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| Titel |
Seismic anisotropy of the lithosphere-asthenosphere system beneath southern Madagascar |
| VerfasserIn |
Miriam Christina Reiss, Georg Rumpker, Frederik Tilmann, Xiaohui Yuan, Elisa Josiane Rindraharisaona |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250105563
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| Publikation (Nr.) |
 EGU/EGU2015-5096.pdf |
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| Zusammenfassung |
| Madagascar is considered as a key region with respect to the assembly and break-up of the
supercontinent Gondwana. Following the collision between East- and West-Gondwana
(~700-650 Ma), its position was central to the Panafrican orogenesis. Madagascar then
separated from East Africa and later from the Indian and Antarctic plates until these
processes came to a halt about 69 Ma ago. Today, Madagascar consists of different
tectonic units; the eastern parts (two thirds of the island) are composed mainly of
Precambian rocks, whereas the western part is dominated by sedimentary deposits.
Furthermore, southern Madagascar is characterized by several NS to NW-SE trending shear
zones.
Madagascar has been the target of a number of geological studies, but seismological
investigations of the presumed complex lithosphere-asthenosphere system and of deeper
upper-mantle structures are sparse. To increase our understanding of these structures and
related tectonic processes, we installed a dense temporary seismic network in southern
Madagascar. It consisted of 25 broadband and 25 short-period stations, which were in
operation for up to 2 years between 2012 and 2014. The broadband stations crossed the island
along an east-west profile; the eastern section was supplemented by a network of short-period
stations.
Here we present results from shear-wave splitting analyses to infer the seismic anisotropy
of the lithosphere-asthenosphere system in response to deformational processes. The
polarization of the fast shear wave and the delay time between the fast and slow waves
provide constraints on the anisotropic fabric. For our study, we use SKS-phases from up to 12
events recorded at the temporary stations and from 10 events at the permanent GEOFON
station VOI. We first apply a single-event splitting analysis by minimizing the transverse
component. For stations that do not show a significant azimuthal dependence of the
splitting parameters, we also apply a joint inversion involving all recorded SKS
waveforms. Our preliminary results exhibit delay times between 0.4 and 1.5 s. In the
center of the E-W profile, fast axes are mainly oriented NNW-SSE, whereas east
of the Ranotsara shear zone, fast axes are oriented NE-SW. Additionally, we will
apply splitting analysis of Ps phases as well as waveform modelling to resolve the
possible influence of the crust on the anisotropy inferred from the SKS phases. |
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