ALBERT

Description: The Bohemian Massif (BM) is the largest coherent surface exposure of basement rocks in central Europe. It is a geodynamically active part of the Hercynian orogenic belt representing a collage of magmatic arcs and micro-continents caused by the collision of Laurasia (Laurentia-Baltica) and Africa (Gondwana). The general northwest direction of accretion is typical of the northern part of the Hercynian belt. Irregularly-shaped colliding blocks resulted in a very complicated structure of convergence, lithospheric subduction, and crustal shortening, followed by extensional processes and rifting. The western part of the Bohemian Massif is the well-known health and resort landscape of Bohemia, Saxonia, and Bavaria, with Karlovy Vary (Karlsbad) as the flagship of the famous spa towns of the region (Figure 1). Allegedly, the Emperor Charles IV founded the spa in the years 1347–1349 at the site, which was already well known for its hot springs. For centuries, 12 springs in Karlovy Vary ranging in temperatures between 42°C and 72°C have been exploited, especially for the treatment of digestive system disorders and metabolic diseases.

Description: In 2004 and 2005 a passive seismic experiment was carried out in the northern and northeastern part of the Bohemian Massif (Sudetes) to study the lithospheric structure. We present results from Ps and Sp receiver function analyses. With one exception, Moho depth at stations in the northwestern part of the study area varies between 28 and 32 km. Thicker crust up to 35 km was mapped toward the south (Moldanubian unit) and toward the east (Moravo–Silesian and Brunovistulian units) confirming results from previous active seismic measurements. There exists a relatively sharp step in Moho depth between units of the central Sudetes (~ 30 km) and the Moravo–Silesian unit (~ 35 km). The vp/vs ratios inverted from primary and multiple Moho Ps conversions hint for different crustal compositions of the units. Toward the Carpathian thrust we have no clear indications for any crustal root or slab beneath the western Carpathians. However, our data suggests a deepening of the Moho or at least a complicated crust–mantle transition in this area. Additional Ps phases were observed between 6 and 10 s delay time in the Sudetes. These phases cannot be explained by Moho reverberations, but are most probably caused by low velocity zones in the middle crust or lithospheric mantle as shown by modeling of theoretical receiver functions. The stations showing these abnormal phases are located in the area of Permo-Carboniferous basins on probably Teplá–Barrandian crust. Therefore we assume that the phases hint at a mid-crustal low velocity zone between 16 and 20 km depth, which is interpreted as a felsic solidified magma reservoir of the Permo-Carboniferous volcanism beneath the Sudetic Basins. Sp receiver functions show phases with negative polarity at 9 to 12 s lead time on average, which we interpret as lithosphere–asthenosphere boundary at about 80 to 110 km depth.

Description: Splitting of shear waves is considered to be evidence of their propagation through an anisotropic medium. We evaluated splitting parameters of core-mantle refracted shear waves (SKS) along with their particle motions (PM), recorded during the passive seismic experiments AlpArray-EASI (2014-2015) and the AlpArray Seismic Network (2016-2019). The study area covers the western part of the Bohemian Massif and the Eastern Alps in about 200 km broad NS-oriented transect. Careful signal preprocessing includes several steps: automated identification of SKS waveforms, filtering and quality check. Special attention is paid to checking the geographical orientation of seismometers at all stations. Parameters of anisotropy – shear-wave split delay time and direction of the fast split shear waves in the LQT coordinate system are evaluated in 3D by two modified splitting methods, the eigenvalue and the transverse energy methods. To improve the splitting analysis results, we also include so-called null splits, i.e., results for directions where SKS waves do not split or their splitting is close to null. In the case of waveforms with a low signal-to-noise ratio (SNR) we apply a more robust PM method. The modified version of splitting methods (Vecsey et al., 2008) allows us to retrieve the 3D orientation of large-scale anisotropic structures in domains of the mantle lithosphere and to detect abrupt changes of their fabrics as well as present-day deformations within the sub-lithospheric part of the upper mantle.

Description: 〉Body-wave recordings from passive seismic experiments AlpArray-EASI (2014-2015) and AlpArray Seismic Network (2016-2019) are evaluated to image the upper mantle large-scale anisotropy and to model the lithosphere-asthenosphere boundary (LAB) beneath the western part of the Bohemian Massif and the Eastern Alps. We analyze P-wave traveltime deviations and interpret them along with results of splitting parameters from core-mantle refracted shear waves at 240 broad-band stations in about 200 km broad and 540 km long band along 13.3° E longitude. The body-wave anisotropic parameters are inverted jointly for 3D self-consistent anisotropic-velocity models of individual mantle lithosphere domains, assuming hexagonal symmetry with inclined ‘slow’ or ‘fast’ axes. To control the depth extent of the fabric, we apply coupled 3D anisotropic-isotropic P-wave tomography (Munzarova et al., 2018). Anisotropic signals in body waves are consistent within sub-regions and often sharply change at tectonic boundaries. The coincidence of boundaries of the anisotropic models of the mantle lithosphere domains with main tectonic features, correlation of the anisotropy depth extent with the LAB models as well as a decrease of anisotropy strength in the sub-lithospheric mantle support fossil origin of the directionally varying component of detected anisotropic fabrics of the continental mantle lithosphere.

Description: We use seismic waveform data from the AlpArray Seismic Network and three other temporary seismic networks, to perform receiver function (RF) calculations and time-to-depth migration to update the knowledge of the Moho discontinuity beneath the broader European Alps. In particular, we set up a homogeneous processing scheme to compute RFs using the time-domain iterative deconvolution method and apply consistent quality control to yield 112 205 high-quality RFs. We then perform time-to-depth migration in a newly implemented 3D spherical coordinate system using a European-scale reference P and S wave velocity model. This approach, together with the dense data coverage, provide us with a 3D migrated volume, from which we present migrated profiles that reflect the first-order crustal thickness structure. We create a detailed Moho map by manually picking the discontinuity in a set of orthogonal profiles covering the entire area. We make the RF dataset, the software for the entire processing workflow, as well as the Moho map, openly available; these open-access datasets and results will allow other researchers to build on the current study. How to cite. Michailos, K., Hetényi, G., Scarponi, M., Stipčević, J., Bianchi, I., Bonatto, L., Czuba, W., Di Bona, M., Govoni, A., Hannemann, K., Janik, T., Kalmár, D., Kind, R., Link, F., Lucente, F. P., Monna, S., Montuori, C., Mroczek, S., Paul, A., Piromallo, C., Plomerová, J., Rewers, J., Salimbeni, S., Tilmann, F., Środa, P., Vergne, J., and the AlpArray-PACASE Working Group: Moho depths beneath the European Alps: a homogeneously processed map and receiver functions database, Earth Syst. Sci. Data, 15, 2117–2138, https://doi.org/10.5194/essd-15-2117-2023, 2023.