Mapping and Interpreting the Uppermost Mantle Reflectivity Beneath Central and South‐West Iberia

Abstract

• Two uppermost mantle P-wave reflections observed beneath the SW-Iberian Peninsula at 50-75 (H) km and 90-110 (L) km depth. • 'H' corresponds to an increase in velocity and is likely the Hales discontinuity. 'L' is related to velocity decrease at the LAB. • Full-wavefield modeling shows the Hales discontinuity corresponds to a zone of randomly distributed thin bodies with small Vp variations.

Crustal Imbrication in an Alpine Intraplate Mountain Range: A Wide‐Angle Cross‐Section Across the Spanish‐Portuguese Central System

“…The prominent P M P phase reveals the existence of a conspicuous upper mantle discontinuity, probably a few kilometers thick. This arrival has been identified previously in other areas of the Iberian Massif (Ayarza et al., 2010; Díaz et al., 1993; Palomeras et al., 2009; Palomeras, Ayarza, et al., 2021) and has been explained as the reflection of a layered or gradient transition zone consistent with the spinel‐lherzolite to garnet‐lherzolite transition (Ayarza et al., 2010; Palomeras, Ayarza, et al., 2021), attributed to the Hales discontinuity. To our knowledge, the continuity, reflectivity, and length of this phase are unprecedented, not only in the Iberian Massif but probably at a worldwide scale.…”

“…The depth of the Hales discontinuity depends on the mantle geochemistry, but it can be generally placed at ∼65 km depending on mantle depletion (O’Neill, 1981). In addition, a gradual conversion from spinel to garnet may explain the characteristics of the reflections coming off this boundary (high amplitude and long coda), pointing to some sort of complexity (i.e., layering where the amount of spinel with respect to garnet is variable, Ayarza et al., 2010; Palomeras, Ayarza, et al., 2021), thus producing constructive seismic interferences. Regardless of only being imaged in one of the shots, the data set presented here contains one of the best recorded reflections of the so‐called Hales discontinuity (P M P in Figure 3a).…”

Four decades of geophysical research on Iberia and adjacent margins

“…The LAB map, defined as the maximum negative velocity gradient below a fast lid, shows depths less than 85 km for most of the Iberian Massif, locally reaching values of less than 70 km beneath the northeast Central Iberian Zone. In a recent study using controlled source profiles over the SW of Iberia (Palomeras et al, 2021), the lithospheric thickness was estimated to be around 90 km beneath the Central Iberian Zone, a value closer to those proposed by studies based in different methodologies. Civiero et al (2019) have used a tomographic technique based on the inversion of the S-SKS relative arrival times to investigate the shear wave structure beneath Iberia, Morocco and the Canary Islands.…”

“…This band was not observed in a collocated normal incidence profile, which showed a seismically transparent mantle (Ayarza et al, 2010). The ALCUDIA wide-angle and normal incidence profiles, placed to the north of the IBERSEIS profile and sampling also the Iberian Central Massif, show also seismic phases reflected at two subcrustal discontinuities (Palomeras et al, 2021). The arrivals of the first phase have been modeled as resulting from a ~10 km thick reflective band with its top located around 50 km to the south of the section and deepening to 57 km to the north.…”

“…These reflected phases show that the uppermost mantle beneath the Iberian Massif is heterogeneous and includes at least two discontinuities between the Moho and depths of about 100 km. Accordingly to the interpretation of Palomeras et al (2021), the origin of the upper discontinuity is a transition from spinel to garnet lherzolite, while the deeper one, located at 105-110 km depth would correspond to the base of the lithosphere.…”