Vladislav Babuška

Models of seismic anisotropy in the deep continental lithosphere

Seismological observations (SKS-wave polarizations, systematic P-residual variations, azimuthal dependence of Pn- and surface-wave velocities or a dispersion of surface waves) are not consistent with isotropic, if laterally heterogeneous, upper-mantle structure. Therefore, an anisotropy should be considered as an a priori aspect of future large-scale studies of mantle structure. Most studies of anisotropy, however, have assumed horizontal or vertical axes of symmetry, but such orientations cannot explain bipolar patterns of spatial variations of P residuals, which we have observed at many seismological stations. On the basis of the petrophysical properties of real upper-mantle rocks we consider anisotropy formed either by hexagonal or by orthorhombic aggregates composed of olivine, orthopyroxene, and clinopyroxene. Rotations of the aggregates about vertical and horizontal axes allow us to find the three-dimensional orientations of symmetry axes that fit combinations of both P and S seismological observations in Central Europe and in western North America. The orientations with plunging symmetry axes (velocity extremes) seem to be consistent across large, spatially uniform tectonic units and change abruptly at important suture zones.

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.

Seismic anisotropy of the lithosphere around the Trans-European Suture Zone (TESZ) based on teleseismic body-wave data of the TOR experiment

Abstract A passive teleseismic experiment (TOR), traversing the northern part of the Trans-European Suture Zone (TESZ) in Germany, Denmark and Sweden, recorded data for tomography of the upper mantle with a lateral resolution of few tens of kilometers as well as for a detailed study of seismic anisotropy. A joint inversion of teleseismic P-residual spheres and shear-wave splitting parameters allows us to retrieve the 3D orientation of dipping anisotropic structures in different domains of the sub-crustal lithosphere. We distinguish three major domains of different large-scale fabric divided by first-order sutures cutting the whole lithosphere thickness. The Baltic Shield north of the Sorgenfrei–Tornquist Zone (STZ) is characterised by lithosphere thickness around 175 km and the anisotropy is modelled by olivine aggregate of hexagonal symmetry with the high-velocity (ac) foliation plane striking NW–SE and dipping to NE. Southward of the STZ, beneath the Norwegian–Danish Basin, the lithosphere thins abruptly to about 75 km. In this domain, between the STZ and the so-called Caledonian Deformation Front (CDF), the anisotropic structures strike NE–SW and the high-velocity (ac) foliation dips to NW. To the south of the CDF, beneath northern Germany, we observe a heterogeneous lithosphere with variable thickness and anisotropic structures with high velocity dipping predominantly to SW. Most of the anisotropy observed at TOR stations can be explained by a preferred olivine orientation frozen in the sub-crustal lithosphere. Beneath northern Germany, a part of the shear-wave splitting is probably caused by a present-day flow in the asthenosphere.

Seismic Anisotropy and Velocity Variations in the Mantle beneath the Saxothuringicum-Moldanubicum Contact in Central Europe

We report on results of a passive seismic experiment undertaken to study the 3-D velocity structure and anisotropy of the upper mantle around the contact zone of the Saxothuringicum and Moldanubicum in the western margin of the Bohemian Massif in central Europe. Spatial variations of P-wave velocities and lateral variations of the particle motion of split shear waves over the region monitor changes of structure and anisotropy within the deep lithosphere and the asthenosphere. A joint interpretation of P-residual spheres and shear-wave splitting results in an anisotropic model of the lithosphere with high velocities plunging divergently from the contact of both tectonic units. Lateral variations of the mean residuals are related to a southward thickening of the lithosphere beneath the Moldanubicum.

An array study of lithospheric structure across the Protogine zone, Värmland, south-central Sweden — signs of a paleocontinental collision

Abstract A small seismological array was installed on both sides of the Protogine Zone (PZ) in Varmland, south-central Sweden, to study the structure of the mantle lithosphere and lateral variations of its anisotropy. No distinct isotropic velocity anomalies were detected by tomography in the upper mantle around the PZ. The observed velocity variations depending on the direction of propagation can be explained by anisotropy within the subcrustal lithosphere on both sides of the suture. The best solution of a joint analysis of anisotropic inferences from teleseismic P-residual spheres and an inversion of shear-wave splitting parameters, resulted in 3D self-consistent anisotropic models of blocks of the subcrustal lithosphere. The anisotropic structures within the lithosphere are approximated by hexagonal models ( k P =5%) with low-velocity symmetry axes. The high-velocity planes dip to the E in a region westward of the PZ and to the NW eastward of the PZ. The PZ can be interpreted as a steep and narrow suture cutting the whole lithosphere and separating the two anisotropic blocks of different origin.

Joint interpretation of upper-mantle anisotropy based on teleseismic P-travel time delays and inversion of shear-wave splitting parameters

Abstract We inverted shear-wave splitting parameters and simultaneously analysed delays of teleseismic longitudinal waves to obtain a self-consistent three-dimensional (3-D) image of the anisotropic upper mantle beneath the continents. Efficiency of the simultaneous 3-D analyses of P-residual spheres and shear-wave polarizations is demonstrated on data from two regions, southern Sweden and the western USA. The anisotropic inferences of the subcrustal lithosphere are, to a first approximation, represented by homogeneous hexagonal or orthorhombic media with plunging symmetry axes.

Elastic anisotropy of igneous and metamorphic rocks

РезюмеПриведены результаты экспериментального исследованиь упругой анизотропии главных типов изверженных и метаморфических горных пород. Анизотропия определялась из скоростей продольных волн, измеренных на образцах формы многогранника и шара. Пространственная катина упругой анизотропии некоторых типов горных пород была изучена в зависимости от текстуры и предпочтительной ориентировки кальцита, доломита и кварца. На образцах мрамора показывается закономерная зависимость пространственного распределения скоростей продольных волн от ориентировки зерен кальцита. Из результатов вытекает, что упругая анизотропия кроме своего значения для геофизики как геофизической характеристики может предоставить новые дополнительные информации для петроструктурного анализа.

A comparison of measured and calculated elastic anisotropies of marble

РезюмеРасчитаны упругие константы и скорости продольных волн в мраморе, учитываь предпочтительную ориентировку кальцита. Расчитанные скорости сравнивались со скоростями, измеренными в 133 независимых направлениях на шаровидном образце мрамора. Наблюдается хорошое совпадение как по величине измеренных и расчитанных скоростей, так и по пространственному распределению упрутой анизотропии.

An apparatus for investigating the elastic anisotropy on spherical rock samples

РезюмеОписывается установка для пространственного изучения упругой анизотропии горных пород. Измерения проводятся на ориентированных образцах в форме шара. При этом определяется время прохождения упругого импульса в образце. Зависимостя времени прохождения от угла поворота образца регистрируется на самописце. Рассматриваются ошибки определения скоростиP-волн. Величина их не превышает 0,3%. Предлагаемая система измерений позволяет определить пространственную картину упругой анизотропии горной породы и истинные значения экстремумов скоростей, что важно для определения козффициента анизотропии.