Seismic anisotropy of the lithosphere/asthenosphere system beneath southern Madagascar

Abstract:

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 Pan-African orogeny and later to the break-up between East-Africa, India and Antarctica. Today, Madagascar consists of different tectonic units; the eastern two thirds of the island are composed mainly of Precambian rocks, whereas the western part is dominated by sedimentary deposits. Southern Madagascar is characterized by several NS to NW-SE trending shear zones. To increase our understanding of these structures and related tectonic processes, we installed a dense temporary seismic network in southern Madagascar. It consisted of 50 stations, which were in operation for up to 2 years between 2012 and 2014.

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 core phases from up to 22 events. We first apply a conventional 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 waveforms from several events. Our 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 zone, fast axes are oriented NE-SW. We apply full-waveform FD modeling to examine the effects of various anisotropic models of the crust and mantle. Our results indicate that recently proposed mantle flow models are insufficient to explain the small scale variations of splitting parameters observed along our profile. Our observations are best characterized by asthenospheric anisotropy, consistent with the absolute plate motion direction of ~50°, in combination with a block of fossil anisotropy located within the lithosphere exhibiting a symmetry axis of -40°. The latter may be related to the indentation of the Antananarivo block into the metasedimentary southern part of Madagascar during the Pan-African orogeny.