Mapping the mantle transition zone beneath the Ibero-Maghrebian region with P-wave receiver functions

Thermal and compositional anomalies are known to drive changes in the thickness of the mantle transition zone (MTZ), as they modify the P-T conditions under which phase transitions occur. Typically, the phase changes defining the upper and lower boundaries of the MTZ take place at 410 km and 660 km respectively, thus yielding a standard MTZ thickness of 250 km. These phase transitions have opposite Clapeyron slopes, so while a cold temperature anomaly makes the 410 discontinuity shallower and the 660 deeper, a hot anomaly has the contrary effect. In this ongoing study we use P-wave receiver functions to map the MTZ discontinuities below southern Iberia and northwestern Africa and identify anomalous regions that can be linked to regional structures in the mantle. In this area, convergence of the African and Eurasian plates led to the subduction of the ancient Tethys oceanic lithosphere, from which a remnant slab is stalled below the Gibraltar arc, introducing thermal and chemical heterogeneities that alter the MTZ.

Roughly 33000 receiver functions were obtained from 501 seismic stations, from both permanent and temporary deployments, including four seismic profiles with high density of stations (interstation distances from 2 to 10 km). We constructed a grid of N-S and W-E sections with a spacing of 0.25x0.25 degrees by depth-migrating and projecting the receiver functions with a phase-weighted common conversion point stacking method. An algorithm for the automatic detection of the MTZ discontinuities was then used, and the results allowed us to obtain preliminary 2D and 3D maps containing the depth, width and number of peaks of the pulses attributed to the 410 and 660 discontinuities. Overall, the MTZ reaches its maximum thickness under the Alboran basin (290 km), but the region with anomalous thickness extends well into southeastern Iberia. This feature is attributable to a cold temperature anomaly and matches the position where tomographic studies locate the stalled Tethys slab. Two small areas in the Gulf of Cadiz also stand out for displaying a MTZ thickness of 290 km, coinciding with a region where the 410 discontinuity splits in two pulses. On the contrary, the MTZ is generally thinner than usual towards the south and west of the Alboran basin, especially in sections of the Rif and the Strait of Gibraltar where it can reach 205 km. Our results show, though, that while changes in the 410 discontinuity can be correlated with the tectonic configuration of the region and known anomalies in the mantle such as the Tethys slab, the 660 displays a much more unpredictable pattern.