Tectonometamorphic evolution of the Bohemian Massif: evidence from high pressure metamorphic rocks

The Variscan orogenic belt, of which the Bohemian Massif is a part, is typically recognized for its characteristic low pressure, high temperature metamorphism and a large volume of granites. However, there are also bodies of high pressure rocks (eclogites, garnet peridotites and high pressure granulites) which are small in size but widely distributed throughtout the Massif. Initially the high pressure rocks were considered to be relicts of a much older orogenic event, but the increasing data derived from isotopic and geochronological investigations show that many of these rocks have Palaeozoic protoliths. Metamorphic ages from the high pressure rocks define no single event. Instead, a number of discrete clusters of ages are found between about 430 Ma and the time of the dominant low pressure event at around 320–330 Ma.Most of the eclogite and granulite facies rocks are assigned to allochthonous nappes that arrived close to the end of the low pressure event, but before final granite intrusion. The nappes contain a mixture of different units and the relationship between rocks with high pressure relicts and host gneisses with no apparent signs of deep burial is still problematic. Some of the high pressure rocks retain evidence of multiple stages of partial re-equilibration during uplift. Moreover, it can be shown in certain instances that host gneisses also endured a multistage metamorphic development but with a peak event convergent with one of the breakdown stages in the enclosed rocks with high pressure relicts. It thus appears that the nappe units are composite bodies probably formed during episodic intracrustal thrusting. Fluids derived from prograde dehydration reactions in the newly under thrusting slab are taken to be the catalysts that drove the partial re-equilibrations.On the scale of the whole Massif it can be seen within the units with high pressure relicts that the temperature at the peak recorded pressure and that during the breakdown are variable in different locations. It is interpreted that regional metamorphic gradients are preserved for given stages in the history and thus the present day dismembered nappe relicts are not too far removed from their original spatial distribution in an original coherent unit. From the temperature information alone it is highly probable that the refrigerating underthrusting slab was situated in the north-west. However, this north-west to south-east underthrusting probably represents the major 380–370 Ma event and is no guide to the final thrusting that emplaced the much thinned nappe pile with high pressure relicts.Granite genesis is attributed to the late stage stacking, during the final Himalayan-type collision stage, of thinned crust covered by young, water-rich, sediments — erosion products of the earlier orogenic stages. Regional metamorphism at shallow depths above the voluminous granites was followed by final nappe emplacement which rejuvenated the granite ascent in places. Correspondence to: P. J. O'Brien     

Ratios of stable carbon isotopes in some High Arctic plants and lake sediments

¹³C/¹²C ratios for a number of High Arctic vascular plants (51 determinations), mosses (11), and freshwater algae (11) show considerable variation, particularly among the freshwater algae (range from-6.9 to -36.3). In some cases the stable carbon ratios on modern and fossil materials provide guidance as to whether marine waters formerly occupied a given pond or lake basin. In other cases the ¹³C values for algae collected along the present-day shore of a pond or lake bear no relation to the values obtained on constituents preserved within the bottom sediments, suggesting that major changes have occurred in the last few thousand years.Geological Survey of Canada Contribution No. 17291. Contribution No. 41 from the Cape Herschel Project.     

Permanent Scatterers for landslide investigations: outcomes from the ESA-SLAM project

Within the SLAM project (Service for Landslide Monitoring), launched in 2003 by the European Space Agency (ESA) the Permanent Scatterers (PS) technique, a multi-image interferometric approach, coupled with the interpretation of aerial-photos and optical satellite images, was carried out for landslide investigations. The PS analysis was applied at a regional scale as support for landslide inventory mapping and at local scale for the monitoring of single well-known slope movements. For the integration of the PS measurements within a landslide inventory the Arno river basin (Italy) was chosen as test site for the presence of a high number of mass movements (to date about 300 areas at high landslide risk and more than 27,000 individual landslides mapped by the institutional authorities). About 350 SAR images have been interferometrically processed by means of the PS technique, with the detection of about 600,000 PS. The use of optical images contributed spatial meaning to the point-wise information provided by the PS, making it easier to identify terrain features related to slope instability and the landslide boundaries. Here we describe the employed methodology and its impact in the updating of a preexisting landslide inventory. 6.8% of the total number of landslides were characterized by ground displacement measurements from the PS: 6.1% of already mapped landslides and 0.8% of new unstable areas detected through the PS analysis. Moreover, most of the PS are located in urban areas, showing that the proposed methodology is suitable for landslide mapping in areas with a quite high density of urbanization, but that over vegetated areas it still suffers from the limitations induced by the current space-borne SAR missions (e.g. temporal de-correlation). On the other hand, the use of InSAR for the monitoring of single slow landslides threatening built-up areas has provided satisfactory results, allowing the measurement of superficial deformations with high accuracy on the landslide sectors characterized by a good radar reflectivity and coherence.     

Patterns in the Turkish reinsurance claim data, 1998–2002: An application of the extreme value theory

The devastating 1999 Marmara and Düzce earthquakes led to a significant increase in the earthquake studies in Turkey in geological, engineering and financial aspects. Extreme Value Theory (EVT) has a range of applications from stock market changes to natural disasters like floods and hurricanes. Here EVT is fitted to the ordinary and earthquake reinsurance claims of Turkey.     

Synchrotron IR study of hydrous ringwoodite (γ-Mg2SiO4) up to 30 GPa

High-pressure synchrotron infrared (IR) absorption spectra were collected between 650 and 4,000 cm⁻¹ at ambient temperature for hydrous Mg-ringwoodite (γ-Mg₂SiO₄) up to 30 GPa. The main feature in the OH⁻ stretching region is an extremely broad band centred at 3,150 cm⁻¹. The hydrogen bond is strong for most protons and the most probable site for protonation is the tetrahedral edge. With increasing pressure, this band shifts downward while decreasing its integrated intensity until disappearance at a pressure of 25 GPa. Only one band at 2,450 cm⁻¹ and an absorption plateau persist with a maximum wavenumber of 3,800 cm⁻¹. This behaviour is reversible upon pressure release. We interpret this as a second-order phase transition occurring in hydrated Mg-ringwoodite at high pressure (beyond ∼ 25 GPa). This result is compatible with the observation by Kleppe et al. (Phys Chem Miner 29:473–476, 2002a) who suggested the presence of Si–O–Si linkages and/or partial increase in the coordination of Si. Beyond the phase transition, the protons are delocalized and their environment on the ringwoodite structure is probably quite different from that at low pressure. Data obtained in situ at high pressures and temperatures are needed to better understand the effect of protonation on the structure and to better constrain this phase transition.     

西藏俄卡地银多金属异常区地球化学

异常形态、分布严格受推覆断裂控制,范围大、浓集中心明显,浓度变化及因子载荷表明。区内找Ag、Pb有利,而Sb又为其最佳指示元素。     

Electrical aerosol spectrometer of Tartu University

The electrical aerosol spectrometer (EAS) of the parallel measuring principle at Tartu University is an efficient instrument for rapid measurement of the unstable size spectrum of aerosol particles. The measuring range from 10 nm to 10 μm is achieved by simultaneously using a pair of differential mobility analyzers with two different particle chargers. The particle spectrum is calculated and measurement errors are estimated in real time by using a least-squares method. Experimental calibration ensures reliability of measurement. The instrument is well suited for continuous monitoring of atmospheric aerosol.     

Impact of Southern Oscillation on the Frequency of Monsoon Depressions in the Bay of Bengal

The impact of Southern Oscillation on thecyclogenesis over the Bay of Bengal duringthe summer monsoon has been investigated.The analysis of correlation coefficients(CCs) between the frequency of monsoondepressions and the Southern OscillationIndex (SOI) reveals that more depressionsform during July and August of El Niñoyears. Due to this, the seasonal frequencyof monsoon depressions remains little higherduring El Niño epochs even though thecorrelations for June and September are notsignificant. The CCs for July and August aresignificant at the 99% level.The El Niño-Southern Oscillation (ENSO)is known to affect Indian MonsoonRainfall (IMR) adversely. The enhancedcyclogenesis over the Bay of Bengal duringJuly and August is an impact of ENSO whichneeds to be examined closely. Increasedcyclogenesis over the Bay of Bengal may bereducing the deficiency in IMR duringEl Niño years by producing more rainfallover the eastern parts of India duringJuly and August. Thus there is a considerablespatial variation in the impact of ENSOon the monsoon rainfall over India and El Niñoneed not necessarily imply a monsoonfailure everywhere in India.The area of formation of monsoon depressionsshifts eastward during El Niño years.Warmer sea surface temperature (SST) anomaliesprevail over northwest and adjoiningwestcentral Bay of Bengal during premonsoon andmonsoon seasons of El Niño years.May minus March SOI can provide useful predictionsof monsoon depression frequencyduring July and August.     

Spectroscopic evidence for pressure-induced phase transitions in diopside

 Raman spectra of diopside were collected from atmospheric pressure to 71 GPa. The pressure dependences of 22 modes were determined. Changes occurred in the spectra at three different pressures. First, at approximately 10 GPa, the two Raman modes at 356 and 875 cm⁻¹ disappeared, while the mode at 324 cm⁻¹ split into two modes, diverging at this pressure with significantly different pressure shifts; second, at approximately 15 GPa, a small (1 to 2 cm⁻¹) drop in several of the frequencies was observed accompanied by changes in the pressure dependency of some of the modes; and third, above 55 GPa, the modes characteristic of chains of tetrahedrally coordinated silicon disappeared, while those for octahedrally coordinated silicon appeared. The first change at 10 GPa appears to be a C2/c to C2/c transition involving a change in the Ca coordination. The third change above 55 GPa appears to be a change in the silicon coordination. At 15 GPa, it is suggested that a change in compressional mechanism takes place. Received: 14 November 2000 / Accepted: 9 January 2002