Alpine Nappes Research Articles

The Vienna Basin: Petroleum Systems, Storage and Geothermal Potential

The Vienna Basin is located in the transition zone between the Alps and the Carpathians. Its tectonostratigraphic structure is complex, and includes from base to top crystalline basement, autochthonous Mesozoic sediments, Cenozoic foreland basin deposits, the Alpine nappe system and a thick Neogene basin fill. The Vienna Basin area hosts one of the major hydrocarbon provinces of Central Europe, but also a significant geothermal potential. The petroleum system is mainly based on the Upper Jurassic Mikulov source rock in the authochthonous succession, which became mature during nappe stacking and Neogene basin subsidence. Migration along faults and via basement highs filled oil and gas reservoirs in all tectonostratigraphic units. Despite a 100-year-long exploration history, new discoveries are still being made. Two geothermal systems are known in the Vienna Basin. One system is based on hot fluids ascending along the major faults in the southern part of the basin and has been used for balenological purposes since Roman times. The other is based on deeply buried, highly permeable carbonate rocks in the Alpine wedge (e.g. Hauptdolomite Formation) and clastic reservoirs in the Neogene basin fill. Current projects aim to supply the city of Vienna with geothermal heat from Lower Miocene fluvial conglomerates.

From nappe stacking to strike-slip deformation: Alpine structural overprints refined by cave and karst geology in the Danubian thin-skinned units (Southern Carpathians, Romania)

Caves are ideal environments for preserving quantifiable deformational indicators in orogenic areas, as they are conditioned by regional tectonics. The caves and karst of Isverna, developed in the Barremian-Aptian limestones of the Danubian sedimentary nappes (Southern Carpathians, SW Romania), expose formerly undetected evidence of compressional tectonics, overlapping older structures related to décollement and deformation of the underlying Turonian-Senonian tectonic mélange. The Danubian domain (the distal part of the Moesian Platform) was incorporated into the Carpathian Orogen during the Late Cretaceous subduction of the Ceahlău–Severin Ocean and collision with the continental Dacia mega-unit. Subsequent Eocene–Oligocene orogen–parallel extension led to the development of a metamorphic core complex and detachment faults, constructing a complicated arcuate fault system around the Moesian Platform during the Late Miocene strike-slip deformation. The integrated analysis of structural, kinematic, and geomorphological data indicates a connection between the strike-slip deformation and the subsequent shortening, exhumation and surface exposure of Cerna Nappe within the Isverna shear zone. Four main evolutionary stages of the Danubian thin-skinned units in the central Mehedinți Mountains were distinguished from cave and karst geology, and illustrated in a detailed 3D model: i) Initial décollement and mélange deformation during the Alpine nappe stacking; ii) Extensional décollement toward SE; iii) Dextral shearing and WNW-ESE contraction; iv) Karstification and cave development. Most structural and kinematic markers recorded within the limestones of the Cerna Nappe date from stage (iii), whereas older structures were better preserved in the tectonic mélange of the Lainici Nappe. The resulting model could be further integrated into the polyphase tectonic evolution of the Southern Carpathians and their relation with the Moesian Platform. This study demonstrates the utility of caves and karst for constraining the chronology of tectonic deformations in complex structural systems and for reconstructing more refined conceptual 3D models.

Large-volume Permian felsic volcanism in the Tisza Mega-unit (East-Central Europe): Evidence from mineralogy, petrology, geochemistry, and geochronology

Permian felsic volcanic rocks are widespread in the Tisza Mega-unit (Carpathian–Pannonian region), covering a relatively large area from southern Transdanubia (Hungary) to the Apuseni Mountains (Romania). The present occurrence and structural position of these Permian volcanic rocks are the result of a complex tectonic evolution (including Alpine nappe stacking and Neogene extension); meanwhile, multiple hydrothermal effects and deformation modified their microtexture and chemical composition. In this study, the same emphasis was given to Permian felsic volcanism as well as the subsequent fluid- and/or deformation-related processes in the Tisza Mega-unit to extend our knowledge in regional issues (e.g., geochronology, stratigraphy, and lithologic correlations) and to investigate if the variably altered volcanic rocks can be used in petrogenetic studies.The zircon U-Pb dating yields ages of ∼270.4–262.9 Ma for the Apuseni samples and ∼268.4–260.2 Ma for those of the Pannonian Basin. Whole-rock geochemistry, considering immobile trace elements, since most of the major element compositions were severely modified by alteration processes, indicates that the studied rocks are dacite to rhyolite. They belong to a voluminous silicic volcanism during the Middle Permian, most probably related to rifts.Petrography, whole-rock geochemistry, and X-ray powder diffraction mineralogy revealed that the predominant alteration of the studied rocks is hydrothermal sericitization, locally superimposed by mylonitization in greenschist to subgreenschist conditions during Alpine orogeny. The K-Ar dating of the separated illite fractions confirms that the ductile deformation acted during the Turonian nappe stacking, as the main Alpine deformation phase in the study area.On a regional scale, the Permian magmatism in the Tisza Mega-unit looks significantly younger than the similar magmatic activity of the stable Europe (∼300–290 Ma).

The Portofino Conglomerate (Eastern Liguria, Northern Italy): Provenance, Age and Geodynamic Implications

The Portofino Conglomerate (PC) cropping out in the Eastern Liguria is an approximately 500 m thick, very gently folded succession mainly composed of poorly bedded and mostly matrix-supported conglomerates. It stratigraphically rests on the Helminthoid Flysch (UA3) thrusted onto the Antola Unit. We vertically distinguished three mostly ruditic litho/petrofacies: (i) Paraggi (fP) with carbonate clasts from an Helminthoid Flysch succession; (ii) Monte Pallone (fMP) with prevailing carbonate and meta-carbonate clasts and minor quartz-rich (meta)siliciclastic and high-pressure–low-pressure (HP-LP) metabasite clasts; and (iii) Monte Bocche (fMB) with dominant quartz-rich (meta)siliciclastic, meta-carbonate clasts, and minor granitoid elements and medium-temperature–high-temperature (MT-HT) regional metamorphic rocks. The middle-upper Eocen age of Paraggi litho/petrofacies is constrained by well-preserved microforaminifers (e.g., Globigerinatheka) recovered in the matrix. During its sedimentation, the directions of the paleocurrents would indicate that the PC underwent a counterclockwise rotation coeval with the first Cenozoic rotational phase of the Sardinia–Corsica system (50–30 Ma) and then stopped before the sedimentation of the Monte Pallone and Monte Bocche litho/petrofacies. The vertical compositional variation in the sedimentary inputs suggested that the PC is the result of a progressive deepening of the erosional level of a tectonic pile that can be located in the Ligurian Alps Chain. We considered the PC as the likely apical part of a submarine fan deposited in a piggy-back/thrust-top basin within the Alpine nappe stack. This sedimentary body was later tectonically transported eastward with its UA3 Helminthoid Flysch substrate (similarly to Epiligurian Units of the Northern Apennines) onto the Apenninic orogenic system (i.e., the Antola Unit).

Rift-related paleogeography of the European margin in the Eastern Alps (Central Tauern Window)

Continent-derived tectonic units in the Tauern Window of the Alps exhibit stratigraphic and structural traces of extension of continental margins eventually leading to the opening of the Alpine Tethys. In this study, we reassess lithostratigraphic data from the central part of the Tauern Window to reconstruct the post-Variscan evolution of this area, particularly the rift-related geometry of the European continental margin. The lithostratigraphy of the Alpine nappes reflects systematic variations of the structure of the European margin. The lowest tectonic units (Venediger nappe system, Eclogite Zone and Trögereck Nappe) are characterized by a thick succession of arkose-rich Bündnerschiefer-type sediments of probably Early Cretaceous age that we interpret as syn-rift sequence and which stratigraphically overlies thinned continental basement and thin pre-rift sediments. In contrast, the highest tectonic unit derived from Europe (Rote Wand Nappe) preserves a thick pre-rift sedimentary sequence overlying thinned continental basement, as well as a thick syn- to post-rift succession characterized by turbiditic Bündnerschiefer-type sediments of probable Cretaceous age. These observations point towards a highly segmented structure of the European rifted margin. We propose that this involved the formation of an outer margin high, partly preserved in the Rote Wand Nappe, that was separated from the main part of the European margin by a rift basin overlying strongly-thinned continental crust. The along-strike discontinuity of the Rote Wand Nappe is proposed to reflect the lateral variation in thickness of the outer margin high that resulted from margin-parallel segmentation of the European continental crust during highly oblique rifting antecedent to the opening of Alpine Tethys.

Collision with Gondwana or with Baltica? Ordovician magmatic arc volcanism in the Marmarosh Massif (Eastern Carpathians, Ukraine)

The pre-Alpine Marmarosh Massif is a tectonically complex unit of the crystalline basement within the Eastern Outer Carpathians. In the eastern (Ukrainian) segment of this massif, two units have been identified—the Bilyi Potok Nappe and the Dilove Nappe. Petrological investigations coupled with zircon U–Pb dating were performed on metavolcanic rocks (porphyroids) and their phyllite host rocks, sampled from three locations within the Dilove Nappe. The geochemical characteristics of the meta-rhyodacite porphyroids revealed a volcanic arc affinity of the protolith, with U–Pb zircon ages of 452.8 ± 1.5 Ma and 461.5 ± 1.6 Ma and zircon saturation temperatures in the range of 823–892 °C. The phyllite host rocks (meta-tuff) yield a U–Pb zircon maximum estimate for the eruption age at 584.7 ± 2.9 Ma. Peak amphibolite-facies metamorphism (M1) was estimated at the pressure of 600–900 MPa with a temperature range of 560–600 °C. Retrogression (M2), possibly related to Alpine nappe stacking and shearing, is assumed to have taken place at 200–250 MPa and 384–222 °C. The volcanic arc is interpreted as an early Caledonian arc that was subsequently accreted to the margin of Baltica during the closure of the easternmost Tornquist Ocean rather than Cenerian (early Paleozoic) orogenic events on the margin of East Gondwana.

Deformation of the European Plate (58-0 Ma): Evidence from Calcite Twinning Strains

We present a data set of calcite twinning strain results (n = 209 samples; 9919 measured calcite twins) from the internal Alpine nappes northwestward across the Alps and Alpine foreland to the older extensional margin along the Atlantic coast in Ireland. Along the coast of Northern Ireland, Cretaceous chalks and Tertiary basalts are cross-cut by calcite veins and offset by calcite-filled normal and strike-slip faults. Both Irish sample suites (n = 16 with four U-Pb vein calcite ages between 70–42 Ma) record a sub-horizontal SW-NE shortening strain with vertical extension and no strain overprint. This sub-horizontal shortening is parallel to the margin of the opening of the Atlantic Ocean (~58 Ma), and this penetrative fabric is only observed ~100 km inboard of the margin to the southeast. The younger, collisional Alpine orogen (~40 Ma) imparted a stress–strain regime dominated by SE-NW sub-horizontal shortening ~1200 km northwest from the Alps preserved in Mesozoic limestones and calcite veins (n = 32) in France, Germany and Britain. This layer-parallel shortening strain (−3.4%, 5% negative expected values) is preserved across the foreland in the plane of Alpine thrust shortening (SE-NW) along with numerous outcrop-scale contractional structures (i.e., folds, thrust faults). Calcite veins were observed in the Alpine foreland in numerous orientations and include both a SE-NW layer-parallel shortening fabric (n = 11) and a sub-vertical NE-SW vein-parallel shortening fabric (n = 4). Alpine foreland strains are compared with twinning strains from the frontal Jura Mountains (n = 9; layer-parallel shortening), the Molasse basin (n = 26; layer-parallel and layer-normal shortening), Pre-Alp nappes (n = 39; layer-parallel and layer-normal shortening), Helvetic and Penninic nappes (Penninic klippe; n = 46; layer-parallel and layer-normal shortening plus four striated U-Pb calcite vein ages ~24 Ma) and calcsilicates from the internal Tauern window (n = 4; layer-normal shortening). We provide a chronology of the stress–strain history of the European plate from 58 Ma through the Alpine orogen.

A regional scale Cretaceous transform fault zone at the northern Austroalpine margin: Geology of the western Ammergau Alps, Bavaria

Abstract We reinvestigated parts of the northern Austroalpine margin and provided structural and kinematic field data in order to interpret the kinematic relationship between the Cenoman-Randschuppe (CRS) marginal slice, Falkensteinzug (FSZ), Tannheim- and Karwendel thrust sheets occurring in a narrow strip at the northern front of the northwestern Northern Calcareous Alps (NCA). As a consequence, we propose a revised model for the tectonic evolution of the northern Austroalpine margin. As thrusting propagates from SSE to NNW (Cretaceous orogeny), the Karwendel thrust sheet (including its frontal part, the FSZ) was emplaced onto the Tannheim thrust sheet in the Albian, deduced from (i) upper-footwall deposits, the youngest sediments below the Karwendel thrust (Tannheim- and Losenstein Fms.), and (ii) thrust-sheet-top deposits unconformably overlying the deeply eroded northern Karwendel thrust sheet (Branderfleck Fm.). The future CRS marginal slice was, at that time, part of the foreland of this Early Cretaceous Alpine orogenic wedge. Pervasive overprint by sinistral shear within the CRS marginal slice and northern Tannheim thrust sheet suggests sinistral W-E striking transform faults cutting across this foreland, decoupling CRS marginal slice and FSZ from the main body of the NCA and enabling an independent evolution of the CRS marginal slice from the Early Cretaceous onwards. Subsequent Late Cretaceous and younger shortening leads to successive incorporation of Arosa zone, Rhenodanubian Flysch (RDF) and Helvetic units into the Alpine nappe stack; the Tannheim thrust representing the basal thrust of the NCA. Growth strata within thrust-sheet-top deposits (Branderfleck-Fm.) give evidence for refolding of thrust sheet boundaries. In a typical thin-skinned fold-and-thrust belt, deformation should cease towards the thrust front, whereas within the NCA it increases. An Austroalpine thrust front controlled by E-trending transform faults could cause an increase in deformation towards the most external NCA and explain the absence of the Arosa zone between Allgäu and Vienna. Such faults would most probably also cut out Lower Austroalpine units. Therefore, RDF and CRS marginal slice are juxtaposed; the latter found in the tectonic position of the Arosa zone. The presence of transform faults underlines the strong imprint of the opening of the North Atlantic Ocean on the depositional setting and tectonic evolution of the NCA.

Crustal exhumation and depocenter migration from the Alpine orogenic margin towards the Pannonian extensional back-arc basin controlled by inheritance

The formation and deformation history of back-arc basins play a critical role in understanding the tectonics of plate interactions. Furthermore, opening of extensional back-arc basins during the overall convergence between Africa and Europe is a fundamental process in the overall tectonic evolution of the Mediterranean and adjacent areas. In this frame, Miocene tectonic evolution of the western Pannonian Basin of Central Europe and its connection to inherited Cretaceous structures of the Eastern Alpine nappes are presented. Revision of published and addition of new structural and thermochronological data, as well as seismic profiles from the western Pannonian Basin is complemented by high-resolution thermo-mechanical numerical modeling in order to propose a new physically consistent tectono-sedimentary model for the basin evolution. The onset of extension is dated as ~25–23 Ma, and higher rates are inferred between 19 and 15 Ma at the south-western part of the area (Pohorje, Kozjak Domes, Murska Sobota Ridge, and Mura-Zala Basin). Rift initiation involved the exhumation of the middle part of the Austroalpine nappe pile along low-angle detachment faults and mylonite zones. The Miocene low-angle shear zones could reactivate major Cretaceous thrust boundaries, the exhumation channel of ultra-high-pressure rocks of the Pohorje Dome, or Late Cretaceous extensional structures. Miocene extension was associated with granodiorite and dacite intrusions between 18.64 and 15 Ma. The Pohorje pluton intruded at variable depth from ~4 to 16–18 km and experienced ductile stretching, westward tilting, and asymmetric exhumation of its eastern side. Terrestrial early Miocene (Ottnangian to Karpatian, 19–17.25 Ma) syn-rift depositional environment in supradetachment basins evolved to near-shore and bathyal one by the middle Miocene (Badenian, 15.97–12.8 Ma). Deformation subsequently migrated eastwards to the western part of the Transdanubian Range (Keszthely Hills) and to newly formed grabens. In this formerly emerged terrestrial area active faulting started at 15–14.5 Ma and continued through the late Miocene almost continuously up to ~8 Ma but basically terminated in the Mura-Zala Basin by ~15 Ma (early Badenian). These observations suggest a ~200 km shift of active faulting, basin formation, and related syn-tectonic sedimentation from the SW (Pohorje and Mura-Zala Basin) toward the Pannonian Basin center. Building on the above described observational and modeling data makes the Pannonian Basin an ideal natural laboratory for understanding the coupling between deep Earth and surface processes.

Az Alföld metamorf aljzata: a köztes tömegtől a tarka mozaikig

A hozzáférhető fúrómaganyag mennyiségének növekedésével, valamint a kőzettani módszerek fejlődésével párhuzamosan az elmúlt évtizedekben alapvetően megváltozott a modellünk az Alföld metamorf képződményekből álló medencealjzatáról. A korábbi egyveretű, statikus aljzat kép helyét egy nagyon eltérő metamorf fejlődéstörténetű egységekből álló, tarka mozaik vette át. Ez a kőzettani sokszínűség az 1960-as évekig feltételezett merev, „köztes tömeg” elképzeléssel szemben a variszkuszi orogenezis óta a szerkezeti mozgásokban aktív szerepet játszó metamorf litoszféra dinamikájának eredményeként alakulhatott ki. Dolgozatunkban nyolc aljzati mintaterületen mutatjuk be a felépítő fő metamorf képződményeket, azok evolúcióját, valamint térbeli kapcsolatrendszerüket. A vizsgált területek Jánoshalma, Kiskunhalas-Tázlár, Szank, Csólyospálos-Kömpöc, Szeghalom, Mezősas-Furta, Dorozsma és Algyő. A kőzetblokkok térbeli helyzetének rekonstrukciója során a részletes petrológiai elemzés mellett négy esetben lyukgeofizikai információt is integráltunk a modellekbe.
 Az eredmények alapján az Alföld metamorf aljzatát a vizsgált területeken különböző szerkezeti mintázatok jellemzik. Kiskunhalason alsó szerkezeti helyzetű ortogneisz-domináns egység fölött széles, különböző kiindulási kőzettípusok deformációjával keletkezett milonitzóna, majd azt követően kis metamorf fokú fillit következik. Az eltérő fejlődésű egységeket elválasztó képlékeny nyírási zóna lapos szögű, közel északi dőlésű. A Szank, a Szeghalom, valamint a Mezősas-Furta területeket három, eltérő metamorf P-T (nyomás-hőmérséklet) fejlődést mutató egység építi fel, melyeket töréses deformációval jellemzett kataklázit zónák választanak el egymástól. Ezek a DNy-i dőlésű nyírási zónák feltételezhetően az alpi takarórendszer valamely egységéhez tartoznak. Szintén kréta korú feltolódásként értelmezzük a Dorozsma területen felismert aljzati blokkokat elválasztó széles kataklázit zónát. Itt, valamint a szomszédos Algyő területen ezt a felépítést felülírják a medence harmadidőszaki keletkezésével szinkron metamorf magkomplexum kialakulásához köthető szerkezetek. Mindezek komplex eredményeként jelenleg inkompatibilis metamorf fejlődésű egységek alkotják az Alföld déli részének metamorf aljzatát.
 A különböző mintaterületeken azonosított szerkezeti mintázatok megfelelő alapot biztosítanak az Alföld metamorf aljzatának litosztratigráfiai újragondolása során is. Így az egyes, litológiailag önmagukban is rendkívül heterogén egységeket (komplexumokat) célszerű ezen mintázatok alapján definiálni és lehatárolni.

The connection between the Alps and the Carpathians beneath the Pannonian Basin: Selective reactivation of Alpine nappe contacts during Miocene extension

Abstract The Cretaceous basement of the broader NW Pannonian Basin, including the Vienna Basin, has an Alpine nappe structure that can be correlated between the Eastern Alps and the Western Carpathians. For the first time in this region, based on the systematic integration of petroleum industry vintage 2D reflection seismic data with well data from Austria, Hungary and Slovakia, we document various modes of interaction between pre-existing thrust and superimposed normal faults. Whereas in some cases the pre-existing Cretaceous nappe contacts became extensional low-angle normal faults, the typical scenario is that the Miocene normal faults obliquely cut across them. The style of this negative inversion, i.e. contraction followed by extension exploiting the same fault system, is based on the orientation and geometry of the Alpine nappes in relation to the successor extensional basins. We expand our earlier findings made in the Hungarian sector of the NW Pannonian Basin where Neoalpine (Neogene) low-angle normal faults did indeed interact with abandoned Eoalpine (Cretaceous) and Mesoalpine (Paleogene) thrust fault planes, but in a more selective manner than expected before. The subsurface examples of this paper are located in the Austrian part of the Vienna Basin, the Hungarian, Austrian and Slovakian segments of the broader Danube Basin. Our new regional and local scale subsurface observations also helped to define a very large, but so far unrecognized Eocene antiformal nappe stack beneath the Danube Basin. Presently this is a highly dismembered and dominantly subsurface structure concealed beneath the NW Pannonian Basin between the Eastern Alps and the Western Carpathians. This finding has important implications for the understanding of the Alpine evolution of the broader region.

Metamorphic Response to Alpine Thrusting of a Crustal-scale Basement Nappe in Southern Calabria (Italy)

Abstract Structural, geophysical and age data indicate that the tilted cross-section of the Variscan continental crust exposed in the Serre of southern Calabria forms the uppermost Alpine nappe (‘Serre nappe’) of three Variscan basement slices derived from the southern European margin. This Alpine nappe stack is a fragment of the western Mediterranean Alps and rests now, after Miocene emplacement, on top of the Apennine carbonate platform. We report for the first time a P–T path for prograde Alpine metamorphism, which is restricted to the two lower nappes (Castagna and Bagni nappes) that are squeezed between cooler tectonic units, the Serre nappe above and the Apennine platform below. Therefore, we attribute their metamorphism to tectonic loading and concomitant shear heating during Eocene south-directed overthrusting of the crustal-scale Serre nappe. In the underlying Castagna nappe, Alpine metamorphism is only locally recorded, mainly by new growth of garnet, forming at the expense of retrogressed Variscan biotite dated at 43 Ma. The local existence of Alpine besides relict Variscan mineral assemblages in the strongly but heterogeneously overprinted rocks allows for characterization of metamorphic evolutions during both the Alpine and Variscan orogeneses in the former intermediate level of the Variscan crust of Calabria. The metamorphic evolutions have been reconstructed through P–T pseudosection modelling for Al-rich metasediments. In the Castagna nappe, rarely preserved Variscan garnet–sillimanite–biotite–ilmenite–plagioclase–quartz (± K-feldspar ± Si-poor white mica) assemblages formed under amphibolite-facies subsolidus conditions (650 ± 60 °C, 4·0 ± 0·5 kbar). During subsequent decompression and cooling to greenschist-facies conditions garnet was replaced by biotite–sillimanite and later by white mica–chlorite intergrowths. Retrogression of Variscan biotite is evidenced by the exsolution of ilmenite along grain boundaries and cleavages, textures that were subsequently overgrown by Alpine garnet coexisting with Si-rich white mica, rare chloritoid (in metapelites), and hornblende (in metagreywackes). Alpine garnet shows prograde zoning and is Ca-rich, and thus is distinct from unzoned and Ca-poor Variscan garnet porphyroblasts. Estimated conditions (520 ± 40 °C, 8·0 ± 1·0 kbar) record elevated pressures during Alpine lower amphibolite-facies metamorphism. In the lowermost Bagni nappe, rare prograde-zoned, Ca-rich garnet in strongly retrogressed mylonitic quartz-phyllites allows isopleth thermobarometry, which indicates lower amphibolite-facies conditions (555 ± 10 °C, 7·4 ± 0·3 kbar) resembling those for Alpine garnet growth in the Castagna nappe. The similar clockwise P–T paths for prograde Alpine metamorphism and the consistent peak pressures of 7–9 kbar in the Castagna and Bagni nappes point to a joint short-lived metamorphism during overthrusting of the crustal-scale Serre nappe within the south European margin during the north-directed subduction of the Alpine Tethys. South-directed overthrusting of the now tilted Variscan crustal section of the Serre along the up to 500 m thick mylonite horizon of the Curinga–Girifalco Line is in agreement with seismic data indicating an extended, few kilometres thick low-velocity zone (Bagni and Castagna nappes and mylonites of the Curinga–Girifalco Line) below the exposed lower crustal section of the Serre nappe. Alpine tectonic transport direction, timing and metamorphic conditions described here are consistent with those reported from the Aspromonte area in southernmost Calabria, suggesting a coeval Alpine history characterized by metamorphism owing to nappe loading and concomitant shear heating. The Alpine subduction–erosion–accretion processes inferred here for the Calabrian basement nappes resemble those proposed for the Dent Blanche nappe system in the Western Alps.

Relationship between magnetic fabrics and deformation of the Miocene Pohorje intrusions and surrounding sediments (Eastern Alps)

The Miocene deformation history of magmatic and host metamorphic rocks and surrounding sediments was reconstructed by measuring meso- and microscale structures and anisotropy of magnetic susceptibility (AMS) data in order to constrain the structural evolution of the Pohorje pluton during the onset of lithospheric extension at the Eastern Alps–Pannonian Basin transition. Principal AMS axes, lineation and foliation are very similar to mesoscopic lineation and foliation data from the main intrusive body and from some dykes. Although contribution from syn-magmatic texture is possible, these structures were formed during the cooling of the pluton and associated subvolcanic dykes just shortly after the 18.64 Ma pluton intrusion. Dykes emplaced during progressively younger episodes reflect decreasing amount of ductile strain, while firstly mesoscopic foliation and lineation, and then the tectonic AMS signal gradually disappears. In the structurally highest N–S trending dacite dykes, the AMS fabric only reflects the magmatic flow. The Miocene sediments underwent the same, NE–SW to E–W extension as the magmatic and host metamorphic rocks as indicated by both AMS and fault-slip data. All these events occurred prior to ~ 15 Ma, i.e., during the main syn-rift extension of the Pannonian Basin and during the fastest exhumation of the Tauern and Rechnitz windows, both demonstrating considerable extension of diverse crustal segments of the Alpine nappe pile. After a counterclockwise rotation around ~ 15 Ma, the maximum stress axis changed to a SE–NW orientation, but it was only registered by brittle faulting. During this time, the overprinting of a syn-rift extensional AMS texture was not possible in the cooled or cemented magmatic, metamorphic and sedimentary rocks.

Metamorphic pressure variation in a coherent Alpine nappe challenges lithostatic pressure paradigm

Pressure–temperature–time paths obtained from minerals in metamorphic rocks allow the reconstruction of the geodynamic evolution of mountain ranges under the assumption that rock pressure is lithostatic. This lithostatic pressure paradigm enables converting the metamorphic pressure directly into the rock’s burial depth and, hence, quantifying the rock’s burial and exhumation history. In the coherent Monte Rosa tectonic unit, Western Alps, considerably different metamorphic pressures are determined in adjacent rocks. Here we show with field and microstructural observations, phase petrology and geochemistry that these pressure differences cannot be explained by tectonic mixing, retrogression of high-pressure minerals, or lack of equilibration of mineral assemblages. We propose that the determined pressure difference of 0.8 ± 0.3 GPa is due to deviation from lithostatic pressure. We show with two analytical solutions for compression- and reaction-induced stress in mechanically heterogeneous rock that such pressure differences are mechanically feasible, supporting our interpretation of significant outcrop-scale pressure gradients.

A regional Alpine graphite décollement level beneath the NW Pannonian Basin

The so-called Transdanubian Conductivity Anomaly (TCA) of the Hungarian part of the NW Pannonian Basin has been well known for more than five decades. The exceptionally low resistivity (i.e. 1–2 Ωm) zone has a very large areal extent (on the order a few thousand km2) and it is an entirely subsurface anomaly occurring at depth between circa 3–15 km, with no corresponding outcrops. Various geological explanations of this enigmatic crustal-scale geophysical anomaly range from invoking sub-horizontal Alpine nappe contacts to sub-vertical dikes with graphite and/or saline fluid content. Only one possible analogue outcrop area was considered for the high conductivity anomaly so far, namely the Drauzug/Gailtal area of the Eastern Alps in Austria, some 300 km to the West from the TCA area. Previous attempts to find correspondence between the TCA and prominent seismic reflectors seen on 2D seismic reflection profiles were based on data acquired by research institutions. This study systematically correlates, for the first time, the TCA with 2D industry seismic reflection data in the same area. Our new results show a very strong correlation between the subsurface extent and location of the TCA with various sub-horizontally oriented Cretaceous Alpine nappe surfaces. In addition, we draw on the latest structural correlation of the Alpine nappe stack of the Transdanubian Range with its proper tectonic counterpart in the Eastern Alps.At the southern edge of the Upper Austroalpine units in northern Styria, in the Veitsch Nappe of the Greywacke Zone, numerous graphite localities are known historically. These laterally extensive graphite units in NW Styria formed as the result of greenschist-grade metamorphism of a Carboniferous coal sequence during the Cretaceous. For the first time, we describe here one well penetration of possibly age-equivalent graphitic units in NW Hungary. Correlation of the magnetotelluric anomaly with the distinct reflection seismic signature suggests that the same Palaeozoic graphitebearing Upper Austroalpine units should be present at 3–15 km depth in our study area.Therefore we propose that the best explanation for the observed extent and geometry of the TCA is the presence of graphite in subhorizontal, tectonically thinned detachment surfaces at the base of the Upper Austroalpine nappe edifice of NW Hungary

Evaluating igneous sources of the Taveyannaz formation in the Central Alps by detrital zircon U–Pb age dating and geochemistry

Late Palaeogene syn-tectonic volcanic products have been found in the Northern Alpine foreland basin and in the South Alpine hemipelagic basin. The source of abundant volcanic fragments is still in debate. We analyzed the geochronology and geochemistry of detrital zircons, and evaluated their temporal and genetic relationships with potential volcanic sources. The study shows that the detrital zircon U–Pb age patterns have two major age groups: a dominance (ca. 90%) of pre-Alpine zircons was found, as commonly observed in other Alpine flysch formations. These zircons apparently derived from erosion of the early Alpine nappe stack in South Alpine and Austroalpine units. Furthermore, a few Neo-Alpine zircons (ca. 10%) have ages ranging from Late Eocene to Early Oligocene (~ 41–29 Ma). Both source materials were mixed during long riverine transport to the basin margins before being re-deposited by gravity flows. These Palaeogene ages match with the activity of Peri-Adriatic magmatism, including the Biella volcanic suite as well as the Northern Adamello and Bergell intrusions. The values of REE and 176Hf/177Hf(t) ratios of the Alpine detrital zircons are in line with the magmatic signatures. We observe an in time and space variable supply of syn-sedimentary zircons. From late Middle Eocene to Late Eocene, basin influx into the South Alpine and Glarus (A) basins from the Northern Adamello source is documented. At about 34 Ma, a complete reorganisation is recorded by (1) input of Bergell sources into the later Glarus (B) basin, and (2) the coeval volcaniclastic supply of the Haute-Savoie basin from the Biella magmatic system. The Adamello source vanished in the foreland basin. The marked modification of the basin sources at ~ 34 Ma is interpreted to be initiated by a northwestern shift of the early Alpine drainage divide into the position of the modern Insubric Line.

Detrital zircon age patterns from turbidites of the Balagne and Piedmont nappes of Alpine Corsica (France): Evidence for an European margin source

At the front of metamorphic Cenozoic Alpine nappe of Schistes Lustrés, Western Alpine Corsica (France) exposes non- to very low grade metamorphic nappes, such as the Piedmont nappes, Upper nappes, and the Balagne nappe. The provenance of the Balagne nappe remains still opened: an origin close to the Corsican continental margin; or an origin far East from the Corsican margin toward the “Apenninic” oceanic domain. This would constrain that the Balagne ophiolite be derived from the opposite OCT (Ocean - Continent transition), close to a microcontinent located to the East of the Mesozoic Corsican margin. A systematic U-Pb dating of 586 detrital zircon grains collected from the turbidites in the Balagne and Piedmont nappes has been performed to constrain the source of sediments. The zircon grains yield U-Pb age spectra ranging from Neoarchean to Late Paleozoic with age peaks at 2600Ma, 2080Ma, 1830Ma, 910Ma, 600Ma, 560Ma, 450Ma, 330Ma and 280Ma with different continental model ages (TDM2) from 3.5Ga to 1.0Ga. The variety of composition of the Corsican batholith, unique in its present Mediterranean environment, and in spite of Alpine transcurrent movements, provide a key to analyze the detrital zircon age distribution patterns of sedimentary rocks. These new results i) confirm the lack of any Cretaceous zircon that validates absence of a magmatic arc of this age, at least in the surroundings of the turbiditic formations from the Balagne and the Piedmont nappes; ii) fully support an European provenance of detritus of the Balagne nappe, iii) put forward evidence that no ophiolitic zircon was contained neither in the Cretaceous nor in the Eocene turbidites samples, and iv) question both the deposition of the Piedmont Narbinco flysch within the ocean continent transition and its possible relationships with the Late Cretaceous Pyrenean basins.

SHALLOW HYDROCARBON INDICATIONS ALONG THE ALPINE THRUST BELT AND ADJACENT FORELAND BASIN: DISTRIBUTION AND IMPLICATIONS FOR PETROLEUM EXPLORATION

Shallow oil and gas shows are common in the Alpine thrust front (including the Flysch Zone) and the North Alpine Foreland Basin in Switzerland, southern Germany and Austria, but have not hitherto been evaluated systematically. In the vertically‐drained Vienna Basin and the easternmost part of the Flysch Zone, shallow oil and gas shows and seeps often coincide with deeper‐lying hydrocarbon accumulations, and gas shows occur along major faults – for example within the urbanised area of the city of Vienna. The number of gas shows decreases in the Vienna Basin away from (to the south of) the subcrop of the main thermogenic source rock (the Upper Jurassic Mikulov Formation); however shallow accumulations of microbial gas occur in that area. To the west, along the northern margin of the laterally‐drained North Alpine Foreland Basin, oil shows have been recorded in both Austria and Switzerland; microbial gas shows are common in addition to thermogenic hydrocarbons. Typically the shows form regional clusters along river valleys and occur above shallow gas accumulations.A Lower Oligocene organic‐rich interval represents the main source of oil / condensate and thermogenic gas in the Upper Austrian part of the North Alpine Foreland Basin, whereas the composition of oil shows within the Calcareous Alps to the south indicates the presence of mature Mesozoic source rocks within the Alpine nappes. This implies the presence of an additional, as‐yet untested petroleum system. Thermogenic gas, occurring in Permo‐Triassic evaporitic rocks in the Calcareous Alps, as well as microbial gas in younger sediments, has frequently been encountered during salt mining and tunnelling activities.A surprising discrepancy has been found in different parts of the study area between the number of hydrocarbon shows and the number of economic fields. Whereas the number of fields and shows are approximately in proportion in the Vienna Basin and the Austrian sector of the North Alpine Foreland Basin, shows appear to be “under‐represented” in Germany. By contrast in Switzerland, despite a high number of shows especially in the North Alpine Foreland Basin and the Jura fold‐and‐thrust belt, no economic production has been established to date. Future exploration will show whether this is due to poor reservoir/trap quality, or if undiscovered resources are in fact present. The presence of oil shows generated from Mesozoic and Oligocene source rocks in the SW German and Swiss parts of the North Alpine Foreland Basin suggests the occurrence of multiple petroleum systems; these systems should be delineated in future studies.Few surface seeps have been recorded in less populated parts of the study area such as the high Alps, possibly due to sampling bias. However, this bias does not explain the low frequency of recorded hydrocarbon shows in the German part of the North Alpine Foreland Basin. This may be because the geological setting there is in general less favourable for the migration of thermogenic gas into shallow reservoirs and its preservation in shallow traps.

Depositional architecture of a confined, sand-rich submarine system: the Bric la Croce – Castelnuovo turbidite system (Tertiary Piedmont Basin, Oligocene, NW Italy)

Submarine sand-rich slope fans within confined basins have long been recognized as components of deepwater depositional systems and, in some areas, they host important hydrocarbon accumulations. The Bric la Croce – Castelnuovo turbidite system (BCTS) of northern Italy offers an opportunity to study an exposed sand-rich slope fan system, allowing reconstruction of the main architectural elements of a fan, from slope to basin floor. The system belongs to the Langhe Basin, the major depocentre of the TPB (Tertiary Piedmont Basin), an episutural basin set upon a complex of Alpine nappes of the Liguria – Piemonte Western Alps.The system consists of three main erosional and depositional zones from up- to down-stream (from SE to NW): i) a base of slope zone; ii) a channel-lobe transition zone; iii) a lobe zone. Downslope and upslope migration of these sectors determines, through time, their superimposition and gives rise to a generation of depositional units of different hierarchical order ranging from metre-thick simple facies sequences to decametres composite facies sequence, expression of single and composite architectural elements as channel and lobe complexes. The vertical organisation of the BCTS reflects its overall forestepping. Thin-bedded turbidites interpreted as distal lobes are overlain by stacked sandsheets and aggradational channels attributed to the mid-fan setting. At the top of the series, coarse-grained sandy channels correspond to the maximum of forestepping.The logged sections and the geological mapping provide a valuable data set that can be used to study the spatial-temporal relationships between facies of channelized and nonchannelized strata (lobe deposits) along a depositional profile, cut roughly parallel to the main palaeocurrent flow. The sands were derived from south and flowed toward the north. The portion of the BCTS which crops out is approximately 160 m thick and extends for about 10-12 km in a SW-NE direction.Research involved a detailed analysis of facies (genetic facies related to depositional processes), physical stratigraphy of stacking and correlation patterns. The detailed correlation panels allowed evaluating how facies changed along the depositional profile and this was considered as the expression of the flow transformation during its movement. Based on these considerations, it has been possible to reconstruct different facies tracts, which are related to flows with different volume and efficiency recording a coeval phase of sedimentation in the channel, channel-transition and lobe zones.

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.