Ambient noise Rayleigh wave tomography across the Madagascar island

Based on recently obtained data set of the RHUM-RUM (Re ́union Hotspot and Upper Mantle– Re ́unions Unterer Mantel) seismological experiment, in addition to previously available data sets, Rayleigh wave group velocity maps across the entire island at periods between 5 and 30 s are generated using the ambient noise tomography technique. Cross-correlating noise records yielded over 1900 Rayleigh wave cross-correlation functions from which group velocities were measured to perform surface wave tomography. Dispersion curves extracted from group velocity tomographic maps are inverted to compute a 3-D shear velocity model of the region. The velocity maps have shown relative improvement in imaging the three sedimentary basins in the western third of the island compared to those of previous studies. The Morondava basin southwest of the island is the broadest and contains the thickest sedimentary rocks while the Antsirinana basin at the northern tip is narrowest and thinnest. The lithosphere beneath the island is characterized by a heterogeneous crust, which appears thickest at the centre but thins away towards the margins. A combined effect of uneven erosion of the crust and rifting accommodates our observations along the east coast. Average 1-D shear velocity models in six different tectonic units, support the causes of low velocity zones observed in the west coast of the island and reveal an intermediate-to-felsic Precambrian upper and middle crust consistent with findings of previous seismic studies. The findings, especially at short periods provide new constraints on shallow crustal structure of the main island region. 

Figure: Shear wave velocity maps in the sedimentary layer and crust at depths of 5–23 km

Citation

N. I. Adimah and S. Padhy; Geophys. J. Int. (2020) 220, 1657–1676 https://doi:10.1093/gji/ggz542