Displays

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 347 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. Outside this 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. We invert differences between observed and predicted travel times for P-wave velocities inside the box. Velocity is discretized on a regular grid with an average spacing of about 25 km. The misfit reduction reaches values of up to 75% 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 extending down to a depth of 300 km are located below the central and eastern Alps, separated by a clear gap below the western part of the Tauern window. Further to the east the model indicates a possible high-velocity connection between the eastern Alps and the Dinarides. Regarding the lateral position of the central and eastern Alpine slabs, our results confirm previous studies. However, there are differences regarding depth extent, dip angles and dip directions. Both structures dip very steeply with a tendency towards northward dipping. 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 AlpArray Working Group: Teleseismic P-wave travel time tomography of the Alpine upper mantle using AlpArray seismic network data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13779, https://doi.org/10.5194/egusphere-egu2020-13779, 2020.

In the frame of the Alparray project we analyse teleseismic data from permanent and temporary stations of the greater Alpine area to study the structure of the crust and the uppermost mantle. We use S-to-p and P-to-s converted waves below the seismic stations which are aligned along the arrival times of the generating P and SV signals. The broadband data used are unfiltered, amplitude normalized and sign corrected. Profiles of migrated data are constructed through the entire Alpine area and compared with results of tomographic, controlled-source and receiver function studies. Thereby we provide additional constraints regarding the ongoing controversies regarding the configuration of the various slabs whose existence was postulated by previous authors within the larger Alpine area including the Western Carpathians. Special attention is given to the possibility of a reversal of subduction polarity in the eastern Alps.

How to cite: Kind, R., Schmid, S., Yuan, X., and Working Group, A.: Negative Velocity Gradients in the uppermost Mantle below the larger Alpine Area, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9787, https://doi.org/10.5194/egusphere-egu2020-9787, 2020.

The Alpine-Dinarides are a complex orogenic system, with its tectonic evolution controlled by the ongoing convergence between Eurasian and African plates with the Adriatic microplate wedged between them. Our study focuses on the upper mantle of the wider Alpine-Dinarides region, and we present surface-wave tomography of two overlapping subregions, interpreting the seismic velocity features in the context of regional geodynamics.

In the first part, we use records of 151 teleseismic earthquakes (2010-2018) at 98 stations distributed across the wider Dinarides region. Surface-wave phase velocities are measured in the range of 30 – 160 s by the two-station method at pairs of stations aligned along the great circle paths with the epicenters. We apply several data-quality tests before the dispersion curves are measured. We use Rayleigh waves recorded on both radial and vertical components. Only the dispersions measured coherently at both components are used for the tomography. In total, we reach the number of 9000 phase velocity measurements for the period of 50 s. Tomographic results including resolution estimates are provided for various frequencies; the local dispersion curves are inverted for depths from the surface down to 300 km. Results are shown as maps for various depths and as cross-sections along several profiles of shear-wave velocities in the whole region.

The other study focuses on the Alps. The AlpArray seismic network stretches hundreds of kilometers in width and more than thousand kilometers in length. It is distributed over the greater Alpine region (Europe) and consists of around 250 temporary and around 400 permanent broadband stations with interstation distances around 40 km. The earthquakes are selected between years 2016-2019. The methodology differs from the Dinarides case in a sense, that while before we used many earthquakes and less stations pairs (due to sparser station coverage), for the Alps, we use less earthquakes (32) and many more stations pairs (tens of thousands) making use of the dense station coverage of the AlpArray network.

Results of the depth inversion of the local dispersion measurements for the Alps are compared with local surface-wave phase-velocity measurement obtained from the (sub)array approach.

How to cite: Kolínský, P., Belinić, T., Stipčević, J., Bianchi, I., Fuchs, F., Bokelmann, G., and Working Group, T. A.: Upper mantle structure beneath the Dinarides and the Alps from surface wave tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18776, https://doi.org/10.5194/egusphere-egu2020-18776, 2020.

We present a 3D probabilistic model of shear wave velocity and radial anisotropy of the European crust and uppermost mantle mainly focusing on the Alps and the Apennines.

The model is built using continuous seismic noise recorded between 2010 and 2018 at 1521 broadband stations, including the AlpArray network (Hetényi et al., 2018).

We use a large dataset of more than 730 000 couples of stations representing as many virtual source-receiver pairs. For each path, we calculate the cross-correlation of continuous vertical- and transverse-components of the noise records in order to get the Green’s function. From the Green’s function, we then obtain the group velocity dispersion curves of Love and Rayleigh waves in the period range 5 to 149 s.

Our 3D model is built in two steps. First, the dispersion data are used in a linearized least square inversion providing 2D maps of group velocity in Europe at each period. These maps are obtained using the same coverage for Love and Rayleigh waves. Dispersion curves for both Love and Rayleigh waves are then extracted from the maps, at each geographical point. In a second step, these curves are jointly inverted to depth for shear velocity and radial anisotropy. The inversion in done within a Bayesian Monte-Carlo framework integrating some a priori information coming either from PREM (Dziewonski and Anderson 1961) or the recent 3D shear wave model of Lu et al. 2018 performed for the same region.

Therefore, this joint inversion of Rayleigh and Love data allows us to derive a new 3D model of shear velocity and radial anisotropy of the European crust and uppermost mantle. The isotropic part of our model is consistent with the shear velocity model of Lu et al. 2018. The 3D radial anisotropy model of the region adds new constraints on the deformation of the lithosphere in Europe. Here we present and discuss this new radial anisotropy model, with particular emphasis on the Apennines.

How to cite: Alder, C., Debayle, E., Bodin, T., Paul, A., Stehly, L., Pedersen, H., and Dubuffet, F. and the the AlpArray working group: Radial anisotropy in Europe from surface waves ambient noise tomography and transdimensional hierarchical inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20109, https://doi.org/10.5194/egusphere-egu2020-20109, 2020.

Convergence between the European and African plates formed the Alps and the neighbouring mountain belts. We present results 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, as well as seismic anisotropy study along a ca. 200 km broad and 540 km long north-south transect (crossing the Bohemian Massif in the north, the East-Alpine root, and reaching the Adriatic Sea in the south), image the steeply northward dipping East-Alpine root, dominated by the Adriatic plate, steady southward thickening of the lithosphere beneath the Bohemian Massif and distinct regional variations of mantle lithosphere fabrics modelled in 3D. These characteristics imply complex, domain-like architecture of the collisional zone of the European/Adriatic plates beneath the Alps. Thanks to the close spacing of the AlpArray stations and high-quality data, the high-resolution tomography resolved for the first time two neighbouring high-velocity northward-dipping heterogeneities beneath the Eastern Alps, instead of one thick root of the lithosphere. The southern one, which we relate to the Adriatic plate, is more distinct, the northern one is less pronounced, it delaminates at ~100km depth and diminishes in direction toward the Central Alps. It may represent a remnant of an early phase subduction of the European plate with the switched polarity (relative to the polarity in the Western Alps), or a preceding phase of the Adriatic subduction.

How to cite: Plomerová, J., Žlebčíková, H., Hetényi, G., Vecsey, L., Babuška, V., Working Group, T. A.-E., and Working Group, T. A.: Tomography image of double high-velocity heterogeneity beneath the Eastern Alps from the AlpArray data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7728, https://doi.org/10.5194/egusphere-egu2020-7728, 2020.

The AlpArray gravity research group (AAGRG) focuses on compiling a homogeneous surface-based gravity dataset across the Alpine area, on creating digital data sets for Bouguer-, Free Air- and isostatic anomalies. In 2016/17 all ten countries around the Alps have agreed to contribute with point/gridded gravity data and/or gravity data processing techniques to recompilation of the Alpine gravity in an area from 2° East to 23° East and 50° North to 42° North. For this recompilation, the group was able to rely on existing national data. For the Ivrea zone in the western Alps, newly surveyed data were also integrated into the database.

The AAGRG decided to present the data set of the recalculated gravity fields on a 2 km x 2 km and 4 km x 4 km grid for the public. The final products will also include the calculated values for mass corrections of the measured gravity at each grid point. This allows users to use later customized densities for their own calculations of mass corrections between the physical surface and the ellipsoidal reference. The densities used are 2 670 kg/m³ for landmasses, 1 030 kg/m³ for water masses above and  -1 640 kg/m³ below the ellipsoid. The correction radius was set to the Hayford zone O2 (167 km). In the future, the calculation of long-distance effects of topography/bathymetry and its compensating masses (roots) are planned. The new Bouguer anomaly will be station completed (CBA) and compiled according to the most modern criteria and reference frames (both location and gravity). The concept of ellipsoidal heights implicitly includes the geophysical indirect effect. Atmospheric corrections are also considered. Special emphasis was put on the numerous lakes in the study area. They partly have a considerable effect on the gravity of stations that lie at their edges (for example, the rather deep reservoirs in the Alps). In the Ligurian and the Adriatic seas, ship data of the Bureau Gravimétrique International were used. Although not unproblematic, these data got the preference over satellite data.

 It is the aim of the work of the AAGRG to release a gravity database that can be used for high-resolution modeling, interdisciplinary studies from local to regional to continental scales, as well as for joint inversion with other datasets.

How to cite: Götze, H.-J. and the AlpArray Gravity Research Group: Completed in Spring 2020: AAGRG´s new recompilation of the Alpine gravity field, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5024, https://doi.org/10.5194/egusphere-egu2020-5024, 2020.

The Alpine orogen and its forelands comprise a multitude of crustal blocks from different tectonic providences and different physical properties. This implies that the thermal configuration of the lithosphere would also be expected to vary significantly throughout the region. Temperature is a key controlling factor for rock strength via thermally activated creep and it exerts a first order influence on the depth of the brittle-ductile transition zone, the lower bound to the seismogenic zone and the spatial distribution of seismicity. Here we present new results from INTEGRATE, a project in the DFG priority program Mountain Building in 4 Dimensions, as part of the AlpArray initiative, which aims to gain a better understanding of the structure, temperature and rheology of the crust and the uppermost mantle beneath the Alps and their forelands using multiple 3D modelling techniques. The overall goal is to test different hypotheses on the configuration of the lithosphere and its relation to the distribution of deformation and related seismicity in the Alpine region. We build on previous work of a 3D density differentiated structural model of the region that is consistent with deep seismic data and gravity, to calculate the 3D conductive steady state thermal field of the Alps and their forelands. The model is unique in using different thermal parameters for different tectonic domains and is validated with a dataset of wellbore temperatures from across the region. Comparing recorded seismicity to the calculated thermal field we find a systematic clustering of the deep seismic activity that correlates with different isotherms within individual crustal blocks, reflecting the presence of different dominant lithologies. These inferred lithologies in conjunction with the calculated temperatures and the previous 3D density-structural model of the region, can be used to shed light on the lateral changes in crustal strength within the Alps and their forelands, helping to explain the observed patterns of deformation.  

How to cite: Spooner, C., Scheck-Wenderoth, M., Cacace, M., Götze, H.-J., and Luijendijk, E.: The thermal field across the Alpine orogen and its forelands and the relation to seismicity , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8730, https://doi.org/10.5194/egusphere-egu2020-8730, 2020.

The present-day deformation in the Central Alps is dominated by vertical uplift, at rates up to 1.5 mm/yr as indicated by high-precision levelling and GPS data. Understanding the driving mechanisms of this neotectonic uplift and its link to seismicity in the Central Alps requires accurate locations of current deformation processes within the upper crust. Especially the question if and how deformation in the crystalline basement is coupled with deformation in the overlaying nappe systems is key to understand the neotectonic processes. Seismicity provides important information on deformation in the uppermost crust, however, an accuracy of focal depths in the order of few kilometers and less is required to distinguish sources in the basement from sources in the sedimentary cover.

In this study, we demonstrate how insufficient crustal velocity models and inconsistent seismic phase selection can lead to biased hypocenter solutions, which hamper such high-resolution seismotectonic interpretations. We propose a relocation procedure combining a new high-resolution Pg and Sg 3D crustal model of the Central Alps with a dynamic seismic phase selection to overcome this bias and to improve accuracy of hypocenter solutions. The new tomographic model is based on more than 60,000 Pg and 30,000 quality-checked Sg phases of earthquakes, which occurred in the greater Central Alpine region between 1996 and 2019. In combination with a nonlinear, probabilistic earthquake location algorithm, the model was used to relocate more than 18’000 earthquakes, which occurred in this region over the past 36 years. The derived catalog includes a consistent error and quality assessment, calibrated against ground-truth events like quarry blasts.

The relocated seismicity in the Central Alps is interpreted together with additional information from the tomographic model, focal mechanisms, geophysical, geological and geodetic data. We focus our interpretation on the eastern Aar massif as well as on the Rawil depression, located in-between the outcropping Aar and Aiguilles-Rouge massifs. Both regions were recently affected by remarkable seismic events. The ML4.6 Urnerboden earthquake of 2017 occurred near the eastern termination of the Aar massif, while a sequence of about 350 events occurred in the Rawil earthquake lineament near the Sanetschpass in November 2019. Both sequences provide unique insights into active faults in the Central Alps and we image systems of sub-vertically oriented strike-slip faults of variable strike, which root in the crystalline basement in both regions. Our results document the existence of active strike-slip fault systems in the External Crystalline Massifs of the Central Alps in regions of maximum change in uplift rates. We therefore discuss possible models relating the observed strike-slip kinematics to buoyancy-driven vertical tectonic processes.

How to cite: Diehl, T., Kissling, E., Lee, T., Schmid, S., and Herwegh, M.: New 3D Pg and Sg Velocity Models for High-Resolution Seismotectonic Interpretations in the Central Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11166, https://doi.org/10.5194/egusphere-egu2020-11166, 2020.

In the Western Alpine arc, GNSS measurements indicate that the far field convergence responsible for the Oligo-Miocene continental collision is now over. However, seismicity and slow deformation are still active. Former collisional tectonic features, such as the Penninic Front, are nowadays reactivated as normal faults. Indeed, geodetic and seismotectonic studies show that the inner part of the chain is undergoing transtensional deformation, although local compression is observed in the foothills at the periphery of the arc. Due to the low to moderate seismicity of the Western Alps, the stress and strain fields remain partly elusive.

The aim of the present study is to quantitatively assess the current seismic stress and strain fields within the Western Alps, from a probabilistic standpoint. We used a new set of more than 30,000 Alpine earthquakes recorded by the dense local Sismalp seismic network since 1989. We first computed well-constrained focal mechanisms (f.m.) for more than 2,000 events with Ml ranging from 0.5 to 4.9 based on first motion (P-wave) polarity. This is the first time that such a huge focal mechanism dataset can be analyzed in the Alps. Corresponding events have been localized using a 3D velocity model (B. Potin, 2016). The global distribution of P and T axes dips confirms a vast majority of dextral transtensive focal mechanisms in the overall Alpine realm. We interpolated these results based on a Bayesian interpolation method, providing a probabilistic 2D map of the styles of seismic deformation in the Western Alps. Compression is robustly retrieved only in the Pô plain where seismicity depth differs from the shallow seismicity of the Western Alps. Extension is localized at the center of the belt. Importantly, extension is clustered instead of continuous along the belt. We then summed seismic moment tensors in homogeneous volumes of crust, to obtain seismic strain rates directly comparable to geodetic ones. Last, we inverted f.m. together in specific areas to obtain principal stress directions. A major outcome is the orientation of the extension, which is surprisingly oblique to the arc, rather than normal, as commonly thought.

These results bring new insights on the geodynamic processes driving the seismotectonic activity of the Western Alps, such as the relative contributions of crustal tectonics and deep processes.

How to cite: Mathey, M., Sue, C., Potin, B., Pagani, C., Bodin, T., Husson, L., Hannouz, E., Baize, S., and Walpersdorf, A.: Seismic deformation in the Western Alps : new insights from high resolution seismotectonic analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7546, https://doi.org/10.5194/egusphere-egu2020-7546, 2020.

Gaining insight into the regional stress field and deformation in the crust is challenging. As we cannot measure these directly, we rely on proxy measurements and numerical modelling to infer their orientation. For the Alpine-Pannonian-Carpathian junction, only a limited number of studies exist that provide such insights. They are based on either the interpretation of sparse and point-wise measurements of local stress-field orientations or on numerical modelling that aims to satisfy tectonic and geological constraints.

We infer seismic azimuthal anisotropy that relates to the orientation of the regional stress-field and crustal deformation from ambient-noise-derived Rayleigh waves in the region. This approach provides a spatially broad and independent measurement that complements previous studies. We use Rayleigh-wave group-velocity residuals after isotropic inversion at 5s and 20s center period, which are sensitive to crustal structure at different depths. They allow us to gain insight into two distinct mechanisms that result in fast orientations. At shallow crustal depths (5s), fast orientations in the region are N/S to NNE/SSW, roughly normal to the Alps. This effect is most likely due to the formation of cracks aligned with the present-day stress field. At greater depths (20s), fast orientations rotate towards NE, almost parallel to the major fault systems that accommodated the lateral extrusion of blocks in the Miocene. This is coherent with the expected direction of aligned crystal grains during crustal deformation occurring along the fault systems and the lateral extrusion of the central part of the Eastern Alps.

How to cite: Schippkus, S., Zigone, D., Bokelmann, G., and Working Group, A.: Stress-field orientation and crustal deformation in the Vienna Basin region (Alpine-Pannonian-Carpathian junction), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2295, https://doi.org/10.5194/egusphere-egu2020-2295, 2020.

The eastern Southern Alps are part of the deformed leading edge of the Adriatic plate indenting the European plate to the north. Neogene deformation in the eastern Southern Alps is partitioned into three, kinematically linked fold-and-fault systems: (1) The Giudicarie Belt, (2) the Valsugana Thrust System and (3) the external fold-and-thrust systems of the orogenic front, including the strike-slip Schio-Vicenza Fault. We aim to constrain fault kinematics from the Southern Alpine orogenic front to the Northern Giudicarie Fault to better understand deformation of the Adriatic indenter since Miocene time.

The Giudicarie Belt is a sinistral transverse zone characterized by NNE-oriented faults. Some of these faults originated in the Mesozoic as NNE-SSW trending normal faults, which were inverted during Alpine orogeny. Most of the Mesozoic normal faults are oriented oblique to sub-parallel to the main Neogene shortening direction, which led to strain partitioning between thrust and strike-slip faults. This significant strike-slip component complicates kinematic and structural restoration of geological cross-sections in 2-D because rock units moved into and out of the section trace, distorting in-section shortening estimates.

To assess lateral variations in shortening and quantify strain partitioning along and across the strike of the Giudicarie Belt, we constructed and balanced a network of closely spaced cross-sections perpendicular to the main structural trend. Seven 2-D NNW-SSE cross-sections from the Northern Giudicarie Fault to the Southern Alpine orogenic front reveal that the amount of Neogene NNW-SSE shortening varies from 11 km in the vicinity of the Adige embayment to 27 km further NE, with most shortening (20 to 26 km) accommodated within the Valsugana and Giudicarie systems. Shortening differs on either side of the Trento-Cles, Schio-Vicenza (4 km difference) and Ballino-Garda (7 km difference) strike-slip faults. These faults are inherited Mesozoic faults that coincide with significant stratigraphic thickness variations, which we constrained along orogen-parallel cross-sections. The SW-NE variation in shortening is inferred to have been taken up by these sinistral strike-slip faults, but also including the Northern Giudicarie Fault, for which we estimate the minimum amount of slip to be 19 km.

Exposure of Pre-Permian basement in the hanging wall of thrusts indicates a thick-skinned style of deformation. Forward modelling using the MOVE Suite Software indicates that the depth of the detachments within the Pre-Permian basement is no greater than 20 km. A recently located cluster of minor seismic events (2017-2018) within the study area is aligned between 5 and 15 km along the modelled detachments. These earthquake clusters occur within the external fold-and-thrust systems of the orogenic front, suggesting that ongoing shortening is taken up within this system and that the Valsugana and Giudicarie systems are inactive today.

How to cite: Verwater, V., Handy, M. R., Le Breton, E., Picotti, V., Najafabadi, A. J., and Haberland, C.: Neogene kinematics and structural evolution of the Giudicarie Belt and eastern Southern Alpine orogenic front (Northern Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7840, https://doi.org/10.5194/egusphere-egu2020-7840, 2020.

A popular model for the exhumation of HP-UHP rocks is the ‘extruding wedge’ model, where a crustal slice is bounded at its base by a ‘thrust shear-sense’ fault and to the top by a ‘normal shear-sense’ fault. In the Western Alps, the late Eocene Combin Shear Zone (CSZ) allowed extrusion of a wedge made by the Briançonnais-Piemonte-Liguria (‘Penninic’) stack.

Geological mapping has established the geometry and continuity of the CSZ from the frontal part of the Dent Blanche Tectonic System to the western boundary of the Sesia Zone. The CSZ has been cut during the Miocene by the brittle Aosta-Ranzola Fault, with an estimated downthrow of the northern block of c. 2.5 km with respect to the southern block. Consequently, the sections observed north (Monte Rosa) and South (Gran Paradiso) of the Aosta Fault display different structural levels in the Alpine nappe stack. The CSZ has been folded (Vanzone phase) during the final part of its history (i.e. when displacement along the CSZ was no more taking place), due to the indentation of the Adriatic mantle. This offers us the unique opportunity to study the change in deformation mechanisms along the shear zone (for a distance parallel to its displacement of about 50 km).

Salient characteristics of the CSZ are the following. (i) The thickness of the ductile shear zone increases from NW (frontal part of the Dent Blanche) to SE (frontal part of the Sesia Zone), from a few hundred metres to several kilometres. The type of lithologies pervasively reworked by the ductile shear changes along strike (dominantly calcschists from the topmost oceanic units in the Combin Zone, possibly up to the whole of the ‘Gneiss Minuti’ in the frontal Sesia Zone). (ii) The main ductile deformation along the CSZ was taking place at greenschist-facies conditions, overprinting eclogite-facies to greenschist-facies deformations of Cretaceous to Middle Eocene age. The CSZ is cutting and reworking eclogite-facies structures developed in its hangingwall (Sesia) as well as in its footwall (Zermatt). (iii) Ductile displacement along the CSZ is associated with the development in its footwall of south-east-verging, kilometre-scale, folds (Mischabel phase). The sedimentary sequences of the Pancherot-Cime Bianche-Bettaforca Unit may be used to estimate the minimum amount of ‘normal shear sense’ displacement of the order of 15-20 km.

A kinematic model integrating slab roll-back, ‘thrust shear-sense’ at the base and ‘normal shear-sense’ displacement on top of the Eocene eclogite-facies stack will be presented.

How to cite: Ballèvre, M. and Manzotti, P.: Exhumation of HP/UHP rocks by normal ductile shearing on top of the Eocene extruding wedge, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10771, https://doi.org/10.5194/egusphere-egu2020-10771, 2020.

Rapid rock exhumation in mountain belts is often associated with crustal-scale normal faulting during late-orogenic extension. The process of normal faulting advects hot footwall rocks towards the Earth's surface, which shifts isotherms upwards and increases the geothermal gradient. When faulting stops, this process is reversed and isotherms move downwards during thermal relaxation. Owing to these temporal changes of the geothermal gradient, it is not straightforward to derive the history of faulting from mineral cooling ages (Braun, 2016). Here, we combine thermochronological data with thermokinematic modeling to illustrate the importance of syntectonic heat advection and posttectonic thermal relaxation for a crustal-scale normal fault in the European Alps. The N–S trending Brenner fault defines the western margin of the Tauern Window and caused the exhumation of medium-grade metamorphic rocks during Miocene orogen-parallel extension of the Alps (Rosenberg & Garcia, 2011; Fügenschuh et al., 2012). We analyzed samples from a 2-km-thick crustal section, including a 1000-m-long drillcore. Zircon and apatite (U-Th)/He ages along this transect increase with elevation from ~8 to ~10 Ma and from ~7 to ~9 Ma, respectively, but differ by only ~1 Myr in individual samples. Thermokinematic modeling of the ages indicates that the Brenner fault became active 19±2 Ma ago and caused 35±10 km of crustal extension, which is consistent with independent geological constraints. The model results further suggest that the fault slipped at a total rate of 4.2±0.9 km/Myr and became inactive 8.8±0.4 Ma ago. Our findings demonstrate that both syntectonic heat advection and posttectonic thermal relaxation are responsible for the cooling pattern observed in the footwall of the Brenner normal fault.

References

Braun, J., 2016, Strong imprint of past orogenic events on the thermochronological record: Tectonophysics, v. 683, p. 325–332.

Fügenschuh, B., Mancktelow, N., Schmid, S., 2012, Comment on Rosenberg and Garcia: Estimating displacement along the Brenner Fault and orogen-parallel extension in the Eastern Alps: Int. J. Earth Sci., v. 101, p. 1451–1455.

Rosenberg, C.L., Garcia, S., 2011, Estimating displacement along the Brenner Fault and orogen-parallel extension in the Eastern Alps: Int. J. Earth Sci., v. 100, p. 1129–1145.

Wolff, R., Hetzel, R., Dunkl, I., Anczkiewicz, A.A., Pomella, H. 2020, Fast cooling of normal-fault footwalls: rapid fault slip or thermal relaxation? Geology, v. 48, doi:10.1130/G46940.1.

How to cite: Wolff, R., Hetzel, R., Dunkl, I., Anczkiewicz, A. A., and Pomella, H.: Fast cooling of normal-fault footwalls: rapid fault slip or thermal relaxation?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2196, https://doi.org/10.5194/egusphere-egu2020-2196, 2020.

Many convergent orogens such as the eastern European Alps display an asymmetric doubly-vergent wedge geometry. Loci of deepest exhumation are located above the overriding retro-wedge, whereas increased fault activity occurs in the pro-wedge on the subducting plate. The main drainage divide separates steeper from more gently sloping topography on the two wedges of different critical taper. We performed apatite and zircon (U-Th)/He analyses densely spaced along the TRANSALP geophysical transect in combination with thermo-kinematic models based on cross-section balancing. Our new low temperature thermochronology data and thermo-kinematic model results underline (i) deepest levels of exhumation across the Tauern Window until the Pliocene and (ii) higher Late Neogene exhumation rates south of the Periadriatic Fault relative to the north, while seismic activity is focussed across the Southern Alps. Our proposed mantle-to-surface link positions the retro-wedge north of the Periadriatic Fault subsequent to subduction polarity reversal during continental collision. Present-day drainage divide migration trends and imaged locations of mantle-lithospheric slabs beneath TRANSALP suggest ongoing, slow slab reversal since Adriatic indentation in the Eastern Alps. 

How to cite: Eizenhöfer, P. R., Glotzbach, C., Büttner, L., Kley, J., and Ehlers, T. A.: Neogene Exhumation History along TRANSALP: Insights from Low Temperature Thermochronology and Thermo-Kinematic Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9714, https://doi.org/10.5194/egusphere-egu2020-9714, 2020.

First unified complete Bouguer anomaly map of AlpArray area compiled from terrestrial gravity data is in preparation. The following steps to calculate the first version of the map were performed: 1. unification of different spatial, height and gravity systems, 2. getting available detailed (mainly LiDAR-based) elevation models and their transformation from physical to ellipsoidal heights, 3. calculation of mass corrections (gravity effect of the topography between the surface and ellipsoid level) with density 2 670 kg/m³, 4. calculation of bathymetric corrections for water masses below the ellipsoid (correction density -1 640 kg/m³), 5. calculation of lake correction for great alpine lakes (correction density -1 670 kg/m³), 6. calculation of the final complete Bouguer anomalies based on normal field (Somigliana formula with GRS80 parameters, free-air correction using Taylor series expansion to the 2^nd order) and particular corrections including also the atmospheric correction.

The quality control of input data was performed based on the height differences between the point data and particular elevation models. Several thousand points with height residuals higher than chosen threshold (±50 m) were excluded. The available detailed local elevation models (resolution 10 – 20 m) were compared with global model MERIT (resolution 25 m).

The most significant methodological innovation is the ellipsoidal heights concept using straightforward calculation of mass/bathymetric corrections in respect to the ellipsoid instead of using the geophysical indirect effect computation. Our specially developed program Toposk was used for mass/bathymetric correction calculation (the standard distance of 166.7 km was used for the first version of the map) as well as for the calculation of lake corrections. Mass corrections amount to hundreds of mGal, while the lake corrections reach more than 5 mGal locally. Atmospheric effect taking into account topography was also calculated and compared with standard atmospheric correction.

How to cite: Zahorec, P., Papčo, J., and Pašteka, R. and the AlpArray Gravity Research Group: Processing steps for the compiling AAGRG gravity maps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16484, https://doi.org/10.5194/egusphere-egu2020-16484, 2020.

Collisional tectonics of the Alps is driven by several slab segments. A detailed imaging of the lithosphere-asthenosphere system beneath the Alps is, however, challenging due to the relatively small size of the slab segments and the highly curved geometry of the Alps. Surface waves, due to their high sensitivity to variations in seismic velocities at lower crustal and upper mantle depth, are well suited to study the Alpine deep structure. New azimuthally anisotropic Rayleigh wave phase velocity maps are calculated from automated inter-station phase velocity measurements in a very broad period range (8 – 350 s). The constructed local dispersion curves are then inverted individually for 1-D shear-wave velocity models using a new implementation of the stochastic Particle Swarm Optimization (PSO) inversion algorithm that enables the calculation of a high-resolution 3-D shear-wave velocity model from the crust down to 300 km beneath the Alps. In the Central Alps, a nearly vertical high velocity anomaly down to depth of 250 km is imaged and interpreted as subducting Eurasian mantle lithosphere. In contrast, low velocities in the Western Alps at depth of approximately 100 km and downwards are supporting the shallow slab break-off model. In the Eastern Alps, the presence of a vertically continuous high-velocity anomaly from 75 km to about 200 km depth beneath the northern Eurasian foreland and the almost continuous extension of a high-velocity anomaly from the Dinarides towards the Eastern Alps hint at a bivergent slab geometry beneath the Eastern Alps caused by subducting mantle lithosphere of both Eurasian and Adriatic origin. There is also evidence for subduction of Adriatic lithosphere to the east beneath the Pannonian Basin and the Dinarides down to a depth of about 150 km. Beneath the northern Apennines, the model indicates an attached Adriatic slab, whereas a slab window is found beneath the central Apennines. The results show that surface wave tomography can contribute to the imaging of complex slab geometries and slab segmentation in the Alpine region.

How to cite: Meier, T., El-Sharkawy, A., and Lebedev, S.: The Alpine Deep Structure from Surface Wave Tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18543, https://doi.org/10.5194/egusphere-egu2020-18543, 2020.

Using AlpArray and European networks, it is now possible to resolve the shape and inherent distortion of teleseismic surface waves across the greater alpine area at an as of yet unprecedented resolution. With well over 1500 available broadband stations within a 20° radius around the central Alps we demonstrate our approach for measuring both phase and amplitude distributions of surface wave signals in the space-frequency domain, leading to structural phase velocity information corrected for possible dynamic effects. Knowledge of the amplitude fields is particularly important to understand wave front deformations and to correct dynamic phase velocity measuremnts.

To diminsh the influence of noise, higher modes, coda waves, or adjacent events on our measurement, we analyse correlations with synthetic fundamental mode signals for a spherically symmetric earth model. The resulting wave field parameters are consequently expressed as amplitude and phase perturbations of the synthetic background wave fields. The measurements are explained and examples for phase and amplitude Rayleigh wave fields are shown and discussed.

Examining the wave field properties it becomes apparent that the dynamic contributions to the eikonal phase velocity are indeed significant, caused by both heterogeneities inside and (far) outside the observed region. Smaller local anomalies are for instance frequently observed around Mount Etna and Vesuvius, with the active vulcanism causing noticable reverberations. Surprisingly, large wave field anomalies are often oriented almost parallel to the propagation direction and can potentially span the entire station distribution, manifesting themselves as contiguous stripes of elevated amplitudes from positive interference of off-axis scattered waves.

How to cite: Tesch, M. and Meier, T.: Phase and Amplitude Rayleigh Wave Fields Measured by AlpArray, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21538, https://doi.org/10.5194/egusphere-egu2020-21538, 2020.

Seismic anisotropy is an important tool for studying geodynamic processes in the Earth, and a common way of constraining it is to analyse shear-wave splitting of seismological body-wave phases,
i.p. SKS. Different techniques exist to quantify shear-wave splitting, but they do not always give the same result, raising the question of how stable they are, and whether there are systematic biases. Furthermore, the strength of the splitting ("splitting delay") has generally been more difficult to determine than the other (the "fast orientation").
A robust technique for determining shear-wave splitting can be set up
based on the splitting intensity method. That technique can in particular also constrain the splitting delay well. Ambient noise can however lead to an underestimation of splitting delay, and it needs to be accounted for, e.g. by a least-squares Wiener filter.
We apply that modified splitting intensity method to data from the AlpArray. We have processed 3 years of teleseismic earthquake data for 336 stations of the AlpArray deployment and additional 315 stations of the Italian network to get a potentially broad and more complete image of anisotropic structures in and outside the Alpine region.
The technique makes restrictive assumptions, e.g. assuming single-layer anisotropy. Yet, the new constraints, especially the one of the splitting delay are rather useful for understanding the deformation under the mountain belt and around it.

How to cite: Bokelmann, G. and Hein, G.: Extracting robust splitting measurements for the AlpArray using the splitting intensity method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16479, https://doi.org/10.5194/egusphere-egu2020-16479, 2020.

SKS-splitting measurements in the European Alps show an anisotropic fast axes parallel/subparallel relative to the mountain-belt. This indicates a mantle flow with a rotational component according to the orogeny under the assumption that the fast axes directly reflect the flow direction. This might be misleading due to a possible crustal contribution of anisotropy. Therefore, we isolate the crustal anisotropy using an improved receiver function method that accounts for anisotropic and structural properties.

The analysis for the crustal anisotropy is based on the stacking method proposed by Kaviani & Rümpker (2015). We modify their approach by introducing a time-selective splitting analysis of the crustal Ps- and PpPs-phases. The stacking is performed to the phases after correction of the anisotropic effect according to the model parameters H, the crustal thickness,  κ, the P-to S-wave velocity ratio, a, the percentage of anisotropy and φ, the fast axis orientation.

The Alps show a considerable Moho-topography due to its mountain root and its complex tectonic history. This can significantly deflect the crustal phases introducing a dominating appearance in the receiver functions. We therefore analyse for a dipping interface (not accounting for anisotropy) and then use an improved model in our analysis to infer the anisotropic properties of the crust.

Knowing the crustal anisotropic contribution we correct for this effect on the XKS-waveforms to isolate the anisotropy of the mantle. The remaining splitting shows an improved approximation of the flow patterns in the asthenosphere, while complexities might still imply an effect of the lithospheric mantle.

We apply our approach to stations of the AlpArray network resulting in a detailed distribution of the crustal anisotropy in the European Alps and show first results for the isolated mantle anisotropy from the corrected XKS-waveforms and the crustal anisotropy from the receiver-function analysis.

How to cite: Link, F. and Rümpker, G.: The mantle flow below the Alps from isolated mantle anisotropy based on differential Ps – XKS Splitting, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2810, https://doi.org/10.5194/egusphere-egu2020-2810, 2020.

The comparison of crustal and mantle shear directions can provide insights into the extent of crustal-mantle coupling and the dynamics guiding surface movements and active tectonics in continental deformation zones. Here we present a first attempt of comparing surface deformation from GNSS and deep deformation from seismic anisotropy observations for the Great Alpine Area, mainly through France, Switzerland, Italy, Germany and Slovenia. The developments of the European GNSS infrastructure, integrating public and private GNSS networks, allow now to precisely determining crustal deformation over the Alps. We present a new 3D surface velocity field obtained from a recent re-analysis of 22 years of GPS data obtained from >800 continuous GNSS stations operating across the Alps and its surroundings. Unlike the crust, the orientation of the strain field within the mantle cannot be directly measured and must be inferred from either mantle earthquakes or seismic observations, such as seismic anisotropy observations. We compiled a new map of SKS directions merging data collected during several experiments and available from different databases, deriving a new continuous mantle deformation pattern over the Great Alpine Region. Geodetically determined displacements of the Earth’s surface reflect the response to different processes acting at different spatial scales. In the comparison between crustal and mantle deformation we accounted for the intrinsic multi-scale characteristics of geodetic deformation measurements, estimating the geodetic strain-rate field using a multi-scale spherical wavelet-based method, where the velocity value at a given point of the Earth’s surface is obtained as a superposition of values obtained at different spatial scales. From the geodetic strain-rate tensors we computed the two planes of shear (or no-length-changes) directions, which are compared with the directions of SKS splitting over the study region.

How to cite: Salimbeni, S., Serpelloni, E., and Pondrelli, S.: Surface and deep deformation of the great Alpine region from GNSS and seismic anisotropy measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13880, https://doi.org/10.5194/egusphere-egu2020-13880, 2020.

The deep dynamics of continental collision is one of the least understood plate tectonic processes. One interesting process that is believed to be a feature of continental collision is a flip in subduction polarity. A prominent location where such a flip is proposed by different seismic tomography studies (e.g Kissling et al., 2006) are the eastern alps.

The aim of our study is to find a particular signature in the seismic anisotropy of the upper mantle that is the result of a temporal subduction polarity reversal. In our case the seismic anisotropy is produced by the LPO of mantle minerals due non-Newtonian deformation rates.

We use the thermo-mechanical 2D finite-difference code FDCON which has been extended to include a free surface with an erosion/sedimentation mechanism. For the geometrical setup an oceanic plate is placed between two continental plates. Subduction of the oceanic plate beneath the right continent is prescribed. The overriding plate (right) is pushed by constant kinematic boundary conditions. Among other parameters we varied a) the plastic strength of sediments (very weak to strong), b) the ductile rheology of the lower crust (felsic or mafic) and c) the convergence velocity of the two continents (1 - 10 cm/yr). From our results we can identify at least two different mechanisms for a subduction polarity switch.

To estimate the full elastic tensor at the grid points of interest, we use a modified version of DREX (Kaminski et al., 2004) that can handle a time dependent flow field. Using the full elastic tensors, we can calculate, e.g with MSAT (Walker & Wookey, 2012), effective delay times and fast shear wave polarization directions for arbitrary azimuths.

Our first results show significant differences between models with and without a subduction polarity reversal.

How to cite: Kruse, J. P., Schmeling, H., Link, F., and Ruempker, G.: Seismic mantle anisotropy associated with subduction polarity reversal: Insights from numerical models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8039, https://doi.org/10.5194/egusphere-egu2020-8039, 2020.

We make use of the AlpArray Seismic Network to study the properties of the ambient-noise field and create a new 3D shear-velocity model of the Alpine crust. The latter will be used to improve our understanding of the tectonic processes that formed the Alps.

From two years of data, more than 150,000 station-station cross-correlations are extracted and used to evaluate strength and directivity of the noise field and its seasonal variations. Phase-velocity measurements for both Love and Rayleigh waves are obtained and the anisotropic phase-velocity structure is imaged. At mid-crustal levels, the strongest azimuthal anisotropy is found underneath the northern Italian Po plain and in the northern Dinarides, with strengths of 10-20% and a fast axis direction pointing NNE in Italy and NE in the Dinarides. In the western and central Alps we find an approximately NE direction and a strength of 5%; the eastern Alpine fast axis point toward the north with strengths of 2-5%.

We apply a probabilistic inversion to resolve the 3D shear-velocity structure of the crust. The homogeneous and dense station setup results in a shear-velocity model of unprecedented resolution for the uppermost 60 km of the crust underneath the entire orogen. By using data in the period range between 2 and 100s, we are able to better constrain shallow structures, such as the sedimentary basins, and to link surface-geological features to velocity variations observed at depth.

How to cite: Kästle, E., Molinari, I., Boschi, L., and Working Group, A.: Azimuthally anisotropic ambient-noise tomography using the AlpArray seismic network, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18008, https://doi.org/10.5194/egusphere-egu2020-18008, 2020.

How to cite: Reuber, G., El-Sharkawy, A., Paffrath, M., Ebbing, J., Friederich, W., Meier, T., and Kaus, B.: Constraining the dynamics of the present-day Alps with 3D geodynamic inverse models - model version 0.2, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21535, https://doi.org/10.5194/egusphere-egu2020-21535, 2020.

The direction and location of subducting slab segments in the Alpine area is highly debated.  Here, we use seismic crustal depth estimates and different upper mantle tomographies to define hypotheses for the geometry of the subducting slab segments. Based on a new surface wave tomography of the upper mantle in the Alpine region, we also include a new hypothesis with a long Eurasian slab in the central Alps, a short slab segment in the western Alps, and bivergent subduction in the eastern Alps. In addition, we consider the south-dipping slab segment beneath the northern Apennines.  

Next, we study the possible slab related effects of the various considered slab geometries on the gravity field. Specifically, we calculate the gravity effects at the surface and at satellite altitude. In addition to the vertical gravity effect we also show gravity gradients. Two approaches are compared.  First, we convert seismic velocities directly to density using accepted conversion factors. Such direct conversion results in relatively scattered gravity anomalies. In the second approach, we assign density contrasts to predefined slab geometries. Starting from simple models with a constant slab density, we increase the complexity by considering temperature and pressure related density changes according to mantle composition. For such models, the density contrast of the slabs to the ambient mantle diminishes with depth. These models based on predefined slab geometries allow to analyse contributions by the different slab segments independently in greater detail. Combining the slab models with recent 3D crustal models of the Alps is needed in order to establish realistic density models of the Alpine realm for geodynamic applications.

How to cite: Lowe, M., Ebbing, J., El-Sharkawy, A., and Meier, T.: Gravity effect of Alpine slab segments based on geophysical and petrological modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7791, https://doi.org/10.5194/egusphere-egu2020-7791, 2020.

In the Eastern Alps, teleseismic tomography suggests that there is a switch from European subduction in the west to Adriatic subduction in the east. The dense SWATH-D seismic network is located in the central-eastern Alps between around 10°E and 14.5°E where a change in the dip direction was suggested to occur (e.g. Lippitsch et al. 2003; Mitterbauer et al. 2011). The receiver function method is particularly sensitive to velocity contrasts and so is suited to imaging the interfaces associated with subduction. New receiver function migrations from SWATH-D stations (supplemented by the AlpArray Seismic Network and the EASI profile) show no evidence for Adriatic subduction in the Eastern Alps. Instead, a southward dipping interface [or pair of interfaces with opposite polarity] which we interpreted as subducting  European lower crust can be traced below the Eastern Alps to a minimum depth of 120 km along the extent of SWATH-D. This suggests that in the Alps the polarity flip in subduction does not occur or is located east of our study region beyond 14.25°E, much further east than tomography suggests.

How to cite: Mroczek, S., Tilmann, F., Yuan, X., Pleuger, J., and Heit, B.: No polarity switch? Continental subduction of European crust below the Eastern Alps imaged by receiver functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13785, https://doi.org/10.5194/egusphere-egu2020-13785, 2020.

We perform P-to-S receiver function analysis to determine a detailed map of the crust-mantle boundary in the Eastern Alps–Pannonian basin–Carpathian mountains junction. We use data from the AlpArray Seismic Network, the Carpathian Basin Project and the South Carpathian Project temporary seismic networks, the permanent stations of the Hungarian National Seismological network, stations of a private network in Hungary as well as selected permanent seismological stations in neighbouring countries for the time period between 2004.01.01. and 2019.03.31. Altogether 221 seismological stations are used in the analysis. Owing to the dense station coverage we can achieve so far unprecedented resolution, thus extending our previous work on the region. We applied three-fold quality control, the first two on the observed waveforms and the third on the calculated radial receiver functions, calculated by the iterative time-domain deconvolution approach. The Moho depth was determined by two independent approaches, the common conversion point (CCP) migration with a local velocity model and the H-K grid search. We show cross-sections beneath the entire investigated area, and concentrate on major structural elements such as the AlCaPa and Tisza-Dacia blocks, the Mid-Hungarian Fault Zone and the Balaton Line. Finally, we present the Moho map obtained by the H-K grid search method and pre-stack CCP migration and interpolation over the entire study area, and compare results of two independent methods to prior knowledge.

How to cite: Kalmár, D., Hetényi, G., and Bondár, I. and the AlpArray Working Group: Moho depth determination in the Eastern Alps-Pannonian basin-Carpathian mountains region based on H-K grid search method and CCP migration of receiver functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3334, https://doi.org/10.5194/egusphere-egu2020-3334, 2020.

We used Rayleigh wave ambient noise tomography to investigate the crust and uppermost mantle structure of the Pannonian Basin. The Pannonian Basin and the surrounding orogens are located within the arcuate Alpine–Carpathian mountain chain in Central Europe. It is a back-arc basin characterized by a thinned lower crust and an updoming mantle. Benath the basin both the crust and the lithosphere have smaller thickness than the continental average. Imaging the velocity structure of the crust and upper mantle may help us to better understand the structure and formation of the Carpathian–Pannonian region.

We used data from the permanent seismological stations of the broader Central European region together with the AlpArray Seismic Network (AASN) and analysed one-year seismic data from 2017. More than 18 thousand vertical component noise cross-correlation functions were calculated and Rayleigh wave inter-station phase velocity curves were determined using an automated measuring algorithm. Anisotropic phase velocity tomographic imaging were carried out for the whole Pannonian Basin between 2 and 40s periods (~5-60 km).

The locations of the retrieved phase-velocity anomalies consistent with the well-known geologic and tectonic structure of the area (deep basins and orogenic belts) and are comparable to recent 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: Lithospheric structure of the Pannonian Basin using Rayleigh wave ambient noise tomography – preliminary results, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10726, https://doi.org/10.5194/egusphere-egu2020-10726, 2020.

The Liguro-Provencal-basin was formed as a back-arc basin of the retreating Calabrian-Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica-Sardinia block at roughly 32–24 Ma is associated with rifting, shaping the Ligurian Sea. It is highly debated though, whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin.

To investigate the velocity structure of the Ligurian Sea a network (LOBSTER) of 29 broadband Ocean Bottom Seismometer (OBS) was installed jointly by GEOMAR (Germany) and ISTerre (France). The LOBSTER array measured continuously for eight months between June 2017 and February 2018 and is part of the AlpArray seismic network. AlpArray is a European initiative to further reveal the geophysical and geological properties of the greater Alpine area.

We contribute to the debate by surveying the type of crust and lithosphere flooring the Ligurian Sea.
Because of additional noise sources in the ocean, causing instrument tilt or seafloor compliance, OBS data are rarely used for ambient noise studies. However, we extensively pre-process the data to improve the signal-to-noise ratio. Then, we calculate daily cross-correlation functions for the LOBSTER array and surrounding land stations. Additionally, we correlate short time windows that include strong events. The cross-correlations of these are dominated by earthquake signals and allow us to derive surface wave group velocities for longer periods than using ambient noise only. Finally, phase velocity maps are obtained by inverting Green’s functions derived from cross-correlation of ambient noise and teleseismic events, respectively. The phase velocity maps show strong heterogeneities for short periods (5-15 s, corresponding to shallow depths). Causes for these include varying sediment thickness, fault zones and magmatism. For longer periods (20-80 s) the velocity structure smoothens and reveals mantle velocities north-northwest of the basin centre. This might hint on an asymmetric opening of the basin. We do not see strong indications for an oceanic spreading centre in the Ligurian basin.

How to cite: Wolf, F. N., Lange, D., Kopp, H., Dannowski, A., Grevemeyer, I., Crawford, W., Froitzheim, N., Thorwart, M., and Paul, A. and the AlpArray Working Group: Crust and upper mantle structure of the Ligurian Sea revealed by ambient noise tomography using ocean bottom seismometer data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5374, https://doi.org/10.5194/egusphere-egu2020-5374, 2020.

The first leg of the SEFASILS cruise took place in November 2018 onboard the RV Pourquoi-Pas ? Up-to-date multichannel and wide-angle seismic data were acquired offshore Monaco, from margin to basin, aiming at providing a renewed vision of the complex North Ligurian backarc system. The compressive and extensive tectonic phases that have closely alternated in time and space over the last 45 My have yielded fairly contrasting structures, whose understanding is rendered even more challenging by the strong overprint of the Messinian salt tectonics. There is ample evidence of a compressive reactivation of the North Ligurian margin since 5 Ma at least, especially to the East, along the Gulf of Genoa. Such deformation is associated with some notable seismicity originating from faults and mechanisms that remain poorly apprehended. Yet, this seismicity peaked at one historical Mw ~6.6-6.9 destructive event (1887 Ligurian earthquake). The main objective of the SEAFASILS effort is a better characterization of the crustal structures, and chiefly, of the active crustal faults and their potential interplay with salt tectonics beneath the margin and the northernmost part of the basin, both featuring seismicity. Linking these aspects with broader-scale lithospheric processes within the Southern Alps/Northern Apennines, addressed in the AlpArray initiative, is also of great importance. Here we present preliminary results of these seismic investigations, with time and prestack depth migrated MCS data. Emphasis was put on the construction of some suitable velocity models to get optimal focusing of structures from surface to depth. Some active crustal tomographic velocities derived from the dense OBS deployment providing complementary insight will also be presented.

How to cite: Dessa, J.-X., Beslier, M.-O., Schenini, L., Sambolian, S., Canva, A., Ribodetti, A., Operto, S., Bachir Miguil, M., Chamot-Rooke, N., Corradi, N., Delescluse, M., Déverchère, J., and Larroque, C.: The active North Ligurian domain: new geophysical insight from the SEFASILS cruise, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10047, https://doi.org/10.5194/egusphere-egu2020-10047, 2020.

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

Within the framework of the AlpArray research initiative and its German component “4D Mountain building” (SPP2017 4D-MB) a long-term experiment was conducted in the Ligurian sea to investigate the lithosphere structure and the seismicity in the Ligurian basin. The passive seismic network was operated by France and Germany and consisted of 29 broad-band ocean bottom stations. It was in operation between June 2017 and February 2018. At the end of the experiment two active seismic profiles were conducted additionally.

A cluster of 15 events with magnitudes lower than 2.5 occurred in the centre of the Ligurian Basin. The earthquakes are located at a depth of 20 km to 35 km, i.e. 10 - 25 km below the Moho. The cluster was not continuously active but had several active periods which lasted between 2 and 5 days.

A fault plane solution could be determined of the larger events in the cluster. The mechanism is a thrust faulting. Smaller events should have a similar mechanism due to the highly coherent waveforms. A similar mechanism was observed for the Mw=4.9 earthquake on 07.07.2011 which occurred 50 km east of the cluster. Both solutions show a SW-NE striking, northwest dipping fault plane. This indicates a convergence in NW-SE direction between Corsica and Eurasia.

How to cite: Thorwart, M., Dannowski, A., Kopp, H., Lange, D., Crawford, W., Paul, A., and Wolf, F. N.: Seismicity cluster below the Moho indicates thrust faulting in the central Ligurian Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19205, https://doi.org/10.5194/egusphere-egu2020-19205, 2020.

We propose a new analysis of the W-Alpine seismicity based on space and time distributions along the Alpine arc. The overall area bears witness of a relatively important seismic activity localized along the so-call Briançonnais and Piemontais seismic arcs, but also along alignments corresponding to individualized active fault, e.g. in front of the Belledonne massif, and locally in form of important seismic swarms (Ubaye, Maurienne, Mont-Blanc). The regional tectonic regime is well analyzed (see for instance Mathey et al., this session), with detailed mapping of both the stress and strain fields. However, actual available studies do not take into account the time and space distributions. Our study is developed using several available datasets covering various time spans and various strategies (local and regional seismic networks, template matching, historical seismic catalogue). We focus firstly on the space distribution of the activity along the arc, taking into account: (i) the simple occurrence of seismic events to calculate regional density maps, also investigating the B-value mapping; and (ii) the energy density, using the seismic moment fluxes per surface unit as a proxy. On this basis, we secondly analyze the time evolution of the seismicity, which is actually limited by the available dataset’s time span. Our integrated analyze focusses on 3 primary targets: (i) to compare the information arising from the different databases; (ii) to compare the most active zones in terms of earthquake occurrence vs. seismic energy released; (iii) to unravel potential evolutions or establish relative steadiness in alpine seismicity through time. This work will finally allow to better understand and discuss the Alpine seismicity’s mechanisms, in relation with the actual dynamics of this orogen.

How to cite: Sue, C., Mathey, M., Hannouz, E., Walpersdorf, A., and Baize, S.: Earthquake density along the Western Alpine Arc, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18574, https://doi.org/10.5194/egusphere-egu2020-18574, 2020.

We take advantage of the new large seismic data set provided by the AlpArray Seismic Network (AASN) as part of the AlpArray research initiative (www.alparray.ethz.ch), to provide consistent and precise hypocenter locations and uniform magnitude calculations across the greater Alpine region. The AASN is composed of more than 650 broadband seismic stations, 300 of which are temporary. The uniform station coverage provides an unique occasion to study the laterally strongly variable seismicity that is presently monitored and reported by a dozen individual observatories. A homogeneous earthquake catalog in terms of location and magnitude is a prerequisite to improve our understanding of seismo-tectonics and the seismic hazard in the greater Alpine region.

Our catalog covers four years of seismicity with a targeted magnitude of completeness of 2.5 from 2016 to 2019 and results from scanning ∼1000 broadband stations (∼60 TB of data). First, we detect and analyse events in the region using the STA/LTA based detector of the SeisComP3 monitoring system in off-line mode. Later, after an initial location has been obtained, we apply a high-quality semi-automated re-picking approach defining the consistent phase arrival times in combination with timing uncertainties and phase identification assessment. This automatic re-picking framework is implemented with the QUAKE library (Bagagli et al., 2019), an object-oriented Python package that exploit different waveform information both frequency- and energy- related by taking advantage of different well-established picking algorithms. The QUAKE picker has been tuned and tested against a consistent phases reference data set (P-, S- and secondary phases) of ∼2500 phases manually picked for 10 events (M≥ 2.5) homogeneously distributed in the region.

Subsequently, the high-quality automatic picks of selected well-locatable earthquakes are used to calculate a minimum 1D P-wave velocity model for the region with appropriate stations corrections. Finally, all events are relocated with the NonLinLoc algorithm in combination with the updated 1D model and a final estimate of the magnitude is given. We compare our locations and magnitudes with existing regional and local earthquake catalogs (ISC, EMSC, national catalogs) to assess and discuss the completeness and quality of the derived AlpArray research catalog.

How to cite: Molinari, I., Bagagli, M., Diehl, T., Kissling, E., Clinton, J., Scarabello, L., Giardini, D., Wiemer, S., and Working Group, T. A.: A consistent and uniform research earthquake catalog for the AlpArray region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5762, https://doi.org/10.5194/egusphere-egu2020-5762, 2020.

The AlpArray seismic network (AASN) was operated from 2016 to 2019 by a European initiative aiming for new insights into the orogenesis of the Alps as well as into past and recent geodynamic and tectonic processes. The network included more than 620 temporary and permanent broadband stations with a spacing of 50 - 60 km. It was accompanied by the even denser Swath-D seismic network in the Eastern Alps (~150 stations with 15 km spacing). While the extensive network provides an excellent station coverage for seismicity studies, the large number of stations (up to 100) poses new challenges to MT inversions. Automated quality control and the choice of appropriate configurations becomes crucial for the inversion process. Weak to moderate magnitude events and the complex heterogeneous tectonic setting in the Alps force us to push the limits of full waveform moment tensor inversions.

We develop semi-automatic, adaptive approaches for a standardized quality assessment of large seismic networks and for the selection of appropriate waveform fitting targets and frequency ranges. The earthquake source optimization framework ‘Grond’ uses a Bayesian bootstrap-based probabilistic inversion scheme with flexible integration of different waveform attributes in time and frequency domain to provide full or deviatoric moment tensor solutions including uncertainties. The entire workflow from station quality control to moment tensor inversion can handle more than 100 stations simultaneously. The large number of stations allows to study the influence of azimuthal gaps. Further, we are able to compare the inversion results of various methods and configurations in time- and frequency domain using different frequency ranges and epicentral distances. We inverted approximately 100 full moment tensor solutions for events down to Mw 3.1 occurring within the operating time of the AASN. For this magnitude range a combination of frequency-domain spectra and time-domain waveform fitting of surface waves (Z, R and T component, 0.02-0.07 Hz) provides most stable results. In case of distorted absolute amplitudes a combination of frequency spectra and maximum cross-correlation fitting proved to be useful. We find that for smaller events (Mw < 3.0) surface waves are not observed and higher frequency body waves are strongly influenced by complex heterogeneities along the travel path. To extend the source analysis to even weaker events the standard MT inversion approach is combined with network similarity cluster analyses, enabling the association of weaker events to larger ones and therefore the reconstruction of the geometry of active faults.

How to cite: Petersen, G., Cesca, S., Heimann, S., Niemz, P., and Dahm, T. and the AlpArray working group: Pushing the limits of CMT inversion with large seismic networks: Challenges and results for small to moderate earthquakes in the Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4785, https://doi.org/10.5194/egusphere-egu2020-4785, 2020.

Seismotectonic models that combine all the relevant seismotectonic data (e.g., hypocenter locations and velocity models, focal mechanisms and moment-tensors, faults, geodetic data, and in-situ/regional stress data) constitute a pre-requisite to better understand the interplay between stress, faulting and seismicity of a region. This study is a contribution to the multiannual project SeismoTeCH funded by the Swiss Geophysical Commission (SGPK) and coordinated by the University of Bern to produce an integrative seismotectonic model for the entire territory of Switzerland. In this context, our aim is to provide an up-to-date, high-quality, and consistent catalog of first-motion focal mechanisms computed by the Swiss Seismological Service (SED) since 1976. For this purpose, we developed a quality classification scheme for existing mechanisms based on a priori independent information (mainly applied to the oldest mechanisms in the catalog) combined with statistical methods such as HASH (Hardebeck and Shearer, 2002) and probabilistic source mechanisms scanner algorithms (Massin and Malcom, 2018) to probe the solution space and translate probability density functions to a discrete quality rating.

Tests on selected problematic mechanisms are also carried out in order to assess the sensitivity of the focal mechanisms to the velocity models used to calculate location and take-off angles. In particular, we compare existing solutions using the standard 3D P-wave model of the SED with solutions based on recently derived high-resolution 3D Pg+Sg models. These tests are functional to understand the benefits of developing an updated full crustal velocity model for first-motion focal mechanisms calculations, in particular in relations to the focal depths and the accuracy of take-off angles.

Finally, to improve the completeness of the existing catalog, we explore new methodologies that would incorporate automated (possibly real-time) and semi-automated techniques for expanding the calculation of first-motion focal mechanisms (and moment tensors) to events of smaller magnitude. The Anzere/Sanetschpass sequence of November 2019 is used to assess and develop these new methods. As a preliminary result of these combined efforts, we present here a high-resolution map of strain-based deformation across Switzerland. This work represents a benchmark for future regional-scale stress inversion and sets the basis for the development of a consistent, fully accessible, and dynamic focal mechanisms database for Switzerland.

How to cite: Lanza, F., Diehl, T., Deichmann, N., Massin, F., Clinton, J., Herwegh, M., and Wiemer, S.: Assessing and improving focal mechanisms in Switzerland: Towards a comprehensive seismotectonic model of the Central Alps and their foreland , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6833, https://doi.org/10.5194/egusphere-egu2020-6833, 2020.

The AlpArray programme "Mountain Building Processes in 4D" is an interdisciplinary project aimed to image the structure of the Alps and understand their formation. The goal is to be able to model the entire crust-mantle system in three dimensions, and investigate its evolution through time. Seismicity can reveal spatial and temporal patterns of faulting and thereby help to understand the current tectonic structure and motions in the Earth's crust. The south-eastern Alps are of special interest as they include the current plate boundary between Adria and Eurasia, but their undelying structure is poorly resolved and seismicity seems to be scarce. Being able to detect the smallest earthquakes is therefore of key importance.

Swath-D was an AlpArray complementary experiment in which approximately 150 broadband seismic stations were deployed in the Eastern Alps from late 2017 to late 2019. With a station spacing of around 15 km, it is much denser than the AlpArray Backbone network. In this work, data from these stations, combined with publicly available broadband data from the region, were used to detect, localize, and characterize microseismic events. A combination of energy-based detection and template matching was applied to both discover previously unidentified seismic activity and yield a high number of detections. An efficient GPU-based implementation was of critical importance to handle computationally demanding detection methods and the large data volume. Here, we present our methods and workflow, and a new map of seismicity in the south-eastern Alps.

How to cite: Hofman, R., Kummerow, J., Cesca, S., Wassermann, J., and Plenefisch, T. and the AlpArray Working Group: Local Seismicity in the Eastern Alps From GPU-Based Template Matching, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18781, https://doi.org/10.5194/egusphere-egu2020-18781, 2020.

In the framework of the AlpArray project more than 600 broadband stations have been installed and operated in the Alps and the surroundings. Together with the permanent stations in the area it is one of the most densely spaced seismic networks worldwide. Thereby, it offers an excellent opportunity to investigate the seismicity and seismotectonics of the Alpine chain. Due to the huge number of stations focal mechanisms can be calculated even for small magnitude earthquakes with high accuracy. The focal mechanisms are one important key to reveal the contemporary stress field and thus contribute to a better understanding of the geodynamic processes of the Alps.

In our study we focus on small to intermediate earthquakes in the Northern Alps, namely on four distinct sub-regions. These are from West to East the Lake Constance, the Arlberg region, the area of Garmisch-Partenkirchen and the broader region of Innsbruck. In order to calculate the focal mechanisms, we apply the FOCMEC program (Snoke, 2003), which inverts for a pure double-couple source. P-polarities as well as amplitude ratios of SH to P are used as input parameters for the inversion. Thanks to the dense network a good coverage of the focal sphere is achieved in most cases.

Altogether, we calculated focal mechanisms for 25 earthquakes in the magnitude range between 2.5 and 3.5 from the time period 2016 to 2019. Most of the focal mechanisms represent reverse or strike-slip faulting, normal faulting events are rather rare. The mechanisms are analysed with respect to lateral changes along the Northern Alpine. On one hand we compare the mechanisms with mechanisms of older studies as well as with moment tensors of events of slightly larger magnitudes. Those events are the scope of another subproject in the framework of the AlpArray (Petersen et al., 2019). On the other hand, we compare our mechanisms with geological indicators, namely orientation of faults. Finally, the focal mechanisms are used as input to invert for the stress field.

How to cite: Plenefisch, T. and Barth, L. and the AlpArray working group: Focal mechanisms for small to intermediate earthquakes in the northern part of the Alps and their seismotectonic interpretation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12066, https://doi.org/10.5194/egusphere-egu2020-12066, 2020.

The region around the town of Albstadt, SW Germany, is one of the most seismically active regions in Central Europe. In the last century alone three earthquakes with a magnitude greater than five happened and caused major damage. The ruptures occur along the Albstadt Shear Zone (ASZ), an approx. 20-30 km long, N-S striking fault with left-lateral strike slip. As there is no evidence for surface rupture the nature of the Albstadt Shear Zone can only be studied by its seismicity.

To characterize the ASZ we continuously complement the earthquake catalog of the State Earthquake Service of Baden-Württemberg with additional seismic phase onsets. For the latter we use the station network of AlpArray as well as 5 additional, in 2018/2019 installed seismic stations from the KArlsruhe BroadBand Array. We inverted for a new minimum 1D seismic velocity model of the study region. We use this seismic velocity model to relocalize the complemented catalog and to calculate focal mechanisms.

The majority of the seismicity happens between the towns Tübingen and Albstadt at around 9°E in a depth range of about 1.5 to 16 km and aligns north-south. Additionally, we see a clustering of events at the towns Hechingen and Albstadt. The dominating focal mechanism is strike-slip, but we also observe minor components of normal and reverse faulting.
Our results image the ASZ by its mainly micro-seismic activity between 2011 and 2018 confirming the N-S striking character, but also indicating a more complex fault system.

We thank the State Earthquake Service in Freiburg for using their data (Az. 4784//18_3303).

How to cite: Mader, S., Reicherter, K., Ritter, J., and Working Group, T. A.: The nature of the Albstadt Shear Zone, Germany, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7165, https://doi.org/10.5194/egusphere-egu2020-7165, 2020.

The objective of this work was to relocate the entire seismicity of the Pannonian Basin with the Bayesloc algorithm, a Markov-Chain Monte Carlo inversion scheme using a Bayesian statistical framework.

In the Hungarian National Seismological Bulletin the magnitudes and event locations are determined with the iLoc location algorithm using the 3D global RSTT velocity model, and we used these locations as initial coordinates. In our work, we have used all of the instrumentally registered seismic events between 1996 and 2019 in the Pannonian Basin.

During data preprocessing we used graph theory to measure data connectivity. Similar to all multiple-event location methods, Bayesloc performs better when events are recorded on a common network.

We used several hundreds of ground truth events (quarry blasts, mine explosions, earthquakes) to tie down the seismicity pattern to known ground truth locations by giving them tighter prior distributions.

Based on the day-time peak on the origin-hour distribution of the bulletin earthquakes we assume that there are anthropogenic events labeled as earthquakes in the catalog, therefore we created a „Suspected explosions (SX)” group to set prior constrains.

The results show that the events around the mines are dramatically better clustered. The prior constraints contributed remarkably to the outcome of the relocation. We show that the results present an improved view of the seismicity of the region.

How to cite: Czecze, B. and Bondár, I.: Relocation of seismicity of the Pannonian Basin using the Bayesloc multiple event location algorithm between 1996 and 2019, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16663, https://doi.org/10.5194/egusphere-egu2020-16663, 2020.

How to cite: Wéber, Z., Czecze, B., Gráczer, Z., Süle, B., Szanyi, G., Bondár, I., and Working Group, T. A.: Comprehensive analysis of the March 7, 2019 Somogyszob, Hungary earthquake cluster, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18040, https://doi.org/10.5194/egusphere-egu2020-18040, 2020.

Changes in the deep lithosphere (e.g., slab break-off or a switch in subduction polarity) potentially result in orogen-wide structural reorientations and changes in the pace and location of exhumation and Earth surface processes. In this project we combine bedrock thermochronology and balanced cross sections with thermo-kinematic modelling to reconstruct the cooling and exhumation history along geophysical profiles crossing the Central and Eastern Alps. Available thermochronological data together with new apatite and zircon (U-Th)/He ages taken along the NFP-20E, TRANSALP and EASI profile is used to test and improve existing across-strike, orogen-wide balanced cross sections. This ‘structural thermochronology’ method yields reliable information about the structural and kinematic evolution of the Alps since continental collision. As an example, thermochronological data along TRANSALP can be fitted with a kinematic model suggested by balanced cross sections and both datasets suggest a general shift from pro- to retro-wedge deformation, potentially related to a switch in subduction polarity.

How to cite: Glotzbach, C., Eizenhoefer, P., Kley, J., and Ehlers, T. A.: Structural thermochronology along geophysical transects through the Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6781, https://doi.org/10.5194/egusphere-egu2020-6781, 2020.

Reconstructing past elevations of mountain ranges improves our understanding of crustal- and mantle-scale geodynamic processes involved in formation of orogenic belts. Recent studies suggest that slab breakoff beneath the Central Alps occured ~30 Ma ago (e.g. Schlunegger and Castelltort, 2016), while the breakoff reached the Eastern Alps about 10 Ma later (~20 Ma, e.g. Handy et al., 2015). The proposed west-to-east slab tear migration would imply variations in topography. This raises the question of a diachronous surface uplift history for the Central and Eastern Alps. Although being extremely well studied over the last century and serving as a prime exemple for orogenic belt evolution, there are very few investigations addressing the Neogene paleoelevation history of the European Alps. Obtained paleoelevation constraints suffer from inconsistency and range from average elevations of 2300 m (Kocsis et al., 2007) to at least 5000 m (e.g. Sharp et al., 2005).

In order to provide quantitative robust paleoelevation estimates for the Mid-Miocene Central Alps we applied stable isotope paleoaltimetry on authigenic soil carbonates from the Northern Alpine Foreland Basin (NAFB) and contrast these with syntectonic high-Alpine fault zone mica. This δ-δ paleoaltimetry approach benefits from the advantage of comparing a low-elevation site and a high-elevation site of the same age which allows us to circumvent the basic issue of climate bias in paleoaltimetry studies.

We obtained stable isotope (δ¹⁸O) records of pedogenic carbonate from the Swiss Molasse Basin and δD values of fault mica from the Simplon Fault Zone, for the Middle Miocene (15.5 – 14.0 Ma). The key element in conducting stable isotope paleoaltimetry is the prevailing temperature during low-elevation proxy material formation. Here we present new Mid-Miocene paleoelevation data for the European Central Alps based on precisely defined clumped isotope (∆₄₇) derived carbonate formation temperatures and new low-elevation stable isotope records. A conservative approach renders Mid-Miocene Central Alps mean elevation of approximately 4000 m, which contrasts modern Alpine topography with average elevations of ca. 2000 m in the Central Alps (Kühni and Pfiffner, 2001).

How to cite: Krsnik, E., Methner, K., Löffler, N., Kempf, O., Fiebig, J., and Mulch, A.: New paleoelevation constraints on the Mid-Miocene Central Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10060, https://doi.org/10.5194/egusphere-egu2020-10060, 2020.

The relief history of mountain belts is strongly influenced by the interplay of tectonics and surface processes, which both shape Earth´s landscapes. In this context, the quantification of the rates of long-term and short-term processes is key for understanding landscape evolution and requires the application of methods that integrate over different timescales. In this study, we apply low-temperature thermochronology and cosmogenic nuclides to quantify the geological and geomorphic evolution of an elevated low-relief landscape in the Eastern Alps, the so-called Nock Mountains, which are situated to the east of the Tauern Window. The low-temperature thermochronological data yield zircon fission track and zircon (U-Th)/He cooling ages of 93.4±12.9 and 77.8±7.8 Ma, respectively, which we interpret to reflect late Cretaceous cooling after Eo-Alpine metamorphism. Apatite fission track and (U-Th)/He ages are significant younger and range from 36.8 to 31.3 Ma. Time-temperature history modelling of the cooling ages suggests enhanced cooling in the Eocene followed by thermal stagnation. Thus, the rocks of the study area have been in near surface position (2-3 km) since the Late Eocene. Enhanced cooling in the Eocene is probably related to an increasing relief due to shortening, folding and thrusting in the Eastern Alps triggered by the onset of collision between the European margin and the Adriatic microplate. Under the assumption that rock exhumation occurred solely by erosion, the long-term average erosion rate derived from the thermochronological data is ~50-90 mm/kyr. Catchment-wide erosion rates derived from cosmogenic ¹⁰Be in river sediments  range from 83±7 to 205±18 mm/kyr and hence are lower than in other parts of the Alps. As the ¹⁰Be-derived erosion rates and the long-term rates derived from thermochronology agree despite the different timescales over which the two methods integrate, our new data suggest that erosion rates did not change significantly over the last ~40 Ma. This is remarkable because within this time span numerous tectonic processes and glacial-interglacial cycles affected the study area. To investigate the deglaciation history after the Last Glacial Maximum in the Nock Mountains, we sampled glacially polished quartz veins for ¹⁰Be exposure dating. The first four exposure ages obtained so far cluster between 14.5±1.4 and 16.8±1.6 ka. We interpret these ages as the record the retreat of the ice cover in the study area shortly after the Oldest Dryas stadial.

How to cite: Wölfler, A., Reimers, S., Hampel, A., Glotzbach, C., and Dunkl, I.: Evolution of a low-relief landscape in the Eastern Alps constrained by low-temperature thermochronology and cosmogenic nuclides, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4632, https://doi.org/10.5194/egusphere-egu2020-4632, 2020.

Based on new structural field data and Raman micro-spectroscopy on carbonaceous material a major detachment juxtaposing Drauzug-Gurktal Nappe System (DGN) against the transgressive Permo-Mesozoic cover sequence of the Ötztal-Bundschuh Nappe System (BN, Stangalm Mesozoic s. str.) in the area SE of Flattnitz (Carinthia, Austria). An Eo-alpine top-SE kinematic has been identified.

The hanging wall unit comprise lithologies of the DGN phyllites, conglomerates and graphite schists (Stolzalpe nappe), which have experienced only low grade greenschist deformation. Raman constrains 350°C±40°C.

The footwall unit consists of dolomitic ultra-mylonites, calcitic marble mylonites, meta-conglomerates and quarzites (Stangalm Mesozoic and Kuster nappe), which have experienced at least four main deformation phases. The oldest structures (D1) corresponding to Eo-Alpine nappe stacking are overprinted by (D2) isoclinal recumbent folds with E-W oriented shallow dipping fold axis and an axial plane schistosity, dipping shallowly to WSW. Ductile to brittle-ductile top to the E shearing (D3) is indicated by ESE-trending stretching lineation, C-type shear bands, stylolites, crystal- and shape preferred orientations of mineral grains. Late brittle deformation (D4) is recorded in steep joint sets with dip-directions to NW. Raman constrains 480°C±40°C.

The detachment zone comprises a complicate zone of high strain including units from DGN folded together within the Stangalm Mesozoic, which have experienced the same deformation as the BN.

How to cite: Werdenich, M., Iglseder, C., Grasemann, B., Rantitsch, G., and Huet, B.: A large-scale detachment system in the central Eastern Alps (Upper Austroalpine Unit, Austria), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19144, https://doi.org/10.5194/egusphere-egu2020-19144, 2020.

We present preliminary results on the meso- and micro-structural evolution of high-strain rocks of the Houillère Zone and Pierre-Avoi Unit outcropping along the Swiss-Italy boundary ridge, to the west of the Grand Saint Bernard Pass.

The stack of Middle and External Pennidic units is folded by polyphasic folds, developed at least partly under low-grade metamorphic conditions. Different generations of folds show isoclinal to open geometries. Fold axes are subhorizontal, trending NE-SW, and the overall fold interference pattern can be generally classified as a type 3 (Ramsay). At the microscale, an important deformation mechanism is pressure solution cleavage, consistent with relatively low-temperature conditions.

Brittle-ductile shear zones, characterized by anastomosing bands of very fine-grained fault rocks, with pressure solution seams and SCC’ shear bands, exploit the weak and strongly anisotropic phyllosilicate-rich layers, particularly in the black schists of the Houillère Zone.

Brittle high-angle faults crosscut ductile and semi-brittle features and show an oblique-normal kinematics. These faults are particularly well developed in the more competent rocks of the Pierre-Avoi Unit (e.g. massive carbonates, metaconglomerates and metasandstones).

A continuous horizon, a few metres thick, with a high density of quartz veins, can be followed in the internal and upper part of the Houillère Zone. This horizon is folded, at least by the younger open folds, and constitutes a major marker to study the large-scale structure of this unit.

How to cite: Pini, D., Arienti, G., Pozzi, M., Monopoli, B., and Bistacchi, A.: Meso- and micro-structural analysis of the Briançonnais Front in the Grand Saint Bernard area (Aosta Valley - Italy, and Valais - Switzerland), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22548, https://doi.org/10.5194/egusphere-egu2020-22548, 2020.

We present a new geological and structural map of the Gran Sometta -Tournalin ridge (Valle d’Aosta). In this area we have Pennidic ophiolitic units of the Combin (Co) and Zermatt-Saas (ZS) zones. In addition, in this area the continental cover sequence of the Pancherot-Cime Bianche-Bettaforca (PCB) unit crops out, close to the base of the Combin zone. The PCB and Co are characterized by Alpine greenschist facies assemblages, while the ZS is characterized by eclogitic assemblages. The greenschist and HP complexes are juxtaposed along the extensional Combin Fault Zone.

Our detailed 1:5000 map allowed reconstructing in 3D, and with a high level of detail, the spatial and crosscutting relationships between metamorphic layering (e.g. calcschists and metabasites in the Co), ductile foliations and shear zones, semi-brittle features (e.g. extensional crenulation cleavage – ECC - along the Combin Fault Zone), and post-metamorphic brittle faults.

The metamorphic layering and foliations are sub-horizontal in this area, and the ECC associated to the Combin Fault results in large components of horizontal stretching. These features are crosscut by two sets of high-angle normal faults, of Oligocene and Miocene age (according to literature), and, thanks to the favourable exposure and numerous structural data, we have been able to reconstruct these structures and their relationships in 3D.

How to cite: Pozzi, M., Arienti, G., Losa, A., and Bistacchi, A.: Structural and geological mapping of the Gran Sometta-Tournalin ridge (Aosta Valley, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22550, https://doi.org/10.5194/egusphere-egu2020-22550, 2020.