Earth and Planetary Science Letters 308 (2011) 211–217

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Earth and Planetary Science Letters

j ourna l homepage: www.e lsev ie r.com/ locate /eps l

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

A. Koulali a,⁎, D. Ouazar a, A. Tahayt b, R.W. King c, P. Vernant d, R.E. Reilinger c, S. McClusky e, T. Mourabit f,J.M. Davila g, N. Amraoui h

a LASH, Ecole Mohammadia d'Ingénieurs, BP 765, Rabat, Moroccob CNRST, Institut National de Géophysique, Rabat, Moroccoc EAPS, MIT, Cambridge, MA, United Statesd Géosciences Montpellier, CNRS-University of Montpellier, Montpellier, Francee The Australian National University, Canberra, Australiaf University Abdelmalek Essaadi, FST, Tangier, Moroccog Real Observatorio de la Armada, San Fernando, Cadiz, Spainh ANCFCC, Rabat, Morocco

⁎ Corresponding author. Tel.: +212 537 77 26 47; faxE-mail address: koulali@chandler.mit.edu (A. Koulal

0012-821X/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.epsl.2011.05.048

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 March 2011Received in revised form 23 May 2011Accepted 25 May 2011Available online 12 June 2011

Editor P. Shearer

Keywords:GPSactive faultsplate boundarydeformationgeodynamicsAlboran

We use velocities from 65 continuous stations and 31 survey-mode GPS sites as well as kinematic modeling toinvestigate present day deformation along the Africa–Iberia plate boundary zone in the westernMediterranean region. The GPS velocity field shows southwestward motion of the central part of the RifMountains in northern Morocco with respect to Africa varying between 3.5 and 4.0 mm/yr, consistent withprior published results. Stations in the southwestern part of the Betic Mountains of southern Spain movewest–southwest with respect to Eurasia (∼2–3 mm/yr). The western component of Betics motion isconsistent with partial transfer of Nubia–Eurasia plate motion into the southern Betics. The southwardcomponent of Betics motion with respect to Iberia is kinematically consistent with south to southwest motionof the Rif Mountains with respect to Africa. We use block modeling, constrained by mapped surface faults andseismicity to estimate the geometry and rates of strain accumulation on plate boundary structures. Ourpreferred 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 widetranspressive boundary in the westernmost Mediterranean, with deformation mainly accommodated by theGloria–Azores fault system to the West and the Rif–Tell lineament to the East. Block boundaries encompassaspects of earlier interpretations suggesting three main deformation styles: (i) extension along the NE–SWtrending Trans-Alboran shear zone, (ii) dextral strike-slip in the Betics corresponding to a well defined E–Wseismic lineament, and (iii) right lateral strike-slip motion extending West to the Azores and right-lateralmotion 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 convergenceand sub-crustal dynamic processes associated with the long history of subduction of the Neotethys oceanlithosphere. The dextral slip identified in the Betic Mountains in Southern Spain may be related to the offshorefault that produced the Great 1755 Lisbon Earthquake, and as such may represent a significant seismic hazardfor the West Mediterranean region.

: +212 537 77 88 53.i).

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The African–Eurasian plate boundary in the most western part ofthe Mediterranean lies at the transition from the oceanic transformplate boundary to the west in the Atlantic to the continental plateboundary comprising the Iberia and Maghreb regions. The generaltectonic framework of this region has been related to Eurasia–Africaconvergence that began during the Cretaceous (Dewey et al., 1988).

This convergence is juxtaposed with extension within the Alboranbasin reflecting the complex deformation in this plate boundary zone(Jolivet and Faccenna, 2000; Platt and Vissers, 1989). Along thetransition zone between Morocco and Spain, seismicity is broadlydistributed over ∼300 km (Fig. 1), where earthquakes are of moderateto low magnitude and mostly occur at shallow depths (0–40 km)(Buforn et al., 1995; Stich et al., 2003). Recent geophysical studies areproviding much more detailed information on the crustal andsubcrustal properties of the westernMediterranean, including seismicprofiles (Simancas et al., 2003), seismic tomography (e.g.; (Blanco andSpakman, 1993); (Calvert et al., 2000); (Spakman andWortel, 2004)),gravity modeling (Ayarza et al., 2005) and heat flow (Rimi et al.,

-200-150-100 -50 0Earthquake depth

Al Hoceima 24 Feb. 2004

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Fig. 1. General structural setting map: seismicity and major faults in the Iberian–Maghreb region. Faults in this figure as well as in the following figures are taken from Gomez et al.(2000); Serpelloni et al. (2007). Topography and bathymetry are from SRTM (http://topex.ucsd.edu/www_html/srtm30_plus.html). The blue line corresponds to the proposedkinematic model (see text for discussion). The seismicity is from NEIC catalog (http://earthquake.usgs.gov/earthquakes/eqarchives/epic/). Focal mechanisms are from Harvard CMTcatalog (http://www.seismology.harvard.edu/CMTsearch.html). Abbreviations are: AA: Anti Atlas, HA: High Atlas, MA: Middle Atlas, GC: Golf of Cadiz, ME: Meseta, GB: GoringeBank, HF: Horseshoe Fracture zone.

212 A. Koulali et al. / Earth and Planetary Science Letters 308 (2011) 211–217

1998). Different models have been proposed to explain how thetopographic symmetry around Gibraltar could develop coevally withextension of the Alboran Sea and shortening of the Betics and Rifmountains, 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 ofan eastward-subducting slab (Faccenna et al., 2004; Gutscher et al.,2002; Lonergan and White, 1997; Royden, 1993; Spakman andWortel, 2004). Recent GPS data ((Fadil et al., 2006; Tahayt et al.,2008; Vernant et al., 2010)) emphasized the importance of dynamicprocesses below the crust for driving surface deformation, includingdelamination and southward rollback of the subducted Africanlithosphere beneath the Alboran/Rif domains (Perouse et al., 2010).

In this paper we combine the GPS data from surveys carried out inMorocco and from new continuous stations from Morocco andsouthern Spain to determine a new velocity field across the mostwestern part of the Africa–Eurasia plate boundary. We present akinematic model for the Rif–Alboran–Betic complex and westernMediterranean plate boundary zone using a block model that includesrigid block rotation and elastic strain accumulation on blockboundaries (e.g. (McCaffrey, 2002; Meade and Hager, 2005)). Thenewmodel defines a single block including the southern Betics,westernAlboran Sea, and the central Rif Mountains (BAR block) that is distinct

from both Africa and Iberia. The northern boundary of the BAR blockextends∼500 km roughly east–west across southern Spain and offshoreacross the Gulf of Cadiz with right-lateral strike slip of ∼4 mm/yr. Thisstructuremay be associatedwith the fault that produced the Great 1755Lisbon Earthquake, and as such may represent a potentially significantseismic hazard in this region.

2. GPS observations

We analyze data from 65 continuously recording GPS stationsextending from Morocco to southern Spain (Fig. 2). The continuousGPS stations are part of the MIT network in Morocco, the AndalusiaPositioning Network known as RAP (Red Andaluza de Posiciona-miento), the Murcia Region Network, the Valencia region Red ErrvaNetwork, and some continuous stations from the IGS and EUREFnetworks that were included in order to determine velocities withrespect to the stable Eurasian and Nubian plates. Campaign GPS datapresented here are an extension of previous surveys (Fadil et al., 2006;Vernant et al., 2010), observed between 1999 and 2009. The GPSobservations were processed with the GAMIT/GLOBK software suite(Herring et al., 2009; http://gpsg.mit.edu/simon/gtgk). We analyzedthe GPS data using a three-step approach (McClusky et al., 2003). Inthe first step we used GPS phase observations for the westernMediterranean region to estimate daily positions using loose a priori

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Fig. 2. GPS site velocities with respect to Africa and 95% confidence ellipses.

213A. Koulali et al. / Earth and Planetary Science Letters 308 (2011) 211–217

constraints. In the second step we combined these positions and theircovariances with similar solutions for over 200 global GPS stationscomputed as part of MIT's processing for the International GNSSService (IGS) (ftp://everest.mit.edu/pub/MIT_GLL). We then exam-ined the position time series for outliers and steps due to earthquakesor antenna changes and corrected these as appropriate. In the thirdstep we combined all of the data in a single solution, estimatingpositions, velocities, and steps for 1999–2009. In both the second andthird steps we mapped the loosely constrained solution into a well-constrained reference frame by minimizing the position and velocitydifferences of selected sites with respect to a priori values defined bythe IGS05 realization of the ITRF2005 reference frame (Altamimi et al.,2007) or an assumption of zero horizontal motion for 13 and 20 siteswith respect to Eurasia or Africa (Nubia), respectively. In the velocitysolution we accounted for temporally correlated errors by including arandom-walk component estimated for each continuous site using the“realistic sigma” algorithm of Herring (2004) (see also Reilinger et al.,2006), themedian value of the stations used for survey-mode stations.TheWRMS fit of our solution with respect to the IGS05 is 0.43 mm/yr,and 0.55 mm/yr for the north and east components respectively.Uncertainties for rates quoted in the text are one-sigma and shown onthe figures are 95% confidence intervals unless stated otherwise.Coordinates, velocities, and uncertainties for all sites presented in thisstudy are given in Supplementary Table 1. Fig. 2 presents the GPSvelocity solution in an Africa (Nubia) plate reference frame. Weobserve 4 main velocity patterns: The region south of the RifMountains, including the Atlas Mountain system and the Meseta, is

deforming slowly, with less than 1.5±0.6 mm/yr differential motionwith respect to Africa. GPS sites in western Morocco, indicate small,but systematic motion toward the ESE with respect to Africa.However, stations in the central part of the Rif Mountains indicate asignificant southwestward motion (3.5 to 4.0±0.3 mm/yr). Thisanomalous motion was reported previously by Fadil et al. (2006).North of the Alboran Sea, stations in the southwestern Betics showwest–southwest motion (∼3 mm/yr). In the vicinity of the Gibraltararc, ALGC and CEUT indicate slow motion towards the SE relative toAfrica with less than 1±0.4 mm/yr. Velocities relative to Eurasia(Fig. 3) exhibit a systematic WNW–ESE motion of most stationslocated in the southwest Betic Mountains, which is distinct from themotions of sites in central, eastern and western Iberia where theexpected NW–SE Eurasia motion direction dominates. This behaviorsuggests that deformation mechanisms affecting this region could beassociated (or coupled) with those seen in the Rif, forming a largedistributed deformation zone. To first order, the velocity field in theSW Betic–Alboran–Rif system, in both the Eurasia and Africa referenceframes, indicates clockwise rotation of the Alboran Sea and surround-ing parts of the Betic and Rif domains extending west of the NE–SWseismic lineament joining southern Spain with the Rif–Tell compres-sive segment of the Africa–Iberia plate boundary.

3. Block modeling

In order to relate GPS velocities in the interseismic period to long-term, geologic deformation, we use kinematic block models to

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Fig. 3. GPS site velocities with respect to Eurasia and 95% confidence ellipses.

214 A. Koulali et al. / Earth and Planetary Science Letters 308 (2011) 211–217

estimate block motions and elastic strain accumulation on blockbounding faults. We assume that the active lithospheric deformationzone is composed of a series of finite elastic blocks bounded by faults.We use the program DEFNODE (McCaffrey, 2005), in which faults arerepresented in 3-D by nodes and surface deformation due to lockedfaults during the interseismic period is modeled as due to dislocationsin an elastic half-space (Okada, 1985). In our model, the locations andlocking depths for block boundaries are prescribed. Accordingly, wesolve for angular velocities of elastic blocks on a sphere and slip rateson block bounding faults to give the best possible fit to the GPSvelocities. Minimization is done using simulated annealing with adownhill simplex method McCaffrey (2005).

Using newGPS data from continuous stationswell distributed in thesouthern Betics as well as in eastern and western Iberia, we suggest anew model geometry based on seismicity, mapped faults and the GPSvelocity field (Figs. 1 and 4; Supplementary Table 1), revising prior GPS-based blockmodels (Fadil et al., 2006; Vernant et al., 2010). Three plateswere used to describe the regional motion: Eurasia, Africa and anintermediate block between them (Betics–Alboran–Rif; BAR Block). Thenorthern BAR Block boundary corresponds to a seismic lineament(National Earthquake Information Center [NEIC] Catalogue) extendingfrom the Gulf of Cadiz to the eastern Betics, a distance of ∼600 km. Theeastern boundary of the Alboran Block corresponds to the NE–SWalignment of seismicity extending approximately 300 km, connectingthe northern BAR Block boundary with the E–W to ENE–WSW AlgeriaTell–Atlas fault system, forming a Z-shaped seismic belt. The southernboundary is coincident with active thrust and strike-slip structures

along the southern Rif mountain boundary (Chalouan and Michard,2004). Recent swath bathymetric studies report a 600 km longdeformation zone named the South West Iberian Margin (SWIM)Fault that traverses theGulf of Cadiz intersecting theHorseshoe FractureZone near the probable epicenter of the Great 1755 Lisbon Earthquake(Rosas et al., 2009; Zitellini et al., 2009).We suggest that the SWIMFaultconnects the southern Rif ridges with the main Azores–Gloria Fault. Allmodel boundaries are vertical dislocations, except the southern Rifboundary, which has a dip of 30° to the north. We extend fault nodes to15 km depth, as in Vernant et al. (2010), based on the shallow depth ofseismicity in the region.

The model provides a good fit to the data (Figs. 4 and 5). In ourinversion, 121 GPS vectors were fit with a residual normalized rootmean square (NRMS) of 2.523 and weighted normalized root meansquare (WRMS) of 0.68 mm/yr. Systematic residuals from our model,although small, may represent model deficiencies and/or intrablockdeformation. As indicated in Fig. 5, the eastern boundary of theAlboran block in our model, which corresponds to the limit of theAlboran shear zone, shows a left lateral strike-slip (∼3.8 mm/yr) andextension (2.0 mm/yr). This is consistent with the previous studies ofSerpelloni et al. (2007) and Stich et al. (2006) where moment tensorinversion solutions show predominantly left-lateral trans-tension.This boundary follows the seismicity lineament across the Al Hoceima2004 earthquake trend and the active seismogenic zone south ofSpain, roughly following the concentration of earthquake epicentersnear Almeria (ALMR). We chose this location for the NE BAR Blockboundary rather than the mapped Carboneras Fault trace because of

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Modeled velocityObserved velocity

Fig. 4. Observed (red) and modeled (black) GPS velocities (see text for discussion). Model boundaries are blue dashed lines. Faults are vertical and assigned locking depths of 15 kmexcept for the faults south of the Rif that have a 30° dip down to the North.

215A. Koulali et al. / Earth and Planetary Science Letters 308 (2011) 211–217

the very low historical seismicity associated with this fault (Stich etal., 2006), and very small slip rates during the last 100 ka (∼0.05–0.1 mm/yr) estimated along the Carboneras Fault (Bell et al., 1997).Although the fault responsible for the 2004 Al Hoceima earthquake isambiguous [conjugate left-lateral with NE strike, and right-lateralwith WNW strike, (Tahayt et al., 2009), or purely right-lateral withWNW strike, (Cakir et al., 2006)], in either case, the inferred stressdrop is consistent with the eastern block boundary geometry andsense of motion.

The northern BAR Block boundary displays right-lateral strike-slip(3.8–4.0 mm/yr) and compression (0.1–3.6 mm/yr). We extend thisboundary offshore across the Gulf of Cadiz to the Horseshoe FractureZone and the proposed location of the Mw ~8.5–8.7, Great 1755Lisbon Earthquake (Martinez Solares and Arroyo, 2004). This locationfor the block boundary finds support from seismic studies indicatingthat both onshore and offshore faulting in SW Iberia have similartectonic deformation styles (Stich et al., 2010; Terrinha et al., 2009).The southern Alboran block boundary follows the E–W orientedthrust faults of the Pre-Rif-ridges (Fig. 1), where our model predictsleft-lateral strike-slip of 1.9 mm/yr with a compressional componentvarying between 2.1 and 2.8 mm/yr. This result is consistent withshortening reported form tectonic studies (Chalouan and Michard,2004; Moratti et al., 2003). Our block model extends the mappedthrust faults bounding the southern Rif offshore to the south of theGulf of Cadiz, where our model indicates slow left-lateral strike slip

(1.1 mm/yr)with an insignificant fault-normal component, consistentwith the low level of seismicity on this fault in comparison to the NBAR block boundary (Fig. 1). In contrast, (Zitellini et al., 2009), basedon bathymetric and seismic reflection investigations, reported dextralmotion along a mapped structure near this block boundary (SWIMFault). While inconsistent with our model results, the sense of slipmay be difficult to constrain given the slow slip rate. Ourmodel differsfrom previously proposed geodetic interpretations (Fadil et al., 2006;Vernant et al., 2010) in two respects. First, we removed the roughlyN–S western Rif block boundary in western Morocco and the AlboranSea for which there is no evidence of surface faulting or seismicity andinstead connect the southern Rif block boundary to the Gloria Faultoffshore to the west in the Gulf of Cadiz. Second, we use only oneblock in the Alboran domain instead of the 2-blockmodel proposed byVernant et al. (2010).

4. Discussion and conclusion

The GPS velocity field presented here gives a detailed view ofpresent-day motion around the Rif–Alboran–Betic system. We reportthe same unexpected southwestward motion of the Rif region withrespect to Africa shown by prior studies (Fadil et al., 2006; Vernant etal., 2010), with a slower motion, relative to Africa, in the Atlas andMeseta domain with less than 1±0.6 mm/yr convergence across theHigh Atlas Mountains. The southwest Betics of south Spain are

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Fig. 5. Residual velocities (observed minus predicted). Strike-slip and fault-normal slip on block bounding segments; rates in mm/yr (fault normal component in brackets; negativefor left lateral and compression).

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similarly characterized by westward motion with respect to Eurasia,and southward motion with respect to Africa. Furthermore, GPSvelocities for stations in the southwest Betics and in Central Rif show aclockwise rotation. On this basis, we suggest that both sides of theAlboran basin are responding to the same driving mechanisms. Basedon GPS velocities and block modeling we can distinguish three mainactive deformation zones in the western Mediterranean; (i) the NE–SW trending Trans-Alboran shear zone (Stich et al., 2006), character-ized by left-lateral strike-slip and extension, consistent with historicalseismicity and mapped structures (Morel and Meghraoui, 1996; Stichet al., 2003; Tahayt et al., 2008); (ii) dextral strike-slip on an ∼E–Wstriking fault system extending ∼500 km along the Betics in S Spain;(iii) right lateral strike slip along the Gloria–Azores Transform Faultwest of the BAR Block and east along the Algerian Tell. The velocityfield with respect to Africa shows clockwise rotation along thesouthwestern Betic and the Rif regions.

Our results suggest a kinematic model consisting of a widetranspressive zone between the Tell–Rif seismicity lineament to theeast where moderate to large earthquakes occur (e.g., the Mw=7.3,Al Asnam earthquake, (Meghraoui et al., 1988)) and the oceanictransform plate boundary extending from the Terceira Ridge to theGloria Fault. The northern and southern boundaries of the BAR Blockintersect the Gloria Fault near thewestern side of the Gulf of Cadiz andthe proposed epicenter of the 1755 Lisbon Earthquake (Mw ≃8.5–8.7;(Gutscher et al., 2006; Martinez Solares and Arroyo, 2004)). Weestimate right-lateral, strike-slip motion at ∼3.8–4.0 mm/yr (Fig. 5)

with a compressional component along the offshore segment of thenorthern BAR Block boundary, consistent with seismic studies in theregion (Stich et al., 2010). Although strain, and hence the geometry offaulting in the eastern-Atlantic, offshore of Spain and Morocco, isunconstrained by GPS and may well be more complex than the simplemodel we present (Fig. 1), following independent seismic studies(Geissler et al., 2010; Stich et al., 2007), we speculate that the northernBAR block boundary might be related to the source of the 1755Earthquake further raising concerns for seismic hazards in this region.

Supplementarymaterials related to this article can be found onlineat doi: 10.1016/j.epsl.2011.05.048.

Acknowledgments

Weare thankful to all colleagueswho contributed to theGPS surveysinMorocco since 1999 and to UNAVCO for technical support.Wewouldlike to acknowledge ANCFCC, RAP-Andalusia, Murcia Region Network,and the Valencia region Red Errva Network for archiving and providingus with continuous GPS data. We are grateful to A. Fadil for fruitfuldiscussions and to Rob McCaffrey for providing his code DEFNODE. Webenefited from careful and constructive reviews from Wim Spakmanand an anonymous reviewer. This researchwas supported in part by theNational Science Foundation (EAR-9814964, EAR-0208200, and EAR-0408728), a FulbrightVisiting Student Fellowship (A.Koulali Idrissi) andSpanishMCINN projects: CGL2006-10311-C03-02, CSD2006-00041 andCGL2010-19803-C03-02.

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