A number of seismic experiments sampling the Spanish Iberian Massif (western Iberia) have recorded sub-Moho events. These reflectors are indicative of a heterogeneous upper mantle. Control source seismic wide-angle shot records show strong sub-Moho arrivals at distant offsets (>180 km) with amplitudes significantly higher than the Pn, and a relatively long coda. Forward modeling indicates that these arrivals reflect off an interface in the 60-80 km depth range. In order to reproduce the observed reflected amplitudes at the appropriate offsets, a high impedance contrast is needed. To achive such a high impedance velocities much higher than the ones estimated from petrological analysis of upper mantle xenoliths are required. To reconcile the observed phases and the estimated velocities from the petrological studies, we propose that the observed amplitudes and codas can be the result of constructive interference effect most probably caused by the tuning effect produced by a gradient heterogeneous zone. We aim to test and validate this hypothesis by synthetic modeling of the seismic response using a 2D finite difference code to solve the wave equation. The velocity model obtained from the forward modeling was modified introducing stochastic lensing at 60-80 km depth. The heterogeneities were smaller than ¿ of the wavelength of the seismic waves in thickness and had a velocity contrast of +/-0.2 km/s. The resulting synthetic wide-angle shots reproduce the observed phases in both, amplitude and coda. This seismic discontinuity is consistent in depth with the Hales discontinuity, the phase transition from spinel lerzholites to garnet lerzholites. Therefore, we propose that the observed sub-Moho reflectivity beneath the western Iberia correspond to layering or lenses where the spinel to garnet ratio varies. Funding resources: EU EIT-RawMaterials Ref: 17024_20170331_92304; MINECO: CGL2016-81964-REDE CGL2014-56548-P: JCYL: SA065P17).

AGU Fall Meeting 2019 in San Francisco, 9-13 december 2019