Abstract

In this study, we analyse the 410-km and 660-km upper-mantle transition zone discontinuities as seen from seismic P-to-s wave conversions beneath the Eurasian-African Plate boundary at south Spain and north Morocco. For this purpose we use teleseismic events recorded at 43 broad-band seismic stations deployed mainly by the TopoIberia project. The conversions from the upper-mantle discontinuities arrive in the P-wave coda together with other signals and are usually identified on stacked receiver functions. We build a new processing approach which is leaned on receiver functions and which is based on cross-correlation and stacking techniques to efficiently detect and extract signals by means of their coherence, slowness, traveltime and polarity. In order to add consistency and robustness to the detections, our final results are based on a joint analysis of two different cross-correlation functionals and receiver functions. This permits to assess errors and to bridge observation gaps due to the breakdown of any of the techniques inherent to signal and noise characteristics. Finally, discontinuity depths are determined using time corrections obtained from a 3-D velocity model. We present topography maps for the 410-km and 660-km discontinuities which show a thickening of the transition zone beneath the plate boundary towards Morocco. The transition zone thickness is about global average beneath south Spain (240-250 km) and is thicker beneath east Morocco (250-275 km). This is mainly due to a deeper 660-km discontinuity, while the topography of the 410-km discontinuity is smaller. In the Alboran Sea we find an up to 25 km deflection of the 660-km discontinuity which suggests that the Alboran Sea heterogeneity or slab is still sufficiently cold to depress the post-spinel phase transition. We finally discuss the results in order to add new constraints on temperature and composition to seismic velocity anomalies observed in the transition zone beneath south Spain and north Morocco.