Ranges and Basins in the Iberian Peninsula: Their Contribution to the Present Topography

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Ranges and basins in the Iberian Peninsula: Their contribution to the present topography

Article in Geological Society London Memoirs · January 2006 DOI: 10.1144/GSL.MEM.2006.032.01.13

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JAUME VERGE´ S & MANEL FERNA` NDEZ Group of Dynamics of the Lithosphere (GDL), Institute of Earth Sciences ‘Jaume Almera’, CSIC, 08028 Barcelona, Spain (e-mail: [email protected])

Abstract: The Iberian Peninsula, at the western end of the Alpine–Himalayan Belt, displays a complex structure with mountain ranges of diverse structural trends and sedimentary basins between them. The Iberian Peninsula also shows an elevated mean topography, the highest in Europe. In this short paper, we investigate the Alpine evolution of the Iberian Peninsula since Mesozoic times, when Iberia was isolated as an independent plate. This occurred from Albian (formation of the northern plate boundary) to Oligocene times (end of the Pyrenean Orogeny). Iberia was squeezed between Africa and Europe during Tertiary times and all previously established Mesozoic extensional basins were inverted, as were some of the Hercynian structures. The opening of the Valencia Trough, cutting the eastern margin of the Iberian Peninsula, began in Oligocene times. Concomitant crustal and lithospheric stretching during the Neogene along the eastern margin of Iberia produced limited uplifts, some of which are still active. The modern topography of the Iberian Penin- sula was developed mainly as the result of three main tectonothermal mechanisms since late Palaeozoic times: variations in crustal densities, and possibly mantle depletion, inherited from the Hercynian Orogeny; crustal and lithospheric thickening during Tertiary compression; and upper mantle thinning during the Neogene–Quaternary.

The Iberian Peninsula constitutes the westernmost segment of the between Africa and Iberia changes progressively from pure right- 12 000 km long Alpine–Himalayan Belt formed as a result of lateral strike-slip along the Gloria Fault to a diffuse transpressive the Tertiary closure of the Tethys Ocean during the collision of boundary from the Gorringe Bank to the Gulf of Cadiz region India, Arabia and Africa with Asia and Europe (e.g. Dercourt (e.g. Argus et al. 1989). et al. 1986). The present structure of the Iberian Peninsula developed through Rifting initiated during Triassic times (c. 250 Ma) and culmi- the interplay of several geodynamic processes related to the Atlan- nated in crustal break-up along the Atlantic margin (e.g. Ziegler, tic opening, the formation of two plate boundaries limiting the 1988, 1992). Continental break-up of the African Plate occurred Iberian Plate, the north–south Africa–Europe convergence, and during the Late Jurassic (c. 156 Ma). The Atlantic Ocean propa- the concomitant rapid retreat and consumption of the oceanic gated northwards through the proto-Azores–Gibraltar plate Tethyan realms. Different geodynamic processes related to these boundary, producing the continental rupture of the Iberian Plate large-scale tectonic events were to some extent coeval over particu- in Early Cretaceous times (c. 118 Ma; e.g. Srivastava et al. lar morphotectonic regions. Both the diversity of geodynamic 1990). The mid-Cretaceous northern boundary of the Iberian processes and their potential conjunction complicate the unravelling Plate formed along the oceanic lithosphere of the Atlantic Ocean of the evolution of the Iberian Plate in general and, in particular, the and its eastern continuation along the continental lithosphere of southern plate boundary between Iberia and Africa (Betic the Pyrenees (Fig. 1). Cordillera, Rif, Alboran Sea and Gulf of Cadiz tectonic units). Towards the end of Late Cretaceous (chron 33, 80 Ma) Africa This paper documents in brief the Alpine evolution of the onshore shifted its motion northwards, initiating convergence with Iberian Peninsula mountain ranges and sedimentary basins, Eurasia with the consumption of the Tethys Ocean (e.g. Dercourt emphasizing the geodynamic processes that created positive topo- et al. 1986). At the westernmost termination of the Alpine– graphic relief. This evolution took involved the following major tec- Himalayan Belt, the Iberian Plate underwent a protracted defor- tonic events: (1) formation of extensional Mesozoic basins at the mation phase, resulting in orogenic belts along the plate bound- intersection of the proto-Atlantic and the Tethys oceans; (2) gener- aries (Bay of Biscay–Pyrenees and Azores–Gibraltar) and ation of Late Cretaceous–Tertiary fold-and-thrust belts and basins severe intraplate deformation. Several large Tertiary sedimentary by the northwards motion of Africa; (3) formation of basins by basins developed on the Iberian Plate close to the bounding moun- Neogene extension along the eastern margin of the proto-Western tain chains (Friend & Dabrio 1996). Most of these basins began as Mediterranean. The paper concludes with the present topographic flexural basins and continued as intermontane basins during the configuration of the Iberian Peninsula and its heritage from Hercy- growth of the complex Iberian mountain system (Fig. 1). nian times including the relatively recent lithospheric thinning Iberia initially moved together with the African Plate, from latest along the Mediterranean province of Spain. Cretaceous to mid-Eocene times (chron 19, 42 Ma), deforming Two recently published books on the geology of Spain give a mainly the Bay of Biscay–Pyrenees plate boundary. From mid- detailed description of the mountains and basins documented in Eocene to the end of Oligocene times (chron 6c, 24 Ma), it moved this brief paper (Gibbons & Moreno 2002; Vera 2004). Andeweg independently and both plate boundaries were active. Subsequently, (2002) has also illustrated the evolution of the palaeostress field during the last 24 Ma, most of the deformation was accommodated in the Iberian Peninsula through the Cenozoic. along the complex and poorly understood plate boundary between Iberia and Africa, leading to the formation of the Betics, the Gibraltar Arc, and the Rif. The end of the Oligocene also coincided with exten- Mesozoic extensional basins sion along the proto-Western Mediterranean Sea, which affected the entire eastern margin of the Iberian Plate. This extension formed the Preceding the opening of the central Atlantic during mid-Jurassic oceanic lithosphere below the Liguro-Provençal Basin north of times (chron BSMA at c. 170 Ma), the Iberian Peninsula (Iberian the Paul Fallot Fault. To the south of this fault, thinned lithosphere Plate) was deformed by large-scale stretching that resulted in below the Valencia Trough and Alboran Sea and oceanic lithosphere numerous extensional basins with different orientations. Rift below the Algeria Basin formed (Fig. 1). The present contact systems developed along the western margin of the Iberian Plate

From:GEE,D.G.&STEPHENSON, R. A. (eds) 2006. European Lithosphere Dynamics. Geological Society, London, Memoirs, 32, 223–234. 0435-4052/06/$15.00 # The Geological Society of London 2006. 223 224 J. VERGE´ S & M. FERNA` NDEZ

The separation between Europe and Africa for this period is about 240 km along this transect (e.g. Boccaletti et al. 1977; Olivet 1996; see position B for Africa in Fig. 2). The proposed position of Africa provides very little room for the restored Betic domain, thus creating a significant space problem, which has already been recognized (e.g. Andrieux et al. 1971; Mauffret et al. 1989; Frizon de Lamotte et al. 1991; Lonergan & White 1997; Spakman & Wortel 2000). Andrieux et al. (1971) proposed a model, still used with modifications by a number of workers, in which the Alboran Block was displaced towards the west by lateral extrusion during the north–south convergence of Africa and Iberia. Crustal and lithospheric thinning was the common process that formed the extensional basins. The thinned regions constituted weaker zones at the end of the Cretaceous just before the onset of Tertiary compression. Most of these extensional basins were tectonically inverted, preserving their original basin orientation. An extensive distribution of Triassic evaporites controls the geometry of the thrust system in both previous tectonic basins and structural highs, as in the case of the southern end of the Iberian Fig. 1. Map of Western Europe with location of principal orogenic chains related Chain along the Altomira thrust system (the western boundary to the Africa–Europe collision (based on Verge´s&Sa`bat 1999). The Pyrenean of the Iberian Range; Fig. 3). Range corresponds to the westernmost limit of the about 12 000 km long Q7 Alpine–Himalayan Belt. CCR, Catalan Coastal Ranges; B-C, Basco-Cantabrian thrust belt; CM, Cantabrian Mountains; P.F.F., Paul Fallot Fault. Alpine Orogeny: Tertiary compressive belts and sedimentary basins (offshore Galicia and Portugal; e.g. Malod & Mauffret 1990), within the continental plate (Iberian and Catalan rifts; e.g. Salas In Late Cretaceous times, the northern motion of Africa against et al. 2001), and along the two plate boundaries (Pyrenean rift in Europe significantly deformed the Iberian Plate along the pre- the north and the rifted south Iberian margin; Fig. 2). viously extended Mesozoic basins. These basins, of diverse orien- The opening of the central Atlantic produced an eastwards tations and sizes, developed along both the plate boundaries and motion of Africa along the former Azores–Gibraltar transform within the interior of the plate. Along the northern Iberian Plate fault. The Alpine–Tethys Ocean opened along the eastern margin, the Pyrenees represent a continental collisional orogeny margin of the Iberian Plate (e.g. Stampfli et al. 2002). This with limited northwards subduction, whereas a more complex opening led to the cessation of rifting processes occurring within region deformed in the Southern Iberian Plate, including the the Iberian Plate at the end of the Late Jurassic. A renewed Betics and Rif, the Alboran Sea, and the Gulf of Cadiz. The phase of extension took place before the opening of the North western Iberian margin along the Portuguese coast represents a Atlantic during the early Aptian (chron M0 at about 118 Ma). slightly inverted margin, especially in its southern segment (e.g. Pyrenean rift events occurred before the onset of ocean formation Alves et al. 2003; Zitellini et al. 2004). During the roughly in the Bay of Biscay during Aptian–Albian times. The northern north–south Alpine convergence the interior of the Iberian Penin- Iberian Plate transform boundary propagated eastwards and the sula deformed while mostly preserving the original trends of the northern segment of the Alpine–Tethys formed along the south- previously extended basins: NE–SW in the Catalan Coastal eastern margin of Western Europe (Stampfli et al. 2002). These Ranges, NW–SE in the Iberian Range, NE–SW in the Central processes ended by the end of Coniacian at about 86 Ma. System, and north–south in the Altomira Range (Munõz Martıń The Iberian rift system formed a linked configuration of exten- & De Vicente 1998; Fig. 3). All these fold-and-thrust systems sional basins with different orientations as observed in the NE are connected and at the intersections of any two of them there is corner of Spain at the eastern end of the east–west-trending Pyr- always a linking zone in which the two different trends coexist enees, the NW–SE-trending Figueres–Montgrı´ branch (F-M in as well as intermediary trends. The linkages between all the com- Fig. 2), and the NE–SW-directed Catalan Basin (Fig. 2). pressive fold-and-thrust belts indicate the synchronicity, at least The Iberian Peninsula at the end of Cretaceous times (Fig. 2), partially, of several of these deformational events (Fig. 3). before the onset of Africa–Europe convergence, shows extended These connections between the different thrust systems that regions within the plate as well as along its margins. It is interesting shape the present Iberian Peninsula have been described since to note that, during this period, at the end of Mesozoic extension, the early 1980s in several transects crossing the Iberian Peninsula. there are about 125–150 km of separation between the central The linkage of different thrust systems and the partial synchroni- and eastern sides of France and Spain, and at least 35 km of exten- city of the deformation are agreed upon by most workers (e.g. sion in the Iberian Basin (Salas & Casas 1993; Salas et al. 2001). Guimera` 1984; Banks & Warburton 1991; Anadoń & Roca The restoration of the Prebetic and Subbetic units shows that 1996; Casas Sainz & Faccenna 2001). The remarkable repetition their former, common, southeastern boundary was at least 90 km of NE–SW trending chains in the central part of the Iberian to the SSE of its present position (restoration according to Garcıá- Peninsula was interpreted as being produced by lithospheric Hernańdez et al. 1980). If we add the Internal Betics to the recon- folding during the Neogene (Cloetingh et al. 2002), in contrast struction by unfolding them to a minimum of double their present to crustal and lithospheric thickening. width (shortening of 50%), the SE boundary of the Internal Betics In the following sections, we describe the principal compressive restores to about 210 km to the SE of its present position. Adding a mountain systems of the Iberian Peninsula and their associated counter-clockwise rotation of about 258 to fit reasonably well with sedimentary basins starting in the Pyrenees (see regional transect the palaeomagnetic rotations observed in the Betics (Platt et al. of Roca et al. 2004) and ending in the Betics (see regional transect 2003) before the end of the late Tortonian (Krijgsman & Garce´s of Frizon de Lamotte et al. 2004). Between these, all the smaller 2004), then the SE border of the Internal Betics restores to about mountain ranges developed in an interior tectonic setting, within 300 km to the SE (Fig. 2). This restoration is approximately in the Iberian Plate. Casas Sainz & Faccenna (2001) have published agreement with the proposed restoration by Platt et al. (2003). an overview of these compressive mountain chains. BASINS & RANGES, IBERIAN PENINSULA 225

Fig. 2. Reconstructed map at the end of Late Cretaceous times to show the distribution of principal sedimentary basins in and around the Iberian Peninsula. Ca, Cameros Basin; Ma, Q8 Maestrat Basin; Col, Columbretes Basin; F-M, Figueres–Montgrı´ Basin; NPFZ, North Pyrenean Fault Zone. Position and extent of Columbretes Basin after Roca (1996). Sardinia is shown in its restored position prior to Neogene opening of the Gulf of Lyons (Olivet 1996). The Balearic Islands are shown in their restored position prior to Neogene opening of the Valencia Trough (Verge´s&Sa`bat, 1999). Position A of Africa corresponds to the present position and position B to the restored position (e.g. Boccaletti et al. 1977; Olivet 1996).

Pyrenees along the extremely thinned lithosphere of the North Pyrenean Fault Zone (Munõz 1992). The northern and southern thrust The partial subduction to the north of the Iberian lithosphere systems tectonically inverted the earlier Mesozoic basins together underneath the Europe along the Iberia–Europe plate boundary with their extensional fault systems. The location and extent of shaped the large-scale Pyrenean fold-and-thrust belt (Choukroune the early Mesozoic (Late Triassic) evaporites also exerted & Team, 1989; Roure et al. 1989; Munõz 1992; Beaumont et al. an important influence on the geometry and extent of the 2000). The Pyrenean orogen is asymmetrical and double-sided, fold-and-thrust belts in the Iberian Peninsula. Most of the import- with the most significant thrust system developed towards the ant Pyrenean shortening processes lasted for about 40 Ma and south, on top of the subducted zone as in most orogenic belts were partitioned along several thrusts describing an overall (e.g. Capote et al. 2002; Verge´s et al. 2002; Fig. 3). foreland-directed propagation of deformation. Maximum shorten- Although Africa compressed the entire Iberian Plate during its ing occurred across the Central Pyrenees and decreased towards northwards shift, the Pyrenees recorded the initial stages of gener- the west (e.g. Verge´s et al. 2002). alized compression around Santonian times (e.g. Puigdefa`bregas & The northwards subduction of Iberia below Europe continued Souquet 1986), about 35 Ma after the end of major rifting events towards the western Pyrenees (Teixell 1998) and the Cantabrian along the Pyrenean branch of the North Atlantic in Albian times. Mountains (Fernańdez-Viejo et al. 2000; Pedreira et al. 2003). The geometry at different scales of the northern boundary of However, the degree of shortening decreased towards the west the Iberian Plate clearly controlled the structural evolution of (e.g. Teixell 1998) and the age of initial deformation was the compressive belt. The northwards subduction was initiated younger in the same direction (e.g. Verge´s et al. 2002). 226 J. VERGE´ S & M. FERNA` NDEZ

Fig. 3. Tectonic map of the Iberian Peninsula based on Rodrı´guez-Ferna´ndez (2004) and the detailed tectonic map of WebColor the Pyrenees of Verge´s et al. (1995). Tertiary mountain ranges: Pyrenees, Cantabrian Mountains (CM), Catalan Coastal Ranges (CCR), Iberian Range, Central System (CS), and Betics. Foreland basins: Ebro, Duero, Tajo, and Guadalquivir (GB). Iberian Massif: South Portuguese Zone, Ossa Morena Zone, Central Iberian Zone, Asturian Leonese Zone (ALZ), and Cantabrian Zone (CZ). AP, As Pontes Basin, located in NW Spain; CR, Ciudad Rodrigo Basin, located in the SW side of the Duero Basin. A, Altomira fold system, forming the western margin of the Iberian Range.

The South Pyrenean flexural foreland basin was underfilled and Roca & Guimera` 1992). This thick-skinned style of tectonics marine from 55 to 37 Ma and then became overfilled and continen- affected the entire crust (Sa`bat et al. 1997; Roca et al. 2004) pro- tal, starting with the deposition of mid–late Eocene Cardona eva- ducing a frontal monocline with relatively high topography porites until the end of shortening during the Oligocene (Lo´pez-Blanco et al. 2001). The oblique position of these basins (Puigdefa`bregas & Souquet 1986; Puigdefa`bregas et al. 1992; with respect to the direction of compression also produced a Verge´s et al. 1995, 1998). At mid–late Eocene times (c. 37 Ma), limited sinistral strike-slip component (e.g. Anadoń et al. 1985; uplift of the western Pyrenees triggered the end of the foreland Guimera` & Alvaro 1990). basin stage and originated an intermontane basin bounded by the Pyrenees, the Catalan Coastal Ranges and the Iberian Range (e.g. Burbank et al. 1992; Garcıá-Castellanos et al. 2003). A long period of lacustrine deposition that lasted through the Oligo- Iberian Range cene and most of the Miocene characterized this endorheic period (Riba et al. 1983). An internal fluvial network delivered sediments The Iberian Range, with a NW–SE trend (nearly orthogonal to the to the Ebro Basin, which was characterized by a large central lake Catalan coastal system), shows a complex structure involving (e.g. Anadoń et al. 1979; Arenas & Pardo 1999). The end of defor- cover and basement units (Alvaro et al. 1979; Casas Sainz & mation occurred during late Oligocene times (c. 24.7 Ma; Meigs Faccenna, 2001; Fig. 3). The thrust system along the chain et al. 1996) although major basement uplift, based on fission-track shows double vergence corresponding to the inversion of previous cooling ages, ended at about 30 Ma (Fitzgerald et al. 1999). The Mesozoic extensional faults. The NW termination of the range late Oligocene–early Miocene age of younger compression in plunges beneath the Duero Basin and continues at depth below the western Pyrenees was synchronous with extensional processes the frontal thrust of the Cantabrian Mountains. The northern affecting the eastern Pyrenees related to the formation of the tectonic displacement of the Iberian Range in its northwestern Western Mediterranean basins. During late Miocene times, the terminus is about 30 km (Casas Sainz 1993; Guimera` et al. endorheic Ebro fluvial system opened towards the Mediterranean 1995). The Sierra de Altomira on the southwestern side of the Sea (e.g. Coney et al. 1996; Garcıá-Castellanos et al. 2003). Iberian Range flanks the Tajo Basin to the east (Fig. 3). This almost north–south-trending fold-and-thrust system is detached above Triassic evaporites (Munõz Martıń & De Vicente 1998). Catalan Coastal Ranges Central System The Catalan Coastal Ranges have a NE–SW trend (Alpine– Tethys Ocean trend) and display the effects of multiple tectonic The Central System, with a NE–SW structural trend, comprises an events, including Eocene compression and Oligocene–Miocene uplifted block, like a large pop-up structure (Vegas et al. 1990), extension, the latter related to the opening of the Valencia with relatively high topography and associated with crustal thick- Trough (e.g. Anadoń et al. 1985; Fig. 3). The Catalan Coastal ening (Mezcua et al. 1996; Fig. 3). This block bounds two large Ranges represent the inversion of earlier extensional structures, sedimentary basins in the central part of the Iberian Peninsula: with the inversion affecting both cover and basement units (e.g. the Duero Basin to the NW and the Tajo Basin to the SE. The BASINS & RANGES, IBERIAN PENINSULA 227

Central System thrusts the Duero and Tajo basins to the NW and forms an intricate system of tectonic thrust imbricates and SE, respectively (e.g. Querol 1989; De Vicente et al. 1996). chaotic units, which possibly correspond to an imbricate thrust The Duero Basin is filled by a maximum of 2.5 km of Oligocene system emplaced in a marine depocentre filled with numerous and Miocene continental deposits, mostly from the Cantabrian olistoliths and olistostromes (e.g. Azanõn et al. 2002). Mountains (e.g. Alonso Gavilań et al. 2004). The age of the Contacts between units in the External Betics are principally younger sediments in the basin is late Miocene at about 9.6 Ma foreland-directed thrusts involving different cover palaeogeogra- (Krijgsman et al. 1996). The Ciudad Rodrigo Basin, at the SW ter- phical domains mostly detached above Triassic evaporites. The mination of the Duero Basin, opened to the Atlantic at the Oligo- cover–basement contact is a major hinterland-directed thrust in cene–Miocene boundary (Santisteban et al. 1996; its location is the east and centre of the Betics (Banks & Warburton 1991), shown in Fig. 3). Subsequently, after 9.6 Ma, the Duero River cap- which changes to a foreland-directed one to the west, where it tured the closed Duero Basin and opened it to the Atlantic basin. over-thrusts the Subbetic Unit as well as the Campo de Gibraltar The Tajo Basin filled with 2–3 km of continental deposits Unit. This hinterland-directed thrust was a response to tectonic ranging in age from the latest Oligocene to the latest Miocene wedging produced by the antiformal stack of basement units flat- (e.g. Alonso Zarza et al. 2004). tening on top of the Triassic detachment level (Banks & Warbur- ton 1991). As for the rest of the mountain ranges of the Iberian Peninsula, the Triassic evaporites constitute an excellent detach- Other compressive areas ment level between basement and cover rocks. Several relatively small intermontane basins filled with Neogene Towards the NW corner of the Iberian Peninsula, small transpres- deposits are located along the contact between the Internal Betics sional basins (such as the As Pontes basin) were filled with alluvial and the External Betics (e.g. Iribarren et al. 2003). The Neogene to to lacustrine deposits during late Oligocene and earliest Miocene Quaternary Guadalquivir Basin, to the NW and WSW of the Exter- times (e.g. Cabrera et al. 1996; Fig. 3). The western boundary of nal Betics, corresponds to a foreland basin in front of the Betics the Duero Basin also formed during this period, closing it. thrust system (e.g. Bera´stegui et al. 1998; Garcıá-Castellanos et al. 2002). Hercynian rocks of the Iberian Massif constitute the Betics NW boundary of the basin.

The Betic Cordillera, trending generally ENE–WSW, corresponds to part of the former northern Africa–Iberia plate boundary, devel- Neogene formation of the Western oped on top of Iberian crust and cropping out now in the southern Mediterranean basins part of the Iberian Peninsula (Figs 1 and 3). Although some aspects of the evolution of the Betic Cordillera are well known, there is Beginning in mid-Oligocene times the opening of the Valencia still no agreement about the mechanisms that created it (see dis- Trough created an extensional fault system paralleling most of cussion of models by Calvert et al. 2000). the eastern coast of northeastern Spain (e.g. Roca et al. 2004; The Betic Cordillera is divided into the Internal Betics, compris- Fig. 3). This system cut obliquely across the Early Tertiary com- ing metamorphic basement, the External Betics, consisting of pressive Catalan Coastal Ranges and cut the SE termination of cover rocks, and the Guadalquivir Foreland Basin (e.g. Azanõn the Iberian Range almost perpendicularly, forming an extensional et al. 2002). The Alboran Sea, with a complex tectonic history, arrangement of basins parallel to the Mediterranean coast (e.g. formed as a result of Neogene extension. Neogene to Quaternary Roca et al. 1999). Concomitant uplift of segments of the Catalan depositional sequences fill curved, elongated and deep basins Coastal Ranges as well as of the SE margin of the Ebro Basin (e.g. Comas et al. 1999). initiated the development of the present landscape configuration The Internal Betics comprise a tectonic pile of three different tec- (e.g. Morgan & Ferna`ndez 1992; Lewis et al. 2000; Gaspar- tonic units separated by thrusts (Nevado–Fila´bride at the base, Escribano et al. 2004). Fission-track studies indicate that more Alpuja´rride in the middle, and Mala´guide at the top). Each unit than 1.5 km of uplift occurred, responsible for the significant dis- displays a different degree of Alpine metamorphism, decreasing section of this margin (Juez-Larre´ & Andriessen 2002). from the bottom to the top. The Nevado–Fila´bride unit was affected To the SE of the Iberian Plate, an early Miocene large-scale system by HP–LT metamorphism indicating that the rocks were located at of normal faults connected to the Alboran Sea extensional system depths of 50–70 km (the metamorphic evolution of the Internal cuts the rear flank of the Internal Betics antiform. Most of these Betics has been described by Comas et al. (1992)). The meta- large normal faults are subparallel to the Internal Betics antiformal morphic units crop out as large antiforms that exhibit east–west thrusts, reactivating some of them (e.g. Platt & Vissers 1989; trends in contrast to the ENE–WSW regional direction of the Exter- Garcıá-Duenãs et al. 1992; Crespo-Blanc et al. 1994; Comas et al. nal Betics. Towards the western end of the Internal Betics, relatively 1999). The Alboran Basin is filled by up to 8 km of early Miocene– large massifs of peridotites have been incorporated into the Quaternary sedimentary sequences (e.g. Comas et al. 1999). Alpuja´rride thrust system (e.g. Ronda Peridotites; Tubıá et al. 1997). The External Betics constitute a system of thrust sheets carrying different Mesozoic SE Iberian passive margin palaeogeographical Present-day topographic configuration units towards the foreland. The Subbetic Zone is located to the SE of the Iberian Peninsula and the Prebetic Zone to the NW (Garcıá-Hernańdez et al. 1980). The stratigraphy of the External Betic units includes Triassic to The present-day mean elevation of the Iberian Peninsula, slightly Miocene successions. The Prebetic Zone is mostly composed of over 600 m, is almost certainly the highest in Western Europe. shallow-water deposits whereas the Subbetic Zone units are Smith (1996) pointed this out but did not propose a good solution deeper and pelagic. The Campo de Gibraltar Unit consists of ter- for what is sustaining it. However, the integration of the tectonic rigenous deposits forming an Oligocene and early Miocene accre- structure of the Iberian Peninsula with its topography (Fig. 4), tionary prism developed above the Subbetic Zone towards the Bouguer anomalies (Fig. 5), and existing 2D and 3D lithospheric WSW (e.g. Crespo-Blanc & Campos, 2001; Bonardi et al. 2003) models can explain most of this high topography. that was actively deformed until late Tortonian times (e.g. The greatest negative Bouguer anomalies of the Iberian Gra`cia et al. 2003). The external part of the Prebetic Zone is com- Peninsula show a very good match with its principal Tertiary posed mainly of Triassic evaporates, unconformably overlain by mountain chains such as the Pyrenees, the Iberian Range, the an incomplete and discontinuous succession of Mesozoic to Central System and the Betics (e.g. Casas Sainz & Faccenna Neogene deposits. The most external part of the Prebetic system 2001; Cloetingh et al. 2002). Available geophysical modelling 228 J. VERGE´ S & M. FERNA` NDEZ indicates that these negative anomalies, corresponding to com- To the west and SW of the Iberian Range there is a very large pressive systems, are primarily explained by crustal thickening, segment of the Iberian Peninsula with an elevated topography which in most cases is combined with lithospheric thickening between 600 and 1000 m (Fig. 4). This region corresponds to such as has been inferred for the Pyrenees (Zeyen & Ferna`ndez the Central Iberian Zone of the Variscan Iberian Massif and 1994) and the Central Betics (Torne et al. 2000). Cloetingh includes the Tajo and Duero basins, and also shows a significant et al. (2002) proposed an alternative mechanism, lithospheric negative Bouguer anomaly (Fig. 5), suggesting that the crust of folding, as the main cause of alternating mountain ranges and this region is either thickened or less dense than the surrounding basins of the central part of Iberia, but combined crustal and litho- crust, or a combination of both (Ferna`ndez et al. 1998). The spheric modelling, as for the Pyrenees by Zeyen & Ferna`ndez former interpretation can be applied to the NE domain (Duero (1994), does not support this interpretation. and Tajo basins) where thick Tertiary sedimentary successions

WebColor Fig. 4. Combined tectonic and topographic map of the Iberian Peninsula. BASINS & RANGES, IBERIAN PENINSULA 229

Fig. 5. Combined tectonic and Bouguer WebColor anomaly map of the Iberian Peninsula (from Mezcua et al. 1996). are present. However, to account for the regional distribution of of about 200 m. Field studies demonstrate an increase in crustal the negative Bouguer anomaly and elevated topography the density for these two Hercynian domains. A 2D lithospheric hypothesis of a less dense crust for the whole of the Central model, which integrates elevation, gravity, geoid and heat-ﬂow Iberian Zone is more realistic. data, indicates that the present lithospheric structure in these two Finally, in the southwestern Iberian Peninsula, the Ossa Morena domains, with relatively high crustal density, must be underlain and South Portuguese zones are dominated by a Bouguer anomaly by a thinned lithosphere or by a depleted lithospheric mantle maximum (Fig. 5), coinciding with an average topographic height (Ferna`ndez et al. 2004). According to the model, the mass deﬁcit 230 J. VERGE´ S & M. FERNA` NDEZ

Fig. 6. Lithospheric thickness for the Valencia Trough (Ayala et al. 2003) and Alboran Sea (Torne et al. 2000) WebColor superimposed on the tectonic map of the Iberian Peninsula. The volcanic centres along the Mediterranean coast coincide with the main entrances of the 70 km thick lithosphere line (note that all these ﬁelds contain alkaline volcanic rocks). The Campo de Calatrava volcanic province crops out in the centre of Iberia. (Abbreviations as in Fig. 2; v.p., volcanic province).

related to mantle thinning or depletion implies a lithospheric sublithospheric channelling starting in the Cape Verde and buoyancy-driven uplift of about 120–150 m to fit the observed ending in the northern North Sea (Ziegler 1990). elevation and geoid data (Ferna`ndez et al. 2004). Thus, the present-day topography of the Iberian Peninsula was On the Mediterranean margin of Iberia, crustal and lithospheric mostly acquired by means of three main tectonothermal mechanisms mantle thinning show dissimilar patterns. Thin lithospheric effective since the Late Paleozoic: variations of crustal densities and mantle underlies onshore Iberia at three main localities (marked possibly mantle depletion inherited from the Hercynian Orogeny, by the 70 km thick lithosphere contour; Fig. 6): in NE Spain, on crustal and lithospheric thickening during Tertiary compression, the southern flank of the Valencia Trough, and in SE Spain. Posi- and upper mantle thinning during the Neogene–Quaternary. tive Bouguer anomalies, crustal and lithospheric thinning, high However, the present landscape of the Iberian Peninsula has also topography, and asthenospheric volcanism characterize these been sculpted by the opening of numerous endorheic basins. The three regions. most spectacular of these openings occurred in the Ebro Basin In NE Iberia, several studies show that thinning was more intense during the early late Miocene (Coney et al. 1996; Lewis et al. in the lithospheric mantle than in the crust, producing additional 2000; Garcıá-Castellanos et al. 2003). The best preserved basin is uplift as well as basic volcanism (e.g. Cabal & Ferna`ndez 1995; the Duero Basin, in which river incision is still relatively minor. Lewis et al. 2000; Ayala et al. 2003; Fig. 7). Radiometric ages for volcanic rocks of the NE volcanic province indicate that there was a migration of volcanism to the SW and west from 14 to 0.011 Ma (e.g. Saula et al. 1994; Lewis et al. 2000; Martı´ 2004). Summary This indicates that lithospheric thinning and concomitant uplift started during late mid-Miocene times and is still active at present. Between a widespread Triassic phase of extension and the opening At the southern end of the Valencia Trough, the SE volcanic pro- of the central Atlantic in Late Jurassic times, Iberia was affected by vince, in Levante (Fig. 6), shows ages ranging from 8 to 1 Ma extensional processes that created numerous sedimentary basins (Ancochea & Huertas 2004). Towards the SE of Iberia, 3D crustal with different orientations, including the Pyrenees, Catalan and and lithospheric modelling shows that this area is also supported Iberian basins, and the Betic basin on the southern margin of dynamically (Torne et al. 2000). The SE volcanic province in Iberia. The Alpine–Tethys Ocean opened at this time, forming Almerıá and Murcia shows only a few alkaline volcanic edifices the eastern margin of the Iberian Plate. (Lo´pez-Ruiz et al. 2004; Fig. 6). An additional extensional phase, forming the western margin of The lithospheric geometry and the existence of onshore alkaline Iberia, took place before the opening of the North Atlantic during volcanic provinces along the coast as well as in the interior of the early Aptian. This transform boundary propagated eastwards Iberia (Campo de Calatrava volcanic province; Fig. 6) has been from the Bay of Biscay and constituted the northern plate bound- interpreted as being indicative of a long-lived deep process ary of the Iberian Plate, resulting in its isolation. These processes related to the opening of the Atlantic Ocean (Oyarzun et al. ended by the end of the Coniacian. 1997). According to Oyarzun et al. (1997), the thinning Basins formed during these extensional phases were inverted along the Western Mediterranean region corresponds to a long, during Late Cretaceous–Tertiary compression, which started in BASINS & RANGES, IBERIAN PENINSULA 231

Fig. 7. Map of the NE corner of the Iberian Peninsula to show two areas affected by Neogene and Quaternary uplift (from Lewis et al. 2000). The SW area is limited to the footwall of the Oligocene– early Miocene normal faults that parallel the coastline. The NE area is wider and affects part of the previous area and both the footwall and hanging wall of the late Miocene–Quaternary NW–SE-trening system of normal faults. The volcanism migrated westwards from the Emporda` Basin to the la Selva Basin and ﬁnally to La Garrotxa, where it is as young as 0.011 Ma.

Santonian times. The extent and orientation of the basins con- processes affecting the eastern margin of the Iberian Peninsula are trolled the size, width and trends of the compressive belts, which still active at present. normally show a double vergence. In almost all the Iberian fold-and-thrust belts, Triassic evaporites provided an excellent This is a contribution of the Group of Dynamics of the Lithosphere (GDL), Depart- decoupling level between the basement and the cover units. ment of Geophysics and Tectonics, Institute of Earth Sciences ‘Jaume Almera’, CSIC. Partial support for this paper was provided by MCYT projects The Iberian mountain chains produced lithospheric flexural REN2001-3868-C03-02/MAR, REN2002-11230-E-MAR, NATO Grant bending and thus foreland basins, especially in front of the Pyre- EST-CLG978922. 2001 SGR 00339, and project 2001 SGR 00339 Grup d’Estruc- nees and the Betics: the Ebro and Aquitaine basins flank the Pyr- tura i Processos Litosfe`rics, funded by the Comissionat per Universitats i Recerca of enees and the Guadalquivir Basin lies to the north of the Betics. the Generalitat de Catalunya, Grups de Recerca Consolidats, II Pla de Recerca de The Duero and Tajo basins developed to the south of the Cantab- Catalunya. We finally thank an anonymous reviewer for constructive remarks and rian Mountains and to the south of the Central System, suggestions. respectively. The Ebro, Duero and Tajo basins became intermontane basins at different stages of their evolution by the closure of their Atlantic References connections. The basins were filled by radial fluvial systems ALONSO GAVILA´ N,G.,ARMENTEROS,I.,CARBALLEIRA,J.,CORROCHANO, feeding central lakes. ´ Starting in the late Oligocene, the opening of the Valencia A., HUERTA,P.&RODRIGUEZ, J. M. 2004. Cuenca del Duero. In: VERA, J. A. (ed.) GeologıádeEspanã. SGE-IGME, Madrid, 550–556. Trough initiated the development of a system of normal faults ALONSO ZARZA, A. M., CALVO, J. P., SILVA,P.G.&TORRES, T. 2004. and linked basins aligned with the present Mediterranean coastline Cuenca del Tajo. In:VERA, J. A. (ed.) Geologıá de Espanã. of Iberia. SGE-IGME, Madrid, 556–561. The deformation history of the Iberian Peninsula produced a sig- ALVARO, M., CAPOTE,R.&VEGAS, R. 1979. Un modelo de evolucioń nificant mean elevation that is the highest in Europe. Tertiary com- geotectońica para la Cadena Celtibe´rica. Acta Geolo`gica Hispa`nica, pressional processes contributed strongly to an increase of mean Volum Homenatge a Lluı´s Sole´ i Sabarı´s, 14, 172–177. elevation, but inherited Hercynian lithospheric structures as well ALVES, T. M., GAWTHORPE, R., HUNT,D.W.&MONTEIRO, J. H. 2003. as late Cenozoic upper mantle thinning related to the opening of Post-Jurassic tectono-sedimentary evolution of the Northern the Western Mediterranean also contributed to the high average Lusitanian Basin (Western Iberian margin). Basin Research, 15(2), elevation of the Iberian Peninsula. Some of the thermo-mechanical 227–249. 232 J. VERGE´ S & M. FERNA` NDEZ

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