Origin and significance of the Permian high-K calc-alkaline magmatism in the central-eastern Southern Alps, Italy

The Atesina Volcanic District, the Monte Luco volcanics, and the Cima d'Asta, Bressanone-Chiusa, Ivigna, Monte Croce and Monte Sabion intrusions, in the central-eastern Southern Alps, form a wide calc-alkaline association of Permian age (ca. 280–260 Ma). The magmatism originated during a period of post-orogenic extensional/transtensional faulting which controlled the magma ascent and emplacement. The magmatic products are represented by a continuum spectrum of rock types ranging from basaltic andesites to rhyolites, and from gabbros to monzogranites, with preponderance of the acidic terms. They constitute a metaluminous to weakly peraluminous series showing mineralogical, petrographic and chemical characteristics distinctive of the high-K calc-alkaline suites. In the MORB-normalized trace element diagrams, the most primitive volcanic and plutonic rocks (basaltic andesites and gabbros with Mg No.=66 to 70; Ni=25 to 83 ppm; Cr=248 to 679 ppm) show LILE and LREE enriched patterns with troughs at Nb–Ta and Ti, a distinctive feature of subduction-related magmas. Field, petrographic, geochemical and isotopic evidence (initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7057 to 0.7114; ε_Nd values from −2.7 to −7.4; ∂¹⁸O values between 7.6 and 9.5‰) support a hybrid nature for both volcanic and plutonic rocks, originating through complex interactions between mantle-derived magmas and crustal materials. Only the scanty andalusite–cordierite and orthopyroxene–cordierite bearing peraluminous granites in the Cima d'Asta and Bressanone-Chiusa intrusive complexes can be interpreted as purely crustal melts (initial ⁸⁷Sr/⁸⁶Sr=0.7143–0.7167; initial ε_Nd values between −7.9 and −9.6, close to average composition of the granulitic metasedimentary crust from the Ivrea Zone in the western Southern Alps). Although the Permian magmatism shows geochemical characteristics similar to those of arc-related suites, palaeogeographic restorations, and geological and tectonic evidence, seem not to support any spatial and/or temporal connection with subduction processes. The magmatism is post-collisional and post-orogenic, and originated in a regime of lithospheric extension and attenuation affecting the whole domain of the European Hercynian belt. A change in the convergence direction between Gondwana and Laurasia, combined with the effects of gravitational collapse of the Hercynian chain, could have been the driving mechanism for lithosphere extension and thinning, as well as for upwelling of hot asthenosphere that caused thermal perturbation and magma generation. In the above context, the calc-alkaline affinity and the orogenic-like signature of the Permian magmatism might result from extensive contamination of basaltic magmas, likely derived from enriched lithospheric mantle source(s), with felsic crustal melts.     

Permian continental basins in the Southern Alps (Italy) and peri-mediterranean correlations

The Late Carboniferous to Permian continental successions of the Southern Alps can be subdivided into two main tectono-sedimentary Cycles, separated by a marked unconformity sealing a Middle Permian time gap, generally estimated at over 10 Ma. The lower cycle (1), between the Variscan crystalline basement and the Early Permian, is mainly characterised by fluvio-lacustrine and volcanic deposits of calc-alkaline acidic-to-intermediate composition, which range up to a maximum thickness of more than 2,000 m. The upper cycle (2), which is devoid of volcanics, is mostly dominated through the Mid?–Late Permian by alluvial sedimentation which covered the previous basins and the surrounding highs, giving rise to the subaerial Verrucano Lombardo-Val Gardena (Gröden) red-beds, up to about 800 m thick. The palaeontological record from the terrigenous deposits of both the above cycles consists mainly of macro- and microfloras and tetrapod footprints. The age of the continental deposits is widely discussed because of the poor chronological significance of a large number of fossils which do not allow reliable datings; however, some sections are also controlled by radiometric calibrations. The comparison with some selected continental successions in southern Europe allows to determine their evolution and set up correlations. A marked stratigraphic gap shows everywhere between the above-mentioned Cycles 1 and 2. As in the Southern Alps, the gap reaches the greatest extent during the Mid-Permian, near the Illawarra Reversal geomagnetic event (265 Ma). In western Europe, however, such as in Provence and Sardinia, the discussed gap persists upwardly to Late Permian and Early Triassic or slightly younger times, i.e. to the onset of the “Alpine sedimentary Cycle”, even though in northeastern Spain (Iberian Ranges, Balearic Islands) this gap results clearly interrupted by late Guadalupian–Lopingian deposits. The above two major tectonosedimentary cycles reflect, in our view, two main geodynamic events that affected the southern Europe after the Variscan orogenesis: the Late Carboniferous–Early Permian transformation of the Gondwana–Eurasia collisional margin into a diffuse dextral transform margin and the Middle–Late Permian opening of the Neotethys Ocean, with the onset of a generalised extensional tectonic regime and the progressive westward marine ingression.     

Permian deposits of the Southern Alps as product of initial alpidic taphrogenesis

Sedimentological and morphotectonic investigations of Permian deposits between the Val Camonica and the Karawanken Mountains indicate the development of a complex rift-system which initiated the alpidic sedimentation phase.The extensive distribution of regoliths and palaeosoils at the base of the Permian deposits demonstrate the existence of a consolidated and subsequently truncated crust. The senile topography which prevailed at the beginning of the Permian had to be modified by fault movements to provide the erosional and transport energy required for the deposition of psephytes and other siliciclastica (scarp foot fans). The formation of syndepositional graben structures, diagnostic for an overall distentional regime, led to the development of three morphotectonic units. Commencing in the early Permian with fissure volcanism, a central, northeasterly trending rift developed. The western margin of it was modified by rift-parallel faulting the resulting trough serving as receptacle for lacustrine sediments. The area to the east of the central rift was separated by a pronounced graben shoulder, the eastern perimeter of which supported a marine carbonate platform.Rejuvenation of tectonism near the middle of the Permian established a new, east-southeast trending rift-branch and expelled the sea from the eastern basin. The ensuing fluviatile regimes, draining mainly in southerly to south-westerly directions resulted in a general equalisation of the landscape.In the late Permian, the eastern part of the area became inundated by a shallow, evaporitic sea, but fluviatile conditions prevailed in the west until the onset of the Triassic. Current directions indicate a south-westerly outlet path for these systems and thus the existence of another, presumably marine base level in that direction.

---

Zusammenfassung Neue sedimentologische und morphotektonische Untersuchungen der Permablagerungen zwischen dem Val Camonica und den Karawanken weisen auf die Entwicklung eines komplexen Grabensystems hin, welches den Beginn der alpidischen Sedimentationsphase einleitete.Die große Verbreitung von Regolithen und Paläoböden an der Basis der Permabfolgen belegen die Existenz einer weitgehend eingerumpften, variszisch konsolidierten Kruste. Somit mußte die zu Beginn des Perms vorherrschende, senile Topographie durch synsedimentäre Bruchtektonik modifiziert werden, um jene Erosions- und Transportenergie bereitzustellen, welche für die Ablagerung von Fanglomeraten und anderen Siliziklastika erforderlich waren. Die Bildung syngenetischer Grabenstrukturen ist Ausdruck eines regionalen Dehnungsregimes, welches zur Entwicklung von drei morphotektonischen Einheiten führte. Beginnend mit ausgedehntem Spaltenvulkanismus entwickelte sich im unteren Perm ein nordöstlich verlaufendes Grabensystem. Die westliche Flanke wurde durch rift-parallele Verwerfungen modifiziert, die daraus resultierenden Tröge wurden vornehmlich mit lakustrinen Sedimenten gefüllt. Das Gebiet östlich der zentralen Rift war von dieser durch eine ausgeprägte Grabenschulter getrennt, auf deren flach nach Osten abdachenden Ausläufern marine Plattformkarbonate zur Ablagerung gelangten.Verstärkte tektonische Aktivität um die Mitte des Perms führte zur Anlage eines neuen, ost-südost verlaufenden Zweiges des Riftsystems und zur Verdrängung des Meeres aus dem östlichen Becken. Das darauf folgende fluviatile Regime, das hauptsächlich in südbis südwestliche Richtungen entwässerte, resultierte in einer allgemeinen Einebnung des Terrains.Die gegen Ende des Perms von Osten her einsetzende marine Transgression führt zur Ablagerung von evaporitischen Sedimenten im östlichen Becken. Im westlichen Bereich dagegen, beherrschen fluviatile Bedingungen bis zur beginnenden Trias weiterhin die Sedimentation. Strömungsazimuthe zeigen einen Abfluß dieser Systeme nach Südwest und somit die Existenz einer zusätzlichen Erosionsbasis in dieser Richtung an.

---

Résumé De nouvelles recherches sédimentologiques et morphotectoniques sur les dépôts permiens entre le Val Camonica et les Karawanken attirent l'attention sur le développement d'un système complexe de grabens qui caractérise le stade initial de la sédimentation du cycle alpin.L'extension importante des régolithes et des paléosols à la base de la série permienne confirme l'existence d'une crôute continentale consolidée à l'époque varisque puis fortement rabotée. La topographie sénile, qui précomine au début du Permien a, du fait d'une tectonique cassante, subi des modifications telles que les capacités d'érosion et de transport ont été suffisantes pour que s'y déposassent les fanconglomérates et les silico-clastites qui s'y rencontrent.Débutant par un volcanisme fissurai très répandu un système de grabens de direction Nord-Est Sud-Ouest s'est développé. Le bord ouest en a été modifié par l'existence d'accident rejouant parallèlement à la bordure du rift. Les bassins qui en ont résulté ont été pour l'essentiel remplis par des sédiments lacustres.Le domaine situé à l'Est du rift central en était séparé par un épaulement bien marqué. Sur le flanc de celui-ci, qui s'enfonçait doucement vers l'Est se déposaient des formations carbonatées marines de plateforme.Vers le milieu du Permien une activité tectonique plus importante a provoqué la formation d'un rameau d'orientation Est-Sud-Est Ouest-Nord-Ouest du système de rift et le retrait de la mer du bassin oriental. Le régime fluviatile qui s'ensuivit et qui se déversait vers le Sud et le Sud-Ouest s'est traduit par un aplanissement généralisé de la surface.La transgression marine qui, vers la fin du Permien s'est étendue à partir de l'Est, a conduit au dépôt de sédiments évaporitiques dans le bassin oriental. Dans le domaine occidental par contre les conditions fluviatiles ont prédominé jusqu'au début du Trias. Les directions de courant indiquent que ce système fluviatile s'écoulait vers le Sud-Ouest et donc qu'un niveau de base existait dans cette direction.

---

- , . , . , , (scarp foot fans). , . , , . , . ; , , . , , . . . . . , . , . , . , , .

     

The transition between the two major Permian tectono-stratigraphic cycles in the central Southern Alps: results from facies analysis and U/Pb geochronology

The Val Daone Conglomerate (VDC) is a continental clastic unit that crops out eastwards of the central Southern Alps, from the NE sector of the Collio Basin to the W as far as the Tione Basin to the E. This significant but as yet relatively unknown formation lies just above the regional unconformity that marks the boundary between the two Permian major tectono-sedimentary cycles (TSU1 and TSU2) and grades upwards paraconformably (?) to the fluvial red beds of Verrucano Lombardo/Val Gardena Sandstone, generally associated with Late Permian times. Recent palynological investigations on the VDC suggested a Guadalupian age (late Roadian–early Wordian), owing to the remarkable presence of diversified pollen associations; therefore, this sedimentary unit is to date the first one ascribed, on a palaeontological basis, to the Middle Permian in the entire Southern Alps domain. A detailed facies analysis of VDC shows deposition in amalgamated alluvial fan-braided and fluvial environments with wide channels and longitudinal bars. In its type area and Val Rendena, the VDC rests unconformably above the last volcanic episode of the TSU1. LA ICP-MS U–Pb dating on zircon from two samples of such topmost Lower Permian volcanic rocks, known as Ponte Murandin dacitic lava and Malga Plan rhyodacitic Ignimbrites (Tione Basin), provided Concordia ages of 278 ± 2 Ma (MSWD = 0.01) and 279 ± 2 Ma (MSWD = 0.16), respectively. As well as allowing us to better define the duration of the time gap between the two Permian megacycles in the central Southern Alps in almost 10 Ma, these radiometric age determinations are also significant because they enable us to regionally link the coeval volcanic bodies which crop out in the Collio Basin to the SW and in the “Athesian Volcanic Group” to the NE, respectively.     

The Variscan post-collisional volcanism in Late Carboniferous–Permian sequences of Ligurian Alps, Southern Alps and Sardinia (Italy): a synthesis

In the considered wide sector of the West-Mediterranean southern Europe, the collisional phase of the Variscan orogeny during Late Carboniferous and Permian times was followed by magmatic intrusive and effusive activity and sedimentation into intracontinental, alluvial to lacustrine basins originated by wrench- to normal-fault systems. The first volcanic cycle (generally Late Carboniferous-Early Permian in age) is represented by early calc-alkaline andesites and rhyolites, in variable amounts, and by following large volume of rhyolites, and by dacites. Both andesites and rhyolites show K-normal and high-K calc-alkaline character. The origin of the liquids of the first cycle is ascribed to partial melting processes at the mantle–crust interface telescoped within a thickened crust. The melting is considered as the consequence of thermal re-equilibration following isostatic disequilibrium and the subsequent collapse of the orogenic belt; the ascent of liquids occurred in a (trans-)tensional regime. The second magmatic cycle is represented by alkaline magmatism, and exhibits typical anorogenic features consistent with a rifting regime. This event was no more related with the collapse of the Variscan belt, but rather to the post-Variscan global re-organization of plates that evolved during Late Triassic times to the neo-Tethyan rifting. In both cycles, important differences in timing, areal distribution and outpoured volumes arise.     

Carbonates in Upper Permian evaporites of the Northern Calcareous Alps,Austria

Thin black dolomites occur within the evaporitic Alpine Haselgebirge Formation (Late Permian) of the central and eastern Northern Calcareous Alps, Austria. They have been compared with the southalpine Bellerophon Formation byTollmann (1964). Samples obtained from the gypsum-anhydrite deposit of Wienern am Grundlsee, Styria, are mostly unfossiliferous mudstones (dolomicrite and dolomicrospar) devoid of sedimentary structures. Only few samples contain a low-diversity ostracode fauna. Electron microprobe and X-ray diffraction analysis indicate well-ordered and near-stoichiometric dolomite (49.9±0.2 mol% CaCO₃). The oxygen isotopic composition varies between –3.0 and –5.7 PDB. The ¹³C ratios fall into two distinct groups: –0.9 to +1.9, and –3.5 to –6.8 PDB. The carbon isotope data of the first group indicate a predominantly marine origin of the carbonates. The significantly depleted ¹³C ratios of the second group are probably due to freshwater influx into the basin and/or sulfate reduction. Sulfur and strontium isotope data from intercalated anhydrite and gypsum preclude an entirely non-marine origin of the brines. The light ¹⁸O ratios are probably due to early diagenetic sulfate reduction and/or dolomite recrystallization during moderate burial. A detailed interpretation of the depositional environment is rendered impossible because of the pervasive alpine tectonics. However, northalpine Bellerophon carbonates appear to be restricted to areas of major anhydrite deposition, and are absent in the halitedominated facies.

---

Zusammenfassung Im zentralen und östlichen Abschnitt der Nördlichen Kalkalpen Österreichs treten geringmächtige, bituminöse Dolomite im Verband der salinaren Alpinen Haselgebirge Formation (Oberperm) auf. Sie wurden vonTollmann (1964) mit der südalpinen Bellerophon Formation verglichen. Bei den untersuchten Proben von der Gips- und Anhydritlagerstätte Wienern am Grundlsee (Steiermark) handelt es sich fast durchwegs um fossilleere Mikrite und Mikrosparite, die keinerlei Sedimentstrukturen aufweisen. Nur sehr wenige Proben führen Ostrakoden. Nach Mikrosonden-Analysen und röntgendiffraktometrischen Untersuchungen bestehen diese mikrokristallinen Dolomite aus fast stöchiometrischen, gut geordneten Dolomit (49.9±0.2 Mol% CaCO₃). Die Sauerstoffisotopenwerte schwanken von –3.0 bis –5.7 PDB und die ¹³C Verhältnisse fallen in zwei Gruppen: –0.9 bis +1.9, und –3.5 bis –6.8 PDB. Die Werte der ersten Gruppe indizieren eine im wesentlichen marine Bildung der Karbonate. Die deutlich leichteren ¹³C-Werte der zweiten Gruppe können durch Süßwassereinfluß während der Sedimentation und/oder durch Sulfatreduktion erklärt werden. Schwefel- und Strontium-Isotopenwerte, gemessen an Anhydriten und Gipsen im Verband mit den Dolomiten, schließen eine rein nicht-marine Herkunft der Salzlösungen aus. Die negativen ¹⁸O-Verhältnisse der Dolomite sind wahrscheinlich auf diagenetische Prozesse (Sulfatreduktion und/oder Umkristallisation) zurückzuführen. Die enorme alpidische Deformation dieser Evaporitserien gestattet es nicht, das ursprüngliche Ablagerungsmilieu der Karbonate zu rekonstruieren. Die nordalpinen Bellerophon Karbonate scheinen jedoch an Ablagerungsbereiche mit mächtiger Anhydritbildung gebunden zu sein, und fehlen primär in der Halit-dominierten Fazies.

---

Résumé En Autriche, dans les secteurs central et oriental des Alpes calcaires du nord, existent des dolomies bitumineuses de faible puissance en relation avec la formation évaporitique alpine de l'Haselgebirge (Permien supérieur).Tollmann (1964) les a comparées à la formation sud-alpine de Bellérophon. Des échantillons provenant des gisements de gypse et d'anhydrite de Wienern sur le Grundlsee (Styrie) montrent des mudstones (dolomicrites à dolomicrosparites) dépourvus de fossiles et de structures sédimentaires. Seuls quelques échantillons contiennent une faune à ostracodes peu diversifiée. Les examens à la microsonde et aux rayons X montrent une dolomite bien ordonnée et quasi-stoechiométrique (49,9±0,2% moléculaire de CaCo₃). La composition isotopique de l'oxygène varie entre –3,0 et –5,7 PDB. Les rapports ¹³C se répartissent en deux groupes: –0,9 à +1,9 et –3,5 à –6,8 PDB. Ces données indiquent pour le premier groupe une origine surtout marine des carbonates. Les valeurs moins élevées des rapports du second groupe sont dues probablement à l'arrivée d'eau douce dans le bassin et/ou à une réduction des sulfates. Les valeurs isotopiques du soufre et du strontium, mesurées sur les gypses et anhydrites associés aux dolomies, excluent une origine entièrement continentale pour les saumures. Les faibles rapports ¹⁸O sont dûs probablement à une réduction diagénétique précoce des sulfates et/ou à la recristallisation de la dolomite pendant l'enfouissement. Une interprétation détaillée des conditions de dépôt est rendue impossible par l'ampleur de la déformation tectonique alpine. Il semble toutefois que des carbonates nord-alpins de type «Bellérophon» soient liés aux aires de forte sédimentation d'anhydrite et soient absents dans les faciès à halite dominante.

---

Haselgebirge ( ) . Tollmann (1964) . , Grundlsee, , , - . . - , - , (49.9–0,2 % ₃). –3,0 –5,7% PDB, ¹³C : –0,9 –1,9% –3,5 –6,8% PDB. . / . , , , . ¹⁸ , , : / . . , , » « .

     

Strontium isotopes in magnesites from Permian and Triassic strata,Eastern Alps

The Sr isotopic composition of sediment-hosted magnesites in Permian and Scythian series of Upper Austroalpine units (Eastern Alps) has been determined. The results suggest diagenetic-metasomatic magnesite formation by Mg-rich pore solutions. The depositional environment of the magnesite-bearing rocks is reflected by different isotopic compositions with initial ⁸⁷Sr/⁸⁶Sr ratios close to contemporaneous sea water in marine sediments (0.7071–0.7083) and higher ratios being typical for lacustrine and coastal environments influenced from the hinterland (0.7133–0.7139). Coarser grained recrystallized magnesite and magnesite veins show a distinct increase in their Sr isotopic ratios (0.7202–0.7220) which can be attributed to metamorphic fluids of Eoalpine age. The findings of these magnesite occurrences, which have been affected only by very low-grade metamorphism, are compared with similar results from spar magnesites associated with metamorphosed Paleozoic sedimentary rocks. A similar genetic model is proposed for the first magnesite mineralization in those Paleozoic strata, but with variably intense later recrystallisation during metamorphism.     

Eoalpine and mesoalpine tectonics in the Southern Alps

According to structural and stratigraphic data, eoalpine and mesoalpine tectonics can be clearly recognized in the Southern Alps and distinguished from the better known neoalpine deformation.The eoalpine phase (Late Cretaceous) was generated by N-S compression. It deformed basically the central-western Southern Alps producing overthrusts to the west and flower structures by sinistral transpression in the Giudicarie Belt. Deposition of the coeval flysch successions was controlled by the trend of this belt. This tectonic pattern persisted until Middle Eocene.The mesoalpine phase (Eocene) was generated mainly by a ENE-WSW compression and produced thrust geometries with N-S or NW-SE direction in the eastern part of the Southern Alps. The coeval Eocene Flysch also followed this trend, filling the foredeep basin. This deformation is considered to be the front of the Dinarids, which began to be deformed since Late Cretaceous until at least Early Oligocene.The neoalpine tectonics inherited the eoalpine and mesoalpine structures and produced the major part of the deformation accounting for the present structural framework of the Southern Alps.

---

Zusammenfassung Die eo- und mesoalpidische Tektonik in den Südalpen ist strukturell und stratigraphisch erkennbar und von den bekannteren neoalpidischen Bewegungsbildern deutlich zu unterscheiden.Die eoalpidische Phase (Obere Kreide) entspricht einer N-S Kompression, die sich in den zentralen bis westlichen Südalpen bemerkbar macht. Im westlichen Bereich treten Überschiebungen auf, das Judikarische Gebiet wird hingegen durch die von linkslateralen Transpressionen bewirkten Verwerfungsbündel gekennzeichnet, die auch die altersgleichen Flysch-Ablagerungen kontrollieren. Diese tektonische Phase ist bis zum Eozän aktiv.Die mesoalpidische Phase (Eozän) ist auf eine ENE-WSW Kompression zurückzuführen. Sie verursachte in den östlichen Südalpen N-S bis NW-SE gerichtete Überschiebungsbilder. In gleichorientierten vorozeanischen Becken kommt der eozäne Flysch vor. Dieses Deformationsbild kennzeichnet schon in der Oberen Kreide die Dinariden-Front.Die neoalpidische Tektonik vererbte die eo-mesoalpidischen Gefüge-Elemente und verursachte im wesentlichen den gegenwärtig erkennbaren Gefügeplan der Südalpen.

---

Résumé Dans les Alpes Méridionales, des données stratigraphiques et structurales permettent de reconnaître les tectoniques éoalpine et mésoalpine et de les distinguer des déformations néoalpines, mieux connues.La phase éoalpine (Crétacé supérieur) a été engendrée par une compression N-S. Celle-ci a affecté surtout la partie centre-occidentale des Alpes Méridionales en produisant des charriages à l'ouest et des structures de décrochement sénestres dans la chaîne des Giudicarie, dont l'orientation a déterminé la sédimentation du flysch concommittant. Cette tectonique s'est poursuivie jusqu' à l'Eocène.La phase mésoalpine a été engendrée surtout par une compression ENE-WSW; elle a produit, dans la partie est des Alpes Méridionales, des charriages de direction N-S à NW-SE. Le flysch éocène a suivi cet alignement structural, en remplissant le bassin de l'avant-fosse. Cette déformation est considérée comme le front des Dinarides, dont la formation a commencé dés le Crétacé supérieur.La tectonique néoalpine a hérité des structures éoalpines et mésoalpines et est responsable de la plus grande part de la structure actuelle des Alpes Méridionales.

---

- , , . ( ) . , , — , . . () ENE WSW; , N — S NW — SE . - . . , , - .

     

Reconstruction of the geodynamic evolution of the Northern Calcareous Alps by means of paleomagnetism

We review the paleomagnetic studies published on the Northern Calcareous Alps (NCA) and also include data from two theses that were not published yet. Moreover, we present new data from upper Santonian to Maastrichtian Gosau deposits from the Neue Welt Area in the easternmost part of the NCA (D = 325.8. I = 37.1, α₉₅ = 8.3, positive fold test, thus indicating counterclockwise rotation and inclination flattening). Although several of the recently published studies provide valuable data, presently the paleomagnetic data base of the NCA does not provide a sufficient frame work for a quantitatively backed paleogeographic model. By thus documenting the state of research, we want to outline major problems regarding the paleomagnetic characterization of the NCA and also specify the kind of paleomagnetic future work that is particularly needed within this mountain range.     

Crustal-scale magmatic systems during intracontinental strike-slip tectonics: U, Pb and Hf isotopic constraints from Permian magmatic rocks of the Southern Alps

The Southern Alps host volcano-sedimentary basins that formed during post-Variscan extension and strike-slip in the Early Permian. We present U–Pb ages and initial Hf isotopic compositions of magmatic zircons from silicic tuffs and pyroclastic flows within these basins, from caldera fillings and from shallow intrusions from a 250 km long E–W transect (Bozen–Lugano–Lago Maggiore) and compare these with previously published data. Basin formation and magmatism are closely related to each other and occurred during a short time span between 285 and 275 Ma. The silicic magmatism is coeval with mafic intrusions of the Ivrea-Verbano Zone and within Austroalpine units. We conclude that deep magma generation, hybridisation and upper crustal emplacement occurred contemporaneously along the entire transect of the Southern Alps. The heat advection in the lower crust by injected mantle melts was sufficient to produce crustal partial melts in lower crustal levels. The resulting granitoid melts intruded into the upper crust or rose to the surface forming large caldera complexes. The compilation of Sr and Nd isotopic data of these rocks demonstrates that the mantle mixing endmember in the melts may not be geochemically enriched but has a depleted composition, comparable to the Adriatic subcontinental mantle exhumed to form the Tethyan sea floor during Mesozoic continental breakup and seafloor spreading. Magmatism and clastic sedimentation in the intracontinental basins was interrupted at 275 Ma for some 10–15 million years, forming a Middle Permian unconformity. This unconformity may have originated during large-scale strike-slip tectonics and erosion that was associated with crustal thinning, upwelling and partial melting of mantle, and advection of melts and heat into the crust. The unconformity indeed corresponds in time to the transition from a Pangea-B plate reconstruction for the Early Permian to the Late Permian Pangea-A plate assembly (Muttoni et al. in Earth Planet Sci Lett 215:379–394, 2003). The magmatic activity would therefore indicate the onset of >2,000 km of strike-slip movement along a continental-scale mega-shear, as their model suggests.     

Potassium-argon ages from the Ceneri Zone,Southern Swiss Alps

The Ceneri Zone is a unit of the crystalline basement of the Southern Alps. Its northern boundary is the Tonale Line segment of the Periadriatic Line, an important tectonic lineament separating the Oligocene and younger features of the Central Alps from the older metamorphic and structural trends of the Southern Alps. Unmetamorphosed Permian and younger sedimentary units lap onto the Southern Alpine basement from the south.Potassium-argon results from the Ceneri Zone define a Hercynian age pattern typical for the basement of continental Europe. This pattern extends to within at least 100 meters of the Tonale Line. Thus, amphibolite facies metamorphism in this region occurred around 325 m.y. ago. The geochronologic similarity of the Southern Alps to many other European regions must be taken into account in megatectonic theories.In detail, the Hercynian age pattern of the Ceneri Zone is complicated. Some hornblendes have apparent ages between the Hercynian and a Caledonian value (430 m.y.). They probably retained some radiogenic argon during the Hercynian upper amphibolite facies metamorphism. In addition, mica results between 200 and 300 m.y. have a strong geographic correlation. Apparently, the northwestern portion of the Ceneri Zone was reheated or mildly metamorphosed during the Upper Triassic to Lower Jurassic. A relationship between these ages and 170–180 m.y. ages from the neighboring Ivrea-Verbano Zone seems likely. No geologic evidence for any post-Hercynian event has been noted as yet in the Ceneri Zone.     

Benthic foraminifera from the upper Collio Formation (Lower Permian, Lombardy Southern Alps): implications for the palaeogeography of the peri-Tethyan area

New palaeontological evidence points to a temporary marine transgression in the Early Permian into the Collio Basin, a major palaeogeographic feature of the present-day Southern Alps. The thick volcaniclastic succession filling the basin (Collio Formation) is widely held as deposited in alluvial to lacustrine settings. Rare calcareous foraminifers were recently found in a single sandstone interval, containing phosphate nodules, from the uppermost Collio Formation. A temporary seaway, necessary for the foraminifera to spread into a continental basin, implies that (i) the Collio Basin lake was not only an intramontane (as commonly viewed), but also a coastal lake, and (ii) its altitude did not exceed the amplitude of a first-order sea-level rise, that is, about 100 m. These constraints, along with striking similarities as to tectonic context, accumulation rates and geochemical signature, suggest that the Collio Basin was a California-type basin, resembling in particular the present-day Salton Sea (CA, USA).     

Late Permian non-marine–marine transitional profiles in the central Southern Permian Basin, northern Germany

The transition from Rotliegend to Zechstein within the Southern Permian Basin is one from continental desert to a marine environment. During the Upper Rotliegend II a huge playa lake existed there. This lake was temporarily influenced by precursors of the Zechstein transgression. Therefore the mega-playa evolved into a sabkha system. One of these early marine ingressions is known from an outcrop in Schleswig-Holstein. Laminated silt- and claystones, deposited within a standing water body, are intercalated in siltstones of a salt-flat environment. The lake sediments are characterised by high frequency cyclicity, shown by the sedimentary record and also by palaeontological data. The section contains fresh water as well as brackish-marine and marine fauna. Climatically forced cycles interact with marine incursions. After the Zechstein transgression had flooded the basin completely, sedimentation was controlled by sea-level fluctuations. Two sections, in the southern North Sea and in Schleswig-Holstein, are presented in this paper. Cyclicities with different frequencies controlled the sedimentation of the Kupferschiefer (T1) and the Werra Carbonate (Ca1). Sediments of the North Sea sequence were deposited within a shallow bay at the margin of an elevation. Therefore, the high frequency cyclicity became obvious within the sedimentary patterns and in the faunal content.     

Southern Alps Cu-Au hydrothermal system,Westland, New Zealand

The Southern Alps of New Zealand is an actively rising mountain belt which displays a thermal anomaly adjacent to the Alpine Fault, the Australian-Pacific plate boundary. Extensive fluid movement occurs in this uplift zone, resulting in metallic vein mineralization. Gold mineralization is confined to greenschist facies rocks, while younger veins in amphibolite facies rocks near the Alpine Fault are enriched in copper. Transport and deposition of metals in this complex hydrothermal system is governed by interaction between rising metamorphic fluids and downward-percolating meteoric fluid. Metamorphic fluids have equilibrated with graphitic schist country rock and are relatively reduced. Infiltration and mixing of meteoric water increases oxygen activity and decreases sulphur activity in the fluid. Oxidised meteoric water heats up and dissolves Cu during downward percolation. This Cu is deposited as the fluid becomes more reduced. Hence, there is a progressive increase in copper content in the middle portions of the hydrothermal system, especially in the more permeable highly fractured rocks near the Alpine Fault.     

Post-granulite facies monazite growth and rejuvenation during Permian to Lower Jurassic thermal and fluid events in the Ivrea Zone (Southern Alps)

U-Pb analyses of single monazite grains from two granulite facies metapelites in the Ivrea Zone (Southern Alps) reveal the presence, in both samples, of at least three different ages and prove that earlier interpretations of supposedly concordant monazite data as cooling ages are unwarranted. One group of monazite data defines a subconcordant discordia line with an upper intercept age of 293.4 ± 5.8 Ma and a lower intercept age of 210 ± 14 Ma. The upper intercept is interpreted as the real cooling age of the monazites. The lower intercept is interpreted as an episode of fluid-driven Pb-loss, indicated by the presence of internal and external corrosion structures not only of the monazites but also of the zircons in the same samples that are also rejuvenated at 210 ± 12 Ma. Another group of monazite data lies above the concordia. The presence of excess ²⁰⁶Pb indicates that these crystals have grown below the monazite blocking temperature, thus after the granulite facies metamorphism. The age of growth of the new monazite crystals is approached by their ²⁰⁷Pb/²³⁵U ages that range between 273 and 244 Ma. The two groups of post-cooling age (post-293.4 ± 5.8 Ma) monazite data correspond to two distinct late- and post-Variscan geotectonic regimes that affected the Southern Alps, (1) Permian transtension with decompression and anatectic melting; (2) Upper Triassic to Lower Jurassic rifting with geographically dispersed hydrothermal activity and alkaline magmatism. Received: 7 July 1998 / Accepted: 4 November 1998     

Permian high-temperature metamorphism in the Western Alps (NW Italy)

International Journal of Earth Sciences - During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial...     

Conical folds and apparent rotations in paleomagnetism (a case study in the Southern Pyrenees)

The so-called apparent rotation was defined as the angular deviation between a local paleomagnetic direction (after the standard bedding correction) and their corresponding paleomagnetic reference [J. Geophys. Res. 85 (1980) 3659]. In this paper, we make a theoretical exploration on this concept and we conclude that (depending on the number, sequence, orientation and magnitude of the deformation axes that have affected to the rock volume) the apparent rotation may be the addition of a vertical-axis rotation plus a spurious rotation. The later is an error whose origin is the inappropriate application of the bedding correction during the restoration (which does not fit the reverse sequence of deformations). Then, Apparent rot. (s.l.) (δ)=Spurious rot. (θ)+Vertical-axis rot. (β).Conical folds are complex geometries that cannot be restored by using the bedding correction. However, appearance of apparent and spurious rotations has not been studied even though the presence of this kind of folds is very common in fold and thrust belts. In this paper, we show a way to restore these structures and its associated paleomagnetic data by means of forward modelling on a stereographic projection. The modelling has to be based on a good characterization of the geometry (fold axis orientation) and understanding of the kinematics of the fold. General modelling has also allowed us to predict the apparent rotation in conical synclines. Its magnitude depends on the semiapical angle and on the degree of development of the fold; the sense of the rotation (clockwise or counter-clockwise) will depend on the sense of rotation the fold axis.The western External Sierras provide an excellent case study of apparent rotations due to the presence of a conical fold in the footwall (Ebro foreland basin) of the South Pyrenean sole thrust. In addition, a vertical-axis clockwise rotation up to 47° (32° in average) has been detected in the hagingwall. An apparent rotation up to 28° (20° in average) is observed in the footwall of the structure when a simple bedding correction is used. This deviation does not fit with the expected Ebro basin direction (reference) and is caused by the effect of the Riglos conical syncline, developed by the flexure of the foot wall ramp of the South Pyrenean sole thrust. The forward modelling carried out considering the geometry and kinematics (non significant rotations in the autochthonous foot wall) of this structure predicts very well the paleomagnetic observations in the field (geographic coordinates) with angular departures of only 5° (in average). The only application of the bedding correction would introduce errors (spurious rotations up to 21°, 12 in average) related to the conical geometry that would not allow the differentiation of these distinct structural units.     

Revision of the stratigraphy of Permian and supposed Permian rocks of Southern Scotland

The formal stratigraphy of the Permian basins of southern Scotland is described for the first time. These rocks consist of piedmont breccias and lavas, overlain by wadi-fan and dune complexes, and alluvial fan and floodplain sediments. The sequences are inferred to be Stephanian to Lower Permian in age and correlatable with similar continental sediments in England and with the Lower Rotliegendes of Germany and the North Sea.

---

Zusammenfassung Erstmals wird hier eine stratigraphische Gliederung der permischen Beckensedimente im südlichen Schottland gegeben. Diese Gesteine bestehen aus Piedmont-Brekzien und Laven, die von Fanglomeraten und Dünenkomplexen überlagert werden. Alluviale Fächer und Schwemmflächen-Sedimente gehören hierzu. Diese Serien sollen ein Alter von Stephan bis Unter-Perm aufweisen und vergleichbar mit ähnlichen kontinentalen Sedimentfolgen in England und im Unter-Rotliegenden von Mitteleuropa sein.

---

Résumé La stratigraphie des formations des bassins permiens du Sud de l'Ecosse est ici décrite pour la première fois. Ces roches sont des brèches de piedmont et des laves, surmontées par un complexe de dépôts d'épandage et de dunes, et par des cones et des dépôts alluviaux. Les couches seraient à rapporter au Stéphanien et au Permien inférieur, et à correler avec des sédiments continentaux semblables en Angleterre et avec le Rothliegend inférieur d'Allemagne et de la Mer du Nord.

---

. Piedmont , . . - ; .

     

Fusulinid biostratigraphy of the Lower Permian Zweikofel Formation (Rattendorf Group; Carnic Alps,Austria) and Lower Permian Tethyan chronostratigraphy

The Zweikofel Formation of the Rattendorf Group in the Carnic Alps (Austria) is 95–102 m thick and consists of a cyclic succession of thin‐ to thick‐bedded fossiliferous limestone and intercalated thin intervals of siliciclastic sediment. The siliciclastic intervals were deposited in a shallow marine nearshore environment. The variety of carbonate facies indicates deposition in a shallow neritic, normal‐saline, low‐ to high‐energy environment. The Zweikofel Formation is characterized by a paracyclic vertical arrangement of facies and represents sedimentary sequences that are not well understood elsewhere in the Tethys. Fusulinids and conodonts from the upper Grenzland and Zweikofel formations in the Carnic Alps clearly suggest that what has been called ‘Sakmarian’ in the Tethys includes both the Sakmarian and Artinskian stages of the Global Time scale. Fusulinids from the lower part of the Zweikofel Formation at Zweikofel closely resemble those of the Grenzland Formation and approximately correlate with the upper part of the Sakmarian and lower part of the Artinskian of the Global Time scale. The upper part of the Zweikofel Formation correlates approximately with the lower‐middle (?) parts of the Artinskian Stage of the Global Time scale. A new regional Hermagorian Stage of the Tethyan scale is proposed between the Asselian and Yakhtashian. The lower boundary of the Hermagorian Stage is proposed to be located at the base of bed 81 in the 1015 section of Darvaz (Tadzhikistan). The boundary between the Hermagorian and Yakhtashian stages is placed at the base of bed 73 in the Zweikofel section at Zweikofel, Carnic Alps. In the Darvaz region, Tadzhikistan, the type area for the Yakhtashian Stage, this boundary has never been precisely defined. The entire fusulinid assemblage of the upper part of the Grenzland and Zweikofel formations reported herein includes 62 species of 18 genera, of which one subgenus and 12 species and subspecies are new. Copyright © 2012 John Wiley & Sons, Ltd.     

Equilibrium-line altitudes of late Quaternary glaciers in the Southern Alps,New Zealand

Equilibrium-line altitudes (ELA's) of former glaciers in the Tasman River-Lake Pukaki drainage basin of the Southern Alps were reconstructed from glacial-geologic data on former ice limits by using an assumed accumulation-area ratio of 0.6 ± 0.05. Late Holocene (Neoglacial) ELA's were depressed 140 m below present levels, whereas those of four late Pleistocene ice advances were depressed 500 m (Birch Hill), 750 m (Tekapo), 875 m (Mt. John), and 1050 m (Balmoral). Reconstructed ELA gradients are approximately parallel to one another and range from 19 to 23 m km^?1. Although vertical movement on active faults and isostatic tilting due to deglaciation have both contributed to modification of reconstructed ELA gradients from their original values, the maximum resulting effect probably amounts to less than 2.0 m km^?1 and is undetectable from present data.