Micropaleontology of some Permian localities in the Tethyan realm: Inventory of foraminifers and calcareous algae, biostratigraphy and paleogeography

This inventory of Permian foraminifers and calcareous algae has been made for more than 5000 samples collected in six geographical areas (Fig.1). Almost all of them were taken from stratigraphical sections. This important and unique collection is kept at the Geological Museum of Lausanne (Switzerland) and are available on request to any further scientific investigations*. Several people carried out the field work. However, only one person is reponsible for the determinations in generea and species of these samples. Therefore, this work method offers a good homogeny of namings and has saved a lot of discussions over nomenclature. Some colleagues may not agree with one or more of the namings and we accept their dissensions. Nevertheless we adopted this method because we are sure that the homogeny of namings vouches for best correlations in both biostratigraphical and paleogeographical investigations. We already used with success this data set in geodynamical paleo-reconstruction maps (Jenny & Stampfli, 2000) (Chapter 3) and managed the same data through the Biograph Program (Savary & Guex, 1999) (Chapter 2). This program is able to compile a lot of data to extract maximal sets of intersecting taxa ranges. The objective was to establish discrete sequences of coexistence interval of taxa and ten to ensure good correlations for the different studied area

The north-subducting Rheic Ocean during the Devonian: consequences for the Rhenohercynian ore sites

Base metal mining in the Rhenohercynian Zone has a long history. Middle-Upper Devonian to Lower Carboniferous sediment-hosted massive sulfide deposits (SHMS), volcanic-hosted massive sulfide deposits (VHMS) and Lahn-Dill-type iron, and base metal ores occur at several sites in the Rhenohercynian Zone that stretches from the South Portuguese Zone, through the Lizard area, the Rhenish Massif and the Harz Mountain to the Moravo-Silesian Zone of SW Bohemia. During Devonian to Early Carboniferous times, the Rhenohercynian Zone is seen as an evolving rift system developed on subsiding shelf areas of the Old Red continent. A reappraisal of the geotectonic setting of these ore deposits is proposed. The Middle-Upper Devonian to Early Carboniferous time period was characterized by detrital sedimentation, continental intraplate and subduction-related volcanism. The large shelf of the Devonian Old Red continent was the place of thermal subsidence with contemporaneous mobilization of rising thermal fluids along activated Early Devonian growth faults. Hydrothermal brines equilibrated with the basement and overlying Middle-Upper Devonian detrital deposits forming the SHMS deposits in the southern part of the Pyrite Belt, in the Rhenish Massif and in the Harz areas. Volcanic-hosted massive sulfide deposits (VHMS) formed in the more eastern localities of the Rhenohercynian domain. In contrast, since the Tournaisian period of ore formation, dominant pull-apart triggered magmatic emplacement of acidic rocks, and their metasomatic replacement in the apical zones of felsic domes and sediments in the northern part of the Iberian Pyrite belt, thus changing the general conditions of ore precipitation. This two-step evolution is thought to be controlled by syn- to post- tectonic phases in the Variscan framework, specifically by the transition of geotectonic setting dominated by crustal extension to a one characterized by the subduction of the supposed northern slab of the Rheic Ocean preceding the general Late Variscan crustal shortening and oroclinal bending

Organization of pre-Variscan basement areas at the north-Gondwanan margin

Pre-Variscan basement elements of Central Europe appear in polymetamorphic domains juxtaposed through Variscan and/or Alpine tectonic events. Consequently, nomenclatures and zonations applied to Variscan and Alpine structures, respectively, cannot be valid for pre-Variscan structures. Comparing pre-Variscan relics hidden in the Variscan basement areas of Central Europe, the Alps included, large parallels between the evolution of basement areas of future Avalonia and its former peri- Gondwanan eastern prolongations (e.g. Cadomia, Intra-Alpine Terrane) become evident. Their plate-tectonic evolution from the Late Proterozoic to the Late Ordovician is interpreted as a continuous Gondwana-directed evolution. Cadomian basement, late Cadomian granitoids, late Proterozoic detrital sediments and active margin settings characterize the pre-Cambrian evolution of most of the Gondwana-derived microcontinental pieces. Also the Rheic ocean, separating Avalonia from Gondwana, should have had, at its early stages, a lateral continuation in the former eastern prolongation of peri-Gondwanan microcontinents (e.g. Cadomia, Intra-Alpine Terrane). Subduction of oceanic ridge (Proto-Tethys) triggered the break-off of Avalonia, whereas in the eastern prolongation, the presence of the ridge may have triggered the amalgamation of volcanic arcs and continental ribbons with Gondwana (Ordovician orogenic event). Renewed Gondwana-directed subduction led to the opening of Palaeo-Tethys

Ollo de Sapo Cambro-Ordovician volcanics from the Central Iberian basement—A multiphase evolution

The Cambro-Ordovician rhyodacitic to dacitic volcanics from the Central Iberian basement, currently known as Ollo de Sapo (toads eye), have been reported as a specific group of felsic porphyritic rocks with blue quartz and large phenocrysts of K- feldspar, in a partly vitreous or fine-grained matrix. Interpreted to form Cambro- Ordovician volcanic domes, they are accompanied by tuffs, ignimbrites and products of reworking in a near-surface environment. The coarse- to fine-grained rocks exhibit rather large K-feldspar phenocrysts, plagioclase and rounded blue quartz, representing former corroded phenocrysts. Their colouration indicates unmixing of TiO2 at around 900°C during cooling from relatively high crystallisation temperatures, indicating their origin at hot lower crustal conditions. We propose at least a two-step evolution (1) starting around 495 Ma in the lower crust of a collapsing cordillera, generating a phenocryst-rich mush and adiabatic melting of the lower crustal protolith to produce the spectacular Ollo de Sapo porphyrites, before (2) magma ascent and crustal extension leading to a different thermal regime around 483 Ma

Patterns of ash (Fraxinus excelsior L.) colonization in mountain grasslands: the importance of management practices

International audienceWoody colonization of grasslands is often associated with changes in abiotic or biotic conditions or a combination of both. Widely used as fodder and litter in the past traditional agro-pastoral system, ash (Fraxinus excelsior L.) has now become a colonizing species of mountain grasslands in the French Pyrenees. Its present distribution is dependent on past human activities and it is locally controlled by propagule pressure and abiotic conditions. However, even when all favourable conditions are met, all the potentially colonizable grasslands are not invaded. We hypothesize that management practices should play a crucial role in the control of ash colonization. From empirical field surveys we have compared the botanical composition of a set of grasslands (present and former) differing in management practices and level of ash colonization. We have displayed a kind of successional gradient positively linked to both ash cover and height but not to the age of trees. We have tested the relationships between ash presence in grassland and management types i.e. cutting and/or grazing, management intensity and some grassland communities' features i.e. total and local specific richness and species heterogeneity. Mixed use (cutting and grazing) is negatively linked to ash presence in grassland whereas grazing alone positively. Mixed use and high grazing intensity are directly preventing ash seedlings establishment, when low grazing intensity is allowing ash seedlings establishment indirectly through herbaceous vegetation neglected by livestock. Our results show the existence of a limit between grasslands with and without established ashes corresponding to a threshold in the intensity of use. Under this threshold, when ash is established, the colonization process seems to become irreversible. Ash possesses the ability of compensatory growth and therefore under a high grazing intensity develops a subterranean vegetative reproduction. However the question remains at which stage of seedling development and grazing intensity these strategies could occur

Paleotectonic and paleogeographic signification of melanges: examples from the Tethyan realm

There are two different but complementary ways to study accretion-related melanges, originating in the contexts of convergent margin (subduction/obduction): i) a local and mechanical approach, where the mélange is seen as a structural object; the aim is then to get information on the large-scale melange-forming processes, by unravelling its internal structure and complex organisation, and by explaining the timing of the different deformation phases; ii) a regional and historical approach, where the melange is considered as a witness of the history of the regional geology; the aim is to get as much information as possible on the various lithologies found in the melange, in order to reconstruct the evolution through space and time of the ocean(s) and adjacent margins contemporaneous of its formation. We want to put the emphasis on this second approach, and on the role of fossil accretion-related melanges from a paleotectonic and paleogeographic perspective. It is then necessary to give them a geodynamic signification, and to go further than the classic descriptive definition. In summary, one can distinguish between three ideal types of mélanges, regarding the origin of the material and their genesis: i) melanges related to the lower plate, reworking lithologies from the subducting oceanic lithosphere into the accretionary prism; ii) mélanges related to the upper plate, reworking lithologies both from the arc and the prism, called here fore-arc type melanges; iii) debris-flow mélanges deposited on the lower plate, and reworking ophiolitic fragments in front of an advancing obducting ophiolite. These three types of melange may be seen as ideal end-members of a simple ternary classification of melanges, based on their geodynamic setting. Examples of melanges taken from the Tethyan realm (Greece, Turkey, etc...) support this view, and their field recognition appear

Paleozoic evolution of pre-Variscan terranes: From Gondwana to the Variscan collision

The well-known Variscan basement areas of Europe contain relic terranes with a pre- Variscan evolution testifying to their peri-Gondwanan origin (e.g., relics of Neoproterozoic volcanic arcs, and subsequent stages of accretionary wedges, backarc rifting, and spreading). The evolution of these terranes was guided by the diachronous subduction of the proto-Tethys oceanic ridge under different segments of the Gondwana margin. This subduction triggered the emplacement of magmatic bodies and the formation of backarc rifts, some of which became major oceanic realms (Rheic, paleo- Tethys). Consequently, the drifting of Avalonia was followed, after the Silurian and a short Ordovician orogenic event, by the drifting of Armorica and Alpine domains, accompanied by the opening of the paleo-Tethys. The slab rollback of the Rheic ocean is viewed as the major mechanism for the drifting of the European Variscan terranes. This, in turn, generated a large slab pull force responsible for the opening of major rift zones within the passive Eurasian margin. Therefore, the µrst Middle Devonian Variscan orogenic event is viewed as the result of a collision between terranes detached from Gondwana (grouped as the Hun superterrane) and terranes detached from Eurasia. Subsequently, the amalgamated terranes collided with Eurasia in a second Variscan orogenic event in Visean time, accompanied by large-scale lateral escape of major parts of the accreted margin. Final collision of Gondwana with Laurussia did not take place before Late Carboniferous time and was responsible for the Alleghanian orogeny

Gondwana-derived microcontinents — the constituents of the Variscan and Alpine collisional orogens

The European Variscan and Alpine mountain chains are collisional orogens, and are built up of pre-Variscan "building blocks" which, in most cases, originated at the Gondwana margin. Such pre-Variscan elements were part of a pre-Ordovician archipelago-like continental ribbon in the former eastern prolongation of Avalonia, and their present-day distribution resulted from juxtaposition through Variscan and/or Alpine tectonic evolution. The well-known nomenclatures applied to these mountain chains are the mirror of Variscan resp. Alpine organization. It is the aim of this paper to present a terminology taking into account their pre-Variscan evolution at the Gondwana margin. They may contain relics of volcanic islands with pieces of Cadomian crust, relics of volcanic arc settings, and accretionary wedges, which were separated from Gondwana by initial stages of Rheic ocean opening. After a short-lived Ordovician orogenic event and amalgamation of these elements at the Gondwanan margin, the still continuing Gondwana-directed subduction triggered the formation of Ordovician Al-rich granitoids and the latest Ordovician opening of Palaeo-Tethys. An example from the Alps (External Massifs) illustrates the gradual reworking of Gondwana-derived, pre-Variscan elements during the Variscan and Alpine/Tertiary orogenic cycles