New evidence of delamination in the Western Alboran Sea

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Journal of Geodynamics

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ew evidence of delamination in the Western Alboran Seaeodynamic evolution of the Alboran domain and its margins

oussef Timoulali a,∗, Hamou Djellitb, Youssef Hahoua,acer Jaboura, Redouane Merroucha

Centre National pour la Recherche Scientifique et Technique (CNRST), Rabat, MoroccoCentre de Recherche en Astronomie, Astrophysique et Géophysique (CRAAG), Alger, Algeria

r t i c l e i n f o

rticle history:eceived 28 January 2013eceived in revised form 22 October 2013ccepted 24 October 2013vailable online xxx

eywords:lboran Seaeismic tomographyrustal velocityectonics blocks

a b s t r a c t

The presence of continuous upper crustal blocks between the Iberian Betics and Moroccan Rif in the west-ern and middle Alboran Sea, detected with tomography, can add new information about the lithospherestructure and geodynamic evolution in this region. A large volume of seismic data (P and S wave arrivaltimes) has been collected for the period between 1 December 1988 and 31 December 2008 by 57 stationslocated in northern Morocco (National Institute of Geophysics, CNRST, Rabat), southern Portugal (Insti-tuto de Meteorologia, Lisbon) and Spain (Instituto Geografico National, Madrid) and used to investigatethe lithosphere in the western Alboran Sea region. We use a linearized inversion procedure compris-ing two steps: (1) finding the minimal 1-D model and simultaneous relocation of hypocenters and (2)determination of local velocity structure using linearized inversion. The model parameterization in thismethod assumes a continuous velocity field. The resolution tests indicate that the calculated images givenear true structure imaged at 5 km depth for the Tanger peninsula, the Alhoceima region and southernSpain. At 15, 30 and 45 km depth we observe a near true structure imaged in northern Morocco, andsouthern Spain. At 60 and 100 km, southern Spain and the SW region of the Alboran Sea give a near truestructure. The resulting tomographic image shows the presence of two upper crustal bodies (velocity6.5 km/s) at 5–10 km depth between the Betics, Rif, western and central Alboran Sea. Low velocities atthe base of these two bodies favor the presence of melt. This new evidence proves that the Tethysianocean upper crust was not totally collapsed or broken down during the late Oligocene–early Miocene.These two blocks of upper crust were initially one block. The geodynamic process in the eastern of theMediterranean is driven by slab rollback. The delamination process of the lithospheric mantle terminateswith the proposed slab rollback in the western part of the Mediterranean. This can be explained by theremoval of the major part of the lithosphere beneath the area, except in the SW part of the Alboran Sea
where a small part of the lithospheric mantle is still attached and is extends and dips to SE beneaththe Rif, slowly peeled back to the west. A second detached lithospheric mantle is located and extendsto eastern part of the Rif and dips to the SE. The removal of lithosphere mantle from the base of thecrust was replaced and heated by extrusion of asthenospheric material coming from depth to replace thepart of crust detached. A combination of isostatic surface/topographic uplift and erosion induced a rapid
of dee
exhumation and cooling
. Introduction

The Western Mediterranean evolution has been explained by variety of geodynamic process ranging from west- directed slab

Please cite this article in press as: Timoulali, Y., et al., New evidence of delAlboran domain and its margins. J. Geodyn. (2013), http://dx.doi.org/10.10

ollback (Royden, 1993; Lonergan and White, 1997; Gutscher et al.,002; Faccenna et al., 2004; Jolivet et al., 2006; Bezada et al., 2013),reak-off of subducting ocean plate (Blanco and Spakman, 1993;

∗ Corresponding author. Tel.: +212 663299660; fax: +212 537778678.E-mail addresses: [email protected], [email protected] (Y. Timoulali),

[email protected] (H. Djellit).

264-3707/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.jog.2013.10.005

p crustal rocks.© 2013 Elsevier Ltd. All rights reserved.

Zeck, 1996, 1999) to convective removal or delamination model(Platt and Vissers, 1989; Vissers et al., 1995; Seber et al., 1996;Mezcua and Rueda, 1997). Calvert et al. (2000) proposed that exten-sion was initiated by detachment of over-thickened continentallithosphere during late Oligocene-Early Miocene.

With the expansion of seismic networks during the last twodecades in Morocco, Spain and Portugal, a large volume of seis-mic data have been collected in this region and used to investigate

amination in the Western Alboran Sea. Geodynamic evolution of the16/j.jog.2013.10.005

the lithospheric structure in the Alboran region. In this paper, wefirst present the tectonic context, seismic data selection and meth-ods necessary for the 3-D structure velocity. Since no 1-D velocitymodel of the All Alboran region exists, we used the European PM2

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mailto:[email protected]



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odel (Spakman et al., 1993a,b) to perform relocation. We then useomography to accurately obtain the velocity map and crustal struc-ure of the Alboran region. The P wave arrival times are inverted toefine the main features of the deep structure in the Alboran Seand to image the crust and upper mantle velocity. The results areompared with studies that have been carried out in this complexegion and we discuss the implication of new results from this studyor the evolution of Alboran lithospheric structure.

. Tectonic setting

The western Alboran Sea corresponds to the western part of thelate boundary between Eurasia and Africa. This region which isounded by The Betic–Rif arcuate mountain belt (southern Spain,orthern Morocco) (Fig. 1) has been interpreted as a collisional oro-en, partly collapsed through convective removal of its lithosphericantle root, or else as resulting of the African plate subduction

eneath Iberia, with further extension due either to slab break-ff or to slab retreat. During the Jurassic-Early Cretaceous, the yetndeformed Alboran Terrane was part of a larger, Alkapeca micro-ontinent bounded by two arms of the Tethyan-African oceanicomain. During the late Eocene-Oligocene, the Alkapeca-Iberiaollision triggered back-thrust tectonics, then NW-dipping sub-uction of the African margin beneath the Alboran Terrane. Thisaghrebian-Apenninic subduction resulted in the Mediterranean

asin opening, and drifting of the deformed Alkapeca fragmentshrough slab roll back process and back-arc extension (Michardt al., 2002). A number of geological, seismic, gravimetric andagnetometric studies have contributed to a better knowledge

f the deep structure of this region. The seismic refraction pro-le made in the area (Working Group for deep seismic sounding

n Spain 1974–1975, 1977; Ansorge et al., 1978; Working Groupor deep seismic sounding in the Alboran Sea 1974–1975, 1978;anda and Ansorge, 1980; Banda et al., 1983) indicate that whilehe Betic Cordillera crust is somewhat thick (35–38 km), the crusthins progressively toward the Iberian Massif (near 30-km). Therustal thickness beneath Alboran Sea range from 10 to 20 km


nd beneath the Rif Mountain are approximately 25–35 km. Withhis crustal thickness the heat flow in the Alboran Sea increasesotably (Polyak et al., 1996). The crustal seismicity is scattered

n the West Alboran especially under The Rif and the Betic. The

Fig. 1. Simplified geologic map. The square insert s

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intermediate – depth earthquakes are concentrated along N–S lineat 4.5◦ W also in the central part of the Alboran Sea (Buforn andSanz de Galdeano Udias, 1995; Seber et al., 1996; Calvert et al.,2000). Seismic studies have determined P and S wave velocities inthe crust and the mantle, as well as their principal structural fea-tures (Marillier and Mueller, 1985; Krishna et al., 1992; Blanco andSpakman, 1993; Paulssen and Visser, 1993; Plomerova et al., 1993;Calvert et al., 2000). These studies show the presence of an anoma-lous mantle below the Betic–Rif cordillera and Alboran Sea. Thegravity study confirmed this anomalous in the Betic–Rif cordillera(Hatzfeld, 1976; Torné and Banda, 1992). The Western Mediter-ranean evolution has been explained by a variety of geodynamicprocess ranging from west-directed slab rollback (Royden, 1993;Lonergan and White, 1997; Gutscher et al., 2002; Faccenna et al.,2004; Jolivet et al., 2006; Bezada et al., 2013), break-off of subduct-ing ocean plate (Blanco and Spakman, 1993; Zeck, 1996, 1999) toconvective removal or delamination model (Platt and Vissers, 1989;Vissers et al., 1995; Seber et al., 1996; Mezcua and Rueda, 1997).Calvert et al. (2000) who proposed that extension was initiated bydetachment of over-thickened continental lithosphere during lateOligocene–Early Miocene.

3. Tomography study

3.1. Data selection and methodology

The arrival times data used in this study were obtained by theMoroccan, Portuguese and Spanish seismic networks from 1988 to2008. In order to determine hypocentral parameters and find thelocal velocity structure of the epicentral area, P and S waves arrivalsat 57 stations located in northern Morocco (National Institute ofGeophysics, CNRST, Rabat), southern Portugal (Instituto de Meteo-rologia, Lisbon) and southern Spain (Instituto Geografico National,Madrid) are used (Fig. 2). We used the travel times of 58,774 Pg andPn waves arrivals collected from 5972 earthquakes. Pg consistingof direct P wave energy from crustal earthquake, Pg wave is usu-ally found in a velocity window between 5.0 and 6.5 km/s (Calvert


et al., 2000). Pn consists of P wave energy guided in the mantle lid,generally in regions of slower Pn (7.5–7.9 km/s).

The preliminary estimate of origin times and hypocentral coor-dinates is determined by the Hypocenter 3.2 program (Lienert,

hows the location of the present study area.

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and S-wave arrival times in iterative, damped, least squares inver-sion for hypocenters and 3-D velocity structure. In this approach,the model parameterization assumes a continuous velocity field

ig. 2. Locations of permanent stations in north of Morocco (triangle) and South ofortugal (inverse triangle) and Spain (square)

994). The crustal seismicity is scattered in the West Alboran espe-ially under the Rif and the Betic associated to the collisionalalpine) zone (Fig. 3). We use in this study the linearized inver-ion procedure (Fig. 4) that includes the following two steps: (1)nding the minimal 1-D model using VELEST (Kissling et al.,994), and (2) simultaneous relocation of hypocentres andetermination of local velocity structure using linearized inver-ion (Thurber, 1983; Eberhart-Phillips, 1990). Since no 1-Delocity model of the All Alboran region exists, we used theuropean PM2 model (Spakman et al., 1993a,b) to performelocation. The minimum 1-D model estimation by VELEST:


issling et al. (1994) defines the optimal 1-D model. The 1- velocity represents the least squares solution of the travel

imes in a coupled hypocenter – velocity model. In this model,

ig. 3. The preliminary estimate of epicentral distribution, for the period988/12/01–2008/12/31), determined by hypocenter 3.2.

Fig. 4. Chart describing the processes of linearized inversion for hypocenter reloca-tion and model estimation.

the layer velocities are approximately equal to the averagevelocity in the 3-D tomographic solution within the same range.In the linearized approach, we use the SIMULPS program origi-nally developed by Thurber (1983) using approximate ray tracing(ART) and pseudo-bending algorithms. This method was furtherimproved by Eberhart-Phillips (1990) for forward modeling of P-


Fig. 5. Initial and final 1D model velocity P.

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Please cite this article in press as: Timoulali, Y., et al., New evidence of delamination in the Western Alboran Sea. Geodynamic evolution of theAlboran domain and its margins. J. Geodyn. (2013), http://dx.doi.org/10.1016/j.jog.2013.10.005



4 Y. Timoulali et al. / Journal of Geodynamics xxx (2013) xxx– xxx

Fig. 6. Final epicentral (top) and hypocentral (bottom) relocation, for the period 1988/12/01–2008/05/10), by using 3-D model.

Fig. 7. The synthetic model test results for Vp at depth 0, 5,15,30,45, 60 and 100 km, the depth of the layer is shown in the top of each map.

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y linearly interpolating between velocities values defined at theodes of a 3-D grid (Foulger and Toomey, 1989). Nodal locationsre fixed prior to an inversion. The velocity at any interveningoint is determined by linear interpolation among the surroundingight grid points. Values at the velocity nodes are systematicallyerturbed during the inversion. Except for the outermost nodes,hich are always fixed, every node location can either be kept fixed

r included in the inversion. The ART algorithm selects the pathith the least travel time from a suite of circular arcs connecting

he source and the receiver. The iterative pseudo-bending methodne-tunes the ray path obtained by ART to better approximate therue path dictated by the local velocity gradients (Um and Thurber,987).

For the relocation of local events, an initial 1-D velocity modelas constructed from a reference model (PM2) determined for

urope (Spakman et al., 1993a,b). Each event was relocated usinghe same starting epicentral location and origin time, but using

ultiple starting hypocentral depths of 0, 5, 15, 30, 45, 60 and00 km. Some criteria are used to remove poorly constrainedvents: the location with the high weighted RMS residual (>1.0 s)nd any event that has a horizontal or vertical shift more than 10 kms discarded.

The final 1-D model velocity is shown in Fig. 5 and the final loca-ion when the two processes are performed (3-D model) is shownn Fig. 6. The crustal seismicity is scattered in the west Alboraneneath the Rif and the Betic and the seismic activity is abundantnd present at both shallow and intermediate depths. In the Rif theeismic activity is relatively limited to shallow depths. The inter-ediate seismic activity is present from depths of 60–150 km and

oncentrated along a N–S line at 4.5◦ W also beneath the center oflboran Sea.

.2. Resolution analysis and synthetic tests

It is commonly recognized that the tomography inversion resultust be assessed with a resolution. By the concept of resolution,e try to know how the true structure is reconstructed in the cal-

ulated image. The simplest means to evaluate the resolution ofomography is to invert a synthetic set of travel times and comparehe results with the known velocity structure (Zhao and Negishi,998; Zhao et al., 2001). In this study, the resolution and check-oard tests are used as additional techniques to improve resolution.or the resolution tests, we use the spread function (SF) of the aver-ging vector for the assessment parameter resolution (Foulger andoomey, 1989). A small value of spread function indicates localizedveraging of velocity.

The checkboard test is the ability to rebuild the initial syntheticodel by the real seismic data. In the checkboard test, we first

roceed with a variation of the order of ±10% for the P wave inhe original 1-D model velocity to create synthetic data from this

odel (the random errors is not added). With the calculation callednverse modeling, we tried to rebuild the initial synthetic modelrom physical data (seismic data). As a result, we try to know howhe initial synthetic model is reconstructed in the calculated image.he quality of this test is verified by knowing at what level theearly true velocity structure is present in the calculated imageTimoulali and Meghraoui, 2011).

The resolution test result shows that the calculated images give aearly true velocity structure from 0- to 60-km depth in the Alboranegion. The checkboard test for our region is shown in Fig. 7 and thepread function analysis is shown in Fig. 8.


. Crust and upper mantle velocity

The resulting tomography image (Figs. 9–12) has a prominentigh-velocity anomaly that shows a maximum increase in P wave

Fig. 8. Test of resolution spread function at depth 0, 5,15,30,45, 60 and 100 km, thedepth of the layer is shown in the top of each map.

velocity of approximately 10% (blue color) and prominent lowvelocity anomaly that shows a maximum decrease in P wave veloc-ity of approximately 10% (red color). The most significant positiveanomaly (fast crustal structure) imaged in the west Alboran Sea islocated at 5–10 km depth. It shows a continuous high crustal veloc-ity (6.5 km/s) located between the Betic and the Rif in the westernand middle part of the Alboran Sea (Fig. 9). The most significantnegative anomaly (slowest crustal structure) is located from 15 to30 km depth. In these zones, the Pn velocity varies from 7.0 to 7.5showing low velocity zones parallel to the Moroccan coast and inthe Betic and NW Betic cordillera. We can see a zone with highvelocity from 15 to 30 km located in the Iberian massif and in themedian and oriental part of the Rif massif. From 45 to 60 km depth,the Pn velocity shows a high velocity in the western and middle ofAlboran Sea.

5. Synthesis of result and implication for lithosphericstructure

The most striking feature of velocity model we obtained in theupper crust is the significant positive anomaly that appears from 5to10 km (Fig. 9a). This crustal anomaly is characterized by 6.5 km/svelocity and probably not collapsed or broken down during the


late Oligocene–early Miocene extensional event (Timoulali et al.,2013). As we know, the convergence between Africa and Eurasiabegan sometime in the Cretaceous (Dewey et al., 1989; Srivastavaet al., 1990) and produced a zone of thick crust (internal zones) that

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Fig. 9. (a) Upper crust velocity (6.5 km/s) shows the upper crust is not collapsed in the Western and in the middle of Alboran Sea. The depth of the layer is shown at top leftcorner of each map. Blue and red colors denote fast and slow velocities, respectively. (b) At the surface, high velocities (5.5–6.2 km/s) represent the cooling extrusion thatreaches the surface like in Nevado-Filabride, Ronda and Beni Bousera. (For interpretation of the references to color in this figure legend, the reader is referred to the webversion of the article.)

Fig. 10. Exhumed and cooling asthenospheric materiel at 15 km depth presented by a high velocity perturbation (7 km/s).

Fig. 11. Inflow of hot asthenospheric with low velocity (7 km/s) at 20 km depth after the lower crust part was removed.

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Fig. 12. Lithospheric body with high velocity that has descended into the upper mantle from 45 to 100 km

Fig. 13. Vertical cross section along (a) South Betics cordillera 37◦ N, (b) Alboran sea 36◦ N, (c) Rif mountain 35.57◦ N showing the absolute velocity.

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as located in the area between present day Morocco and southernpain. During the late Oligocene–early Miocene the belt collapsedlong with limited injection of basaltic dikes beneath the BeticsTorres-Roldan et al., 1986).

At 0–5 km depth, the negative anomaly that appears in thepper crust with low velocities (4.5–5 km/s) represent marineeposition when the western portions of west Alboran Basin sub-ided below sea level (Comas et al., 1992). The positive anomaliesith high velocities (5.5–6 km/s) represent the cooling extrusion

hat reaches the surface like in Nevado-Filabride, Ronda and Beniousera (Fig. 9b)

From 10 to 15 km, the most striking feature is the positivenomaly with a fast Pn waves (7.0–7.5 km/s) shown in Fig. 10. Wenterpret this positive lithospheric anomaly as a cooling astheno-pheric material. In the West Alboran Sea a combination of isostaticurface/topographic uplift and erosion induce a rapid exhumationnd cooling of deep crustal rocks (Platt et al., 1998). In 20–30 kmepth, the most striking feature is the negative anomaly with alower Pn (7.0–7.5 km/s) shown in Fig. 11. We interpret this nega-ive lithospheric anomaly as a hot asthenopheric material comingrom depth to replace the part of crust detached. Theses anomaliesre associated with region of warm or absent mantle lithosphere orith the presence of melt (Hearn, 1996). The high heat flow in Alb-

ran basin (Polyak et al., 1996) can be explained by the presence ofot ductile material. In the geodynamic model this extrusion of hotsthenosphere materiel is interpreted as an uplift and extension inhe Alboran area. (Thompson et al., 1997a,b).


From 45 to 60 km the most striking features are the posi-ive anomalies with fast Pn waves (7.5–8.0 km/s) associated toithospheric body that has descended into the upper mantleFig. 12). The profile in the south-eastern Betics (Fig. 13a) shows

ofile 37◦ N, (b) profile 36◦ N, (c) profile 35.57◦ N and (d) profile 35.46◦ N from theespectively. (For interpretation of the references to color in this figure legend, the

the thickness of crust is relatively thin (∼24 km) it is also in agree-ment with the work of Banda and Ansorge (1980). The profile inthe Alboran basin (Fig. 13b) indicates that the crustal thickness isrelatively thin (15–20 km) in agreement with previous proposalsthat the continental crust beneath the Alboran is thin (e.g. WorkingGroup for deep seismic sounding in the Alboran Sea 1974–1975,1978).

The cross sections NE/SW and NW/SE (Fig. 15, section 1, 2)shows clearly two bodies, the first body oriented NW–SE (section 2),dipping in SE and still attached in the SW part of Alboran. This bodyextends in the southern part of Alboran Sea beneath the Rif andslowly peeled back to the west. The second crustal body is located inthe south beneath the Rif, oriented NE–SW (section 1) and dippingSW (Timoulali et al., 2013). These two bodies can be interpretedas what remains of the detached lithospheric mantle of LigurianOcean that separated Iberia and Africa. The profile along 35,46◦ Nin the (Fig. 14d) shows also a body oriented in NW–SE and it isdipping in SE in the eastern part of the Rif.

The profile along 5◦ W in (Fig. 16a) shows that the Alboran crusthas a N–S trend and extends from the Iberic complex in the north tothe Rif in the south. The contact between the Iberic and the Alborancrust is clearly determined in the SW Iberic region. Two crustalbodies are detected in southern part, the first is oriented NW–SEdipping in SE beneath the Rif. The second is a new crustal bodylocated in the SW of the Rif, and dipping beneath the SW of the Rif(Timoulali et al., 2013).

Different models have been proposed in an effort to explain


the geodynamic evolution of this region from retreating slabsubduction or slab rollback of subducting ocean plate (Royden,1993; Zeck, 1996; Lonergan and White, 1997) to convectiveremoval or delamination (Platt and Vissers, 1989; Vissers et al.,

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ig. 15. Upper crust body (velocity 6.5 km/s) defined between the Iberian Betics aody oriented NE-SW and Section 2, the crustal body oriented NW-SE located bene

995; Seber et al., 1996; Mezcua and Rueda, 1997); the latest modelrom Calvert et al. (2000), proposing extension was initiated byetachment of over-thickened continental lithosphere.

The new evidence presented by the two upper crustal bodiesvelocity 6.5 km/s) at 5–10 km depth located in the western andn the middle of the Alboran Sea proves that, the Tethysian ocean


pper crust was not totally collapsed or broken down during theate Oligocene–early Miocene. These two blocks of upper crust

ere initially one block. The geodynamic process in the easternf the Mediterranean is driven by slab rollback. The delamination

ig. 16. Vertical cross section of P-wave velocity perturbation (in percent) along (a) proolors denote fast and slow velocities, respectively. (For interpretation of the references to

Moroccan Rif in western and Middle od Alboran Sea. Section 1 shows the crustale Rif.

process of the lithospheric mantle terminates with the proposedslab rollback in the western part of the Mediterranean. This modelcan be explained by the removal of the major part of the litho-sphere beneath the area, except in the SW part of the Alboran Seawhere a small part of the lithospheric mantle is still attached and isextends and dips to SE beneath to the Rif (Fig. 14d) (Fig. 15, section


2). A second detached lithospheric mantle is located and extendsto eastern part of the Rif and dips to the SE (Fig. 16a). These twodetached lithospheric mantle bodies are slowly peeled back to thewest. The removal of lithosphere mantle from the base of the crust

file 5◦ W, (b) profile 4◦ W, (c) profile 3.4◦ W from Morocco to Iberia. Blue and red color in this figure legend, the reader is referred to the web version of the article.)

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as replaced and heated by extrusion of asthenospheric materialoming from depth to replace the part of crust detached. A com-ination of isostatic surface/topographic uplift and erosion induce

rapid exhumation and cooling of deep crustal rocks. The mostbvious examples are Nevado-Filabride, Ronda, Beni Bousera.

With this new evidence, we can better explain the complex pat-ern of anisotropy and the fast direction of Pn velocity (Diaz et al.,998, 2010; Calvert et al., 2000; Serrano et al., 2005; Buontempot al., 2008) which is related to the main features of the upper crustlocks from 5 to10 km depth and the lower crust dipping SE from5 to 60 km depth shown by this study.

. Conclusion

The use of the Pg and Pn waves in the tomography study gives aew evidence to explain delamination model in the west Alboranea.

The most significant crustal anomaly imaged in the west AlboranSea is presented by two upper crust blocks (velocity 6.5 km/s) andlocated at 5–10 km between the Betic and the Rif in the westernand middle part of the Alboran Sea.

The geodynamic process in the western part of Mediterranean isdriving by delaminated lithospheric mantle which terminate theslab rollback that moves from the east of the Mediterranean.

This model can be explained by the fact that most part of thelithosphere was removed beneath the area. Except in the SW partof the Alboran Sea where the detached lithospheric mantle is stillattached, and is extends and dips to SE beneath the Rif. A seconddetached lithospheric mantle is located and extends to easternpart of the Rif and dips to the SE.

The removal of lithosphere mantle from the base of the crustwas replaced and heated by extrusion of asthenospheric materialcoming from depth to replace the part of crust detached.

The uplifting in the Alboran sea area followed this delaminationprocess.

cknowledgment

This study was supported by the CNRST, PROTARS D15/41. Somef the figures were prepared using the public domain GMT softwareWessel and Smith, 1998).

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New evidence of delamination in the Western Alboran Sea

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