Mapping and Interpreting the Uppermost Mantle Reflectivity Beneath Central and South‐West Iberia. Issue 4 (27th April 2021)

• Record Type: Journal Article
• Authors: Palomeras, I.; Ayarza, P.; Andrés, J.; Álvarez‐Valero, A. M.; Gómez‐Barreiro, J.; Díaz, J.; Alcalde, J.; Carbonell, R.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 4(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-04-27
• Subjects: Continental lithospheric mantle -- Hales discontinuity -- P‐wave velocity model; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2020JB019987
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: At least two sub‐Moho reflectors have been identified in different seismic refraction and wide‐angle reflection experiments conducted in western Iberia since the early 1990s. The wavelet kinematics and characteristics of the shallowest event are probably produced by an increase in P‐wave velocity that forward modeling places at ∼70–75 km depth beneath the Ossa‐Morena Zone (OMZ) shallowing up to 50–60 km beneath the Central Iberian Zone (CIZ). Synthetic modeling suggests that the coda and amplitude of this arrival may correspond to a ∼10‐km‐thick heterogeneous layer. We used a two‐dimensional second‐order finite‐difference acoustic full wavefield modeling scheme with an input model which includes a layer of randomly distributed bodies thinner than one‐fourth of the wavelength of the source waves and Δ Vp = ±0.1 km/s at the considered depth range. The resulting synthetic data reproduce well the observed amplitudes and codas because of the constructive interferences caused by this heterogeneous zone. The origin of this layer also discussed here in detail, is very likely related to the phase transition from spinel to garnet lherzolite, the so‐called Hales discontinuity. A second reflection also observed in some of the experiments suggests the presence of a velocity inversion at greater depths. Forward modeling places this discontinuity at around 90 km depth beneath the OMZ, deepening to 105–110 km depth beneath the southeast CIZ and shallowing up to 80 km depth in the northeast CIZ. The observed characteristics of this event are consistent with those of the lithosphere‐asthenosphere boundary. Plain Language Summary: The continental upper mantle is considered to be homogeneous, mainly composed of peridotite. However, compositional changes exist, and the seismic reflectivity observed with different seismic methods suggests the presence of certain localized layering within the lithospheric mantle. Western Iberia (Spain) has been sampled by multiple active source seismic surveys that illuminated the uppermost mantle, showing strong upper mantle reflections. The arrival time and nature of the shallowest among the upper mantle reflections indicate that it corresponds to a positive velocity contrast placed at ∼50–75 km depth (depending on the tectonic zone). To reproduce the recorded characteristics of the signal we introduced a ∼10‐km‐thick heterogeneous layer in the velocity model consisting of randomly distributed thin bodies with small velocity variations at the considered depth. The characteristics of this layer (contrast in physical properties and depth) agree with those of the transition from spinel to garnet lherzolite, the so‐called Hales discontinuity. The seismic datasets also recorded a second deeper reflection, whose origin can be attributed to a velocity decrease with depth at ∼90–110 km depth. Due to its characteristics and observed depth, we interpreted this reflector as the lithosphere‐asthenosphere boundary. Key Points: Two uppermost mantle P‐wave reflections observed beneath the SW‐Iberian Peninsula at 50–75 km (H) and 90–110 km (L) 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