Intraplate deformation in the NW Iberian Chain: Mesozoic extension ...

Journal of the Geological Society, London, Vol. 161, 2004, pp. 291–303. Printed in Great Britain.

Intraplate deformation in the NW Iberian Chain: Mesozoic extension and Tertiary contractional inversion ` 1, R A M O ´ N MAS2 & A ´ NGELA ALONSO3 J OA N G U I M E R A Departament de Geodina`mica i Geofı´sica, Universitat de Barcelona, Martı´ i Franque´s s/n, E-08028 Barcelona, Spain (e-mail: [email protected]) 2 Departamento de Estratigrafia, Universidad Complutense, Ciudad Universitaria, E-28040 Madrid, Spain 3 Departamento de Ciencias da Navegacioń e da Terra, Universidade de A Corunã, Campus da Zapateira s/n, E-15071 A Corunã, Spain 1

Abstract: The Iberian Chain developed within the Iberian plate during the Palaeogene and Early Miocene as a result of convergence between the African and Eurasian plates. It is a fold-and-thrust belt, which involves the Hercynian basement and the Mesozoic and Cenozoic cover. A generalized cross-section of the NW part of the Chain, of 195 km length, is presented. Mesozoic basins, developed on the Hercynian basement, display thickness variations across normal faults that bounded them and that can be recognized in the field. The Tertiary contraction deformed and inverted the Mesozoic basins, and it is inferred to have produced a thrust sheet about 5 km thick in its frontal (northeastern) part, which thickens to the SW. The total Tertiary shortening in the section is 66.6 km (26%). The structure and the crustal shortening and thickening of the Iberian Chain are explained by a major upper-crustal thrust system with simple flat-and-ramp geometry, which may branch to the Pyrenees or the Betics. This is combined with internal deformation of the thrust sheet. The contribution of the Iberian Chain shortening to the convergence of the African and Eurasian plates during the Tertiary is about one-half of that of the Pyrenees, and should be taken into account in any reconstruction of the kinematics of these plates. Keywords: Iberian Chain, inversion tectonics, crustal shortening, cross-sections.

thick sequences of Upper Permian and Mesozoic continental and shallow marine clastic rocks, carbonates and minor evaporites. These sediments rest on the regional post-Hercynian unconformity, which truncates folded and thrust low-grade Palaeozoic metasediments and intrusive rocks belonging to the Hercynian basement. The Mesozoic sedimentary sequences of the Iberian Chain show large lateral thickness changes from less than 1000 m to more than 6000 m and facies changes over distances of a few kilometres, thus indicating deposition in tectonically active basins.

This paper discusses the geological evolution of the NW Iberian Chain and, from this, provides insight into the evolution, during Tertiary times, of the Iberian plate and the plates surrounding it, that is, the African and Eurasian plates. To do this, the Mesozoic extension and subsequent Tertiary contraction is studied, paying attention to the pattern of Mesozoic faulting and its influence on Mesozoic palaeogeography, the inversion of the Mesozoic basins during Tertiary contraction, the structures formed during this process, and the crustal structure and shortening that can be deduced from previous geophysical data. The Iberian Chain is a double-vergent fold-and-thrust belt thrusting towards its flanking Tertiary basins. It resulted from the Tertiary inversion of extensional Mesozoic basins located within ´ lvaro et al. 1979; Guimera` 1984; Guimera` & the Iberian plate (A ´ Alvaro 1990). Its overall structure is defined by two major anticlinoria or arches trending NW–SE with wavelengths of 71– 119 km (Fig. 1); within these arches Tertiary contractional deformation involves basement. The cores of both arches are located on a thickened crust, as deduced from gravity anomalies (Salas et al. 2001). Hence, both arches are crustal-scale structures. In the northwestern part of the chain, the arches are neatly separated by the Almazań basin (Fig. 1b), which has a complex synclinal structure (Casas Sainz et al. 2000), but they merge towards the SE (Fig. 1). The northern arch, where the present study area is located, contains several units: the Cameros unit in the NW, the Aragonese Branch, and the Maestrat basin in the SE. The southern arch is mainly formed by the Castilian Branch of the Iberian Chain (Fig. 1). The basins of the Mesozoic Iberian Rift System (which include the Iberian Chain and extensive parts of the surrounding Tertiary basins and the Mediterranean Valencia trough) contain

Geology of the study area To characterize the structure of the area, a map and a general cross-section at the scale of 1:50 000 have been constructed (Figs 2 and 3); in two specific areas, maps and sections at 1:10 000 scale have also been drawn. The general cross-section (Fig. 3) has been length and area balanced and restored to the state previous to the Tertiary contraction and to specific times during the Mesozoic (Fig. 3). From these results we propose two structural hypotheses for the crustal thickening in the NW Iberian Chain during the Tertiary contraction. The overall structure of the Aragonese Branch in the study area (Figs 2 and 3) consists of a complex anticlinorium or arch more than 60 km wide, which involves the Hercynian basement and the Mesozoic and Cenozoic cover. This arch has two limbs and a wide, flat inner zone; in the Tertiary basins outside it (Almazań to the south and Ebro to the north) the pre-Tertiary rocks reach depths of 2 and 2.5 km below sea level, respectively (Sańchez Navarro et al. 1990; Guimera` et al. 1995). The study area contains two regions with different Mesozoic stratigraphic 291

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eastern region there is the NW–SE-striking high-angle Tablado fault, which, like the Malache fault, we interpret as a Mesozoic normal fault (Figs 2 and 3), as discussed below. In the SW region, south of the south Cameros thrust, the thickness of Mesozoic rocks varies from 500 to 2000 m. Basement-involved folds have a wavelength of 3–5 km and plunge to the NW, deforming the Palaeogene to Lower Miocene rocks of the northern part of the Almazań basin. Palaeozoic and Mesozoic rocks of that region are the substratum of the Almazań basin. In both the NE and SW regions, many of the basement-involved anticlines contain, in their northern limb, high-angle faults separating blocks with Mesozoic series of different thickness, usually thicker in the northern block of the fault (Fig. 3). These high-angle faults are interpreted as folded Mesozoic extensional faults.

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Fig. 1. Location of the study region. (a) Structural sketch map of the Iberian Chain. (b) Geological map of the NW Iberian Chain. Locations of the cross-section (Fig. 11), the gravity profile (Fig. 10) of Salas & Casas (1993), the seismic line A8011, the seismic-reflection line (X–X9) of Pedreira et al. (2003) and the detailed map of the NW Aragonese Branch are shown.

successions and structures of different dimensions. The central and northeastern region is the SE continuation of the inverted Mesozoic Cameros basin (Casas Sainz 1993; Guimera` et al. 1995). It is bounded by the north and south Cameros thrusts and contains up to 4000 m of Mesozoic rocks, deformed by basement-involved folds with a wavelength of 10–13 km (Fig. 2). The north Cameros thrust is covered by the undeformed Upper Miocene rocks of the Ebro basin, but the location of the thrust can be deduced from the Magalloń oil well (Lanaja 1987; Sańchez Navarro et al. 1990), many water wells and from geoelectrical surveying (San Romań Saldanã 1994). It emplaces Mesozoic rocks over 2500 m of Palaeogene and Lower Miocene rocks of the Ebro basin; the Mesozoic series in its footwall is 700 m thick (in the Magalloń well), much thinner than in the Cameros basin (Fig. 3). The south Cameros thrust ends, to the SE, within the study region, and the Malache high-angle fault, with a northern downthrown block that contains a thicker Mesozoic series than the southern block, appears at the SE of the thrust end (Fig. 2). We interpret this pattern as a Mesozoic normal fault zone whose inversion gave rise, along strike to the NW, to the south Cameros thrust. In the middle of this north-

Four depositional supersequences have been recognized in the ´ lvaro et al. 1979; Vilas et al. Mesozoic of the Iberian Chain (A 1983; Salas & Casas 1993; Roca et al. 1994): (1) Triassic (Upper Permian–Hettangian), lying on the Hercynian basement; (2) Lower to upper Jurassic (Sinemurian–Oxfordian); (3) Upper Jurassic to upper Cretaceous (Kimmeridgian–Middle Albian); (4) Upper Cretaceous (Upper Albian–Maastrichtian). Palaeogeography and stratigraphic columns for the Aragonese branch during the Mesozoic are shown in Figure 4. Upper Permian to Triassic Megacycle (onset of rifting in the Mesozoic Iberian basin). The regional post-Hercynian unconformity is generally overlain by Lower Triassic rocks, although Upper Permian clastic and volcanic rocks are present locally (Arribas 1985; Rey & Ramos 1991). The Triassic record is represented by the Germanic facies: Buntsandstein, Muschelkalk and Keuper. Lower to Upper Jurassic Megacycle. Three units mostly formed by limestones and marls of marine origin are distinguished: J1 (Lias), J2 (Dogger) and J3 (Lower and Middle Malm). The base of the megacycle includes everywhere a unit of Lower Lias carbonate breccias, which unconformably overlies the Triassic units (Aurell et al. 1992); the angular unconformity at outcrop or map scale can be observed in many places (Fig. 5). Uppermost Jurassic to Lower Cretaceous Megacycle. Three unconformity-bounded stratigraphic sequences have been recognized (Alonso & Mas 1988): (1) the Tithonian–Berriasian Sequence (K1 and K2 in Fig. 6c); (2) the Lower Aptian Sequence (K3 lower part), which lies unconformably on the former; (3) the Upper Aptian–Lower Albian Sequence (K3, upper part). Upper Cretaceous Megacycle. This megacycle is mostly preserved SW of the south Cameros fault. In the northeastern corner of the study area only small outcrops can be found, as a result of the deep erosion of the inverted Cameros basin. It began with continental facies (sands and shales, K4 unit), of Late Albian age. Successive broad epicontinental carbonate platforms (K5 unit) developed later (Alonso et al. 1993). The total thickness ranges between 450 and 550 m.

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Fig. 2. Geological map of the NW part of the Aragonese Branch. Location of Figures 3, 5 and 6 are shown. A stereogram of the poles of bedding planes around the cross-section of Figure 3 and their best-fit plane is also shown. The map is based on our own studies and those of Ruiz Fernańdez de la Lopa et al. (1991), Nararro Va´zquez (1991), Rey & Ramos (1991), Martıń Fernańdez & Esnaola Go´mez (1973) and Hernańdez Samaniego et al. (1980).

Mesozoic extensional structure Figure 3 shows partially restored sections from the pre-contractional stage to the stage before the Mesozoic deposition. The main basins and faults are represented in the section. From NE to SW, the faults on the section are as follows: the north Cameros fault, separating two very different Mesozoic successions, 700 m in the footwall and about 4000 m in the hanging wall; the Tablado fault, bounding the Moncayo basin during the Triassic (mostly the Buntsandstein); the south Cameros fault, separating two different Jurassic series. This fault, which is the continuation of the southern border of the Cameros basin, may separate a thicker Upper Jurassic–Lower Cretaceous series in the NE, now almost totally eroded after the strong Tertiary inversion. South of it, an area with Mesozoic faulted blocks 3–5 km wide is present (Fig. 3); the main faults are the Cardejoń fault (which is the SW boundary of the Jurassic and Lower Cretaceous rocks), the

Carabantes fault and the La Alameda fault, south of which no Mesozoic rocks older than Late Albian (K4 unit) are preserved. The original geometry of the deformed Mesozoic normal faults is inferred from their present attitude, as explained below. Two specific areas have been mapped in detail, because more detailed structures and relationships can be observed. Figure 5 shows a map of the Triassic and Lower Jurassic rocks of the southernmost Moncayo basin and its south-flanking Tablado fault. A local unconformity in the Muschelkalk (TrM) facies can be observed (Fig. 5, around point C), perhaps produced by NE tilting related to the Beratoń normal fault, antithetic to the Tablado fault. The Lowermost Jurassic Cortes de Tajunã Fm (L1, calcareous and dolomitic sedimentary breccias of Early Hettangian age) is not affected by several hectometre- to kilometrescale normal faults that affect the Triassic rocks. Low-angle unconformities can be observed between the Triassic and Jurassic rocks at outcrop scale.

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Fig. 3. (a) Geological section through the NW part of the Aragonese Branch (for location see Fig. 2), and restoration to: (b) the pre-contractional stage, showing normal faults that affect the youngest Cretaceous rocks; (c) the end of the Cretaceous; (d) prior to deposition of the K4 unit; (e) the end of the Jurassic; (f) the end of the Triassic; (g) prior to deposition of Upper Permian and Triassic rocks, assuming a flat topography.

The second area is located around the south Cameros fault (Fig. 8). North of the fault a 980 m thick marine Jurassic succession lies within the inverted Cameros basin. South of it, Mesozoic extensional and Tertiary contractional structures can be observed. The thickness of the Jurassic series (775 m at most) decreases to the north: in the two thrust sheets just south of the south Cameros fault, Upper Cretaceous rocks (Ks) lie on the Dogger (D2) and on the Liassic sequences (L1) at localities A and B (Fig. 6), showing the erosion of most of the Jurassic series. We explain this as the result of erosion of the uplifted footwall of the south Cameros fault. South of Ciria, several hectometre-scale faults bounding small grabens of Lower Cretaceous rocks (K1 and K2) can be observed. In the southern outcrops (around locality C, Fig. 6), the combination of the dip of beds (508 SW) and the NE topographic slope makes the map of this area a section perpendicular to the bedding. Up to 560 m

of terrigenous and calcareous Lower Cretaceous rocks are found in grabens bounded by NE–SW normal faults. A thinner series of these rocks occurs on top of horsts of Jurassic rocks. Tilting of these blocks to the SE can also be observed; Marine Lower Aptian and overlying clastic Lower Albian (K3) formations bury these structures, and are not affected by them. At locality D, a normal fault affects Upper Cretaceous rocks. From the partially restored sections, a total Mesozoic and post-Mesozoic extension between the pin points (La Alameda and north Cameros faults) of 3.9 km is deduced (Fig. 3g), over an initial length of 45 km. This implies a stretching factor (â) of 1.09. These values do not take into account extension produced by slip along the north Cameros fault; two end possibilities can be envisaged (Fig. 3b) assuming that: (1) the north Cameros fault had a SW dip of 608, in which case the additional extension would be 1.4 km and the total stretching factor 1.12; or (2)


Fig. 4. Palaeogeographical maps of the NW Aragonese Branch during the Mesozoic, and stratigraphic columns. (a) Triassic; (b) Jurassic; (c) Early Cretaceous. Names of the main faults and basins are shown. Grey patterns indicate basins or areas of differing sedimentary records. Dashed pattern indicates areas where no sediments of that age have been preserved.

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1980). This has been applied to the stratigraphic series of the two main sedimentary areas recognized: north of the south Cameros thrust (up to 4000 m of Mesozoic sediments), and north of the Cardejoń fault (up to 1900 m; Fig. 6), obtaining stretching values of 1.22 and 1.13, respectively.

Tertiary contractional stage Stratigraphy Almazań basin. This basin is filled with up to 3500 m of continental rocks, as can be deduced from seismic profiles (Guimera` et al. 1995; Casas Sainz et al. 2000). They can be grouped into five tectonosedimentary units (unconformitybounded units; Armenteros et al. 1989; Bond 1996; Casas Sainz et al. 2000). The four lower units are involved in the contractional structure and range from Palaeocene(?) to Early Miocene in age. They are generally covered by the youngest Neogene unit (up to 200 m of rocks), although towards the northern basin margin, within the study area, the four lower units have an extensive outcrop. Calatayud basin. The continental deposits that fill the Calatayud basin are not crossed by the present section, but appear in the southernmost part of Figure 2. They are of Mid–Late Miocene to Pliocene age. Ebro basin. Eight tectonosedimentary units of continental rocks have been recognized in the southern margin of the Ebro basin (Munõz-Jimeńez & Casas-Sainz 1997). The lower five sequences, ranging from the Palaeogene to Mid-Miocene in age, are affected by the Cameros thrust (Guimera` et al. 1995); the upper three sequences are of Late Miocene to Pliocene age (Munõz-Jimeńez & Casas-Sainz 1997) and unconformably overlie the northern Iberian Chain contractional structures.

Contractional structures

Fig. 5. Detailed geological map and cross-section of the Beratoń area. (For location, see Fig. 2.) Point C locates the intra-Muschelkalk angular unconformity.

during the Mesozoic the north Cameros fault had the same geometry as the Tertiary thrust, in which case an additional extension of 12.1 km would have been produced and the total Mesozoic stretching would be 1.35. These values are deduced assuming only macroscopic strain. Another independent way of estimating the Mesozoic extension is by analysing the basin subsidence using a back-stripping method (Sclater & Christie

Most of the contractional structures recognized are folds at several scales, and there are some thrusts. The largest folds have a wavelength of 5–13 km and involve basement. They are asymmetrical, NE-vergent and, as stated above, most of their frontal (NE) limbs contain faults that bound Mesozoic basins; these faults now have a high-angle dip, sometimes vertical, because they were mostly folded, rather than reactivated, during the Tertiary contraction. The Mesozoic layers, probably SW-tilted after the Mesozoic extension, were also folded. Many of these folds (La Alameda, Reznos and Sierra del Tablado anticlines; Fig. 2) have a back-limb dipping about 308 SW and can be explained by a fault propagation fold model (Suppe 1985) with a fault ramp of 308 SE, or a NE-vergent detachment fold model (Jamison 1987), which can have similar geometries at the surface. The Moncayo anticline cannot be explained as a Suppe’s fault propagation fold, because the dip of both fold limbs is far from the values required; a detachment fold or a fault-bend fold origin can be adduced in this case. Mesozoic (mostly Upper Cretaceous) and Tertiary cover rocks are also folded by hectometre-scale folds, most of them detached at the Escucha and Utrillas Formations. At the SW end of the section (Fig. 3) there are good examples of these folds. North of the Tablado fault, but only north of the Moncayo anticline in the section presented here, a pervasive cleavage is developed in the Mesozoic rocks; it is spaced in limestones and sandstones and more penetrative in shales (Gil Imaz 1999). This cleavage is roughly parallel to the axial plane of the folds


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Fig. 6. Detailed geological map of the Ciria area. (For location, see Fig. 2.)

described previously and has a reverse-fan disposition (Casas Sainz & Gil-Imaz 1998; Gil Imaz 1999). A very low-grade metamorphism, which developed chloritoid, affects cleaved rocks. Some chloritoids post-date the cleavage, and have been

dated by Mantilla Figueroa (1999) at about 50 Ma (Early–MidEocene). The recrystallization therefore developed during the Tertiary contraction. Only a few thrusts can be observed at surface, mostly NE-


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Fig. 7. Folding of a half-graben (a) by a fault propagation fold (b). (c, d) Sections of folded of Mesozoic normal faults, which bound the Mesozoic basins, by fault-propagation folds. (For location, see Fig. 3a).

vergent except for the largest one, which is the south Cameros thrust. This thrust accommodated the inversion of the south flank of the Cameros basin and separates two areas with very different Mesozoic sequences (Figs 2, 3 and 5). As a result, Triassic (Keuper) rocks in the hanging wall thrust onto Upper Cretaceous rocks in the footwall (Fig. 6). A complexly deformed area can be observed in its footwall (the same one that displays the complex Mesozoic extensional structure described above), containing folds and NE-vergent thrusts. One of those thrusts eventually cuts the south Cameros thrust (Fig. 6, point E). To draw the balanced cross-section and to restore it, linelength conservation of beds and Mesozoic basins bounded by extensional faults with moderate dip are assumed. Except perhaps for the north Cameros fault, as discussed above, no evidence for low-angle normal faults was found. The datum used is the base of the Upper Cretaceous sequence, because it is homogeneous wherever present. Constant thicknesses of Jurassic and Cretaceous rocks were assumed where the deep erosion has only left isolated remains of these rocks. To explain the basement-involved folds, fault propagation folding (Suppe 1985) has been assumed. The asymmetry of the folds and the limb dips

support this model (Fig. 7c and d). Figure 7a and b shows the folding of a half-graben by a fault propagation fold; the initially normal fault became almost vertical and the layers tilted against the fault changed their sense of dip to form a syncline in the hanging wall of the folded fault. From this model, the geometry of the deformed Mesozoic normal faults has been reconstructed from their present disposition assuming passive deformation by shear parallel to the frontal limb of the fault-propagation fold, whereas the Mesozoic beds are assumed to be folded by layerparallel slip. The section has been balanced and restored, for the contractional structures, between two pins (Fig. 3a), one located in the Almazań basin (in the SW), perpendicular to the beds, and the other in the north Cameros thrust (in the NE). The shortening between the two pin lines is 16.6 km (21%). This value refers only to the internal deformation of the thrust sheet, produced by the major structures, without considering the north Cameros thrust displacement. This displacement cannot be easily estimated, because of the post-tectonic Tertiary cover and the lack of good seismic profiles in that area of the Ebro basin. From the cross-section, a minimum of 8.9 km of horizontal shortening can be deduced taking the reference of the top of the Upper Cretaceous sequence at the frontal ramp of the thrust (Fig. 3). To estimate the total displacement of the north Cameros thrust sheet, we utilize the geometry assumed for its sole thrust. Figure 8 shows two possible depth continuations of the north Cameros thrust. The overall structure of the cross-section can be summarized as a flat anticlinorium about 46 km wide (the flat top dipping 1.58 SW, derived from the altitude of the basement at the Alameda and Moncayo anticlines, as shown in Fig. 8a) with an emergent thrust in its northern part, and the substratum of the two bounding Tertiary basins at a similar depth (about 2.5 km). The first possibility assumes that the ‘flat anticlinorium’ is the result of an initial geometry involving one flat and one large ramp (Fig. 8b). In that case the total displacement of the thrust sheet would be the width of the rear (SW) monocline; this is about 9 km, a value smaller than the internal shortening of the thrust sheet deduced from the surface structures (16.6 km). That possibility should therefore be rejected. The second possibility assumes an initial geometry consisting of a double flat–ramp and that the rear monocline is a fault-bend fold on the ramp. In this case a 37 km total displacement of the thrust sheet is implied. Taking into account the 16.6 km of the thrust sheet internal shortening, this means a total slip of 53.6 km for the thrust sheet. Such a total slip implies that a flat of that width continues to the south beneath the Castilian Branch, underlying most of the SW arch of the Iberian Chain (Fig. 9), and also that the thrust sheet involves only the uppermost part of the crust. No intermediate thrust geometries between the two proposed are reasonable, because to produce the flat core of the anticlinorium they would imply a perfect fit of different ramps and flats in both hanging wall and footwall. The structure of the southern part of the section (Fig. 9) is derived from the interpretation of seismic reflection profiles for the Almazań basin (Guimera` et al. 1995; Casas Sainz et al. 2000), geological maps at 1:50 000 scale for the Castilian Branch (Adell Argile´s et al. 1981, 1982) and seismic reflection profiles for the Tajo basin (Querol Muller 1989). The additional shortening to the SW of Figure 3a, in the section of Figure 10, is 13 km, so the total shortening of the section between the Ebro and the Tajo basins is 66.6 km (Fig. 9). The temporal and geometric relationship between the structures studied and the Tertiary rocks in the NW Iberian Chain indicates that contractional deformation started during the Eocene and lasted until Mid-Miocene time. A cover of undeformed


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Fig. 8. (a) Outline of the overall structure of the Aragonese Branch along the cross-section of Figure 3. (b, c) Two possibilities for sole thrust geometry deduced from the overall structure.

Fig. 9. Geological cross-section through the NW Iberian Chain, from the Ebro basin to the Tajo basin. The subcrop structure of the Almazań basin is based on the interpretation of the seismic reflection profile A8011. (For location, see Fig. 1b.)

Middle–Upper Miocene to Pliocene rocks is present in all the Tertiary basins (Guimera` et al. 1995; Munõz-Jimeńez & Casas Sainz 1997; Casas-Sainz et al. 2000). In the Iberian Chain, the generalized transport direction has been estimated to be roughly to the north (0–108E), (Guimera` & ´ lvaro 1990; Casas-Saínz 1993; Guimera` et al. 1995). The A cross-sections of Figures 3 and 10 are drawn perpendicular to the NW–SE-striking structures of the Aragonese and Castilian branches, so they are oblique to the generalized transport direction. We assume that the transport direction on the NW–

SE-striking north and south Cameros thrusts, in the Aragonese Branch, is oblique to their strike, so they are oblique ramps. We do not know the value of that obliquity, because of the commonly observed change of transport direction between frontal and oblique ramps. Hence, the shortening calculated in the crosssection is less than the shortening parallel to the transport direction. Taking into account the dimensions and highly cylindrical nature of the folds present in the area studied (from the bedding data included in the stereogram of Fig. 2, a half-apical angle of 88.58 is deduced), the strike-slip component of faults

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Fig. 10. (a) Gravity profile through the NW Iberian Chain. (b) Density model of the crust, deduced from (a). After Salas & Casas (1993). (For location, see Fig. 1b.)

during the Tertiary contraction would not produce significant changes in the shortening calculated parallel to the cross-section.

Discussion on the major-scale structure of the NW Iberian Chain Information on the crustal structure of the Iberian Chain can be obtained from the Bouguer anomaly maps compiled by Salas & Casas (1993) and Mezcua et al. (1996). These maps show that the Iberian Chain is associated with a regional gravity low (a minimum of less than 110 mGal) that contrasts with the relative gravity high of the Ebro, Duero and Tajo basins ( 30 to 40 mGal) and the pronounced high of the Valencia Trough, in the western Mediterranean. The large negative anomalies of the Iberian Chain are consistent with a thickened crust that reaches a maximum thickness of 43 km in the Sierra de Albarracıń area, about 150 km SSE of the study section (Salas & Casas 1993). This crustal thickness has been interpreted as the result of the crustal thickening during Tertiary contraction (Salas & Casas 1993; Guimera` et al. 1996). To explain this thickening a thrust ´ lvaro 1990; system involving only the upper crust (Guimera` & A Salas et al. 2001) or the whole crust (Salas & Casas 1993; Casas Sainz et al. 2000) has been proposed. Using direct modelling of gravity data, Salas & Casas (1993) obtained a gravity profile (Fig. 10) that is roughly parallel to the

cross-section of Figure 9 (see Fig. 1b for location of both). In this profile, the Moho lies at a depth of about 40 km in the Aragonese and Castilian branches, whereas there is a Moho high below the Almazań and Ebro basins, where it has a depth of 34 km. A seismic wide-angle reflection profile through the Demanda area (Pedreira et al. 2003; see Fig. 1 for location) displays a thickening of the middle crust beneath the frontal thrusts of the Iberian Chain, with Moho depths of 40–42 km beneath the mountain front. In the Duero basin, west of the Iberian Chain structures, some 400 km to the NW of the study section, the Moho depth is 31 km, as deduced from wide-angle seismic and gravity profiles (Fernańdez Viejo 1998, figs 3.12 and 3.17). In the north–central Ebro basin, at the southern end of the Ecors–Pyrenees deep reflection seismic profile, the Moho depth is 34 km and there is a 2 km thickness of Tertiary basin infill (Bera´stegui et al. 1993). The substratum of the Tertiary basins in the cross-section of Figure 9 has a maximum depth of 2.2 km below sea level in the Tajo and Almazań basins and 2.7 km in the Ebro basin (Figs 3 and 10). In the Ebro basin, flexure of the crust was produced mainly by the Pyrenean load, but also by the emergent thrust sheet of the Aragonese Branch (Millań et al. 1995). The Tajo basin is not represented in this gravity profile, but the Salas & Casas (1993, fig. 5) gravimetry map shows a lower anomaly there than in the Loranca basin, and the pre-Tertiary substratum


is below 2 km. Both are consistent with a thinner crust. Independent evidence for a crustal thickening below the Aragonese and Castilian branches is the average elevation, which is 1700–2300 m in the former region and 1200 m in the latter. In summary, the major structure of the NW Iberian Chain consists of two NW–SE-oriented major anticlinoria separated by a synclinorium. The lower-crustal structure mimics the surface structure of the cross-section of Figure 9; it shows two crustal roots under the anticlinoria and a thinner crust under the synclinorium, indicating a locally isostatically compensated crust. The chain is bounded by a major thrust to the north (the north Cameros or north Iberic thrust), which, as deduced in this work, involves only the upper crust. To explain the crustal structure interpreted from the gravity profile in terms of the structure deduced from the cross-section, two hypotheses are proposed (Fig. 11). Both assume that (1) the Castilian Branch anticlinorium (the SW arch) is a north-vergent fault-bend fold on a ramp-and-flat thrust geometry similar to that shown in Figure 9, and (2) the crustal thickness was 30 km prior to the crustal thickening during the Tertiary contraction (based on the evidence of the Mesozoic extension and the passage from marine to continental rocks at the end of the Cretaceous, as discussed by Guimera` et al. (1996) and Salas et al. (2001)). The first hypothesis assumes a double ramp-flat geometry for an upper-crustal thrust deepening to the SW (Fig. 11a, 1). The displacement of the thrust sheet produces two anticlines and a syncline (Fig. 11a, 2). The rear (SW) anticline shows a geometry and dimensions and implies a crustal thickening similar to that deduced in the gravity profile for the Castilian Branch, whereas beneath the frontal anticline the crustal thickening is less than that deduced in the Aragonese Branch. Assuming another coeval thrust, with a flat–ramp–flat geometry on the footwall of the previous one, a crustal thickening similar to that deduced from the gravity profile is obtained (Fig. 11a, 3). To obtain a more realistic section, 3 km of Tertiary rocks have been added on the sectors where Tertiary basins are present, and local isostatic compensation is applied. The result of this simple model (Fig. 11a, 4) shows: (1) two crustal roots below the two anticlines

Hypothesis 1

301

similar to those in the gravity profile, (2) relief in the anticlines similar to that observed in the Iberian Chain; (3) no tectonic crustal thickening below the NE and SW Tertiary basins and at the centre of the synclinal basin. Tertiary sediments cause subsidence of the basin substrata 2.3 km below sea level, as a result of the sediment load. This hypothesis implies that the lower part of the crust is not shortened beneath the Iberian Chain, but it is displaced southwards, towards the Betic orogen, in southern Spain. The second hypothesis assumes a crustal wedge thinning to the south containing several flats and ramps (Fig. 11b, 1). By displacing this wedge (Fig. 11b, 2), and adding 3 km of sediment in the low areas and applying local isostatic compensation, a crustal thickness similar to that of the first hypothesis is obtained (Fig. 11b, 3). In this case, the lower part of the crust is displaced to the north, to the Pyrenees. Both hypotheses explain the dominant north vergence of the Iberian Chain, with the major thrust bounding it to the north, and both illustrate how the surface structure and the crustal thickness in the Iberian Chain can be explained in a simple manner by means of a thrust system with a simple flat-and-ramp geometry involving only the upper part of the crust. No significant internal deformation of the thrust sheets, either the hanging wall or the footwall, is needed to produce the large anticlinoria, the syncline separating them, and the crustal roots below the former, although internal deformation can occur. In the cross-section studied, the internal deformation of the thrust sheet is mainly located in its frontal part, in the Aragonese Branch, and is about half of the total displacement of the thrust sheet. To discuss whether the hypotheses presented are compatible with data outside the Iberian Chain, we have to take into account that the Pyrenees deformed during the Palaeogene and Early Miocene (Munõz 1992; Beaumont et al. 2000), when subduction of the lower crust occurred, coeval with the deformation of the Iberian Chain. This prompted Banks & Warburton (1991) and ´ lvaro (1990) to associate the Iberian Chain Guimera` & A deformation with that of the Pyrenees by means of a mid-crustal detachment. The Betic orogen involved the Iberian plate during

Hypothesis 2

A

B

30 km

1

1

30 km

30 km

2 65 km

30 km

30 km 40 km

Relief = 1.4 km

41.9 km

2

30 km

3

Relief = 1.4 km

Relief = 1.9 km

0

40 km 30 km

3

30 km

43 km

30 km

r = 2.74 r = 3.2

30 km 40 km

65 km

Rocks sedimented = 3 km Subsidence = 2.3 km Relief = 0.7 km

0 r = 2.74 r = 3.2

43 km

40 km

30 km

Rocks sedimented = 3 km subsidence = 2.3 km Relief = 0.7 km Relief = 1.7 km

30 km

30 km

30 km 41.9 km

4

Continental crust (including Mesozoic)

Fig. 11. Two hypotheses on the crustal structure of the NW Iberian Chain resulting from the Tertiary contraction.

Tertiary

302


the Late Aquitanian and Serravallian (De Ruig et al. 1987; Ott d’Estevou et al. 1988; Fontbote´ et al. 1990; Roca et al. 1996), later than the deformation of the Iberian Chain. Nevertheless, subduction of Africa beneath Iberia starting in the Late Eocene has been proposed (Zeck 1996; Casas Sainz & Faccenna 2001). Both subductions north and south of Iberia are consistent with the kinematics of the plate boundaries between Eurasia, Iberia and Africa proposed by Roest & Srivastava (1991), who deduced that between the late Eocene and latest Oligocene Iberia acted as an independent plate, having two convergent plate boundaries in the Pyrenees and the Betics. Summing up, to both the north and the south of the Iberian plate, convergent plate boundaries were active during the deformation of the Iberian Chain and, hence, the Iberian Chain thrust system can be related to either of them or even to both. In the latter case, a combination of our two hypotheses should be envisaged. The total Alpine shortening estimated from the study crosssection (66.6 km, 26%) is similar to the 75 km (20.5%) 12 km of shortening estimated by Guimera` et al. (1996) and Salas et al. (2001) in a crustal area balance by comparing a gravity crustal profile of Salas & Casas (1993), about 150 km SE of the study cross-section, with its restoration to latest Mesozoic time. Comparing these values with those obtained in the Pyrenees (165 km or 44% of shortening in the central Pyrenees, Beaumont et al. 2000; 75–80 km or 37% in the western Pyrenees, Teixell 1998), the contribution of the Iberian Chain shortening to the convergence between African and Eurasian plates during the Tertiary is about one-half of the mean shortening of the Pyrenees, and this should be taken into account in any reconstruction of the kinematics of these plates.

Conclusions The NW Iberian Chain, and by implication the whole Iberian Chain, is an example of the evolution of an intraplate area, beginning with a Mesozoic extensional period and ending with a Tertiary contractional event as a result of the collision between Iberia and Europe that gave rise to the Pyrenees. Mesozoic rocks in the Aragonese Branch were deposited in basins bounded by normal faults, which have been recognized and mapped in the field, and which display thickness variations produced by this setting. These basins changed their dimensions with time. Mesozoic extension can be estimated from major structures to have a minimum stretching of 1.12 and a maximum of 1.35. Using a back-stripping analysis, the stretching values vary from 1.13 to 1.22. Tertiary contraction in the Aragonese Branch produced a basement-involved thrust sheet about 5 km thick, which implies a total shortening of 53.6 km in the cross-section direction (NE– SW). The geometry of the sole thrust is inferred to have a flat of 37 km width dipping 1.58 to the south, joining two ramps dipping 208 to the south. Internal deformation of the thrust sheet is responsible for 16.6 km of the total shortening by means of thrusts formed by the inversion of the Mesozoic basin-bounding normal faults, as well as by newly formed thrusts and folds, all of which involved the Hercynian basement. Many Mesozoic normal faults were not inverted, but instead were deformed in the frontal limbs of asymmetric, NE-verging folds, interpreted as fault-propagation folds associated with blind thrusts. The total Tertiary shortening estimated in a NE–SW section across the entire NW Iberian Chain is 66.6 km (26%). The sole thrust geometry deduced in the Aragonese Branch cross-section indicates that the surface structure and the crustal thickness of the Iberian Chain can be explained by a major upper- and mid-

crustal thrust system with simple flat-and-ramp geometry; the upper thrust of the crustal system forms the sole thrust of the superficial structures. This thrust system can be related to the Pyrenean or to the Betic orogen by means of a mid-crustal detachment. The hypothesis presented illustrates how the surface structure and the crustal thickness in the Iberian Chain can be explained in a simple manner by means of a thrust system with a simple flatand-ramp geometry involving only the upper part of the crust. No internal deformation of the hanging wall or the footwall of this crustal-scale thrust is needed to produce the crustal thickening, although such internal deformation can also occur. The contribution of the Iberian Chain shortening to the convergence between African and Eurasian plates during the Tertiary is about one-half of that of the Pyrenees, and this should be taken into account in any reconstruction of the kinematics of these plates. We are grateful to T. Lawton, A. Teixell, H. de Boorder, J. A. Munõz and an anonymous reviewer for their suggestions, which improved the original manuscript. This work was funded by the projects BTE200204453-CO2 and REN2001-1734-CO3-03/MAR of the MCyT and 2001SGR-000074 (Generalitat de Catalunya, Grup de Recerca de Geodina`mica I Ana`lisi de Conques).

References Adell Argile´s, F., Gonza´lez Lodeiro, F. & Tena-Da´vila Ruiz, F. 1981. Mapa Geolo´gico de Espanã, 1:50 000. Nu´m 461 (Siguënza). Instituto Tecnolo´gico Geominero de Espanã, Madrid. Adell Argile´s, F., Gonza´lez Lodeiro, F. & Tena-Da´vila Ruiz, F. 1982. Mapa Geolo´gico de Espanã, 1:50 000. Nu´m 434 (Barahona). Instituto Tecnolo´gico Geominero de Espanã, Madrid. Alonso, A. & Mas, J.R. 1988. La transgresioń aptiense al sur del Moncayo. Lı´mites de las Provincias de Soria y Zaragoza. In: Volumen de Comunicaciones del III Congreso Geolo´gico de Espanã, Granada, 11–14. ´ lvaro, M., Capote, R. & Vegas, R. 1979. Un modelo de evolucioń geotectońica A para la cadena Celtibe´rica. Acta Geologica Hispanica, 14, 172–181. Armenteros, I., Dabrio, C.J., Guisado, R. & Sańchez de Vega, A. 1989. Megasecuencias sedimentarias del Terciario del borde oriental de la Cuenca de Almazań (Soria–Zaragoza). In: Dabrio, C.J. (ed.) Paleogeografia de la Meseta Norte durante el Terciario. Studia Geologica Salmanticensa, 5, 107–127. Alonso, A., Floquet, M., Mas, R. & Meleńdez, A. 1993. Late Cretaceous Carbonate Platforms: Origin and evolution. Iberian Range, Spain. In: Simo´, T., Scott, R.W. & Masse, J.P. (eds) Cretaceous Carbonate Platforms. AAPG Memoirs, 56, 297–316. Arribas, J. 1985. Base litoestratigra´fica de las facies Buntsandstein y Muschelkalken la rama aragonesa de la Cordillera Ibe´rica (Zona Norte). Estudios Geolo´gicos, 41(1-2), 47–57. Aurell, M., Meleńdez, A. & San Romań, J. et al. 1992. Tectońica sinsedimentaria distensiva en el lı´mite Tria´sico–Jura´sico en la Cordillera Ibe´rica. In: III Congreso Geolo´gico de Espanã y VIII Congreso Latinoamericano de Geologia, Salamanca, Actas. 1, 50–54. Banks, C.J. & Warburton, J. 1991. Mid-crustal detachment in the Betic and Pyrenean systems of eastern Spain. Tectonophysics, 191, 275–289. Beaumont, C., Munõz, J.A., Hamilton, J. & Fullsack, P. 2000. Factors controlling the Alpine evolution of the central Pyrenees inferred from a comparison of observations and geodynamical models. Journal of Geophysical Research, 105(B4), 8121–8145. Bera´stegui, X., Losantos, M., Munõz, J.A. & Puigdefa`bregas, C. 1993. Tall geolo`gic del Pirineu central. Servei Geolo`gic de Catalunya, Barcelona. Bond, J. 1996. Tectono-sedimentary evolution of the Almazań Basin, NE Spain. In: Friend, P.F. & Dabrio, C. (eds) Tertiary Basins of Spain: the Stratigraphy Record of Crustal Kinematics. Cambridge University Press, Cambridge, 203–213. Casas Sainz, A.M. 1993. Oblique tectonic inversion and basement thrusting in the Cameros Massif (Northern Spain). Geodinamica Acta, 6(3), 202–216. Casas Sainz, A.M. & Faccenna, C. 2001. Tertiary compressional deformation of the Iberian plate. Terra Nova, 13, 281–288. Casas Sainz, A.M. & Gilmaz, A. 1998. Extensional subsidence, contractional folding and subsequent inversion of the Eastern Cameros Basin (NW Spain). Geologische Rundschau, 86, 802–818. Casas Sainz, A.M., Corte´s-Gracia, L. & Maestro-Gonza´lez, A. 2000.

I N T R A P L AT E D E F O R M AT I O N, N W I B E R I A Intraplate deformation and basin formation during the Tertiary within the northern Iberian plate: origin and evolution of the Almazań basin. Tectonics, 19(2), 258–289. De Ruig, M.J., Mier, R.M. & Stel, H. 1987. Interference of compressional and wrenching tectonics in the Alicante region, SE-Spain. Geologie en Mijnbouw, 66, 201–212. Fernańdez Viejo, G. 1998. Estructura cortical de la Cordillera Canta´brica y su transicioń a la Cuenca del Duero a partir de datos de sı´smica de refraccioń/ reflexioń de gran ańgulo. Doctoral thesis, Universitat de Barcelona. Fontbote´, J.M., Guimera`, J., Roca, E., Sa`bat, F., Santanach, P. & Fernańdez-Ortigosa, F. 1990. The Cenozoic geodynamic evolution of the Valencia trough (western Mediterranean). Revista de la Sociedad Geolo´gica de Espanã, 3, 249–259. Gil Imaz, A. 1999. La estructura de la Sierra de Cameros: deformacioń dućtil y su significado a escala cortical. PhD thesis, Universidad de Zaragoza. Guimera`, J. 1984. Palaeogene evolution of deformation in the north-eastern Iberian Peninsula. Geological Magazine, 121, 413–420. ´ lvaro, M. 1990. Structure et e´volution de la compression alpine Guimera`, J. & A dans la Chaıˆne Ibe´rique et la Chaıˆne coˆtie`re catalane (Espagne). Bulletin de la Socie´te´ Geólogique de France, 6(2), 339–348. Guimera`, J., Alonso, A. & Mas, R. 1995. Inversion of an extensional-ramp basin by a newly formed thrust: the Cameros basin (N Spain). In: Buchanan, J.G. & Buchanan, P.G. (eds) Basin Inversion. Geological Society, London, Special Publications, 88, 433–453. Guimera`, J., Salas, R., Verge´s, J. & Casas, A. 1996. Mesozoic extension and Tertiary compressive inversion in the Iberian Chain: results from the analysis of a gravity profile. Geogaceta, 20(7), 1691–1694. Hernańdez Samaniego, A., Aragone´s Valls, E., del Olmo Zamora, P., Aguilar Toma´s, M. & Ramı´rez del Pozo, J. 1980. Mapa Geolo´gico de Espanã, 1:50 000. Nu´m 352 (Tabuenca). Instituto Tecnolo´gico Geominero de Espanã, Madrid. Jamison, W.R. 1987. Geometric analysis of fold development in overthrust terranes. Journal of Structural Geology, 9(2), 207–219. Lanaja, J.M. 1987. Contribucioń de la exploracioń petrolı´fera al conocimiento de la geologıá de Espanã. IGME, Servicio de Publicaciones del Ministerio de Industria y Energia, Madrid. Mantilla Figueroa, L. C. 1999. El metamorfismo hidrotermal de la Sierra de Cameros (La Rioja—Espanã): petrologıá, geoquı´mica, geocronologıá y contexto estructural de los procesos de interaccioń fluı´do-roca. PhD thesis, Universidad Complutense de Madrid. Martıń Fernańdez, M. & Esnaola Go´mez, J.M. 1973. Mapa Geolo´gico de ´ lvega). Instituto Tecnolo´gico Geominero de Espanã, 1:50 000. Nu´m 351 (O Espanã, Madrid. Mezcua, J., Gil, A. & Benarroch, R. 1996. Estudio Gravime´trico de la Penıńsula Ibe´rica y Baleares. Ministerio de Fomento, Instituto Geogra´fico Nacional, Madrid. Millań, H., Den Bezemer, T. & Verge´s, J. et al. 1995. Present-day and middle Lutetian flexural modelling in the eastern Pyrenees and Ebro basin. Marine and Petroleum Geology, 12, 917–928. Munõz, J.A. 1992. Evolution of a continental collision belt: ECORS Pyrenees crustal balanced cross section. In: McClay, K.R. (ed.) Thrust Tectonics. Chapman and Hall, New York, 235–246. Munõz-Jimeńez, A. & Casas-Sainz, A.M. 1997. The Rioja Trough (N Spain): tecto-sedimentary evolution of a symmetric foreland basin. Basin Research, 9, 65–85. Navarro Va´zquez, D. 1991. Mapa Geolo´gico de Espanã, 1:50 000. Nu´m 380

303

(Borobia). Instituto Tecnolo´gico Geominero de Espanã, Madrid. Ott d’Estevou, Ph., Montenat, Ch., Ladure, F. & Pierson d’Autrey, L. 1988. E´volution tectono-se´dimentaire du domaine pre´be´tique oriental (Espagne) au Mioce`ne. Comptes Rendus de l’Acade´mie des Sciences, Se´rie II, 307, 789–796. Pedreira, D., Pulgar, J.A., Gallart, J. & Dıáz, J. 2003. Seismic evidence of alpine crustal thickening and wedging from the western Pyrenees to the Cantabrian Mountains (north Iberia). Journal of Geophysical Research, 108(B4), 2204 10.1029/2001JB001667. Querol Muller, R. 1989. Geologıá del subsuelo de la Cuenca del Tajo. Escuela Tećnica Superior de Ingenieros de Minas de Madrid, Departamento de Ingenierıá Geolo´gica, Madrid. San Romań Saldanã, J. 1994. Estudio hidrogeolo´gico del interfluvio Queiles– Jaloń (Zaragoza). PhD thesis, Universidad de Zaragoza. Rey, D. & Ramos, A. 1991. Estratigrafıá y sedimentologıá del Pe´rmico y Tria´sico del sector Deza–Castejoń (Soria). Revista de la Sociedad Geologica de Espanã, 4(1-2), 105–125. Roca, E., Guimera`, J. & Salas, R. 1994. Mesozoic extensional tectonics in the southeast Iberian Chain. Geological Magazine, 131(2), 155–168. Roca, E., Anadoń, P., Utrilla, R. & Va´zquez, A. 1996. Rise, closure and reactivation of the Bicorp–Quesa evaporite diapir, eastern Prebetics, Spain. Journal of the Geological Society, London, 153, 311–321. Roest, W.R. & Srivastava, S.P. 1991. Kinematics of the plate boundaries between Eurasia, Iberia, and Africa in the North Atlantic from the Late Cretaceous to the present. Geology, 19, 613–616. Ruiz Fernańdez de la Lopa, V., Lendıńez Gonza´lez, A. & Martıń Herrero, D. 1991. Mapa Geolo´gico de Espanã, 1:50 000. Nu´m 408 (Torrijo de la Canãda). Instituto Tecnolo´gico Geominero de Espanã, Madrid. Salas, R. & Casas, A. 1993. Mesozoic extensional tectonics, stratigraphy, and crustal evolution during the Alpine cycle of the eastern Iberian basin. Tectonophysics, 228, 33–55. Salas, R., Guimera`, J., Mas, R., Martıń-Closas, C., Meleńdez, A. & Alonso, A. 2001. Evolution of the Mesozoic Central Iberian Rift System and its Cainozoic inversion (Iberian Chain). In: Ziegler, P.A., Cavazza, W., Robertson, A.H.F. & Crasquin-Soleau, S. (eds) Peri-Tethys Memoir 6: Peri-Tethyan Rift/Wrench Basins and Passive Margins. Me´moires du Museúm National de l’Histoire Naturelle, 186, 145–185. San Romań Saldanã, A. 1994. Estudio hidrogeolo´gico del interfluvio Queiles– Jaloń (Zaragoza). PhD thesis, Universidad de Zaragoza. Sańchez Navarro, J.A., San Romań Saldanã, J., de Miguel Cabeza, J.L. & Martinez Gil, F.J. 1990. El drenaje subterrańeo de la Cordillera Ibe´rica en la depresioń del Ebro: aspectos geolo´gicos. Geogaceta, 8, 115–117. Sclater, J.M. & Christie, P.A.F. 1980. Continental stretching: an explanation of the post Mid-Cretaceous subsidence of the central North Sea basin. Journal of Geophysical Research, 85, 3711–3739. Suppe, J. 1985. Principles of Structural Geology. Prentice–Hall, Englewood Cliffs, NJ. Teixell, A. 1998. Crustal structure and orogenic material budget in the west central Pyrenees. Tectonics, 17(3), 395–406. Vilas, L., Alonso, A., Arias, C., Garcıá, A., Mas, J.R., Rincoń, R. & Meleńdez, N. 1983. The Cretaceous of the south-western Iberia ranges (Spain). Zitteliana, 10, 245–254. Zeck, H.P. 1996. Betic–Rif orogeny: subduction of Mesozoic Tethys lithosphere under eastward drifting Iberia, slab detachment shortly before 22 Ma, and subsequent uplift and extensional tectonics. Tectonophysics, 254, 1–16.

Received 16 April 2003; revised typescript accepted 24 October 2003. Scientific editing by Alex Maltman