3.4.5. Present-day stress regime and topography

Much of Iberia is dominated by a NW-SE directed compressional stress field that had come into evidence during the Middle and Late Miocene (Fig. 54a) (Galindo-Zaldivar et al., 1993; De Vicente et al., 1996; Ribeiro et al., 1996; CSN, 1998; Andeweg, 2002; De Vicente et al., 2006). In the NE part of the Peninsula, along the Pyrenean domain, the Ebro Basin and the Valencia Trough, the trajectories of maximum horizontal compressional stresses are deflected into a N-S direction (De Vicente et al., 1996; Jurado and Müller, 1997; Schindler et al., 1998; Goula et al., 1999; De Vicente et al., 2006). Zones of active extension occur in the Iberian Chain and the Valencia Trough.

In the Betic foreland, earthquake focal mechanisms suggest possible reactivation of NW-SE striking normal faults and WNW-ESE trending right-lateral and SSE-NNW trending left lateral strike-slip faults. Whilst in the easternmost parts focal mechanisms suggest activation of normal faults, strike-slip focal mechanisms become increasingly frequent going westward.

This is compatible with the NW-SE maximum horizontal compressional stress field of the southern parts of the Iberian Peninsula, the Alboran Sea and the Rif of Morocco (Fig. 54). In Algeria, the most common focal mechanisms indicate NW-SE compression, while in the Gulf of Cadiz and Gorringe, stress solutions are indicative for a strike-slip regime and NW-SE directed compression.

The regional stress field of Iberia reflects a combination of forces related to collisional coupling of Africa, Iberia and Europe, and Atlantic ridge-push (Andeweg, 2002). NW-ward movement of Africa at rates of 3.3–5 mm yr-1 is apparently compensated by crustal deformation in the seismically active Maghrebian, Betic and Pyrenean zones, as well as by deformation of cratonic Iberia. Moreover, first results of GPS surveys point toward a consistent NW-directed horizontal displacement of Iberia at rates of some 5 mm/yr-1 (Fernandes et al., 2000).

The topography of cratonic Iberia is characterized by a succession of roughly NE–SW trending highs and lows that strike normal to the present-day stress trajectories and parallel similar trending Bouguer gravity anomalies (Cloetingh et al., 2002). The magnitude of Plio-Pleistocene vertical motions and results of precision levelling suggest that processes controlling topography development are still on-going and exert a first-order control on the present surface topography of Iberia. Observed Bouguer gravity anomalies reflect long-wavelength depth variations of intra-lithospheric density interfaces, such as the crust–mantle boundary (Cloetingh and Burov, 1996), and thus mirror deformation of the entire lithosphere. This raises the question whether the observed Plio-Quaternary vertical motions are related to lithospheric folding (Andeweg and Cloetingh, 2001) or whether they are related to upper mantle thermal perturbations evidenced by mantle tomography (Fig. 55; Sibuet et al., 2004; Spakman and Wortel, 2004).

In order to assess whether the average high elevation of Iberia results from stress-induced lithospheric folding and/or potential large-scale asthenospheric thermal anomalies, giving rise to a gravity signal with a wavelength of over 500 km, TOPO-EUROPE intends to image the structure of the lithosphere and the mantle beneath entire Iberia and its surrounding areas by detailed seismic tomography and magnetotelluric profiling (Figs. 56, 57).

Results will permit to assess and model dynamic processes controlling the neotectonic deformation of Iberia, the resulting topography and its repercussions on drainage systems. Furthermore seismotectonic and paleoseismic studies will aim at identifying active fault zones and at a refinement of available seismic hazard maps.