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Scherwellendoppelbrechungsanalyse von Registrierungen der Stationen des seismologischen Netzwerkes an der Neumayer Station, Antarktis: seismische Anisotropie und die tektonische Entwicklung des Kontinentalrandes Queen Maud Lands = Shear-wave splitting analysis on registrations of the Neumayer Station seismological network, Antarctica: seismic anisotrophy and the tectonic evolution of the Queen Maud Land continental margin
Müller, C. (2000). Scherwellendoppelbrechungsanalyse von Registrierungen der Stationen des seismologischen Netzwerkes an der Neumayer Station, Antarktis: seismische Anisotropie und die tektonische Entwicklung des Kontinentalrandes Queen Maud Lands = Shear-wave splitting analysis on registrations of the Neumayer Station seismological network, Antarctica: seismic anisotrophy and the tectonic evolution of the Queen Maud Land continental margin. Ber. Polarforsch. Meeresforsch. 374: 1-224
In: Berichte zur Polar- und Meeresforschung = Reports on Polar and Marine Research. Alfred-Wegener-Institut für Polar- und Meeresforschung: Bremerhaven. ISSN 1618-3193, more
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Keyword
    Marine

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  • Müller, C.

Abstract
    Seismic anisotropy in the upper earth's mantle is a global observable phenomenon. Anisotropic fabrics originate from deformation induced lattice preferred orientation of cristal structures of intrinsic anisotropic mantle minerals (olivin and orthopyroxen). The formation of these anisotropic structures allows insights into recent as well as fossile geodynamical processes. Thus, these structures informations about tectonic evolutional processes may be deduced. A method for investigating seismic anisotropy is the analysis of shear-wave splitting from teleseismic events. Shear-wave splitting originates from linear polarized S-waves traversing an anisotropic medium. The linear polarized wave splits into two orthogonal polarized waves which travel with different velocities. From these modified wave forms simple anisotropic structures can be deduced. The splitting parameters ø (direction of fast velocity) and δt (traveltime delay for a simple model of azimuthal anisotropy are retrieved via specific analysis methods. These investigations were performed on recordings from seismographs of the Neumayer Station on seismological network. Since five of these seven stations are situated on the floating plate of the Ekström Ice Shelf, only registrations from the on grounded ice deployed seismographs Watzmann (WAZ) and Olymp (OLY) could be used. In total, 58 core phases (SKS, SKKS, PKS) could be analyzed. These waves pass the liquid, outer core as compressional waves and are converted to linear polarized S- Waves when entering the core mantle boundary to the ascending part of the ray path. These waves possess the advantage of not being contaminated by source side anisotropy, the initial polarization direction is known from the back azimuth of the ray, and a steep arrival angle beneath the station. Additionally, 17 carefully chosen S- and ScS-waves from deep focus hypocenters events were analyzed. At both stations an average travel time delay of δt = 1.1 sec was found which is a signicant anisotropy effect corresponding well to the global average of continental observations. A slight azimuthal variation of the single measurements results indicates a more complex anisotropic structure. The results of investigations from WAZ and OLY registrations are very consistent. The results are discussed regarding the tectonic evolution of the continental margin of Dronning Maud Land. This region has major importance for understanding details of integration and disintegration of the supercontinent Gondwana. The S-wave splitting investigations reveal fast polarization directions nearly perpendicular to the absolute plate motion direction. Thus, anisotropy does not originate from recent dynamics and must have ancient origin. These fossile structures originated in Precambriam times as part of the former Zimbabwe-Kaapvaal-Grunehogna Craton. Later major tectonic events (Kibarian mountain building event, Pan-African overprint) show no or vanishing effects. Impacts of rifting processes during Gondwana break-up or accompanying activity of a mantle plume are documented. This influence is manifested in mapping small-scale, fossile convective currents. A transform deformation along the continental margin (Explora Escarpment) cannot be ruled out, but anisotropy directions can hardly be explained by this. From azimuthal variations of the anisotropy parameters, a two-layer model was constructed which explains anisotropy structures of Precambrian origin in the upper layer and a lower layer which originated from break-up mechanisms. Additional investigations on core phases recorded at the stations PMSA (Palmer Station, Antarctic Peninsula) and SPA (South Pole Station) also indicate strong evidence of seismic anisotropy. Nevertheless, the used amount of data is too small to discuss the results in a comprehensive tectonic/geodynamic framework. However, delay times measured at PMSA are exceptional large which only can be explained by recent, asthenospheric mantle flow.

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