Seismic anisotropy and velocity structure beneath the southern half of the Iberian Peninsula

Abstract

Travel times of 11,612 Pn arrivals collected from 7675 earthquakes are inverted to image the uppermost mantle velocity and anisotropy structure beneath the southern half of the Iberian Peninsula and surrounding regions. Pn phases are routinely identified and picked for epicentral distances from 200 to 1200 km. The method used in this study allows simultaneous imaging of variations of Pn velocity and anisotropy. The results show an average uppermost mantle velocity beneath the study area of 8.0 km/s. The peninsular area covered by the Iberian massif is characterized by high Pn velocity, as expected in tectonically stable regions, indicating areas of the Hercynian belt that have not recently been reactivated. The margins of the Iberian Peninsula have undergone a great number of recent tectonic events and are characterized by a pronouncedly low Pn velocity, as is common in areas greatly affected by recent tectonic and magmatic activity. Our model indicates that the Betic crustal root might be underlined by a negative anomaly beneath the southeastern Iberian Peninsula. In the Atlantic Ocean, we find a sharp variation in the uppermost mantle velocities that coincides with the structural complexity of the European and African plate boundary in the Gulf of Cadiz. Our results show a very pronounced low-velocity anomaly offshore from Cape San Vicente whereas high velocities are distributed along the coast in the Gulf of Cadiz. In the Alboran Sea and northern Morocco, the direction of the fastest Pn velocity found is almost parallel to the Africa-Eurasia plate convergence vector (northwest-southeast) whereas to the north, this direction is almost parallel to the main trend of the Betic Cordillera, i.e. east-west in its central part and north-south in the curvature of the Arc of Gibraltar. This suggests that a significant portion of the uppermost mantle has been involved in the orogenic deformation that produced the arcuate structure of the Betic Cordillera. However, we assume that the Neogene extension had no major influence on a lithospheric scale in the Alboran Sea. Our results also show a quite complex pattern of anisotropy in the southwest Iberian lithospheric mantle since the relationship between the direction of fastest Pn velocity and major Hercynian tectonic trends cannot be directly established.

A seismological evidence for the northwestward movement of Africa with respect to Iberia from shear-wave splitting

“…The wide range of the fast velocity directions observed at this station as well as station PVAQ to the south may also be favored by the Variscan shearing processes affecting the region. In the Alboran Sea and northern Morocco, the direction of the fastest Pn velocity detected by Serrano et al (2005) is almost parallel to the AfricaEurasia plate convergence vector (NW-SE), which are consistent with the fast velocity directions observed at RTC in northwest Morocco and SFS near the Strait of Gibraltar. Results of Serrano et al (2005) also show a quite complex pattern of anisotropy in the southwest Iberian lithospheric mantle.…”

“…In the Alboran Sea and northern Morocco, the direction of the fastest Pn velocity detected by Serrano et al (2005) is almost parallel to the AfricaEurasia plate convergence vector (NW-SE), which are consistent with the fast velocity directions observed at RTC in northwest Morocco and SFS near the Strait of Gibraltar. Results of Serrano et al (2005) also show a quite complex pattern of anisotropy in the southwest Iberian lithospheric mantle. Contrarily, Buontempo et al (2008) detected ENE-WSW fast directions in the Betic domain, nearly parallel to the trend of the mountain belt.…”

Shear-wave velocity structure of the south-eastern part of the Iberian Peninsula from Rayleigh wave analysis

“…Furthermore, using geology, elevation, gravity data, geoid, heat flux and seismic data, The Iberian massif shows relatively higher velocity values (4.7-4.8 km/s) than the Alpine domain (4.5-4.6 km/s). These values agree well with the results found by Badal et al (1993), González et al (1998), Calvert et al (2000) and Serrano et al (2005). This high velocity is interpreted as that the Iberian massif has not recently reactivated, in opposition to the Alpine domain.…”

“…Moreover, the S-wave velocity structure obtained in this study for the south-eastern part of Iberia will complement previous geophysical studies performed in this area such as: geodynamic evolution (Galindo-Zaldívar et al 1999;Andeweg 2002;Braga et al 2003;Galindo-Zaldívar et al 2003;Sanz de Galdeano and Alfaro 2004), seismic refraction and gravity data analysis (Suriñach and Vegas 1988;Banda et al 1993), seismic reflection (García- Dueñas et al 1994;Carbonell et al 1998), studies performed with magnetotelluric and deep seismic data (Pous et al 1999(Pous et al , 2004Almeida et al 2001;Carbonell et al 2003;Simancas et al 2003), seismic tomography (Blanco and Spakman 1993;Dañobeitia et al 1998;Serrano et al 1998;Gurría and Mezcua 2000;Serrano et al 2002) and seismic velocity structure (Calvert et al 2000;Serrano et al 2005).…”

Crustal structure beneath the Rif Cordillera, North Morocco, from the RIFSIS wide‐angle reflection seismic experiment

“…[] and Serrano et al . [] used seismic regional waveforms to map Pn velocities along the Africa‐Iberia boundary. A robust low‐Pn (<7.8 kms −1 ) velocity anomaly is imaged beneath the Betics, in contrast with the relatively normal values beneath the Alboran Sea.…”