Добірка наукової літератури з теми "Lower alpine crust"

Abstract. The Iberian Central System, formed after the Alpine reactivation of the Variscan Iberian Massif, features maximum altitudes of 2500 m. It is surrounded by two foreland basins with contrasting elevation: the Duero Basin to the north, located at 750–800 m, and the Tajo Basin to the south, lying at 450–500 m. The deep crustal structure of this mountain range seems to be characterized by the existence of a moderate crustal root that provides isostatic support for its topography. New seismic data are able to constrain the geometry of this crustal root, which appears to be defined by a northward lower-crustal imbrication of the southern Central Iberian crust underneath this range. Contrarily to what was expected, this imbrication also affects the upper crust, as the existing orogen-scale mid-crustal Variscan detachment was probably assimilated during the Carboniferous crustal melting that gave rise to the Central System batholith. In addition, the lower crust might have thinned, allowing coupled deformation at both crustal levels. This implies that the reactivated upper-crustal fractures can reach lower-crustal depths, thus allowing the entire crust to sink. This new model can explain the differences in topography between the Central System foreland basins. Also, it provides further constraints on the crustal geometry of this mountain range, as it seems to be that of an asymmetric Alpine-type orogen, thus hindering the existence of buckling processes as the sole origin of the deformation. The results presented here have been achieved after autocorrelation of seismic noise along the CIMDEF (Central Iberian Massif DEFormation Mechanisms) profile. Although the resolution of the dataset features limited resolution (0.5–4 Hz, stations placed at ∼ 5 km), this methodology has allowed us to pinpoint some key structures that helped to constraint the deformation mechanisms that affected Central Iberia during the Alpine orogeny.

Abstract. The southern Alpine regions were affected by several magmatic and volcanic events between the Paleozoic and the Tertiary. This activity undoubtedly had an important effect on the density distribution and structural setting at lithosphere scale. Here the gravity field has been used to create a 3D lithosphere density model on the basis of a high-resolution seismic tomography model. The results of the gravity modeling demonstrate a highly complex density distribution in good agreement with the different geological domains of the Alpine area represented by the European Plate, the Adriatic Plate and the Tyrrhenian basin. The Adriatic-derived terrains (Southalpine and Austroalpine) of the Alps are typically denser (2850 kg m−3), whilst the Alpine zone, composed of terrains of European provenance (Helvetic and Tauern Window), presents lower density values (2750 kg m−3). Inside the Southalpine, south of the Dolomites, a well-known positive gravity anomaly is present, and one of the aims of this work was to investigate the source of this anomaly that has not yet been explained. The modeled density suggests that the anomaly is related to two different sources; the first involves the middle crust below the gravity anomaly and is represented by localized mushroom-shaped bodies interpreted as magmatic intrusions, while a second wider density anomaly affects the lower crust of the southern Alpine realm and could correspond to a mafic and ultramafic magmatic underplating (gabbros and related cumulates) developed during Paleozoic extension.

SUMMARY We present an improved crustal Vs model and Moho depth map using ambient noise wave-equation tomography. The so-called ‘ambient noise wave-equation tomography’ is a method to invert seismic ambient noise phase dispersion data based on elastic waveform simulation, which accounts for 3-D and finite-frequency effects. We use cross-correlations of up to 4 yr of continuous vertical-component ambient seismic noise recordings from 304 high-quality broad-band stations in the Alpine region. We use model LSP_Eucrust1.0 obtained from traditional ambient noise tomography as initial model, and we iteratively improve the initial model by minimizing frequency-dependent phase traveltime differences between the observed and synthetic waveforms of Rayleigh waves in the period range 10–50 s. We obtain the final model after 15 iterations with ∼65 per cent total misfit reduction compared to the initial model. At crustal depth, the final model significantly enhances the amplitudes and adjusts the shapes of velocity anomalies. At Moho and upper-mantle depth, the final model corrects an obvious systematic velocity shift of the initial model. The resulting isovelocity Moho map confirms a Moho step along the external side of the external crystalline massifs of the northwestern Alps and reveals underplated gabbroic plutons in the lower most crust of the central and eastern Alps. Ambient noise wave-equation tomography turns out to be a useful tool to refine shear wave velocity models obtained by traditional ambient noise tomography based on ray theory.

The layered structure of the Earth's crust in South Bulgaria is a result of the complicated geologic development, and of changes in the physical properties of the rocks under high pressures and temperatures. The following layers are distinguished (from bottom to top): Layer "A" (dehydrated lower crust) with layer velocities from 6.4 to 6.7 km/s, and characterized by low-velocity ductile deformations: Layer "B" (hydrated upper crust) with layer velocities 5.9–6.0 km/s, and characterized by homogeneous strains, as well as by deformations by cataclastic flow; low-velocity layer which developed only partially in the west part of Rhodope Massif, and is deformed by superplastic flow; presumably, it compensated differently directed movements in adjacent layers during some deformational stages; granitoid-metamorphic crust characterized mainly by brittle deformations. This structure probably resulted from multiphase deformations during several tectonic cycles. The dynamic environment changed considerably, compression stages being replaced by extension stages; in some stages different crust layers behaved differently, e. g. with an extension environment in the granitoid-metamorphic crust, and compression – within the deeper levels. Crustal thickening is due to different mechanisms but has been bound mainly to continental collision. With reaching a crytical thickness, isostasy-bound uplift led to thinning by erosion. Other mechanisms of crustal thinning are bound to environments of generalized extension and rifting, homogeneous deformation, or non-homogeneous ductile deformation with necking. As a result of multiphase crustal thickening, Rhodope Massif formed as a peculiar crustal lensoid body. The collisional orogen on the Balkan Peninsula, at least during the Alpine cycle, had a transitional character – from Alpine towards the Himalayan-Tibethan type, the Rhodope Massif playing the part of plateau.

Abstract Magma-poor ocean-continent transitions at distal rifted margins record complex stratigraphic interactions engendered by extreme crustal thinning and mantle exhumation. The Tasna ocean-continent transition, exposed in the Middle Penninic Tasna nappe in eastern Switzerland, is so far the only known example where the lateral transition from continental crust to exhumed serpentinized mantle lithosphere is exposed and not overprinted by later Alpine deformation. This paper presents sedimentological, structural, and petrographical observations and detrital zircon provenance data to document: (1) the processes controlling continental hyperextension and mantle exhumation; and (2) the facies, depositional systems, sediment sources, delivery pathways, and depositional stacking patterns associated with magma-poor ocean-continent transitions. Our results show that the basement units of the Tasna ocean-continent transition are composed of prerift upper and lower crust and subcontinental mantle rocks juxtaposed as part of the continental crustal thinning process. The absence of pervasive, synrift deformation in the lower-crustal rocks indicates that the thinning was likely achieved by deformation along localized shear zones before being exhumed at the seafloor by brittle, late extensional detachment faulting and not by any form of lower-crustal flow. The age of the first sediments deposited on the continental crust and exhumed mantle, the so-called Tonschiefer Formation, is considered to be Late Jurassic. A key observation is that the restored morpho-tectonic and sedimentary evolution of the Tasna ocean-continent transition shows the intercalation of downdip, transported platform-derived sediments and along-axis–derived siliciclastic sediments originating from the recycling of prerift sediments, local basement, and/or extra-Alpine sources.

Abstract The occurrence of a Lower Cretaceous flysch group, cropping out from the Gibraltar Arc to the Balkans with a very similar structural setting and sedimentary provenance always linked to the dismantling of internal areas, suggests the existence of only one sedimentary basin (Alpine Tethys s.s.), subdivided into many other minor oceanic areas. The Maghrebian Basin, mainly developed on thinned continental crust, was probably located in the westernmost sector of the Alpine Tethys. Cretaceous re-organization of the plates triggered one (or more) tectonic phases, well recorded in almost all the sectors of the Alpine Tethys. However, the Maghrebian Basin seems to have been deformed by Late- or post-Cretaceous tectonics, connected with a “meso-Alpine” phase (pre-Oligocene), already hypothesized since the beginning of the nineties. Field geological evidence and recent biostratigraphic data also support this important meso- Alpine tectonic phase in the Sicilian segment of the Maghrebian Chain, indicated by the deformations of a Lower Cretaceous flysch sealed by Lower Oligocene turbidite deposits. This tectonic development is emphasized here because it was probably connected with the onset of rifting in the southern paleomargin of the European plate, the detaching of the so-called AlKaPeCa block (Auct.; i.e. Alboran + Kabylian + Calabria and Peloritani terranes) and its fragmentation into several microplates. The subsequent early Oligocene drifting of these microplates led to the progressive closure of the Maghrebian Basin and the opening of new back-arc oceanic basins, strongly controlled by extensional processes, in the western Mediterranean (i.e. Gulf of Lion, Valencia Trough, Provençal Basin and Alboran Sea).

Abstract. Normal incidence seismic data provide the best images of the crust and lithosphere. When properly designed and continuous, these sections greatly contribute to understanding the geometry of orogens and, along with surface geology, unraveling their evolution. In this paper we present the most complete transect, to date, of the Iberian Massif, the westernmost exposure of the European Variscides. Despite the heterogeneity of the dataset, acquired during the last 30 years, the images resulting from reprocessing the data with a homogeneous workflow allow us to clearly define the crustal thickness and its internal architecture. The Iberian Massif crust, formed by the amalgamation of continental pieces belonging to Gondwana and Laurussia (Avalonian margin), is well structured in the upper and lower crust. A conspicuous mid-crustal discontinuity is clearly defined by the top of the reflective lower crust and by the asymptotic geometry of reflections that merge into it, suggesting that it has often acted as a detachment. The geometry and position of this discontinuity can give us insights into the evolution of the orogen (i.e., of the magnitude of compression and the effects and extent of later-Variscan gravitational collapse). Moreover, the limited thickness of the lower crust below, in central and northwestern Iberia, might have constrained the response of the Iberian microplate to Alpine shortening. Here, this discontinuity, featuring a Vp (P-wave velocity) increase, is observed as an orogen-scale boundary with characteristics compatible with those of the globally debated Conrad discontinuity.

The Corno Alto–Monte Ospedale magmatic complex crops out at the eastern border of the Adamello batholith, west of the South Giudicarie Fault (NE Italy). This complex includes tonalites, trondhjemites, granodiorites, granites and diorites exhibiting an unfoliated structure suggesting passive intrusion under extensional-to-transtensional conditions. Major, minor elements, REE and isotopic analyses and geochemical and thermodynamic modelling have been performed to reconstruct the genesis of this complex. Geochemical analyses unravel a marked heterogeneity with a lack of intermediate terms. Samples from different crust sections were considered as possible contaminants of a parental melt, with the European crust of the Serre basement delivering the best fit. The results of the thermodynamic modelling show that crustal melts were produced in the lower crust. Results of the geochemical modelling display how Corno Alto felsic rocks are not reproduced by fractional crystallization nor by partial melting alone: their compositions are intermediate between anatectic melts and melts produced by fractional crystallization. The tectonic scenario which favored the intrusion of this complex was characterized by extensional faults, active in the Southalpine domain during Eocene. This extensional scenario is related to the subduction of the Alpine Tethys in the Eastern Alps starting at Late Cretaceous time.

Three samples of meta-acidic rocks with pre-Alpine metamorphic relicts from the Sesia-Lanzo Zone eclogitic continental crust were investigated using stepwise controlled elemental maps by means of the Quantitative X-ray Maps Analyzer (Q-XRMA). Samples were chosen with the aim of analysing the reacting zones along the boundaries between the pre-Alpine and Alpine mineral phases, which developed in low chemically reactive systems. The quantitative data treatment of the X-ray images was based on a former multivariate statistical analytical stage followed by a sequential phase and sub-phase classification and permitted to isolate and to quantitatively investigate the local paragenetic equilibria. The parageneses thus observed were interpreted as related to the pre-Alpine metamorphic or magmatic stages as well as to local Alpine re-equilibrations. On the basis of electron microprobe analysis, specific compositional ranges were defined in micro-domains of the relict and new paragenetic equilibria. In this way calibrated compositional maps were obtained and used to contour different types of reacting boundaries between adjacent solid solution phases. The pre-Alpine and Alpine mineral parageneses thus obtained allowed to perform geothermobarometry on a statistically meaningful and reliable dataset. In general, metamorphic temperatures cluster at 600–700 °C and 450–550 °C, with lower temperatures referred to a retrograde metamorphic re-equilibration. In all the cases described, pre-Alpine parageneses were overprinted by an Alpine metamorphic mineral assemblage. Pressure-temperature estimates of the Alpine stage averagely range between 420 to 550 °C and 12 to 16.5 kbar. The PT constraints permitted to better define the pre-Alpine metamorphic scenario of the western Austroalpine sectors, as well as to better understand the influence of the pre-Alpine metamorphic inheritance on the forthcoming Alpine tectonic evolution.

Population dynamics of chamois (genusRupicapra, subfamily Caprinae) can be influenced by infectious diseases epizootics, of which sarcoptic mange is probably the most severe in the Alpine chamois (Rupicapra rupicapra rupicapra). In this study, skin lesions and cellular inflammatory infiltrates were characterized in 44 Alpine chamois affected by sarcoptic mange. Dermal cellular responses were evaluated in comparison with chamois affected by trombiculosis and controls. In both sarcoptic mange and trombiculosis, a significantly increase of eosinophils, mast cells, T and B lymphocytes, and macrophages was detected. Moreover, in sarcoptic mange significant higher numbers of T lymphocytes and macrophages compared to trombiculosis were observed. Lesions in sarcoptic mange were classified in three grades, according to crusts thickness, correlated with mite counts. Grade 3 represented the most severe form with crust thickness more than 3.5 mm, high number of mites, and severe parakeratosis with diffuse bacteria. Evidence of immediate and delayed hypersensitivity was detected in all three forms associated with diffuse severe epidermal hyperplasia. In grade 3, a significant increase of B lymphocytes was evident compared to grades 1 and 2, while eosinophil counts were significantly higher than in grade 1, but lower than in grade 2 lesions. An involvement of nonprotective Th2 immune response could in part account for severe lesions of grade 3.

Des images géophysiques récentes dans les Alpes montrent une signature sismique particulière du sommet du panneau plongeant crustal à 40 km de profondeur. Une augmentation de vitesse brutale des ondes S est corrélée à une forte probabilité de présence de contraste d’interface dans la tomographie et à une conversion négative dans les données « stackée » des fonctions-récepteur. Le but de cette thèse est d’évaluer si des changements de minéralogie et de texture de la croûte continentale inférieure peuvent expliquer cette signature sismique. Pour cela, nous avons calculé les variations de vitesses sismiques « bulk », générées par les changements minéralogiques durant l’enfouissement de roches représentatives de la croûte inférieure européenne le long de profils pression-température typiques de zone de convergence. Nous avons étudié l’évolution de l’anisotropie des mêmes roches à l’échelle macroscopique en fonction de la pression et la température, à partir de mesures directes. Ces mesures ont été comparées aux calculs d’anisotropie couramment effectués à partir de cartographies d’orientations cristallographiques à l’échelle de la lame mince. Le but ultime de ces exercices est de comprendre quelles propriétés contrôlent les vitesses sismiques effectives des roches à l’échelle kilométrique. Nous avons finalement tenté de déceler, à cette dernière échelle, l’anisotropie des roches dans les données de fonctions-récepteur à partir de leur décomposition harmonique. Nous montrons que la transformation des roches du faciès des amphibolites à celui des granulites de haute pression permet d’expliquer l’augmentation de vitesse du modèle tomographique et que ce front est décalé d’une dizaine de kilomètre le long du panneau plongeant en comparaison des prédictions thermodynamiques. A travers une modélisation thermocinétique de zone de convergence, nous évaluons le profil thermique du panneau plongeant lors du passage de la subduction à la collision et expliquons ce décalage par des effets cinétiques. Les mesures directes comparées aux calculs d’anisotropie indiquent que la différence attendue entre anisotropie intrinsèque et effective est plus importante dans les roches du faciès des amphibolites, où litage et CPO se renforcent, que dans celles du faciès des granulites où l’anisotropie résulte surtout de l’anisotropie intrinsèque. A l’échelle kilométrique, la transformation amphibolite vers granulite est susceptible de s’accompagner d’une diminution de l’anisotropie, en plus d’une augmentation de vitesse. A travers la décomposition harmonique, nous montrons que la baisse de visibilité du réflecteur associé au Moho, aux stations à l’aplomb du panneau plongeant, se fait au profit de la mise en évidence d’une direction rapide intra- panneau plongeant et orientée perpendiculairement à son pendage. Puisque cette transformation est visible tant dans les données de fonctions- récepteur que dans les modèles de tomographie, nous en déduisons que l’épaisseur du front de réaction est de l’ordre du kilomètre
Recent geophysical images in the Alps show a distinctive seismic signature of the top of the crustal dipping panel at 40 km depth. A sharp increase in S-wave velocity is correlated with a high probability of interface contrast in tomography and negative conversion in stacked receiver- function data. The aim of this thesis is to assess if changes in the mineralogy and textural properties of the lower continental crust can explain this seismic signature. To this end, we calculated bulk seismic velocity variations, generated by mineralogical changes during burial of rocks representative of the European lower crust along pressure- temperature profiles typical of convergence zones. We studied the evolution of the macroscopic anisotropy of the same rocks as a function of pressure and temperature, using direct measurements. These measurements are compared with anisotropy calculations commonly performed from thin- section crystallographic orientation maps. The ultimate aim of these exercises is to understand which properties control the effective seismic velocities of rocks at kilometer scale. Finally, we have attempted to detect the anisotropy of rocks at this latter scale in receiver-function data from their harmonic decomposition. We show that the transformation of rocks from amphibolite to high-pressure granulite facies explains the increase in velocity of the tomographic model, and that this front is shifted by around ten kilometers along the slab, compared with thermodynamic predictions. Using thermokinetic modelling of convergence zones, we evaluate the thermal profile of the dipping panel during the transition from subduction to collision, and explain this offset by kinetic effects. Direct measurements compared with anisotropy calculations indicate that the expected difference between intrinsic and effective anisotropy is greater in amphibolite facies rocks, where layering and CPO are enhanced, than in granulite facies rocks, where anisotropy results mainly from intrinsic anisotropy. At kilometer scale, amphibolite-to-granulite transformation is likely to be accompanied by a decrease in anisotropy in addition to an increase in velocity. Through harmonic decomposition, we show that the reduced visibility of the Moho, at stations above the dipping panel, is to the benefit of highlighting a fast intra-slab direction oriented perpendicular to its dip. Since this transformation is visible both in the receiver-function data and in the tomography models, we deduce that the thickness of the reaction front is of the order of a kilometer

ABSTRACT The Cenozoic accretionary complex in the Calabrian Arc, southern Italy, contains hectometric- to kilometric-scale exposures of basalt, gabbro, and serpentinite that have been interpreted as dismembered fragments of Alpine Tethys ocean crust because of their incomplete nature with respect to the traditional view of a complete ophiolite sequence. We present new geologic mapping, geochemistry, and geochronology of one of these units at Timpa di Pietrasasso near the town of Terranova di Pollino in the Basilicata region that exposes Jurassic Tethyan pillow basalt and chert that are separated from gabbro and serpentinite by a fault. The gabbro in the footwall is Permian in age, indicated by U-Pb zircon ages of 284 ± 6 Ma, 293 ± 6 Ma, and 295 ± 4 Ma, linking it to gabbros that underplated continental crust after the Permo-Carboniferous Variscan Orogeny. The gabbro first underwent amphibolite-facies metamorphism, then developed a greenschist-facies mylonitic foliation near the fault surface that is crosscut by undeformed Jurassic-aged dikes of Tethyan origin, indicating that deformation is early Tethyan or pre-Tethyan in age. The underlying serpentinite is tectonically interleaved with blocks of Variscan lower crust, indicating that the missing upper plate of the extensional detachment complex was continental in origin. These features indicate that the Timpa di Pietrasasso unit preserves a low-angle detachment fault that developed in a hyperextended continental margin of the Alpine Tethys.

In 1901, Karl Zittel, president of the Bavarian Royal Academy of Sciences, declared that “Suess has secured almost general recognition for the contraction theory” of mountain-building. This was wishful thinking. Suess’s Das Antlitz der Erde was indeed an influential work, but by the time Suess finished the final volume (1904), the thermal contraction theory was under serious attack. Problems were evident from three different but equally important quarters. The most obvious problem for contraction theory arose from field studies of mountains themselves. As early as the 1840s, it had been recognized that the Swiss Alps contained large slabs of rock that appeared to have been transported laterally over enormous distances. These slabs consisted of nearly flat-lying rocks that might be construed as undisplaced, except that they lay on top of younger rocks. In the late nineteenth century, several prominent geologists, most notably Albert Heim (1849 –1937), undertook extensive field work in the Alps to attempt to resolve their structure. Heim’s detailed field work, beautiful maps, and elegant prose convinced geological colleagues that the Alpine strata had been displaced horizontally over enormous distances. In some cases, the rocks had been accordioned so tightly that layers that previously extended horizontally for hundreds of kilometers were now reduced to distances of a few kilometers. But in even more startling cases, the rocks were scarcely folded at all, as if huge slabs of rocks had been simply lifted up from one area of the crust and laid down in another. Heim interpreted the slabs of displaced rock in his own Glarus district as a huge double fold with missing lower limbs, but in 1884 the French geologist Marcel Bertrand (1847–1907) argued that these displacements were not folds but faults. Large segments of the Alps were the result of huge faults that had thrust strata from south to north, over and on top of younger rocks. August Rothpletz (1853–1918), an Austrian geologist, realized that the Alpine thrust faults were similar to those that had been earlier described by the Rogers brothers in the Appalachians. By the late 1880s, thrust faults had been mapped in detail in North America, Scotland, and Scandinavia.

The French Uplands were built by the Hercynian orogenesis. The French Massif Central occupies one-sixth of the area of France and shows various landscapes. It is the highest upland, 1,886 m at the Sancy, and the most complex. The Vosges massif is a small massif, quite similar to the Schwarzwald in Germany, from which it is separated by the Rhine Rift Valley. Near the border of France, Belgium, and Germany, the Ardennes upland has a very moderate elevation. The largest part of this massif lies in Belgium. Though Brittany is partly made up of igneous and metamorphic rocks, it cannot be truly considered as an upland; in the main parts of Brittany, altitudes are lower than in the Parisian basin. Similarities of the landscape in the French and Belgian Uplands derive from two major events: the Oligocene rifting event and the Alpine tectonic phase. The Vosges and the Massif Central are located on the collision zone of the Variscan orogen. In contrast, the Ardennes is in a marginal position where primary sediments cover the igneous basement. Four main periods are defined during the Hercynian orogenesis (Bard et al. 1980; Autran 1984; Ledru et al. 1989; Faure et al. 1997). The early Variscan period corresponds to a subduction of oceanic and continental crust and a highpressure metamorphism (450–400 Ma) The medio- Variscan period corresponds to a continent–continent collision of the chain (400–340 Ma). Metamorphism under middle pressure conditions took place and controlled the formation of many granite plutons: e.g. red granites (granites rouges), porphyroid granite, and granodiorite incorporated in a metamorphic complex basement of various rocks. The neo-Variscan period (340–320 Ma) is characterized by a strong folding event: transcurrent shear zones affected the units of the previous periods and the first sedimentary basins appeared. At the end of this period, late-Variscan (330–280 Ma), autochthonous granites crystallized under low-pressure conditions related to a post-collision thinning of the crust. Velay and Montagne Noire granites are the main massifs generated by this event. Sediment deposition in tectonic basins during Carboniferous and Permian times occurred in the Massif Central and the Vosges: facies are sandstone (Vosges), shale, coal, and sandstone in several Stephanian basins of the Massif Central, with red shale and clay ‘Rougier’ in the south-western part of the Massif Central.