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131.
Magnetometer array studies have led to the discovery and mapping of the Southern Cape Conductive Belt (SCCB) crossing the southern tip of Africa from west to southeast coasts. The SCCB lies just south of the Namaqua-Natal Belt of cratonic rocks remobilized about 1000 m.y. B.P. It is shown that it coincides with a zone of weakness which has been exploited by three major geosynclinal accumulations over some 600 m.y. Relationships between the SCCB and the basement geochronology, geology and tectonics are considered in detail. These relationships support the view that the conductive belt was formed by an accumulation of marine sediments and oceanic lithosphere at the top of a Proterozoic subduction which stopped about 1000 to 800 m.y. B.P. Associated with this subduction we propose a Proterozoic range of Andean mountains, whose roots are now exposed in the Namaqua-Natal Belt. Later subduction further south, near the present south coast, is proposed to account for the intrusion, between the south coast and the SCCB, of the Cape Granites in the time interval 600-500 m.y. B.P. There is some evidence for a third, yet more distant, subduction episode off Permian Gondwanaland. After outlining this tectonic history, the paper turns to a closer examination of the hypothesis that the Southern Cape Conductive Belt consists of partly serpentinized basalt accumulated at the top of a Proterozoic subduction. A large static magnetic anomaly, which correlates with the SCCB over most of its length, is well fitted by a model which strongly supports this hypothesis. Bouguer gravity anomalies along western and central profiles likewise support the hypothesis. A discussion follows of the process of formation of the proposed block of serpentinized marine rocks, beginning with serpentinization of the crust near oceanic ridges by reaction of warm, porous, newly-extruded basalt with seawater convecting through it. The serpentinized basalt is stable at crustal temperatures and pressures and so is transported in the seafloor until it reaches a subduction, where it accumulates because of its low density. Examples of such accumulations are cited. Finally, it is shown that any iron in the olivine and pyroxene in the original basalt precipitates, when these silicates are hydrated to serpentine, as magnetite which is the dominant mineral conferring high electrical conductivity and high magnetic susceptibility on serpentinites. In particular the Beattie static magnetic anomaly requires, in our model calculation, a very high susceptibility readily attainable in basalt at 15–20% serpentinization. The authors know of no other rock able to provide this high susceptibility. A similar percentage of serpentinization provides the density required to model the gravity anomalies. 相似文献
132.
F. Wagner H. -R. Wenk H. Kern P. Van Houtte C. Esling 《Contributions to Mineralogy and Petrology》1982,80(2):132-139
Carbonate rocks deform preferentially by twin gliding on e={01ˉ18} and slip on r ={10ˉ14} and f={02ˉ21}. In polycrystalline aggregates strong textures develop. We report on experimentally produced textures in triaxial
plane strain geometry with orthorhombic symmetry at 200° C and 400° C. Pole figure of the experimentally deformed specimens
are compared quantitatively with theoretical simulations based on the Taylor theory using both slip and mechanical twinning
as mechanisms. Agreement at low and high temperature is satisfactory and documents that models developed for f.c.c. metals
can be applied to low symmetry minerals provided that deformation mechanisms are known and that mechanical twinning is properly
accounted for. Comparison with experimental results indicates that strain was nearly homogeneous at the conditions considered
and the same may apply to many geological textures. Three texture types are described which are differentiated mainly by the
relative importance of e twinning. 相似文献
133.
On February 13, 1981 a relatively strong earthquake occurred in the Lake Vänern region in south-central Sweden. The shock had a magnitude ofML = 3.3 and was followed within three weeks by three aftershocks, with magnitudes 0.5 ≤ ML ≤ 1.0. The focal mechanism solution of the main shock indicates reverse faulting with a strike in the N-S or NE-SW direction and a nearly horizontal compressional stress. The aftershocks were too small to yield data for a full mechanism solution, but first motions of P-waves, recorded at two stations, are consistent for the aftershocks. Dynamic source parameters, derived from Pg- and Sg-wave spectra, show similar stress drops for the main shock (2 bar) and the aftershocks (1 bar), while the differences in seismic moment (1.5·1020 resp. 4·1018dyne cm), fault length (0.7 resp. 0.2 km) and relative displacement (0.15 resp. 0.03 cm) are significant. 相似文献
134.
Naturally deformed clinoamphiboles from the Selbu-Tydal and Forsbäck-Tärnaby areas of the Scandinavian Caledonides exhibit a well defined subgrain microstructure. From a transmission electron microscopy study (TEM), the subgrain boundaries are shown to consist of arrays of positive and negative screw dislocations with Burgers vector
. Locally expanded loops are present having long screw segments. The subgrain boundaries are parallel to rational crystallographic planes of the type (hk0). The density of isolated dislocations within the subgrains is low. In addition planar defect structures parallel to (010) and bounded by screw dislocations with
are observed. Based on metamorphic criteria the P−T values at the time of the amphibole growth have been estimated at 450°–600°C and 4–6 kbar, and these represent maximum conditions for the deformation. The present results indicate that slip on (hk0) [001] is an operative deformation mechanism in naturally deformed clinoamphiboles. 相似文献
135.
PCB levels in blood of common terns (Sterna hirundo) from Terneuzen, a breeding colony in The Netherlands with relatively low breeding success, were significantly higher than in birds from two Dutch reference colonies, one nearby (Oesterdam) and one further away (the Isle of Griend). However, a detailed analysis of the patterns of PCB congeners in birds from the different colonies indicated that birds from Terneuzen and to a lesser extent birds from the Oesterdam showed Ah-receptor mediated activity of P450 iso-enzymes. It is concluded that the induction of this enzyme activity in Terneuzen may be related to PCBs as well as other non-identified compounds. In contrast to this, it is likely that at the Oesterdam non-identified compounds are mainly responsible for the enzyme activity, resulting in a shift of PCB congener patterns. 相似文献
136.
This study attempts to reconstruct the history of the Collinabos landslide, a landslide with a fresh morphology that is representative for more than 150 dormant, deep-seated (> 3 m) landslides in the Flemish Ardennes (Belgium). A geomorphological map was created based on LIDAR (Light Detection and Ranging)-derived maps and detailed field surveys. The map showed that the landslide consisted of three zones with significant differences in surface topography. The northern landslide zone 1 is characterised by at least five reverse slopes, whereas zones 2 and 3, the southern landslide zones, have only two reverse slopes and a convex foot. Electric resistivity profiles measured in zones 1 and 2 revealed that the differences in surface topography were not related to differences in internal structure as both parts of the landslide were initiated as a rotational earth slide with a surface of rupture at 15 m deep, where the displaced material broke apart in two blocks. However, two shear surfaces of reactivations within landslide debris were only distinguished in the accumulation area of zone 1. The observed differences in surface morphology can be caused by a temporary conversion of a forest into cropland in zone 2. It is suggested that reverse slopes of smaller reactivations within landslide debris were obliterated during the agricultural activities. AMS radiocarbon dating of organic material found in ponds located in reverse slopes generally resulted in relatively recent dates (i.e. 1400–1950 Cal AD) suggesting that several of the small local reactivations occurred in that period. One dating at 8700–8440 Cal BP of organic matter collected in a reverse slope in zone 1 suggests that an initiation under periglacial conditions cannot be excluded for the Collinabos landslide. By combining different technologies, this study provides valuable information for a better understanding of dormant landslides. 相似文献
137.
Embedding reach-scale fluvial dynamics within the CAESAR cellular automaton landscape evolution model 总被引:2,自引:2,他引:2
We introduce a new computational model designed to simulate and investigate reach-scale alluvial dynamics within a landscape evolution model. The model is based on the cellular automaton concept, whereby the continued iteration of a series of local process ‘rules’ governs the behaviour of the entire system. The model is a modified version of the CAESAR landscape evolution model, which applies a suite of physically based rules to simulate the entrainment, transport and deposition of sediments. The CAESAR model has been altered to improve the representation of hydraulic and geomorphic processes in an alluvial environment. In-channel and overbank flow, sediment entrainment and deposition, suspended load and bed load transport, lateral erosion and bank failure have all been represented as local cellular automaton rules. Although these rules are relatively simple and straightforward, their combined and repeatedly iterated effect is such that complex, non-linear geomorphological response can be simulated within the model. Examples of such larger-scale, emergent responses include channel incision and aggradation, terrace formation, channel migration and river meandering, formation of meander cutoffs, and transitions between braided and single-thread channel patterns. In the current study, the model is illustrated on a reach of the River Teifi, near Lampeter, Wales, UK. 相似文献
138.
A. Foubert D. Van Rooij D. Blamart J.-P. Henriet 《International Journal of Earth Sciences》2007,96(1):141-158
Three provinces, characterized by the presence of carbonate mounds interpreted as cold-water coral banks have been reported
in Porcupine Seabight, SW of Ireland and were recently subjected to many detailed studies. This contribution discusses the
use of X-ray imagery and physical properties in deciphering palaeoceanographic, sedimentological and biological processes.
Physical property core logging and X-ray imagery are used to identify and describe sedimentation regimes and so their respective
palaeoceanographic and palaeoclimatological settings in two mound provinces, respectively the Belgica mound province and the
Magellan mound province. Both provinces show at present time clear differences in the hydrodynamic environment. This study
confirms that also during the past the oceanographic and sedimentological environment of both provinces differ clearly. Impacts
of glacial–interglacial variations and locally derived ice rafting events (IRE), comparable with the North Atlantic Heinrich
events (HE) have been recognized in both provinces. Moreover, the combination of X-ray imagery, magnetic susceptibility, gamma
density and P-wave velocity makes it possible to estimate the coral content and coral distribution in unopened cores localized
on top of carbonate mounds. A comparison between on-mound and off-mound cores in both provinces allowed revealing some mechanisms
of mound evolution and coral growth versus time. 相似文献
139.
140.
The purpose of this study is to quantify the dispersivity in the longitudinal direction by upscaling pore scale mixing over a network domain and to verify the dispersivity with that obtained through the more rigorous upscaling technique, the Brownian particle tracking model (BPTM). We model a porous medium with a network of pore-units that are comprised of pore bodies and bonds of finite volume. Such a pore-unit is assumed to be a mixing cell with the steady state flow condition for a single fluid. Dispersivity can be obtained by solving the mixing cell model (MCM) for the concentration in each pore-unit and by averaging the concentrations for a large number of pore units (as a function of time and space). A minimal size of network that ascertains an asymptotic value of dispersivity was determined and verified with large size pore networks. This numerically computed dispersivity is compared with the results from the BPTM for the same porous medium and flow conditions. We show that the dispersivity obtained from the MCM is equally reliable for the heterogeneous pore-networks and can be estimated as a function of pore size heterogeneity. For homogeneous networks with the MCM, the iteration time step plays an important role. On the other hand, for networks with the BPTM, the assumption of intra-bond velocity profile affects the results. 相似文献