New GPS constraints on active deformation along the Africa–Iberia plate boundary

https://doi.org/10.1016/j.epsl.2011.05.048Get rights and content

Abstract

We use velocities from 65 continuous stations and 31 survey-mode GPS sites as well as kinematic modeling to investigate present day deformation along the Africa–Iberia plate boundary zone in the western Mediterranean region. The GPS velocity field shows southwestward motion of the central part of the Rif Mountains in northern Morocco with respect to Africa varying between 3.5 and 4.0 mm/yr, consistent with prior published results. Stations in the southwestern part of the Betic Mountains of southern Spain move west–southwest with respect to Eurasia (∼ 2–3 mm/yr). The western component of Betics motion is consistent with partial transfer of Nubia–Eurasia plate motion into the southern Betics. The southward component of Betics motion with respect to Iberia is kinematically consistent with south to southwest motion of the Rif Mountains with respect to Africa. We use block modeling, constrained by mapped surface faults and seismicity to estimate the geometry and rates of strain accumulation on plate boundary structures. Our preferred plate boundary geometry includes one block between Iberia and Africa including the SW Betics, Alboran Sea, and central Rif. This geometry provides a good fit to the observed motions, suggesting a wide transpressive boundary in the westernmost Mediterranean, with deformation mainly accommodated by the Gloria–Azores fault system to the West and the Rif–Tell lineament to the East. Block boundaries encompass aspects of earlier interpretations suggesting three main deformation styles: (i) extension along the NE–SW trending Trans-Alboran shear zone, (ii) dextral strike-slip in the Betics corresponding to a well defined E–W seismic lineament, and (iii) right lateral strike-slip motion extending West to the Azores and right-lateral motion with compression extending East along the Algerian Tell. We interpret differential motion in the Rif–Alboran–Betic system to be driven both by surface processes related the Africa–Eurasia oblique convergence and sub-crustal dynamic processes associated with the long history of subduction of the Neotethys ocean lithosphere. The dextral slip identified in the Betic Mountains in Southern Spain may be related to the offshore fault that produced the Great 1755 Lisbon Earthquake, and as such may represent a significant seismic hazard for the West Mediterranean region.

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

References (47)

  • E. Buforn et al.

    Seismotectonics of the Ibero Maghrebian region

    Tectonophysics

    (1995)
  • A. Calvert et al.

    Geodynamic evolution of the lithosphere and upper mantle beneath the Alboran region of the western Mediterranean: constraints from travel time tomography

    J. Geophys. Res.

    (2000)
  • A. Chalouan et al.

    The Alpine Rif Belt (Morocco): a case of mountain building in a subduction–subduction–transform fault triple junction

    Pure Appl. Geophys.

    (2004)
  • J.F. Dewey et al.

    Kinematics of the western Mediterranean

    Geol. Soc. Lond. Spec. Publ.

    (1988)
  • C. Faccenna et al.

    Lateral slab deformation and the origin of the western Mediterranean arcs

    Tectonics

    (2004)
  • A. Fadil et al.

    Active tectonics of the western Mediterranean: evidence for roll-back of a delaminated subcontinental lithospheric slab beneath the Rif Mountains Morocco

    Geology

    (2006)
  • W.H. Geissler

    Focal mechanisms for sub-crustal earthquakes in the Gulf of Cadiz from a dense OBS deployment

    Geophys. Res. Lett.

    (2010)
  • F. Gomez et al.

    Role of the Atlas Mountains (northwest Africa) within the African–Eurasian plate boundary zone

    Geology

    (2000)
  • M.A. Gutscher et al.

    Evidence for active subduction beneath Gibraltar

    Geology

    (2002)
  • M. Gutscher et al.

    The Gibraltar Arc seismogenic zone (part 2): constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by tsunami modeling and seismic intensity

    Tectonophysics

    (2006)
  • T.A. Herring et al.

    Introduction to GAMIT/GLOBK, Release 10.4

    Mass. Instit. of Tech.

    (2009)
  • L. Jolivet et al.

    Mediterranean extension and the Africa–Eurasia collision

    Tectonics

    (2000)
  • L. Lonergan et al.

    Origin of the Betic–Rif mountain belt

    Tectonics

    (1997)
  • Cited by (0)

    View full text