The Yunquera and Saucillo Units in the western Betic Internal Zone: regional significance

Background

One of the first divisions of the Alpujarride tectonic units in the western part of the Betic Cordillera corresponds to Hoeppener et al. (1964)Hoeppener, R.; Hoppe, P.; Dürr, S.H. & Molla,t H. (1964). Ein Querschnitt durch die Betischen Kordilleren bei Ronda (SW Spanien). Geologie en Mijnbouw, 43: 282-298 and Dürr (1967)Dürr, S.H. (1967). Geologie der Sierra de Ronda und ihrer Südwestlichen Ausläufer (Andalousien). Geologica Romana, 6: 73 pp who distinguished, from bottom to top, the Blanca, the Ronda Peridotites, and the Casares units (in this case, the Casares unit comprises the later called Jubrique unit and the Benarrabá Imbrications) (Fig. 3). Later, Navarro-Vila & Tubía (1983)Navarro-Vila, F. & Tubía, J.M. (1983). Essai d’une nouvelle différenciation des Nappes Alpujarrides dans le secteur occidental des Cordillères Bétiques (Andalousie, Espagne). Comptes Rendus Académie des Sciences Paris, 296: 111-114 and Tubía (1988)Tubía, J.M. (1988). Estructura de los Alpujárrides occidentales: Cinemática y condiciones de emplazamiento de las peridotitas de Ronda. Parte I: Características litológicas. Boletín Geológico y Minero, 99 (2): 165-212 distinguished the unit of Ojén at the bottom of the complex, with the Guadaiza unit above it, and the Los Reales unit on top of the Guadaiza.

Balanyá (1991)Balanyá, J.C. (1991). Estructura del dominio de Alborán en la parte norte del arco de Gibraltar. Tesis Doctoral, Universidad de Granada, 232 pp and Balanyá & García-Dueñas (1991)Balanyá, J.C. & García-Dueñas, V. (1991). Estructuración de los Mantos Alpujárrides al W de Málaga (Béticas, Andalucía). Geogaceta, 9: 30-33 joined the Ojén and Guadaiza units into the Guaro nappe, and above it, they described the Bermeja unit (the peridotites), the Jubrique unit and, at the top, the Benarrabá tectonic imbrications. These authors grouped the last units, from Bermeja to the Imbrications, into the Los Reales nappe.

Sanz de Galdeano (2017Sanz de Galdeano, C. (2017). Implication of the geology of the Guadaiza and Verde valleys (Malaga Province, Betic Cordillera) on the position of the Ronda peridotites and the structure of the Alpujárride Complex. Boletín Geológico y Minero, 128 (4): 517-539. https://doi.org/10.21701/bolgeomin.128.4.006 , 2019)Sanz de Galdeano, C. (2019). Paleogeographic reconstruction of the Betic-Rif Internal Zone: An attempt. Revista de la Sociedad Geológica de España, 32 (2): 107-128 considered that the Guaro nappe does not have a real and independent existence, as it is the case with the Blanca and Mijas units (the Sierra de Mijas is situated farther to the East, Fig. 2). Instead, the author sees all these units as forming part of the Jubrique or Reales nappe (this last name is usually given when the peridotites situated at the bottom are also considered). Above them, the Intermediate units and the Malaguide Complex are located.

The Yunquera unit, although forming part of this region, is generally not cited in the previous divisions because it is situated immediately more to the N and its extension is reduced, approximately of 15-16 km². This last sector was briefly described by Blumenthal (1930)Blumenthal, M. (1930). Beiträge sur Geologie der betischen Cordilleres beiderseits des Rio Guadalhorce. Betische und penibetische Zone zwischen Antequera und Yunquera, Prov. Málaga. Eclogae Geologicae Helvetiae, 20: 41-293. Dürr et al. (1960)Dürr, S.; Hoeppener, R.; Hoppe, P. & Kockel, F. (1960). Géologie des montagnes entre le río Guadalhorce et le Campo de Gibraltar (Espagne méridionale). Livre a la mémoire du Professeur Paul Fallot, Memoire Hors sèries, Societé Geologique de France, 1: 209-227; Hoeppener et al. (1964)Hoeppener, R.; Hoppe, P.; Dürr, S.H. & Molla,t H. (1964). Ein Querschnitt durch die Betischen Kordilleren bei Ronda (SW Spanien). Geologie en Mijnbouw, 43: 282-298, and later Dürr (1967)Dürr, S.H. (1967). Geologie der Sierra de Ronda und ihrer Südwestlichen Ausläufer (Andalousien). Geologica Romana, 6: 73 pp cited their existence as a tectonic unit.

Bourgois (1978)Bourgois, J. (1978). La transversale de Ronda, Cordillères Bétiques, Espagne. Données géologiques pour un modèle d’évolution de l’Arc de Gibraltar. Annales Scientifiques de l’Université de Besançon, Géologie, 3e série, 30: 1-445 considered that in the area of Yunquera the Alpujarride Complex is formed by one unit type Los Reales, and, in the northern part, by another unit equivalent to the Casares unit (in the sense of Didon et al., 1973Didon, J.; Durand-Delga, M. & Kornprobst, J. (1973). Homologies géologiques entre les deux rives du détroit de Gibraltar. Bulletin de la Societé Géologique de France, S7-15 (2): 77-105. https://doi.org/10.2113/gssgfbull.S7-XV.2.77 , that is, an Intermediate unit). Martín-Algarra (1987)Martín-Algarra, A. (1987). Evolución geológica alpina del contacto entre las Zonas Internas y las Zonas Externas de la Cordillera Bética. Tesis Doctoral, Universidad de Granada, 1171 pp used the name of Yunquera for the first unit, and that of Saucillo, for the second one. This author considered the possibility that the Yunquera unit was equivalent to the Blanca unit situated southwards (Fig. 2). These same names were later used by Del Olmo et al. (1991)Del Olmo Sanz, A.; Moreno Serrano, F.; Campos Fernández, J.; Estévez, A.; García Dueñas, V.; García Rossell, L.; Martín-Algarra, A.; Orozco, M. & Sanz de Galdeano, C. (1991). Mapa geológico de España, escala 1:50.000, nº 1051 (Ronda), I.G.M.E., 56 pp, although their geologic map was originally made in the year 1981. Finally, Esteban et al. (2005)Esteban, J.J.; Cuevas, J.; Tubía, J.M.; Gil Ibarguch,i J.I &, Seward D. (2005). Metamorfismo, exhumación y termocronología de la unidad de Yunquera (Alpujárrides occidentales. Cordilleras Béticas). Revista de la Sociedad Geológica España, 18: 63-76 studied the Yunquera unit, particularly from the petrologic point of view, describing also the process of exhumation and thermochronology. They indicate ages of T-t evolution from the Permian to the early-middle Miocene.

Geological setting

In this western area of the Betic Internal Zone, in addition to the Alpujarride and the Malaguide Complexes, it is necessary to cite the Dorsal (Figs. 1 and 2), which can be considered as the fourth tectonic complex of the Internal Zone, narrowly linked to the Malaguide Complex, so much so that the Mesozoic cover of this last complex, in some cases, cannot be distinguished from the Dorsal units. The Dorsal, present in the Betics and the Rif (Durand-Delga & Foucault, 1967Durand-Delga, M. & Foucault A. (1967). La dorsale Bétique, nouvel élément paléogéographique et structural des Cordillères Bétiques au bord Sud de la Sierra Arana (Province de Grenade, Espagne). Bulletin de la Societé Géologique de France, 7 (5): 723-728. https://doi.org/10.2113/gssgfbull.S7-IX.5.723 ), is divided into many tectonic units. In the western part of the Betics, this complex was named Rondaides by Blumenthal (1928)Blumenthal, M. (1928). Sur le dispositif des nappes de récouvrement de la Serranía de Ronda (Andalousie). Eclogae Geologicae Helvetiae, 21 (2): 358-363 and Frontal units (Serrano, 1998Serrano, F. (1998). La Cordillera Bética en la provincia de Málaga. In: Itinerarios por espacios naturales de la provincia de Málaga. Una aproximación al conocimiento de su Geología y de su Botánica, (Rebollo, M.; Serrano, F.; Nieto, J.M. & Cabezudo, B., Eds.). Studia Malacitana, Servicio de Publicaciones, Universidad. de Málaga, 75-111; Jabaloy et al. 2019Jabaloy-Sánchez A.; Martín-Algarra A.; Padrón-Navarta J.A.; Martín-Martín M.; Gómez-Pugnaire M.T.; Sánchez-Vizcaíno, V.L. & Garrido C.J. (2019). Lithological Successions of the Internal Zones and Flysch Trough Units of the Betic Chain. In: The Geology of Iberia: a Geodynamic approach, Regional Geology Reviews (Quesada C. & Oliveira, J.T., Eds.). Springer Nature, Switzerland. 377-432. https://doi.org/10.1007/978-3-030-11295-0_8 ), but for the moment, that of Dorsal is the more used one. Other units of this area correspond to the so called Predorsal, linked to the Dorsal, but paleogeographically occupying a more external position. In this article they have not being differentiated and are considered as forming a single ensemble.

In the study area, the Sierra de las Nieves and other neighboring mountains belong to the Dorsal (Fig. 2). This Sierra is mainly formed by Triassic-Jurassic carbonates, and it is thrust by the Alpujarride and Malaguide Complexes. The Dorsal, in turn, is thrust over the External Zone.

The External Zone corresponds to the Mesozoic and Cenozoic south and southeastern sedimentary cover of the Paleozoic Iberian Massif, and it is divided into Prebetic and Subbetic. The Prebetic is situated nearer this massif, and it is not represented in the study area. The Subbetic generally corresponds to marine sediments and in many places it can be further divided into Internal, Middle and External units, according to their regional position. In the study area it correspond to the Internal Subbetic, which is also called Penibetic.

Also, it is necessary to mention the units called Campo de Gibraltar, or more generally known as Flyschs units. They appear in the south of Italy, and pass from Tunicia to the northern part of the Rif (Alcalá et al., 2013Alcalá, F.J.; Guerrera, F.; Martín-Martín, M.; Raffaelli, G. & Serrano, F. (2013): Geodynamic implications derived from Numidian-like distalturbidites deposited along the Internal-External Domain Boundary of the Betic Cordillera (S Spain). Terra Nova, 25, 119-129 https://doi.org/10.1111/ter.12014 ; Martín Martín et al., 2020Martín-Martín, M.; Guerrera, F.; Miclaus, C. & Tramontana, M. (2020). Similar Oligo-Miocene tectono-sedimen-tary evolution of the Paratethyan branches represented by the Moldavidian Basin and Maghrebian Flysch Basin. Sedimentary Geology, 396: 105548. https://doi.org/10.1016/j.sedgeo.2019.105548 ), and are also present in the region of the Campo de Gibraltar, hence their name in the Betics. They correspond mainly to Cenozoic sediments tectonically superimposed over any other units previously cited. In the study area a particular type of these Flyschs units, the Neonumidian, is present. Moreover, near this area the Formation d’Argiles à Blocks (the Clays Formation with blocks) also appears. Both units are defined by Bourgois (1978)Bourgois, J. (1978). La transversale de Ronda, Cordillères Bétiques, Espagne. Données géologiques pour un modèle d’évolution de l’Arc de Gibraltar. Annales Scientifiques de l’Université de Besançon, Géologie, 3e série, 30: 1-445. According to this author, these formations have an olistostromic character, presenting abundant kilometric olistoliths. The age of these formations is Burdigalian.

To help in the understanding of the relations of the previous cited complexes and units, Fig. 4 present a simplified general scheme of their structure at the western end of the Betic Internal Zone.

Finally, three types of unconformable deposits can be mentioned: the Alozaina formation (Bourgois et al., 1972 aBourgois, J.; Chauve, P.; Lorenz, C.; Monnot, J.; Peyre, Y.; Rigo, E. & Rivière, M. (1972a). La formation d’Alozaina. Sèrie d’âge oligocene et aquitanien transgressive sur le Betique de Malaga (région de Alozaina-Tolox, province de Málaga, Espagne). Comptes Rendus de l’Académie des Sciences Paris, 275: 531-534), the Brecha de la Nava - the Nava Breccias- (Martín-Algarra & Estévez, 1984Martín-Algarra, A. & Estévez, A. (1984). La Brèche de la Nava: dépôt continental synchrone de la structuration pendant le Miocène inférieur des zones internes de l’Ouest des Cordillères bétiques. Comptes Rendus Académie des Sciences Paris, 299-2 (8): 463-466), and the Millanas (Bourgois et al., 1972bBourgois, J.; Chauve, P.; Magne, J.; Monnot, J.; Peyre, Y.; Rigo, E & Rivière, M. (1972b). La formation de las Millanas. Série burdigalienne transgressive sur les Zones Internes des Cordillères bétiques occidentales. (Région d’Alozaina-Tolox, province de Malaga, Espagne). Comptes Rendus Académie des Sciences Paris, 275: 169-172) or Cártama formation (Sanz de Galdeano et al., 1993Sanz de Galdeano, C.; Serrano, F.; López Garrido, A.C. & Martín Pérez J.A. (1993). Palaeogeography of the Late Aquitanian-Early Burdigalian Basin in the Western Betic Internal Zone. Geobios, 26 (1): 43-55. https://doi.org/10.1016/S0016-6995(93)80005-C ). A revision of these formations can be consult in Jabaloy et al. (2029)Jabaloy-Sánchez A.; Martín-Algarra A.; Padrón-Navarta J.A.; Martín-Martín M.; Gómez-Pugnaire M.T.; Sánchez-Vizcaíno, V.L. & Garrido C.J. (2019). Lithological Successions of the Internal Zones and Flysch Trough Units of the Betic Chain. In: The Geology of Iberia: a Geodynamic approach, Regional Geology Reviews (Quesada C. & Oliveira, J.T., Eds.). Springer Nature, Switzerland. 377-432. https://doi.org/10.1007/978-3-030-11295-0_8 . All of them have a lower Miocene age.

Aims of the article

Its first purpose is to describe the geometry of the Yunquera area. This description is mainly referred, but not exclusively, to the tectonic unit of this name. The second purpose is to try to reconstruct the kinematics able to produce this geometry and to identify the possible scenario in which the area was involved. Moreover, a comparison of the lithologic characteristics of the tectonic units concerned will allow to position them in relation with other units of this area, and it will also contribute to their paleogeographic reconstruction.

The Yunquera unit

Gneisses and schists

 ⌅

Towards the bottom of this formation there are gneisses, in which well formed feldspars can be seen, that are very abundant in some levels. This texture changes progressively towards the top, passing gradually to schists. There are also visible garnets, different types of micas, including biotite, and also sillimanite.

The dominant color is brown, with the exception of some gneisses in which the abundance of feldspar give the rock a generally lighter color. The conserved thickness of these rocks is very variable, from nothing in the northern parts of the unit, to more than 100 m in other parts. Originally they would have had more than several hundred of meters of thickness, but that which can be observed at present is much less due to tectonic and erosion effects.

The age attributed to the schists is Paleozoic, while that of the gneisses probably corresponds to the age of the emplacement of the peridotites, then forming an aureole to which the gneisses form part. This age is discussed, varying from the Aquitanian (Esteban et al., 2011Esteban, J.J.; Cuevas J.; Tubía, J.M.; Sergeev, S. & Larionov A. (2011). A revised Aquitanian age for the emplacement of the Ronda peridotites (Betic Cordilleras, southern Spain). Geological Magazine, 148 (1): 183-187. https://doi.org/10.1017/S0016756810000737 ) to the Permian (Sanz de Galdeano & Ruiz-Cruz, 2016Sanz de Galdeano, C. & Ruiz Cruz, M.D. (2016). Late Palaeozoic to Triassic formations unconformably deposited over the Ronda peridotites (Betic Cordilleras): Evidence for their Variscan time of crustal emplacement. Estudios Geológicos, 72(1): e043. https://doi.org/10.3989/egeol.42046.368 ).

Against the opinion of some authors (Del Olmo et al., 1991Del Olmo Sanz, A.; Moreno Serrano, F.; Campos Fernández, J.; Estévez, A.; García Dueñas, V.; García Rossell, L.; Martín-Algarra, A.; Orozco, M. & Sanz de Galdeano, C. (1991). Mapa geológico de España, escala 1:50.000, nº 1051 (Ronda), I.G.M.E., 56 pp; Martín-Algarra, 1987Martín-Algarra, A. (1987). Evolución geológica alpina del contacto entre las Zonas Internas y las Zonas Externas de la Cordillera Bética. Tesis Doctoral, Universidad de Granada, 1171 pp) neither the schists nor the gneisses, contain intercalated levels of marbles in their interior. Only near the top of the schists these levels begin to appear, marking a short transition to the marbles. In this transition, a thin level of orthogneiss, about 30 cm thick (Fig.5A), exists near the kilometric point 36.7 in the road (A-366) to the E of Yunquera. In other places these levels of former volcanic rocks correspond to the Triassic, or Permian (Puga & Torres-Roldán, 1989Puga, E. & Torres Roldán, R.L. (1989). Geochemistry and age relationships of metamorphosed mafic sills from Sierra de Enmedio and Sierra de Carrascoy (Eastern Betic Zone, southeastern Spain). Estudios Geológicos, 45, 325-336. https://doi.org/10.3989/egeol.89455-6504 ).

Figure 5.  A: view of the orthogneiss level situated near the top of the schists in the Yunquera unit. B: phyllites of the Saucillo unit. The white dotted line indicates the position of the vertical foliation, disposed in an ENE-WSW tectonic band. C: thrust of the peridotites over the Yunquera unit affected by an oblique dextral fault forming part of the later shear zone. D: oblique dextral fault in the contact between the Dorsal and the Yunquera unit. It is part of the shear zone that, among other effects, thinned and cut the Dorsal. The position of the photos are indicated in Fig. 6.

Both types of rocks, paragneisses and schists have not been differentiated in Figs. 6 and 7, being indicated with the same color, because it is not possible to check at every point of this lithological formation, owing to the structure, the existence of soil cover, and the transitions from one type of rock to another.

Figure 6.  Geologic map of the Yunquera area. Its position is indicated in Fig. 2. The situation of the cross sections of Fig. 7 are marked. Letters A to G correspond to the position of the photos of Figs. 5 and 8.

Figure 7.  Geologic cross sections of the study area. Their positions are indicated in figure 6. The symbol (x/o) indicates dextral strike slip faults.

The Saucillo unit

Under this name, rests of other units of intermediate character are actually also included, as is it mentioned a little later in this article.

The Malaguide units

The plural designation of “units” used here refers to the fact that this complex appears in the southern part of the study area and also in the northern, in the strike slip area.

In the south it is formed by Paleozoic rocks, lutites, greywackes, and quartzites with a brown color. In this area more than 100 m of thickness is conserved.

In the strike slip area the Silurian-Devonian limestones (calizas alabeadas) (Mäkel, 1985Mäkel, G.H. (1985). The geology of the Malaguide Complex and its bearing on the geodynamic evolution of the Betic-Rif orogen (southern Spain and northern Morocco). GUA papers of Geology, 22 (1): 1-263) have been previously mentioned, although in this case they are slightly metamorphosed and situated in the Intermediate units. There are also red lutites and quartzites attributable to the Triassic, disposed in tectonically elongated bodies. Some carbonatic rests also could belong to this complex.

Moreover the described units, in this area there are others not directly studied, although their lithology is now very briefly indicated here, in order to complete the geologic context.

The Dorsal and the Predorsal (or Frontal Units)

At the bottom of the sequence a thick dolomitic formation appears. Above it there are limestones and dolomites, locally with marls, limestones with chert, and other higher formations. The age of these rocks spans from the upper Triassic to the Jurassic and up to the Cretaceous; even younger ages are cited in very local points. The limestones with chert, and some thin interbedded marl levels, correspond to the lower Jurassic. The total thickness of these formations, although variable, is in excess of a thousand meters.

In some areas, particularly those in contact with the Alpujarride units, the above mentioned rocks of the Dorsal have been metamorphosed, passing to marbles, calc-schists and even phyllites. For this reason, in some cases, it is not possible to distinguish them from equivalent Alpujarride rocks, the lower marbles of the Yunquera unit. This means that in some places the cartography here presented has the value of an interpretation rather than an exact determination.

In the Dorsal it is necessary to mention the existence of the Nava Breccias, unconformably deposited over the previous formations, made up by carbonatic clasts of the same domain, some of them metamorphosed, and someone of Alpujarride origin (Martín-Algarra & Estévez, 1984Martín-Algarra, A. & Estévez, A. (1984). La Brèche de la Nava: dépôt continental synchrone de la structuration pendant le Miocène inférieur des zones internes de l’Ouest des Cordillères bétiques. Comptes Rendus Académie des Sciences Paris, 299-2 (8): 463-466). According to these authors its age is lower Miocene. Del Olmo et al. (1991)Del Olmo Sanz, A.; Moreno Serrano, F.; Campos Fernández, J.; Estévez, A.; García Dueñas, V.; García Rossell, L.; Martín-Algarra, A.; Orozco, M. & Sanz de Galdeano, C. (1991). Mapa geológico de España, escala 1:50.000, nº 1051 (Ronda), I.G.M.E., 56 pp called Jarro Breccias the deposits that lay over the Yunquera unit, which can be considered totally equivalent to the Nava Breccias (Martín-Algarra, 1987Martín-Algarra, A. (1987). Evolución geológica alpina del contacto entre las Zonas Internas y las Zonas Externas de la Cordillera Bética. Tesis Doctoral, Universidad de Granada, 1171 pp). In Fig. 6 two small outcrops of these breccias have been indicated, and they are significantly less than those indicated in the map of Del Olmo et al. (1991)Del Olmo Sanz, A.; Moreno Serrano, F.; Campos Fernández, J.; Estévez, A.; García Dueñas, V.; García Rossell, L.; Martín-Algarra, A.; Orozco, M. & Sanz de Galdeano, C. (1991). Mapa geológico de España, escala 1:50.000, nº 1051 (Ronda), I.G.M.E., 56 pp. In this area the thickness of these breccias is very variable, probably not reaching more than 10 m.

The Penibetic (Internal Subbetic, External Zone)

In the area of study only Cretaceous to Cenozoic limestones, marlstones and marls appear (Del Olmo et al., 1991Del Olmo Sanz, A.; Moreno Serrano, F.; Campos Fernández, J.; Estévez, A.; García Dueñas, V.; García Rossell, L.; Martín-Algarra, A.; Orozco, M. & Sanz de Galdeano, C. (1991). Mapa geológico de España, escala 1:50.000, nº 1051 (Ronda), I.G.M.E., 56 pp). Most of the levels are of salmon color, but there are also others of white tones. Under these rocks, the Jurassic is mainly carbonatic.

The Neonumidian

This unit corresponds to a Flysch sequence, formed by sandstones, lutites, clays, locally conglomerates, and contains sedimentary klippes of variable lithology, such as microcodium limestones. The dominant color is brown. Although very similar in its field aspect to the Flysch units present in the Gibraltar area (Campo de Gibraltar units), its age is Burdigalian (Bourgois, 1978Bourgois, J. (1978). La transversale de Ronda, Cordillères Bétiques, Espagne. Données géologiques pour un modèle d’évolution de l’Arc de Gibraltar. Annales Scientifiques de l’Université de Besançon, Géologie, 3e série, 30: 1-445).

Finally, there are also alluvial and colluvial sediments, in many cases not indicated in Fig. 6.

Structure of the Yunquera unit

The structure of this unit can be deduced from the map of the Fig. 6 and the cross sections of Fig. 7. This unit is tectonically superimposed to the Dorsal, but the formations conserved at its bottom vary. In the northern part, the lower marbles of the Yunquera unit are directly in contact with the Dorsal, while in the southern part, the gneisses and schists are conserved, placed over the Dorsal. This means that the bottom of the Yunquera unit is cut in a beveling fashion.

The S-SE border of the Yunquera unit is limited by peridotites tectonically superimposed, but the last movements observed at this contact (in the more eastern part of the studied area) correspond to an oblique-dextral fault (Fig. 5 C) with striae of 40º plunging to the SW (Fig. 5 D).

In the cartography of this unit (Fig. 6) there are two aspects that are not precisely defined. The first one is found in its NW border, in the area of Jarro and Arca points. There, the dolomitic marbles of the Dorsal and those of the Yunquera unit are in contact, the latter ones sit over those of the Dorsal. Due to the similarities in terms of facies of the marbles in both units, the separation between these formations is only approximate. The uninterrupted forest mantle existing in that location contributes to this possible imprecision.

The second aspect corresponds to the small tectonic windows located to the NE of Yunquera (Fig. 6 and cross section 2 of Fig. 7). The existence of these windows is indicated in Bourgois (1978)Bourgois, J. (1978). La transversale de Ronda, Cordillères Bétiques, Espagne. Données géologiques pour un modèle d’évolution de l’Arc de Gibraltar. Annales Scientifiques de l’Université de Besançon, Géologie, 3e série, 30: 1-445 and Del Olmo et al. (1991)Del Olmo Sanz, A.; Moreno Serrano, F.; Campos Fernández, J.; Estévez, A.; García Dueñas, V.; García Rossell, L.; Martín-Algarra, A.; Orozco, M. & Sanz de Galdeano, C. (1991). Mapa geológico de España, escala 1:50.000, nº 1051 (Ronda), I.G.M.E., 56 pp, although their shape and number varies. Here, a different shape is presented, although it is only one approximation. There really exist tectonic windows where formations of the Dorsal crop out, for instance marble limestones with chert, rocks that don’t exist in the Yunquera unit. Some of these small windows are limited by faults, generally normal faults. But it is necessary to insist on the fact that the shape of these windows presented in Fig. 6 is only one approximation, mainly made to indicate their existence more than their detailed shape.

Structure of the Saucillo unit

This unit, in which there are also fragments of several other Intermediate units and of the Malaguide Complex, is located in the northern part of the study area. It is tectonically placed over the Yunquera unit and also over the Dorsal and Predorsal. In turn, the Neonumidian is thrust over the Saucillo unit and also over the Subbetic.

But this structure of superposition of units is at present almost entirely affected by a wide dextral strike slip fault zone (Fig. 6 and cross sections 4-6 of Fig. 7), forming vertical tectonic bands, a flower structure. The cartography of this area in Fig. 6 is only one approximation because, as it happens in other parts of the region, it is covered by forest and scrub, making it difficult to follow the contacts, even though it probably expresses the existing structure well enough.

It is worth to indicate that thin shreds of Neonumidian are located within this flower structure, together with those of the Saucillo unit, that is to say, the Neonumidian is also clearly imbricated in the structure.

Some kinematic indicators have been found in this area of the Saucillo unit. In its eastern part the bottom of the unit is observable, situated over the Predorsal/Dorsal. The contact corresponds to a low angle fault, with striae trending N50ºE, and plunging towards the SW (Fig. 8 E). More than 1 km to the west, striae with dip angles that vary from 45º to 0º (horizontal) can be found. Similar features exist in the nearest Subbetic formations (Fig. 8 F, G). Also, to the SW of the Convento, in the tectonic contact existing between the Dorsal and the Subbetic, equivalent structures have been found, including in between small remnants of the Saucillo unit and even of the Neonumidian, not indicated in Fig. 6, due to their small size.

The hidden cut of the Dorsal

In the study area the Dorsal appears at the E and NW borders of the Yunquera unit. In both sides the Dorsal is extensively exposed, forming mountains higher than 1500 m, while the area of Yunquera, in many areas is lower than 900 m (Fig. 6). That is to say, the area where the Yunquera unit is conserved corresponds comparatively to a depressed sector.

This depressed area does not correspond to a kind of flexure in which the Dorsal is solely bent; on the contrary, in this area the Dorsal is strongly sheared, in such a way that it disappears entirely in the sector of the Saucillo unit, being this last unit, and also practically the Yunquera unit, in direct contact with the Subbetic. This is expressed in the cross sections 4-6 of the Fig. 7. In this sense, Fig. 9 indicates the geometry of this shear cutting the Dorsal and puts it in its regional context.

The sense of displacement of this zone of thinning in which the Dorsal was cut, is the previously indicated one of N50º, plunging southwards. This measure has been obtained not only from the bottom and the splinters of the Saucillo unit, but also from other sectors, i.e. north of Jorox, in the contact between the Yunquera unit and the Dorsal, and between the Yunquera and the peridotites (Fig. 5, C and D).

Different features of the formation of the alpine structures in the Yunquera area

The thrust of the Malaguide Complex over the Alpujarride Complex is the most important tectonic alpine feature in this region. The rest of the structures, though also important, must be considered linked to the effects related to the westward displacement of the Betic Internal Zone.

Regionally, the Dorsal is thrust over the Malaguide Complex, being the Alpujarride Complex in a lower position, as it happens in the Jubrique-Gaucín area, the more western part of the Internal Zone (Fig. 4). But on the contrary, this position of the Dorsal is not conserved in the Yunquera area. In fact, in the entire Sierra de las Nieves, i.e. in the study area, the Dorsal is widely thrust by the Alpujarride units, the Yunquera, the peridotites and the Saucillo units. And due to this superimposition of the Alpujarride Complex, the Dorsal underwent an important increase of temperature that likely has been the source of its metamorphism in the sector of contact with the Alpujarride.

The reason for this “abnormal” thrust of the Alpujarride Complex over the Dorsal is to be found in the gradual and progressive westward displacement of the Internal Zone occurred during the end of the Oligocene and part of the lower Miocene. This movement first caused the thrust and the final embedding of the Dorsal against the Subbetic, but, simultaneously, the pressure exerted by the Alpujarride and Malaguide Complexes continued, culminating into also thrusting over the Dorsal.

But the evolution of deformation in this area was not yet finished: after the thrusting of the Alpujarride Complex over the Dorsal, and also linked to the progressive westward drift of the Internal Zone, the previously mentioned thinning and cut of the Dorsal began. This happened because this drifting of the Internal Zone was not homogeneous. The displacement was greater in the area of Yunquera compared with the sector located directly to the NE. This differential movement produced the thinning of the Dorsal. The cut had a sense of displacement of N230º (N50º, top to the SW), also coincident with the direction obtained in the fault zone affecting the Saucillo unit. Both aspects were linked. In fact, this fault zone begins exactly in the zone of the cut of the Dorsal.

The aforementioned differential displacement implied in the cut of the Dorsal is here interpreted as produced by the movements of very important regional E-W dextral strike slip faults (Figs. 2, 9 and 10) which facilitated in each case a greater advance of the southern segments of the Internal Zones in relation to those situated in the northern sectors (Sanz de Galdeano, 1996Sanz de Galdeano, C. (1996). The E-W segments of the contact between the External and Internal Zones of the Betic and Rif Cordilleras and the E-W corridors of the Internal Zone (a combined explanation). Estudios Geológicos, 52: 123-136. https://doi.org/10.3989/egeol.96523-4260 and 2020Sanz de Galdeano, C. (2020). La costa E-O de Andalucía. Discusión sobre su significado geológico. Boletín Geológico y Minero, 131: In press). This happened in the study area and also to the south of the Sierra de las Nieves (Fig. 9), where the Dorsal forms a reversed syncline whose southern part is metamorphosed by the thrust of the Alpujarride Complex (Dürr, 1967Dürr, S.H. (1967). Geologie der Sierra de Ronda und ihrer Südwestlichen Ausläufer (Andalousien). Geologica Romana, 6: 73 pp, Piles Mateo et al., 1978Piles Mateo, E.; Chamón, C. & Estévez-González, C. (1978). Mapa y memoria explicativa de la hoja 1065 (Marbella) del Mapa Geológico Nacional a escala 1:50.000, IGME, 65 pp; Martín-Algarra, 1987Martín-Algarra, A. (1987). Evolución geológica alpina del contacto entre las Zonas Internas y las Zonas Externas de la Cordillera Bética. Tesis Doctoral, Universidad de Granada, 1171 pp), (there the Jubrique/Los reales unit is represented not only by the peridotites but also by the rest of its lithological sequence).

As indicated previously, the Yunquera unit is a local duplication of the Alpujarride domain. In the study area, before its individualization, the Yunquera unit occupied the frontal part of the Alpujarride Complex during the thrusting over the Dorsal (Fig. 4, area in red color). In its advance, and superimposition, the basal part of the Yunquera unit was partially cut, being at the same time progressively embedded. Then the whole of the Alpujarride domain also thrusted over this embedded sector, forming the “small” slice of the Yunquera unit.

But probably, the present bevelling of the bottom of the Yunquera units was formed in two stages, first during its emplacement, and, partially, later during the cut and shearing of the Dorsal, something that also affected this unit.

The Saucillo unit (also containing rests of other Intermediate units) is situated over the Yunquera unit. In this regard, it has the same tectonic position than the Intermediate units in the area of Gaucín (Didon et al., 1973Didon, J.; Durand-Delga, M. & Kornprobst, J. (1973). Homologies géologiques entre les deux rives du détroit de Gibraltar. Bulletin de la Societé Géologique de France, S7-15 (2): 77-105. https://doi.org/10.2113/gssgfbull.S7-XV.2.77 ; Sanz de Galdeano et al., 1999Sanz de Galdeano, C.; López Garrido, A.C. & Andreo, B. (1999). The stratigraphic and tectonic relationships of the Alpujarride and Malaguide complexes in the western Betic Cordillera (Casares, prov. of Malaga, South Spain). Comptes Rendus Académie des Sciences Paris, 328: 113-119. https://doi.org/10.1016/S1251-8050(99)80006-8 and 2001Sanz de Galdeano, C.; Andreo, B.; García-Tortosa, F.J. & López-Garrido, A.C. (2001). The Triassic palaeogeographic transition between the Alpujarride and Malaguide complexes, Betic-Rif Internal Zone (S Spain, N Morocco). Palaeo, 167: 157-173. https://doi.org/10.1016/S0031-0182(00)00236-4 ), where they are also placed over the Jubrique-Los Reales Alpujarride unit. But the difference is that the Saucillo unit is tectonically placed over the Dorsal, contrarily to what happens in the area of Gaucín (Fig. 4).

The position of the Neonumidian unit is generally clear: it is tectonically placed over the rest of the units, but it is involved in the northern strike slip shear zone, inside which it locally appears forming thin vertical splinters.

The age of formation of the different structures

The thrust of the Malaguide Complex over the Alpujarride Complex was the first structure that formed, regionally, as it pertains to the present study. It is related with the main alpine tectonic structuring of the Betic Internal Zone, and took place before the end of the Oligocene (Durand-Delga & Fontboté, 1960Durand-Delga, M. & Fontboté, J.M. (1960). Le problème de l’âge des Nappes Alpujarrides d’Andalousie. Revue de Géographie Physique et de Géologie Dynamique, 2 (3): 181-187, 1980Durand-Delga, M. & Fontboté, J.M. (1980). Le cadre structural de la Méditerranée occidentale. Mémoires du Bureau de Recherches Géologique et Minières, 115: 67-85), although several authors thinks that it continued till the end of the Aquitanian (Martín-Algarra, 1987Martín-Algarra, A. (1987). Evolución geológica alpina del contacto entre las Zonas Internas y las Zonas Externas de la Cordillera Bética. Tesis Doctoral, Universidad de Granada, 1171 pp; Jabaloy et al., 2019Jabaloy-Sánchez A.; Martín-Algarra A.; Padrón-Navarta J.A.; Martín-Martín M.; Gómez-Pugnaire M.T.; Sánchez-Vizcaíno, V.L. & Garrido C.J. (2019). Lithological Successions of the Internal Zones and Flysch Trough Units of the Betic Chain. In: The Geology of Iberia: a Geodynamic approach, Regional Geology Reviews (Quesada C. & Oliveira, J.T., Eds.). Springer Nature, Switzerland. 377-432. https://doi.org/10.1007/978-3-030-11295-0_8 ). This affirmation, the first main alpine structure, would be false if the emplacement of the peridotites occurred during the Aquitanian (Esteban et al. 2011Esteban, J.J.; Cuevas J.; Tubía, J.M.; Sergeev, S. & Larionov A. (2011). A revised Aquitanian age for the emplacement of the Ronda peridotites (Betic Cordilleras, southern Spain). Geological Magazine, 148 (1): 183-187. https://doi.org/10.1017/S0016756810000737 , among other authors) or in any Cenozoic times, but for Sanz de Galdeano & Ruiz-Cruz (2016)Sanz de Galdeano, C. & Ruiz Cruz, M.D. (2016). Late Palaeozoic to Triassic formations unconformably deposited over the Ronda peridotites (Betic Cordilleras): Evidence for their Variscan time of crustal emplacement. Estudios Geológicos, 72(1): e043. https://doi.org/10.3989/egeol.42046.368 this happened many time before, at the end of the Carboniferous, beginnings of the Permian. In fact, analyzing the regional position of the peridotites, they are situated under the Jubrique unit, even when they thrust, for instance, the Dorsal in the Sierra de las Nieves or the Blanca unit in the Sierra Alpujata.

This was, the final Oligocene - Aquitanian, the initial moment, while the last moment of structuring in the study area corresponds to the thrust of the Neonumidian over the rest of the tectonic units and the cut of the Dorsal.

As pointed out before, the Neonumidian is considered Burdigalian (Bourgois, 1978Bourgois, J. (1978). La transversale de Ronda, Cordillères Bétiques, Espagne. Données géologiques pour un modèle d’évolution de l’Arc de Gibraltar. Annales Scientifiques de l’Université de Besançon, Géologie, 3e série, 30: 1-445) and immediately during this time it was tectonically emplaced. This is confirmed approximately 20 km to the E, in a sector located to the north of Cártama, where the Neonumidian thrusts over early marine Burdigalian sediments (Sanz de Galdeano et al., 1993Sanz de Galdeano, C.; Serrano, F.; López Garrido, A.C. & Martín Pérez J.A. (1993). Palaeogeography of the Late Aquitanian-Early Burdigalian Basin in the Western Betic Internal Zone. Geobios, 26 (1): 43-55. https://doi.org/10.1016/S0016-6995(93)80005-C ), abruptly cutting their sedimentation. Moreover, the Neonumidian is included in the form of splinters in the strike slip fault zone of the Saucillos area, something that occurred during the Burdigalian, probably not reaching the Langhian.

Consequently, the interval of time in which the studied structures were formed spans from the end of the Oligocene to the Burdigalian. And the image obtained is that their formation was a continuous and not interrupted process.

On the whole, this process can be described as follows:

Just before the end of the Oligocene the main structuring of the complexes of the Betic Internal Zone was achieved (the Nevado-Filabride Complex was thrust by the Alpujarride Complex, and this last by the Malaguide Complex). Immediately, the processes of the westward drift of this Internal Zone began, and the Betic External Zone underwent its oblique impact. This was the moment of the superimposition of the Dorsal/Predorsal over the External Zone, the Subbetic. But the westward drift continued and, with the Dorsal embedded against the Subbetic, then, the Malaguide and Alpujarride complexes (together with the Intermediate units) thrust over the Dorsal in the study area, and metamorphosed it partially.

According to the age proposed by Martín-Algarra & Estévez (1984)Martín-Algarra, A. & Estévez, A. (1984). La Brèche de la Nava: dépôt continental synchrone de la structuration pendant le Miocène inférieur des zones internes de l’Ouest des Cordillères bétiques. Comptes Rendus Académie des Sciences Paris, 299-2 (8): 463-466 for the Nava Breccias (containing metamorphic clasts inherited from the Dorsal) and the age of the Alozaina Formation (Bourgois et al., 1972aBourgois, J.; Chauve, P.; Lorenz, C.; Monnot, J.; Peyre, Y.; Rigo, E. & Rivière, M. (1972a). La formation d’Alozaina. Sèrie d’âge oligocene et aquitanien transgressive sur le Betique de Malaga (région de Alozaina-Tolox, province de Málaga, Espagne). Comptes Rendus de l’Académie des Sciences Paris, 275: 531-534), this thrust of the Alpujarride Complex occurred during the Aquitanian. But the process of structuring continued (because the westward drift continued), and then, during the Burdigalian, the Neonumidian arrived, it thrust over the rest of units and, at the same time, the Internal Zone broke in important E-W dextral strike slip faults. This was the time when the Dorsal was totally cut in the Yunquera area, moving its southern part to the SW and, directly related to this, the strike slip zone affecting the Saucillo area also was formed.

This is, approximately, the sequence of deformation in the study area, a continuous sequence, practically uninterrupted during the entire process.

The relationship of the Yunquera and Saucillo units with the rest of the Alpujarride Complex in its western part

As indicated in the background, the Yunquera unit has been compared to the so called Reales or Jubrique and Blanca units. These comparisons were made taking into account the respective tectonic positions and the lithologic sequences.

The tectonic position is clear for the Yunquera unit. It is a slice situated under the whole mass of the Alpujarride Complex in this area. Regarding the lithologic sequences existing in this Alpujarride western area, there is great similarity between all of the outcrops. In all the areas indicated in the figure 10, there are gneisses in the lower parts of the metapelitic sequences (with the exception of the units located in the western frontal areas, the Intermediate units, mainly situated to the west of Jubrique). In many locations, peridotites are found under these gneisses, suggesting that the gneisses were the aureole covering the peridotites.

Only in the sectors of Santi Petri, Cártama (Mon, 1971Mon, R. (1971). Estudio geológico del extremo occidental de los Montes de Málaga y de la Sierra de Cártama (Prov. de Málaga). Boletín Geológico y Minero, 82-83: 132-146; Chamón et al., 1978Chamón, C.; Quinquer, R.; Crespo, V.; Aguilar, M. & Reyes, J.L. (1978). Hoja geológica 1:50.000 nº 1052 (Alora). I.G.M.E., 71 pp) and Yunquera (see Fig. 10 for these names), the peridotites are not visible under the gneisses. In the eastern part of the Cártama sector there are peridotites located over the Cártama unit thanks to a thrust of fragile character (the Majada Vieja unit, Mon, 1971Mon, R. (1971). Estudio geológico del extremo occidental de los Montes de Málaga y de la Sierra de Cártama (Prov. de Málaga). Boletín Geológico y Minero, 82-83: 132-146) (Fig. 8 H). This superposition is here interpreted as being due to the effects of two dextral faults limiting the Cártama sierra. In the Yunquera unit the absence of visible peridotites is interpreted as the effect of its beveled bottom. In the other two sectors, Santi Petri and Cártama, the peridotites are not visible because the erosion is not deep enough to allow for them to be seen. But very probably they exist.

The area of Sierra Blanca (and the equivalent of Sierra de Mijas) has been considered a unit thrust by the Guadaiza unit. But the Guadaiza unit does not exist as a separate tectonic unit (Sanz de Galdeano, 2017Sanz de Galdeano, C. (2017). Implication of the geology of the Guadaiza and Verde valleys (Malaga Province, Betic Cordillera) on the position of the Ronda peridotites and the structure of the Alpujárride Complex. Boletín Geológico y Minero, 128 (4): 517-539. https://doi.org/10.21701/bolgeomin.128.4.006 ), rather, it is the same Jubrique /Los Reales unit. And Sierra Blanca and Mijas actually correspond to huge reversed folds, representing to the top of the lithologic sequences, whose lower parts are directly located to the south (Sanz de Galdeano & López-Garrido, 2016Sanz de Galdeano, C. & López-Garrido, A.C. (2016). Geometry of the contact of the peridotites of Sierra Alpujata with the Sierra Blanca succession (Alpujarride Complex, Betic Internal Zone). Geogaceta, 60: 7-10; Sanz de Galdeano, 2019Sanz de Galdeano, C. (2019). Paleogeographic reconstruction of the Betic-Rif Internal Zone: An attempt. Revista de la Sociedad Geológica de España, 32 (2): 107-128). This means that both sectors had originally the peridotites at their bottom and, directly above the peridotites, the gneisses and the rest of the sequence are there.

Over the gneisses, in every sector of this western Alpujarride region, there are brown schists and, above them, phyllites. But the thickness and even the aspect of the phyllites are variable. For instance, in the Jubrique area to the west of the Genal River, more than 100 m of thickness of these rocks can be seen, having gray blue color. More or less the same happens in the Carratraca area, while in other sectors, for instance in Yunquera, these rocks practically do not exist under the marbles. However, this feature probably results from the combination of lithologic facies and metamorphic degree.

The marbles are not conserved in all the sectors of this region. For instance, in Santi Petri area they have being tectonically cut for the superimposition of the Malaguide Complex. Nevertheless, in Sierra Blanca and Mijas they are greatly conserved. But on the whole, these marbles are equivalent in all of the sectors of this region.

The revision of the lithologic sequences of this western part of the Alpujarride Complex, allows for the conclusion that inside it (apart from the Intermediate units) the different outcrops existing are in reality equivalent, although the Yunquera unit can be differentiated as a tectonic slice.

Regarding the Saucillo unit, this one is easily correlatable, as it was done, with the Intermediate Units (or Benarrabá imbrications), occupying the same tectonic position. In the Saucillo area, rests of several of these units actually also exist, although very tectonized inside the strike slip fault zone.