Azimuthal variation of Pg velocity in the Moldanubian, Czech Republic: observations based on a multi-azimuthal common-shot experiment

S U M M A R YThe Variscan orogeny is the major Middle to Late Palaeozoic tectonometamorphic event in central Europe, and the Bohemian Massif is the largest exposure of rocks deformed during this orogeny. The Bohemian Massif consists of the Saxothuringian, Barrandian and Moldanubian units. Adjacent to this massif in the southeast, the Western Carpathians form an arc-shaped mountain range related to the Alpine orogeny during the Cretaceous to Tertiary. The complex crustal-scale geological structure of the Variscan Bohemian Massif and the Western Carpathians, and especially their contact, were analysed in this study employing the data of the SUDETES 2003 international seismic refraction experiment. The analysed seismic data were acquired along the 740 km long, NW-SE oriented S04 profile that crossed the Bohemian Massif and the Western Carpathians before terminating in the Pannonian Basin. The data were interpreted by 2-D trial-and-error forward modelling of P waves, and additional constraints on crustal structure were provided by gravity modelling.The complex velocity structure derived in our analysis included low velocities of 5.85 km s −1 at the contact of the Saxothuringian and Barrandian units that reflect the presence of lowdensity granites. There are distinct lateral variations in deep crustal structure in the transition between the Bohemian Massif and the Western Carpathians. The abrupt change of the crustal thickness in this transition zone may be associated with the Pieniny Klippen Belt, a deep-seated boundary between the colliding Palaeozoic lithospheric plate to the north and the ALCAPA microplate to the south. In the upper crust of this transition, low velocities of 4 km s −1 extend to 6 km and represent the sedimentary fill of the Carpathian Flysch and Foredeep that thins towards the foreland. This basin is also expressed as a pronounced gravity low. The Moho in the Carpathians reaches a depth of 32-33 km. In contrast, in the Pannonian Basin the Moho rises to a depth of 25 km, which corresponds to the Pannonian gravity high.

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“…Azimuthal anisotropic studies are a matter of other investigations (e.g. Růžek et al 2003; Vavryčuk et al 2004).…”

Section: Discussion Of the Geological And Tectonic Implicationsmentioning

confidence: 99%

From the Variscan to the Alpine Orogeny: crustal structure of the Bohemian Massif and the Western Carpathians in the light of the SUDETES 2003 seismic data

Hrubcová

Środa

Grad

et al. 2010

Geophysical Journal International

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“…In the neighbouring areas of the Tauern Window, the Bohemian Massif in the Czech Republic and in the marginal zone of the EEC in SE Poland, the anisotropy of the upper crust can reach 5–10 per cent (Ru°žek et al 2003; Lüschen et al . 2004; Vavryčuk et al 2004; Bleibinhaus & Gebrande 2006; Środa 2006).…”

Section: Discussion Of Significant Structures Derived From P Wavesmentioning

confidence: 99%

Crustal structure of the Eastern Alps and their foreland: seismic model beneath the CEL10/Alp04 profile and tectonic implications

Grad

Brückl

Majdański

et al. 2009

Geophysical Journal International

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SUMMARY The subject of this paper concerns the seismic modelling of the crustal structure in the transition zone from the Bohemian Massif, across the Molasse basin and the Eastern Alps to the Southern Alps, mainly on the territory of Austria. The CEL10/Alp04 profile crosses the triple point of the European plate, Adriatic microplate and the recently identified Pannonian fragment. The seismic data along the presented profile originate from two large experiments: CELEBRATION 2000 and ALP 2002. The wavefield observed in the Eastern Alps is more complex than in the Bohemian Massif. Strong first arrivals (Pg) are distinct up to 60–90 km offset and are characterized by large variations in apparent velocity and amplitude. The contact between the Molasse basin and the Eastern Alps represents a barrier for seismic waves. Mid‐crustal reflections (Pc) are usually recorded at short distance intervals (20–50 km) and are also characterized by variations in apparent velocity and amplitude. The Moho reflections are usually strong and well correlated, while Pn arrivals are only fragmentarily recorded. Detailed 2‐D forward modelling of all refracted, post‐critical and reflected phases, identified in the correlation process, was undertaken using a ray‐tracing technique. The P‐wave velocity in the crystalline upper crust of the Bohemian Massif and Molasse basin is about 6.15 km s−1, which is slightly higher than in the Alpine area (about 6.0 km s−1). Below the northern accretionary wedge of the Eastern Alps low‐velocity sediments penetrate towards the southwest (SW) down to about 10 km depth. In the middle crust of the Alpine part, a reflective zone was modelled by a lamellae structure with alternating high and low velocities and thicknesses of about 2–3 km. The lower crust in this part of the model is more homogeneous, with a velocity of about 6.9 km s−1. In the Bohemian Massif, a high‐velocity (HV) body (VP∼ 7 km s−1) of a few kilometres thickness was delineated in the depth interval 18–23 km. The crustal thickness along the CEL10/Alp04 profile changes from about 42–44 km in the SW (Alpine part), to around 40 km in the central part of the profile (Molasse basin), and 38–40 km in the NE (Bohemian Massif). The velocity in the uppermost mantle determined from Pn wave traveltimes is about 8 km s−1 along the whole profile. The interpretation of the seismic wavefield is supplemented by an existing 3‐D P‐wave velocity model of the area. Main features derived by 2‐D modelling (low velocities beneath the accretionary wedge, high velocities in the lower crust of the Bohemian Massif, Moho topography) well correlate with the 3‐D model. Furthermore, the 3‐D model allows assessing the lateral extent of significant features alongside the CEL10/Alp04 profile. This area is affected by both collision and escape tectonics. The high‐reflectivity zone in the middle crust is explained by intermediate to mafic intrusions, rather than by ductile extensional deformation as generally observed in the lower crust.

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“…Gravity modeling complemented the velocity analysis. In this study we concentrate on velocity variations along the profile; azimuthal anisotropic studies are a matter of other investigations [e.g., Růžek et al , 2003; Vavryčuk et al , 2004; Plomerová et al , 1984]. …”

Section: Introductionmentioning

confidence: 99%

Crustal and uppermost mantle structure of the Bohemian Massif based on CELEBRATION 2000 data

et al. 2005

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[1] The deep structure of the Bohemian Massif (BM), the largest stable outcrop of Variscan rocks in central Europe, was studied using the data of the international seismic refraction experiment Central European Lithospheric Experiment Based on Refraction (CELEBRATION) 2000. The data were interpreted by seismic tomographic inversion and by two-dimensional (2-D) trial-and-error forward modeling of P and S waves. Additional constraint on crustal structure was given by amplitude modeling using the reflectivity method and gravity modeling. Though consolidated, the BM can be subdivided into several tectonic units separated by faults, shear zones, or thrusts reflecting varying influence of the Cadomian and Variscan orogeneses: the Saxothuringian, Barrandian, Moldanubian, and Moravian. Velocity models determine three types of crust-mantle transition in the BM reflecting variable crustal thickness and delimiting contacts of tectonic units in depth. The NW area, the Saxothuringian, has a highly reflective lower crustal layer above Moho with a strong velocity contrast at the top of this layer. This reflective laminated lower crust reaches depths of 26-35 km and is characteristic for the Saxothuringian unit, which was subject to eastward subduction. The Moldanubian in the central part is characterized by the deepest (39 km) and the most pronounced Moho within the whole BM with a strong velocity contrast 6.9-8.1 km s À1 . A thick crust-mantle transition zone in the SE, with velocity increase from 6.8 to 7.8 km s À1 over the depth range of 23-40 km, seems to be the characteristic feature of the Moravian overthrusted by the Moldanubian during Variscan collision.Citation: Hrubcová, P., P. Ś roda, A. Š pičák, A. Guterch, M. Grad, G. R. Keller, E. Brueckl, and H. Thybo (2005), Crustal and uppermost mantle structure of the

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Azimuthal variation of Pg velocity in the Moldanubian, Czech Republic: observations based on a multi-azimuthal common-shot experiment

Cited by 18 publications

References 27 publications

From the Variscan to the Alpine Orogeny: crustal structure of the Bohemian Massif and the Western Carpathians in the light of the SUDETES 2003 seismic data

From the Variscan to the Alpine Orogeny: crustal structure of the Bohemian Massif and the Western Carpathians in the light of the SUDETES 2003 seismic data

Crustal structure of the Eastern Alps and their foreland: seismic model beneath the CEL10/Alp04 profile and tectonic implications

Crustal and uppermost mantle structure of the Bohemian Massif based on CELEBRATION 2000 data

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