Moment Tensor Inversion of Regional Phases: Application to a Mine Collapse

—?The aim of our study consists of analyzing potentially non-double-couple seismic events recorded at regional distances. In order to define the nature of the seismic source, a moment tensor inversion is carried out as this method is general enough not to initially constrain the source mechanism. In this paper we present an application to a seismic event induced by a mine collapse which occurred near the town of Halle in Germany. Because of its induced nature, many parameters such as the location and geometry of this seismic source are known. This information allows us to test the influence of inadequate propagation modeling on the moment tensor obtained from the inversion. Green's functions have been computed with the reflectivity method in a flat layered medium, using the European model EurID (Du et? al., 1998; Dufumier et al., 1997). From the inversion of P-wave seismograms recorded by the German Regional Seismic Network will, we obtained a source time function which can be decomposed into two subevents. The first one has a large isotropic part and a deviatoric mechanism with near vertical nodal planes. No volume change is observed for the second subevent, but a deviatoric component opposite of the first one. The addition of S-waves does not change the results of the inversion which are stable. Surface waves were not used because of their poor dispersion curves. Based on the moment tensor obtained from these inversions, the physical process at the source is compatible with a large cavity collapse.     

Stable isotope studies and processes of carbonate formation in Hungarian alkali basalts and lamprophyres: evolution of magmatic fluids and magma-sediment interactions

Processes of carbonate formation have been related to C and O isotopic compositions in the Mesozoic alkali basalt (Mecsek Mts.) and lamprophyre (Transdanubian Range) suites of Hungary. In the studied magrnatic rocks, carbonates are present as ocelli, amygdales, xenoliths, veins and groundmass carbonate. C and O isotope studies of these types of carbonate have yielded information on the origin of the carbonates and indicated the following processes of formation that determined the δ¹³C and δ¹⁸O values of the carbonates:(1)Crystallization of magmatic carbonate. Textural characteristics and δ¹³C values suggest formation of magmatic carbonate in alkali basalt and lamprophyre dikes, whereas the δ¹⁸O compositions of these carbonates indicate low temperature oxygen isotope exchange with magmatic fluids.(2) Assimilation of sedimentary carbonate by silicate magmas. Even completely recrystallized amygdales and ocelli of basalts and lamprophyres have preserved their sedimentary δ¹³C values. In contrast, variations in the extent of mobilization and isotope exchange with magmatic fluids are reflected in differences in the ranges of the δ¹⁸O values of amygdales, ocelli and veins, and can be attributed to different amounts of fluids involved in the magmatic events.(3) Low temperature alteration of magmatic rocks caused only ¹⁸O-enrichment in the carbonate amygdales of basalts and the groundmass carbonates of lamprophyres, indicating that no externally-derived CO₂ was present in the alteration fluids.(4) Degassing of magma and magmatic fluid. Correlations between δ¹³C and δ¹⁸O data, magma crystallization depths and amygdale sizes in the alkali basalts suggest that CO₂ degassing has been responsible for the negative δ¹³C and positive δ¹⁸O shifts observed. A similar trend was found in the lamprophyres, but the extent of the δ¹⁸O shift indicates that in these rocks H₂O degassing also played an important role.     

Geochronology of granulites from the south Itabuna-Salvador-Curaçá Block,São Francisco Craton (Brazil): Nd isotopes and U–Pb zircon ages

This work provides five new U–Pb zircon dating and the corresponding Nd isotope data for felsic granulites from the south Itabuna-Salvador-Curaçá Block (ISCB), in the São Francisco Craton, Brazil. Three major sets of felsic granulites can be recognised. The oldest set is tonalitic in composition and of TTG affinity. It is Archaean in age with magmatic zircon cores dated at 2675 ± 11 Ma by LA-ICPMS and up to ca 2.7–2.9 Ga by SHRIMP on an other sample. It exhibits epsilon Nd values between ?8 and ?11 at 2.1 Ga. This Nd signature is similar to that of granulites found in the western Archaean Jequié Block. Cartographically, this set of Archaean terrains represents at least 50% of the granulites in the studied area. The second set corresponds to a Palaeoproterozoic calc-alkaline tonalitic suite with zircon ages from 2019 ± 19 Ma to 2191 ± 10 Ma and epsilon Nd values between ?3 and ?4 at 2.1 Ga, corresponding partially to a newly formed crust. The third set of granulites is also Palaeoproterozoic. It is shoshonitic to monzonitic in composition and synchronous with the high grade metamorphism dated by metamorphic zircons at 2086 ± 7 Ma (average of five samples). The Nd isotope signature for this alkaline set is similar to that of the Palaeoproterozoic calc-alkaline one. Nd isotopes appear to be a very efficient tool to distinguish Archaean from Palaeoproterozoic felsic protoliths in granulitic suites of the Itabuna-Salvador-Curaçá Block (ISCB). Finally, the southern part of the ISCB is composed of a mixture of Archaean and Palaeoproterozoic protoliths, in similar amounts, suggesting that it was probably an active margin between 2.1 and 2.2 Ga located on the eastern border of the Archaean Jequié Block. A major crustal thickening process occurred at ca 2.09 Ga in the ISCB and seems significantly younger towards the west, in the Jequié granulites, where an average of 2056 ± 9 Ma is determined for the high grade event.     

Observer presence may alter the behaviour of reef fishes associated with coral colonies

Although underwater visual census (UVC) is the most frequently used technique for quantifying reef fish assemblages, remote video analysis has been gaining attention as a potential alternative. In the South Atlantic Ocean, Millepora spp. (class Hydrozoa) are the only branching coral species; however, little is known about the ecological role that they play for reef fish communities. We compared these two observation methods (remote video and UVC) to estimate reef fish abundance and species richness associated with colonies of the fire‐coral Millepora alcicornis at Tamandaré Reefs, Northeast Brazil. Additionally, the two different techniques were used to compare species behaviour in association with fire‐corals in order to examine the biases associated with each technique and provide useful information for behavioural ecologists studying fish–coral associations. There were no differences in reef fish abundance or species richness sampled by remote video or UVC. However, a significant difference in the behaviour of associated fish was recorded between the two methods. In the presence of a diver carrying out a UVC, fish were observed spending more time sheltered amongst the coral branches compared with passively swimming on coral colonies with the remote video technique. Specifically, on the remote video recordings agonistic interactions between fish and passive swimming accounted for 33.3% and 22.2% of the census time, respectively. By comparison, when observed by a diver fish spent 34.8% of their time sheltering amongst the coral branches. We demonstrate that both techniques are similarly effective for recording fish abundance and species richness associated with fire‐corals. However, differences were observed in the ability of each method to detect the behaviour of coral‐associated fishes. Our findings show that behavioural ecologists studying complex fish–coral associations need to ensure that their aims are clearly defined and that they choose the most appropriate technique for their study in order to minimize methodological biases.     

Orthogneisses due to irrotational extension,a case from the Sudetes,Bohemian massif

Orthogneisses occurring in the core of the Orlica-Kodzko dome, NE part of the Bohemian massif, exibit a penetrative, N-S stretching lineation defined by mostly ductile elongation of quartz and dilation of K-feldspar crystals of the former granite, now turned into quartz rods and variously elongated K-feldspar porphyroclasts. N-S stretching of the granite seems to be inconsistent with W-E tectonic transport shown by major folds and thrusts developing concurrently in its schistose envelope. The two major lithologies of the dome, differing in their fabrics and rheological properties, offered a drastically different response to an overall shortening. It is interpreted that folds due to buckling evolved in the mantle rocks and after a certain definite range of incremental shortening suffered extension parallel to their hinges. At that time the statistically isotropic granite body was subjected to the same N-S extension and was deformed in common with its schistose mantle under conditions of irrotational strain which represents an early but significant part of the protracted deformation history related to the granite-gneiss transformation.

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Zusammenfassung Die Orthogneise, die im NE-Teil der Böhmischen Masse den Kern der Adlergebirge-Kodzko-Kuppel bilden, zeigen eine N-S-ausgerichtete Streckungslineation. Die Streckungsfaser ist an plastisch gedehnten Quarz-Aggregaten zu erkennen sowie an Klastenzügen von Kalifeldspat. Große Falten und Überschiebungen in der Schieferhülle des Granits hingegen geben eine W-E Richtung des tektonischen Transports an. Diese entspricht nicht der Längung des Granits.Zur Deutung werden die unterschiedlichen rheologischen Eigenschaften des Granits und seiner Hüllgesteine herangezogen, die auf Unterschieden der Zusammensetzung und des Gefüges beruhen. Diese Unterschiede bewirken, daß die Hüllgesteine zunächst Biegefaltung erleben bis weitere Einengung durch Faltung unmöglich wird. Daraufhin werden die Hüllgesteine parallel zur Faltenachse in N-S-Richtung gedehnt. Diese Dehnungsrichtung ist allein und von Anfang an den isotropen Graniten überprägt.Die Deformation erfolgte irrotational und nahezu als achsialsymmetrische Streckung. Sie stellt ein frühes, aber wichtiges Stadium der Umwandlung dieses Granits zu Gneisen dar.

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Résumé Les orthogneiss qui constituent le coeur du dôme d'Orlica-kodzko (région nord-est du massif de Bohème) présentent une linéation d'étirement pénétrative orientée nordsud. Cette linéation est exprimée principalement par l'allongement ductile en bâtonnets d'agrégats de quartz et par l'alignement de porphyroclastes de feldspath potassique. D'autre part, dans l'encaissant schisteux du granite, les grands plis et les charriages indiquent une direction ouest-est du transport tectonique, ce qui ne semble donc pas correspondre à la structure de l'orthogneiss central.Ce contraste est interprété par la différence entre les propriétés rhéologiques des deux ensembles lithologiques, qui ont répondu de manières différentes au processus de raccourcissement régional. Dans les roches de l'enveloppe, se sont formés d'abord des plis de courbure, jusqu'à un certain degré de raccourcissement, après quoi, ce processus n'étant plus possible, elles ont subi un étirement parallèle aux axes de ces plis. Le corps granitique isotrope, au contraire, a réagi dès le début par une extension nord-sud. Ces conditions de déformation irrotationnelle, à symétrie quasi-axiale, représentent un stade précoce, mais significatif de la transformation du granite en orthogneiss.

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Convection of geothermal fluids in the timanfaya volcanic area (Lanzarote,Canary Islands)

Summary A mathematical model has been derived to study the superficial thermal anomalies to be found in Lanzarote (605°C at 13 m depth) in association with the convection of geothermal fluids. The model is valid for a wide range of conditions, in particular for those found beneath the Timanfaya volcano (active between 1730 and 1736). Geological and geophysical data suggest that the heat source is related to a cylindrical magma body with a radius of 200±100 m and a top temperature of 850±100°C at a depth of 4±1 km.Energy is transported through fractures by magmatic volatiles and/or by water vapour coming from a deeply located water table: in such a convection system, a fluid flow of 10 l/m² day, which corresponds to a thermal flux of 130 W/m², is sufficient to explain the temperature anomalies observed at the surface. The relationships between gas flow and the surface temperatures, as well as the thermal gradients in the conducting fracture are also discussed.     

Entropy‐based equation to assess hillslope sediment production

A new equation to assess hillslope sediment production, based on physical and probabilistic approaches, is presented. The equation, which allows the computation of the delivery ratio for every event, considers the physical variables of travel distance, stream power, settling velocity and gross erosion. The probability density function that arises from the new formulation is solved using the principle of maximum entropy. Based on data from five watersheds in both tropical and temperate zones, the new delivery parameter K_v is calibrated and associated with vegetation cover and conservation practice. The proposed equation is rationally based in relation to parameter K_v. The entropy‐based equation was applied to assess sediment yield in two other experimental watersheds, showing good predictability for the set (mean absolute error of 20·8%). No systematic error was found in the analysed data. The entropy‐based equation showed good predictability for long‐term sets of data and for high‐erosivity events, but did not perform well for the low‐erosivity ones. Copyright © 2007 John Wiley & Sons, Ltd.     

Peralkaline acid tendencies in Gran Canaria (Canary Islands)

The study of a volcanic series from the island of Gran Canaria (Canary Islands) in which alkaline and peralkaline, saturated and undersaturated rocks coexist, is reported here. Materials with high volatile content (ignimbritic trachytes) were first emitted and the series ended with the eruption of phonolitic lavas. The average peralkalinity index in these rocks is typically about 1.0 and, therefore, peralkaline rocks coexist with non-peralkaline ones. However, a maximum in peralkalinity is found in the ignimbritic rocks of the lower part of the series. In spite of the evident acid peralkaline tendencies of these rocks, it does not seem appropriate to classify them as pantellerites or comendites. Nor are they consistent with the genetic processes proposed for rocks of similar composition and oceanic environment.The crystallization of the feldspars controls the variation trends among the different magmas but the fractionation alone does not sufficiently explain the genesis of successive fluids. Various factors seem to point to the important role which a gas-transfer process causing a geochemical stratification inside the magmatic chamber may have played.The occurrence of peralkaline silicics at Gran Canaria, which is located for away from the active Mid-Atlantic ridge, is not related to transitional basalts. These rocks are a deviation from the main undersaturated alkalic trend which characterizes the volcanism of the Canary Islands, their genesis being related to the realization of favourable local volcanic conditions.