Alpine Orogenic Cycle Research Articles

The Permian-Triassic succession of the Montagnola Senese Ridge (Middle Tuscan Ridge, Italy): a perspective for late Palaeozoic magmatism and continentalisation in the western Tethys

ABSTRACT During the Permian-Triassic transition, dramatic environmental changes occurred, including the segmentation of Pangaea and the beginning of the Alpine orogenic cycle with widespread magmatism and tectonic activity. The Permian-Triassic succession deposited in the northern (palaeo-) Gondwana is now exposed in the inner Northern Apennines where it crops out discontinuously in the Middle Tuscan Ridge, a NS-oriented morphotectonic feature mainly formed by blue-schist and green-schist facies metamorphic rocks. In this paper, we describe a previously unreported late Palaeozoic succession exposed in the central part of the Middle Tuscan Ridge: the Montagnola Senese Ridge. This succession provides information concerning the enigmatic late Palaeozoic units exposed in southern Tuscany, and permits the age constraint of continentalization at the Permian-Triassic boundary with associated magmatism. Specifically, by integrating stratigraphic, structural, and petrographic data, we describe a siliciclastic, volcanoclastic-to-carbonate succession which recorded the transition from marine sedimentation to the typical continental, red-beds of the Verrucano tectofacies. Additionally, deformational features, metamorphism and age of the succession are discussed in terms of its tectono-sedimentary evolution and implications for the Palaeozoic-Triassic setting of the central Pangaea sector currently exposed in southern Tuscany. The results allow us to recognize a middle-late Permian pre-Verrucano succession, and to date the transition to continental facies as latest Permian. The microflora permits correlation of the pre-Verrucano succession to the late Palaeozoic units of the inner Northern Apennines, and establishes the diachronous nature of the Verrucano tectofacies exposed in the inner Northern Apennines. Finally, the thermal maturity of the organic matter indicates a maximum temperature of 387 ± 30°C attained during metamorphism.

Geology of the Magura Nappe in the Osielec area with emphasis on an Eocene olistostrome with metabasite olistoliths (Outer Carpathians, Poland)

The Magura Nappe in the Polish sector of the Outer Carpathians consists of four tectonic subunits characterized by differing development of facies. From the south to the north, they include the Siary, Raca, Bystrica and Krynica subunits. The sedimentary succession in the Raca Subunit in the vicinity of the village of Osielec is composed of Campanian–Palaeogene flysch deposited in the Magura Basin. In this succession, the Middle Eocene Pasierbiec Sandstone Fm consists of thick-bedded sandstones and conglomerates with occasional intercalations of thin-bedded shale-sandstone flysch. Within the Pasierbiec Sandstone Fm at Osielec there is an olistostrome, rich in pebbles and cobbles of exotic rocks. In addition, large blocks of Neoproterozoic metabasites and boulders of Palaeogene organogenic limestones were found. The discovery of metabasites raised the possibility that the rocks in question could be evidence of supposed oceanic crust in the basement of the Magura sedimentary basin, because of the suggestion that they represent the Alpine orogenic cycle. This concept was abandoned when investigations of the absolute age of the metabasites gave a date of ca. 600 Ma. In the Osielec area, there are two tectonic thrust sheets in the Raca Subunit, namely the Osielczyk Thrust Sheet in the north and the Bystra Thrust Sheet in the south; they are folded and cut by a transverse system of strike-slip and oblique faults. The Osielczyk Thrust Sheet was overthrust northwards on to the Siary Subunit.

Triassic magmatism on the transition from Variscan to Alpine cycles: evidence from U–Pb, Hf, and geochemistry of detrital minerals

New data on the U–Pb geochronology and Hf-isotopic composition of detrital zircons, along with the geochemistry of garnets, chromian spinel and feldspars from Permian and Triassic volcano-sedimentary sequences in the Southern and Eastern Alps reveal similarities in the nature and tectonomagmatic setting of the Early Permian and the Middle Triassic magmatic events. Detrital zircon U–Pb geochronology displays magmatic activity at 290–280 Ma (Early Permian) and at 245–235 Ma (Middle Triassic). The two-stage Hf-depleted mantle model ages obtained from Permian and Triassic zircons cluster around 1.7–1.6 and 1.3–0.8 Ga. Epsilon Hafnium analysis on dated detrital zircons suggests mixing of mantle and crustal components into the parental magmas of Permian and Triassic magmatism, the latter showing less crustal contribution. The composition of detrital feldspars from the Permian and Triassic volcaniclastic sandstones reveals provenance from intermediate to acidic igneous rocks. Garnet in Middle and Late Triassic beds suggests the incorporation of material comparable to the mafic rocks of the Ivrea-Verbano Zone into the parental melts of the Triassic magmatism. Chromian spinel in syn-magmatic Carnian sandstones indicates an origin associated with MORB-type peridotites and therefore, corroborates a mantle contribution to the Middle Triassic magmatism. The integration of detrital zircon U–Pb ages, Hf isotopes and composition of detrital minerals suggests that continuous lithospheric stretching at the transition from the post-Variscan to the Alpine orogenic cycle, with periods of strike-slip movements, presumably led to an asymmetrical continental rifting process, in which the Middle Triassic magmatic episode represents the earliest stage of the Alpine Tethys oceanic domain.

40Ar–39Ar laser dating of ductile shear zones from central Corsica (France): Evidence of Alpine (middle to late Eocene) syn-burial shearing in Variscan granitoids

The island of Corsica (France) plays a central role in any reconstruction of Western Mediterranean geodynamics and paleogeography but several key aspects of its geological evolution are still uncertain. The most debated topics include the interpretation of the Corsican orogen as the result of an east- or west-directed subduction, and the actual involvement of the Variscan basement of Corsica in the Alpine orogenic cycle. This study integrates 40Ar–39Ar laserprobe, mesostructural, microtextural, and microchemical analyses and places relevant constraints on the style, P–T conditions, and timing of Alpine-age, pervasive ductile shear zones which affected the Variscan basement complex of central Corsica, a few kilometers to the west of the present-day front of the Alpine nappes. Shear zones strike ~NNE–SSW, dip at a high angle, and are characterized by a dominant sinistral strike-slip component. Two of the three investigated shear zones contain two texturally and chemically resolvable generations of white mica, recording a prograde (burial) evolution: (1) deformed celadonite-poor relicts are finely overgrown by (2) a celadonite-rich white mica aligned along the main foliation. White mica from a third sample of another shear zone, characterized by a significantly lower porphyroclast/matrix ratio, exhibits a nearly uniform high-celadonite content, compositionally matching the texturally younger phengite from the nearby shear zones. Mineral-textural analysis, electron microprobe data, and pseudosection modeling constrain P–T conditions attained during shearing at ~300°C and minimum pressures of ~0.6GPa. In-situ 40Ar–39Ar analyses of coexisting low- and high-celadonite white micas from both shear zones yielded a relatively wide range of ages, ~45–36Ma. Laser step-heating experiments gave sigmoidal-shaped age profiles, with step ages in line with in-situ spot dates. By contrast, the apparently chemically homogenous high-celadonite white mica yielded concordant in-situ ages at ~34Ma, but a hump-shaped age spectrum, with maximum ages of ~35Ma and intermediate- to high-temperature steps as young as ~33–32Ma. Results indicate that the studied samples consist of an earlier celadonite-poor white mica with a minimum age of ~46Ma, overgrown by a synshear high-celadonite white mica, developed at greater depth between ~37 and 35Ma; faint late increments in shearing occurred at ≤33–32Ma, when white mica incipiently re-equilibrated during exhumation. Results suggest that ductile shearing with a dominant strike-slip component pervasively deformed the Corsican basement complex during the emplacement and progressive thickening of the Alpine orogenic wedge and broaden the extent of the domain affected by the Alpine tectonometamorphic events. Integration of petrological modeling and geochronological data shows that the Variscan basement of central Corsica, close to the Alpine nappes, was buried during the late Eocene by ≥18km of Alpine orogenic wedge and foreland deposits. Our results, combined with previously published apatite fission-track data, imply an overburden removal ≥15km from the late Eocene (Priabonian) to the early Miocene (Aquitanian), pointing to a minimum average exhumation rate of 1.3–1.5mm/a.

Geometry and kinematics of the Roisan-Cignana Shear Zone, and the orogenic evolution of the Dent Blanche Tectonic System (Western Alps)

The Dent Blanche Tectonic System (DBTS) is a composite thrust sheet derived from the previously thinned passive Adriatic continental margin. A kilometric high-strain zone, the Roisan-Cignana Shear Zone (RCSZ) defines the major tectonic boundary within the DBTS and separates it into two subunits, the Dent Blanche s.s. nappe to the northwest and the Mont Mary nappe to the southeast. Within this shear zone, tectonic slices of Mesozoic and pre-Alpine meta-sediments became amalgamated with continental basement rocks of the Adriatic margin. The occurrence of high pressure assemblages along the contact between these tectonic slices indicates that the amalgamation occurred prior to or during the subduction process, at an early stage of the Alpine orogenic cycle. Detailed mapping, petrographic and structural analysis show that the Roisan-Cignana Shear Zone results from several superimposed Alpine structural and metamorphic stages. Subduction of the continental fragments is recorded by blueschist-facies deformation, whereas the Alpine collision is reflected by a greenschist facies overprint associated with the development of large-scale open folds. The post-nappe evolution comprises the development of low-angle brittle faults, followed by large-scale folding (Vanzone phase) and finally brittle extensional faults. The RCSZ shows that fragments of continental crust had been torn off the passive continental margin prior to continental collision, thus recording the entire history of the orogenic cycle. The role of preceding Permo-Triassic lithospheric thinning, Jurassic rifting, and ablative subduction processes in controlling the removal of crustal fragments from the reactivated passive continental margin is discussed. Results of this study constrain the temporal sequence of the tectono-metamorphic processes involved in the assembly of the DBTS, but they also show limits on the interpretation. In particular it remains difficult to judge to what extent pre-collisional rifting at the Adriatic continental margin preconditioned the efficiency of convergent processes, i.e. accretion, subduction, and orogenic exhumation.

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.

Fault‐slip analysis in the metaophiolites of the Voltri Massif: constraints for the tectonic evolution at the Alps/Apennine boundary

AbstractThe metaophiolitic Voltri Massif underwent a complex tectono‐metamorphic evolution, resulting from subduction to collision events during the Alpine orogenic cycle, and was subsequently involved in the first stages of the Apennine Orogeny. So this is a key area to investigate the late orogenic Alpine tectonics and its time relationship with the first Apenninic deformation events.We performed detailed mapping, fault‐slip analysis and study of fault rocks in a study area located at the eastern border of the Voltri Massif. Here we recognized two main generations of reverse shear zones (RSZ). The first one (RSZ1) is associated to mylonites and protomylonites in greenschist to low‐greenschist metamorphic facies and is characterized by a ductile to ductile–brittle behaviour; RSZ1 display either top‐to‐N–NW or to‐SW kinematics. The second generation of reverse shear zones (RSZ2) is characterized by fault breccia, crush breccia or protocataclasite; RSZ2 sense of tectonic transport ranges from top‐to‐NW, ‐SW and ‐NE.RSZ1 structures are locally folded by 10 m‐scale, open chevron folds with fold axes gently plunging to SW that likely represent the last phases of deformation during Alpine collision.RSZ2 structures locally reactivate RSZ1, with a more brittle behaviour and produce a huge volume of fault rock. They are coeval with low‐greenschist to zeolite‐facies metamorphic conditions. The RSZ2 fit a transpressional regime active at the eastern border of the Voltri Massif at least during the late Oligocene–early Miocene. Copyright © 2008 John Wiley & Sons, Ltd.

Poly-orogenic multi-stage metamorphic evolution inferred via P– T pseudosections: An example from Aspromonte Massif basement rocks (Southern Calabria, Italy)

The metamorphic evolution of a key sector of the western Mediterranean internal Alpine orogenic belt (southern Calabrian Peloritani Orogen) is identified and described by means of P– T pseudosections calculated for selected metapelite specimens, showing evidence of multi-stage metamorphism. Attention focused on the two lowermost basement nappes of the Aspromonte Massif (southern Calabria), which were differently affected by poly-orogenic multi-stage evolution. After a complete Variscan orogenic cycle, the upper unit (Aspromonte Peloritani Unit) was involved in a late-Alpine shearing event. In contrast, the several underlying metapelite slices, here grouped together as Lower Metapelite Group, show exclusive evidence of a complete Alpine orogenic cycle. In order to obtain reliable P– T constraints, an integrated approach was employed, based on: a) garnet isopleth thermobarometry; and b) theoretical predictions of the P– T stability fields of representative equilibrium assemblages. This approach, which takes into account the role of the local equilibrium volumes in controlling textural developments, yielded reliable information about P– T conditions from early to peak metamorphic stages, as well as estimates of the retrograde trajectory in the pseudosection P– T space. According to inferred detailed P– T paths, the evolution of the Aspromonte Peloritani Unit is characterised by a multi-stage Variscan cycle, subdivided into an early crustal thickening stage with P– T conditions ranging from 0.56 ± 0.05 GPa at 570 ± 10 °C to 0.63–0.93 GPa at 650–710 °C (peak conditions) and evolving to a later crustal thinning episode in lower P– T conditions (0.25 GPa at 540 °C), as documented by the retrograde trajectory. Conversely, the prograde evolution of the rocks of the Lower Metapelite Group shows evidence of a HP-LT early Alpine multi-stage cycle, with P– T evolving from 0.75–0.90 GPa at 510–530 °C towards peak conditions, with pressure increasing northwards from 1.12 ± 0.02 GPa to 1.24 ± 0.02 GPa, and temperatures of 540–570 °C. A late-Alpine mylonitic overprint affected the rocks of both the Aspromonte Peloritani Unit and the Lower Metapelite Group. This overprint was characterised by an initial retrograde decompression trajectory (0.75 ± 0.05 GPa at 570–600 °C), followed by a joint cooling history, ranging from 0.38 ± 0.14 at temperature from 450 to 520 °C. These inferred results were then used: a) to interpret the Lower Metapelite Group as a single crystalline basement unit exclusively affected by a complete Alpine orogenic cycle, according to the very similar features of P– T paths, comparable petrography and analogous structural characteristics; b) as a tool for more reliable correlations between the Aspromonte Massif, the other Calabrian terranes and the north African Orogenic Complexes. They may therefore consider a contribution to the geodynamic modelling of the western Mediterranean.

A zircon study from the Rhodope metamorphic complex, N-Greece: Time record of a multistage evolution

Zircons from an eclogite and a diamond-bearing metapelite near the Kimi village (north-eastern Rhodope Metamorphic Complex, Greece) have been investigated by Micro Raman Spectroscopy, SEM, SHRIMP and LA-ICPMS to define their inclusion mineralogy, ages and trace element contents. In addition, the host rocks metamorphic evolution was reconstructed and linked to the zircon growth domains. The eclogite contains relicts of a high pressure stage (ca. 700 °C and > 17.5 kbar) characterised by matrix omphacite with Jd 40–35. This assemblage was overprinted by a lower pressure, higher temperature metamorphic event (ca. 820 °C and 15.5–17.5 kbar), as indicated by the presence of clinopyroxene (Jd 35–20) and plagioclase. Biotite and pargasitic amphibole represent a later stage, probably related to an influx of fluids. Zircons separated from the eclogite contain magmatic relicts indicating Permian crystallization of a quartz-bearing gabbroic protolith. Inclusions diagnostic of the high temperature, post-eclogitic overprint are found in metamorphic zircon domain Z2 which ages spread over a long period (160 – 95 Ma). Based on zircon textures, zoning and chemistry, we suggest that the high-temperature peak occurred at or before ca. 160 Ma and the zircons were disturbed by a later event possibly at around 115 Ma. Small metamorphic zircon overgrowths with a different composition yield an age of 79 ± 3 Ma, which is related to a distinct amphibolite-facies metamorphic event. The metapelitic host rock consists of a mesosome with garnet, mica and kyanite, and a quartz- and plagioclase-bearing leucosome, which formed at granulite-facies conditions. Based on previously reported micro-diamond inclusions in garnet, the mesosome is assumed to have experienced UHP conditions. Nevertheless, (U)HP mineral inclusions were not found in the zircons separated from the diamond-bearing metapelite. Inclusions of melt, kyanite and high-Ti biotite in a first metamorphic zircon domain suggest that zircon formation occurred during pervasive granulite-facies metamorphism. An age of 171 ± 1 Ma measured on this zircon domain constrains the high-temperature metamorphic event. A second, inclusion-free metamorphic domain yielded an age of 160 ± 1 Ma that is related to decompression and melt crystallization. The similar age data obtained from the samples indicate that both rock types recorded a high-T metamorphic overprint at granulite-facies conditions at ca. 170 – 160 Ma. This age implies that any high pressure or even ultra-high pressure metamorphism in the Kimi Complex occurred before that time. Our findings define new constraints for the geodynamic evolution for the Alpine orogenic cycle within the northernmost Greek part of the Rhodope Metamorphic Complex. It is proposed that the rocks of the Kimi Complex belong to a suture zone squeezed between two continental blocks and result from a Paleo-ocean basin, which should be located further north of the Jurassic Vardar Ocean.

Crustal evolution of Western and Central Europe

Abstract A new Moho depth map has been assembled for Western and Central Europe and the Western Mediterranean area that is exclusively based on published regional Moho depth maps. Tectonic overlays summarize Caledonian and Variscan tectonic units, Permo-Carboniferous fault systems and magmatic provinces, Mesozoic and Cenozoic rift-wrench systems, areas of intraplate compression, the outlines of Alpine orogens and the distribution of oceanic crust. Based on a comparison of these overlays with the Moho depth map we assess processes that controlled the evolution of the crust in the various parts of Europe through time. The present-day crustal configuration of Western and Central Europe results from polyphase Late Palaeozoic to recent lithospheric deformation that overprinted the margin of the Proterozoic East European Craton and particularly the Caledonian and Variscan crustal domains. Following consolidation of the Caledonides, their crustal roots were destroyed in conjunction with Devonian wrench tectonics and back-arc rifting. During the Permo-Carboniferous tectonomagmatic cycle, wrench faulting disrupted the crust of the Variscan Orogen and its foreland and the lithosphere of these regions was thermally destabilized. Late Permian and Mesozoic re-equilibration of the lithosphere-asthenosphere system was interrupted by the development of the Arctic-North Atlantic, Tethyan and associated rift systems. During the Alpine orogenic cycle, intraplate compressional stresses controlled basin inversion-related crustal thickening and lithospheric folding, as well as the evolution of the Rhine-Rhone rift system. Variably deep crustal roots characterize the Alpine orogenic chains. Neogene back-arc extension disrupted the eastern Pyrenees, Betic-Balearic, Apennine and Dinarides orogens.

‘Verrucano’ and ‘Pseudoverrucano’ in the Central-Western Mediterranean Alpine Chains: palaeogeographical evolution and geodynamic significance

Abstract The Anisian-Carnian Verrucano Group of the Tuscan Metamorphic Units and the Triassic-Hettangian Pseudoverrucano Formation of the homonymous unit are mainly continental redbeds occurring in Tuscany at the base of the Alpine orogenic cycle. A study carried out throughout the Apennine, Maghrebian and Betic Chains emphasized the presence in all these orogenic belts of deposits more or less coeval and similar both to the metamorphic Verrucano and to the unmetamorphosed Pseudoverrucano. Thus, the distinction of Verrucano and Pseudoverrucano successions has a palaeogeographical and geodynamic importance at the scale of the Western Mediterranean. Both successions developed during the continental rift stage of Pangaea, which led to later break-up at the edges of a future microplate, interposed between the Europe, Africa and Adria-Apulia plates, but they are characterized by different tectonometamorphic evolution. Pseudoverrucano-like deposits, devoid of Alpine metamorphism, characterize the highest tectonic units of the nappe stack and they overthrust units bearing Verrucano-like deposits. These latter show an Alpine tectonometamorphic history marked during the Miocene by intense deformation and HP/LT metamorphism (at pressures in the range of 0.8–2 GPa), followed by a retrograde phase associated with decompression, suggesting subduction and subsequent exhumation of continental crust. Intriguing palaeogeographical problems arise from the analysis of Verrucanobearing units, because the same evolution seems to characterize both units considered to belong to a realm similar to that of the north-verging Austroalpine nappe system and some units referred to the south-verging fold-thrust belt derived from the Adria-Apulia palaeomargin.

“Siderite mineralization of the Gemericum superunit (Western Carpathians, Slovakia): review and a revised genetic model” [Ore Geology Reviews 24, 267–298]—a discussion

Radvanec et al. (2004) provided a revised version of their metamorphic-hydrothermal model for siderite and associated mineral deposits of the Western Carpathians. Compared to the previous version (Grecula, 1995; Grecula et al., 1996), the upgraded model emphasizes the dominant role of retrograde Variscan metamorphic processes and admits remobilization of the Variscan siderite and other ore components during the Alpine orogenic cycle. Similar to the previous versions, the Variscan ore-forming processes genetically related to metamorphism are believed to dominate not only in the siderite vein and metasomatic deposits, but also in the superimposed vein sulphides (p. 292). The revised model is unsustainable in light of recent knowledge. The major pitfalls are summarized and commented as follows.

Electron-microprobe dating of monazites from Western Carpathian basement granitoids: plutonic evidence for an important Permian rifting event subsequent to Variscan crustal anatexis

Accessory monazites from 35 granitoid samples from the Western Carpathian basement have been analysed with the electron microprobe in an attempt to broadly constrain their formation ages, on the basis of their Th, U and Pb contents. The sample set includes representative granite types from the Tatric, Veporic and Gemeric tectonic units. In most cases Lower Carboniferous (Variscan) ages have been obtained. However, a much younger mid-Permian age has been recorded for the specialised S-type granites of the Gemeric Unit, and several small A- and S-type granite bodies in the Veporic Unit and the southern Tatric Unit. This distinct Permian plutonic activity in the southern part of the Western Carpathians is an important, although previously little considered geological feature. It appears to be not related to the Variscan orogeny and is interpreted here to reflect the onset of the Alpine orogenic cycle, with magma generation in response to continental rifting. The voluminous Carboniferous granitoid bodies in the Tatric and Veporic units comprise S- and I-type variants which document crustal anatexis accompanying the collapse of a compressional Variscan orogen sector. The Variscan magmas were most likely produced through the remelting of a subducted Precambrian volcanic arc-type crust which included both igneous and sedimentary reworked volcanic-arc material. Although the 2σ errors of the applied dating method are quite large and typically ±10–20 Ma for single samples, it would appear from the data that the Variscan S-type granitoids (333–367 Ma) are systematically older than the Variscan I-type granitoids (308–345 Ma). This feature is interpreted in terms of a prograde temperature evolution in the deeper parts of the post-collisional Variscan crust. In accordance with recently published zircon ages, this study shows that the Western Carpathian basement must be viewed as a distinct "eastern" tectonomagmatic province in the Variscan collision zone, where the post-collisional crustal melting processes occurred ~20 Ma earlier than in the central sector (South Bohemian Batholith, Hohe Tauern Batholith).

Cretaceous evolution of a metamorphic core complex, the Veporic unit, Western Carpathians (Slovakia): P–T conditions and in situ40Ar/39Ar UV laser probe dating of metapelites

AbstractAlpine metamorphism, related to the development of a metamorphic core complex during Cretaceous orogenic events, has been recognized in the Veporic unit, Western Carpathians (Slovakia). Three metamorphic zones have been distinguished in the metapelites: 1, chloritoid + chlorite + garnet; 2, garnet + staurolite + chlorite; 3, staurolite + biotite + kyanite. The isograds separating the metamorphic zones have been modelled by discontinuous reactions in the system K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH). The isograds are roughly parallel to the north‐east‐dipping foliation related to extensional updoming along low‐angle normal faults. Thermobarometric data document increasing P–T conditions from c. 500 °C and 7–8 kbar to c. 620 °C and 9–10 kbar, reflecting a coherent metamorphic field gradient from greenschist to middle amphibolite facies. 40Ar/39Ar data obtained by high spatial resolution in situ ultraviolet (UV) laser ablation of white micas from the rock slabs constrain the timing of cooling and exhumation in the Late Cretaceous. Mean dates are between 77 and 72 Ma; however, individual white mica grains record a range of apparent 40Ar/39Ar ages indicating that cooling below the blocking temperature for argon diffusion was not instantaneous. The reconstructed metamorphic P–T–t path is ‘clockwise’, reflecting post‐burial decompression and cooling during a single Alpine orogenic cycle. The presented data suggest that the Veporic unit evolved as a metamorphic core complex during the Cretaceous growth of the Western Carpathian orogenic wedge. Metamorphism was related to collisional crustal shortening and stacking, following closure of the Meliata Ocean. Exhumation was accomplished by synorogenic (orogen‐parallel) extension and unroofing in an overall compressive regime.

Preliminary evidence for a Hercynian age of the Versoyen complex, western Alps

Abstract To date the magmatic event that generated the Versoyen mafic complex, four fractions of zircon from a cross-cutting leucogabbro dike, has been analyzed by the U–Pb method, defining a regression line that intercepts the concordia curve at 309 ± 6 (2 σ) Ma and 3 240 ± 34 Ma. These two ages can be interpreted to date, respectively, emplacement of the leucogabbro into the Versoyen complex, and the age of inherited Archean zircon cores, present in the newly formed crystals. The age of 309 Ma suggests that both Versoyen mafic magmatism and subsequent eclogite facies metamorphism belong to the Hercynian, and not the Alpine orogenic cycle.

Petroleum systems of Alpine-Mediterranean foldbelts and basins

Abstract The Alpine-Mediterranean area hosts a number of large and smaller hydrocarbon provinces in platforms, rifts, foreland basins and the frontal parts of thrust belts. These hydrocarbon provinces are related to a wide range of source rocks occurring in Palaeozoic, Mesozoic and Cenozoic series. Source rocks are alternatively localized in prerift, syn-rift or passive-margin sequences, the age of which is highly variable. Moreover, syn-orogenic foreland basin sequences can contain important source-rocks and are frequently the locus of biogenic gas generation. Most Mesozoic passive margins of the Western Tethys were destroyed during the Alpine orogenic cycle and were overthrusted by or incorporated into orogenic wedges, the external parts of which host a number of highly diversified petroleum systems. Late Variscan and Alpine compressional intraplate deformation had severe repercussions on the hydrocarbon habitat of some of the Peri-Tethyan platforms. Late Cretaceous and Palaeogene intracratonic and back-arc rifts host some important petroleum provinces; the Euphrates, Abu Gharadig and Sirte basins rely for hydrocarbon charge on syn-rift source rocks whereas post-rift series contain the source rocks of the Black Sea back-arc basin. Oligocene and younger rift systems host the petroleum provinces of the Gulf of Suez, Pelagian Shelf, Valencia Trough and Rhine Graben; these rely variably on pre- and syn-rift source rocks. The Neogene Pannonian system of transtensional back-arc basins relies for hydrocarbon charge mainly on syn-rift source rocks whereas the Vienna Basin is exclusively charged by Mesozoic post-rift source rocks. The complex evolution of the Western Tethys belt, contrasted thermal regimes and successive episodes of sedimentary and tectonic burial account for the great diversity of its petroleum systems. Each hydrocarbon province is characterized by a very distinct scenario for the accumulation of source rocks, timing of their maturation, hydrocarbon expulsion and migration from effective kitchens to potential trapping domains, and the preservation of hydrocarbon accumulations. Future exploration in the external parts of the Alpine orogenic belts, their forelands and beneath the Neogene fill of the Pannonian Basin is likely to yield further oil and gas discoveries. In contrast to the Black Sea Basin, the hydrocarbon potential of the Algero-Provençal, Tyrrhenian and Aegean back-arc basins is questionable. The thick sedimentary sequences of the deep East Mediterranean Basin may contain effective source rocks.

Heat sources of Cretaceous metamorphism in the Eastern Alps — a discussion

The Alpine orogenic cycle is characterised by a series of early metamorphic events that occurred in the Cretaceous — the Eoalpine events. A collection of data on peak metamorphic conditions. PT paths and on relationships between structure and metamorphism for these events in the Eastern Alps shows that: (1) Eoalpine metamorphism occurred largely simultaneously with pervasive deformation; (2) the Eoalpine thermal peak was reached near the pressure peak: (3) lower-grade, structurally higher rocks reached their metamorphic peak earlier than higher-grade, structurally lower rocks and there is a wide spread of metamorphic ages; (4) low-grade metamorphic conditions are probably more perturbed than high-grade conditions: and (5) there is a conspicuous regional distribution of metamorphic grade with amphibolite and eclogite facies metamorphism occurring predominantly in two regions located symmetrically west and east of the Tauern window. Possible heating mechanisms are discussed that can satisfy these observations and a review of previous thermal models is provided. It is shown that models which rely on conductive heat transfer, cannot account for all of these observations. In order to assess other heat sources, the effects of shear heating are evaluated here with simple model calculations and a straightforward comparison of length and time scales of dissipative heat production. The results suggest that shear heat, episodically released during the stacking of the Austroalpine nappe pile. may be a significant contributor to the observed thermal perturbation.

Determination of high spatial resolution argon isotope variations in metamorphic biotites

The circumstances of preservation of relict metamorphic minerals and fabrics which have subsequently been involved in later metamorphic cycles remain enigmatic because the kinetic parameters are not well understood. Textures and chemistry cannot always give direct information on the conditions attained in the successive events. We show in this study that 40 Ar 39 Ar laser studies can, in contrast, provide thermal history determinations and successfully discriminate between geological models. Two samples have been examined from the Seconda Zona Diorito Kinzigitica (IIDK) of the Sesia Zone, Italian Alps. A unit which preserves relict Hercynian metamorphic assemblages and is now located structurally above rocks which preserve Alpine eclogite and greenschist facies assemblages. The circumstances of this preservation remain controversial, with proposed models falling into two main categories. One hypothesis advocates that the IIDK was juxtaposed into its present position late in the Alpine orogeny and experienced no significant heating event. Alternatively the IIDK has been in its present position for the duration of the Alpine orogenic cycle and has been exposed to high temperatures but due to sluggish reaction kinetics, it failed to reequilibrate and develop Alpine metamorphic minerals. A high spatial resolution 40 Ar 39 Ar study using the ultraviolet laser ablation microprobe (UVLAMP) technique was undertaken with the aims of reassessing the metamorphic history of the IIDK and investigating the behavior of argon within the unit during its thermal evolution. 128 analyses have been obtained from ten profiles, between 60 and 275 μm long, across biotite grains which show apparent ages ranging from 514 to 161 Ma within individual crystals. These apparent age variations are systematic according to location in the grain, with the oldest ages at the grain edges and the youngest in the cores. These apparent age variations can not be explained by chemical alteration and require diffusion of excess argon into the grains. The age profiles show a remarkable fit to theoretically derived curves for volume diffusion of argon. Time integrated diffusion values obtained for the profiles fall between 2 × 10 −6 cm −6 and 5 × 10 -6 cm 2; this variation is interpreted as being due to spatial and temporal fluctuations in the availability of argon from the grain boundary network. Diffusion, and hence profile development, initiated at different times within the biotites, consequently producing the range of time integrated diffusion values. The temperatures at which the profiles developed have been estimated using published biotite diffusion parameters to model possible thermal histories. Using the assumptions of either a short thermal pulse event or continuous cooling from high temperatures, the models suggest that the profiles developed at temperatures less than 300°C. Possible geological models are proposed which document the thermal and excess argon history of the IIDK from the Hercynian through to the Alpine orogeny. The study shows the key role of detailed argon isotope studies in assessing the thermal evolution of rocks with relict metamorphic assemblages.

The evolution of the “Serie dei Laghi” (Strona-Ceneri and Scisti dei Laghi): the upper component of the Ivrea-Verbano crustal section; Southern Alps, North Italy and Ticino, Switzerland

The Serie dei Laghi is the upper-intermediate component of the exposed crustal section of Massiccio dei Laghi in the Southern Alps, the latter including also the Ivrea-Verbano Zone. Its oldest event is a sedimentary cycle at least of Early Paleozoic age. It starts with a pelitic sequence (Scisti dei Laghi) that later suffered a tectono-metamorphic event of rather high pressure. This event is recorded by rare mineral and textural relics detected in the Scisti dei Laghi of the M. Gambarogno area. Later, an arenaceous sequence (Strona-Ceneri) was deposited. It is now represented at the base by basaltic metatuffites resulting from the reworking of an ophiolite belt, then followed by paragneisses with detrital phenoclasts (Ceneri gneiss) and by fine-grained gneisses (“Gneiss minuti”). During the Ordovician, conspicuous tonalitic to granitic masses were emplaced. The induced thermal disturbance, and the circulation of pegmatitic stage magmatic residuals, caused contact metamorphism and granitization in the sedimentary country rocks. During the Hercynian, a main prograde event generated the regional foliation, with isoclinal geometry and transposed character. A syn-kinematic stage produced medium-grade parageneses of relatively high pressure; static recrystallization and sillimanite growth then followed. During the uplift, due to the combined action of hydration and decompression, margarite-bearing coronitic textures grew around kyanite. A retrograde metamorphism was then recorded by the appearance of greenschist facies parageneses, at particular structural sites. The formation of the Cossato-Mergozzo-Brissago (CMB) and Pogallo fault system against the Ivrea-Verbano Zone, as well as the “Schlingen” folds in the northeast, is of late-Hercynian age. Along the CMB fault, calc-alkaline dykes and stocks (appinites) were intruded, probably towards the end of the upheaval, but deep enough to induce partial melting in their country rocks (between Val d'Ossola and Valsesia). The Hercynian evolution is concluded by the Early Permian magmatic cycle that gave rise to both extrusive rhyolitic sheets and to granitic epiplutonic bodies accompanied by mafic dykes. In Mesozoic times the Serie dei Laghi was dissected by many extensional faults. These partly reactivated the Hercynian lineaments, such as the CMB and Pogallo Lines. During the Alpine orogenic cycle, the Serie dei Laghi was affected by S-verging cataclastic overthrusts.

Iberia Versus Europe--Effects of Continental Break-Up and Round-Up on Hydrocarbon Habitat: ABSTRACT

Based on the continuity of foldbelts and the positions of intermountain continental nuclei and transcontinental megashears, a close Pangea fit is proposed for the central and north Atlantic borderlands. The Variscan arch segment missing between Brittany and Galicia in the Gulf of Gascony (Biscaye) can tentatively be identified with the Flemish Cap block off Newfoundland. At the same time the northwest African-Gondwana border (central Morocco) was located some 800 km farther to the west-northwest, as compared to its present position in southwestern Europe (Iberia). During the opening of the central and northern segments of the Atlantic Ocean (Jurassic and Cretaceous) and during the closure of the western Mediterranean basin, i.e., the thrust of Africa toward southern Europe (Tertiary), the European continental mass underwent deformation in the transtensive and transpressive modes, which reactivated parts of its inherited structural network. The trailing south European continental margin was partially dismembered into loosely bound continental blocks, to be assembled again during the subsequent Alpine orogenic cycle. These events can be compared with processes known in the northernmost and western segments of the North American continent. Mechanisms are proposed for the formation and deformation of inter- and intraplate basins by way of moderate shifts (wrenching)more » and slight rotations, the direction of which changed during the Mesozoic-Tertiary according to the global stress field. The above evolution and mechanisms had multiple and decisive effects on hydrocarbon generation, habitat, and accumulation.« less