The origin of magnesite deposits from the Polish Foresudetic Block ophiolites: preliminary δ13C and δ18O investigations

Twenty-three magnesite samples from vein deposits, found in ultramafic rocks related to ophiolites in the north-east Bohemian Massif, have been examined for carbon and oxygen isotope ratios. In these rocks two stages of antigorite serpentinization took place prior to magnesite precipitation. δ¹⁸O_SMOW in magnesites ranges from 24.5 to 27.9%o (corrected for acid fractionation factor) which leads us to conclude a meteoric origin of the ore-forming solution. This inference is further supported by δ¹³C values (?18.7 to ?4.4%o) which point to a biogenic origin of carbon in the magnesites.     

Residential preferences in the city of Warsaw,Poland

Warsaw was heavily damaged during World War II. Post-War socialist reconstruction emphasized public multi-family housing. A 1987 survey revealed that about one-third of Warsaw residents lived in single-family units, though most expressed their preferences to live in single family houses free from noise pollution and in an aesthetically acceptable environment. In two areas, the CBD and the Praga North, a large proportion of residents preferred to live in existing multi-family units, though such preference was much greater among the blue collar workers than white collar.     

Different modes of the Late Cretaceous–Early Tertiary inversion in the North German and Polish basins

Several selected seismic lines are used to show and compare the modes of Late-Cretaceous–Early Tertiary inversion within the North German and Polish basins. These seismic data illustrate an important difference in the allocation of major zones of basement (thick-skinned) deformation and maximum uplift within both basins. The most important inversion-related uplift of the Polish Basin was localised in its axial part, the Mid-Polish Trough, whereas the basement in the axial part of the North German Basin remained virtually flat. The latter was uplifted along the SW and to a smaller degree the NE margins of the North German Basin, presently defined by the Elbe Fault System and the Grimmen High, respectively. The different location of the basement inversion and uplift within the North German and Polish basins is interpreted to reflect the position of major zones of crustal weakness represented by the WNW-ESE trending Elbe Fault System and by the NW-SE striking Teisseyre-Tornquist Zone, the latter underlying the Mid-Polish Trough. Therefore, the inversion of the Polish and North German basins demonstrates the significance of an inherited basement structure regardless of its relationship to the position of the basin axis. The inversion of the Mid-Polish Trough was connected with the reactivation of normal basement fault zones responsible for its Permo-Mesozoic subsidence. These faults zones, inverted as reverse faults, facilitated the uplift of the Mid-Polish Trough in the order of 1–3 km. In contrast, inversion of the North German Basin rarely re-used structures active during its subsidence. Basement inversion and uplift, in the range of 3–4 km, was focused at the Elbe Fault System which has remained quiescent in the Triassic and Jurassic but reproduced the direction of an earlier Variscan structural grain. In contrast, N-S oriented Mesozoic grabens and troughs in the central part of the North German Basin avoided significant inversion as they were oriented parallel to the direction of the inferred Late Cretaceous–Early Tertiary compression. The comparison of the North German and Polish basins shows that inversion structures can follow an earlier subsidence pattern only under a favourable orientation of the stress field. A thick Zechstein salt layer in the central parts of the North German Basin and the Mid-Polish Trough caused mechanical decoupling between the sub-salt basement and the supra-salt sedimentary cover. Resultant thin-skinned inversion was manifested by the formation of various structures developed entirely in the supra-salt Mesozoic–Cenozoic succession. The Zechstein salt provided a mechanical buffer accommodating compressional stress and responding to the inversion through salt mobilisation and redistribution. Only in parts of the NGB and MPT characterised by either thin or missing Zechstein evaporites, thick-skinned inversion directly controlled inversion-related deformations of the sedimentary cover. Inversion of the Permo-Mesozoic fill within the Mid-Polish Trough was achieved by a regional elevation above uplifted basement blocks. Conversely, in the North German Basin, horizontal stress must have been transferred into the salt cover across the basin from its SW margin towards the basins centre. This must be the case since compressional deformations are concentrated mostly above the salt and no significant inversion-related basement faults are seismically detected apart from the basin margins. This strain decoupling in the interior of the North German Basin was enhanced by the presence of the Elbe Fault System which allowed strain localization in the basin floor due to its orientation perpendicular to the inferred Late Cretaceous–Early Tertiary far-field compression.     

Stochastic analysis of lithologic sequences: Bed thickness distribution

A stochastic model of bed thickness distribution is given. The following scheme of sedimentation and bed formation is assumed: (1)sedimentation consists of an unending sequence of alternating periods of deposition and erosion; (2)succession of sedimentary environments is described in terms of a transition probability matrix; (3)each depositional period accumulates a bed of sediments; (4)net accumulation exceeds erosion; (5)amounts of deposition and erosion are independent random variables. The equations for transformation of bed thickness distribution from sedimentary environment to geologic record are obtained. The sedimentological significance of these equations is discussed.     

Analysis of Shallow and Deep Earthquake Doublets in the Fiji-Tonga-Kermadec Region

For the Fiji-Tonga-Kermadec area and for the period from January 1977 to July 2003, the Harvard CMT catalogue lists 1022 shallow, 410 intermediate and 633 deep earthquakes of moment magnitude from 4.9 to 8.0. The magnitude threshold, above which the catalogue is complete, is 5.3–5.4, and the number of earthquakes of magnitude above this value is 691 for shallow, 329 for intermediate and 476 for deep events, respectively. The proportion of earthquakes associated with doublets and multiplets against the total number of earthquakes is approximately the same in both data sets and therefore all earthquake pairs were considered regardless of their magnitude. We investigated all the pairs of earthquakes that occurred at a centroid distance of less than 40, 60 or 90 km from each other and within a time interval of 200, 300 or 450 days, depending on their magnitude. We found 208 pairs of shallow, 31 of intermediate and 92 of deep events. To ascertain whether these earthquakes in pairs are not connected by chance, the possibility of their occurrence in an uncorrelated Poissonian catalogue was considered. It was assumed that in such a catalogue the inter-event time is exponentially distributed, the earthquake magnitude follows the Gutenberg-Richter relation, and the distribution of centroid distances between the events in pairs is controlled by its non-parametric kernel estimate. The probability of the appearance of the observed proportion of doublets of shallow earthquakes in the Poissonian catalogue was found to be very low. The low probability of occurrence in a semi-random catalogue, created by randomising centroid locations in the actual data set, also indicates major importance of the distance criterion used for a doublet specification. In general, shallow earthquakes tend to form pairs at shorter distances and within shorter time intervals than deep earthquakes. Both the distance and the time intervals do not depend on the magnitude of involved events. The largest number of pairs of deep earthquakes is observed at a depth of about 600 km, and the proportion of deep events associated with doublets against the number of all events increases with depth. From comparison of the focal mechanism of earthquakes in pairs, measured by the 3-D rotation angle, it follows that deep earthquakes forming pairs have a more diverse focal mechanism than shallow events; the rotation angle for three quarters of shallow pairs and only for about one third of deep pairs is reasonably small. The azimuth between two events forming a doublet is in about 60–65% of cases close to the strike of one of nodal planes of the first or the second event.     

Back-analysis for initial ground stress field at a diamond mine using machine learning approaches

Exact knowledge for ground stress field guarantees the construction of various underground engineering projects as well as prediction of some geological hazards such as the rock burst. Limited by costs, field measurement for initial ground stresses can be only conducted on several measure points, which necessitates back-analysis for initial stresses from limited field measurement data. This paper employed a multioutput decision tree regressor (DTR) to model the relationship between initial ground stress field and its impact factor. A full-scale finite element model was built and computed to gain 400 training samples for DTR using a submodeling strategy. The results showed that correlation coefficient r between field measurement values and back-analysis values reached 0.92, which proved the success of DTR. A neural network was employed to store the global initial ground stress field. More than 600,000 node data extracted from the full-scale finite element model were used to train this neural network. After training, the stresses on any location can be investigated by inputting corresponding coordinates into this neural network.

     

Bias,variance and computational properties of Kijko’s estimators of the upper limit of magnitude distribution,M<Subscript>max</Subscript>

It is often assumed in probabilistic seismic hazard analysis that the magnitude distribution has an upper limit M _max, which indicates a limitation on event size in specific seismogeneic conditions. Accurate estimation of M _max from an earthquake catalog is a matter of utmost importance. We compare bias, dispersion and computational properties of four popular M _max estimators, introduced by Kijko and others (e.g., Kijko and Sellevoll 1989, Kijko and Graham 1998, Kijko 2004) and we recommend the ones which can be the most fruitful in practical applications. We provide nomograms for evaluation of bias and standard deviation of the recommended estimators for combinations of sample sizes and distribution parameters. We suggest to use the bias nomograms to correct the M _max estimates. The nomograms of standard deviation can be used to determine minimum sample size for a required accuracy of M _max.     

SHRIMP zircon geochronology and geochemistry of the Orlica-Śnieżnik gneisses (Variscan belt of Central Europe) and their tectonic implications

Abstract

The Orlica-?nie?nik dome comprises large orthogneiss bodies interbedded with amphiblite-grade metasediments and minor metavolcanics. New U-Pb and Pb-Pb SHRIMP zircon ages for two major gneiss units of the dome, the ?nie?nik and Giera?tów gneiss, yielded similar ages of ca. 500 Ma. This is interpreted to reflect the magmatic crystallization age from the same or similar igneous precursors, in agreement with the geochemical characteristics of these rocks. Some zircon cores in both gneisses, interpreted to be inherited xenocrysts, have ages of ca. 530–540 Ma, and, additionally, of ca. 565 Ma and 2.6 Ga in the ?nie?nik gneiss. Igneous grains in both gneiss types have high-U rims, which are dark under cathodoluminescence. They are much better developed in the Giera?tow gneiss and they yield a well-defined weighted mean U-Pb age of 342 ± 6 Ma. These high-U rims are interpreted to have grown close to the peak of HT metamorphism which is responsible for the migmatitic texture of the Giera?tow gneiss. The Visean HT-LP metamorphism in the Orlica-?nie?nik dome is interpreted as a result of rapid uplift and decompression due to overthrusting of high grade rocks over the Moravo-Silesian nappe pile. Our data support geodynamic models that ascribe a predominant influence in the tectonic evolution of the West Sudetes to the Variscan oro- genic events. This is suggested by the inheritance of zircon xenocrysts from the Cadomian basement and by the Late Cambrian- Early Ordovician magmatic event, both typical of the Armorican terrane assemblage, as well as by the Early Carboniferous age of the metamorphism. © 2000 Éditions scientifiques et médicales Elsevier SAS