The Alpine orogenic belt is the result of the continental collision and convergence between the Adriatic microplate and European plate during the Mesozoic. The Alps orogenic belt has a complex tectonic history and the deformation in and around the Alps are significantly affected by several microplates (e.g., Adriatic and Iberia) and blocks, in particular the Apennines, Betics, Dinarides. The mantle transition zone is delineated by seismic velocity discontinuities around the depths of 410 and 660 km which are generally interpreted as polymorphic phase changes in the olivine system and garnet-pyroxene system.The subduction depth of the European plate and the origin of the mantle flow behind the plate plays crucial roles for our understanding of regional geodynamic (Zhao et al., 2016; Hua et al., 2017). Therefore, we use receiver function method to study the seismic features of discontinuities beneath the Western Alps to constrain the structure of subducted plate and study the geodynamic origin of the low velocity anomaly behind the subduction zone and its relationship with the high-relief topography. 

This study uses data collected from 293 permanent and temporary broadband seismic stations (e.g., CIFALPS). Teleseismic events are selected from 30^o to 90^o epicentral distrance with magnitudes (Mw) between 5.3 and 9.0. Data are carefully checked by automated and manual procedures to to give a total of 24904 receiver functions. Both 1D velocity model of the IASP91 and 3D velocity model of the EU60 (Zhu et al., 2015) are used for time-to-depth migration. The results show that using 3D velocity model to image the two discontinuities may obtain a more accurate structure image of the mantle transition zone.

In the northern part of the study area, along the alpine orogenic belt, we find a localized arc-shaped thinning area with a depressed 410 discontinuity, which is attributed to hot mantle upwellings. The uplift is hardly seen on the 660 discontinuity, suggesting that the thermal anomaly is unlikely to be interpreted as a mantle plume. The uplift of the 410-km can be interpreted as the European plate subducting to the depth of the upper transition zone. The depression of the 660-km  is likely attributed to the remnants from the oceanic mantle lithosphere that detached from the Eurasian plate after closure of the Alpine Tethys. Our results show a good agreement between the thinning area of MTZ and the area of topographic uplift, the mantle upwelling promotes the temperature increase which is conducive to the uplift of topographic.

Reference

Zhao L , Paul A , Marco G. Malusà, et al. Continuity of the Alpine slab unraveled by high-resolution P-wave tomography. Journal of Geophysical Research: Solid Earth, 2016, 121.

Hua, Y., D. Zhao, and Y. Xu (2017), P wave anisotropic tomography of the Alps, J. Geophys. Res. Solid Earth, 122, 4509–4528, doi:10.1002/2016JB013831.

Zhu H,Bozdag E and Tromp J.Seismic structure of the European upper mantle based on adjoint tomography.Geophys. J. Int. 2015, 201, 18–52

How to cite: Liu, D., Zhao, L., Paul, A., Yuan, H., Solarino, S., Aubert, C., Dumont, T., Eva, E., Guillot, S., Malusà, M. G., Pondrelli, S., Salimbeni, S., and Schwartz, S.: Receiver Function mapping of mantle transition zone discontinuities beneath Western Alps using scaled 3-D velocity corrections, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8121, https://doi.org/10.5194/egusphere-egu21-8121, 2021.

To constrain seismic anisotropy under and around the Alps in Europe, we study SKS shear-wave splitting from the region densely covered by the AlpArray seismic network. We apply a technique based on measuring the splitting intensity, constraining well both the fast orientation and the splitting delay. 4 years of teleseismic earthquake data were processed automatically (without human intervention), from 724 temporary and permanent broadband stations of the AlpArray deployment including ocean-bottom seismometers. We have obtained an objective image of anisotropic structure in and around the Alpine region, at a spatial resolution that is unprecedented. As in earlier studies, we observe a coherent rotation of fast axes in the western part of the Alpine chain, and a region of homogeneous fast orientation in the central Alps.  The spatial variation of splitting delay times is particularly interesting. On one hand, there is a clear positive correlation with Alpine topography, suggesting that part of the seismic anisotropy (deformation) is caused by the Alpine orogeny. On the other hand, anisotropic strength around the mountain chain shows a distinct contrast between western and eastern Alps. This difference is best explained by the more active mantle flow around the Western Alps. We discuss earlier concepts of Alpine geodynamics in the light of these new observational constraints. 

How to cite: Bokelmann, G., Hein, G., Kolinsky, P., Bianchi, I., and Working Group, A.: Shear-Wave Splitting in the Alpine Region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5377, https://doi.org/10.5194/egusphere-egu21-5377, 2021.

The Alpine orogeny is characterized by tectonic sequences of subduction and collision accompanied by break-off events and possibly preceded by a flip of subduction polarity. The tectonic evolution of the transition to the Eastern Alps has thus been under debate. The dense Swath-D seismic network as complementary experiment to the AlpArray network provides unprecedented lateral resolution to address this open discussion. We analyze shear wave splitting of this data set to get insights into the deformation at depth by studying seismic anisotropy. Previous studies indicate two-layer anisotropy in the Eastern Alps. This is supported by azimuthal pattern of the measured fast axis direction across all stations of the network. The temporary character of the deployment requires a joint analysis of multiple stations to increase the number of events adding complementary information of the anisotropic property of the mantle. We perform a cluster analysis based on a correlation of the remaining transverse energy between all stations. The energy tensor is calculated in the grid search for the best fitting two-layer splitting parameters to the ensemble of events at each station. This leads to two main groups of different two-layer properties separated at 12.5 degrees Longitude. We identify a layer with constant fast axis direction of 60° over the whole area, with a possible dip from West to East. The lower layer in the West shows N-S direction and upper layer in the East 115° alignment. We propose two likely scenarios, both accompanied by a slab break-off in the Eastern part. The continuous layer can either be interpreted as frozen-in anisotropy with lithospheric origin or an asthenospheric flow evading the retreat of the European slab that would precede the break-off event.  In both scenarios the upper layer in the East is result of a channel flow through the gap formed in the slab break-off. The N-S direction is interpreted as asthenospheric flow mainly driven by the subduction of the European plate below Adria.

How to cite: Link, F. and Rümpker, G.: Lithosphere-asthenosphere decoupling in the Central/Eastern Alps from seismic anisotropy beneath the dense SWATH-D network, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14505, https://doi.org/10.5194/egusphere-egu21-14505, 2021.

In the frame of the AlpArray project we analyse teleseismic data from permanent and temporary stations of the greater Alpine region to study seismic discontinuities in the entire lithosphere. We use broadband S-to-P converted signals from below the seismic stations. In order to avoid sidelobes, no deconvolution or filtering is applied and S arrival times are used as reference. We show a number of north-south and east-west profiles through the greater Alpine area. The Moho signals are always seen very clearly, and also negative velocity gradients below the Moho are visible in a number of profiles. The subducting European Moho is visible in the Eastern Alps west of 13.5°E (the eastern edge of the Tauern Window) and reaches there about 60km depth at 47°N. East of about 13.5°E, the image of the Moho changes completely. No south dipping European Moho is found anymore, instead the Moho is shallowing towards the Pannonian Basin. This suggests severe post-nappe emplacement modifications east of about 13.5°E, most probably associated with delamination of the mantle lithosphere within the formerly subducting European slab, i.e. mantle that separated from the crustal parts of the Alpine-West Carpathian orogen during the last ca. 20 Ma when the Pannonian basin formed and the ALCAPA block underwent its E-directed lateral extrusion.

Ratschbacher, L., Frisch, W., Linzer, H.-G. and Merle, O. (1991) Lateral extrusion in the Eastern Alps, Part 2: Structural analysis. Tectonics, vol.10, No.2, 257-271.

How to cite: Kind, R., Schmid, S., Yuan, X., and Heit, B.: The seismic structure of the lithosphere in the greater Alpine area from S-to-P converted waves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-442, https://doi.org/10.5194/egusphere-egu21-442, 2021.

We present potential scenarios of the European and Adriatic plates’ collision that formed the Alps and the neighbouring mountain belts. Our results are based on teleseismic body-wave data from the AlpArray-EASI complementary experiment (2014-2015, Hetényi et al., Tectonophysics 2018) and the AlpArray Seismic Network (Hetényi et al., Surv. Geophys. 2018).  Tomography of seismic velocities in the upper mantle along a ca. 200 km broad and 540 km long north-south transect images steady southward thickening of the lithosphere beneath the Bohemian Massif  and northward dipping East-Alpine lithospheric keel. Thanks to the dense spacing of the AlpArray Seismic Network stations and high-quality data, the high-resolution tomography resolves for the first time two sub-parallel down-going high-velocity heterogeneities beneath the Eastern Alps, instead of a single, thick anomaly. The southern heterogeneity, which we relate to the subducted Adriatic plate, is more distinct than the northern one, which loses its connection with the shallow parts. Moreover, amplitudes and size of this heterogeneity decrease in cross-sections perpendicular to the strike of the Alps when shifting towards the Central Alps. The presented collision scenarios consider the smaller northern heterogeneity as (1) a remnant of a delaminated early phase subduction of the European plate with the reversed polarity relative to that in the Western Alps, (2) a piece of continental and oceanic lithosphere together, or, (3) a fragment of a quite extended lithosphere margin foundering in a preceding phase of the Adriatic subduction.

How to cite: Plomerova, J., Zlebcikova, H., Hetenyi, G., Vecsey, L., Babuska, V., and Working Group, A.: European-Adriatic plate collision in teleseismic tomography of the Eastern Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10724, https://doi.org/10.5194/egusphere-egu21-10724, 2021.

We perform a teleseismic P-wave travel time tomography to examine geometry and slab structure of the upper mantle beneath the Alpine orogen. Vertical component data of the extraordinary dense seismic network AlpArray are used which were recorded at over 600 temporary and permanent broadband stations 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. Mantle phases of 370 teleseismic events between 2015 and 2019 of magnitude 5.5 and higher are evaluated automatically for direct and core diffracted P arrivals using a combination of higher-order statistics picking algorithms and signal cross correlation. The resulting database contains over 170.000 highly accurate absolute P picks that were manually revised for each event. The travel time residuals exhibit very consistent and reproducible spatial patterns, already pointing at high velocity slabs in the mantle.

For predicting P-wave travel times we consider a large computational box encompassing the Alpine region up to a depth of 600 km within which we allow 3D-variations of P-wave velocity. To account for influences of the strongly heterogeneous crust that cannot be resolved with teleseismic data, we integrate a complex three-dimensional crustal model directly into our model. Outside the box we assume a spherically symmetric earth and apply the Tau-P method to calculate travel times and ray paths. These are injected at the boundaries of the regional box and continued using the fast marching method (Rawlinson et al. 2005). We invert differences between observed and predicted traveltimes for P-wave velocities inside the box. Velocity is discretized on a regular grid with a spacing of about 25x25x15 km. The misfit reduction reaches values of over 80% depending on damping and smoothing parameters.

The resulting model shows several steeply dipping high velocity anomalies following the Alpine arc. The most prominent structure stretches from the western Alps into the Apennines mountain range reaching depths of over 500 km. Two further anomalies of high complexity extending down to a depth of 300 km are located below the central and eastern Alps, both being detached from the lithosphere and separated by a clear gap below the western part of the Tauern window. The central anomaly shows mainly southwards dipping, whereas the eastern anomaly is mainly dipping to the northeast. We compare our results to former studies, confirming lateral positions of the anomalies. However, the new results can benefit from the superior resolution capabilities of the dense AlpArray seismic network, providing more accurate insights into depth extent, dip angle and directions. We perform various general, as well as purpose-built resolution tests, to verify the capabilities of our setup to resolve slab gaps as well as different possible slab dipping directions.

How to cite: Paffrath, M. and Friederich, W. and the the AlpArray Working Group: Teleseismic P-wave travel time tomography of the Alpine upper mantle using AlpArray seismic network data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8429, https://doi.org/10.5194/egusphere-egu21-8429, 2021.

The prevailing paradigm of mountain building in the Alps entails subduction of European continental lithosphere some 100km thick beneath the Adriatic plate. Based on recent results of AlpArray, we propose a new model that involves subduction and wholesale detachment of locally much thicker (200-240 km) European lithosphere. Our approach combines teleseismic P-wave tomography and existing Local Earthquake Tomography (LET) to image the Alpine slabs and their connections with the overlying orogenic crust at unprecedented resolution. The images call into question the simple notion that slabs comprise only seismically fast lithosphere and suggest that the mantle of the downgoing European plate is compositionally heterogeneous, containing both positive and negative seismic anomalies of up to 5%. We interpret these as compositional rather than thermal anomalies, inherited from the Paleozoic Variscan orogenic cycle and presently dipping beneath the Alpine orogenic front. In contrast to the European Plate, the lithosphere of the Adriatic Plate is thinner (100-120 km) and has a more poorly defined lower boundary approximately at the interface between positive and negative Vp anomalies.

Horizontal and vertical tomographic slices reveal that beneath the Central and Western Alps, the downgoing European Plate dips steeply to the S and SE and is locally detached from the Alpine crust. However, in the Eastern Alps and Carpathians east of the central Tauern Window, the Alpine slab anomaly occupies the 150-400 km depth interval and dips steeply to the N-NE, having completely detached from the  Alpine orogenic crust. This along-strike change coincides with an abrupt eastward decrease in Moho depth (Kind et al., this session), the Moho being underlain by a pronounced negative Vp anomaly reaching eastward into the Pannonian Basin area. This negative Vp anomaly is interpreted to represent hot upwelling asthenosphere that was instrumental in accommodating Neogene orogen-parallel lateral extrusion of the ALCAPA tectonic unit (upper plate crustal edifice of Alps and Carpathians) to the E.  An Adriatic origin of the northward-dipping, detached slab segment beneath the Eastern Alps is unlikely since its imaged down-dip length (200-300 km) matches estimated Tertiary shortening in the Eastern Alps accommodated by south-dipping subduction of European lithosphere, whereas shortening in the south-vergent eastern Southern Alps is only ≤ 70 km.

A slab anomaly beneath the northernmost Dinarides, laterally adjoining the Eastern Alps, is missing. The slab anomaly beneath the northern Apennines, of Adriatic origin und dipping beneath the Tyrrhenian backarc, hangs subvertically and appears to be almost detached from the Apenninic orogenic crust. Except for its westernmost segment where it meets the Alpine slab, this slab is clearly separated from the latter by a broad extent of upwelling asthenosphere located south of the Alpine slabs beneath the Po Plain, i.e., just south of the Alpine subduction zone. Considered as a whole, the slabs beneath the Alpine chain are interpreted as attenuated, largely detached sheets of continental margin and Alpine Tethyan lithosphere that locally reach down to a slab graveyard in the Mantle Transition Zone (MTZ).

How to cite: Handy, M. R., Schmid, S. M., Paffrath, M., and Friederich, W.: Time for a change of paradigm for Alpine subduction?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11826, https://doi.org/10.5194/egusphere-egu21-11826, 2021.

The Pannonian Basin is located in Central-Europe surrounded by the Alpine, Carpathian, and Dinarides mountain ranges. This is a back-arc basin characterized by shallow Moho depth, updoming mantle and high heat flow. In this study, we present the results of the Rayleigh wave based ambient noise tomography to investigate the velocity structure of the Carpathian-Pannonian region. 

For the ambient noise measurements, we collected the continuous waveform data from more than 1280 seismological stations from the broader Central-Eastern European region. This dataset embraces all the permanent and the temporary (AlpArray, PASSEQ, CBP, SCP) stations from the 9-degree radius of the Pannonian Basin which were operating between the time period between 2005 and 2018. All the possible vertical component noise cross-correlation functions were calculated and all phase velocity curves were determined in the 5-80 s period range using an automated measuring algorithm. 

The collected dispersion measurements were then used to create tomographic images that are characterized by similar velocity anomalies in amplitude, pattern and location that are consistent with the well-known tectonic and geologic structure of the research area and are comparable to previous tomographic models published in the literature.

How to cite: Timkó, M., Wiesenberg, L., El-Sharkawy, A., Wéber, Z., and Meier, T. and the AlpArray Working Group: Crustal and upper mantle velocity structure of the Pannonian Region using Rayleigh wave ambient noise tomography, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14525, https://doi.org/10.5194/egusphere-egu21-14525, 2021.

Adriatic microplate (Adria) has been a major component of the central Mediterranean geodynamic history since at least Late Cretaceous time. Although Cenozoic motion of Adria is relatively well established, there remains several open questions regarding its dynamics. This is especially evident when trying to reconstruct the motion of Adria since early Miocene. Although there is a general consensus about the counterclockwise rotation of Adria with respect to Europe, the amount of this rotation is still a matter of vigorous debate. In order to explain various measurements, several models of Adria motion were introduced: 1) Adria moving in conjunction with Africa, 2) Adria moving independently as a single block and 3) Adria moving independently but divided into two fragments.

Here we explore the third model by using data from temporary seismic station deployments (AlpArray and AlpArray-CASE) and available permanent stations surrounding the Adriatic Sea. We constructed the tomography image of the Adriatic Sea region using the interstation surface wave dispersion measurements from teleseismic events. Additionally, we test the properties of the Central Adriatic crust by analyzing P-receiver functions from mid-Adriatic island seismic stations. Preliminary results show anomalous lithospheric structure in the Central Adriatic dividing Adria into two sections thus indicating that Adria fragmentation has progressed all the way to the base of the lithosphere. 

How to cite: Stipčević, J., Belinić, T., Kolínský, P., Herak, M., Molinari, I., and Dasović, I.: Adria microplate fragmentation: geophysical perspective, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14133, https://doi.org/10.5194/egusphere-egu21-14133, 2021.

The dense SWATH-D seismic network in the Central-Eastern Alps gives an unprecedented window into the collision of the Adriatic and European plates. Previous studies have suggested a Moho gap overlying a subduction polarity switch. This switch, from European subduction in the west to Adriatic subduction in the east, was suggested by teleseismic tomography where low velocity zones in the mantle were interpreted as two slabs with opposite subduction polarity. The TRANSALP profile at 12°E indeed showed a gently southward dipping European Moho truncated by a nearly flat Adriatic Moho in receiver function (RF) images, which clearly indicated southward directed subduction. In contrast, RF images derived from the EASI profile at 13.3°E were interpreted to show Moho topography consistent with underthrusting Adriatic Moho, which would support the hypothesized polarity switch, but the image is actually ambiguous. 

We apply the receiver function method to stations in the dense SWATH-D broadband seismic network, covering approximately the area from 45-49°N and 10-15°E, supplemented by the AlpArray Seismic Network and the EASI data. We construct common conversion point stacks in order to pick the Moho conversion and its multiples.  The 15 km average station spacing has allowed us to fill in areas where previously the Moho was too weak to image. In this more comprehensive image, the asymmetry of the Moho in the TRANSALP profile can be traced to continue to at least the longitude of the EASI profile, suggesting continued southward-directed underthrusting of the European crust along the extent of the Eastern Alps, in conflict with the popular polarity switch hypothesis. At the eastern border of our study area we capture a sharp transition from European to extended Pannonian crust. Here the Adriatic Moho retreats and dips below the Pannonian Moho as it continues beneath the Dinarides.

How to cite: Mroczek, S., Tilmann, F., Pleuger, J., Yuan, X., and Heit, B. and the the AlpArray Working Group: Filling the Moho gap: High resolution crustal structure of the Eastern Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11044, https://doi.org/10.5194/egusphere-egu21-11044, 2021.

Noise cross-correlations provide a good azimuthal coverage, limited only by the distribution of noise sources and the layout of the stations used. It is therefore a promising method to constrain azimuthal anisotropy. As noise cross-correlations consist mainly of surface waves, they are especially sensitive to the crust and provide good depth constraints, as opposed to SKS-splitting data that are more sensitive to the upper mantle. We use the AlpArray network as well as stations from permanent networks all across Europe to perform time-domain beamforming on noise cross-correlations. The extent and density of the AlpArray network allows us to obtain reliable measurements all across the Alps. We divide the area in smaller zones using all stations outside the zone as sources and all stations inside as a sub-array for beamforming. This allows us to estimate the quality of our measurements in a region where strong lateral heterogeneities make measurements challenging, by estimating the magnitude of bias due to heterogeneities using the cos(theta) amplitude and evaluating uncertainties with bootstrap. This way, we measure Rayleigh wave azimuthal anisotropy in several period bands between 15 s and 60 s period. Inversion of dispersion curves in specific areas allows us to constrain the depth of the observed anisotropy. The results are broadly similar to results from SKS-splitting as they are generally parallel to the mountain belt. However, we observe lower anisotropy at short periods (40 seconds and less) in the Alps themselves than in surrounding regions. We also observe several structures in the crust that are not observed with SKS-splitting data. The most striking is a strong and spatially coherent NE-oriented anisotropy to the NW of the Alps that is possibly related to Variscan inheritance (at 40 seconds and less, in the upper and lower crust).  In the Northern Apennines, we observe anisotropy perpendicular to the belt at 30 s period (middle crust) that correlates well with an area of strong radial anisotropy recently observed by Alder et al (in review) at 30 km depth. 

How to cite: Soergel, D., Pedersen, H., Bodin, T., Paul, A., and Stehly, L.: Imaging azimuthal anisotropy in the alpine crust from ambient noise beamforming., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13514, https://doi.org/10.5194/egusphere-egu21-13514, 2021.

We have successfully derived a new 3-D high resolution shear wave velocity model of the crust and uppermost mantle of a large part of W-Europe from transdimensional ambient-noise tomography. This model is intended to contribute to the development of the first 3-D crustal-scale integrated geophysical-geological model of the W-Alps to deepen understanding of orogenesis and its relationship to mantle dynamics.

We used an exceptional dataset of 4 years of vertical-component, daily seismic noise records (2015 - 2019) of more than 950 permanent broadband seismic stations located in and around the Greater Alpine region, complemented by 490 temporary stations from the AlpArray sea-land seismic network and 110 stations from Cifalps dense deployments.

We firstly performed a 2-D data-driven transdimensional travel time inversion for group velocity maps from 4 to 150 s (Bodin & Sambridge, 2009). The data noise level was treated as a parameter of the inversion problem, and determined within a Hierarchical Bayes method. We used Fast Marching Eikonal solver (Rawlinson & Sambridge, 2005) jointly with the reversible jump algorithm to update raypath geometry during inversion. In the inversion of group velocity maps for shear-wave velocity, we set up a new formulation of the approach proposed by Lu et al (2018) by including group velocity uncertainties. Posterior probability distributions on Vs and interfaces were estimated by exploring a set of 130 millions synthetic 4-layer 1-D Vs models that allow for low-velocity zones. The obtained probabilistic model was refined using a linearized inversion. For the ocean-bottom seismometers of the Ligurian-Provencal basin, we applied a specific processing to clean daily noise signals from instrumental and oceanic noises (Crawford & Webb, 2000) and adapted the inversion for Vs to include the water column.

Our Vs model evidences strong variations of the crustal structure along strike, particulary in the subduction complex. The European crust includes lower crustal low-velocity zones and a Moho jump of ~8-12 km beneath the W-boundary of the external crystalline massifs. We observe a deep LVZ structure (50 - 80 km) in the prolongation of the European continental subduction beneath the Ivrea body. The striking fit between the receiver functions ccp migrated section across the Cifalps profile and this new Vs model validate its reliability.

How to cite: Nouibat, A., Stehly, L., Paul, A., Brossier, R., Bodin, T., Schwartz, S., and Working Group, A.: First step towards an integrated geophysical-geological model of the W-Alps: A new Vs model from transdimensional ambient-noise tomography, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3197, https://doi.org/10.5194/egusphere-egu21-3197, 2021.

The recent S-wave velocity tomography undertaken at the scale of the Alps by Nouibat et al. (2021) allows a reappraisal of the deep structure of this mountain belt. These geophysical data highlight the role of crustal geometry in the strain field development observed in the Western Alps. The geophysical imagery shows a standard crustal thickness in the foreland, with slow velocities (<3.6 km.s^-1) in the lower crust. The occurrence of a sharp Moho offset of 5-12 km is detected beneath the External Crystalline Massifs (ECMs). The ECMs do not show any significant crustal thickening in their frontal parts (<35 km), except for the Pelvoux ECM (35-40 km). Beneath the internal zones, east of the Penninic Frontal Thrust, the crustal geometry is more complex with the presence of an European continental slab subducting locally deeper than 80 km beneath the Adria plate. This slab is overlain by a high-pressure metamorphic orogenic prism. The lower part, corresponding to the Ivrea gravimetry anomaly, shows seismic signatures of serpentinized mantle (Vs between 3.8 and 4.3 km.s^-1) whose upper limit is located at 10 km depth below the Dora Maira internal crystalline massif. This new crustal-scale image can be compared to the current deformation pattern, which appears highly partitioned at the scale of the Alpine arc. The internal zones show a transtensional deformation regime, whose activity is distributed along two major seismic lineaments (the ‘Piemontais’ and ‘Briançonnais’ ones). The Alpine European foreland shows a transpressional deformation that is more diffuse and associated with vertical displacements in the ECMs. Beneath the Po plain, the seismic activity is deeper (>40 km), and correlates with a transpressional deformation which is localized along sub-vertical lineaments. The deformation of the orogenic prism appears controlled by a deeper and rigid mantle indenter split in two units by a major subvertical serpentinized structure. The upper unit, which indents horizontally and vertically the crustal orogenic prism, is located between 20 and 45 km depth. The lower unit corresponds to the western boundary of the Adria mantle that pinches directly the European slab. The surface observations and geochronological data suggest that the Moho offstets are superposed on European crustal-scale faults trend inherited from the Variscan orogeny, following the East-Variscan strike-slip system. This structural anisotropy was reactivated during the Alpine orogeny as shear zones in a mainly transpressional regime since about 25-30 Ma, as documented by Ar-Ar data on syn-kinematic mica and U-Pb on monazite. The comparison of current seismicity with the kinematics of exhumed shear zones suggests a continuity of this regime since 25-30 Ma, in response to the Adria plate anticlockwise rotation.

How to cite: Schwartz, S., Nouibat, A., Rolland, Y., Dumont, T., Paul, A., Guillot, S., Malusà, M., Stehly, L., Group, C., and Working Group, A.: Role of the deep crustal scale geometry on Western Alps strain partitioning : Insights from S-wave velocity tomography, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8928, https://doi.org/10.5194/egusphere-egu21-8928, 2021.

The CIFALPS receiver-function (RF) profile in the southwestern Alps provided the first seismological evidence of continental subduction in the Alps, with the detection of waves converted on the European Moho at 75-80 km depth beneath the western edge of the Po basin (Zhao et al., 2015). To complement the CIFALPS profile and enhance our knowledge of the lithospheric structure of the Western Alps, we installed CIFALPS2, a temporary network of 55 broadband seismic stations that operated for ~14 months (2018-2019) across the North-Western Alps (Zhao et al., 2018). The CIFALPS2 line runs from the Eastern Massif Central to the Ligurian coast, across the Mont-Blanc and Gran Paradiso massifs and the Ligurian Alps. Seismic stations were installed along a quasi-linear profile with a spacing of 7-10 km.

We will show 2 receiver-function CCP (common-conversion point) depth-migrated sections along the CIFALPS2 profile, the first one across the Alps, and the second one across the Ligurian Alps and the Po basin. The time-to-depth migration of RF data is based on the new 3-D Vs model of the Greater Alpine region derived by Nouibat et al. (2021) using transdimensional ambient noise tomography on a large dataset including the AlpArray seismic network. Depth sections across the Vs model are also useful for interpreting the RF CCP sections as they have striking similarities.

The images of the lithospheric structure of the NW Alps along CIFALPS2 are surprisingly different from those of the SW Alps along CIFALPS. The deepest P-to-S converted phases on the European Moho are detected at 60-65 km depth beneath the Ivrea-Verbano zone, that is 15 km less than on CIFALPS. The negative polarity converted phase interpreted as the base of the Ivrea body mantle flake on the CIFALPS section is still visible on CIFALPS2, but with a lower amplitude. The RF section confirms the existence of a jump of the European Moho of ~10 km amplitude in less than 10 km distance, which is located within a few km from the western boundary of the Mont Blanc external crystalline massif. All these observations are confirmed by the Vs model that also displays a less deep continental subduction than on CIFALPS, weaker S-wave velocities in the Ivrea body wedge, and the jump of the European Moho.

The Moho beneath the Ligurian Alps is detected at 25-30 km depth both on the RF and on the Vs depth sections. Moving northwards, this Ligurian Moho is separated from the Adriatic Moho by a puzzling S-dipping set of P-to-S converted waves with negative polarity. The crust of the Ligurian Alps is characterized by a set of north-dipping negative-polarity converted waves at 10 to 20 km depth beneath the Valosio massif, which is a small internal crystalline massif of (U)HP metamorphic rocks located north of Voltri. The similarity of this set of negative-polarity conversions to the one observed beneath the Dora Maira massif on the CIFALPS profile suggests that it may be a relic of the Alpine structure overprinted by the opening of the Ligurian sea.

How to cite: Paul, A., Nouibat, A., Zhao, L., Solarino, S., Schwartz, S., Malusà, M., Stehly, L., Aubert, C., Dumont, T., Eva, E., Guillot, S., Pondrelli, S., Salimbeni, S., and Working Group, A.: Striking differences in lithospheric structure between the north- and south-western Alps: insights from receiver functions along the Cifalps profiles and a new Vs model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9391, https://doi.org/10.5194/egusphere-egu21-9391, 2021.

The Alpine orogeny, which was formed by subduction of the European plate beneath the Adria plate, is considered as one of the world’s foremost natural laboratories for the study of orogenic processes. In contrast to other mountain belts, the Western Alpine belt is curved and affected by three-dimensional effects. Due to differences in stress distribution and rheological properties of crustal rocks, the Moho geometry and crustal structure along different sections differ, in particular in the vicinity of the continental subduction complex.

To better understand the configuration of continental subduction along a profile that crosscuts the North-Western Alps, we combine receiver function analysis with computation of synthetic receiver functions and gravity anomaly modeling to precise the subduction structures and estimate a crustal 2D shear wave velocity and density model. Seismic data come from the CIFALPS2 (China-Italy-France Alps seismic survey) temporary experiment, which operated from 2018 to 2020. We use a 2D hybrid waveform simulation method (Zhao et al., 2008) that is reliable and efficient and has a better response to 2D structures compared to conventional 1D waveform inversion methods, in particular for the dipping Moho interface of the subduction complex. We compute synthetic receiver functions for a large set of models compatible with surface geology data, which are then processed to obtain synthetic CCP depth-migrated stacks. Furthermore, we model the Bouguer gravity data along the same profile to obtain preferred density distribution. The nature of rocks in the subduction complex can be inferred from our synthetical models.

Compared to the results of the CIFALPS profile in the Central Western Alps (Zhao et al., 2015), the subduction along the CIFALPS2 profile has a shallower dip angle, which is a significant difference between the two sections. As for velocity and density models, the two sections have a high velocity and high-density wedge in the subduction complex. We argue that the reason for the difference in crustal structures between the two sections may be related to the difference in stress distribution.

Zhao, L., et al. (2008). "A two-dimensional hybrid method for modeling seismic wave propagation in anisotropic media." Journal of Geophysical Research 113(B12).

Zhao, L., et al. (2015). "First seismic evidence for continental subduction beneath the Western Alps." Geology 43(9): 815-818.

How to cite: Mao, Y., Zhao, L., Paul, A., Solarino, S., Aubert, C., Dumont, T., Eva, E., Guillot, S., Malusà, M. G., Pondrelli, S., Salimbeni, S., and Schwartz, S.: Configuration of continental subduction beneath the Western Alps: results using forward modeling of receiver functions and gravity data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8117, https://doi.org/10.5194/egusphere-egu21-8117, 2021.

Accuracy of hypocenter location, in particular focal depth, is a precondition for high-resolution seismotectonic analysis of natural and induced seismicity. For instance, linking seismicity with mapped fault segments requires hypocenter accuracy at the sub-kilometer scale. In this study, we demonstrate that inaccurate velocity models and improper phase selection can bias absolute hypocenter locations and location uncertainties, resulting in errors larger than the targeted accuracy. To avoid such bias in densely instrumented seismic networks, we propose a coupled hypocenter-velocity inversion restricted to direct, upper-crustal Pg and Sg phases. The derived three-dimensional velocity models, combined with dynamic phase selection and non-linear location algorithms result in a highly accurate earthquake catalog, including consistent hypocenter uncertainties. We apply this procedure to about 60’000 Pg and 30’000 Sg quality-checked phases of local earthquakes in the Central Alps region. The derived tomographic models image the Vp and Vs velocity structure of the Central Alps’ upper crust at unprecedented resolution, including small-scale anomalies such as those caused by a Permo-Carboniferous trough in the northern foreland, Subalpine Molasse below the Alpine front or crystalline basement units within the Penninic nappes. The external Aar Massif is characterized by low Vp/Vs ratios of about 1.625-1.675 in the depth range of 2-6.5 km, which we relate to a felsic composition of the uplifted crustal block, possibly with increased quartz content. Finally, we discuss along-strike variations imaged by relocated seismicity in the Central Alps and demonstrate how joint interpretation of velocity structure and hypocenters provides additional constraints on lithologies of upper-crustal seismicity.

How to cite: Diehl, T., Kissling, E., Herwegh, M., and Schmid, S.: New insights into structure and seismicity of the Central Alps from 3D Pg and Sg tomography and improved hypocenter relocations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2902, https://doi.org/10.5194/egusphere-egu21-2902, 2021.

In the framework of the SeismoTeCH project, which aims at advancing our understanding of seismotectonic processes in Switzerland, we present the first 3-D attenuation model of the upper crust for the Central Alps and their northern foreland. The 3-D inversions derive the quality factor Q (1/attenuation) using path attenuation t^∗ observations for 4,192 distributed earthquakes recorded on permanent and temporary stations, including both velocity and acceleration records for the period 2002-2019. We followed a procedure of gradational inversions, in which a series of inversions with increasingly grid complexity are performed, with the goal of obtaining a useful Q model everywhere despite the varied data distribution. The Qs and Qp results show large-scale features in the upper crust, which are consistent with a recently improved high-resolution velocity models of the same region and serve to refine the interpretations of crustal structures from Vp and Vp/Vs. For example, the foreland region of southern Germany and northern Switzerland show a low Q crustal block bounded by high Q regions in the uppermost layer between -2.5 and 2.0 km depth. This markedly correlates with the overlying surface geology, where low Q areas coincide with the Molasse Basin, and the transition between low and high Q regions outline the geological boundary between the Molasse and the Mesozoic sediments towards north and the Alpine front to the south. At depths ranging between 2.0 - 6.5 km, low Q is imaged along the Rhone valley in the Valais in southwest Switzerland. This region presents the transition between the Centrals and Western Alps and hosts the presently seismically most active fault zones. As the attenuation of fractured areas is enhanced by fluids, low Q values may relate here to distributed microfractures that produce greater fracture connectivity and permeability in a relatively higher strain-rate zone. These geophysical constraints seem to support crustal scale fluid flow along fracture networks as manifest by the prominent occurrence of hot springs in this area. On the other hand, the moderate-to-high Qs and Qp (400-800) along with low Vp/Vs ratio and high Vs observed in the external Aar Massif could be indicative of metamorphic processes leading to different Vp/Vs ratios compared to the basement in the northern foreland (Black Forest Massif), and possibly image the continuation of the massif 20-30 km further to the northeast. In combination with recently developed Vp and Vs velocity models, the developed 3-D attenuation models provide additional constraints in terms of composition and physical properties of the uppermost crust of the central Alps as well as crucial input for next generation seismic hazard models of Switzerland, allowing for a more realistic prediction of earthquake related ground motions.

How to cite: Lanza, F., Diehl, T., Eberhart-Phillips, D., Herwegh, M., Fäh, D., and Wiemer, S.: 3-D Qp and Qs Seismic Attenuation for the Central Alps and their Foreland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8028, https://doi.org/10.5194/egusphere-egu21-8028, 2021.

Despite recent tectonic activity, the Alpine mountain range in central Europe is mostly characterized by weak to moderate seismicity. Low earthquake magnitudes and the heterogeneous crust comprising of different tectonic units challenge centroid moment tensor inversions in this region. Thanks to the dense AlpArray seismic network, comprising more than 600 stations across the Alps, as well as the adoption of a flexible, bootstrap-based inversion tool, we were able to reduce the magnitude threshold for moment tensor inversion to Mw 3.0. The inversion set-up was implemented after systematic tests of different frequency bands, distance ranges, input data types and azimuthal gaps. We quantified the uncertainties of centroid locations and moment tensors, and assessed the reliability of potential non double couple components. Here, we present ~80 deviatoric moment tensor solutions and compare our results to strain rates, historic and recent seismic activity as well as to other published focal mechanisms. We identify three main seismically active subregions, namely the Western Alps, the Lake Garda region and the SE Alps, and two clusters further away from the study region, in the Dinarides and the Apennines. Seismicity is particularly low in the NE Alps and in parts of the central Alps. Additionally, we apply a focal mechanism clustering algorithm to the joint catalog, including our moment tensor solutions and those from existing catalogs. While typical E-W to ENE-WSW striking thrust faulting is observed in the Friuli area in the SE Alps, strike-slip faulting with a similarly oriented pressure axis is observed along the central Alps and in the Dinarides. NW-SE striking normal faulting is observed in the NW Alps with a similar strike direction as the dominant normal faulting events in the Apennines. In the W Alps as well as in the SE Alps, rotations of mechanisms are observed. Both, our centroid depths as well as hypocentral depths in existing catalogs indicate that Alpine seismicity is predominantly very shallow, with 80 % of the studied events being shallower than 10 km.

How to cite: Petersen, G. M., Cesca, S., Heimann, S., Niemz, P., Dahm, T., Kühn, D., Kummerow, J., Plenefisch, T., and AlpArray working group, T.: Implications on recent tectonics in the Alps from centroid moment tensor inversion of weak to moderate seismicity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3164, https://doi.org/10.5194/egusphere-egu21-3164, 2021.

The AlpArray seismological experiment is an international and interdisciplinary project to advance our understanding of geophysical processes in the greater Alpine region. The heart of the project consists of a large seismological array that covers the mountain range and its surrounding areas. To understand how the Alps and their neighbouring mountain belts evolved through time, we can only study its current structure and processes. The Eastern Alps are of prime interest since they currently demonstrate the highest crustal deformation rates. A key question is how these surface processes are linked to deeper structures. The Swath-D network is an array of temporary seismological stations complementary to the AlpArray network located in the Eastern Alps. This creates a unique opportunity to investigate high resolution seismicity on a local scale.

In this study, a combination of waveform-based detection methods was used to find small earthquakes in the large data volume of the Swath-D network. Methods were developed to locate the seismic events using semi-automatic picks, and estimate event magnitudes. We present an overview of the methods and workflow, as well as a preliminary overview of the seismicity in the Eastern Alps.

How to cite: Hofman, R., Kummerow, J., Cesca, S., Wassermann, J., and Plenefisch, T. and the AlpArray Working Group: Microseismicity in the Eastern Alps: Preliminary Results From the Swath-D Network, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9829, https://doi.org/10.5194/egusphere-egu21-9829, 2021.

The Alpine orogen and the Apennine system are part of the complex tectonic setting in the Mediterranean Sea caused by the convergence between Africa and Eurasia. Between 30 Ma and 15 Ma the Calabrian subduction retreated in a southeast direction pulling Corsica and Sardinia away from the Eurasian landmass. In this extensional setting, the Ligurian Sea was formed as a back-arc basin. The rifting jumped 15 Ma ago in the Tyrrhenian Sea leaving Corsica and Sardinia in a stable position relative to Eurasia.

Within the framework of the AlpArray research initiative a long-term seismological experiment was conducted in the Ligurian Sea to investigate the lithospheric structure and the seismicity in the Ligurian basin. The passive seismic network consisted of 29 broad-band ocean bottom stations from Germany and France. It was in operation between June 2017 and February 2018.

Two seismicity clusters occurred in the centre of the Ligurian Basin. The 18 earthquakes are located in the lower crust and in the upper-most mantle at depths between 10 km and 16 km. Re-location was performed only using picks from the OBS in the centre of the Ligurian Sea to avoid artifacts from the complex 3D velocity structure of the basin. Mantle refractions Pn and Sn have apparent velocities of 8.2 km/s and 4.7 km/s. The low Vp-Vs-ratio of 1.72 indicates a more brittle behaviour of the mantle material.

Fault plane solutions were determined for four events using also the data of land stations in southern France, Corsica, Sardinia and northern Italy. The focal mechanisms are thrust faulting. Fault planes strike in a NE-SW direction, coinciding with the alignment of the events and the direction of the basin axis.

We interprete the two earthquake clusters related to the inversion of the Ligurian Basin where the basin’s centre is under compression and stresses are taken up by reactivated faults in the crust and uppermost mantle. The compressional forces could be caused by the convergence of Africa and Europe. In general, observations of earthquakes in continental mantle lithosphere are rare and they reveal on the one hand a strengthening of the crust and uppermost mantle during rifting and on the other hand they support the interpretation that rifting failed in the northern Ligurian Basin.

How to cite: Thorwart, M., Dannowski, A., Grevemeyer, I., Lange, D., Kopp, H., Petersen, F., Crawford, W., and Paul, A. and the AlpArray Working Group: Local Seismicity indicates basin inversion in the Ligurian Sea , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6411, https://doi.org/10.5194/egusphere-egu21-6411, 2021.

The north Ligurian margin is a stretched continental margin located at the junction of the Western Mediterranean Sea and the Alpine belt. This region underwent several phases of contrasting deformation styles. The Ligurian basin opened from late Oligocene to early Miocene times, as a result of a back-arc extension induced by the rollback of the subducted Apulian plate. Since then, it has been evolving in the immediate vicinity of the active Alpine orogen, in a regional compressional setting between the Corsica-Sardinia continental block and mainland Europe.

Nowadays, continuous seismic activity, with mainly reverse focal mechanisms, is recorded in the northeastern part of the Ligurian Basin. It is attributed to the compressional phase at work in the Gulf of Genoa since about 5 Myrs, which led to a significant uplift of the north margin documented by a vertical offset of the Messinian stratigraphic markers by more than 1000 m offshore Imperia. Although active seismogenic faults are still poorly known, a fault system outcropping at the foot of the continental slope, offshore Liguria and the French Riviera, is suspected from previous joint high-resolution seismic reflection data interpretation and sismotectonic studies.

The SEFASILS project (Seismic Exploration of Faults And Structures In the Ligurian Sea) aims to better understand the mechanisms of the ongoing tectonic inversion of the margin and the crustal-scale tectonic structures –active or not– marking its evolution.  We also aim to better characterize the sharp transition from the South Alpine belt to the Ligurian basin. Acquiring quality deep seismic data in the Ligurian Sea is challenging due to the complexity of structures beneath the margin and to the screening effect of the thick Messinian evaporitic series interlayered in the sedimentary cover farther seaward. To this end, joint acquisitions of deep, long-streamer multichannel seismic (MSC) reflection data and dense sea-bottom wide angle refraction data (WAS) have been carried out along a 150 km long profile offshore Nice, perpendicularly to the basin’s axis.

The MCS data, thanks to pre- and post-stack migration, highlight faults at the foot of the continental slope rooting deeper than the salt decollement level. A first arrival travel time tomographic inversion of the wide angle data allowed us to build a velocity model of the study area reaching down to the uppermost mantle. Here, we present the results obtained from the joint analysis of MCS and WAS data. On the southern part of our profile some deep reflectivity, closely mirrored by the 7 km/s tomographic isovelocity, likely corresponds to the Moho. It is lost to the north, where shallower reflectivity, which could be interpreted as the base the thick sedimentary cover, coincides with the 5 km/s isovelocity. These two features are separately observed on both sides of what appears to be a major structural discontinuity between two contrasting basement domains, coinciding with an anomalously large salt diapiric complex in the sedimentary cover, also observed farther east in the basin. Such observations and their potential consequences will be discussed, in the light of previous regional studies.

How to cite: Canva, A., Dessa, J.-X., Ribodetti, A., Beslier, M.-O., Schenini, L., Larroque, C., Thinon, I., Sambolian, S., Chamot-Rooke, N., Delescluse, M., and Déverchère, J.: Structural inversion of the North Ligurian margin: results from the SEFASILS experiment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9759, https://doi.org/10.5194/egusphere-egu21-9759, 2021.

The apparent seismic quiescence of the Upper Rhine Graben, as opposed to the Lower Rhine Graben, inspired authors to put the question whether it is real or due to short sampling (Stein et al. 2015). Lack of destructive earthquakes in the Leydecker (2011)  historical catalogue (except two in the very north and south) made us to survey an alternative source of seismic documentation. We carried out archaeoseismological studies on the built environment: on Roman cities of Mogontiacum (Mainz) and Augusta Raurica (Kaiseraugust) on opposite ends of the Upper Rhine Graben, and on Medieval sites: the cathedrals of Mainz, Worms, Speyer, Strasbourg and Basel, the monastery churches of Lorsch, Ladenburg and Achern, altogether at 19 sites. Buildings were checked for seismic deformation. Detailed architectural history of construction, destruction and repair was created for each. Dating of (re)construction was achieved by studying published historical documents. Known earthquake epicenters were re-positioned, intensities corrected (usually raised), and previously unknown, highly destructive events recognized and dated. The 1080 AD Mainz earthquake (I=VI) is shifted to Speyer, causing collapse and rebuilding of the imperial cathedral there (I=IX). An additional earthquake occurred there in early modern times, damaging the newly built parts. The late 12^thcentury has seen the rebuilding of the Strasbourg cathedral: surviving Romanesque parts still carry evidence for earthquake damage, covered by the Gothic cross-nave. A strange belfry was added to the western front to reinforce the two towers in unsatisfactory status in 1384. Destruction of the 1356 Basel earthquake is much larger than previously recorded: churches citywide carry evidence for damage followed by substantial reconstruction. The Upper Rhine Graben was seismically active in the past two millennia: instrumental quiescence is is misleading, causing dangerously low hazard estimates.

How to cite: Kazmer, M. and Reicherter, K.: Millennial seismicity of the Upper Rhine Graben, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1806, https://doi.org/10.5194/egusphere-egu21-1806, 2021.

How to cite: Sonnet, M., Labrousse, L., Bascou, J., and Plunder, A.: Geophysical signature of the alpine slab: Field analogues and direct models , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16332, https://doi.org/10.5194/egusphere-egu21-16332, 2021.

The Alps mountains and its forelands consist of a heterogeneous lithosphere, comprised of a multitude of tectonic blocks from different tectonic provinces with different thermo-physical properties. Patterns of seismicity distribution are also observed to vary significantly throughout the region. However, the relationship between seismicity and lithospheric heterogeneity has been often overlooked in previous studies. We present an overview of recent results that have attempted to address these questions through the use of integrated 3D modelling techniques, thereby including: (i) a gravity and seismic data constrained, 3D, density structural model of the lithosphere; (ii) a 3D thermal model constrained against available wellbore temperature data; and,  (iii) a 3D rheological model of the long-term lithospheric strength and effective viscosities. Our models support the existence of a first-order correlation between the distribution of seismicity (laterally and with depth) and the strength of the lithosphere, with the former being clustered mainly within weaker domains. Beneath the Alps, observed upper-crustal level (i.e., unimodal) seismicity correlates with a weaker lithosphere where plate strength is controlled by the thick crustal root. Whereas in the southern foreland, weaker zones are found preferentially around the stronger Adriatic indenter while in the northern foreland they are located in the crust beneath the the Upper Rhine Graben (URG). We found that this correlation is primarily controlled by resolved thermal gradients and is a function of the tectonic inheritance setting (e.g., UGR), crustal architecture (e.g., thickness of sediments, upper and lower crust) and LAB depth. Sediment thickness and topographic effects controls the shallow thermal filed (0 – 10 km) whereas the deeper thermal field is controlled by the thickness of felsic upper crust (higher radiogenic heat contribution), the mafic lower crust (less radiogenic heat contribution) and basal thermal boundary condition from LAB depth. Seismicity is bounded by specific isotherms, 450 ^oC in the crust and < 600 ^oC in the mantle, except in regions where slabs are imaged by seismic tomography models. This is in contrast to the recent proposition that convergence velocity is a first-order factor controlling seismicity in an orogen rather than its architecture. Fast convergence rates (e.g., Himalayas) have been related to the subduction of the cold crust to deeper crustal depths thereby leading to a deepening of the brittle  domain and to a bimodal (i.e., upper and lower crust) seismicity character. In contrast, slow convergence (e.g., Alps) is thought to lead to a hotter ductile lower crust thus limiting brittle deformation within the upper crust. We therefore end our contribution by opening a discussion on the relative role of convergence rates and lithospheric heterogeneities, inherited and/or developed during orogenesis, in controlling the seismicity. In doing so we carry out a comparison between observed seismicity and lithospheric architecture in the other mountain ranges of the western Alpine-Himalayan collision zone where  convergence velocities are of a similar order of magnitudes as Alps, i.e., the Betics, the Pyrenees and the Apennines but where seismicity is observed to occur both at upper and lower crustal levels.

How to cite: Kumar, A., Spooner, C., Scheck-Wenderoth, M., and Cacace, M.: How seismicity relates to lithospheric heterogeneity in the Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11552, https://doi.org/10.5194/egusphere-egu21-11552, 2021.

Geodetic data play a crucial role in the detection of surface deformation related to active tectonic processes. The present study aimed to investigate the Northeastern Italian sector, characterized by a convergent regime due to the NNW-ward motion of the Adria microplate towards the Eurasian plate, at a rate of ~ 2mm/yr. N-S shortening is accommodated by fold and thrust systems in the Alpine chain and buried below the Friuli-Venetian plain sediments. We used InSAR and GNSS data respectively in 2015-2019 and 2000-2020 time interval to estimate the surface kinematics and deformation pattern of the area. We processed the SAR images acquired by the European satellites Sentinel 1A/B from ascending and descending tracks by using the Stanford Method for Persistent Scatterers (StaMPS). A post-processing of the resulting Line-Of-Sight (LOS) deformation time series was carried out by applying a spatial-temporal filter and calibrating using the velocities provided by GNSS stations. Finally, the post-processed ascending and descending LOS measurements were combined to solve for the vertical and horizontal (east-west) deformation components. We observed a positive vertical signal toward the Alps, in the northern region of Veneto and Friuli-Venezia Giulia. Moreover, we observed a significant negative vertical signal located in the plain and in the coastal zones due to the subsidence that strongly affects these areas. Horizontal velocities with rate of 1-2 mm/yr are observed close to main tectonic structures, especially in the eastern and the northwestern sector of the study area, where GNSS data reveal higher shortening rate.

How to cite: Areggi, G., Merryman Boncori, J. P., Pezzo, G., Serpelloni, E., and Bonini, L.: Surface deformation analysis in Northeast Italy by using PS-InSAR and GNSS data , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12724, https://doi.org/10.5194/egusphere-egu21-12724, 2021.

The Alps have an overall East-West orientation, which changes radically in their western termination, where they rotate southward into a N-S strike, and then eastward into an E-W strike, forming the arc of the Western Alps. This arc is commonly inferred to have formed during collision, due to indentation of the Adriatic plate into the European continental margin. Several models attempted to provide a kinematic explanation for the formation of this arched, lateral end of the Alps. Indeed, the radial nature of the transport directions observed along the arc of the Western Alps cannot be explained by a classic convergence model.
For more than 50 years the formation of this arc was been associated to westward-directed indentation of Adria, accommodated along East-West oriented strike-slip faults, a sinistral one in the South of the arc and a dextral one in the North. The dextral one correspond to the Insubric Fault. The sinistral strike-slip zone, inferred to be localized along the «Stura corridor» (Piedmont, Italy) would correspond to a displacement of 100 to 150 km according to palaeogeographical, and geometric analyses. However, field evidence is scarce and barely documented in the literature.
Vertical axis rotations of the Adriatic indenter also inferred to be syn-collisional could have influenced the acquisition of the morphology of the arc. Paleomagnetic analyses carried out in the Internal Zone and in the Po plain suggest a southward increading amount of counter-clockwise rotation of the Adriatic plate and the Internal Zone, varying from 20°-25° in the North to nearly 120° in the South.
Dextral shear zones possibly accommodating this rotation in some conceptual models is observed in several places below the Penninic Front and affect the Argentera massif to the south. However, the measured displacement quantities do not appear to be equivalent to those induced by such rotations.
The present study aims to constrain the kinematic evolution of the arc of the Western Alps through a multidisciplinary approach. The first aspect of this project is the structural analysis of the area (Stura corridor) inferred to accommodate large sinistral displacements allowing for the westward indentation of the Adriatic indenter. We discuss the general lack of field evidence supporting sinistral strike-slip movements, in contrast to large-scale compilation of structures suggesting the possible occurrence of such displacement. The second part consists of a palaeomagnetic study, in which new data are integred with a compilation of already existing data. This compilation shows that several parts of the arc in the External Zone did not suffer any Cenozoic rotations, hence suggesting that a proto-arc already axisted at the onset collision, as suggested by independent evidence of some paleogeographic reconstruction. Finally, 2D and 3D thermo-mechanical modeling in using the pTatin3D code is used to test which structural (geometrical), and rheological parameters affected the first-order morphology of the Western Alpin arc and its kinematics. The synthesis of these different approaches allows us to propose a new model explaining the kinematics and the mechanisms of formation of the Western Alps arc.

How to cite: Brunsmann, Q., Rosenberg, C., Bellahsen, N., and Le Pourhiet, L.: The arc of the Western Alps : understanding its kinematics and formation mechanisms based on new structural and paleomagnetic datas, and thermo-mechanical modelling., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16529, https://doi.org/10.5194/egusphere-egu21-16529, 2021.

The fact that the western Alps Miocene foreland basin succession is poorly dated impacts directly our understanding of the deformation kinematics of that part of the external part of the Alpine belt (France). Here we propose a multidisciplinary approach aiming at building a robust tectono-stratigraphic framework of the Miocene deposits at the basin scale (northern subalpine massifs, southern Jura, Royans, Bas-Dauphiné and La Bresse basins). Sr isotopes stratigraphy combined with magnetostratigraphy and biostratigraphy enable sequence stratigraphy subdivisions S1 to S8 between the Upper Aquitanian (-21 Ma) and the Tortonian (-9 Ma) dated with a precision <0.5 Ma. These results highlight four different palaeogeographical domains during the Miocene: (i) the oriental domain with depositional sequences S1a to S3 (~21.3 to 15Ma), (ii) the median domain, in which sequences S2, S3, S4 and S5 occurred (~17.8 to 14Ma), (iii) the occidental domain with sequences S2 to S8 (~17.8 to ~9.5Ma); and (iv) the Bressan domain, in which sequences S6 to S8 are found (~ 11.5 to ~9.5Ma).

This revised chronostratigraphy was complemented with a structural and tectono-sedimentary study based on new fieldwork data and a reappraisal of regional seismic profiles, allowing to highlight five major faults zones (FZ). It appears that the oriental, median and occidental paleogeographical domains are delineated by FZ1, FZ2 and FZ3, therefore suggesting a strong interplay between tectonics and sedimentation. Evidences of syntectonic deposits and a westward migration of the depocenters impart the following deformation chronology : a Oligocene compressive phase (P1) corresponding to thrusting above FZ1 rooted east (above) Belledonne, which generated reliefs that limited the early Miocene transgression to the east; an Early- to Middle Miocene W-WNW/E-ESE-directed compressive phase (P2) involving the Belledonne massif basal thrust, which between 18.05 +/- 0.15 Ma and 12Ma successively activated the Salève thrust fault, and the FZ2 to FZ5 from east to west. P2 deeply impacted the Miocene palaeogeographical evolution by a rapid westward migration of depocenters in response to the exhumation of piggy-back basins above the growing fault zones; a last Tortonian phase (P3), less well constrained, apparently implied a significant uplift in the subalpine massifs, combined with the activation of the frontal Jura thrust.

How to cite: Kalifi, A., Leloup, P.-H., Sorrel, P., Galy, A., Demory, F., Spina, V., Huet, B., Lecacheur, K., Grime, R., Pittet, B., and Rubino, J.-L.: Westward propagation of thrusts in the external Western Alps (France) reappraised from an updated chronostratigraphy of the Miocene Molasses, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8681, https://doi.org/10.5194/egusphere-egu21-8681, 2021.

Since two centuries the European Alps are a natural laboratory to study continental lithosphere deformation during mountain building. Since the early studies, a constant question has been to evaluate the importance of vertical versus horizontal displacements in the building of reliefs. Whilst the occurrence of large thrust sheets, as initially proposed from field observations, are now well explained in the frame of plate tectonics, controversies still arise on the precise geometry, amount, and timing of major thrusting during the orogeny.

We present a new detailed 3D structural study of the cover/basement relationships in the Chamonix synclinorium in between the Mont-Blanc (MB) and Aiguilles Rouges (AR) ranges. These massifs are two of the main external basement ranges of the western Alps.  The study allows deciphering the area structural history: the Mesozoic sedimentary cover has been thrust at least 10km NW above the Helvetic Basal Décollement (HBD) before to be offset by late steep thrusts during exhumation in the Miocene.

Such interpretation fundamentally diverges from the classical view of the sedimentary cover of the Chamonix synclinorium being expulsed from a former graben during a single deformation phase and implies that a major thrust phase lasting ~10 Ma has been overlooked. Our observations show that the HBD was a major thrust system active between ~30 and ~20 Ma, possibly until 15 Ma, with a shortening of more than 10km in the south to 20km in the north. It extends below most of the subalpine ranges and emerges in front of the Bauges and within the Chartreuse and Vercors massifs, and was rooted east of the External Cristalline Massifs (Mont-Blanc and Belledonne). During the Miocene, the HBD was cut by steep reverse faults and uplifted above the basement culmination of the External Cristalline Massifs obscuring its continuity and precluding its recognition as a major structure even if it was previously described at several localities.

How to cite: Mercier, A., Leloup, P. H., Courrioux, G., Caritg, S., Lopez, S., and Kalifi, A.: Rediscovery of a major alpine thrust : the Helvetic Basal Decollement, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8595, https://doi.org/10.5194/egusphere-egu21-8595, 2021.

In the front of the Digne thrust, the deformed foreland basin, the well exposed tectonic window of Barles, is still not well understood. This region has undergone a complex tectonic history involving synsedimentary deformation, potential migration of alpine front, late exhumation related to surface processes and potential salt tectonics. Although the stratigraphy and the structural geology of the area is well known, the respective contributions of regional tectonics, salt tectonic and surface processes remain uncertain. The region displays still enigmatic objects emplaced at each step, from the rifting phase to the late exhumation, such as the overturned Liassic Barre de Chine or the overturned Miocene syncline of the Vélodrome. This study aims at understanding the evolution of the foreland Valensole basin from the deposition of first sediments 30Ma ago to late exhumation and relief formation. We focused our work on the emblematic Vélodrome syncline which is also the only place where a continuous sequence of the basin deposits is exposed. The molassic and conglomeratic layers of the Vélodrome form an overturned syncline with a curved axis of which direction changes from EW in the north to NS in the most south-eastern part. The Vélodrome has been studied for more than a century but its history is still debated. If the Vélodrome is often interpreted as a growth fold which explain the observation of progressive unconformities, microstructural analyses (Fournier et al., 2008) suggest that folding postdates sedimentation. Moreover, recent studies (Graham et al., 2012) propose that this spectacular fold formed as a result of salt tectonic. The obliquity of the regional shortening direction regarding the axis direction, the 3-D pattern of the overturned Miocene series and the origin of the progressive unconformities are issues still not resolved. Such a complex tectonic structure as the Vélodrome fold requires a thorough understanding of the 3-D geometries and their evolution through time. Based on field observations and 3-D geometrical modelling (GeoModeller - ©BRGM), we propose a preliminary model of the Vélodrome that brings new insights on this part of the Valensole basin. The implicit approach that offer the GeoModeller and the field structural data-based approach (here more than 2000 structural data) bring an objective and new vision of the geometries in 3-D of the Vélodrome basin and provide arguments to determine the contribution of each geological processes in the tectonostratigraphic evolution of the north margin of the Valensole basin and subsequent shortening at the western subalpine front.

How to cite: Faure, A., Jolivet, L., Gumiaux, C., Allanic, C., Laurent, G., Loget, N., Callot, J.-P., and Guiomar, M.: New insights on the Vélodrome syncline in the western sub-alpine foreland basin of Haute Provence : 3-D geometrical modelling approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12342, https://doi.org/10.5194/egusphere-egu21-12342, 2021.

Ultrahigh-pressure (UHP) continental crustal rocks were first discovered in the Western Alps in 1984 and have since then been observed at many convergent plate boundaries worldwide. Unveiling the processes leading to the formation and exhumation of (U)HP metamorphic crustal rocks is key to understand the geodynamic evolution of orogens such as the Alps.

Previous numerical studies investigating (U)HP rock exhumation in the Alps predicted deep (>80 km) subduction of crustal rocks and rapid buoyancy-driven exhumation of mainly incoherent (U)HP units, involving significant tectonic mixing forming so-called mélanges. Furthermore, these predictions often rely on excessive erosion or periods of divergent plate motion as important exhumation mechanism. Inconsistent with field observations and natural data, application of these models to the Western Alps was recently criticised.

Here, we present models with continuous plate convergence, which exhibit local tectonic-driven upper plate extension enabling compressive- and buoyancy-driven exhumation of coherent (U)HP units along the subduction interface, involving feasible erosion.

The two-dimensional petrological-thermo-mechanical numerical models presented here predict both subduction initiation and serpentinite channel formation without any a priori prescription of these two features. The (U)HP units are exhumed coherently, without significant internal deformation. Modelled pressure and temperature trajectories and exhumation velocities of selected crustal units agree with estimates for the Western Alps. The presented models support previous hypotheses of synconvergent exhumation, but do not rely on excessive erosion or divergent plate motion. Thus, our predictions provide new insights into processes leading to the exhumation of coherent (U)HP crustal units consistent with observations and natural data from the Western Alps.

How to cite: Candioti, L. G., Vaughan-Hammon, J. D., Duretz, T., and Schmalholz, S. M.: Synconvergent and coherent (ultra)high-pressure crustal rock exhumation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11119, https://doi.org/10.5194/egusphere-egu21-11119, 2021.

Lawsonite is a strongly hydrated (12 weight % H₂O) Ca- and Al-rich silicate, exclusively stable along low P/T gradients, typical of subduction zones. The distribution and preservation of lawsonite at the scale of a subduction/collision belt reflect the occurrence of rocks with favourable chemical composition (mainly hydrothermally altered metabasalts and marly limestones (i.e. calcschists), two lithologies especially common in the oceanic units) and their pressure-temperature-fluid history (with preservation favoured by decreasing T during decompression).

The distribution of lawsonite in the Western Alps has been investigated since several decades. In the blueschist-facies units from the South-Western Alps (Queyras, Ubaye), lawsonite is well preserved in the external domain, at the contact with the Briançonnais domain, but is largely pseudomorphed in the more internal domain, at the contact with the Viso Unit. Further North, neither lawsonite nor lawsonite pseudomorphs have been reported in the supposedly blueschist-facies Combin Zone, taken by most studies as an equivalent of the Queyras-Ubaye units. This constitutes a paradox with respect to the overall metamorphic structure of the Alpine belt.

This study documents for the first time several occurrences of lawsonite and garnet in the calcschists from the Combin Zone. Field and metamorphic data (thermodynamic modelling and Raman spectroscopy on carbonaceous material) point to the occurrence of two tectonometamorphic units within the Combin Zone, characterised by distinct geometry, lithological content and Alpine P-T conditions.

In the higher grade unit, lawsonite and garnet were stable at peak P-T conditions (~14-16 kbar and ~460-490 °C) at very low X(CO₂) values. Although lawsonite is systematically pseudomorphed, we have been able to recognize hourglass zoning in lawsonite or preservation of an internal fabric associated with the prograde ductile deformation.

The lower grade unit (~8 ± 1 kbar ~370-400 °C) is discontinuously exposed along the western base of the Dent Blanche nappe and records Alpine P-T conditions similar to those reached by the Dent Blanche nappe (Manzotti et al. 2020).

Our data show that lawsonite is not missing in the Combin Zone, and resolve a paradox about the large-scale metamorphic structure of the Alps.

Manzotti, P., Ballèvre, M., Pitra, P., Müntener, O., Putlitz, B., Robyr, M. (2020). Journal of Petrology, egaa044, https://doi.org/10.1093/petrology/egaa044.

How to cite: Manzotti, P., Ballèvre, M., Pitra, P., and Schiavi, F.: Missing lawsonite found ! Resolving paradoxes of the metamorphic structure of the Western Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6160, https://doi.org/10.5194/egusphere-egu21-6160, 2021.

The Austroalpine Ötztal Nappe shows pervasive Eoalpine and local Variscan high-pressure metamorphism and deformation in its southeastern end, which obscure prior structures. We used magmatic and detrital zircon U-Pb dating by laser ablation ICP-MS to identify the precursor units of the Ötztal Nappe and the relationships among them.

Magmatic protolith dating of granitoid othogneisses in the Ötztal basement yielded Ordovician ages (450 – 470 Ma). The zircons of the Ordovician magmatism are important markers in the detrital zircon record. The paragneisses of the Ötztal basement, in which the Ordovician granitoids intruded, show no Ordovician zircons. The partly calcareous metasediments of the Schneeberg Complex and the Laas Series record some Ordovician detrital zircons. While the Schneeberg Complex is in tectonic contact to the Ötztal Nappe (Klug & Froitzheim, subm.), the Laas Series is the post-Ordovician sedimentary cover of the Ötztal basement. A Permo-Triassic basal metasandstone of the Brenner Mesozoic shows next to a strong Ordovician zircon age population some Variscan and Permo-Triassic zircons.

Zircon dating allowed to identify pre-Ordovician basement with Ordovician intrusions covered by post-Ordovician-pre-Variscan and Permo-Mesozoic sediments. This supports the concept of a non-tectonic unity in the southeastern Ötztal Nappe outside of the Schneeberg Complex.

How to cite: Klug, L., Froitzheim, N., Tomaschek, F., and Lagos, M.: Ordovician zircons as detrital markers in the Ötztal Nappe (Austroalpine, Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2080, https://doi.org/10.5194/egusphere-egu21-2080, 2021.

The Dolomites Indenter (DI) represents the front of the Neogene to ongoing N(W)-directed continental indentation of Adria into Europe. Deformation of the DI is well studied along its rim, documented by important fault zones as, e.g., the Periadriatic fault system (PFS), the Giudicarie belt, and the Valsugana and Montello fault systems. With this study, we aim to investigate the internal deformation of the DI and its eastern continuation towards the Dinarides including the interference of Dinaric SW-directed and Alpine SE-directed folds and thrusts. What also remains unsolved at present is the relationship between deep-seated mantle dynamics and their control on the geometry and internal deformation of the DI. Our approach to unravel this tectonic history is a combination of (i) compilation and acquisition of detailed structural field data within the DI, (ii) collection of a new and comprehensive low-temperature thermochronological dataset covering the entire DI, and (iii) crustal- to lithospheric scale physical analogue experiments.

The existing but limited thermochronological dataset already indicates the presence of relative vertical motions within the DI after the onset of indentation, including mostly Miocene Apatite fission track (AFT) ages along the PFS and the Valsugana fault and two age clusters of Triassic to Jurassic AFT data. One cluster represents the Monti Lessini east of Riva del Garda, the second is located SE of Bozen, in the footwall of the Truden line. Are these Mesozoic AFT age clusters resulting from tectonic vertical movements and/or are they linked to inhomogeneities within the DI, like the Mesozoic platform-basin geometries or the Permian Athesian Volcanic Complex? Ongoing thermochronological investigations aim to clarify these issues.

By using crustal-scale (as a first step) physical analogue models, we aim to study (i) the impact of Jurassic E-W extension and (ii) the effect of crustal strengthening on the NW-SE directed deformation of the DI since Neogene times. Jurassic NNE-SSW trending normal faults led to a platform-basin-topography resulting, from west to east, in the Lombardian basin, Trento platform, Belluno basin, and Friuli platform (Winterer & Bosellini, 1981) but were inverted during Alpine orogeny. Moreover, the Trento platform approximately coincides with the extent of the up to ~2 km thick (Avanzini et al., 2013) Permian Athesian Volcanic Group. We simulate rigid Permian magmatic rocks, which could have led to a critical strengthening of the crust, in our analogue experiments by incorporating an additional strong domain to the lower upper crust. This, together with studying the influence of structural inheritance on the geometry and kinematics of Dinaric and Alpine deformation in the Southern Alps, allows us to model various deformational styles and -wavelengths of the DI during Neogene indentation.

This study will contribute substantially to the understanding of internal deformation and thus enable conclusions to be drawn about the processes at lithospheric scale also addressed by AlpArray.

References:

Avanzini, M. et al. (2013): Note illustrative della carta geologica d'Italia, foglio 026 Appiano. Roma, Servizio Geologico d'Italia, 324 pp.
Winterer, E. L., & Bosellini, A. (1981): Subsidence and Sedimentation on Jurassic Passive Continental Margin, Southern Alps, Italy. AAPG Bulletin, 65(3), 394-421.

How to cite: Sieberer, A.-K., Klotz, T., Pomella, H., Ortner, H., Fügenschuh, B., and Willingshofer, E.: Internal deformation of the Dolomites Indenter, eastern Southern Alps: An integrated field, thermochronology and physical analogue modelling approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13598, https://doi.org/10.5194/egusphere-egu21-13598, 2021.

Despite significant recent progress in the understanding and quantification of the parameters controlling deformation modes in carbonate multilayers within fold-and-thrust belts, the details of early deformation and faulting during the initial stages of large-scale thrusting remain poorly documented and understood. Aiming to narrow this knowledge gap, we have chosen to study the relatively low-strain carbonate multilayer footwall of the Belluno Thrust (BT), one of the most external and S-vergent thrusts of the eastern Southern Alps (Italy). The BT footwall is composed of a c. 600 m thick Meso-Cenozoic multilayer succession of shallow water carbonate and pelagic sedimentary units characterized by strong mineralogical heterogeneity, with calcite (32-98%), sheet silicates (1-27%), and quartz (1-37%) as principal components. Its structural framework reflects cumulative strain due to multiple deformation events and is defined by the superposition of different structures such as i) south-verging asymmetric folds, ii) faulted folds, cut by slip planes with centimetric to metric throw, iii) SC-C’ fabrics in the marly layers, and iv) cataclastic domains.  Structures recording the early shortening increments are generally well preserved mesoscopic upright folds. Asymmetric folds with gently N-dipping backlimbs and steeply S-dipping (or even overturned N-dipping) forelimbs, record further shortening of the early upright and symmetrical folds. Strain is strongly partitioned within the marly layers, with discrete faults commonly defined by multiple slip surfaces forming duplex geometries and SC-C’ fabrics and exploiting millimetric to centimetric marly beds as detachment layers. Thrusts and diffuse reverse faults not associated with any cataclasite localise along the backlimbs of the asymmetric folds, suggesting dominant layer-parallel shortening. Cataclasites develop instead along the thrust surfaces that cut across the steeply dipping (locally even overturned) forelimbs, where cataclastic flow becomes the dominant deformation mechanism. On the vertical forelimbs, cataclasis and strain localisation are commonly associated with veins, which contributed to harden the rock system.  

Based on our systematic observations, we propose that deformation progressively evolved from folding and layer-parallel shortening (initial phases) to faulting and cataclasis (final phases) as a function of the dynamic interplay of the following factors: i) the geometrical relationships between fault orientation, fold attitude (forelimb and backlimb domains) and stress field, ii) the lithotype, which we conveniently account for by referring to the ratio between the cumulative thickness of the outcrop marly layers and the total measured stratigraphic thickness, iii) the involvement of fluids during deformation, iv) the mineral assemblage of the involved layers and v) the geometric framework of the domain localising strain with respect to the principal stress axes orientation. We conclude that these parameters play a major role in guiding strain localisation and partitioning during continuous shortening within fold-and-thrust belts. They also govern the transition from overall aseismic creep to coseismic rupturing at the scale of mesoscopic structures and, possibly, of the entire belt.

How to cite: Zuccari, C., Viola, G., Vignaroli, G., and Aldega, L.: What steers deformation within fold-and-thrust belts: insights from the carbonate multilayer footwall of the Belluno Thrust, Italian eastern Southern Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1068, https://doi.org/10.5194/egusphere-egu21-1068, 2021.

Late-stage orogenic evolution often leads to multiple segmented slab systems, where the relative motion along oblique plate boundaries partitions the crustal strain into strike-slip and reverse faulting. The strain partitioning patterns and mechanics of deformation are thought to be closely related to the rheology inherited from previous tectonic events that affected various orogenic areas. The SE External Dinarides is one place to study such strain partitioning in a less understood tectonic setting. The Dinarides orogenic build-up is characterised by top SW thrusting during Late Cretaceous to Oligocene times. Subsequently, the N to NE indentation of the Adria microplate took place in this area after an early - middle Miocene period of generalized extension and was characterised by N-S to NNE-SSW oriented contraction, which is oblique to the inherited NW-SE oriented structural grain. We have studied the interplay between various structures creating strain partitioning during the Adria indentation in a SE External Dinarides region situated between the Trebinje city in SE Bosnia and Herzegovina and the Tivat city of SW Montenegro.

The post- middle Miocene orogenic evolution is characterised by regional NNW-SSE to N-S dextral strike-slip faulting associated with strain partitioning by the reactivation of NW-SE inherited rheological weak zones (former thrusts, nappe contacts or rheologically weak sediments). Kinematic analyses along individual structures define the strain partitioning pattern by a number of fault groups. The kinematically constrained mechanics of deformation (correlated to strain partition groups) in focus areas depict a gradual SE-ward transfer of deformation in the external thrust sheets of Montenegro. Such migration of deformation is done by an interplay between strike-slip, high-angle reverse faults and thrusts, which are locally associated with moderate block rotations (CW and CCW). The overall analysis demonstrates that oblique motions in advanced orogenic stages do not constrain a single paleostress field, and therefore they should be analysed by an improved kinematic approach aimed to understand strain partitioning and their effects superposed over an inherited structural grain.

How to cite: Feliu, N., Demir, V., Matenco, L., Mrdak, M., Radusinović, S., and Đaković, M.: Strain-partitioning and mechanics of deformation during oblique indentation: Inferences from the SE external Dinarides post- middle Miocene evolution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9814, https://doi.org/10.5194/egusphere-egu21-9814, 2021.

The Dinarides fold and thrust belt on the Balkan Peninsula is the result of the long-lasting convergence between the Adriatic and Eurasian plates since the Mid-Jurassic. Late Jurassic obduction of ophiolites, Early Cretaceous composite nappe stacking, and subsequent continent-continent collision in the latest Cretaceous resulted in folding and thrusting that in the most external part of the Dinarides took place during the Middle Eocene – Oligocene. This extensive last phase of substantial crustal shortening and thickening was associated with flexural foreland basin deposition, resulting in Eo- to early Oligocene syntectonic units. These rocks and older Mesozoic carbonate platform units now form the mountain chain of the external Dinarides. So far, the driving mechanism behind the rock uplift was unknown and it was not clear when the present-day topography formed. Here we show that horizontal marine terraces preserved at elevations of up to 600 m in the external Dinarides are crucial to answer these questions.

We extracted horizontal surfaces, river incision profiles, and the Adriatic and Black Sea catchments from a digital elevation model (DEM). The extracted horizontal surfaces are interpreted as marine terraces because they are degradational, locally preserved in a staircase morphology, neither bedding- nor fault-related, and located close to the present-day Adriatic shoreline. The marine terraces stretch c. 600 km along-strike the entire Dinaric coastal region. Their spatial correlation agrees with the position of a reported positive P-wave tomography anomaly beneath the Dinarides. This up to 180 km deep anomaly correlates also with the thinnest part of the Adriatic lithosphere and the Adriatic-Black Sea drainage divide. The orogen-perpendicular river incisions profiles reveal a symmetric river incision pattern on both sides of the drainage divide. The mean amount of the river incision is equivalent to the mean elevation of the documented marine terraces. All results point to an orogen-wide surface uplift of the Dinarides.

Based on the geological record this post-collisional uplift event can be relatively dated to Oligocene-Miocene (28-17 Ma) and seems to be broadly contemporaneous with the emplacement of igneous rocks with mantle affinity (33-22 Ma) in the internal Dinarides. Previously published geophysical and petrological, as well as the new geomorphological data presented here suggest that the post-collisional reorganization of the Dinarides is attributed to an Oligocene-Miocene mantle delamination event, which results in uplift event affecting the entire Dinarides. We also show that no significant, orogen-scale deformation affected the uplifted Dinarides after the Early Miocene.

How to cite: Balling, P., Grützner, C., Tomljenović, B., Spakman, W., and Ustaszewski, K.: Post‑collisional mantle delamination in the Dinarides implied from staircases of Oligo‑Miocene uplifted marine terraces, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13148, https://doi.org/10.5194/egusphere-egu21-13148, 2021.

Mt. Ivanščica is one of inselbergs in the Internal Dinarides (NW Croatia) in the transitional area with Southern Alps. In this area, NNW-verging Dinaric structures are overprinted by S-verging Alpine structures. Mt. Ivanščica is composed of Mesozoic shallow to deep-marine sedimentary succession of the passive continental margin of Adriatic plate, which was facing the Neotethys ocean, overthrust by ophiolitic mélange. Here, we aim to present new preliminary structural data from pelagic sediments of Ivanščica Mt. in attempt to better understand tectonic history of this part of Internal Dinarides.

Mesozoic succession of Mt. Ivanščica is composed of Triassic clastic, volcanic and carbonate rocks overlain by Upper Triassic to Lower Jurassic shallow-marine carbonates. These are overlain by Jurassic pelagic carbonates and cherts followed by Tithonian−Valanginian pelagic “Aptychus Limestones”. The uppermost part of this succession is composed of Lower Cretaceous Oštrc fm., which conformably overlies the “Aptychus Limestone”. The Oštrc fm. is characterized by turbidites with ophiolitic detritus and represents syn-orogenic deposits presumed as formed in a front of advancing ophiolitic nappe(s).

The focus of our investigation is primarily on structural characteristics of the “Aptichus Limestones” and the Oštrc fm. The character of the contact between the “Aptychus Limestones” and underlying Upper Triassic to Lower Jurassic carbonates is still uncertain. According to Šimunić et al. (1982) “Aptychus limestones” unconformably overlays Triassic carbonates in periclinal geometry, while Babić (1974) suggests continuous condensed pelagic sedimentation throughout the Jurassic. In contrast with previous observations and interpretations, our observations suggest a tectonic contact, characterized by significantly different orientation of bedding and locally marked by fault gauge (clay) seams.

Structural analysis shows numerous gentle to open asymmetric folds in the “Aptychus Limestones” and closed chevron folds and isoclinal folds in overlaying Oštrc fm. Chevron folds and open to gentle asymmetric folds indicate NW vergence in present day orientation with fold axis parallel to the strike of the contact with underlying unit. Although different in shape and size, these folds are likely formed during the same tectonic event while their geometry is controlled by differences in rheological properties. Isoclinal folds occurring exclusively at the contact with ophiolitic mélange are characterized by E-W oriented fold axis and S dipping axial surfaces which is in a contrast with aforementioned folds. Thus, we assume that these folds originated from another, presumably older tectonic event. Bedding in Triassic dolomites uniformly dips towards the SE. Local occurrence of condensed pelagic limestones and radiolarian cherts is interpreted, as rheologically weak horizon ideal to form a décollement that, at least locally, could be interpreted to mark a thrust fault.

Formation of isoclinal folds in the Oštrc fm. and the tectonic contact with ophiolitic mélange is preliminarily attributed to the Aptian-Albian nappe stacking known from the Internal Dinarides. In addition, we assume that the pelagic succession of the “Aptychus Limestones” together with the overlying Oštrc fm. and the ophiolitic mélange are thrusted over the Upper Triassic to Liassic carbonates sometime later, possibly during the final stage of Neotethys closure in the Internal Dinarides.

How to cite: Vukovski, M., Kukoč, D., and Tomljenović, B.: An overview on Structural position of Mesozoic succession of distal Adriatic continental margin on Ivanščica Mt. (NW Croatia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5818, https://doi.org/10.5194/egusphere-egu21-5818, 2021.

The Bükk and Uppony Hills (NE Hungary) are two adjacent structural units with correlations to the Northern Dinarides and Inner Western Carpathians (ALCAPA), respectively. These two units are separated by the Nekézseny Fault, which may therefore be considered as a presently displaced segment of the Dinaric-ALCAPA contact zone (Schmid et al. 2008). Along this contact zone, the Bükk-type Permo-Mesozoic formations are thrust over the Paleozoic and Senonian formations of the Uppony Unit (Schréter 1945). Despite of the Nekézseny Fault being a terrain boundary, its structural evolution has not been studied in details. Preliminary structural data suggested multiple faulting events between the latest Senonian and early Miocene (Fodor et al. 2005), however, the initial age of the contact zone has remained uncertain.

In this study a detailed structural analysis was carried out in order to understand the deformation geometry, kinematics and the timing of movements along the Nekézseny Fault. Our preliminary results show that the Nekézseny Fault developed in response to NW-SE shortening. Low-angle fractures within individual pebbles suggest an early (latest Cretaceous or early Paleogene) age for the NW-SE shortening, as pebble fracturing is limited to the early stage of diagenesis and requires soft or semi-consolidated fine-grained matrix.

The top-to-the-NW emplacement of the Bükk over the Uppony Unit was accompanied by the folding of the Senonian conglomerate in the footwall, where a large, almost isoclinal recumbent fold developed due to the estimated several km of displacement along the main contact zone. Despite of the similarity in the shortening directions, the top-to-the-NW shortening certainly post-dates the penetrative S-SE-vergent contractional structures present throughout the Bükk Hills, that are related to the latest Jurassic to Early Cretaceous nappe stacking and subsequent shortening (Csontos 1999). Microtectonic analysis of the Nekézseny Fault Zone proved that the main contact zone is a strongly distorted cataclastite zone, which suggests a late-stage low-temperature deformation. Similarly younger semi-ductile or low-temperature contractional structures (e.g. kink folds) were recognized in several parts of the Bükk Unit, all of which were dated tentatively to the late Cretaceous (Flórián-Szabó & Csontos 2002, Juhász 2020, McIntosh 2014, Koroknai et al. 2008, Scherman 2018). Our observations indicate that the top-to-the-NW displacement was much more extensive than previously thought and incorporated large part of the Bükk Unit. This shows that the top-to-the-NW displacement represents an important deformation phase, which should be integrated into the Mesozoic structural evolution of the Alpine-Dinaric area.

This study was supported by the research founds NKFIH OTKA 113013 and 134873, the ÚNKP-17-2 and ÚNKP-20-3 New National Excellence Program of the Ministry of Human Capacities.

References:
Csontos (1999): Bulletin of the Hungarian Geological Society 129, 4, 611-651.
Fodor et al. (2005): GeoLines 19: 141-161.
Juhász (2020): TDK thesis, ELTE, Budapest.
Koroknai et al. (2008): Journal of Structural Geology 30, 159-176.
McIntosh (2014): PhD thesis, University of Debrecen, Debrecen.
Scherman (2018): MSc thesis, ELTE, Budapest.
Schmid et al. (2008): Swiss Journal of Geosciences 101: 139-183.
Schréter (1945): Annual Report of the Geological Institute of Hungary, 1941-42: 197-237.

How to cite: Oravecz, É., Juhász, D., Götz, A., Kövér, S., Scherman, B., and Fodor, L.: Structural evolution of the Nekézseny Fault – a displaced segment of the Dinaric-ALCAPA contact zone in NE Hungary (Bükk and Uppony Hills), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11499, https://doi.org/10.5194/egusphere-egu21-11499, 2021.

The Carpathians fold-and-thrust belt results from oblique collision of ALCAPA and Tisza-Dacia plates with the eastern European margin. It formed during the Oligocene and Miocene, propagating laterally from NW to SE as clearly demonstrated by balanced-cross sections (Nakapelyukh et al., 2017; Castellucio et al., 2016; Merten et al., 2010). The coeval development of the foreland basin (Roure et al., 1993) is revealed by an axial transport system that prograded from NW to SE, ultimately supplying sediments to the Black Sea (de Leeuw et al., 2020). However, lacking a regional synthesis and integration of thermochronology data, lateral propagation of exhumation in the orogen has not been demonstrated yet.

 We reconstruct the exhumation history of the entire Carpathians from the Oligocene onwards and link it with the development of the Carpathians foreland basin (CFB) using a source-to-sink approach. We compiled more than 500 apatite and zircon fission-track and (U-Th)/He ages from the literature. This comprehensive database was separated by region (Western, Eastern, and South-Eastern Carpathians) and by tectonic domain (as defined in Schmid et al., 2008). This partitioning allows for the inversion of large datasets, reflects the tectonic complexity of the belt, and avoids spurious spatial correlations (Schildgen et al., 2018). The thermochronology data was inverted using Pecube (Braun et al., 2012) to constrain exhumation rates in a Bayesian approach. We thus obtain estimates of exhumation rates through time along the belt (with their uncertainty) and convert these into bulk  sediment fluxes over time, permitting tracking of sediment routing from the eroding belt to the CFB. Ultimately, these data will be used to unravel deeper geodynamics, including the possible effects of slab detachment on the evolution of the belt and its foreland basin.

Key words: Low-temperature thermochronology, Carpathians, exhumation, source to sink, Pecube inversions.

How to cite: Roger, M., van der Beek, P., de Leeuw, A., and Husson, L.: Diachronous exhumation of the Carpathians from low-temperature thermochronology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2648, https://doi.org/10.5194/egusphere-egu21-2648, 2021.

Within-plate migration of alkaline basaltic centers is generally related to translation of the lithosphere with a hot spot above a largely stationary mantle plume. Here, we report, for the first time, a hitherto unrecognized migration of small-sized Late Miocene to Early Quaternary alkali-basaltic volcanic centers at the transition from Eastern Alps to the Pannonian Basin. The volcanic centers migrated along the South Burgenland High in a regular sequence over a 95 km distance from NNE to SSW between 11 Ma and 1.7 Ma. The basaltic magmatism was also associated with thinning of the crust and lithosphere and an increase of the thermal gradient as previous studies testify. In detail, three stages of migration are recognized as follows: (i) Stage 1 with a 55 km SSW-toward shift of volcanism between 11 and 5 Ma; (ii) Stage 2 with a 35 km-WSW shift and enlargement of the distribution of volcanic centers between ca. 5 and 3.5 Ma; and (iii) Stage 3 with a S-directed shift of ca. 25 km between 3.5 and 1.7 Ma. We propose that this pattern and mechanism of migration of the volcanic centers along the South Burgenland High resulted from thermally induced progressive thinning of the lithosphere over a mantle plume underneath the ALCAPA block, which was moving from SSW to the NNE between 11 and 1.7 Ma, interrupted by a marked eastward shift between 5 and 3.5 Ma (Stage 2). The Stage 1 shift of alkali-basaltic volcanism can be also observed in the Little Hungarian Plain and South Slovakia volcanic fields, here rotated in a sinistral wrench corridor, and the Stage 2 in the Balaton-Bakony of the Pannonian Basin (north of Lake Balaton). The migration of volcanic centers correlates with orientation and timing of regional shortening phases and inversion of the Alpine-Carpathian-Pannonian system although the overall amount of shortening remains uncertain. The convergence of the Alpine-Carpathian-Pannonian-Dinaric system is driven by the northward motion of the Adriatic microplate. Previous balancing of the shortening of Eastern Alps resulted in an approximately constant convergence rate of ca. 10 mm/yr since 20 Ma ago although the system changed to overall compression since ca. 6 Ma and lower shortening rates can be expected. Interestingly, the reasonable estimated migration rates of volcanic centers are all in a similar order of magnitude between 6.5 and 13.8 mm/yr as the mentioned 10 mm/yr. This is nearly an order of magnitude larger than present-day rates of 1 – 2 mm/yr with E to NE-directed motion, but similar to the global hotspot-fixed reference frame, which implies a ca. northeast-directed motion of >10 mm/yr. Although this could represent an explanation of observed migration of volcanic centers, we exclude it because of the short tracks (maximum 95 km) and small volumes of volcanic products implying rather local plumes in the upper mantle.

Acknowledgements: This work has been supported by the Austrian Science Fund (FWF), grant no. M-1343.

How to cite: Neubauer, F. and Cao, S.: Migration of Late Miocene to Quaternary alkaline magmatism at the Alpine-Pannonian transition area: Significance for deformation rates and decoupling of lithospheric levels, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2015, https://doi.org/10.5194/egusphere-egu21-2015, 2021.

The Pannonian Basin is a continental extensional basin system with various depocentres within the Alpine–Carpathian–Dinaridic orogenic belt. Along the western basin margin, exhumation along the Rechnitz, Pohorje, Kozjak, and Baján detachments resulted in cooling of diverse crustal segments of the Alpine nappe stack (Koralpe-Wölz and Penninic nappes); the process is constrained by variable thermochronological data between ~25–23 to ~15 Ma. Rapid subsidence in supradetachment sub-basins indicates the onset of sedimentation in the late Early Miocene (Ottnangian? or Karpatian, from ~19 or 17.2 Ma). In addition to extensional structures, strike-slip faults mostly accommodated differential extension between domains marked by large low-angle normal faults. Branches of the Mid-Hungarian Shear Zone (MHZ) also played the role of transfer faults, although shear-zones perpendicular to extension also occurred locally.

During this period, the distal margin of the large tilted block in the hanging wall of the detachment system, the pre-Miocene rocks of the Transdanubian Range (TR) experienced surface exposure, karstification, and terrestrial sedimentation. The situation changed after ~15–14.5 Ma when faulting, subsidence, and basin formation shifted north-eastward. Migration of normal faulting resulted in fault-controlled basin subsidence within the TR which lasted until ~8 Ma.

3D thermo-mechanical lithospheric and basin-scale numerical models predict similar spatial migration of the depocenters from the orogenic margin towards the basin center. The reason for this migration is found in the interaction of deep Earth and surface processes. A lithospheric and smaller crustal-scale weak zones inherited from a preceding orogenic structure localize initial deformation, while their redistribution controls asymmetric extension accompanied by the upraising of the asthenopshere and flexure of the lithosphere. Models suggest ~4–5 Myr delay of the onset of sedimentation after the onset of crustal extension and ~150–200 km of shift in depocenters during ~12 Myr. These modeling results agree well with our robust structural and chronological data on basin migration.

Simultaneously with or shortly after depocenter migration, the southern part of the former rift system, mostly near the MHZ, underwent ~N–S shortening; the basin fill was folded and the boundary normal faults were inverted. The style of deformation changed from pure contraction to transpression. The Baján detachment could be slightly folded, although its synformal shape could also be considered a detachment corrugation. Deformation was dated to ~15–14 Ma (middle Badenian) in certain sub-basins while in other sub-basins deformation seems to be continuous throughout the late Middle Miocene from ~15 Ma to ~11.6 Ma.

Another contractional pulse occurred in the earliest Late Miocene, between ~11.6 and ~9.7 Ma while the western part of the TR was still affected by extensional faulting and subsidence. All these contractional deformations can be linked to the much larger fold-and-thrust belt that extends from the Southern and Julian Alps through the Sava folds region in Slovenia. Contraction is still active, as indicated by recent earthquakes in Croatia.

Mol Ltd. largely supported the research. The research is supported by the scientific grant NKFI OTKA 134873 and the Slovenian Research Agency (research core funding No. P1-0195).

How to cite: Fodor, L., Balázs, A., Csillag, G., Dunkl, I., Héja, G., Kelemen, P., Kövér, S., Németh, A., Nyíri, D., Oravecz, É., Selmeczi, I., Trajanova, M., Vrabec, M., and Vrabec, M.: Migration of deformation, subsidence, and basin formation in the SW Pannonian Basin (central Europe) and the change to contractional deformation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13300, https://doi.org/10.5194/egusphere-egu21-13300, 2021.