute WeckmannTim Vietor Volker Haak 《Physics of the Earth and Planetary Interiors》2003,138(2):71-90
The Namibian margin is dominated by the late Proterozoic to early Cambrian fold belts of the Damara Orogen, which wrap around and separate the Congo and Kalahari Cratons. This mosaic of relatively ‘soft’ fold belts and ‘hard’ cratons apparently controlled the path for the opening of the South Atlantic in the early Cretaceous. The continents split along the coast-parallel fold belts of the Damara Orogen while the inland fold belt (Damara Belt) was effected by extension and widespread igneous intrusion but never developed to the rift stage. This paper is concerned with the interpretation of magnetotelluric (MT) data along a 200 km NW-SE profile across the Damara Belt in NW Namibia. The regional, two-dimensional electrical resistivity model and the induction vector data exhibit three distinctive zones: (i) a generally very resistive upper crust which is typical for the granites and metasediments of the Damara Belt, (ii) two subvertical conductors in upper to mid-crustal levels which correlate with major tectonic zone boundaries and (iii) a highly conductive middle to lower crust in the southern part of the profile. The geometry of the conductive structures could reflect a regional shear system in which upper crustal listric faults pass into a detachment zone in the middle crust. We interpret the high electrical conductivity in terms of graphite (or other forms of mineralization) enrichment along the shear planes. This zone of crustal weakness may have originated in Pre-Damara times and had probably experienced several episodes of crustal reactivation before the intrusion of basaltic dike swarms during the Cretaceous rifting and magmatism associated with the opening of the South Atlantic. 相似文献
110.
Stephan Königer Volker Lorenz Harald Stollhofen Richard A. Armstrong 《International Journal of Earth Sciences》2002,91(2):341-356
Thin, widespread, fallout tuff layers interbedded within fluvio-lacustrine successions of the Carboniferous-Permian Saar-Nahe Basin provide important tephrostratigraphic markers. In addition, radiogeochronometric data derived from the tuffs serve as calibration points for the adjustment to regional chronostratigraphy and to numerical time scales. The Pappelberg-Tuff in the Meisenheim Formation (Glan Group) has been dated by U/Pb zircon SHRIMP technique at 297.0Dž.2 Ma. Taking the Carboniferous/Permian boundary at 296 Ma, the Meisenheim Formation coincides approximately with this boundary. Consequently, underlying strata, lithostratigraphically regarded as the basal part of the 'Rotliegend', chronostratigraphically belong to the Upper Carboniferous. Bed thicknesses, grain size and sorting characteristics of the tuffs and the absence of contemporaneously emplaced volcanics within the Saar-Nahe Basin point to an extrabasinal derivation of the wind-drifted volcanic ash. Decreasing grain sizes of juvenile pyroclastic particles towards the north suggest source areas south of the basin within 300 km distance. The majority of the tuffs are rhyolitic to rhyodacitic and indicate petrographic and geochemical affinities to Moldanubian S-type granitoids, in particular to highly differentiated two-mica granites, and related volcanic effusives. Within the time frame considered here, such potential source rocks were emplaced in the northern and central Black Forest (SW Germany) and the northern Vosges (E France) at 100-150 km distance south of the Saar-Nahe Basin. 相似文献