Jaroslava Plomerová

Mapping the lithosphere–asthenosphere boundary through changes in surface-wave anisotropy

Abstract The alignment of olivine crystals is considered as the dominant source of seismic anisotropy in the subcrustal lithosphere and asthenosphere. Different components of large-scale anisotropy can be traced in depth distributions of the radial and azimuthal anisotropy of surface waves. We propose a global model of the lithosphere–asthenosphere boundary (LAB) as a transition between a ‘frozen-in’ anisotropy in the lithosphere to anisotropy in the sublithospheric mantle related to the present-day flow. Due to different orientations of velocity maxima in the anisotropic subcrustal lithosphere and the asthenosphere, the velocity contrast related to the LAB can increase in particular directions. Because of their long wavelengths and horizontal propagation, surface waves suffer from poor lateral resolution. However, surface waves with various wavelengths allow us to map gross features of the LAB with a good vertical resolution. We estimate depths to the LAB to be between 200 and 250 km for the Precambrian shields and platforms, around 100 km for the Phanerozoic continental regions and 40–70 km beneath oceans from the world-wide depth distribution of the radial and azimuthal anisotropy of surface waves.

Gravity modelling of the lithosphere in the Eastern Alpine-Western Carpathian-Pannonian Basin region

Abstract Gravity models illustrate changes in the degree of continental convergence in the Eastern Alpine-Western Carpathian region, and modifications to the lithosphere due to the plate convergence and subsequent Pannonian Basin extension. Analysis of the continental collision zone incorporates a kinematic model of ocean basin closure, whereby gravity anomalies and topography are viewed as part of a continuum of continental crustal shortening, erosion and isostatic rebound. Thick crust and high topography in the Eastern Alps, along with a broad Bouguer anomaly of −140 mGal amplitude, are consistent with about 175 km of crustal shortening, followed by 10 km of isostatic rebound. Eastward, crustal thicknesses and gravity anomaly widths and amplitudes are less, so that only about 50 km of continental crustal shortening and 4 km of rebound occurred in the Western Carpathians. Preservation of thick flysch deposits and small isostatic rebound are attributable to the high-density, shallow mantle of the intact continent-ocean transition zone. Seismic delay time studies have suggested that, relative to the average thickness of the region, the lithosphere thickens by about 70 km beneath the Eastern Alps and thins by about 60 km under the Pannonian Basin. In both regions, gravity anomalies cannot be explained fully without considering this large relief on the lithosphere/asthenosphere boundary. The Eastern Alpine crustal root, which extends 15 km below the average depth for the region, overcompensates the topography and results in gravity anomalies that are 40 mGal lower than those observed; the extra 70 km of lithosphere provides excess mass that achieves isostatic equilibrium and accounts for the 40 mGal difference. Observed gravity anomalies and local isostasy are also consistent with thin crust and thin lithosphere beneath the Pannonian Basin, whereby the 60 km of extra asthenosphere provides a large part of the compensation for the elevated mantle. Regional cross sections suggest that shallowing of the lithosphere/asthenosphere boundary, associated with Pannonian Basin extension, has propagated northward beyond the Carpathians, to within the European Platform. Crustal thinning, however, appears to be confined to exotic terranes of the Carpathian interior, so that crustal structure in the Eastern Alps and Outer Carpathians is a remnant of the earlier collision orogen.

Images of lithospheric heterogeneities in the Armorican segment of the Hercynian Range in France

The Armorican Massif is located in western France. It is a part of the Hercynian Range formed in several phases of SE-NW compression from Devonian to Carboniferous time (ca. 400-300 Ma). The main tectonic features are the North and South Armorican shear zones with WNW-ESE strike. In the framework of the GeoFrance3D program, we have installed 35 seismological stations along two NS lines crossing the main Hercynian WNW-ESE fault systems. The data set collected during the experiment allows us to compute a 3D P-velocity model and to map S-wave seismic anisotropy at depth using teleseismic shear waves splitting measurements. The South Armorican shear zone (SASZ) is characterized by a strong (4-5%) velocity contrast and a fast shear wave azimuth parallel to its strike. The North Armorican shear zone (NASZ) does not show any significant lithospheric signature. The tomographic image and anisotropy results suggest that the Armorican lithosphere consists of the juxtaposition of distinct lithospheric blocks assembled in a subduction type process predating the Hercynian Orogeny. D 2002 Elsevier Science B.V. All rights reserved.

Temporary Array Data for Studying Seismic Anisotropy of Variscan Massifs – The Armorican Massif, French Massif Central And Bohemian Massif

We present the first results of a comparison of deep lithosphere structure of three Variscan massifs - the Armorican Massif, French Massif Central and Bohemian Massif, as revealed by recent tomographic studies of seismic anisotropy. The data originate from several field measurements made in temporary arrays of stations equipped with both short-period and broadband seismometers with digital recording. The study is based on teleseismic body waves and a joint inversion of anisotropic data (P-residual spheres, the fast shear-wave polarizations and split times) and demonstrates that the three Variscan massifs appear to consist of at least two parts with different orientation of large-scale fabric derived from seismic anisotropy. The boundaries of anisotropic lithospheric domains are related to prominent tectonic features recognised on the surface as sutures, shear zones or transfer fault zones, as well as grabens, thus indicating that some of them extend deep through the entire lithosphere.

Three-dimensional velocity model of the crust of the Bohemian Massif and its effects on seismic tomography of the upper mantle

We have compiled a representative three-dimensional P-velocity model of the crust of the Bohemian Massif (BM) to provide a basis for removing effects of the crustal structure in teleseismic tomography of the upper mantle. The model is primarily based on recently published 2D velocity models from findings of wide-angle refraction and near-vertical reflection seismic profiles of CELEBRATION 2000, ALP 2002, and SUDETES 2003 experiments. The best fitting 3D model of the BM crust (NearNeighbour model) is complemented by velocities according to the reference Earth model at sites where data are sparse, which precludes creating artificial heterogeneities that are products of interpolation method. To test the model, we have performed tomographic inversions of the P-wave travel times measured during the BOHEMA II experiment and compared the results obtained with and without crustal corrections. The tests showed that the presented crustal model decreases magnitudes of velocity perturbations leaking from the crust to the mantle in the western part of the BM. The tomographic images also indicated a highvelocity anomaly in the lower crust or just beneath the crust in the Brunovistullian unit. Such anomaly is not described by our model of the crust since no seismic profile intersects this part of the unit. The tests also indicated that crustal corrections are of the great importance especially for interpretations of the uppermost mantle down to depths of about 100 km.

Deep structure of the lithosphere beneath the territory of Bulgaria

РезюмеРaсчumaны временные невязкu Р волн nрuхо¶rt;ящuх nо¶rt; рaзнымu aзuмуmaмu u улaмu. Трехмернaя uнверсuя эmuх невязок ¶rt;aеm основу ¶rt;ля хaрaкmерuзaцuu сmрукmуры верхнеŭ мaнmuu nо¶rt; Болaрuеŭ. Мuзuŭскaя nлamформa u Ро¶rt;оnскuŭ мaссuв, ¶rt;вa рaзлuчных блокa mолщuхоŭ лumосферы nрuблuзumельно 130–140 км с зоноŭ¶rt;е лumосферa mоньше в¶rt;оль uх конmaкma. Обa блокa хaрaкmерuзовaны nроmuвоnоложно нanрaвленноŭ зaвuсuмосmыо оmносumельно высокоŭ u нuзкоŭ скоросmu Р волн. Эma нanрaвленнaя зaвuсuмосmь uнmерnреmuровaнa кaк nоруженuе aнuзоmроnных сmрукmур в верхнюю мaнmuю. Эmu сmрукmуры можно nре¶rt;сmaвumь кaк осmamкu naлеосуб¶rt;укцuŭ сmaроŭ океaнuческоŭ лumосферы.SummaryTeleseismic P residuals calculated for waves arriving from various azimuths and angles of incidence, and a 3-D inversion of the residuals provided the basis for characterizing the uppermost mantle structure beneath Bulgaria. The Moesian Platform and the Rhodopean Massif are two different blocks characterized by a lithosphere thickness of about 130–140 km with a zone of lithosphere thinning along their contact. Both units have opposite patterns of the directional dependence of relatively high and low P velocities. This directional dependence is interpreted by dipping anisotropic structures in the subcrustal lithosphere, which probably represent remnants of paleosubductions of an old oceanic lithosphere.

Saxothuringian-Moldanubian Suture and Predisposition of Seismicity In The Western Bohemian Massif

We modelled the thickness and seismic anisotropy of the subcrustal lithosphere from the variations of P-wave delay times and the shear-wave splitting observed at seismological observatories and portable stations in the western part of the Bohemian Massif. The Saxothuringian lithosphere is characterized by a total thickness between 90 and 120 km, the Moldanubian lithosphere is generally thicker –120-140 km, on the average. The subcrustal lithosphere of both units is characterised by divergently dipping anisotropic structures and the suture between them is marked by a lithosphere thinning to about 80km. Within the subcrustal lithosphere a complex structure of the transition of both units extends to about 150 km toward the south. We suggest that the Saxothuringian-Moldanubian suture has created a zone of mechanical predisposition for the Tertiary Ohře (Eger) Graben, as well as for the occurrence of earthquake swarms in the region. Most earthquakes occur within the brittle part of the upper crust above the crossing of the suture between the Saxothuringian in the north and the Moldanubian and the Tepl´-Barrandian in the south, with the tectonically active Mariánské Lázně fault.

Joint inversion of teleseismic P waveforms and surface-wave group velocities from ambient seismic noise in the Bohemian Massif

Joint inversion of teleseismic P-waveforms and local group velocities of surface waves retrieved from ambient seismic noise has been performed to model velocity structure of the crust and uppermost mantle of the Bohemian Massif. We analysed P-waveforms of 381 teleseismic earthquakes recorded at 54 broadband seismic stations located on the territory of the Czech Republic and in its close surroundings. Group velocities of Rayleigh and Love surface waves were obtained by cross-correlating long-term recordings of seismic noise. The basis for waveform inversion is the well-known methodology of P-to-S receiver functions constructed from converted phases. Due to instabilities in direct inversion of receiver functions caused by the necessity of applying deconvolution, we propose an alternative formulation to fit observed and calculated radial components of P waveforms. The joint inversion is transformed into a search for the minimum of the cost function defined as a weighted sum of waveform and group velocity misfits. With the use of the robust stochastic optimizer (Differential Evolution Algorithm), neither derivatives nor a starting model are needed. The task was solved for 1D layered isotropic models of the crust and the uppermost mantle. We have performed a sequence of inversions with models containing one, two, three and four layers above a half-space. By using statistical criteria (F-test) we were able to select the simplest velocity models satisfying data and representing local geological structures. Complex crustal models are typical for stations located close to boundaries of major tectonic units. The relatively low average P to S wave-velocity ratio is in agreement with the generally accepted view that the BM crust is predominantly felsic.

Velocities of seismic waves propagating through the Bohemian massif from foci in Poland

РезюмеВ сmamье сmрояmся регuонaльные временa nробегa волн Pn, Pg, Sn u Sg nрuхо¶rt;ящuх в Чешскuŭ мaссuв с северо-восmокa. Покaзaно, чmо ¶rt;о рaссmоянuя в 5° оm эnuценmрa регuонaльныего¶rt;огрaфы ¶rt;aюm более короmкuе временa nробегa nо срaвненuю со сmaн¶rt;aрmнымu u скоросmu рaсnросmрaненuя волн, блuзкuе к скоросmям nолученным uз сmaн¶rt;aрmныхго¶rt;огрaфов. Дaеmся соnосmaвленuе локaльных uзмененuŭ скоросmеŭ рaсnросmрaненuя волн с mекmонuкоŭ Чешского мaссuвa.

Geodynamics of Lithosphere and Earth’s Mantle: Seismic Anisotropy as a Record of the Past and Present Dynamic Processes

Plate tectonics has significantly broadened our view of the dynamics of continental evolution, involving both the processes currently active at the surface and those extending deep into the interior of the Earth. Seismic anisotropy provides some of the most diagnostic evidence for mapping past and present deformation of the entire crust-mantle system.