High-resolution mapping of the mantle transition zone and its interaction with subducted slabs in the Ibero-Maghrebian region

Abstract

The mantle transition zone (MTZ) beneath Iberia and NW Maghreb has been precisely mapped using more than 56000 high-quality P-wave receiver functions calculated from the data collected by permanent seismic networks and multiple temporary deployments in the region. Three-dimensional depth migration using both regional and global tomographic models has allowed us to obtain robust and continuous measurements of the MTZ thickness and the depth of the 410 and 660 discontinuities. We found the MTZ thickened by as much as 30 km in the Mediterranean coast due to the effect of the cold Gibraltar-Alboran and Alpine-Tethys slabs. Coinciding with expected water-enriched MTZ areas near the subducted slabs there is evidence for partial melting atop the 410 in at least three low-velocity layers (LVL). Partial melting is also likely in a LVL in the uppermost lower mantle under the Alpine-Tethys slab, while we attribute other intra-MTZ LVL to increased mineralogical heterogeneity. We link a thinning of the MTZ at the rear of the Gibraltar-Alboran slab to mantle upwelling, and a band of depressed 410 along its southern boundary as an area of hot toroidal flow. A discontinuous region of depressed 410 following the Atlas Mountains also supports mantle upwelling beneath this range. Areas with LVL atop a depressed 410 discontinuity correlate well with active intraplate volcanism, suggesting a possible MTZ source. We also found that deep-focus seismicity occurs where the 660 discontinuity starts to deepen at the westernmost edge of the Gibraltar-Alboran slab.