Periadriatic Research Articles

Not too old to rock: ESR and OSL dating reveal Quaternary activity of the Periadriatic Fault in the Alps

The Periadriatic Fault system (PAF) ranks among the largest post-collisional structures of the European Alps. Recent geodetic data suggest that a fraction of the Adria–Europe convergence is still being accommodated in the Eastern Alps. However, the historical seismicity records along the easternmost segment of the PAF are ambiguous, and instrumental records indicate that seismotectonic deformation is mostly concentrated in the adjacent Southern Alps and Dinarides. Both electron spin resonance (ESR) and optically stimulated luminescence (OSL) dating methods can be used to date coseismic slip (with a combined range covering a few decades to a couple of million years) in slowly deforming fault zones, such as the PAF. Since the saturation doses of the quartz ESR signals are larger than quartz and feldspar OSL, ESR enables establishing a maximum age of the last resetting event of the system, while OSL allows constraining their minimum age when the signal is in saturation. We collected fault gouge samples from three localities along the easternmost segment of the PAF. For ESR, we measured the signals from the Al center in quartz comparing the results from the single aliquot additive dose (SAAD) and single aliquot regenerative (SAR) dose protocols. For OSL, we recorded the infrared stimulated luminescence signal at 50°C (IR50) and post-infrared infrared-stimulated luminescence signal at 225 °C (pIRIR225). Our dating results indicate that the studied segment of the PAF system accommodated seismotectonic deformation during the Quaternary, with a maximum age for the last resetting event of the system ranging from 1075 ± 48 to 552 ± 26 ka (ESR SAR) and minimum ages in the range from 196 ± 12 to 281 ± 16 ka (saturation of pIRIR225). The obtained ages suggest that the studied segment of the PAF could be considered at least as a potentially active fault.Graphical abstract

Revealing Subtle Active Tectonic Deformation: Integrating Lidar, Photogrammetry, Field Mapping, and Geophysical Surveys to Assess the Late Quaternary Activity of the Sava Fault (Southern Alps, Slovenia)

We applied an interdisciplinary approach to analyze the late Quaternary activity of the Sava Fault in the Slovenian Southern Alps. The Sava Fault is an active strike-slip fault, and part of the Periadriatic Fault System that accommodated the convergence of Adria and Europe. It is one of the longest faults in the Southern Alps. Using high-resolution digital elevation models from lidar and photogrammetric surveys, we were able to overcome the challenges of assessing fault activity in a region with intense surface processes, dense vegetation, and relatively low fault slip rates. By integrating remote sensing analysis, geomorphological mapping, structural geological investigations, and near-surface geophysics (electrical resistivity tomography and ground penetrating radar), we were able to find subtle geomorphological indicators, detect near-surface deformation, and show distributed surface deformation and a complex fault pattern. Using optically stimulated luminescence dating, we tentatively estimated a slip rate of 1.8 ± 0.4 mm/a for the last 27 ka, which exceeds previous estimates and suggests temporal variability in fault behavior. Our study highlights the importance of modern high-resolution remote sensing techniques and interdisciplinary approaches in detecting tectonic deformation in relatively low-strain rate environments with intense surface processes. We show that slip rates can vary significantly depending on the studied time window. This is a critical piece of information since slip rates are a key input parameter for seismic hazard studies.

Stress Patterns and Crustal Anisotropy in the Eastern Alps: Insights From Seismological and Geological Observations

AbstractIn the Eastern Alps, the indentation of the Adriatic promontory since the Cenozoic affected the kinematics of separate crustal domains bounded by faults that accommodate lateral extrusion processes and differential shortening. Deciphering the pattern of crustal stresses in the orogen interior is challenging, due to the lack of in situ stress measurements at crustal depths. We define stress regimes and the orientations of the most‐compressive horizontal stress (SHmax) by integrating published results with new data, including stress analysis from fault plane solutions, estimation of crustal anisotropy through the shear wave splitting analysis, paleostress determination from fault slip data, and computation of cumulative seismic displacements. The retrieved regional SHmax are generally consistent with N‐S convergence. In the northern part of the study area, current stress orientations are almost parallel to paleo‐SHmax, suggesting a rather uniform compressional regime since the late Cenozoic. Conversely, sharp deflections and divergence with paleo‐SHmax appear at the western border of the Adriatic promontory across major transpressive and extensional shear zones and in the Southalpine domain, indicating a change in tectonically induced second‐order stresses. A current strike‐slip regime with subordinate orogen‐parallel seismic displacements affects a belt to north of the Periadriatic Lineament and NE‐extension characterizes the Ortles‐Engandine region. Seismic anisotropy locally exhibits fault‐parallel fast axes (Brenner‐Giudicarie fault‐systems, Dinaric and Southalpine thrusts), whereas stress‐induced anisotropy parallel to SHmax characterizes the southern part of the orogen. Cumulative seismic displacements are small compared to geodetic ones, and unravel partitioning of deformation into second‐order transpressive and extensional belts in response to indentation.

Anisotropic Reversible‐Jump McMC Shear‐Velocity Tomography of the Eastern Alpine Crust

AbstractThe eastern Alpine crust has been shaped by the continental collision of the European and Adriatic plates beginning at 35 Ma and was affected by a major reorganization after 20 Ma. To better understand how the eastern Alpine surface structures link with deep seated processes, we analyze the depth‐dependent seismic anisotropy based on Rayleigh wave propagation. Ambient noise recordings are evaluated to extract Rayleigh wave phase dispersion measurements. These are inverted in a two step approach for the azimuthally anisotropic shear velocity structure. Both steps are performed with a reversible jump Markov chain Monte Carlo (rj‐McMC) approach that estimates data errors and propagates the modeled uncertainties from the phase velocity maps into the depth inversion. A two layer structure of azimuthal anisotropy is imaged in the Alpine crust, with an orogen‐parallel upper crust and approximately orogen‐perpendicular layer in the lower crust and the uppermost mantle. In the upper layer, the anisotropy tends to follow major fault lines and may thus be an apparent, structurally driven anisotropy. The main foliation and fold axis orientations might contribute to the anisotropy. In the lower crust, the N‐S orientation of the fast axis is mostly confined to regions north of the Periadriatic Fault and may be related to European subduction. Outside the orogen, no clearly layered structure is identified. The anisotropy pattern in the northern Alpine foreland is found to be similar compared to SKS studies which is an indication of very homogeneous fast axis directions throughout the crust and the upper mantle.

Raman Spectroscopy of carbonaceous material thermometry in the Carnic Alps (Austria, Italy)

Abstract The Carnic Alps (eastern Southern Alps) provide a classical area to study polyphase very-low- to low-grade metamorphism within the Variscan belt of Europe. Temperature indicators collected during the past three decades map the general metamorphic structure of a mountain chain affected by three major tectonic events (Variscan and Alpine thrusting, Oligocene transpression). Thermometric data obtained by Raman Spectroscopy of carbonaceous material (RSCM) described in this study extend the already published database, provide a map of metamorphic isotherms, and are interpreted in the view of current tectonic concepts. The RSCM temperatures of this study describe a gradient between ca. 460 °C in the tectonically deepest segments, bordered by the Periadriatic Fault System, and temperatures of ca. 200 °C in Permian- Triassic boundary strata of the Gartnerkofel-1 core. Mapped isotherms indicate three domains with different thermal histories, characterized by Variscan imbrication of an accretionary wedge, Permo-Mesozoic burial, and Oligocene contact metamorphism.

Seismic wave attenuation (1/Qp) in the crust underneath the Eastern and eastern Southern Alps (Europe): imaging effects of faults, fractures, and fluids

We present a novel three-dimensional model of compressional wave attenuation (1/QP) for the Eastern and eastern Southern Alps in Europe that includes the eastern part of the Adriatic indenter, termed here the Dolomites Sub-Indenter. Our approach employed waveform data from the SWATH-D network, a dense temporary network operational between 2017 and 2019, as well as selected stations of the larger AlpArray Seismic Network. A spectral inversion method using frequency-independent quality factor QP, was applied to derive 3578 path-averaged attenuation values (t*) from 126 local earthquakes. These were then inverted using the damped least square inversion (local earthquake tomography) for the attenuation structure. The resulting QP model, which builds on and complements a previously calculated 3-D velocity model (VP and VP/VS), exhibits good resolution down to ~ 20 km depth. Several anomalies can be correlated with the distribution of other physical parameters (VP and VP/VS) and regional tectonic features. Notably, the Friuli-Venetian region exhibits the highest attenuation (lowest QP) anomaly, coinciding with low VP values and increased VP/VS. This anomaly is likely associated with a high density of faults and fractures, as well as the presence of fluid-filled sediments along the active thrust front in the eastern segment of the Southern Alps. Another intriguing observation is the low attenuation (high QP) anomaly along the northwestern edge of the Dolomites Sub-Indenter (NWDI), located south of the Periadriatic fault and east of the Giudicarie fault, where seismicity is notably absent. This anomaly coincides with Permian magmatic rocks at the surface and may be a measure of their strength at depth.Graphical

S Hmax orientation in the Alpine region from observations of stress-induced anisotropy of nonlinear elasticity

SUMMARY The orientation of SHmax is commonly estimated from in situ borehole breakouts and earthquake focal mechanisms. Borehole measurements are expensive, and therefore sparse, and earthquake measurements can only be made in regions with many well-characterized earthquakes. Here, we derive the stress-field orientation using stress-induced anisotropy in nonlinear elasticity. In this method, we measure the strain derivative of velocity as a function of azimuth. We use a natural pump-probe (NPP) approach which consists of measuring elastic wave speed using empirical Green’s functions (probe) at different points of the earth tidal strain cycle (pump). The approach is validated using a larger data set in the Northern Alpine Foreland region where the orientation of maximum horizontal compressive stress is known from borehole breakouts and drilling-induced fractures. The technique resolves NNW-SSW to N-S directed SHmax which is in good agreement with conventional methods and the recent crustal stress model. We confirm that the NPP method can be applied to dense large-scale seismic arrays. The technique is then applied to the Southern Alps to understand the contemporary stress pattern associated with the ongoing deformation due to counterclockwise rotation of the Adriatic plate with respect to the European plate. Our results explain why the two major faults in Northeastern Italy, the Giudicarie Fault and the Periadriatic Line (Pustertal–Gailtal Fault) are currently inactive, while the currently acting stress field allows faults in Slovenia to deform actively. We have demonstrated that the pump-probe method has the potential to fill in the measurement gap left by conventional approaches, both in terms of regional coverage and in depth.

The Obir caves adjacent to the Periadriatic Fault in southern Austria: Uplifted hypogene caves formed by carbonic acid speleogenesis

Hypogene karst caves, formed independently of surface processes, have garnered increasing attention in recent decades, revealing a wider distribution than previously assumed. While most studies have relied on morphological observations, this research combines morphological and geochemical approaches to present new insights from the Obir caves in the Northern Karawank Mountains, Austria. These caverns, discovered during mining activities, lack natural entrances and developed along a prominent master horizon about 500 m above the current spring level. Notably, large shafts within the caves originate in narrow fissures and terminate blindly at the top, also suggesting a phreatic regime during their formation, with some areas being modified by vadose waters later on. A comprehensive study of drill cores indicates a stable isotope halo in the first few centimeters of the wall rock, characterized by significantly depleted δ13C and moderately depleted δ18O values compared to the pristine limestone, accompanied by bleaching of the host rock. This geochemical evidence of wall rock alteration is only present where the original phreatic solutional morphology is preserved and absent where vadose waters resulted in rugged cave wall surfaces.Considering the lack of evidence for the involvement of sulfuric acid or thermal waters, the formation of the Obir caves is attributed to carbonic acid speleogenesis. This interpretation is supported by the observed strong C isotope shift in the wall rock and the presence of CO2-bearing springs both north and south of the Periadriatic Fault. In contrast to previous studies of carbonic acid speleogenesis suggesting large-scale upwelling of aggressive waters, we propose an alternative mechanism: the injection of carbon dioxide gas into limestone situated in the phreatic zone. These injections likely occurred along subvertical tectonic fissures, triggered by the tectonic transpression of the Northern Karawank Mountains during the middle Miocene. Subsequent tectonic uplift and the incision of valleys led to a drop in the groundwater table, widespread collapse of larger cave rooms and localized vadose overprint.This study highlights the strength of combining mega- to the microscale cave morphological observations with systematic geochemical analyses of wall rock alteration in identifying caves formed by carbonic acid speleogenesis.

A wealth of P–T–t information from metasediments in the HP–UHP terrane of the Pohorje Mountains, Slovenia, elucidates the evolution of the Eastern Alps

AbstractContrasting views exist in regard of the evolution of metamorphic rocks in the southeastern Pohorje Mountains (Mts), located in the southeastern Eastern Alps. Major debated points are whether micaschists have experienced ultrahigh‐pressure metamorphism in the Late Cretaceous (Eo‐Alpine) and whether they were continuously exhumed or experienced a multiple subduction–exhumation process from that time on. Therefore, we studied micaschist sample 18Slo39 with two generations of garnet and phengitic muscovite from this area. Our detailed study of this rock included petrographic observations, chemical analyses of minerals with the electron microprobe, pseudosection modelling, conventional geothermometry, and monazite in‐situ U‐Th‐Pb dating using laser‐ablation inductively coupled plasma (ICP) mass spectrometry. The following results were obtained: The studied micaschist was subject to a peak pressure of 1.31 ± 0.14 GPa at 603 ± 26°C in Eo‐Alpine times: 90.62 ± 2.78 (2σ) Ma (Stage I). Contact metamorphism at pressure–temperature conditions of 0.66 ± 0.10 GPa and 577 ± 23°C was induced by the intrusion of the Pohorje pluton (Stage III). We determined an early Miocene age of 18.33 ± 0.43 (2σ) Ma for this intrusion. Based on this study and the previously reported data for a micaschist (16Slo12) taken in the vicinity of sample 18Slo39, a geodynamic model is proposed for the region of the Pohorje Mts considering Eo‐Alpine subduction of oceanic crust and European continental crust, of which the micaschist was part of. Another high‐pressure event in the Eocene (Stage II) was the result of intracontinental subduction because of transpression by the Periadriatic fault system that separates the Eastern Alps from the Southern Alps. This type of subduction gave rise to magma generation and ascent to form the Pohorje pluton, which caused contact metamorphism in its vicinity.

The Meran-Mauls Fault: Tectonic switching from compression to transpression along a restraining bend of the Periadriatic Fault

The WSW-ENE Meran-Mauls Fault (MMF) represents a break in the broadly E-W trending Periadriatic Fault (PAF), separating the Southern Alps from the N-verging wedge of the European Alps. Understanding the MMF evolution is mandatory to reconstruct the role of the entire PAF during the evolution of the belt since the late Cenozoic.Structural and microstructural analyses and paleostress calculations based on fault-slip data suggest the occurrence of four evolutionary stages for the MMF: 1) top-to-the-SE mylonites; 2) top-to-the-SE brittle faulting; 3) dextral faulting with re-activation of previous structures; 4) N–S compression associated with conjugate sets of mainly strike-slip fault systems. Paleostress reconstructions point to a σ2-σ3 permutation from stage 2 to stage 3, resulting in the switch from pure thrusting to strike-slip, followed by an anticlockwise rotation of the principal stress axes during stage 4, in a strike-slip regime related to N–S compression. Geochronological and thermochronological data point to 39-22 Ma age for stage 1, 22-17 Ma for stage 2, less than 17 Ma for stage 3. Our reconstruction strongly supports the interpretation of the MMF as a restraining bend of the PAF, which was weakly reactivated along its eastern portion as a dextral fault and later displaced by NNE-SSW left-lateral faults.

Alpine convergence record in the Carboniferous Badstub Formation, Upper Austroalpine basement nappes, Austria

ABSTRACT Within the Carboniferous of Nötsch, the Badstub Formation is a clastic sequence of the Upper Austroalpine basement nappes exposed few kilometres north of the Periadriatic Fault System (Carinthia, Austria). Although these rocks preserve pristine sedimentary features, multi-scale structural analysis disclosed a syn-metamorphic foliation in fine-grained rocks, sets of mineralized faults, veins, and corona textures. Vein fillings and coronas contain equilibrium mineral assemblages with prehnite, pumpellyite, chlorite, phengitic mica, winchite, and riebeckite. Thermodynamic modelling and geothermometry constrain metamorphic conditions at 260–310°C and 0.25–0.50 GPa that are consistent with a temperature/depth ratio of about 20°C km−1. In a 2D thermomechanical model this thermal state is reached either in the upper or lower continental plate during convergence. Thus, during the Alpine convergence, the Badstub rocks were buried from the shallow crust at depths between 13 and 18 km either by ablative oceanic subduction or by continental subduction of the passive margin, and eventually stacked into the orogenic wedge at the Adria margin. During the downward and upward paths, the Badstub rocks were translated as a coherent poorly strained block. This first quantitative constraint on metamorphism for these Carboniferous rocks is consistent with the Upper Austroalpine basement nappes being a tectonic system that recorded the Alpine convergence under eclogite to prehnite–pumpellyite facies conditions.

Seismic Activity in the Celje Basin (Slovenia) in Roman Times—Archaeoseismological Evidence from Celeia

Searching for unknown earthquakes in Slovenia in the first millennium, we performed archaeoseismological analysis of Roman settlements. The Mesto pod mestom museum in Celje exhibits a paved Roman road, which suffered severe deformation. Built on fine gravel and sand from the Savinja River, the road displays a bulge and trench, pop-up structures, and pavement slabs tilted up to 40°. The city wall was built over the deformed road in Late Roman times, supported by a foundation containing recycled material (spolia) from public buildings, including an emperor’s statue. We hypothesize that a severe earthquake hit the town before 350 AD, causing widespread destruction. Seismic-induced liquefaction caused differential subsidence, deforming the road. One of the nearby faults from the strike-slip Periadriatic fault system was the seismic source of this event.

Differentiating between artificial and natural sources of electromagnetic radiation at a seismogenic fault

Ultralow frequency (ULF) to low frequency (LF) electromagnetic radiation represents one of the most promising effects of brittle rock strain and microcracking that might be potentially helpful for short term earthquake forecasting. In this study the results of a six month monitoring campaign are presented from Obir Cave in the eastern Alps. Direct experimental observations of electromagnetic radiation have been made using a customised broadband data logger installed next to the Obir Fault - this seismogenic fault near the Periadriatic Lineament is known to be related to at least three large prehistoric earthquakes. On the basis of these measurements it has been possible to characterise a number of distinct signals: artificial constant narrowband signals at discrete frequencies; short serial broadband impulses; high energy broadband impulses; and low energy broadband impulses. The narrowband artificial signals were removed from the electromagnetic radiation time series analysis so that the natural signals were enhanced and could be compared more easily to meteorological parameters and rock strain indicators. Critically, the high energy broadband impulses show a strong correlation with lightning activity across much of central Europe and the eastern Mediterranean while the low energy broadband impulses appear to be associated with local rock strain in and around Obir Cave. Unfortunately, it seems certain that some of the essential strain related impulses are likely to have been overprinted by the larger lightning related impulses.

Drainage pattern evolution during continental indentation in Central Alps: Constraints from the sedimentary record of associated deep-water clastic offshoots

This work reconstructs the evolution of a source-to-sink system in a transpressional setting from compositional changes within its deep-water clastic offshoots. Field-based clast counts on 10 conglomerate beds of three transects, combined with petrographical and mineralogical (XRD) analyses on 14 samples, have been used to integrate the already-existing large dataset developed during more than 60 years of study on the Como Conglomerate (the base of the Gonfolite Lombarda Group, Chattian to Aquitanian – Northern Italy). The results shed a new light on the terranes involved in the production of sediments and further constrain the evolution of the fluvial drainage in response to the Cenozoic Adria indentation during the post-collisional phases of Alpine orogeny. In particular, it has been possible to track-back the steps during which paleorivers were connected, and later disconnected, with the basin due to the Oligo-Miocene activity of the right-lateral Periadriatic Fault System (PFS). During its transpressive movement, accommodating the Adria westward indentation, the PFS cumulated a total displacement increasing from ca. 20 km to the west, up to ca. 60 km, to the east. The results of this study also constrain the post-depositional kinematics of the clastic wedge, which was progressively rotated clockwise and thrust to its present-day position, onto the Southalpine Mesozoic sequence. This model represents a fundamental step forward in the comprehension of how tectonic indentation interacts with surface source-to-sink systems, and demonstrates that tectonic induced connections/disconnections of fluvial systems are as fundamental as exhumation processes in the transfer of sediments from source to sink areas.

Constraints on Crustal Structure in the Vicinity of the Adriatic Indenter (European Alps) From Vp and Vp/Vs Local Earthquake Tomography

AbstractIn this study, 3‐D models of P‐wave velocity (Vp) and P‐wave and S‐wave ratio (Vp/Vs) of the crust and upper mantle in the Eastern and eastern Southern Alps (northern Italy and southern Austria) were calculated using local earthquake tomography (LET). The data set includes high‐quality arrival times from well‐constrained hypocenters observed by the dense, temporary seismic networks of the AlpArray AASN and SWATH‐D. The resolution of the LET was checked by synthetic tests and analysis of the model resolution matrix. The small inter‐station spacing (average of ∼15 km within the SWATH‐D network) allowed us to image crustal structure at unprecedented resolution across a key part of the Alps. The derived P velocity model revealed a highly heterogeneous crustal structure in the target area. One of the main findings is that the lower crust is thickened, forming a bulge at 30–50 km depth just south of and beneath the Periadriatic Fault and the Tauern Window. This indicates that the lower crust decoupled both from its mantle substratum as well as from its upper crust. The Moho, taken to be the iso‐velocity contour of Vp = 7.25 km/s, agrees with the Moho depth from previous studies in the European and Adriatic forelands. It is shallower on the Adriatic side than on the European side. This is interpreted to indicate that the European Plate is subducted beneath the Adriatic Plate in the Eastern and eastern Southern Alps.

Two subduction-related heterogeneities beneath the Eastern Alps and the Bohemian Massif imaged by high-resolution P-wave tomography

Abstract. We present high-resolution tomographic images of the upper mantle beneath the Eastern Alps and the adjacent Bohemian Massif (BM) in the north based on recordings from the AlpArray-EASI and AlpArray seismic networks. The tomography locates the Alpine high-velocity perturbations between the Periadriatic Lineament and the Northern Alpine Front. The northward-dipping lithosphere is imaged down to ∼ 200–250 km of depth, without signs of delamination. The small amount of crustal shortening compared to that in the Western Alps and the bimodal character of the positive perturbations with a separation beneath the Tauern Window indicate a dual source of the velocity heterogeneity, most probably formed by a mixture of a fragment of detached European plate and the Adriatic plate subductions. A detached high-velocity heterogeneity, sub-parallel to and distinct from the Eastern Alps heterogeneity, is imaged at ∼ 100–200 km beneath the southern part of the BM. We associate this anomaly with the western end of a SW–NE-striking heterogeneity beneath the south-eastern part of the BM, imaged in models of larger extent. The strike, parallel with the Moldanubian–Brunovistulian mantle–lithosphere boundary in the BM and with the westernmost part of the Carpathian front, leads us to consider potential scenarios relating the heterogeneity to (1) a remnant of the delaminated European plate, (2) a piece of continental-and-oceanic lithosphere mixture related to the building of the BM, particularly to the closure of the old Rheic ocean during the MD–BV collision, or (3) a lithospheric fragment going through to the NW between the Eastern Alps and Western Carpathians fronts in a preceding subduction phase. The study is dedicated to our outstanding and respected colleague Vladislav Babuška, who coined innovative views on the European lithosphere and died on 30 March 2021.

The birth of the Sava Suture Zone: The early geological observations and the context of bimodal magmatism (southern Belgrade outskirts; Andjelkovic, 1973)

This paper presents one of the significant results of the research of Prof. Dr. Milodrag Andjelkovic. The study underlines the first field record providing the evidence of the Upper Cretaceous bimodal magmatism that eventually led towards the definition of the Late Cretaceous ?Sava?Vardar Zone? in 2002 (Pamic, 2002). Currently, the 20-years old ?Sava?Vardar Zone? i.e., the Sava Suture Zone regarded as a crustal assembly formerly intervening the amalgamated Adria and south Eurasian affinities. The pioneering field mapping- based observations contributing the debuted Sava Suture Zone, are in the underestimated report of Prof. Dr. Milodrag Andjelkovic, presenting his observations of the typifying Late Cretaceous bimodal magmatism (published in 1973 in the regional XIX-century established journal ?Annales geologiques de la peninsule Balkanique?; Title: ?Geologija mezozoika okoline Beograda?, translated: The geology of Mesozoic assembly: vicinity of the Belgrade area). in that time, during early 1970?s, the entire geoscience community of former Yugoslavia for a long time denying any Late Cretaceous magmatism within the Vardar Zone, offering a counterargument grounded onto the geosyncline tectonic framework. The 1973 paper was published much prior the constraints on the Late Cretaceous bimodal magmatic intrusions, discussed as the ?Peri Adriatic Sava magmatic arc? or Peri Adriatic Lineament, but only three decades later. The Andjelkovic?s 1973 report represents the first published record, interpreting a number of the Late Cretaceous bimodal magmatic mini-occurrences distributed in the modern-day Sava Suture Zone belt. Despite using the previous tectonic model, the often-neglected descriptions of the Jurassic?Late Cretaceous biostratigraphy and their spatial relationship with the confined magmatic entities, allow the important correlation of a number of Late Cretaceous smallscale basins, positioned to the northwest (Bosnia & Herzegovina, Croatia), and to the south of Belgrade (Central Serbia, North Macedonia). The understanding of this suture zone, bending in the vicinity of Belgrade, is of primary importance, providing the characterization of the terminal Alpine collisional mechanism in the area.

Petrological and geochronological investigations on the individual nappes of the Meran-Mauls nappe stack (Austroalpine unit/South Tyrol, Italy)

Abstract The Meran-Mauls nappe stack is part of the Austroalpine unit in South Tyrol (Italy). There it holds a special position directly in front of the Southalpine Dolomites indenter and west of the Tauern Window. It is situated in the hanging wall of the Southalpine unit, above a NW dipping segment of the Periadriatic fault system, namely the Meran-Mauls fault. Also all other sides are defined by Oligocene-Miocene strike-slip and normal faults. Based on recent mapping the Meran-Mauls nappe stack consists of three nappes separated by NW to NNW dipping shear zones. The lowermost nappe in the southwest is represented by the Schenna (Scena) unit. It is overlain along the Masul shear zone by a nappe consisting of the Hirzer (Punta Cervina) unit and the Pens (Pennes) unit including Triassic (meta)sediments. Separated by the Fartleis fault the St. Leonhard (San Leonardo) unit forms the uppermost nappe. The aim of this study is to describe the individual units and the separating structural elements more properly, based on new structural, petrological, geothermobarometric and geochronological data and to compare these units to other Austroalpine elements in the vicinity. Sillimanite-bearing paragneiss, minor amphibolite and quartzite as well as a distinct marble layer close to its base characterise the Schenna unit. Further, it contains pegmatite dikes, presumably Permian in age. Amphibolite-facies P-T conditions of c. 0.55 ± 0.15 GPa and 600 ± 100°C are thus correlated with a Permian metamorphic imprint. The Masul shear zone mostly consists of mylonitic paragneiss of the Hirzer unit. It is pre-Alpine in age and probably formed during the Jurassic. For the paragneiss of the Hirzer unit upper greenschist- to amphibolite-facies metamorphic conditions of 0.4-0.50 ± 0.15 GPa and 550 ± 70°C are attributed to the Variscan tectonometamorphic imprint. The whole Pens unit represents a shear zone. Due to the occurrence of Permotriassic (meta)-sediments within this shear zone, it is an Alpine structure, as well as the bordering Fartleis fault. Rb/Sr biotite ages yield sometimes partly reset pre-Alpine age values in the whole Meran-Mauls nappe stack, indicating a pervasive anchizonal to lowermost greenschist-facies metamorphic overprint during the Eoalpine tectonometamorphic event. Tectonostratigraphically the Meran-Mauls nappe stack can be attributed to the Drauzug-Gurktal nappe system. The latter forms the uppermost structural element of the Austroalpine nappe stack and thus only shows a weak Eoalpine metamorphic overprint. With respect to its special lithologic composition the Schenna unit can be correlated with the Tonale unit in the southwest and the Strieden-Komplex in the east.

A hidden Oligocene pluton linked to the Periadriatic Fault System beneath the Permian Bressanone pluton, eastern Southern Alps

ABSTRACT The Bressanone (Brixen) pluton, cropping out at the culmination of the Southalpine indenter between the North Giudicarie and the Pustertal-Gailtal faults, is mainly composed of Permian granodiorite to granite, with minor gabbros and diorites in its southern part. New U–Pb SHRIMP zircon ages reveal two distinct crystallization episodes at 289.7 ± 3.2 and 280 ± 2.2 Ma, respectively. The pluton is affected by a hydrothermal potassic to sodic + Cu metasomatic alteration, which has long been ascribed to a late phase of the Permian magmatism. In contrast with this hypothesis, we report new 39Ar–40Ar data for different generations of metasomatic K-feldspar, which indicate formation ages between 35.3 ± 0.3 and 27.8 ± 0.5 Ma. This interval overlaps with the ages of the widespread ‘Periadriatic’ calc-alkaline magmatism, which extends from the Western to the Eastern Alps straddling the Periadriatic Fault System. The observed hydrothermalism has geochemical characteristics compatible with those of the coeval calc-alkaline Periadriatic magmatism. These data altogether suggest the release of fluids from a hidden intrusion during the main stage of the Alpine orogenic magmatism. Our results provide the first evidence of Oligocene magmatic activity in the Southern Alps east of the Giudicarie Line, bridging the gap between the western-central and eastern Alpine magmatic plutons. A fairly continuous Oligocene magmatic belt straddling the Periadriatic Fault System is consistent with a triggering of the magmatism by slab steepening or slab break-off, either of which are considered to be an essential driver for the Miocene lithospheric rearrangement in the Eastern Alps. Our finding is particularly relevant given that the outcropping area of the Bressanone pluton is centred above the imaged subducting lithosphere gap that separates the Western-Central and the Eastern Alps, hence at a location where mantle upwelling should have been easier although no relevant magmatism was found to date.

Imaging structure and geometry of slabs in the greater Alpine area – a P-wave travel-time tomography using AlpArray Seismic Network data

Abstract. We perform a teleseismic P-wave travel-time tomography to examine the geometry and structure of subducted lithosphere in the upper mantle beneath the Alpine orogen. The tomography is based on waveforms recorded at over 600 temporary and permanent broadband stations of the dense AlpArray Seismic Network deployed by 24 different European institutions in the greater Alpine region, reaching from the Massif Central to the Pannonian Basin and from the Po Plain to the river Main. Teleseismic travel times and travel-time residuals of direct teleseismic P waves from 331 teleseismic events of magnitude 5.5 and higher recorded between 2015 and 2019 by the AlpArray Seismic Network are extracted from the recorded waveforms using a combination of automatic picking, beamforming and cross-correlation. The resulting database contains over 162 000 highly accurate absolute P-wave travel times and travel-time residuals. For tomographic inversion, we define a model domain encompassing the entire Alpine region down to a depth of 600 km. Predictions of travel times are computed in a hybrid way applying a fast TauP method outside the model domain and continuing the wave fronts into the model domain using a fast marching method. We iteratively invert demeaned travel-time residuals for P-wave velocities in the model domain using a regular discretization with an average lateral spacing of about 25 km and a vertical spacing of 15 km. The inversion is regularized towards an initial model constructed from a 3D a priori model of the crust and uppermost mantle and a 1D standard earth model beneath. The resulting model provides a detailed image of slab configuration beneath the Alpine and Apenninic orogens. Major features are a partly overturned Adriatic slab beneath the Apennines reaching down to 400 km depth still attached in its northern part to the crust but exhibiting detachment towards the southeast. A fast anomaly beneath the western Alps indicates a short western Alpine slab whose easternmost end is located at about 100 km depth beneath the Penninic front. Further to the east and following the arcuate shape of the western Periadriatic Fault System, a deep-reaching coherent fast anomaly with complex internal structure generally dipping to the SE down to about 400 km suggests a slab of European origin limited to the east by the Giudicarie fault in the upper 200 km but extending beyond this fault at greater depths. In its eastern part it is detached from overlying lithosphere. Further to the east, well-separated in the upper 200 km from the slab beneath the central Alps but merging with it below, another deep-reaching, nearly vertically dipping high-velocity anomaly suggests the existence of a slab beneath the eastern Alps of presumably the same origin which is completely detached from the orogenic root. Our image of this slab does not require a polarity switch because of its nearly vertical dip and full detachment from the overlying lithosphere. Fast anomalies beneath the Dinarides are weak and concentrated to the northernmost part and shallow depths. Low-velocity regions surrounding the fast anomalies beneath the Alps to the west and northwest follow the same dipping trend as the overlying fast ones, indicating a kinematically coherent thick subducting lithosphere in this region. Alternatively, these regions may signify the presence of seismic anisotropy with a horizontal fast axis parallel to the Alpine belt due to asthenospheric flow around the Alpine slabs. In contrast, low-velocity anomalies to the east suggest asthenospheric upwelling presumably driven by retreat of the Carpathian slab and extrusion of eastern Alpine lithosphere towards the east while low velocities to the south are presumably evidence of asthenospheric upwelling and mantle hydration due to their position above the European slab.