Assembly and breakup of Gondwana - seismological signatures in the lithosphere/asthenosphere system of Southern Madagascar (SELASOMA)

点击翻译按钮获取中文摘要

The assembly and destruction of supercontinents in the Wilson cycle is one of the defining paradigms for understanding the plate tectonic history beyond the time recorded by seafloor magnetic lineations. In this context the question arises to what extent the location and mode of breakup is controlled by ancient fabric formed during earlier cycles. Complementary to this issue is the question to what extent major episodes of suturing or rifting overprint earlier fabric, and how these earlier events influence recent kinematics. Whereas these questions have been addressed extensively by geological investigations, the dynamics of these processes are likely to be strongly influenced by the rheology of the mid- and lower crust and the mantle part of the lithosphere, best characterised by seismological investigations. The island of Madagascar occupied a key region in both the assembly and the multi-stage breakup of Gondwanaland (Stern 1994), itself part of the super-continent Pangaea. Madagascar consists of amalgamated continental material comprising several distinct tectonic units, with the oldest rocks being of Archaean age (3.2 Ga). Major geological boundaries generally follow a N-S trend mirroring the geographic orientation of the island. Its Proterozoic to early Paleozoic fabric is characterised by a shear-zone network spanning hundreds of kilometers, with some of the branches running oblique to the N-S trend, in particular in the south of the island. More recently during the Neogene, moderate volcanism has occurred in the Central and Northern part of the island, and there are indications of uplift throughout Eastern Madagascar over the last 10 Ma (Roberts et al. 2012). Although Madagascar is located within the interior of the African plate and far away from major plate boundaries, its seismic activity indicates that some deformation is taking place. Present-day kinematic models based on geodetic data and earthquake moment tensors identify a diffuse N-S-oriented minor boundary separating two microplates, which appears to pass through Madagascar (Stamps et al. 2008). Because of its key role in the assembly of Gondwana, numerous geological investigations have been carried out in Madagascar. However, comparatively little is known about the sub-surface structure as few focused seismological studies have been carried out in Madagascar. In this proposal we aim to characterise the lithospheric structure of southern Madagascar using data from a passive seismic experiment, which we carried out from May 2012 to May 2014. This experiment included an approximately E-W oriented profile of broadband stations across the island (operating for two years), and an areal array of short-period stations (operated for one year). The profile crosses all major geologic provinces and the short-period array will be used to map local seismicity and provide additional control on the 3-dimensional structure of the lithosphere/asthenosphere system.