Imaging the Cape Verde hotspot structure

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Publication . Carvalho, J.; Silveira, Graça; Custódio, Susana; Arroucau, Pierre

The Cape Verde archipelago is an interesting region to study plate-hotspot interactions, considering hotspots as enigmatic surface features that are not easily explained in the plate tectonics theory. The horseshoe geometry and the surrounding seafloor morphology and bathymetry of the Cape Verde archipelago play a significant role in the generation of ambient seismic noise in this region. In order to improve ambient noise based studies, it is important to understand the distribution of the sources of ambient noise, as they affect the estimates of empirical Green's function, and consequently the resulting isotropic and anisotropic tomographic models of velocity and attenuation. We analyzed microseisms recorded in Cape Verde and determined the number of signals polarized as Rayleigh waves, as well as their back azimuth as a function of time and frequency. We determined that the local bathymetry around the Cape Verde Islands strongly amplifies local secondary microseismic sources. Furthermore, we confirmed that Cape Verde stations mostly record local sources. In order to study the properties of the crust and upper mantle below Cape Verde, we performed surface wave studies using two different types of data: ambient noise and teleseismic data. By applying a recent implementation of the surface-wave two-station method we measured the phase velocities of Rayleigh-wave fundamental modes in a broad period range (8–250 s). This method is based on cross-correlating teleseismic earthquake seismograms between pairs of stations. From these signals, we derived a robust, average, phase-velocity curve for the Cape Verde region, which we inverted in order to obtain a shear-wave velocity profile. Our results show significantly low velocities of ∼ 4.2 km/s in the asthenosphere, indicating the presence of anomalously high temperatures and, eventually, partial melting. The temperature anomaly is probably responsible for the thermal rejuvenation of the lithosphere to an effective age as young as about 30 Ma. For ambient-noise tomography we computed phase cross-correlations between all station pairs, which were then stacked, yielding empirical Green's functions. Based on these, we derived group-velocity maps for the period band between 10 - 24 s and the corresponding 3D shear-wave velocity structure. The final tomographic model shows the main velocity anomalies, which are associated with tectonic and volcanic activity in Cape Verde. A clear low-velocity anomaly is observed beneath the active volcanic area of the Fogo - Brava islands. This anomaly extends to the islands of Boavista and Maio, even though with less expression. On the contrary, the islands of the Northwestern group display velocities higher than the average at all depths.

2020-11Tese de doutoramento

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Fundação para a Ciência e a Tecnologia

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PD/BD/114480/2016