Alpine Nappes Research Articles

Early Variscan I-type pluton in the pre-Alpine basement of the Western Alps: The ca. 360 Ma Cogne diorite (NW-Italy)

Located at the internal border of the Grand-Saint-Bernard Zone, the diorite and its aureole lie on top of intensively studied Alpine eclogitic units but this pluton, poorly studied yet, has kept locally almost undeformed. The pluton intruded, at ~360Ma, country-rocks mostly composed of dark shales with Na2O>K2O and minor mafic intercalations of tholeiitic basalt affinity. This association is characteristic of the Vanoise (France) basement series, where available age determinations suggest an Early Paleozoic age. Parts of the pluton, and of its hornfels aureole that is evidenced here for the first time, in the Punta Bioula section of Valsavaranche valley (NW-Italy), have been well-preserved from the Alpine deformation. Syn-emplacement hardening, dehydration-induced, probably prevented strain-enhanced Alpine recrystallization. Magmatic rock-types range continuously from subordinate mafic types at SiO2~48%, of hornblendite with cumulative or appinite affinities, to the main body of quartz diorite to quartz monzonite (SiO2 up to 62%). P–T estimates for the pluton emplacement, based on the abundance of garnet in the hornfelses, using also zircon and apatite saturation thermometry and Al-in-hornblende barometry, suggest T~800–950°C and minimum P in the 0.2–0.5GPa range, with records of higher pressure conditions (up to 1–2GPa?) in hornblendite phlogopite-cored amphibole. The high-K, Na>K, calc-alkaline geochemistry is in line with a destructive plate-margin setting. Based on major element data and radiogenic isotope signature (εNd360 Ma from −1.2 to +0.9, 87Sr/86Sr360 Ma from 0.7054 to 0.7063), the parental magmas are interpreted in terms of deep-seated metabasaltic partial melts with limited contamination from shallower sources, the low radiogenic Nd-content excluding a major contribution from Vanoise tholeiites. There is no other preserved evidence for Variscan magmatism of similar age and composition in the Western Alps, but probable analogs are known in the western and northern parts of French Massif Central. Regarding the Alpine tectonics, not only the age of the pluton and its host-rocks (instead of the Permo-Carboniferous age previously believed), but also its upper mylonitic contact, suggest revisions of the Alpine nappe model. The Cogne diorite allegedly constituted the axial part of the E-verging “pli en retour [backfold] du Valsavaranche”, a cornerstone of popular Alpine structural models: in fact, the alleged fold limbs, as attested here by field and geochemical data, do not belong to the same unit, and the backfold hypothesis is unfounded.

Relationship between a syntectonic granitic intrusion and a shear zone in the Southern Carpathian-Balkan area (Almăj Mountains, Romania): Implications for late Variscan kinematics and Cherbelezu granitoid emplacement

The Carboniferous Cherbelezu batholith (Almăj Mountains, Romania) is a well-preserved but poorly studied intrusion belonging to the Upper Danubian Alpine Nappe. This pluton crops out along a pre-existing major verticalized formation, the Corbu Mylonitic Zone (CMZ). Our study investigates the role of the CMZ on the deformation recorded during the mush emplacement and cooling. A detailed microstructural study of this granitic body, coupled with investigations on both Anisotropy of Magnetic Susceptibility (AMS) and Shape Preferred Orientation (SPO) of biotite subfabric, has been performed. The surrounding rocks preserve evidence that the CMZ has been reactivated as a sinistral strike-slip fault before the pluton emplacement. Microstructural investigations of the granitic facies indicate that the pluton has undergone superimposed deformations during its cooling, from submagmatic to LT conditions. Foliation and lineation patterns obtained by AMS and SPO – both methods giving similar results – reflect either magmatic/submagmatic or solid-state flows. Magmatic flow, preserved in the western and southern parts of the pluton, is characterized by concentric foliation pattern with both divergent and parallel lineations, the latter pointing to an early transcurrent regime. Subsequently, a solid-state deformation, recorded during the pluton cooling and restricted to its eastern and northern parts, argues for the concomitant CMZ activity under a sinistral transpressive regime. This is supported by the P′ and T parameter distributions, especially for SPO results, this technique showing clear advantages for the interpretation of the fabric scalar parameters.

Strain accumulation during basal accretion in continental collision — A case study from the Suretta nappe (eastern Swiss Alps)

Stacked crystalline basement nappes are a typical feature in the core of many orogens. The Suretta nappe, exposed in Eastern Switzerland, consists of Briançon-derived crustal slices which were assembled in a south-dipping subduction zone during the Alpine orogenic cycle. The nappe contains post-Variscan rocks of the Rofna Porphyry Complex (RPC) and Permo-Mesozoic cover sequences, which are ideal to study the strain evolution during Alpine nappe formation. We present new structural cross-sections, finite strain analyses and a retrodeformation for the frontal part of the Suretta nappe. The overall geometry of the Suretta nappe is the result of two main deformation phases: (1) Eocene top-to-the-NNW directed thrusting and folding (Ferrera phase), which is overprinted by (2) backfolding and backshearing (Niemet–Beverin phase). We suspect that this backshearing was caused by the buoyant rise of light continental crust beneath the Suretta nappe and by frictional resistance at the top contact of the Suretta nappe, which had been accreted to the upper plate in the course of nappe stacking. In the lower and interior parts of the Suretta nappe, weakly to undeformed boudins are generally surrounded by L-tectonites indicating WSW–ENE stretching; foliated equivalents reveal a plane strain deformation state. The upper part of the Suretta nappe, which was strongly affected by backshearing, shows flattening strain. Strain data were used to reconstruct the palinspastic evolution of the Suretta nappe at two distinct time slots: Prior to Niemet-Beverin backfolding, the Suretta nappe is characterized by thrust faults and large-scale detachment folds. Before the onset of Ferrera phase nappe stacking, Jurassic normal faults and the RPC intrusion shape trigger the localization of subsequent deformation.Our structural study confirms a complex, polyphase evolution for the Suretta nappe, which might be characteristic for crystalline basement nappes in continental collision zones elsewhere.

Survival of Ancient Landforms in a Collisional Setting as Revealed by Combined Fission Track and (U-Th)/He Thermochronometry: A Case Study from Corsica (France)

AbstractThe age of high-elevation planation surfaces in Corsica is constrained using new apatite (U-Th)/He data, field observations, and published work (zircon fission track, apatite fission track [AFT] data and landform/stratigraphical analysis). Thermal modeling results based on AFT and (U-Th)/He data, and the Eocene sediments uncomformably overlapping the Variscan crystalline basement indicate that present-day elevated planation surfaces in Corsica are the remnants of an erosion surface formed on the basement between ∼120 and ∼60 Ma. During the Alpine collision in the Paleocene-Eocene, the Variscan crystalline basement was buried beneath a westward-thinning wedge of flysch, and the eastern portion was overridden by the Alpine nappes. Resetting of the apatite fission track thermochronometer suggests an overburden thickness of >4 km covering Variscan Corsica. Protected by soft sediment, the planation surface was preserved. In the latest Oligocene to Miocene times, the surface was re-exposed and offset by...

MANTLE ORIGIN OF THE ANTRONA SERPENTINITES (ANTRONA OPHIOLITE, PENNINE ALPS) AS INFERRED FROM MICROSTRUCTURAL, MICROCHEMICAL, AND NEUTRON DIFFRACTION QUANTITATIVE TEXTURE ANALYSIS

The Antrona ophiolite is located in the Italian side of Western Central Alps. In the Alpine nappe stack, it lies at low structural levels, being sandwiched between the overlying continental Monte Rosa Nappe (upper Penninic) and the underlying Camughera-Moncucco continental Unit (middle Penninic). The ophiolite sequence includes serpentinized ultramafites, metagabbros and mafic rocks covered by calcschists. The ultramafic portion of the Antrona ophiolite consists of serpentinized peridotites, with interbedded layers of various mafic/ultramafic rocks, and underlies the mafic rocks and metasediments. In spite of the Alpine tectonic and metamorphic evolution of the Antrona ophiolite and its heavy serpentinization, the ultramafic rocks preserve relict texture and mineralogy that allow discussing the nature of their protoliths. Olivine-clinopyroxene-spinel-bearing serpentinites still retain relict porphyroclastic texture, commonly attributed to mantle peridotites. This inference is supported by quantitative textural analysis of Lattice Preferred Orientation by neutron diffraction, performed for the first time in olivine crystals of Alpine ophiolites, suggesting T conditions > 800°C for the activation of slip systems. Mineral chemistry of fresh olivine, clinopyroxene and spinel also contribute to support the mantle nature of the serpentinite protolith. The Antrona ophiolite may thus be regarded as a fossilized fragment of oceanic lithosphere including mantle rocks, volcanics and deep-see sediments. Although their extention is smaller, the Antrona ophiolite seems to be comparable with a coherent ophiolitic slice, such as Zermatt Saas ophiolite more than with a serpentinite melange.

New 40Ar/39Ar laser single-grain ages of muscovites from mylonitic schists in the Rodna Mountains, Eastern Carpathians, Romania: correlations with microstructure

The Rodna Mountains afford the most internal structural window into the crystalline units of the Eastern Carpathians in Romania. The Rodna Mountains consist of Variscan metamorphic nappes that were restacked in the Alpine phase of Carpathian development forming the Subbucovinian and Infrabucovinian nappes. In order to evaluate age of deformation, ten samples were taken from the zone of greenschist facies mylonitic schist that marks the Alpine tectonic boundary between the Subbucovinian and Infrabucovinian nappes and 40Ar/39Ar laser single-grain ages determined for schistosity-forming muscovite. Microstructural assessment of quartz and muscovite distinguished two deformation events. Single-grain ages from the microstructurally most strongly reworked samples (four samples) give a tight clustering of ages at ca. 95 Ma. The least reworked schists have a broader clustering of ages spanning ca. 200–280 Ma with a late Permian peak and some samples showing outlier ages in the range 200–100 Ma. The relative development of the outliers, which correlates with evidence for increased microstructural reworking, is interpreted to mark progressive isotopic resetting. The ca. 95 Ma ages for the most reworked schists are estimates for the age of the Alpine nappe stacking. The ca. 200–280 Ma ages are similar to those of magmatism, metamorphism, and sedimentation thought to mark post-Variscan-pre-Alpine rifting and ocean basin formation in parts of the Alps and may be the thermal imprint of a related event in the Eastern Carpathians.

Oil–oil and oil-source rock correlations in the Alpine Foreland Basin of Austria: Insights from biomarker and stable carbon isotope studies

The Alpine Foreland Basin is a minor oil and moderate gas province in central Europe. In the Austrian part of the Alpine Foreland Basin, oil and minor thermal gas are thought to be predominantly sourced from Lower Oligocene horizons (Schöneck and Eggerding formations). The source rocks are immature where the oil fields are located and enter the oil window at ca. 4 km depth beneath the Alpine nappes indicating long-distance lateral migration. Most important reservoirs are Upper Cretaceous and Eocene basal sandstones. Stable carbon isotope and biomarker ratios of oils from different reservoirs indicate compositional trends in W–E direction which reflect differences in source, depositional environment (facies), and maturity of potential source rocks. Thermal maturity parameters from oils of different fields are only in the western part consistent with northward displacement of immature oils by subsequently generated oils. In the eastern part of the basin different migration pathways must be assumed. The trend in S/(S + R) isomerisation of ααα-C 29 steranes versus the αββ (20R)/ααα (20R) C 29 steranes ratio from oil samples can be explained by differences in thermal maturation without involving long-distance migration. The results argue for hydrocarbon migration through highly permeable carrier beds or open faults rather than relatively short migration distances from the source. The lateral distance of oil fields to the position of mature source rocks beneath the Alpine nappes in the south suggests minimum migration distances between less than 20 km and more than 50 km. Biomarker compositions of the oils suggest Oligocene shaly to marly successions (i.e. Schoeneck, Dynow, and Eggerding formations) as potential source rocks, taking into account their immature character. Best matches are obtained between the oils and units a/b (marly shale) and c (black shale) of the “normal” Schöneck Formation, as well as with the so-called “Oberhofen Facies”. Results from open system pyrolysis-gas chromatography of potential source rocks indicate slightly higher sulphur content of the resulting pyrolysate from unit b. The enhanced dibenzothiophene/phenanthrene ratios of oils from the western part of the basin would be consistent with a higher contribution of unit b to hydrocarbon expulsion in this area. Differences in the relative contribution of sedimentary units to oil generation are inherited from thickness variations of respective units in the overthrusted sediments. The observed trend towards lighter δ 13C values of hydrocarbon fractions from oil fields in a W–E direction are consistent with lower δ 13C values of organic matter in unit c.

P-T evolution of kyanite eclogite from the Pirin Mountains (SW Bulgaria): implications for the Rhodope UHP Metamorphic Complex

A new occurrence of kyanite eclogite in the Pirin Mountains of southwestern Bulgaria within the rocks belonging to the Obidim Unit of the Rhodope Metamorphic Complex is presented. This eclogite provides important information about the peak-pressure conditions despite strong thermal overprint at low pressure. Textural relationships, phase equilibrium modelling and conventional geothermobarom- etry were used to constrain the metamorphic evolution. Garnet porphyroblasts with inclusions of omphacite (up to 43 mol.% Jd), phengite (up to 3.5 Si p.f.u.), kyanite, polycrystalline quartz, pargasitic amphibole, zoisite and rutile in the Mg-rich cores (XMg = 0.44-0.46) record a prograde increase in P-T conditions from ! 2.5 GPa and 650 ! C to ! 3 GPa and 700-750 ! C. Maximum pressure values fall within the stability field of coesite. During exhumation, the peak-pressure assemblage garnet + omphacite + phengite + kyanite was variably overprinted by a lower pressure one forming symplect- itic textures, such as diopside + plagioclase after omphacite and biotite + plagioclase after phengite. The development of spinel (XMg = 0.4-0.45) + corundum + anorthite assemblage in the kyanite- bearing domains at ! 1.1 GPa and 800-850 ! C suggests a thermal overprint in the high-pressure granulite facies stability field. This thermal event was followed by cooling at ! 0.8 GPa under amphibolite facies conditions; retrograde kelyphite texture involving plagioclase and amphibole was developed around garnet. Our results add to the already existing evidence for ultra high pressure (UHP) metamorphism in the Upper Allochthon of the Rhodope Metamorphic Complex as in the Kimi Unit and show that it is more widespread than previously known. Published age data and field structural relations suggest that the Obidim Unit represents Variscan continental crust involved into the Alpine nappe edifice of the Rhodopes and that eclogite facies metamorphism was Palaeozoic, in contrast to the Kimi Unit where age determinations suggest a Jurassic or Cretaceous age for UHP metamorphism. This implies that UHP metamorphism in the Upper Allochthon of the Rhodopes may have occurred twice, during Alpine and pre-Alpine orogenic events, and that two independent HP⁄UHP provinces of different age overlap in this area.

The Chenaillet Ophiolite in the French/Italian Alps: An ancient analogue for an Oceanic Core Complex?

The Chenaillet Ophiolite in the Franco–Italian Alps represents a well-exposed ocean-floor sequence that was only weakly overprinted by Alpine metamorphism during its emplacement in the Alpine nappe stack. Pillow lavas showing a Mid Ocean Ridge Basalt (MORB) signature overlie tectonically exhumed gabbros and serpentinized mantle rocks similar to oceanic core complexes (OCC) described from present-day slow- to ultraslow-spreading ridges. Based on detailed mapping it can be shown that: 1) syn-magmatic high-temperature shear zones are truncated by oceanic detachment faults; 2) sediments consisting exclusively of clasts derived from the footwall of the detachment are overlain by voluminous lavas, and 3) emplacement of lavas is linked with high-angle faulting that overprint the oceanic detachment fault. Based on stratigraphic and cross cutting relationships we conclude that: 1) a complex relation between high- and low-temperature shear zones exists, 2) oceanic detachment faults may be obliterated by later emplaced of MOR basalts, and 3) high-angle faults may serve at shallow levels as feeder channels for the overlying pillow basalts. These observations raise questions about the relative relations in time and space between magmatic, hydrothermal and tectonic processes at spreading systems. We propose that the observed change from exhumation along an oceanic detachment fault to high-angle faulting associated with extrusion of volcanic rocks may represent an evolution within a life cycle of a spreading system that may be comparable to present-day slow- to ultraslow-spreading ridges.

Geological relationships and U-Pb zircon and 40 Ar/ 39Ar tourmaline geochronology of gneisses and tourmalinites from the Nevado–Filabride complex (western Sierra Nevada, Spain): Tectonic implications

The Nevado–Filabride complex, the lowest complex in the Betic hinterland, forms a stack of Alpine nappes. The tectonic units consist of metasedimentary sequences whose ages are not well constrained. Gneiss bodies included in the sequences have been one of the few sources of geochronological data in this metamorphic complex. New radiometric data from U-Pb zircon dating on gneisses from the western Sierra Nevada confirm the presence of late Carboniferous intrusive rocks in the Betic hinterland. These results, combined with available data from the literature and a detailed structural analysis, suggest that the gneisses represent a single late Variscan magmatic event. Evidence for a close genetic relation between gneisses and tourmalinites is provided by field and petrographic observations, in conjunction with geochemical data, U-Pb zircon (314 ± 7; 304 ± 23 Ma) and 40Ar/ 39Ar tourmaline (319.85 ± 5.81; 317.85 ± 3.67 Ma) geochronology. A pre-late Carboniferous age for the basal formation of the Nevado–Filabride sequence can be inferred. The gneiss protolith and the graphite schist are considered to be the boron source and the precursor of the tourmalinites, respectively. Superposed tectonic units rather than a continuous Palaeozoic sequence is supported by the occurrence of Palaeozoic rocks at the top of the Nevado–Filabride complex. The nappe tectonics, as evidenced from the contractional character of the unit boundaries and the superposition of higher-grade on lower-grade metamorphic rocks, is additionally supported by the superposition of older on younger rocks as revealed from radiometric data.

Jura, Alps and the boundary of the Adria subplate

The thin-skinned Jura fold/thrust belt is the frontal part of a thick-skinned late Miocene Alpine nappe. Important late Miocene structures in the Alps that help define this nappe are the External Massifs in the north, the Orobic thrusts and the Piemontese Southern Alps in the south, the Giudicarie–Brenner Line in the east and, more vaguely, the south end of the Belledonne Massif in the west. A bottom boundary compatible with the data is the Brittle–Ductile Transition (BDT). This Jura nappe is separated from the (latest Oligocene)–early Miocene Helvetic nappe system most obviously by the late Miocene Orobic–Giudicarie–Brenner fault complex. In contrast, the distinctive boundary of the early Miocene thick-skinned Helvetic nappe was the Iorio–Tonale Line (IT) in the south, merging into the Piemontese Southern Alps at what may be termed the “Locarno Singularity”–the point of minimum width of the thick-skinned Alps; the northern boundary passed through the Gotthard–and Montblanc Massifs. Motion along the southern boundary was dextrally transpressive, associated with complex strain partitioning patterns. One part of these patterns is the confusing entity comprising the Piemontese Southern Alps and the Lepontine “dome”. The slivers composing the Piemontese Southern Alps are dragged dextrally by the Iorio–Tonale strike–slip at the “Locarno Singularity” and thereby exhibit an important characteristic trait of “exotic terranes” — crustal slivers dragged along strike–slip faults. The Lepontine “dome” around the Locarno Singularity on the north side of the Iorio–Tonale Line displays the characteristics of a backstop in thrust systems as modeled in sandbox experiments. The special boundary functioning as a backstop, compatible with the transpressive kinematics, was the Ivrea body, a strong mafic–ultramafic–granulitic complex which in the early Miocene migrated dextrally along the Iorio–Tonale Line, forcing detachment to descend into the hot middle crust and producing an upsurge of the hot masses. In the late Miocene Jura phase this particular impediment had moved to the west, enabling décollement at the Brittle–Ductile Transition to propagate across the Insubric Line into the Lombardic domain. Large-scale strain partitioning manifestations resemble those of the equally dextrally transpressive southern boundary of the Caribbean plate, in spite of a difference in scales. The foremost of these manifestations are regional right-slip faults slicing through a broad band of contraction, accompanied by collapse basins.

Alpine tectonics in the Calabrian–Peloritan belt (southern Italy): New 40Ar/ 39Ar data in the Aspromonte Massif area

This study provides new 40Ar/ 39Ar geochronological constraints on the age of the Alpine tectonics in the Aspromonte Massif (southern part of the Calabrian–Peloritan belt). This massif exposes the upper units of the Calabride Complex which originated from the European continental margin. The Calabride Complex was incorporated in the Alpine orogenic wedge and then integrated into the Apennines and Maghrebides fold-and-thrust belts. Throughout the Calabride Complex there is evidence for a two stage tectonic history, which remains however rather poorly dated: Alpine nappe stacking is followed by extensional reworking along the former thrust contacts or along new detachment surfaces. Our new ages suggest that exhumation of the uppermost units, which accompanied nappe stacking, probably started at 45 Ma and that the deepest units were almost completely exhumed at 33 Ma. This kinematics probably corresponds to syn-orogenic extension while the end of exhumation is clearly related to the extensional tectonics dated at 28.6 Ma along detachment structures. Our geochronological data reveal a very short lag time between accretional and extensional processes in this part of the Mediterranean Alpine orogenic belt. The direction of extension, when the units are restored to their initial position (i.e. before the opening of the Western Mediterranean basins and the bending of the arc) is NNE–SSW. Such a direction does not fit with the eastward slab-retreat model generally put forward to explain extension in the Western Mediterranean. In contrast, we provide evidence for roughly N–S middle Oligocene extension in the accretionary prism, not previously described in this part of the Mediterranean domain.

The manganese and polimetalic sulphures exploration impact in the Gândacu - Suh rzelul Mare mining zone (north-west of Suceava District, north of Eastern Carpathians) on the environment

The mining activity in Iacobeni – Cârlibaba area dates back as far as the XIII-th century, the first documentary certification being before 1241. After 1775 (when Bucovina was annexed like province to the Austrian Empire) Iacobeni –Cârlibaba area was scientifically examined by Austrian geologists. Geologically and structurally, the Gândacu – Suh rzelul Mare mining area belongs to the Crystalline Mesozoic Zone of the Eastern Carpathians. The Crystalline Mesozoic Zone is made up of a system of Alpine Nappes, called the Eastern Central Carpathian Nappes (S ndulescu, 1984). The exploration and geological surveying work was done in the metamorphic basement of the Sub-Bucovinian Nappe. In the Iacobeni – Cârlibaba mining area, the manganese ore deposits are located in the epimetamorphic Tulghe 2 level (Tg2) of Tulghe lithogroup and the polimetalic sulphures ore deposits in the Tulghe 3 level (Tg3). Geographically, the area Gândacu – Suh rzelul Mare is situated in the Suhard Mountains, on the right slope of the Golden Bistri a river, from an administrative point of view belonging to Cârlibaba, Cioc ne ti and partially to Iacobeni. The prospection works outlined three research perimeters: Puiu - Suh rzel, Humoru and Diaca – Gândacu. The geological study in the area Puiu – Suh rzel showed the presence of polimetalic sulphures ore in acid riolitic metavulcanites, sericite or sericite-chloritic schists. In the same perimeter, in the superior drainage basin of the Recele brook, its discovered a manganous level in black quartzite with intercalations of sericite-graphitic schists. The manganese ore is predominantly siliceous, made up of lens and nests in parallel disposition. On the left slope of the Suh rzelul Mare brook it uncovered polychrome limestone ores. In the Humoru and Diaca – Gândacu perimeters the prospection works highlighted a primary ore of manganese consisting of rhodonite, rhodochrosite, manganese silicates, quarts, calcite, pyrite, etc. The secondary ore of manganese, resulted from the alteration of the primary ore, appears sporadically and it is made up of manganese and iron oxides and hydroxides. The prospection and research works in the three perimetres ended in the interval 1991-1996.

Calcite twinning strains in Alpine orogen flysch: Implications for thrust-nappe mechanics and the geodynamics of Crete

Recent uplift in Crete, along the Hellenic trench, has exposed the internal portions of Alpine nappes that were transported south along north-dipping thrusts from the middle Cretaceous to the Oligocene (100–20 Ma). Flysch deposited along the leading, southern edge of the Alpine foreland basin dates the timing of nappe transport and contains twinned calcite in synorogenic limestones and calcite veins that record the stress-strain fields associated with nappe transport and stacking for a 17 Ma period between 35 and 18 Ma. The flysch ages young away from the craton thereby documenting thin-skin thrusting and nappe motion, for the first time, toward the trench. Fossiliferous marine limestones and calcareous shales were deposited in marginal basins on the leading edges of four thrust-nappes in Crete: the higher and older flysch deposits (Asteroussia, Pindos) are underlain by younger flysch deposits of the Tripolitza and Plattenkalk nappes. The ages of these flysch deposits are well constrained and the flysch sequences are now found deformed between nappes without visible signs of Alpine metamorphism, except the lowest Plattenkalk flysch. Field observations in the flysch deposits include variable bedding, vein, fold axis, and kinematic orientations. Twinned calcite in the flysch limestones (16 samples, n = 425) and veins (23 samples, n = 612) generally preserve subhorizontal, in-transport shortening and vertical extension; where there is a strain overprint (high negative expected values [NEV]), a vertical shortening strain with transport-parallel extension (~N-S) is preserved. Strain magnitudes are greater in the vein sample suite, and differential stress magnitudes responsible for twinning are −230 bars for the entire sample suite. The tectonic evolution of the region involved thin-skinned shortening with south-vergent nappe formation, from north to south, followed by thrust motion on the Cretan detachment. Today, metamorphosed rocks (i.e., Phyllite-Quartzite unit [PQU]) overlie the basal Plattenkalk nappe placing earlier exhumed rocks of the Asteroussia, Tripolitza, and Pindos nappes on top of high-pressure (HP)-metamorphic rocks of the PQU without the deposition of any flysch.

Variscan Tectonics in the Malaguide Complex (Betic Cordillera, Southern Spain): Stratigraphic and Structural Alpine versus Pre‐Alpine Constraints from the Ardales Area (Province of Malaga). II. Structure

Unraveling structural features related to different orogenic cycles is fundamental for a better understanding of collisional orogens, particularly concerning the role of structural inheritance and extent of crustal reworking. In the Internal Domains of the Betic Cordillera, the geometry and kinematics of pre‐Alpine contractional deformation in the Paleozoic basements are rather poorly known. At Ardales, a strongly deformed area corresponding to part of the Alpine suture zone of the Betic Cordillera, the Malaguide Complex consists of two distinct, superposed Alpine tectonic units, including both Paleozoic basement and Mesozoic‐Tertiary cover rocks. Within the Alpine footwall unit, stratigraphic data document a tectonic superposition of two subunits made of different Paleozoic successions, which was later fossilized by Triassic redbeds. Structural analysis indicates that the contact between these subunits is an east‐directed overthrust and that the internal structure of the lower subunit is consistent with bulk noncoaxial strain dominated by top‐to‐the‐E shearing. These features show a marked contrast with the well‐known Alpine nappe transport direction to the N‐NW, with tectonic features developed along the Internal‐External Domain boundary and with the structural trends obtained from the analysis of Mesozoic‐Tertiary Malaguide rocks of the area. This study demonstrates that east‐directed thrusting and shearing within Paleozoic Malaguide rocks resulted from Variscan tectonics and that careful, detailed, and integrated stratigraphic and structural analysis is necessary to avoid confusion between older and newer orogenic cycles in the Internal Domain Units of the Betic Cordillera.

Block tilting of the North Provence early Cretaceous carbonate margin: stratigraphic, sedimentologic and tectonic data

AbstractIn the western part of the Castellane tectonic arc, the so-called “ Provence platform area “, corresponding to the foreland of the Alpine nappes (figs. 1–2), is marked by Tithonian-Berriasian shallow water carbonates capped by hemipelagic sediments deposited from the Valanginian up to the Aptian-Albian. A detailed biostratigraphic study of the Berriasian succession, based on calcareous algae and foraminifera, allows us to distinguish a Lower to Middle Berriasian, with Clypeina sulcata, Clypeina isabellae and Holosporella sarda, from an Upper Berriasian with Pfenderina neocomiensis, Danubiella cernavodensis, Falsolikanella campanensis and Macroporella praturloni (fig. 3). We performed a field survey of 30 sites located from Quinson to the west, and Escragnolles to the east (figs. 4–5) including the study of measured stratigraphic sections and the collection of samples for biostratigraphic interpretations. These stratigraphic investigations show that below the Valanginian beds, the Berriasian platfom carbonate succession, is locally incomplete, i.e. Upper Berriasian beds are frequently absent. During the Early and Middle Berriasian, depositional environments are marked by a strong bathymetric instability, with frequent subaerial exposure events, and a significant marine restriction; by contrast, during the Late Berriasian, the overall biological diversity increases and water agitation as well, which means a significant marine opening towards the basin. The Upper Berriasian hiatus is consequently regarded as the result of a Berriasian/Valanginian and/or a lowermost Valanginian erosion (fig. 6). The spatial distribution of complete or truncated Berriasian successions identifies east-west bands, in each band truncated series are located northward and complete series are located southward. Bands are limited by thrust or strip faults interpreted as palaeofaults reactivated during the Alpine orogeny (fig. 7). These fault-bounded blocks, 3 to 10 km in width, known as the Aiguine, La Palud-sur-Verdon, Carajuan-Audibergue and Peyroulles-La Foux blocks, are southerly rotated by 1 to 2o. We regard this structural architecture as the result of basinward tilting of blocks. Due to their rotation, the uplifted parts were eroded whereas the depressed parts were protected against erosion (fig. 8). Such a dynamic behavior reflects a distensive tectonic regime, which has been active at least during the Valanginian, that is after the drowning of the North-Provence carbonate platform. These structural events are considered as the regional expression of the Neocimmerian tectonic phase coupled with an enhancement of the Atlantic rifting. The orientation of the major Alpine structural elements (folds and faults) of the Castellane arc, is mostly inherited from these early Cretaceous tectonic events.

Tertiary tectono-metamorphic evolution of the European margin during Alpine collison: example of the Leventina Nappe (Central Alps, Switzerland)

The Leventina Nappe represents one of the lowermost exposed units in the Alpine nappe stack and corresponds to a slice of the European margin that was entrained into the Alpine continental accretionary prism during the Tertiary tectonic event. This study yields details regarding the tectonic and metamorphic history of the Leventina Nappe, through detailed analysis of structures and shear zone patterns, and the examination of the Si-content of white mica along a north-south profile. The Leventina Nappe underwent three phases of ductile deformation. Foliation S1 is mostly sub-parallel to the regionally dominant structural fabric (the S2 foliation). S2 foliation is pene tratively developed in the structurally higher portions of the Leventina Nappe toward the Simano Nappe, while it is only weakly developed in the core of the Leventina Nappe. A 50 to 200 m wide mylonite zone, with a D2 top-to-NW sense of shear marks the boundary to the Simano Nappe. Throughout the Leventina Nappe only small-scale D2 shear bands (mm to cm wide) are observed, showing a top-to-NW sense of shear. Deformation phase D3 locally generated a vertical axial plane foliation (S3) associated with the large-scale D3 Leventina antiform.

A synchronous Alpine and Corsica‐Sardinia rotation

We report on the paleomagnetism of 34 sites from lower Oligocene–middle Miocene sediments exposed in the Tertiary Piedmont Basin (TPB, northern Italy). The TPB is formed by a thick (∼4000 m) and virtually undeformed sedimentary succession unconformably lying upon Alpine nappes decapitated by extensional exhumation, which in turn are tectonically stacked over the Adriatic foreland. Paleomagnetic directions from 23 (mostly Oligocene) sites were chronologically framed using new biostratigraphic evidence from calcareous nannoplankton. Our data, along with published paleomagnetic results, show that the TPB rotated ∼50° counterclockwise with respect to Africa in Aquitanian‐Serravallian times. The rotation was likely driven by underneath nappe stacking and was synchronous with (further) bending of the Alpine chain. Both the rotation magnitude and its timing are similar to those documented for the Corsica‐Sardinia microplate. Therefore the formation of the western Alpine arc (or at least part of its present‐day curvature) occurred during the rollback of the Apenninic slab and related back‐arc spreading of the Liguro‐Provençal Basin and drift of the Corsica‐Sardinia block. This suggests a common dynamics driving both the Alpine and the Apennine slab motions. Paleomagnetic data also document that the Adriatic plate has undergone no paleomagnetic rotation since mid‐late Miocene times. Anisotropy of magnetic susceptibility data suggests that the TPB, an enigmatic basin arising from a controversial tectonic setting, formed in an extensional regime characterized by a stretching direction approximately orthogonal to the main trend of the underlying chain.

Tectonic control on diagenesis in a foreland basin: combined petrologic and thermochronologic approaches in the Grès d’Annot basin (Late Eocene–Early Oligocene, French–Italian external Alps)

AbstractCombined petrographic study and apatite fission track analysis (apatite FTA) across the Grès d’Annot basin document maximum temperatures reached by this turbiditic sandstone formation, from around 60 °C at the SW basin margin up to around 200 °C below the Penninic frontal thrust, in relation to burial below the front of Alpine nappes. Increasing diagenetic grade across the basin is primarily expressed by the development of silica transfer by pressure solution and correlated porosity loss. We infer the nappe thickness profile, from the nappe front in the SW up to around 8–10 km in the NE. Apatite FTA dates exhumation of the Grès d’Annot during the Late Miocene, in relation to thrusting of the underlying Argentera and Barrot Palaeozoic massifs. This study illustrates the influence of thrust front propagation on foreland basin diagenesis and exhumation, and highlights potential implications for the evolution of sediment reservoir properties in this context.

The rare earth element distribution over Europe: geogenic and anthropogenic sources

The distribution of rare earth elements (REE) in Europe has been investigated using new geochemical data prepared by the Forum of European Geological Surveys (FOREGS), to estimate baseline values and to identify anomalous levels attributable to anthropogenic sources. Interpolated maps showing REE distributions in subsoils were generated using ArcView™ and classified with the new concentration–area (CA) fractal method available in the software GeoDAS™; they identify three broad ranges for baseline REE levels that are related to regional geological/geomorphological features. A combination of REE spider diagrams and interpolated maps showing REE distributions in floodplain and stream sediments was used to investigate anthropogenic sources of REE in Europe. It is concluded that the large-scale distribution patterns of REE over Europe are entirely attributable to geology. No evidence has been found of REE contamination from likely anthropogenic sources such as the electronics or nuclear industry. Three component colour maps of REE ratios (Gd/Lu, Eu/Eu* and La/Y) were used to identify the major plates of the Variscan orogeny and Variscan granites caught up in Alpine nappes in Italy.