On the role of the Hercynian and Alpine thrusts in the Upper Paleozoic rocks of the Central and Eastern Pyrenees

Abstract

The structure of the Pyrenean pre-Hercynian rocks involved in the “Axial Zone” antiformal stack, results from the association of Hercynian cleavage-related folds and Hercynian and Alpine thrusts. Some of these Alpine and Hercynian thrusts separate thrust sheets in which Upper Paleozoic rocks, Devonian and pre-Hercynian Carboniferous, exhibit different lithostratigraphy and internal structure.

In order to know both, the original Devonian facies distribution and the structural characteristics, the effects of the Alpine and the Hercynian thrusts must be considered. If a conceptual restored cross-section is constructed taking into account both the Alpine and Hercynian thrusts, a different Devonian facies distribution is achieved. Devonian carbonatic successions were originally located in a northernmost position, whereas sequences made by alternations of slates and limestones lie in southernmost areas. Moreover, a N-S variation of the Hercynian structural style appears. In the northern units thrusts are synchronous to folding development and they are the most conspicuous structures. In the intermediate units, thrust postdate cleavage-related folds, and in the southernmost units several folding episodes, previous to the thrusts, are well developed.

We present some examples which enable us to discuss the importance of the Hercynian and Alpine thrusts in the reconstruction of the Pyrenean pre-Alpine geology.     

Postglacial displacements along the Alpine fault, and its significance to the New Zealand tectonic framework

Recent field studies demonstrate the southern and northern parts of the Alpine fault to be dominantly under right-lateral shear. The central portion of this fault is dominantly under compression.The Marlborough—North Island dextral shear zone, together with the Fiordland and NW Nelson sinistral shear zones, demonstrate these shears to result from lateral drag within these zones and is only partially transmitted to the central section of the Alpine fault which is dominantly reverse in character.Regional extension in the North Island west of the shear belt and regional shortening in the South Island indicate clockwise rotation at the east side of the Alpine fault and its extension in the North Island relative to the west side about a “pole” on the Alpine fault in the north of the South Island.     

Plate tectonic processes that governed the mineralization of the Eastern Alps

The notably limited post-Hercynian mineralization of the Eastern Alps is shown to be a direct function of the particular plate tectonic history of the region. The Alpine orogeny can be viewed as consisting of two separate, although overlapping events. Formation and then destruction of a "Penninic Ocean" of Jurassic age left a root of subduced oceanic crust in the Asthenosphere. Cyprus style copper deposits and submarine exhalative tungsten and base metal ore-bodies are the main mineralization episodes that can be related to this Penninic event. In the second stage of the Alpine orogeny the northward subducing Tethyan ocean floor collided with the Penninic remnant causing steepening and deflection of the Benioff-zone. The Alps were thus insulated by the Penninic root from many of the thermal events typical of normal subduction induced orogenies. Minor transport of earlier disseminated mineralization into faults formed in the Alpine tectonism is the dominant manifestation of the limited Alpine thermal event. However, although the geometry of the Alpine orogeny favoured only minor metallization it is also noted that earlier orogenies of the Alpine region are metal poor compared to many orogenic regions. It is suggested therefore that inheritance of the metallogenetic character of the preceeding basement may also play a role in the metalliferous nature of later orogenic episodes.     

Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere

The evolution of the European Cenozoic Rift System (ECRIS) and the Alpine orogen is discussed on the base of a set of palaeotectonic maps and two retro-deformed lithospheric transects which extend across the Western and Central Alps and the Massif Central and the Rhenish Massif, respectively.During the Paleocene, compressional stresses exerted on continental Europe by the evolving Alps and Pyrenees caused lithospheric buckling and basin inversion up to 1700 km to the north of the Alpine and Pyrenean deformation fronts. This deformation was accompanied by the injection of melilite dykes, reflecting a plume-related increase in the temperature of the asthenosphere beneath the European foreland. At the Paleocene–Eocene transition, compressional stresses relaxed in the Alpine foreland, whereas collisional interaction of the Pyrenees with their foreland persisted. In the Alps, major Eocene north-directed lithospheric shortening was followed by mid-Eocene slab- and thrust-loaded subsidence of the Dauphinois and Helvetic shelves. During the late Eocene, north-directed compressional intraplate stresses originating in the Alpine and Pyrenean collision zones built up and activated ECRIS.At the Eocene–Oligocene transition, the subducted Central Alpine slab was detached, whereas the West-Alpine slab remained attached to the lithosphere. Subsequently, the Alpine orogenic wedge converged northwestward with its foreland. The Oligocene main rifting phase of ECRIS was controlled by north-directed compressional stresses originating in the Pyrenean and Alpine collision zones.Following early Miocene termination of crustal shortening in the Pyrenees and opening of the oceanic Provençal Basin, the evolution of ECRIS was exclusively controlled by west- and northwest-directed compressional stresses emanating from the Alps during imbrication of their external massifs. Whereas the grabens of the Massif Central and the Rhône Valley became inactive during the early Miocene, the Rhine Rift System remained active until the present. Lithospheric folding controlled mid-Miocene and Pliocene uplift of the Vosges-Black Forest Arch. Progressive uplift of the Rhenish Massif and Massif Central is mainly attributed to plume-related thermal thinning of the mantle-lithosphere.ECRIS evolved by passive rifting in response to the build-up of Pyrenean and Alpine collision-related compressional intraplate stresses. Mantle-plume-type upwelling of the asthenosphere caused thermal weakening of the foreland lithosphere, rendering it prone to deformation.     

Tectono-metamorphic evolution of the European continental margin involved in the Alpine subduction: New insights from Alpine Corsica,France

In Corsica, continental units (the Lower Units) affected by high-pressure metamorphism represent the remains of the European margin deformed during the Alpine orogeny. In order to document how Alpine deformation and metamorphism changed along the European margin involved in the Alpine subduction, we selected three key areas: the Corte, Cima Pedani, and Ghisoni transects. The three transects show a broadly similar lithostratigraphy. They are characterized by a Variscan basement intruded by Permo-Carboniferous metagranitoids, and by a sedimentary cover including Mesozoic carbonates and middle to late Eocene breccias and sandstones. The three transects recorded a similar deformation history with three deformation phases. Thermo-baric estimations, instead, reveal that each unit was exhumed along an independent retrograde path within the orogenic Alpine wedge. In particular, the lowest units of the Lower Units stack were exhumed along an isothermal path, whereas those located at upper structural levels experienced progressive heating.     

Intraplate deformation and 3D rheological structure of the Rhine Rift System and adjacent areas of the northern Alpine foreland

The lithosphere of the Northern Alpine foreland has undergone a polyphase evolution during which interacting stress-induced intraplate deformation and upper mantle thermal perturbations controlled folding of the thermally weakened lithosphere. In this paper we address relationships among deeper lithospheric processes, neotectonics and surface processes in the Northern Alpine foreland with special emphasis on tectonically induced topography. We focus on lithosphere memory and neotectonics, paying special attention to the thermo-mechanical structure of the Rhine Graben System and adjacent areas of the northern Alpine foreland lithosphere. We discuss implications for mechanisms of large-scale intraplate deformation and links with surface processes and topography evolution.     

The Jura glaciers: palaeogeography in the Würmian circum-Alpine zone

Since the first studies on the glacial formations of the Jura mountains there have been two concepts concerning type of glaciation and ice origin during the Würmian glaciation. Some authors believed that the Jura was totally inundated by ice of Alpine origin, while others believed that the Jura had its own glaciers, originating from a single ice-cap. Several recent studies on both the NW and SE slopes of the range define the problem more accurately. This paper presents a synthesis of the new results from mapping the moraine systems and studying the stratigraphic relationships between Alpine and Jura tills. It is concluded that even the highest level of Alpine glaciers could not lead to a penetration of the Jura. A reconstruction of the extension of the Jura glaciers and their relationship with the Alpine glaciers is given, and we conclude that at the Würmian maximum a local ice-cap was present.     

Alpine climate during the Holocene: a comparison between records of glaciers,lake sediments and solar activity

The European Alps are very sensitive and vulnerable to climate change. Recent improvements in Alpine glacier length records and climate reconstructions from annually laminated sediments of Alpine Lake Silvaplana give the opportunity to investigate the relationship between these two data sets of Alpine climate. Two different time frames are considered: the last 500–1000 years as well as the last 7400 years. First, we found good agreement between the two different climate archives during the past millennium: mass accumulation rates and biogenic silica concentration are largely in phase with the glacier length changes of Mer de Glace and Unterer Grindelwaldgletscher, and with the records of glacier length of Grosser Aletschgletscher and Gornergletscher. Secondly, the records are compared with temporally highly resolved data of solar activity. The Sun has had a major impact on the Alpine climate variations in the long term, i.e. several centuries to millennia. Solar activity varies with the Hallstatt periodicity of about 2000 years. Hallstatt minima are identified around 500, 2500 and 5000 a. Around these times grand solar minima (such as the Maunder Minimum) occurred in clusters coinciding with colder Alpine climate expressed by glacier advances. During the Hallstatt maxima around 0, 2000 and 4500 a, the Alpine glaciers generally retreated, indicating a warmer climate. This is supported by archaeological findings at Schnidejoch, a transalpine pass in Switzerland that was only accessible when glaciers had retreated. On shorter timescales, however, the influence of the Sun cannot be as easily detected in Alpine climate change, indicating that in addition to solar forcing, volcanic influence and internal climate variations have played an important role. Copyright © 2011 John Wiley & Sons, Ltd.     

Tectonically driven fluid flow and associated low-grade metamorphism during the Alpine compression in the eastern Iberian Chain (Spain)

Fluid inclusions and clay mineralogy of the Permo-Triassic rocks from the Espina and Espadà Ranges (SE Iberian Chain, Spain) have been investigated to establish their relationship with hydrothermal fluid circulation during the Alpine Orogeny. Primary fluid inclusions in quartz-filled tension gashes in Permo-Triassic sandstones reveal maximum temperatures around 230 °C and very constant salinities of 8.5% wt. eq. NaCl. Secondary fluid inclusions found in quartz from the Santonian Ba–Cu–Hg deposits show similar compositional and thermodynamic characteristics, denoting an Alpine recrystallization. Clay mineral composition of Permo-Triassic mudrocks is characterized by pyrophyillite, indicating low-grade metamorphic conditions. Field observations and experimental data suggest that the crystallization of quartz in tension gashes, the formation of secondary fluid inclusions and the development of the metamorphism are contemporaneous and related to fluid circulation during the Alpine compression. Fluid flow took place along the Hercynian fault system that was reactivated during the Mesozoic rift stage and inverted during the Alpine deformation.     

Geology of the Bozdag area, central Menderes massif, SW Turkey: Pan-African basement and Alpine deformation

The Menderes massif consists of a Precambrian Core Series that preserves evidence for a polymetamorphic history and a Paleozoic/Mesozoic Cover Series that experienced only the Alpine tectonometamorphic evolution. Structural, petrographic, and geochronologic investigations in the central Menderes massif demonstrate that (a) part of the metamorphic and structural evolution of the Precambrian basement is older than the undeformed 551±1.4-Ma-old Birgi metagranite, and (b) inferred Alpine fabrics overprinting the Cover Series largely have the same attitudes as the old structures in the much older Core Series. The inferred Alpine fabrics include both contractional and extensional structures. Contraction under greenschist to amphibolite facies conditions resulted in the imbrication of the Core and Cover Series and generated an inverted metamorphic sequence by north-directed thrusting. During Alpine extension, most of the south-dipping thrust faults were reactivated as extensional shear zones under decreasing greenschist facies conditions.     

Impacts of Climate Warming on Alpine Glacier Tourism and Adaptive Measures:A Case Study of Baishui Glacier No.1 in Yulong Snow Mountain,Southwestern China

Alpine glaciers usually feature with best hydrothermal condition in mountain climate,and present beautiful glacier scenery,various glacier landforms,rich biodiversity,and easier accessibility,compared with continental glaciers or ice sheets.Nevertheless,Alpine glaciers are more sensitive to climate warming,and climate warming has seriously affected Alpine glaciers and surroundingenvironment.The quality and attractiveness of Alpine glaciers to tourism has been and will continue to be diminished with tourists...     

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.     

Direction changes in thrusting of the Schistes Lustrés in Alpine Corsica

The direction of thrusting contemporaneous with high pressure-low temperature (HP/LT) metamorphism of the ophiolite Schistes Lustrés nappes in Cap Corse, Alpine Corsica has changed from being towards the northwest to towards the southwest during Upper Cretaceous obduction.Similar anticlockwise changes in thrusting have been observed in other regions of Alpine Corsica, Calabria and Southern Betic Cordilleras. A model is proposed for the Alpine evolution of this part of the Western Alps involving a sinistral component of transcurrent movement added to the northwest thrusting. These events have been followed by Eocene backthrusting of nappe of southern-Alpine origin in northwest Cap Corse towards the southeast with associated backfolding of the underlying Schistes Lustrés.     

Controls of surface erosion on the evolution of the Alps: constraints from the stratigraphies of the adjacent foreland basins

 The combined information about the stratigraphies from the foreland basins surrounding the Swiss Alps, exhumation mechanisms and the structural evolution of the Alpine orogenic wedge allow an evaluation of the controls of erosion rates on large-scale Alpine tectonic evolution. Volumetric data from the Molasse Basin and fining-upward trends in the Gonfolite Lombarda indicate that at ∼20 Ma, average erosion rates in the Alps decreased by >50%. It appears that at that time, erosion rates decreased more rapidly than crustal uplift rates. As a result, surface uplift occurred. Because of surface uplift, the drainage pattern of the Alpine hinterland evolved from an across-strike to the present-day along-strike orientation. Furthermore, the decrease of average erosion rates at ∼20 Ma coincides with initiation of a phase of thrusting in the Jura Mountains and the Southern Alpine nappes at ∼50 km distance from the pre-20-Ma thrust front. Coupled erosion-mechanical models of orogens suggest that although rates of crustal convergence decreased between the Oligocene and the present, the reduction of average erosion rates at ∼20 Ma was high enough to have significantly influenced initiation of the state of growth of the Swiss Alps at that time. Received: 8 June 1998 / Accepted: 30 October 1998     

Reconciling late faulting over the whole Alpine belt: from structural analysis to geochronological constrains

The significance of late-stage fracturing in the European Alps in a large geodynamic context is reappraised by studying brittle deformations over the entire belt. In the internal Western Alps, paleostress datasets display a major occurrence of orogen-parallel extension resulting in normal faulting and associated strike-slip mode. There the direction of subhorizontal extension rotates with the bending of the Alpine belt. In the Central Alps, paleostress tensors also indicate orogen-parallel extensional regimes, both in the Bergell area and the Lepontine Dome, where the brittle structures are associated with ductile structures related to the formation of large-scale upright folds that accommodate most of the collisional shortening due to the north-directed component of the movement of the South-Alpine indenter. This brittle deformation phase is of Miocene age and is coeval with the propagation of the Alpine front toward the external Alpine domains. In the Eastern Alps, brittle deformation of the Tauern Window displays an overwhelming occurrence of orogen-parallel normal faulting and associated strike-slip regimes again, which is inferred to be driven by lateral extrusion of the orogenic wedge toward the Pannonian basin, partly due to indentation on the Dolomites indenter. The major orogen-parallel extensional signal of the brittle Cenozoic deformations appears remarkably stable all over the internal Alps. Extensional brittle structures are part of a late phase of collisional deformation, during which the propagation of the Alpine front of the Western Alps and the northward movement of the Southern Alpine and the Dolomites indenters in the Central and Eastern Alps were accommodated by orogen-parallel extension in the inner zones, at the scale of the entire chain.     

Eo-Alpine HP metamorphism in the Permian intrusives from the steep belt of the central Alps (Languard-Campo nappe and Tonale Series)

Abstract

Diorites and granitoids that intruded the Upper Austroalpine units of the central Alps during the Permian display map-pable tectonic imprints and metamorphic transformations that were acquired during the Alpine tectonometamorphic cycle. Superposed heterogeneous deformations interacted with metamorphic re-equilibration stages and created a range of textural types that reflect local deformation gradients: coronitic transformations textures, normally foliated S-tectonites and mylonitic foliations. The three textural types are distinguished on maps recording foliation trajectories of successive deformation phases, which are correlated to the evolution of metamorphic assemblages. Tectonic deformation of Alpine age is represented by three generations of ductile syn-metamorphic structures. The mineral assemblages stable during the first Alpine deformation phase (D1) are Amp_II + P1_II + white mica, + Zo/Czo + Grt + Qtz ± Mg-Ch1 ± Ilm in metadiorites and P1_II + white mica_I + Zo/Czo + Grt + Amp_II + Qtz ± Ilm/Ttn in metagranitoids; the successive foliations D2a and D2b are defined by greenschist facies minerals. Thermobarometric estimates allow T = 500–600 °C and P = 1.1 ± 0.2 GPa conditions to be determined during D1 and T ≤ 350 °C and P ≤ 0.5 GPa during D2. Relict igneous minerals in metadiorites allow to determine intrusive conditions of T = 879 ± 110 °C and P = 0.4–0.7 GPa. Radiometric ages and P/T ratio of Alpine P_maxT_Pmax suggest that the inferred P-T-d-t path may represent the thermal state of the initial Alpine subduction stages. © 2000 Éditions scientifiques et médicales Elsevier SAS     

Ecological stability during the LGM and the mid-Holocene in the Alpine Steppes of Tibet?

Arid and Alpine ecosystems are known for extreme environmental changes during the Late Quaternary. We hypothesize that the world's largest Alpine arid ecosystem however, the Alpine Steppes of the Tibetan highlands, remained ecologically stable during the LGM and the mid-Holocene. This hypothesis is tested by distributional range of plant species, plant life forms and rate of endemism. The set of character species has a precipitation gradient between 50 and 350 mm/a, testifying for resilience to precipitation changes. 83% of the species have a wider vertical range than 1000 m used as a proxy for resilience to temperature changes. 30% of the species are endemic with 10 endemic genera, including plate-shaped cushions as a unique plant life form. These findings are in line with palaeo-ecological proxies (δ¹⁸O, pollen) allowing the assumption that Alpine Steppes persisted during the LGM with 3 to 4 K lower summer temperatures.During the mid-Holocene, forests could have replaced Alpine Steppes in the upper catchments of the Huang He, Yangtze, Mekong, Salween and Yarlung Zhangbo, but not in the interior basins of the north-western highlands, because the basins were then flooded, suppressing forests and supporting the environmental stability of this arid Alpine grassland biome.     

Equilibrium-disequilibrium relations in the Monte Rosa Granite,Western Alps: Petrological,Rb-Sr and stable isotope data

Nine samples from the Monte Rosa Granite have been investigated by microscopic, X-ray, wet chemical, electron microprobe, stable isotope and Rb-Sr and K-Ar methods. Two mineral assemblages have been distinguished by optical methods and dated as Permian and mid-Tertiary by means of Rb-Sr age determinations. The Permian assemblage comprises quartz, orthoclase, oligoclase, biotite, and muscovite whereas the Alpine assemblage comprises quartz, microcline, albite+epidote or oligoclase, biotite, and phengite. Disequilibrium between the Permian and Alpine mineral assemblages is documented by the following facts: (i) Two texturally distinguishable generations of white K-mica are 2 M muscovite (Si=3.1–3.2) and 2 M or 3 T phengite (Si=3.3–3.4). Five muscovites show Permian Rb-Sr ages and oxygen isotope fractionations indicating temperatures between 520 and 560 ° C; however, K-Ar ages are mixed or rejuvenated. Phengite always shows mid-Tertiary Rb-Sr ages, (ii) Two biotite generations can be recognized, although textural evidence is often ambiguous. Three out of four texturally old biotites show mid-Tertiary Rb-Sr cooling ages while the oxygen isotopic fractionations point to Permian, mixed or Alpine temperatures, (iii) Comparison of radiogenic and stable isotope relations indicates that the radiogenic isotopes in the interlayer positions of the micas were mobilized during Alpine time without recrystallization, that is, without breaking Al-O or Si-O bonds. High Ti contents in young muscovites and biotites also indicate that the octahedral (and tetrahedral) sites remained undisturbed during rejuvenation. (iv) Isotopic reversals in the order of O¹⁸ enrichment between K-feldspar and albite exist. Arguments for equilibrium during Permian time are meagre because of Alpine overprinting effects. Texturally old muscovites show high temperatures and Permian Rb-Sr ages in concordancy with Rb-Sr whole rock ages. For the tectonically least affected samples, excellent concordance between quartz-muscovite and quartz-biotite Permian temperatures implies oxygen isotope equilibrium in Permian time which was undisturbed during Alpine metamorphism. Arguments for equilibrium during the mid-Tertiary metamorphism are as follows: (i) Mid-Tertiary Rb-Sr mineral isochrons of up to six minerals exist, (ii) Oxygen isotope temperatures of coexisting Alpine phengites and biotites are concordant.The major factor for the adjustment of the Permian assemblages to Alpine conditions was the degree of Alpine tectonic overprinting rather than the maximum temperatures reached during the mid-Tertiary Alpine metamorphism. The lack of exchange with externally introduced fluid phases in the samples least affected by tectonism indicates that the Monte Rosa Granite stewed in its own juices. This seems to be the major cause for the persistence of Permian ages and corresponding temperatures.     

Lu–Hf garnet systematics of a polymetamorphic basement unit: new evidence for coherent exhumation of the Adula Nappe (Central Alps) from eclogite-facies conditions

The Adula Nappe in the Central Alps is a mixture of various pre-Mesozoic continental basement rocks, metabasics, ultrabasics, and Mesozoic cover rocks, which were pervasively deformed during Alpine orogeny. Metabasics, ultrabasics, and locally garnet–mica schists preserve eclogite-facies assemblages while the bulk of the nappe lacks such evidence. We provide garnet major-element data, Lu profiles, and Lu–Hf garnet geochronology from eclogites sampled along a north–south traverse. A southward increasing Alpine overprint over pre-Alpine garnets is observed throughout the nappe. Garnets in a sample from the northern Adula Nappe display a single growth cycle and yield a Variscan age of 323.8 ± 6.9 Ma. In contrast, a sample from Alpe Arami in the southernmost part contains unzoned garnets that fully equilibrated to Alpine high-pressure (HP) metamorphic conditions with temperatures exceeding 800 °C. We suggest that the respective Eocene Lu–Hf age of 34.1 ± 2.8 Ma is affected by partial re-equilibration after the Alpine pressure peak. A third sample from the central part of the nappe contains separable Alpine and Variscan garnet populations. The Alpine population yields a maximum age of 38.8 ± 4.3 Ma in line with a previously published garnet maximum age from the central nappe of 37.1 ± 0.9 Ma. The Adula Nappe represents a coherent basement unit, which preserves a continuous Alpine high-pressure metamorphic gradient. It was subducted as a whole in a single, short-lived event in the upper Eocene. Controversial HP ages and conditions in the Adula Nappe may result from partly preserved Variscan assemblages in Alpine metamorphic rocks.