ABSTRACT

The Malaga Basin is a subbasin of the Alboran Sea Basin, which in turnrepresents the backarc of the Gibraltar Arc System. It is a half-graben whichshows a very complete Miocene to Holocene sedimentary sequence. In orderto determine the 3D tectonic framework of this half-graben, we interpreteda grid of multichannel seismic commercial profiles, to constrain its geody-namic evolution. A map of the lower to middle Miocene structures that oc-curred during the rifting events as well as structures generated or reacti-vated in compression during Tortonian times shows striking differences re-garding the structural style from the northern to the southern flank of theMalaga Basin and also along strike of this half-graben. Moreover, our re-sults point that the tectonic inversion and compressional deformation withinthe sedimentary infill ceased by end of the Messinian, which contrast withthe southern and eastern areas of the Alboran Basin.

Key-words: Malaga Basin, Alboran Sea, tectonic inversion, multichannelseismic, basin evolution.

RESUMEN

La cuenca de Málaga es una subcuenca de la Cuenca del Mar de Albo-ran, que a su vez representa el retro arco del Sistema del Arco de Gibraltar.Se trata de una semifosa cuya secuencia sedimentaria del Mioceno hasta elHoloceno se encuentra muy completa. Para determinar la arquitectura tec-tónica en 3D de esta semifosa se han interpretado una red de perfiles sís-micos comerciales de sísmica multicanal, lo que permite determinar su evo-lución geodinámica. El mapa resultante, que incluye estructuras formadasen el Mioceno Inferior y Medio correspondientes a la etapa de rift, y estruc-turas generadas o reactivadas en compresión durante el Mioceno Superiormuestra notables diferencias en el estilo estructural entre los flancos sep-tentrional y meridional de la Cuenca de Málaga. Además, nuestros resulta-dos muestran que la inversión tectónica que afecta al relleno sedimentarioen la cuenca de Málaga cesó a finales del Messiniense, hecho que contrastacon lo que se observa en zonas más meridionales y orientales de la Cuencadel Mar de Alboran.

Palabras clave: Cuenca de Málaga, Mar de Alboran, inversión tectónica,sísmica multicanal, evolución de cuenca.

Geogaceta, 54 (2013), 87-90. Fecha de recepción: 31 de enero de 2013ISSN (versión impresa): 0213-683X Fecha de revisión: 25 de abril de 2013ISSN (Internet): 2173-6545 Fecha de aceptación: 24 de mayo de 2013

Copyright© 2013 Sociedad Geológica de España / www.geogaceta.com 87

GEOGACETA, 54, 2013

Introduction

The geodynamic evolution of the Gibral-tar Arc System, which closed to theWest theMediterranean orogenic system, is verycomplex and remains a matter of debate. Inthis scenario, the Alboran Basin, situated ina back-arc position, is a key element to un-derstand the evolution of the wholeGibratar Arc System, which also includes theBetic-Rifean mountain belt and the Gulf ofCadiz accretionary wedge (inset of Fig. 1).

The timing of the main events in theNeogene tectonic evolution of the AlboranSea was established by Comas et al. (1999)and is generally used as a model for thewhole basin. Nevertheless, the Alboran

Basin is made of several sub-basins (op.cit.).A remarkable diachronism in terms of de-formational events has been observed on-shore (e.g. Crespo-Blanc et al., 2007). Ac-cordingly, similar diachronism may be ex-pected in the back-arc position, and it isfeasible that each of the sub-basins showsparticular characteristics in terms of subsi-dence, kinematics and tectonic evolution.For this reason more regional studies needto be done offshore. In this paper we pres-ent the architecture of the Malaga Basin,which represents the north-western branchof the West Alboran Basin (Comas et al.,1999). It is a NE-SW oriented half-grabenthat extends from East of Gibraltar Strait toapproximately the 4ºW meridian (100 km

long, Fig. 1). In this contribution we refinethe age of the structures observed in thisbasin, in particular those associated withthe Miocene rifting and the later compres-sional deformation.

Suades et al. (2012) analysed the topof the basement of the Malaga Basin,meanwhile in this paper we focus on its in-fill. For this study we use a dense grid(around 2km side) of commercial multi-channel seismic profiles oriented NNW-SSEand ENE-WSW. This grid covers the areamapped in figure 1 and is also shown inSuades et al. (2012; Fig. 1). The seismic pro-files were analysed using Kingdom Suitesoftware in order to generate the 3D basinarchitecture.

Tectonic evolution of the Malaga Basin (Alboran Sea):insights from its sedimentary infill

Evolución tectónica de la Cuenca de Málaga (Mar de Alboran): visión a partir del relleno sedimentario

Enric Suades1, Menchu Comas2 and Ana Crespo Blanc1,2

1 Departamento de Geodinámica, Facultad de Ciencias, Universidad de Granada, Fuentenueva s/n, 18071-Granada, España. esuades@ugr.es; acrespo@ugr.es

2 Instituto Andaluz de Ciencias de la Tierra, UGR – CSIC, Av. de las Palmeras, 4, 18100-Armilla, Granada, España. mcomas@ugr.es

Fig. 1.- Structural sketch of the Malaga Basin showing the main faults affecting lower to middle Miocene deposits (units IV and V). Fault traces correspondwith the top of the fault scarps. Top basement contour lines every 500 msec. (TWTT). Inset: location of the Malaga Basin and main domains of the Gibral-tar Arc System. GC: Gulf of Cadiz accretionary wedge.

Fig. 1.- Esbozo estructural de la Cuenca de Málaga mostrando las fallas principales que afectan los depósitos del Mioceno inferior y medio (Unidades IVy V). El trazado de las fallas se corresponde con el inicio del escarpe de falla. Líneas de contorno de la superficie de Basamento cada 500 mseg. El recua-dro muestra la localización de la Cuenca de Málaga así como los dominios principales del sistema del Arco de Gibraltar. GC: prisma de acreción del Golfode Cádiz.

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E. Suades, M. Comas and A. Crespo BlancGEOGACETA, 54, 2013

Geological setting

The extensional evolution of the Albo-ran Basin took place from very LateOligocene onwards and lasts until LowerTortonian (27-9 Ma; Comas et al., 1999).The rifting affected the internal zones ofthe Gibraltar Arc System known as the Alb-oran Domain, a post-metamorphic nappestack whose main complexes are nowbounded by brittle extensional detach-ments (García-Dueñas et al., 1992). TheAlboran Domain is now partially emergeddue to a contractive reorganization fromUpper Miocene (9 Ma) onwards (inset ofFig. 1). Commercial wells (Alb-A1 andAnd-G1, Fig. 1) and results from ODP Site976 show that these metamorphic com-plexes form the basement of the sedimen-tary sequences in the western Alboran Seabasins (Comas et al., 1999, and referencestherein).

Comas et al. (1999) established thatthe sedimentary cover of the Alboran Sea,

and in particular the Malaga sub-basin(Martínez del Olmo and Comas 2008), isbuild up by six major seismostratigraphicunits, bounded by major unconformities.The onset of deposition begins with unit VI,Burdigalian, eventually upper Aquitanian,in age. It is an overpressured olistostromicand clay unit responsible of the shale di-apirism observed in the deepest parts ofthe basin (Fig. 1). Unit VI, together withunits V (Langhian to lower Serravallian) andIV (upper Serravallian to lower Tortonian)represent the syn-rift sequence of thebasin. The R3 unconformity separates themain-rift sequences from units III and II(late Tortonian and Messinian). On top ofthem, a common major erosional uncon-formity is observed (M reflector), which ismainly associated with post-Messinian ero-sion following the Mediterranean salinitycrisis. Finally, the Pliocene to Quaternaryunit I capes the Miocene sedimentary cover.In this paper, the same seismic sub-divisionhas been adopted (Fig. 2).

Basin architecture and tectonicstructures

The overall geometry of the MalagaBasin is well depicted by the basement con-tour map (Fig. 1 in Suades et al., 2012). Itis a narrow and deep half-graben, NE-SWoriented that swings to a NNE-SSW direc-tion towards theWest, following the curva-ture of the Gibraltar Arc. It is limited to theNorth by the actual shelf and to the Southby the basement high called High 976. Fromthe top-of-basement surface, Suades et al.(2012) shown a strong asymmetry betweenthe graben flanks and differences in termsof shape, topographic gradient, and scarpalignment from the eastern to the westernpart of the basin. Similar differences are alsoobserved in the sedimentary architecture ofthe Malaga Basin that led us to differentiatebetween the eastern and the western sec-tors. The boundary between both areas issituated approximately 10km East of theAnd-G1 well (Fig. 1). Two seismic lines rep-

resentative of the relationships between thesedimentary sequence and the structures ofthe basin are presented in figure 2.

The dip-line crossing the eastern sectorof the basin shows both graben flanks (Fig.2A). The southern flank is characterized bya set of faults whose footwall correspondsto the basement High 976 and by the lackof significant faults affecting units V and IV.Sedimentary wedges drawn by unit IV (be-tween shot points 2200 and 2100) revealthe sin-sedimentary activity of this base-ment normal-faulting. In the northern flank,the lack of important normal faults affecting

the sedimentary infill stands out. The mainfaults are located on top of the basementand may correspond to extensional du-plexes. Along this segment of the graben, itis also noticeable the important angular un-conformity observed on top of unit VI(around shot point 2400). The maximumthickness of units V and IV roughly coincideswith the basin axis drawn by the top-of-basement surface. By contrast, unit VIshows a thickness increase towards thenorthern flank probably caused by localcompression from the shale tectonics thataffected this over-pressured unit (Fig. 2A).

Deformation of beds in units III and IVaround shot point 2600 denotes that thiscompression remains until the late Miocene,at least (Fig. 2A). Tectonic inversion of nor-mal faults bounding the footwall of theHigh 976 is also shown in this profile(around shot point 2000).

The dip-line crossing the western partof the graben (Fig. 2B) shows two normalfaults affecting units VI to IV (around shotpoint C1073), which have been reactivatedin compression. It is noticeable that theamount of inversion on those faults is smallas the cut-off observed in the R3 reflector

GEOGACETA, 54, 2013Tectonic evolution of the Malaga Basin (Alboran Sea): Insights from its sedimentary infill

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Fig. 2.- Interpreted seismic profiles of the Malaga Basin (location in figure 1). Units are according to Comas et al. (1999). Reflectors are drawn in purplefor unit VI and in yellow for the rest of the sedimentary sequence.A) Line EAS-098 showing the seismic architecture of the north-eastern part of the basin.B) Line ALB-015 showing the flank North at south-western sector. Faults represented in the map of figure 1 are coloured in light-blue.

Fig. 2.- Perfiles sísmicos interpretados de la Cuenca de Málaga (localización en la figura 1). Unidades según Comas et al. (1999). Los reflectores están mar-cados en rosa para la unidad VI y en amarillo para el resto de la secuencia sedimentaria. A) Línea EAS-098 mostrando la arquitectura símica de la parteNord-Este de la cuenca. B) Línea ALB-015 del flanco norte en el sector Sud-Oeste. Las fallas en azul claro son las que están representadas en el mapa dela figura 1.

is less than 200msec. Such inversion had tooccur during Upper Miocene times andstopped before the Messinian as unit II isnot deformed and its internal reflectorsdraw a progressive onlap. The geometry ofthe reflectors inside unit VI reveals reversefaulting in this unit that may be in part trig-gered by shale tectonics. These faults arerooted on top of the basement, which cor-responds to an extensional detachmentoriginated during the rift stage. The M re-flector shows that post-Messinian erosionwas very important in the western part ofthe profile.

The map of figure 1 shows that in thenorthern flank faults affecting units V andIV are more abundant towards theWest. Inaddition, there is more variability in faultdirections. Note that normal faults labeledas affecting lower to middle Miocene de-posits in this map were traced accordingtheir projection upon the observable faultscarp at depth. Near Marbella High thesenormal faults are mainly NE-SW andWNW-ESE directed coinciding with a basement-top escarpment. A few faults trending N-Sand E-W are also found. Most of thesefaults have listric geometry and root on topof unit VI or along the basement-top sur-face (Fig. 2). Sometimes the basement-topalso shows evidences of active faulting dur-ing deposition of units V and IV. This is thecase of the fault at the top of MarbellaHigh following roughly a NE-SW orienta-tion (Fig. 1). It must be taken into accountthat in the northern flank the post-Messin-ian erosion has removed an importantamount of Neogene deposits that preventsthe preservation of structures that mayhave affected the middle to upper Miocenesequences.

Miocene tectonic evolution:discussion

The architecture of the Malaga Basinshows that the Syn-rift sedimentationstarted at Early Miocene with the deposi-tion of unit VI (Burdigalian, eventuallyupper Aquitanian; Martínez del Olmo andComas, 2008). The main unconformity be-tween this unit and the remaining Miocenesediments and the fact that this unit be-

comes thicker towards the NW (northernflank) suggest that the counter fault whichbounds the southern flank of graben to theSE may have had a minor role during thedeposition of unit VI. From shot point 2150to 2560 (Fig. 2A), the sedimentary wedgeincluded between R5 and R3 reflectors isthe response of a significant normal faultmovement along the counter fault. How-ever, the present-day organization of thestrata from shot point 2150 to the SE de-notes that it was modified by later uplift-ing of the High 976.

The main rifting episode that controlledthe sedimentary infill of the Malaga Basinoccurred from Langhian to Lower Tortonian(units V and IV). During this time interval, inthe eastern part of the basin, most of theextension was accommodated by both thelow-angle detachment on top of the base-ment of the northern flank and its counterfault that bounds the High 976 (Fig. 2A). Inthe western part of Malaga Basin, thenorthwestern flank shows extensionalstructures (now partially inverted) that af-fected both the basement and the wholeMiocene infill (Fig. 2B).

The contractive reorganization in theMalaga Basin, occurred after the formationof R3 unconformity (intra-Tortonian in age),and did not cause significant deformationof the sedimentary infill. Indeed, there is nodeformation of the Plio-Quaternary bedsand only minor tilting is observable in thelate Tortonian/Messinian beds, associatedwith discrete, minor faulting. In fact, upperMiocene reflectors (units II and III) are ti-tled but usually a progressive unconformitycan be observed onlapping unit VI (Fig. 2B).This can be explained by uplifting of thenorthern flank and eventually concomitantshale tectonics (Comas et al., 2012). Thiscontrast with what is observed in otherareas of the Alboran Basin where compres-sion is still ongoing and is the cause of no-ticeable folding and faulting affecting thepost-R3 Late Tortonian to Holocene se-quence (e.g. Martínez-García et al., 2011).

Conclusions

1) We confirmed that the accumulation ofunit VI occurred in the early stages of

the rift on a shallower and wider basinthan the one observed nowadays.

2) The deformation resulting from the ex-tension during Langhian to Lower Tor-tonian was different in each sector ofthe Malaga Basin. For the eastern sec-tor, extension was accommodated by amajor basement counter fault (High976 fault) while the northern flankprobably rotated as the hanging wall. Inthe western part the extension was ac-commodated also by internal faultingwithin the Miocene cover. Those faultsshow a much more complex patternthan the one observed for the easternpart.

3) The compressional deformation in theMalaga Basin started at Upper Torton-ian and seems to have ceased beforethe Plio-Quaternary.

Acknowledgements

This study was supported by projectsRNM-3713, RNM-215, CGL2008-03474E/BTE, CGL2009-11384, CTM2009-07715, CSD2006-00041 (MINECO, & FederFounds). We also thank Ahmed Chalouanand María José Jurado for their constructivereview of the manuscript.

References

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Crespo-Blanc, A., Balanyá, J.C., Expósito, I.,Luján, M. and Díaz Azpiroz, M. (2007). Re-vista de la Sociedad Geológica de España 20,135-152.

García-Dueñas, V., Balanyá, J.C. and MartínezMartínez, J.M. (1992). Geo-Marine Letters12, 88-95.

Martínez del Olmo,W. and Comas, M.C. (2008).Revista de la Sociedad Geológica de España21, 151, 167.

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