The topography of the seismic Lithosphere-Asthenosphere Boundary in Iberia and adjacent regions

The lithosphere-asthenosphere boundary, or LAB, is a key element in plate tectonics as it separates the rigid lithosphere from the underlying convecting mantle. Its origin, though, is still not fully understood mainly due to its various definitions (thermal, compositional, etc.), which lead to different LAB depth predictions or measurements. In this study we use more than 34500 S-wave receiver functions to map the depth of the seismic LAB in Iberia and neighbouring regions. We found that the LAB in Iberia is generally shallow, especially along the eastern coast (70 km from the Gulf of Lion to the Alboran Sea) and more locally in the northwest of the peninsula. The LAB only exceeds 90 km depth in the Western Pyrenees and Iberian Range, where there is significant crustal thickening, and also in the Gulf of Cadiz. However, LAB depth and crustal thickness are not always correlated. Most of the major mountain ranges in the region (the Atlas, the Rif, the Betic System and the Pyrenees) feature areas where there is no lithospheric root, with thickened crust (>40 km) underlain by a shallow LAB (<90 km or even <80 km). The LAB depth gradient revealed that this discontinuity changes sharply along the subduction-transform edge propagator (STEP) fault in the Eastern Betics and the area of continental subduction in the Western Pyrenees. Several sublithospheric negative-velocity gradients (NVGs) also occur near these major lithospheric structures, but their origin seems diverse. The most notable NVG is an eastwards-dipping discontinuity under the Strait of Gibraltar, which we identify as the subducted lithosphere of the Gibraltar-Alboran slab. We link additional NVGs below the Alboran Sea to processes related to the slab and the STEP fault, possibly dehydration melting and/or inflow of hotter mantle materials, but the origin of a fourth NVG below the Western Pyrenees and northern Iberian Range is still up in the air.