Depth dependent azimuthal anisotropy in Madagascar island from ambient noise tomography

摘要

To gain better insight into the structure and complex deformation pattern of Madagascar island, we derived a 3-D heterogeneous and anisotropic velocity model of its crust and uppermost mantle based on ambient noise analyses. The data set comprises similar to 1 year of seismic ambient noise recorded by 63 broad-band seismic stations deployed in and around the island. Data from Ocean Bottom Seismometers were treated for tilt and compliance noise before applying the standard noise processing routine followed by cross-correlation, dispersion measurement, Rayleigh-wave phase-velocity tomography in period range 5-40 s, and 3-D shear wave velocity modelling to 70 km depth. At shallow to mid-crustal depths, our results show fast velocities all over the Precambrian shield of the eastern two-thirds of the island in contrast to the adjacent west coast that is delineated by slow speeds that correlate with the sedimentary basins. In the lower crust and uppermost mantle, relatively fast velocities preponderate the western third of the island indicating a thinned crust and uplifted mantle. We found complex spatial and depth variation in patterns of azimuthal anisotropy across the island. The orientation of fast azimuth is relatively similar in the upper and middle crust. Aligning mostly N-S in the south and approximately NE-SW in the northern areas, it is attributed to alignment of cracks and metamorphic layering that pervades the Precambrian basement. The fast polarization axis in many sub-regions in the lower crust differs from that in the upper to middle crust and uppermost mantle. Apart from the dominant NE-SW fast axis observed in the uppermost mantle consistent with mantle flow, a semi-circular pattern of anisotropy coinciding with widespread intense slow velocities is observed beneath the central Antananarivo terrane. A delaminating lower crust that is initiated by an upwelling asthenosphere hypothesized for this region is invoked to explain our observation.