Transversely isotropic lower crust of Variscan central Europe imaged by ambient noise tomography of the Bohemian Massif

Abstract

Abstract. The recent development of ambient noise tomography, in combination with the increasing number of permanent seismic stations and dense networks of temporary stations operated during passive seismic experiments, provides a unique opportunity to build the first high-resolution 3-D shear wave velocity (vS) model of the entire crust of the Bohemian Massif (BM). This paper provides a regional-scale model of velocity distribution in the BM crust. The velocity model with a cell size of 22 km is built using a conventional two-step inversion approach from Rayleigh wave group velocity dispersion curves measured at more than 400 stations. The shear velocities within the upper crust of the BM are ∼0.2 km s−1 higher than those in its surroundings. The highest crustal velocities appear in its southern part, the Moldanubian unit. The Cadomian part of the region has a thinner crust, whereas the crust assembled, or tectonically transformed in the Variscan period, is thicker. The sharp Moho discontinuity preserves traces of its dynamic development expressed in remnants of Variscan subductions imprinted in bands of crustal thickening. A significant feature of the presented model is the velocity-drop interface (VDI) modelled in the lower part of the crust. We explain this feature by the anisotropic fabric of the lower crust, which is characterised as vertical transverse isotropy with the low velocity being the symmetry axis. The VDI is often interrupted around the boundaries of the crustal units, usually above locally increased velocities in the lowermost crust. Due to the north-west–south-east shortening of the crust and the late-Variscan strike-slip movements along the north-east–south-west oriented sutures preserved in the BM lithosphere, the anisotropic fabric of the lower crust was partly or fully erased along the boundaries of original microplates. These weakened zones accompanied by a velocity increase above the Moho (which indicate an emplacement of mantle rocks into the lower crust) can represent channels through which portions of subducted and later molten rocks have percolated upwards providing magma to subsequently form granitoid plutons.

Crustal S-Wave Velocity Structure Beneath the Northwestern Bohemian Massif, Central Europe, Revealed by the Inversion of Multimodal Ambient Noise Dispersion Curves

“…For a comparison with the picked data, we calculate the theoretical dispersion curves of these 1D velocity models (Figures 13C,D; Supplementary Figure S7C, Supplementary Figure S7D), the results of which clearly demonstrate an unsatisfactory match between the theoretical dispersion curves of these models and the F-J spectrograms (especially at the higher modes). Specifically, the theoretical dispersion curves corresponding to the 1D models under the four arrays extracted from the models of Lu et al (2018) and Kvapil et al (2021) are overall lower than picked data; the two 1D models using the phase velocity dispersion curves for the inversion (Kästle et al, 2018;Růžek et al, 2016), especially the models from Kästle et al (2018) present better fitting results for fundamental mode, but large deviations at the higher modes are still observed for the two models. Our models obtained via the joint inversion of the multimodal dispersion curves match well with the fundamental mode and higher modes for the four arrays.…”

“…In comparison, the S-wave velocities in our models are higher than those in the other models at depths of ~30-40 km. In addition, the S-wave velocities in the models from Kvapil et al (2021) are obviously lower than those in other models (shown in Figure 13, Supplementary Figure S7 and Supplementary Figure S8), even the models of Lu et al (2018), who also used group velocity dispersion curves for inversion. The general group velocity level of the dispersion curves in Kvapil et al (2021) is lower than the level presented in Lu et al (2018), which may be the main reason for the models' discrepancy.…”

“…Recently, some researchers have used the traditional ASNT method to extract phase velocity dispersion curves (e.g., Růžek et al, 2016;Kästle et al, 2018) and group velocity dispersion curves (e.g., Růžek et al, 2016;Lu et al, 2018;Kvapil et al, 2021) to study the velocity structure in the study region and obtained S-wave velocity models. Based on the velocity models of the previous studies, we obtain average 1D S-wave velocity models (Figures 13A,B; Supplementary Figure S7A, Supplementary Figure S7B) beneath the stations of the four arrays (we select the velocity models of the grid points nearest to the stations to calculate the averaged 1D models).…”

“…Some researchers utilized the ASNT method with data from additional seismic stations in Europe to extract phase velocity dispersion curves (e.g., Kästle et al, 2018) and group velocity dispersion curves (e.g., Lu et al, 2018) for the inversion and derived high-resolution S-wave velocity models that present the almost smoothly increasing velocity structure of the crust in the study area. More recently, Kvapil et al (2021) reported that the velocity-drop interface (negative velocity gradient) in the lower part of the crust of the Bohemian Massif (depth of 18-30 km); however, the general group velocity level of dispersion curves is lower than the level presented in Lu et al (2018).…”

“…Nevertheless, the crustal models derived from previous studies for the Bohemian Massif present different, even contradictory results regarding the existence of the LVZ in the middle crust (e.g., Enderle et al, 1998;Wilde-Piórko et al, 2005;Grad et al, 2008;Kolínský et al, 2011;Růžek et al, 2016;Kästle et al, 2018;Lu et al, 2018;Kvapil et al, 2021). To address this inconsistency, in this study, we apply our newly developed multimodal ambient noise dispersion curve tomography method, denoted the frequency-Bessel transform (F-J) method (Wang et al, 2019), to investigate the crustal structure beneath the study area by using available ambient seismic noise datasets.…”