New GPS constraints on active deformation along the Africa–Iberia plate boundary
Research highlights
► We investigate present day deformation along the Africa-Iberia plate boundary. ► GPS velocities show southwestward motion of the central part of the Rif Mountains. ► We show a west–southwest motion of the southwestern part of the Betic Mountains. ► We propose a plate boundary geometry including one block between Iberia and Africa. ► We discuss the implications of the great 1755 earthquake.
Introduction
The African–Eurasian plate boundary in the most western part of the Mediterranean lies at the transition from the oceanic transform plate boundary to the west in the Atlantic to the continental plate boundary comprising the Iberia and Maghreb regions. The general tectonic framework of this region has been related to Eurasia–Africa convergence that began during the Cretaceous (Dewey et al., 1988). This convergence is juxtaposed with extension within the Alboran basin reflecting the complex deformation in this plate boundary zone (Jolivet and Faccenna, 2000, Platt and Vissers, 1989). Along the transition zone between Morocco and Spain, seismicity is broadly distributed over ∼300 km (Fig. 1), where earthquakes are of moderate to low magnitude and mostly occur at shallow depths (0–40 km) (Buforn et al., 1995, Stich et al., 2003). Recent geophysical studies are providing much more detailed information on the crustal and subcrustal properties of the western Mediterranean, including seismic profiles (Simancas et al., 2003), seismic tomography (e.g.; (Blanco and Spakman, 1993); (Calvert et al., 2000); (Spakman and Wortel, 2004)), gravity modeling (Ayarza et al., 2005) and heat flow (Rimi et al., 1998). Different models have been proposed to explain how the topographic symmetry around Gibraltar could develop coevally with extension of the Alboran Sea and shortening of the Betics and Rif mountains, including (i) convective removal of the upper mantle (Calvert et al., 2000, Platt and Vissers, 1989, Seber et al., 1996). (ii) breakoff of a subducting lithospheric slab ((Blanco and Spakman, 1993)), and (iii) backarc extension driven by the westward rollback of an eastward-subducting slab (Faccenna et al., 2004, Gutscher et al., 2002, Lonergan and White, 1997, Royden, 1993, Spakman and Wortel, 2004). Recent GPS data ((Fadil et al., 2006, Tahayt et al., 2008, Vernant et al., 2010)) emphasized the importance of dynamic processes below the crust for driving surface deformation, including delamination and southward rollback of the subducted African lithosphere beneath the Alboran/Rif domains (Perouse et al., 2010).
In this paper we combine the GPS data from surveys carried out in Morocco and from new continuous stations from Morocco and southern Spain to determine a new velocity field across the most western part of the Africa–Eurasia plate boundary. We present a kinematic model for the Rif–Alboran–Betic complex and western Mediterranean plate boundary zone using a block model that includes rigid block rotation and elastic strain accumulation on block boundaries (e.g. (McCaffrey, 2002, Meade and Hager, 2005)). The new model defines a single block including the southern Betics, western Alboran Sea, and the central Rif Mountains (BAR block) that is distinct from both Africa and Iberia. The northern boundary of the BAR block extends ∼500 km roughly east–west across southern Spain and offshore across the Gulf of Cadiz with right-lateral strike slip of ∼ 4 mm/yr. This structure may be associated with the fault that produced the Great 1755 Lisbon Earthquake, and as such may represent a potentially significant seismic hazard in this region.
Section snippets
GPS observations
We analyze data from 65 continuously recording GPS stations extending from Morocco to southern Spain (Fig. 2). The continuous GPS stations are part of the MIT network in Morocco, the Andalusia Positioning Network known as RAP (Red Andaluza de Posicionamiento), the Murcia Region Network, the Valencia region Red Errva Network, and some continuous stations from the IGS and EUREF networks that were included in order to determine velocities with respect to the stable Eurasian and Nubian plates.
Block modeling
In order to relate GPS velocities in the interseismic period to long-term, geologic deformation, we use kinematic block models to estimate block motions and elastic strain accumulation on block bounding faults. We assume that the active lithospheric deformation zone is composed of a series of finite elastic blocks bounded by faults. We use the program DEFNODE (McCaffrey, 2005), in which faults are represented in 3-D by nodes and surface deformation due to locked faults during the interseismic
Discussion and conclusion
The GPS velocity field presented here gives a detailed view of present-day motion around the Rif–Alboran–Betic system. We report the same unexpected southwestward motion of the Rif region with respect to Africa shown by prior studies (Fadil et al., 2006, Vernant et al., 2010), with a slower motion, relative to Africa, in the Atlas and Meseta domain with less than 1 ± 0.6 mm/yr convergence across the High Atlas Mountains. The southwest Betics of south Spain are similarly characterized by westward
Acknowledgments
We are thankful to all colleagues who contributed to the GPS surveys in Morocco since 1999 and to UNAVCO for technical support. We would like to acknowledge ANCFCC, RAP-Andalusia, Murcia Region Network, and the Valencia region Red Errva Network for archiving and providing us with continuous GPS data. We are grateful to A. Fadil for fruitful discussions and to Rob McCaffrey for providing his code DEFNODE. We benefited from careful and constructive reviews from Wim Spakman and an anonymous
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