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

Solid Earth ◽  
2021 ◽  
Vol 12 (5) ◽  
pp. 1051-1074
Author(s):  
Jiří Kvapil ◽  
Jaroslava Plomerová ◽  
Hana Kampfová Exnerová ◽  
Vladislav Babuška ◽  
György Hetényi ◽  
...  
Keyword(s):  
Bohemian Massif ◽  
Lower Crust ◽  
Ambient Noise ◽  
Regional Scale ◽  
Crustal Thickening ◽  
Scale Model ◽  
Significant Feature ◽  
Ambient Noise Tomography ◽  
North East ◽  
The North

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.

scholarly journals Transversely Isotropic Lower Crust of Variscan Central Europe imaged by Ambient Noise Tomography of the Bohemian Massif

2020 ◽  
Author(s):  
Jiří Kvapil ◽  
Jaroslava Plomerová ◽  
Hana Kampfová Exnerová ◽  
Vladislav Babuška ◽  
György Hetényi ◽  
...  
Keyword(s):  
Bohemian Massif ◽  
Lower Crust ◽  
Ambient Noise ◽  
Group Velocity Dispersion ◽  
Transverse Isotropy ◽  
Regional Scale ◽  
Velocity Model ◽  
Significant Feature ◽  
Ambient Noise Tomography ◽  
Low Velocity

Abstract. Recent development of ambient noise tomography, in combination with 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 crust of the Bohemian Massif (BM). The velocity model with a cell size of 22 km is built by 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 model provides compelling evidence for a regional-scale of velocity distribution. The Cadomian part of the region has a thinner crust, while 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 thickenings. 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 anisotropic fabric of the lower crust, which is characterized 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 NW SE shortening of the crust and the late Variscan strike slip movements along the NE SW 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 extrusion 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.

Sensitivity enhancement of the ambient noise tomography and analysis of seasonal variations of noise sources to refine velocities in the lower crust in central Europe

2021 ◽  
Author(s):  
Jiri Kvapil ◽  
Jaroslava Plomerova ◽  
AlpArray Working Group
Keyword(s):  
Central Europe ◽  
Seasonal Variations ◽  
Lower Crust ◽  
Ambient Noise ◽  
Regional Scale ◽  
Dispersion Curves ◽  
Small Scale ◽  
Ambient Noise Tomography ◽  
Noise Sources

<p>The capability of the ambient noise tomography (ANT) to image subtle regional-scale velocity variations <span>in </span>the lower crust is limited by strong directionality of ambient noise sources in central Europe, which affects the quality of dispersion curves. Significant decrease of sensitivity kernels and sparse coverage of long interstation ray-pathes result in lower resolution at longer periods and thus <span>increase</span> uncertainty of the inversion solution <span>in</span> depth. <span>If</span> these well-known ANT <span>limitations</span> are properly addressed, the ANT is able to retrieve reliable high-resolution 3‑D shear velocities of the lower crust.</p><p>In this study we focus on seasonal variations of ambient noise sources in selected sites in different tectonic settings. We analyse ambient noise sources on continusly recorded wavefields from permanent observatories and temporary stations of AlpArray passive experiment with its complementary experiment and PACASE. These seismic networks with densely-spaced stations are well-suited for detailed analysis of period-dependent directionality of ambient noise sources and their effects on FTAN appearance and consequently on the quality of dispersion curves. In the second part of this study, we advocate a concept of layer-stripping during the stochastic inversion (enhanced ANT). It proved to be an efficient technique to explore the model space, particularly in the lower part of the crust. We discuss the sensitivity of the enhanced ANT to the imaged small-scale velocity features in the lower part of the crust, as well as the sensitivity to the sharp or gradational Moho in the models.</p>

Architecture of the crust and lithosphere beneath the Semail Ophiolite from ambient noise tomography and receiver functions: insights on the tectonic evolution of eastern Arabia

2021 ◽  
Author(s):  
Christian Weidle ◽  
Lars Wiesenberg ◽  
Andreas Scharf ◽  
Philippe Agard ◽  
Amr El-Sharkawy ◽  
...  
Keyword(s):  
Late Cretaceous ◽  
Ambient Noise ◽  
Function Analysis ◽  
Oceanic Lithosphere ◽  
Receiver Functions ◽  
Crustal Thickening ◽  
Ambient Noise Tomography ◽  
First Order ◽  
Pan African ◽  
Semail Ophiolite

<p>The Semail Ophiolite is the world<span>‘</span>s largest and best exposed oceanic lithosphere on land and a primary reference site for studies of creation of oceanic lithosphere, initiation of subduction, geodynamic models of obduction, subduction and exhumation of continental rocks during obduction. Five decades of geological mapping, structural, petrological and geochronological research provide a robust understanding of the geodynamic evolution of the shallow continental crust in northern Oman and how the late Cretaceous obduction process largely shaped the present-day landscape. Yet, prior to obduction, other first-order tectonic processes have left their imprint in the lithosphere, in particular the Neoproterozoic accretion of Arabia and Permian breakup of Pangea. Due to the scarcity of deep structure imaging below the ophiolite, the presence and significance of inherited structures for the obduction process remain unclear.</p><p>We discuss a new 3-D anisotropic shear wave velocity model of the crust below northern Oman derived from ambient noise tomography and Receiver Function analysis which allows to <span>resolve</span> some key unknowns in geodynamics of eastern Arabia: (1) <span>Several NE-trending structural boundaries in the middle and lower crust are attributed to the Pan-African orogeny and align with first-order lateral changes in surface geology and topography.</span> (2) The well-known Semail Gap Fault Zone is an upper crustal feature whereas two other deep crustal faults are newly identified. (3) Permian rifting occurred on both eastern and northern margins but large-scale mafic intrusions and/or underplating occurred only in the east. (4) While obduction is inherently lithospheric by nature, its effects <span>are mostly observed at shallow crustal depths, and lateral variations in its geometry and dynamics can be explained by effects on pre-existing Pan-African and Permian structures. (5) Continental subduction and exhumation during late Cretaceous obduction may be the cause for crustal thickening below today‘s topography.</span> (6) Thinning of the continental lithosphere below northern Oman in late Eocene times – possibly related to thermal effects of the incipient Afar mantle plume - provides a plausible mechanism for the broad emergence of the Oman Mountains and in particular the Jabal Akhdar Dome. Uplift might thus be unrelated to compressional tectonics during Arabia-Eurasia convergence as previously believed.</p>

Energy Conversion OWC Devices With Additional Vertical Ducts

2008 ◽  
Author(s):  
Sergio M. Camporeale ◽  
Pasquale G. F. Filianoti
Keyword(s):  
Energy Conversion ◽  
Large Fraction ◽  
Scale Model ◽  
Small Scale ◽  
Wells Turbine ◽  
Small Opening ◽  
North East ◽  
The North ◽  
Pneumatic Power ◽  
Vertical Duct

In breakwaters embodying an OWC connected to the sea through a vertical duct or through a small opening, the oscillations of the water column are due to the wave pressure acting on the outer opening of the vertical duct or on the small opening. In fact, in neither of the two cases waves can enter the plant, like it happens in conventional OWCs. The additional vertical duct extends along the wave-beaten wall, giving the device the characteristic form of a U-conduit; for this reason they were also named U-OWC. Experiments on a small-scale breakwater embodying a U-OWC were carried out in the natural laboratory of Reggio Calabria. The plant is a 1:10 scale model of a breakwater suitable for the North-East Pacific coast. The paper describes new experiments on the U-OWC device connected to a monoplane Wells turbine. During an intensive measurement campaign, more than 260 sea states, 5 min long were recorded in order to characterize the energy conversion process. From the experiments, the analysis of the energy conversion shows that: 1) the system is able to absorb a share up 80% of the incident wave energy; this result is similar to that obtained in previous experiments carried out without the turbine; 2) a large fraction of the energy entering the U-OWC is converted in pneumatic power acting on the turbine, being head losses in the water flow limited; 3) the efficiency of conversion of the pneumatic power in turbine power is relatively low (about 36% of the pneumatic power) due to the small dimensions of the turbine that lead to low Reynolds number and large influence of secondary losses.

Ambient noise tomography of the Central Cantabrian Mountains (NW Spain). New insights from the GEOCANTABRICA-COSTA seismic network.

2020 ◽  
Author(s):  
Jorge Acevedo ◽  
Gabriela Fernández-Viejo ◽  
Sergio Llana-Fúnez ◽  
Carlos López-Fernández ◽  
Luis Pando ◽  
...  
Keyword(s):  
Ambient Noise ◽  
Body Wave ◽  
Processing Technique ◽  
Ambient Noise Tomography ◽  
Cantabrian Mountains ◽  
Nw Spain ◽  
The West ◽  
The North ◽  
New Information ◽  
Nucleation Sites

<p><span><span>The Cantabrian Mountains (NW Spain) are an Alpine chain that was formed as a result of the collision between Iberia and Europe in the Cenozoic. In their central sector, the uplift of the orogen led to the exhumation of a block of Variscan -Paleozoic- basement, the reactivation of Variscan structures and the formation of new E-W oriented fractures. Moreover, the formation of the Cantabrian Mountains involved the development of a crustal root with a thickness of 45-55 km that decreases up to 30-35 km towards the west. The thickening occurs preferentially in the crust that had previously been extended during the two main rifting episodes that affected this area in the Mesozoic. At the surface, the limit between the normal and the thickened crust roughly coincides with the trace of the Ventaniella fault, a subvertical crustal structure that runs for more than 400 km both inland and offshore. </span></span></p><p><span><span>In order to obtain new insights from this complex region, it was installed a network (GEOCANTÁBRICA-COST</span></span><span><span>A, doi:</span></span><span><span><span>10.7914/SN/YR_2019</span></span></span><span><span>) of 13 broadban</span></span><span><span>d stations covering an area of 160x80 km (</span></span><span><span>~40 km spacing) for 8 months.</span></span> <span><span>The phase cross-correlation (PCC) processing technique was used to cross-correlate daily records of the 78 station pairs. After stacking the cross-correlograms, the empirical Green’s functions and the dispersion curves were obtained. Finally, a Rayleigh wave group velocity tomography was performed, retrieving the seismic signature of the Variscan crust and allowing us to extend to the north our previous seismic ambient noise tomography and complete the tomographic model of the central Cantabrian Mountains. To reveal the structure beneath the seismic stations, we also performed ambient noise auto-correlations, successfully retrieving body-wave reflections from the crust-mantle boundary that provide new information about the limits of the crustal root. </span></span></p><p><span><span>The study area presents a lingering, low-magnitude intraplate seismic activity that increases from east to west and extends into the continental shelf. The Ventaniella fault also acts as a seismic barrier to the propagation of earthquakes towards the east while provides nucleation sites along its trace. Thus, another objective of this study was to detect and relocate the local seismicity of the Cantabrian Mountains and the Cantabrian margin activity in particular. Our preliminary catalogue of events, obtained from the automatic analysis of the real-time seismic data with </span></span><span><span><em>SeiscompP3</em></span></span><span><span>, comprises 54 local earthquakes. Seven of them have their epicentres in the Cantabrian margin and, as expected, all were located to the west of the Ventaniella fault.</span></span></p>

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