The Iberian Peninsula is located on the Eurasian Plate, just north of the present-day Africa/Eurasia plate boundary (Fig. 1G). This boundary is located along the Azores-Gibraltar Fracture Zone (AG-FZ), just south of Iberia, and continues eastwards into the Mediterranean (Srivastava et al., 1990a,b).
Analysis of seafloor-spreading magnetic anomalies, aided by paleomagnetic studies (Van der Voo and Zijderveld, 1971; Galdeano et al., 1989), has shown that the plate tectonic setting of the Iberian Peninsula (Fig. 1) changed significantly throughout its postrifting history (Olivet et al., 1984; Klitgord and Schouten, 1986; Malod and Mauffret, 1990; Srivastava et al., 1990a,b; Roest and Srivastava, 1991 ; Sibuet and Collette, 1991). It acted either as an independent plate or as an accreted terrane of the Eurasian or the African Plate (Klitgord and Schouten, 1986; Srivastava et al., 1990a,b; Roest and Srivastava, 1991). For the major part of the Cretaceous Magnetic Quiet Period, when the Bay of Biscay spreading ridge was active (Sibuet and Collette, 1991), Iberia appears to have moved as an independent plate (Fig. 1B). From the Late Cretaceous (sometime before 84 Ma, Chron 34) to the middle/late Eocene (Chron 18), it was probably attached to the African plate, with the Eurasian/African plate boundary extending westward from the Bay of Biscay (Srivastava et al., 1990a,b; Fig. 1C and Fig. 1D). Sometime between Chrons 21 and 13 (possibly at Chron 19, middle Eocene) the plate boundary jumped to the south, extending from King's Trough to the Pyrenees, along the Azores-Biscay Rise and the North Spanish Trough (Fig. 1E). Since the early Oligocene (Chron 13), Iberia has been part of the Eurasian plate, with the Eurasian/African plate boundary located in its present position along the AGFZ (Fig. 1F and Fig. 1G; Klitgord and Shouten, 1986; Srivastava et al., 1990a,b; Roest and Srivastava, 1991).
The western segment of the plate boundary, between the Azores Triple Junction and the Madeira-Tore Rise, is clearly marked by seismic activity along the AGFZ (Fig. 2A; Udias et al., 1976; Udias, 1982; Grimison and Chen, 1986, 1988; Madeira and Ribeiro, 1990). Focal mechanism solutions indicate a mainly northeast-southwest to north-northeast–south-southwest extension near the Azores, changing to dextral strike-slip motion along the AGFZ (Fig. 2B; Udias et al., 1976; Udias, 1982; Grimison and Chen, 1986; 1988). Further to the east, between the Madeira-Tore Rise, Gibraltar, and the western Mediterranean, the seismicity becomes more diffuse and the definition of the plate boundary is more problematical (Fig. 2A; Grimison and Chen, 1988; Bergeron and Bonnin, 1991). East of 12ºW, and in particular near Gorringe Bank, the focal-mechanism solutions indicate a combination of strike-slip and northwest-southeast thrust faulting related to the Africa/Eurasia convergence that began in the Late Cretaceous (late Campanian?) (Fig. 2B; McKenzie, 1972; Fukao, 1973; Grimison and Chen, 1988).
Today, the main seismic activity in western Iberia is located at or close to the Eurasia/Africa boundary, particularly in the Gorringe Bank region (Fig. 2). There is also moderate intraplate seismicity associated with the Tore Seamount and along the offshore extensions of late Variscan basement faults (Fig. 2C). Onshore, the most active faults are the Vilariça Fault, the Seia-Lousã Fault and its probable extension along the Nazaré Fault and the Lower Valley of the Tagus Fault (Fig. 2C). Other faults with associated seismicity include the Messejana Fault (also known as Odemira-Avila Fault) and the Loulé Fault (Fig. 2C; Cabral, 1983; Moreira, 1984; 1985; Oliveira, 1986).