Contemporaneous assembly of Western Gondwana and final Rodinia break-up: Implications for the supercontinent cycle

Geological, geochronological and isotopic data are integrated in order to present a revised model for the Neoproterozoic evolution of Western Gondwana. Although the classical geodynamic scenario assumed for the period 800–700 Ma is related to Rodinia break-up and the consequent opening of major oceanic basins, a significantly different tectonic evolution can be inferred for most Western Gondwana cratons. These cratons occupied a marginal position in the southern hemisphere with respect to Rodinia and recorded subduction with back-arc extension, island arc development and limited formation of oceanic crust in internal oceans. This period was thus characterized by increased crustal growth in Western Gondwana, resulting from addition of juvenile continental crust along convergent margins. In contrast, crustal reworking and metacratonization were dominant during the subsequent assembly of Gondwana. The Río de la Plata, Congo-São Francisco, West African and Amazonian cratons collided at ca. 630–600 Ma along the West Gondwana Orogen. These events overlap in time with the onset of the opening of the Iapetus Ocean at ca. 610–600 Ma, which gave rise to the separation of Baltica, Laurentia and Amazonia and resulted from the final Rodinia break-up. The East African/Antarctic Orogen recorded the subsequent amalgamation of Western and Eastern Gondwana after ca. 580 Ma, contemporaneously with the beginning of subduction in the Terra Australis Orogen along the southern Gondwana margin. However, the Kalahari Craton was lately incorporated during the Late Ediacaran–Early Cambrian. The proposed Gondwana evolution rules out the existence of Pannotia, as the final Gondwana amalgamation postdates latest connections between Laurentia and Amazonia. Additionally, a combination of introversion and extroversion is proposed for the assembly of Gondwana. The contemporaneous record of final Rodinia break-up and Gondwana assembly has major implications for the supercontinent cycle, as supercontinent amalgamation and break-up do not necessarily represent alternating episodic processes but overlap in time.     

Population structure and conservation status of the red gorgonian Paramuricea clavata (Risso, 1826) in the Eastern Adriatic Sea

Gorgonians are important structuring species of the Mediterranean hard‐bottom communities that are threatened by disturbances such as increasing seawater temperature, mucilaginous events and destructive fishing, among others. In this study we assessed for the first time the population structure and conservation status of one of the most common gorgonians in the Eastern Adriatic Sea, the red gorgonian Paramuricea clavata. During late spring 2009, nine populations dwelling between 30 and 50 m depth were examined by SCUBA diving along 200 km of the Croatian coastline. The density ranged between 7 and 20 colonies·m^?2. The mean and maximum colony heights were 31.2 ± 22.7 cm (±SD) and 138 cm, respectively. Two main patterns of P. clavata size frequency distributions were observed: the first one with a higher proportion of juveniles (~30%) observed mostly in the northernmost populations, and the second one with a higher proportion of larger colonies (>25% of colonies >40 cm in height). Regarding the disturbance impact level, the proportion of healthy colonies (with <10% of injured surface) was high in almost all of the studied populations (>60%) and the mean extent of injury (i.e. denuded axis or epibiosis) was 9.7 ± 4% (±SD), indicating low impacts. Contrasting population size structures with high recruitment in mature populations provides new insights into the demographic structure of the Mediterranean gorgonian forests dwelling in their upper bathymetric range (<50 m depth). Furthermore, these size structures and the low impact levels suggest a current favorable conservation status of the studied populations in the Eastern Adriatic Sea and provide a baseline for their monitoring in the future.     

North-South asymmetry in the solar flare index

This paper reports the results of a study of the N-S asymmetry in the flare index using the results of Knoka (1985) combined with our results for the solar cycles 17 to the current cycle 22. By comparing the time-variation of the asymmetry curve with the solar activity variation of the 11-year cycle, we have found that the flare index asymmetry curve is not in phase with the solar cycle and that the asymmetry peaks during solar minimum. A periodic behaviour in the N-S asymmetry appears: the activity in one hemisphere is more important during the ascending part of the cycle whereas during the descending part the activity becomes more important in the other hemisphere. The dominance of flare activity in the southern hemisphere continues during cycle 22 and, according to our findings, this dominance will increase gradually during the following cycle 23.     

La rétromorphose du talc dans les dolomies métallifères de Boukdema (Algérie); discussion du rôle des fluides

Résumé Les échantillons étudiés proviennent du gîte stratiforme de Boukdema (Algérie), où l'on observe notamment l'association dolomite-quartz-talcsphalérite. L'étude des inclusions fluides primaires des quartz contemporains, selon toute vraisemblance, de la dissolution partielle du talc, montre que ceux-ci se sont développés dans des solutions de salinités très variables (entre 4 et 28% en équivalent pondéral NaCl). L'hypothèse d'une ébullition des solutions (par suite d'une chute brutale de pression) rend compte de ce fait inhabituel. Dans le cadre d'un tel modèle, les corrections dues à la pression sont nulles et l'on peut en déduire à partir des mesures de températures d'homogénéïsation que la croissance du quartz s'est amorcée vers 250 °C. Compte tenu de cette indication thermométrique, on discute enfin du rôle relatif des facteurs physiques et chimiques (T, f_{CO 2}, a_Mg/a_Ca) sur la stabilité du talc dans le contexte minéralogique du gîte de Boukdema.

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The samples studied were collected in the Boukdema stratabound deposit (Algeria), where the association dolomite-quartz-talc-sphalerite occurs. Cryoscopic investigations of fluid inclusions in quartz indicate a wide salinity range in mineral forming fluids (4 to 28 weight % NaCl eq). Boiling of hydrothermal fluids possibly explains this unusual characteristic. In this hypothesis thermometric data need no pressure corrections and one may conclude that quartz growth (and simultaneously talc dissolution) occurred in the 250°–200 °C range. On the basis of this last indication, the relative role of T, f_{CO 2}, a_Mg/a_Ca, on talc stability is discussed in the mineralogical environment of Boukdema.

     

National minorities as an element of the demographic and spatial structure of the Alpine-Adriatic-Pannonian region

The paper discusses minorities and their problems along borders of post-WWII Europe and of nation states formed in the early 1990's. One focuses on the socio-geographic conditions of the territory settled by minorities. These territories are most often peripheral, less developed areas of the state and are, because of the constant ignorance of the local culture and language, conflict regions, where interests of the minority and majority population of the state, as well as of the neighboring countries clash. Finally, an attempt of a cross-cultural comparison is undertaken to distinguish among major elements of the minority (protection) arrangements of the Alpen-Adria region and of the areas along the long-time closed borders of Central Europe.     

Geochemical Evaluation of Suitability of Central Anatolian (Turkey) Volcanic Rocks for Rock Fiber Production

Natural Resources Research - It is common to use synthetic fibers to increase the strength of engineering materials. Fibers obtained from rocks have also started to take their place in composite...     

Subsurface Structural Setting and Hydrocarbon Potentiality of the Komombo and Nuqra Basins,South Egypt: A Seismic and Petrophysical Integrated Study

Natural Resources Research - Sedimentary basins in south Egypt, in particular Komombo and Nuqra basins, have been paid more attention during the last few decades, due to their hydrocarbon...     

Comparison of the QBO and F10.7 Solar Flux Effects on Total Mass Density

Geomagnetism and Aeronomy - The comparison of the Quasi Biennial Oscillation (QBO) and F10.7 solar flux effects on Total Mass Density (TMD) obtained from NRLMSIS-00 model for 90 km altitude of...     

Delineation of Shallow Subsurface Structure by Azimuthal Resistivity Sounding and Joint Inversion of VES-TEM Data: Case Study near Lake Qaroun, El Fayoum, Egypt

An azimuthal resistivity survey was conducted at the transition zone between the desert area and the cultivated land near Lake Qaroun, Egypt. This area has been affected by an east-west trending fault system as indicated from the surface geology. Apparent resistivity values were plotted along azimuth on a polar diagram. Resistivity anomalies, for most of the AB/2 values with long axes strike in a direction parallel to the contact between the desert and cultivated lands, indicate the presence of electrical macro-anisotropy, mainly due to the faulting effect, at this area. Vertical electrical soundings (VES) and transient electromagnetic (TEM) measurements were conducted at eight stations along a line that crosses the boundary between the desert and cultivated land. Joint inversion of VES-TEM data was successfully used for identification of the subsurface lithostratigraphic succession and demonstrated the effect of the fault zone on the investigated subsurface medium. Apparent anisotropy coefficients at all current electrode spacings were calculated, plotted against AB/2 values and compared with the geoelectrical cross section. The effect of the fault zone was detected at AB/2 spacings equal to 100 m and extended downward and is largely related to the depth of the fault, as indicated in the constructed cross section.