Estudios geol., 43: 435-443 (1987)

STRIKE-SLIP FAULTS IN THE SOUTHERN BORDER OF THE VERABASIN (ALMERIA, BETIC CORDILLERAS)

C. Sanz de Galdeano (*)

ABSTRACT

The Southern border of the Vera basin is affected by a dextral fault (Cortijo Grandefault) trending N70E. This is crossed by other faults trending NNE-SSW, and particularyat the Eastern end where it is severely bent by the Palomares fault.

The movements of the Cortijo Grande fault started at the latest during the Serravallianwith compression trending from N80W to N60W wich steadily turned to N20W during theMessinian-Pliocene. Fault displacements affect Quaternary deposits in many occasions.Neogene sediments clearly show deformations contemporary to the deposit.

Key words: Strike-slip Jaults. Neogene, BelÍc Cordilleras.

RESUMEN

El borde sur de la depresión de Vera está afectado por una falla dextrorsa (falla deCortijo Grande) de dirección N70E. Esta ha sido cortada por otras fallas de direcciónNNE-SSW, en especial en su terminación oriental donde está fuertemente flexionada porla falla de Palomares.

Los movimientos de la falla de Cortijo Grande se han producido al menos desde elSerravallicnse con compresiones según la dirección N80W a N60W que progresivamenterotaron a N20W en el Messiniense - Plioceno. Los desplazamientos de las fallas llegan aafectar en muchos casos a sedimentos cuaternarios. Los del Neógeno muestran claramenteestas deformaciones coetáneas con el depósito.

Palabras clave: Fallas de desgarre. Neógeno, Cordilleras Bélicas.

Introduction

On thc SE part of the Iberian Peninsula, withinthe Betic Cordilleras, there are important strike­slip faults. Some of these are trending NE-SW toNNE-SSW and have been described on many oc­casions: Bousquet and Philip (1976), Ott d'Este­vou and Montenat (1985), Rutter et al (1986),Weijermars et al (1985) or cited in the context ofthc Betic Cordilleras: Groupe de Recherche Néo­tectonique (1977), Sanz de Galdeano (1983),Boccalctti et al (1985), Sanz de Galdeano et al(1985), etc. Howcver, fractures trending N6ÜE toE-W on the northern flanks of the Sierras Cabre­ra and Alhamilla have not been studied or haveat most been described as faults without any ot­her specification or as reverse faults with localstrike-slip components: Durand-Delga and Font-

bote (1963). Nevertheless, they are large strike­slip faults which be long to the most importantsystem of the Betic Cordilleras. (Sanz de Ga1dea­no, 1983).

Here, features of these fractures in the north ofSierra Cabrera and their relationship with thosetrending NE-SW are described.

From a geological point of view this sector is si­tuated to the east and south of the Sorbas andVera basins respectively, within the interna1 Zü­

nes of the Betic Cordilleras (fig. 1).In the basement of the Sierra Cabrera the Ne­

vado-Filabride is mainly constituted by darkschists and micaschists with garnet, quartzites andonly in very few places by carbonate rocks andtourmaline gneises. However, in the Sierra deSedar (fig. 2), carbonate rocks, gneisses and me­tabasites are much more common, although mi­caschists are also presento

(*) Instituto Andaluz de Geología Mediterránea. e.S.Le. y Opto. de Geodinámica. Fac. de Ciencias. Universidad de Granada.18071, Spain.

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The Alpujarride complex, overlies the Nevado­Filabride and is formed by triassic bluish greyphyllites (although in other arcas there are schitsunder them) and quartzites with some metahasitsand, over them, some carbonate rocks, here withvery linle metamorphism. The Malaguide appearin vcry small outcrops, whose description can besecn in Rondeel (1965) and Garcia-Monzon et al(1974), included in this articIe together with theAlpujarride complex.

The oldest neogene scdimcnts apart the Burdi­galian that form very linle outcroops, are thoseof the Serravallian-Lower Tortonian. They areformed by marine conglomera tes, sands and siltsseveral hundred meters thick. One of their cha­racteristics is that they do not have boulders fromthe Nevado-Filabride but from the Alpujarrideand the Malaguide.

C1early discordant. some marine conglomeratesare deposited, with boulders of up to several cu­bic metres, containing thin beds of sanstones andin some places silts and c1ays. Thc boulders aremostly formcd by the Nevado-Filabride rocks,and of particular interest is the great amount oftourmaline gneisses and metabasites. The thick­ness of this sediments varies very much, rangingfrom 300-400 m thick near Cerro Colorado to al­most nil to the East near the sea. Their age isUpper Tortonian-Messinian? Their characteris­tics and age c10sely resemble those of the Block­Formation of the Granada basin (Viennot, 1930).

Over this, gradually, and at times laterally, ap­pear silts, c1ays and marls. Their thickness variesgreatly, reaching here about 300 m Their age isUpper Tortonian?-Messinian.

Above this sediments and slightly discordant,there are sandstones, sandly limestones and mari­ne conglomerates of varying thickness: 80-100 min this sector and towards Mojacar much less. Ontop of these, there are marls and silts. They areabout lOO m thick, although laterally they alsodecrease considerably. Their age is Messinian.

To the SW of Mojacar, SE and SSE of Turre,there are sediments formed by porous silts withscattered gypsum, with structures which resemblesmall pipes, perhaps algae. They may correspondto the Pliocene or Lower Quaternary.

There are also deposits to the Tyrrhenian agenear to, or even on, the coast and other Quater­nary sedíments such alluvial and colluvial depo­sits.

Tectonics

An important fault, the Cortijo Grande fault,on the Northern border of Sierra Cabrera (figs. 1

C. SANZ DE GALDEANO

to 4) passes through the Colorado and Pilica hillssouth of Turre.

This fault is a set of almost parallel fracturc li­nes affecting the Alpujarride and Neogene sedi­ments within a zone lOO m to 1.500 m thick (fig.3). This zonc IS characterized by bands of intensecataclasis of a few metres to over 30 m of thick­ness, to he observed particularly within the Alpu­jarride phyllites. These bands show incipient ver­tical foliation. Furthermore, phyllites, quartzitesand, sometimes even carbonate beds suddenlyappear in vertical position with thcir edges para­llel to the direction of the bands. Also, there aremany tectonic fragments specially formed by car­bonate and quartzite materials and by Mioccnerocks included within the phyllites or vice vcrsa.These fragments range from less than I m tomore than 1 or 2 km in length (e.g. the Colora­do and Pilica hills). The edges of this tectonicfragments, generally totally c1eancut, are striated.The striae are in general subhorizontal and para­lid to foliation bands and are interpreted as indi­cators of movement directions. They are veryspectacular in some places, as in the Judio hill,however they are present throughout the fault,from the vicinity of Mojacar to the village of LaHuelga. In general, striae are parallel to the di­rection of the fault because the tectonic frag­ments themselves are elongated in this directionand only two class of deviation are found: at theends of the fragments where faces are not parallelto the direction of the fault and in very smallfragments. In the first case, striae may have verydifferent directions. However, pianes associatedto the faults, even if not vertical, usually maintainthe direction of the striae. Very small tectonicfragments can be rotated and the striae are adap­ted surrounding the surfaces of the boulder.

It has to be stressed that some large surfaces,sometimes of over 150 m, contain different sets,of subhorizontal as well as locally subverticalstriae. The striae, calcite and gypsum fibers andlunate tectoglyphs, indicate that the CortijoGrande faults is dextral. This interpretatíon isalso supported by the great number of small tec­tonic fragments with sigmoidal tails. Furthermo­re, there are fractures oblique to the bands of ca­taclastic rocks with dextral displacements, withcould be considered as Riedel fractures (R).

Miocene sediments, especially older ones, andeven those considered as Pliocene or Quaternary,are also affected by these bands. Cataclasis inMiocene sediments is less intense, but the bedsare vertical in places and severe fracturing, ratherthan foliation, is frequent.

The Cortijo Grande fault is limited to both E

STRIKE-SLlP FAULTS IN THE SOUTHERN BORDER OF THE VERA BASIN 439

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and W by faults trending NNE. To the East, thePalomares fault, complex and diversified, is parti­cularly important. Geological and photogeologi­cal studies show that it is formed by a set of faultsand not by a single surface. A description is givenby Bousquet and Philip (1976) and Weijermars etal (1985).

Of particular interest are the effects of thisfault on the Cortijo Grande fault: the latter turnsinto parallelism with the NNE direction of thePalomares fault. AIso, Alpujarride and Neogeneracks have been turned forming a vertical dragfold. The Cortijo Grande fault reappears to theeast of the Palomares fault, which is crassed by

E-W trending fault. This situation is interpretedas two active crass-cutting systems.

To the West, the NNE trending faults whichpass thraugh La Huelga and to the East of theSierra de Alcornia (East of Sierra de Bedar) havec1early sinistral components and a vertical compo­nent of over 200 m. These faults cut the CortijoGrande fault and represent its western termina­tion.

A third system of fractures is trending NW-SE,and unlike other sectors in the Betics, does notfram large fractures but are present at micra andmesotectonic levels.

The Cortijo Grande fault also presents vertical

440

or subvertical striae and other intermediate onesbut those with rakes between O and 20 are muchmore common. Therefore it can be deduced thatthe throw is not very important with regard to theheave. However, the latter cannot be calculateddirectly, although the Cortijo Grande accidentmay reach easily tens of km while that of Palo­mares, taking into account the displacement cxis­ting between the sierras Cabrera and Almenara,reaches 60 km approximately.

Microtectonics

More than a thousand of measurements offaults and joints were carried out. Many of thisfaults appear in well defined conjugated sets, 379had strie and in 178 of these the direction of mo­vement could be deduced.

Measurements cover the whole area, whichcould be subdivided into 12 sectors. The resultsare shown in figs. 4 and 5.

It has to be pointed out that fractures trendingNW-SE have displaced both dextrally and sinis­trally depending on whether (JI was closer to N-S01' to E-W. Similarly, the whole set of fractureshas not been formed by only one system of stres­ses, but at least two systems are necessary to ex­plain its movements and the conjugated setswhich are clearly observed.

In the Alpujarride rocks we have obtained theapproximate direction of N8üW-N5üW, depen­ding the sector, as predominat for (JI' A directionclose to N20W is also deduced (fig. 5, diagrams 1to 4).

In the Serravallian-Tortonian sediments the di­reccion of N80W is deduced to the North of thePilica and Colorado hillocks and in the CortijoGrande sectors. The direction N50W is deducedto the south of Judio hillock. The directionN20W, approximately, is also deduced in this se­diments with small changes from sector to sector(fig. 5, diagrams 5 to 8).

The direction N20W is the only one obtainedin the Messinian outcrops at the bridge over theriver Aguas and in the Alfaix sector (N27W andN15W respectively) (fig. 5, diagrams 9 and 10),were there are also numerous normal faults al­most parallel to the deduced direetion of (J l' Thisfaet is particularly evident in the river Aguaswhere there are a great number of normal eonju­gated faults with N27W as their average strike.So, it is possible to think that the formation ofstrike-slip and normal faults is almost eontempo­rary, with a clear tension perpendicular to the di­reetion of (J l'

e SANZ DE GALDEANO

In Mojacar, at the Western road, the directionsobtained in the Messinian sediments for (TI areN24 E, N5E and N25W (fig. 5, disgram 11). Twoof these, N24E and N5E, without any real signi­ficance 01' somehow unreliahle, since this is a sec­tor in which materials have rotated more that 50"due to the drag effect of the Palomares lines offaults. The direction N25W coincides with othermentioned previously.

In Pliocene-Ouaternary (?) deposits there arejoints which would be compatible with N20W andN70E compressions (fig. 4).

Directions of tensional fractures in conglome­rate pebbles (in Serravallian-Lower Tortonian se­diments outcropping near Cortijo Grande) havealso been mcasured (121 measures). Their poleshave one maximum in the dircction N58E andanother Icss important one in N35E (fig. 5, dia­gram 12).

Cronology of deformations

During the Serravallian and Lower Tortonianthere was not important erosiono since in their se­diments there is not any detrital material inheri­ted from the Nevado-Filabride.

Without any doubt, after the deposit of the Se­rravallian-Lowcr Tortonian sediments and beforethe upper Tortonian, a strong pulsation took pla­ce, as is shown by the faet that the Upper Torto­nian-Messinian sediments are clearly discordanteven over the Alpujarride. There are detrital ma­terials from the Nevado-Filabride, in large boul­ders in many plaees, but apparently thcy comefrom the Sierra de los Filabres (in this case. itsSoutheastern spur which is called Sierra de Bedarand Sierra Alcornia) and apparently before SierraCabrera had risen sufficiently.

On the whole, it seems that the predominantdirection of (JI ranged from N80W to N55W forthe Serravallian (and earlier) and Tortonian ages.These stresses seem to have continued during theMessinian, although a turn in the direction of (JI'

which steadily becomes N30W-N20W, can be de­duced. At the same time, NNE-SSW faults musthave moved a great deal, whilst in this sectorNW-SE faults are not very important, althoughquite numerous at meso and microstructural le­ve\. The NNE-SSW faults may have had an olderhistory, although movements older that the Mes­sinian, when they already had vertical displace­ments of at least 200, are not observed. On theSE edge of the Sierra Alcornia they formed a c1iffon the base of which beach-like Messinian mate­rials were deposi ted.

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Fig. 5.-Diagrams of tectonic measures. Points: Poles of strike-slip and reverse faults. Triangtes: Poles Ol normal faults.Crosses: Indifferentiated striae. D, S, R and N: Striae of dextral, sinistral, reverse and normal faults respectively. In manycases several measures are coincident in a determinated point. The arrows show the directions of (J", deduced, and their sizeis proportional to the importance of the directions obtained. Diagram 12: measures of poles of tensional joints in Lower

Tortonian conglomerates.

STRIKE-SLIP FAULTS 1 THE SOUTHERN BORDER OF THE VERA BASIN 44}

The strong drags (over 2 km) and the f1exurecaused by the Palomares faults on the CortijoGrande fault clearly affect Upper Messinian sedi­ments which proves that the movements are quitemoderno Furthemore, to the North in the Verabasin, in outcrops which are almost kilometricalin size, sediments, possibly from the Lower Qua­ternary and affected by wrench faults trendingNNE-SSW are observed. One of these faultsclearly affects tyrrhenian deposits in the village ofGarrucha. Also in the interior of Sierra Cabrerasorne colluvial deposits are affectcd by strike mo­vements (c1early seen due to the presence of gyp­sum fibers), but on this occasion the fault trendsN70E.

Even fault planes it the Cortijo Grande faultare c1early affccted at several points, such as theJudio hillock, where they are cut by NNE-SSWfaults. It should also be pointed out however thatnear Mojacar, almost E-W fault planes cut NNE­SSW faults with decimetrical, dextral horizontaldisplacements.

In short, forces running N80W which steadilychanged to an average N45W were predominantin this sector during the Serravallian and until theTortonian-Messinian. (TI rotation continued du­ring the Messinian-Pliocene until the approximatedirection of N20W became predominant, as canbe observed in the river Aguas. This compressiondirection extends into the Quaternary, if not untilpresent. However, E-W movements still continuesomehow, as is shown by the existence of defor­mations and sets of faults in Neogene and Qua­ternary sediments compatible with WNW-ESEcompressions.

Conclusions

1. The Northern border of Sierra Cabrera isaffected by a large dextral accident trending ap­proximately N70E: the Cortijo Grande fault. Therocks adjacent to the fault contain cataclasitebands and striations.

2. The Cortijo Grande faults is cut both to theEast (Palomares fault) and to the West, by NNE­SSW faults. The Palomares faults provoked thef1exion and dragging of the Cortijo Grande faultwith a displacement of over 2 km.

3. The (TI direction steadily rotates in timcfrom N80W-N60W, in the Serravallian or evenearlier, to N20W in the Messinian. This last pre­dominant direction seems to have continued upto the present, although are some very modernmovements which prove the existence of com­prcssions, at least of a local and momentary natu­re, with WNW-ESE directions.

Study carried out through the 3184-83 C04-02projcct of the e.A.I.e.Y.T. and 79/84. C4 pro­jcct of the e.A.Le. Y.T. and e.S.Le.

Referencias

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Bous'luet. 1.C. and Philip. H. (1 \>7ó). Ohservations microlecto­ni'lues sur la compression nord-sud '1uaternaire des Cordillc­res bétiques orientales (Espagne méridionale-Are de Gihral­tar). B/I//. Soco Géo/. France (7). 28. 711-724.

Durand Delga, M. and Fontbote, 1.M. (1'Jó'J). Le problcme deráge des nappes alpujarrides ti"Andalousie. Ret'. Géogr.Phys. Céo/. Dy". (2), VIII. 181-187.

García Monzón. G .. Kampschuur W. and Verburg. J. (''J74).Ce%gica/ map of Sorbas (1031). 1:50.000. I.G.M.E., Ma­drid.

Groupe de Recherche néotectoni'lue de rarc de Gihraltar.(1 \>77). L'histoire tectonique récente (Tononien á Ouaternai­re) de rArc de Gihraltar et des bordures de la mer d'Albo­rano Bull. SoC. Géo/. Franee. (7). 19, 575-óI4.

Ott d'Estevou Ph. and Montenat Ch. (IW\5). Evolution structu­rale de la zone béli'lue orienlale (Espagne) du Tortonien aI'Holocene. eR. Acad. Sc. Paris. 300. 11. 8. 3ó3-368.

Rondeel H. E. (1 (5). Ce%gica/ 11l1'esfigllfiolls ill tll(' "·e.\'IemSierra Cabrera and adjoilling Areas. SOlllh-Eastem Spllin.Thesis, Amsterdam, lól p.

Rulter E.H., Maddock R.H., Hall S.H. and Whitc S.H. (198ó).Comparalive Microstructures of Natural and ExpcrimcnlallyProduced C1ay-Bcaring Fault Gougcs. P//re & Applied Geop­hysics, 124, 1/2. 2-30.

Sanz de Galdcano. c., (1983). Los accidentes y fracturas princi­pales de las Cordilleras Béticas. Estudios Ceo/. Madrid. 39.157-165

Sanz de Galdeano. C. Rodríguez Fermíndez, 1. and López Ga­rrido. A.e. (1\>85). A Strike-slip Fault Corridor within the AI­pujarra Mountains (Betic Cordilleras, Spain). Ceo/. R//nds­chau. 74. 641-655.

Viennot, P. (1930). Observations géologiques dans la région deGrenade (Andalousie). Uvre ¡uhilaire Soco Céol. France. 11.633-660.

Weijermars. R., Roep. Th. B.. Van den Eeckhou!. B., PostmaG. and Klcverlaan K. (]\>85). Uplift hislory of a Bctic foldnappe inferred from Neogene-Quaternary sedimentation andtcctonics (in the Sierra Alhamilla and Almeria. Sorbas andTabernas Basins of the Betic Cordilleras, SE Spain). Ce%~~ enMijnho//w. 64, 397-411.

Recihido el 25 de Noviembre de 19f17Aceptado e/ 20 de Diciembre de 19f17