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1.
Strontium chemical diffusion has been measured in albite and sanidine under dry, 1 atm, and QFM buffered conditions. Strontium oxide-aluminosilicate powdered sources were used to introduce the diffusant and Rutherford Backscattering Spectroscopy (RBS) used to measure diffusion profiles. For the 1 atm experiments, the following Arrhenius relations were obtained:
Sanidine (Or61), temperature range 725–1075°C, diffusion normal to (001): D=8.4 exp(−450±13 kJ mol−1/RT) m2s−1. Albite (Or1), temperature range 675–1025°C, diffusion normal to (001): D=2.9 × exp(−224±11 kJ mol−1/RT) m2s−1.
The alkali feldspars in this and earlier work display a broad range of activation energies for Sr diffusion, which may be a consequence of the thermodynamic non-ideality of the alkali feldspar system and/or the mixed alkali effect.  相似文献   

2.
Scaling properties of landslides in the Rif mountains of Morocco   总被引：2，自引：0，他引：2  
M. Rouai  E. B. Jaaidi   《Engineering Geology》2003,68(3-4):353-359
Landslides in the central Rif mountains (Morocco) were analyzed by multifractal analysis. Our results suggest that spatial distribution of landslides in the region is not a homogeneous fractal structure but a heterogeneous one with generalized dimensions D(1)=1.713>D(2)>…>D(12)=1.325. The value of D(12)=D(∞) is the fractal dimension of the most intensive clustering in the heterogeneous fractal set. It is worthwhile to note that we found D(0)<D(1). The analysis of areas affected by sliding from the geological map of Beni Ahmed at a scale 1:50 000 shows the power law size distribution: N(A>a)∝a−1.57. This confirms the scale invariance of sliding and suggests that real landslides may exhibit a Self-Organized Criticality (SOC) behaviour.  相似文献   

3.
Musa Güner 《Tectonophysics》1982,90(3-4):309-312
The palaeomagnetism of basic rocks and sulphide ores has been studied in the Küre area, Pontic Ranges, Turkey. Progressive alternating-field demagnetization revealed a characteristic remanent magnetization in all investigated rock types except a dacite. The following virtual geomagnetic poles were obtained:

Basalt and quartz diabase (oldest): D = 59°, I = +66°, 95 = 4.8, pole 49°N, 93°E. Diabase: D = 210°, I = −15°, 95 = 15.0, pole 47°N, 167°E. Massive sulphide ores: D = 107°, I = +63°, 95 = 8.7, pole 18°N, 80°E. Peridotite: D = 131°, I = +54°, 95 = 10.9, pole 2°S, 72°E. Amphibolitized diabase (youngest): D = 293°, I = +59°, 95 = 12.6, pole 40°S, 145°E.

The longidutinal difference in pole positions between the oldest and the youngest rocks is interpreted as being due to a post-Permian counterclockwise rotation of the studied region in relation to the African continent. In addition, there are indications of local rotational movements within the Küre area.  相似文献   


4.
Garnet–melt trace element partitioning experiments were performed in the system FeO–CaO–MgO–Al2O3–SiO2 (FCMAS) at 3 GPa and 1540°C, aimed specifically at studying the effect of garnet Fe2+ content on partition coefficients (DGrt/Melt). DGrt/Melt, measured by SIMS, for trivalent elements entering the garnet X-site show a small but significant dependence on garnet almandine content. This dependence is rationalised using the lattice strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994. Prediction of crystal–melt partition coefficients from elastic moduli. Nature 372, 452–454], which describes partitioning of an element i with radius ri and valency Z in terms of three parameters: the effective radius of the site r0(Z), the strain-free partition coefficient D0(Z) for a cation with radius r0(Z), and the apparent compressibility of the garnet X-site given by its Young's modulus EX(Z). Combination of these results with data in Fe-free systems [Van Westrenen, W., Blundy, J.D., Wood, B.J., 1999. Crystal-chemical controls on trace element partitioning between garnet and anhydrous silicate melt. Am. Mineral. 84, 838–847] and crystal structure data for spessartine, andradite, and uvarovite, leads to the following equations for r0(3+) and EX(3+) as a function of garnet composition (X) and pressure (P):
r0(3+) [Å]=0.930XPy+0.993XGr+0.916XAlm+0.946XSpes+1.05(XAnd+XUv)−0.005(P [GPa]−3.0)(±0.005 Å)
EX(3+) [GPa]=3.5×1012(1.38+r0(3+) [Å])−26.7(±30 GPa)
Accuracy of these equations is shown by application to the existing garnet–melt partitioning database, covering a wide range of P and T conditions (1.8 GPa<P<5.0 GPa; 975°C<T<1640°C). DGrt/Melt for all 3+ elements entering the X-site (REE, Sc and Y) are predicted to within 10–40% at given P, T, and X, when DGrt/Melt for just one of these elements is known. In the absence of such knowledge, relative element fractionation (e.g. DSmGrt/Melt/DNdGrt/Melt) can be predicted. As an example, we predict that during partial melting of garnet peridotite, group A eclogite, and garnet pyroxenite, r0(3+) for garnets ranges from 0.939±0.005 to 0.953±0.009 Å. These values are consistently smaller than the ionic radius of the heaviest REE, Lu. The above equations quantify the crystal-chemical controls on garnet–melt partitioning for the REE, Y and Sc. As such, they represent a major advance en route to predicting DGrt/Melt for these elements as a function of P, T and X.  相似文献   

5.
Three types of chemically and isotopically distinct pore fluids from the southern San Joaquin basin previously recognized by J.B. Fisher and J.R. Boles also have distinctive 87Sr/86Sr ratios and Sr concentrations. Meteoric fluids have stable isotopic compositions which lie on or near the meteoric water line and low chlorinities. Sr concentrations are between 0.01 and 2.6 mg l−1, and 87Sr/86Sr ratios range from 0.7061 to 0.7078. Diagenetically modified connate marine fluids have δD-and δ18O-values more positive than −35‰ and 0‰, respectively, and have chlorinities generally comparable to seawater. Sr concentration are much higher than the meteoric group (16–198 mg l−1), although the 87Sr/86Sr ratios (0.7070–0.7081) are not distinctive. Mixed meteoric-modified connate fluids have δD, δ18O and chlorinity intermediate between the meteoric and modified connate groups. Sr concentrations are also intermediate, between 16 and 22 mg l−1, but 87Sr/86Sr ratios (0.7080–0.7087) are generally more radiogenic than either the meteoric or modified connate groups.

All of the fluids have 87Sr/86Sr ratios comparable to or lower than Tertiary seawater. Alteration of detrital plagioclase is the probable origin of the low isotopic ratios. Mass-balance calculations based on the Sr data suggest that essentially no transport of Sr occurred during diagenesis of sandstones containing modified connate pore fluids, while large amounts of Sr have been transported out of meteoric reservoirs by fluid flow. The chemically anomalous mixed meteoric-modified connate fluids contain the most radiogenic strontium in the basin. These fluids are spatially associated with major faults, and may represent clay mineral dehydration waters which have been transported upward from greater depth.

These results suggest that the three types of fluids identified by Fisher and Boles represent three distinct mass transport regimes: a largely stagnant deep-basin system containing modified connate pore fluids; an actively recharging meteoric system along the basin flanks; and a third system restricted to the southern basin which may be characterized by largescale cross-formational fluid flow, rather than dilution by meteoric waters.  相似文献   


6.
Critical rainfall thresholds for landslides are powerful tools for preventing landslide hazard. The thresholds are commonly estimated empirically starting from rainfall events that triggered landslides in the past. The creation of the appropriate rainfall–landslide database is one of the main efforts in this approach. In fact, an accurate agreement between the landslide and rainfall information, in terms of location and timing, is essential in order to correctly estimate the rainfall–landslide relationships. A further issue is taking into account the average moisture conditions prior the triggering event, which reasonably may be crucial in determining the sufficient amount of precipitation. In this context, the aim of this paper is exploiting historical landslide and rainfall data in a spatial database for the derivation of critical rainfall thresholds for landslide occurrence in Sicily, southern Italy. The hourly rainfall events that caused landslides occurred in the twentieth century were specifically identified and reconstructed. A procedure was proposed to automatically convert rain guages charts recorded on paper tape into digital format and then to provide the cumulative rainfall hyetograph in digital format. This procedure is based on a segmentation followed by signal recognition techniques which allow to digitalize and to recognize the hyetograph automatically. The role of rainfall prior to the landslide events was taken into account by including in the analysis the rainfall occurred 5, 15 and 30 days before each landslide. Finally, cumulated rainfall duration thresholds for different exceedance probability levels were determined. The obtained thresholds resulted in agreement with the regional curves proposed by other authors for the same area; antecedent rainfall turned out to be particularly important in triggering landslides.  相似文献   

7.
Majority of landslides in the Indian sub-continent are triggered by rainfall. Several attempts in the global scenario have been made to establish rainfall thresholds in terms of intensity-duration and antecedent rainfall models on global, regional and local scales for the occurrence of landslides. However, in the context of the Indian Himalayas, the rainfall thresholds for landslide occurrences are not yet understood fully. Neither on regional scale nor on local scale, establishing such rainfall thresholds for landslide occurrences in Indian Himalayas has yet been attempted. This paper presents an attempt towards deriving local rainfall thresholds for landslides based on daily rainfall data in and around Chamoli-Joshimath region of the Garhwal Himalayas, India. Around 128 landslides taken place in last 4 years from 2009 to 2012 have been studied to derive rainfall thresholds. Out of 128 landslides, however, rainfall events pertaining to 81 landslides were analysed to yield an empirical intensity–duration threshold for landslide occurrences. The rainfall threshold relationship fitted to the lower boundary of the landslide triggering rainfall events is I?=?1.82 D ?0.23 (I?=?rainfall intensity in millimeters per hour and D?=?duration in hours). It is revealed that for rainfall events of shorter duration (≤24 h) with a rainfall intensity of 0.87 mm/h, the risk of landslide occurrence in this part of the terrain is expected to be high. Also, the role of antecedent rainfall in causing landslides was analysed by considering daily rainfall at failure and different period cumulative rainfall prior to failure considering all 128 landslides. It is observed that a minimum 10-day antecedent rainfall of 55 mm and a 20-day antecedent rainfall of 185 mm are required for the initiation of landslides in this area. These rainfall thresholds presented in this paper may be improved with the hourly rainfall data vis-à-vis landslide occurrences and also data of later years. However, these thresholds may be used in landslide warning systems for this particular region of the Garhwal Himalayas to guide the traffic and provide safety to the tourists travelling along this pilgrim route during monsoon seasons.  相似文献   

8.
Mass movements varying in type and size, some of which are periodically reactivated, affect the urban area of Avigliano. The disturbed and remoulded masses consist of sandy–silty or silty–clayey plastic material interbedded with stone fragments and conglomerate blocks. Five landslides that were markedly liable to rainfall-associated instability phenomena were selected.

The relationships between landslides and rainfall were investigated using a hydrological and statistical model based on long-term series of daily rainfall data. The model was used to determine the return period of cumulative daily rainfall over 1–180 days. The resulting hydrological and statistical findings are discussed with the aim of identifying the rainfall duration most critical to landslides.

The concept of a precipitation threshold was generalized by defining some probability classes of cumulative rainfall. These classes indicate the thresholds beyond which reactivation is likely to occur. The probability classes are defined according to the return period of the cumulative rainfall concomitant with landslide reactivation.  相似文献   


9.
 Hydrological landslide-triggering thresholds separate combinations of daily and antecedent rainfall or of rainfall intensity and duration that triggered landslides from those that failed to trigger landslides. They are required for the development of landslide early warning systems. When a large data set on rainfall and landslide occurrence is available, hydrological triggering thresholds are determined in a statistical way. When the data on landslide occurrence is limited, deterministic models have to be used. For shallow landslides directly triggered by percolating rainfall, triggering thresholds can be established by means of one-dimensional hydrological models linked to the infinite slope model. In the case of relatively deep landslides located in topographic hollows and triggered by a slow accumulation of water at the soil-bedrock contact, simple correlations between landslide occurrence and rainfall can no longer be established. Therefore real-time failure probabilities have to be determined using hydrological catchment models in combination with the infinite slope model. Received: 15 October 1997 · Accepted: 25 June 1997  相似文献   

10.
Rosanna De Rosa  Paola Donato  Guido Ventura   《Lithos》2002,65(3-4):299-311
Upper Pollara eruption products (13 ka, Salina Island, Italy) include both homogeneous and heterogeneous pumices resulting from mixing/mingling processes between an HK andesite and a high-SiO2 rhyolite. Representative samples of heterogeneous pumices are collected and analyzed in order to check the correspondence between glass composition and morphological features of the mingling/mixing structures. Image analysis techniques are applied and eight grey color ranges (classes) are extracted from high-resolution scans of pumice. Class 1 (lighter colors) and class 8 (darker colors) show end-member glass compositions, i.e. HK andesite and high-SiO2 rhyolite, respectively. These two classes show spot- to cluster-like morphological structures. Intermediate classes show an HK dacitic to rhyolitic composition and a banding- to fold-like morphology. Fractal analysis by box-counting of the boundary pattern of eight grey classified images is performed over a length scale of 0.028–1.8 cm. Fractal dimension D is between 1.01 and 1.84. Coupled fractal analysis and geochemical data reveal that D increases as the degree of magma interaction (homogenization) increases. This feature well fits the results from numerical models on the convective mixing of fluids driven by thermal convection. We conclude that the increase of D observed in the Upper Pollara samples reflects the transition from fractal mixing to homogenization. End-member magmas (HK andesite and high-SiO2 rhyolite) represent isolated mixing regions, while homogenized magmas represent active mixing regions. In the analyzed pumices, isolated and active mixing regions coexist at scales between 10−4 and 10−2 m. Morphological and compositional features of the Upper Pollara pumices result from turbulence.  相似文献   

11.
The dissolution and precipitation rates of boehmite, AlOOH, at 100.3 °C and limited precipitation kinetics of gibbsite, Al(OH)3, at 50.0 °C were measured in neutral to basic solutions at 0.1 molal ionic strength (NaCl + NaOH + NaAl(OH)4) near-equilibrium using a pH-jump technique with a hydrogen-electrode concentration cell. This approach allowed relatively rapid reactions to be studied from under- and over-saturation by continuous in situ pH monitoring after addition of basic or acidic titrant, respectively, to a pre-equilibrated, well-stirred suspension of the solid powder. The magnitude of each perturbation was kept small to maintain near-equilibrium conditions. For the case of boehmite, multiple pH-jumps at different starting pHs from over- and under-saturated solutions gave the same observed, first order rate constant consistent with the simple or elementary reaction: .

This relaxation technique allowed us to apply a steady-state approximation to the change in aluminum concentration within the overall principle of detailed balancing and gave a resulting mean rate constant, (2.2 ± 0.3) × 10−5 kg m−2 s−1, corresponding to a 1σ uncertainty of 15%, in good agreement with those obtained from the traditional approach of considering the rate of reaction as a function of saturation index. Using the more traditional treatment, all dissolution and precipitation data for boehmite at 100.3 °C were found to follow closely the simple rate expression:

Rnet,boehmite=10-5.485{mOH-}{1-exp(ΔGr/RT)}, with Rnet in units of mol m−2 s−1. This is consistent with Transition State Theory for a reversible elementary reaction that is first order in OH− concentration involving a single critical activated complex. The relationship applies over the experimental ΔGr range of 0.4–5.5 kJ mol−1 for precipitation and −0.1 to −1.9 kJ mol−1 for dissolution, and the pHm ≡ −log(mH+) range of 6–9.6. The gibbsite precipitation data at 50 °C could also be treated adequately with the same model:Rnet,gibbsite=10-5.86{mOH-}{1-exp(ΔGr/RT)}, over a more limited experimental range of ΔGr (0.7–3.7 kJ mol−1) and pHm (8.2–9.7).  相似文献   


12.
Rainfall patterns for shallow landsliding in perialpine Slovenia   总被引：2，自引：0，他引：2  
This paper presents two types of analysis: an antecedent rainfall analysis based on daily rainfall and an intensity-duration analysis of rainfall events based on hourly data in perialpine Slovenia in the ?kofjelo?ko Cerkljansko hills. For this purpose, eight rainfall events that are known to have caused landslides in the period from 1990 to 2010 were studied. Over the observed period, approximately 400 records of landslides were collected. Rainfall data were obtained from three rain gauges. The daily rainfall from the 30 days before landslide events was investigated based on the type of landslides and their geo-environmental setting, the dates of confirmed landslide activity and different consecutive rainfall periods. The analysis revealed that the rainfall events triggering slope failure can be divided into two groups according to the different antecedent periods. The first group of landslides typically occurred after short-duration rainstorms with high intensity, when the daily rainfall exceeded the antecedent rainfall. The second group comprises the rainfall events with a longer antecedent period of at least 7 days. A comparison of the plotted peak and mean intensities indicates that the rainfall patterns that govern slope failure are similar but do not necessarily reflect the rainfall intensity at the time of shallow landslides in the Dav?a or Poljane areas, where the majority of the landslides occurred. Because of several limitations, the suggested threshold cannot be compared and evaluated with other thresholds.  相似文献   

13.
We report Sr, Nd and Pb isotopic compositions of mid-Proterozoic anorthosites and related rocks (1.45-1.65 Ga) and of younger olivine diabase dikes (1.4 Ga) from two complexes on either side of the Grenville Front in Labrador. Anorthositic or diabasic samples from the Mealy Mountains (Grenville Province) and Harp Lake (Nain-Churchill Provinces) complexes have very similar major, minor and trace element compositions, but distinctly different isotopic signatures. All Mealy Mountains samples have ISr = 0.7025−0.7033, εNd = +0.6 to +5.6 and Pb isotopic compositions consistent with derivation from a mantle source depleted with respect to Nd/Sm and Rb/Sr. Pb isotopic compositions for the Mealy Mountains samples are slightly more radiogenic than model mantle compositions. All Harp Lake samples have ISr = 0.7032−0.7066, εNd = −0.3 to −4.4 and variable, but generally unradiogenic 207Pb/204Pb and 206Pb/204Pb compared to model mantle, suggesting mixing between a mantle-derived component and a U-depleted crustal contaminant. Crustal contaminants are probably a variety of Archean high-grade quartzofeldspathic gneisses with low U/Pb ratios and include a component that must be isotopically similar to the early Archean (>3.6 Ga) Uivak gneisses of Labrador or the Amitsoq gneisses of west Greenland. This would imply that the ancient gneiss complex of coastal Labrador and Greenland is larger than indicated by present surface exposure and may extend in the subsurface as far west as the Labrador Trough. If Harp Lake and Mealy Mountains samples were subjected to the same degree of contamination, as suggested by their chemical similarities, then the Mealy contaminants must be much younger, probably early or middle Proterozoic in age. The Labrador segment of the Grenville Front, therefore, appears to coincide with the southern margin of the Archean North Atlantic craton and may represent a pre mid-Proterozoic suture.  相似文献   

14.
Regionalization of rainfall thresholds: an aid to landslide hazard evaluation   总被引：25，自引：2，他引：23  
 Rainfall, soil properties, and morphology are major factors controlling shallow landsliding. A series of meteorological events that triggered soil slips in northern Italy were studied to define rainfall thresholds and to evaluate a possible regionalization. Soil properties, triggering rainfall, and local lithological and morphometrical settings of different sites were used as input to an infiltration model. The approach allows the recognition of several triggering conditions in the Piedmont, Pre-Alpine and Alpine regions. This suggests the need for different rainfall thresholds with respect to those derived with other methods. Intensity versus rainfall duration relationships become particularly important when related to soil permeability and thickness, and demonstrate the role of antecedent precipitation. Events with exceptional water discharge from obstructed road culverts reveal the role played by anthropic structures in triggering such phenomena. Different approaches to slope stability analysis are shown, taking into account bedrock lithology, topography, seepage, and local saturation conditions. Received: 23 October 1997 · Accepted: 25 June 1997  相似文献   

15.
Rainfall is the most relevant factor for the triggering of landslides and it is characterized by an extreme variability. Rainfall analysis is the most frequently adopted approach for predicting landslides. But predicting deep-seated landslides by means of rainfall thresholds is sometimes inadequate due to the complexity of such slopes. In this paper, further study of the loading / unloading response ratio theory applied in landslides prediction was carried out. As the aforementioned effect and character of rainfall was concerned, a prediction for the known Xintan landslide was carried out by means of the loading / unloading response ratio method, in which the action of water was regarded as loading/unloading measures. The results show that it is an effective method for landslides prediction, and it has an advantage over conventional rainfall thresholds or time sequences analysis method.  相似文献   

16.
The results of the 7 years (1994–2000) of monthly monitoring of spring water before and during eruptions show response to volcanic activity. Low salinity and temperature characterize most of the springs, which are located on the flanks of Popocatepetl Volcano. The pH ranges from 5.8 to 7.8 and temperature from 3 to 36 °C. Oxygen and hydrogen isotopic data show that the water is of meteoric origin, but SO42−, Cl−, F−, HCO3−, B, and SO42−/Cl− variations precede main eruptive activity, which is considered linked to influx of magmatic gases and acid fluids that react with sublimates and host rock and mix with the large water system. Na+, Ca2+, SiO2 and Mg2+ concentrations in the water also increased before eruptive activity. The computed partial pressure of CO2 in equilibrium with spring waters shows values higher than air-saturated water (ASW), with the highest values up to 0.73 bar of pCO2. Boron is detected in the water only preceding the larger eruptions. When present, boron concentration is normally under health standard limits, but in two cases the concentration was slightly above. Other components are within health standard limits, except for F− in one spring.  相似文献   

17.
Rainfall thresholds represent the main tool for the Italian Civil Protection System for early warning of the threat of landslides. However, it is well-known that soil moisture conditions at the onset of a storm event also play a critical role in triggering slope failures, especially in the case of shallow landslides. This study attempts to define soil moisture (estimated by using a soil water balance model) and rainfall thresholds that can be employed for hydrogeological risk prevention by the Civil Protection Decentrate Functional Centre (CFD) located in the Umbria Region (central Italy). Two different analyses were carried out by determining rainfall and soil moisture conditions prior to widespread landslide events that occurred in the Umbria Region and that are reported in the AVI (Italian Vulnerable Areas) inventory for the period 1991?C2001. Specifically, a ??local?? analysis that considered the major landslide events of the AVI inventory and an ??areal?? analysis subdividing the Umbria Region in ten sub-areas were carried out. Comparison with rainfall thresholds used by the Umbria Region CFD was also carried out to evaluate the reliability of the current procedures employed for landslide warning. The main result of the analysis is the quantification of the decreasing linear trend between the maximum cumulated rainfall values over 24, 36 and 48?h and the soil moisture conditions prior to landslide events. This trend provides a guideline to dynamically adjust the operational rainfall thresholds used for warning. Moreover, the areal analysis, which was aimed to test the operational use of the combined soil moisture?Crainfall thresholds showed, particularly for low values of rainfall, the key role of soil moisture conditions for the triggering of landslides. On the basis of these results, the Umbria Region CFD is implementing a procedure aimed to the near real-time estimation of soil moisture conditions based on the soil water balance model developed ad hoc for the region. In fact, it was evident that a better assessment of the initial soil moisture conditions would support and improve the hydrogeological risk assessment.  相似文献   

18.
We present a database and a graphical analysis of published experimental results for dissolution rates of olivine, quartz plagioclase, clinopyroxene, orthopyroxene, spinel, and garnet in basaltic and andesitic melts covering a range of experimental temperatures (1100–1500°C) and pressures (105 Pa-3.0 GPa). The published datasets of Donaldson (1985, 1990) and Brearly and Scarfe (1986) are the most complete. Experimental dissolution rates from all datasets are recalculated and normalized to a constant oxygen basis to allow for direct comparison of dissolution rates between different minerals. Dissolution rates (ν) range from 5·10−10 oxygen equivalent moles (o.e.m.) cm−2 s−1 for olivine in a basaltic melt to 1.3·10−5 o.e.m. cm−2 s−1 for garnet in a basaltic melt. Values of ln ν are Arthenian for the experiments examined and activation energies range from 118 to 1800 kJ/o.e.m. for quartz and clinopyroxene, respectively.

The relationship between calculated A/RT for the dissolution reactions, where A is the thermodynamic potential affinity, and values of ν is linear for olivine, plagioclase, and quartz. We interpret this as strong evidence in support of using calculated A as a predictor of ν for, at least, superliquidus melt conditions.  相似文献   


19.
The paleomagnetism of Upper Cretaceous magmatic rocks from 47 collecting sites (172 samples, 692 specimens) in the Apuseni Mountains was studied. After AF cleaning, characteristic magnetizations were identified for various collecting areas in the study zone, which defined a few spatial and temporal units for which paleomagnetic poles could be derived statistically. At 21 sampling sites the paleomagnetic directions showed a high level of intrasite and intersite consistency, with a mean direction of If = −38° and Df = −100°, with 95 = 6°. The paleomagnetic results show that to reach their present-day position the Apuseni Mountains moved to the north, around 14° with respect to Europe, or around 25° with respect to the geographic poles, between the Campanian and, probably, Late Miocene, while a clockwise rotation, of around 80°, was taking place.  相似文献   

20.
To verify paleomagnetic proof for megatectonic translation in the Tethys a large collection of samples from a key area, the Bolzano Quartz Porphyry Plateau in the Southern Alps, was examined. Their natural remanent magnetization was analyzed with thermal, and mainly alternating field demagnetization. The result is a well-established paleomagnetic direction of D: 150° and I: −19.5° (95 = 4.9), obtained from 152 samples from 39 sites distributed over 12 volcanic units. It is argued that the inclination of this result is not significantly different from that which can be extrapolated for the Southern Alps from Early Permian paleomagnetic directions of the stable European shield. Consequently it is concluded that a paleomagnetic indication for megatectonic translation of the Southern Alps is virtually absent. But a large counterclockwise deviation of the declination is evident, and is easily explained by a counterclockwise rotation of 50° of the Southern Alps with respect to stable Europe. Since the paleomagnetic direction of the Early Permian volcanics of the Southern Alps fits in reasonably well with the (poorly known) Early Permian paleomagnetic pattern of Africa, a coherence between both regions is presumed.  相似文献   

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