The birth of the Alps: Ediacaran to Paleozoic accretionary processes and crustal growth along the northern Gondwana margin

New whole-rock geochemical and coupled U–Pb and Lu–Hf LA-ICP-MS zircon data of metasedimentary rocks of the Austroalpine, South Alpine and Penninic basement domains are presented, to disentangle the pre-Variscan tectonic evolution of the proto-Alps. The studied units seem to record distinct stages of protracted Late Ediacaran to Carboniferous tectonosedimentary processes prior to the Variscan collision. In the case of Austroalpine and South Alpine units, nevertheless, no major differences in terms of provenance are observed, since most detrital zircon samples are characterized by a major Pan-African peak. Their detrital zircon spectra record a provenance from the northeastern Saharan Metacraton and the Sinai basement at the northern Arabian-Nubian Shield, being thus located along the eastern Early Paleozoic northern Gondwana margin, whereas sources located further west are inferred for the Penninic Unit, which might have been placed close to the Moldanubian Unit of the Bohemian Massif. In any case, it is thus clear that the Alpine basement remained in a close position to the Gondwana mainland at least during the Early Paleozoic. The Late Ediacaran to Silurian tectonic evolution, which includes Cadomian and Cenerian tectonometamorphic and magmatic processes, seem thus to record a continuum related to a retreating-mode accretionary orogen, with diachronous back-arc basin opening and possibly discrete compressional/transpressional pulses linked to changes in subduction zone dynamics. On the other hand, it is inferred that the Alpine basement essentially comprises Pan-African metasedimentary and subordinate metaigneous rocks, possibly with very few Early Neoproterozoic relics. This basement was significantly reworked during the protracted Paleozoic orogenic evolution, due to anatexis and/or assimilation by mantle-derived juvenile magmatism.

Anisotropy of out-of-phase magnetic susceptibility due to eddy currents in graphite ore and its structural implications

<p>Magnetostatic susceptibility of single crystals of graphite is negative (the mineral is diamagnetic) and strongly anisotropic. The in-phase component of dynamic susceptibility (measured in alternating magnetic field) is also negative, but an order-of-magnitude stronger than the magnetostatic susceptibility. The out-of-phase component, which is no doubt due to electrical eddy currents, is positive and strong. Consequently, if the graphite crystals in graphite ore are oriented preferentially by crystal lattice (LPO), one would expect strong anisotropy of magnetic susceptibility (AMS) of graphite ore in both in-phase (ipAMS) and out-of-phase (opAMS) components. The ipAMS is controlled not only by the LPO of graphite, but also by the preferred orientation of paramagnetic and ferromagnetic minerals of the barren rock, while the opAMS indicates only the LPO of graphite. In graphite ores occurring in the Moldanubian Unit of Southern Bohemia, the in-phase susceptibility ranges from negative values in the order of 10<sup>-5</sup> [SI units] to positive values in the order of 10<sup>-4</sup>. This probably indicates simultaneous control by graphite and paramagnetic and/or ferromagnetic minerals. On the other hand, the out-of-phase susceptibility is much higher, in the order of 10<sup>-4</sup>, and no doubt indicates its graphite control. The degree of ipAMS is moderate, that of opAMS is truly high. The ipAMS foliation is roughly parallel to the metamorphic foliation in ores and wall rocks and the ipAMS lineation is parallel to the mesoscopic lineation. The opAMS is inverse to the ipAMS with the opAMS lineation being perpendicular to the metamorphic foliation. All this indicates a conspicuous LPO of graphite in the ore that was probably created during Variscan regional metamorphism and associated ductile deformation. The opAMS has therefore shown an effective tool for the investigation of the LPO of graphite in graphite ore or graphite-bearing rocks provided that the opAMS is strong enough to be determined with sufficient precision and graphite is the only conductive mineral in the samples investigated.</p>

Shear-Wave Velocity Model of the Bohemian Massif Crust from Ambient Noise Tomography

<p><span><span>The current knowledge of the structure of the Bohemian Massif (BM) crust is mostly based on interpretation of refraction and reflection seismic experiments performed along 2D profiles. The recent development of ambient noise tomography, in combination with dense networks of permanent seismic stations and arrays of passive seismic experiments, provides unique opportunity to build the high-resolution 3D velocity model of the BM crust from long sequences of ambient seismic noise data.</span></span></p><p><span><span>The new 3D shear-wave velocity model is built from surface-wave group-velocity dispersion measurements derived from ambient seismic noise cross-correlations by conventional two-step inversion approach. First, the 2D fast marching travel time tomography is applied to regularise velocity dispersions. Second, the stochastic inversion is applied to compute 1D shear-wave velocity profiles beneath each location of the processing grid.</span></span></p><p><span><span>We processed continuous waveform data from 404 seismic stations (permanent and temporary stations of passive experiments BOHEMA I-IV, PASSEQ, EGER RIFT, ALPARRAY-EASI and ALPARRAY-AASN) in a broader region of the BM (in an area of 46-54</span></span><sup><span><span>0 </span></span></sup><span><span>N 7-21</span></span><sup><span><span>0 </span></span></sup><span><span>E). The overlapping period of each possible station-pair and cross-correlation quality review resulted in more than 21,000 dispersion curves, which further served as an input for surface-wave inversion </span></span><span><span>at h</span></span><span><span>igh-density grid with the cell size of 22 km. </span></span></p><p><span><span>We present the new high-resolution 3D shear-wave velocity model of the BM crust and uppermost mantle with preliminary tectonic interpretations. We compare this model with a compiled P-wave velocity model from the 2D seismic refraction and wide-angle reflection experiments and with the crustal thickness (Moho depth) extracted from P-wave receiver functions (see Kampfová Exnerová et al., EGU2020_SM4.3). 1D velocity profiles resulting from the stochastic inversions exhibit regional variations, which are characteristic for individual units of the BM. Velocities within the upper crust of the BM are ~0.2 km/s higher than those in its surroundings. The highest crustal velocities occur in its southern part (Moldanubian unit). The velocity model confirms, in accord with results from receiver functions and other seismic studies, a relatively thin crust in the Saxothuringian unit, whilst thickness of the Moldanubian crust is at least 36 km in its central and southern parts. The most distinct interface with a velocity inversion at the depth of about 20 to 25 km occurs in the Moldanubian unit. The velocity decrease in the lower crust reflects probably its transversely isotropic structure.</span></span></p>

Terciérní říční sedimenty v širším okolí Jihlavy

Tertiary deposits have been known around Jihlava city for many years. However, they have never been thoroughly studied. Most of the known occurrences are situated in the Jihlava Furrow. During the drilling research, these Tertiary deposits were newly detected in three wells: HGM-1, V1 and V4. Facies analysis, petrography and gamma spectrometry were performed on samples taken from all three wells. The model of basement of Cenozoic deposits has been constructed. Based on facies analysis, 8 lithofacies (7 studied deposits and 1 eluvium) have been described. The study of lithofacies has made the recognition of the depositional environments of the Tertiary deposits possible. The depositional environments of these deposits are interpreted as fluvial channelized gravels and non-channelized deposits (flood plain, crevasse splays, oxbow lake and lake). The channelized gravels have been found by the well HGM-1 in the overburden of the crystalline rocks of the Moldanubian. Their thickness is relatively low (1.7 m) and the channels were probably not deep. The non-channelized deposits are formed by clays, silts and fine-grained sands. In the upper part of the succession a layer of organic deposits (peat) has been detected. The results of petrography have shown that material originated from local sources, especially Moldanubian Unit and Jihlava Massif (gneisses, mica-schistes, granitoids, less importantly pegmatites and phyllites). High concentrations of Th and U have been found in the studied deposits. These high concentrations can be associated with the source rocks. The values of Th /U and Th /K ratios are signifi cantly higher in the studied deposits than in eluvium. This indicates conditions of weathering and sedimentation, and also points to input of the material from several sources. The model of crystalline basement shows that there were probably at least two spatially separated fluvial river systems. One river system was situated in the area of Velký Beranov and Měšín. The preserved remains of the second system are visible in the area of Jihlava city and its peripheral parts (Bedřichov and Pávov).