GEODYNAMIC EVOLUTION OF THE ALPINE IBERIAN MESETA VIA MIXED PROCESSES: VERTICAL MANTLE UPWELLINGS AND HORIZONTAL TECTONICS

 

M. DOBLAS

 Científico Titular del CSIC, Instituto de Geociencias (CSIC-UCM), Facultad de Medicina,
c/ del Doctor Severo Ochoa 7, Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid, Spain, doblas@mncn.csic.es

Abstract

The geodynamic evolution of the late Cenozoic Alpine Iberian Meseta (AIM) might be understood in terms of the close temporal and spatial interaction between vertical mantle upwelling phenomena and horizontal tectonic stresses. The model presented in this paper constitutes a completely new approach to the Alpine evolution of the AIM and Iberian Peninsula as a whole, different from the classical orogenic-related ideas taking only into account the horizontal movements. The origin of the sublithospheric upwelling movements in the AIM will be discussed and might be related to several possible scenarios: 1) The far-reaching effects of a Triassic/Jurassic superplume associated to the opening of the central Atlantic defining a  long-lived anomalous sublithospheric hot residuum sheet attached to the plates; 2) Long-lived mantle insulation effects beneath the stable Iberian craton since the Paleozoic; 3) Edge-driven convection related to the asymmetric opening of the eastern Atlantic leaving an uplifted upper-plate border in Iberia/western Africa;  4) The anticlockwise rotational behaviour of the Iberian peninsula during the Mesozoic/Cenozoic opening of the Bay of Biscay. We will argue that the first hypothesis best accounts for the upwelling characteristics of the Iberian peninsula. Late Cenozoic horizontal tectonic stresses are related to the continued NS convergence between Africa and Eurasia, finally crushing the Iberian plate between the eastern Pyrenees and Betic indenters. This scenario triggered a series of  W-escaping wedge-shaped blocks (according to a Prandtl-cell model) bounded to the N, E, and S by major transcurrent faults, and defining in their western open sides a major W-increasing extensional scenario with: NS-oriented normal faults, tilted, uplifted and collapsed blocks,  and the final exhumation of asthenospheric mantle peridotites in the western Iberian-Atlantic passive margin. Thus, the late Cenozoic evolution of the AIM can be thought of  as a major W-escaping wedge-shaped lithospheric megablock rafting irregularly on top of a long-lived hot and upwelling asthenospheric mantle residuum.

 Keywords: Alpine Iberian Meseta, horizontal stresses, vertical mantle upwellings, hot sublithospheric mantle.

 Introduction

             The Alpine Iberian Meseta (AIM) occupies most of the western/central Iberian realm and it is constituted by the following elements (Fig. 1 A; ITGME, 2001): 1) The Hesperian or Iberian Massif forming the vast majority  of the AIM outcrops, and which belongs to the Variscan orogenic belt of western Europe, now constituting the basement of the Iberian plate. Several blocks of the Hesperian Massif were uplifted by the Alpine tectonic events (the Galician-Asturian-León mountains in the N, the Spanish Central System in the center, and the Sierra Morena in the S). 2) Three major Cenozoic continental  sedimentary basins (from N to S: Duero, Tajo, and La Mancha), as well as a series of minor continental basins in the NW (e.g. Villalba, Sarria, Monforte, Tui, As Pontes, El Bierzo), and the W (e.g. Guadiana, Alagón, Coria,  Lower Tajo). The AIM is bounded to the W by the Atlantic ocean and a Mesozoic basin in western Portugal. Elsewhere, the AIM is characterized by a peripheral subcircular disposition of  bounding high reliefs (Cantabrian and Basque mountains to the N, Iberian Ranges to the E, Betic cordilleras to the S and SE, and Sierra Morena to the S and SW (Fig. 2).

Figure 1. Alpine Iberian Meseta (AIM). A) Simplified geological sketch of Iberia: AL: Algarve; BE: Betics; CR: Catalonian ranges; DB: Duero basin; EB: Ebro basin; GB: Guadalquivir basin; HM: Hesperian Massif; IR: Iberian ranges; LTB: lower Tajo basin; MB: La Mancha basin; OM: Occidental portuguese margin; PY: Pyrenees; SCS: Spanish Central System; TB: Tajo basin. B) NS topographic cross-section of Iberia (Solé Sabaris 1983). C) Hypsometric curve of Iberia showing anomalous mean elevation of 660 m  mostly due to the AIM (Solé Sabaris 1983). D) Selected topographic cross-sections of the AIM (location in A) highlighting the dome-shaped geometry of the AIM (Solé Sabaris 1983).

 

 Figure 2. Topographic maps of Iberia. A) Large-scale topographic map and accompanying sketch outlining the major subcircular pattern defined by some of the highest reliefs of the Peninsula: Cantabrian Mountains, Sierra de la Demanda, Montes Universales, Sierras de Segura/Cazorla, and Sierra Nevada (CDUSCN great circle). B) Detailed topographic map of Iberia (Villa Valentí 1968): 1: less than 100 m. 2: 500 m. 3: 1000 to 1500 m. 4: more than 1500 m. C) Sketch outlining the CDUSCN great circle and an elliptical feature deduced from the topographic map of B.

            The AIM has long been recognized as an intraplate domain with a present anomalously  high topographic elevation averaging 660 m (Fig. 1 B & C; Solé Sabaris  1983; Smith 1996). As a comparison the mean elevation of France (including the french Alps) is around 342m, and in Europe the only country with highest values is of course Switzerland (1300m; Solé Sabaris 1983). The hypsometric curve of the Iberian peninsula clearly shows the relevance of the AIM “high plateau” in determining the higher-than-normal topography of this realm (Solé Sabaris 1983; Fig. 1 C). Moreover, a generalized and large-scale dome-shaped uparching of the AIM  is visible  in most topographic cross-sections (Fig. 1 D; Solé Sabaris 1983). In this sense, the AIM defined a “topographic swell” during the Cenozoic, something similar to what happens with the Neogene Moroccan meseta (Benmohammadi et al., 2007). On a much larger scale, this is also the case for the quasi-stationary Cenozoic African plate which is characterized by vertical movements with a “domes & swells” topography which stands higher (much of it above 1000m) than any other continent (Ebinger and Sleep, 1998). The anomalous high elevation of the AIM swell has been long-lived as shown by the fact that during all the Mesozoic the Hesperian Massif of western Iberia standed as a high emerged domain with no sediment deposition during the marine transgressions (Solé Sabaris 1983).

The origin of the present high topography of the AIM  swell is unclear, and as suggested by Smith (1996), “...the ultimate origin of the topography and what sustains its present height is a key problem in regional Iberian studies.” High topographic swells are often associated with orogenic belts due to crustal thickening by collision and underplating (Smith 1996) or to crustal buckling phenomena (Alvarado, 1983). Though the peninsula is bordered by the Pyrenees to the N and the Betics to the S, the lateral extend of the high topography is much greater than is found adjacent to other orogenic belts of a similar age, such as the Alps or the Carpathians (Smith 1996). Thus, the high topography appears unlikely to be an orogenic effect (Smith 1996). In this sense, the classical assumption that the higher-than-normal altitude of the peninsula might be explained by a generalized uparching/buckling of the Iberian plate compressed by the African and Eurasiatic plates (Alvarado 1983) can be dismissed. According to Smith (1996) the only possible explanation would be in terms of upwelling hotspot activity. However, a  classical hotspot  (Morgan 1972; Wilson 1973) is not present in the Alpine Iberia (Smith 1996). As we will see later in the proposal of our model different scenarios might account for this Iberian enigma, including a long-lived superplume activity (Oyarzun et al. 1997). 

High topography in non-orogenic areas is associated with mantle upwelling phenomena, such as: insulation-related accumulation of heat beneath supercontinental caps (Anderson 1982; Doblas et al. 2002); earth-rotation stresses (Le Pichon and Huchon, 1984; Doblas et al. 1999); cycles of breaking-healing plates (Gurnis, 1988); pulsation tectonics (Sheridan, 1997); mantle contrasted exchange rates (Stein and Hofmann, 1994); disruption of the 670 km thermal boundary layer by the massive arrival of subducted slabs (Larson and Kincaid, 1996); mantle ascent resulting from local plate motions (Humphreys et al. 2000); and, upwelling  mantle plumes and hot-spots (Celal Sëngor, 2005).

Several authors dealing with the AIM recognized that it constitutes a unique area where anomalous asthenospheric mantle upwelling phenomena were active during the Cenozoic. However, no global convincing mechanism has been suggested to account for these phenomena. Alía was the first author to acknowledge the existence of  such processes in the central part of the AIM (Alía 1972, 1976; Alía et al. 1980). Later, other researches also recognized the relevance of anomalous uplifting  phenomena both in the central (Martín Escorza 1980, 1983; Portero & Aznar 1984) and northwestern and southwestern (Martín Serrano 1989; ITGE 1994; Muñoz Jiménez & Sanz Herraíz 1995) sectors of the AIM, as well as in most of eastern Iberia (Simón Gómez 1984, 1989; Sanz de Galdeano 1996) and the western Mediterranean (Doblas & Oyarzun 1990).

            Anomalous  heat conditions existed during the Cenozoic evolution of the AIM as shown by the following elements: 1) A series of  alkaline basaltic volcanics that outcrop either in the S of the AIM (Calatrava; López Ruiz et al. 1993) or in peripheral regions (SE Iberia, Iberian Ranges, and NE Iberia; López Ruiz et al. 2002); 2) Fracture-related hydrothermal alteration of granites and Paleogene sediments in the Spanish Central System and the Duero basin (silicification, alunitization, kaolinitization, syderolithization; Ubanell et al. 1978; Martín Serrano et al. 1996); 3) Opaline cherts within the Neogene Tajo basin (Bustillo & Martín Escorza 1984; Martín Escorza 1983) apparently related to NS- to EW-oriented hydrothermal fracture systems; 4)  Hydrothermal-related Mn-(Co)-Fe deposits in the Calatrava volcanic field (Crespo Zamorano et al. 1995) and Mn deposits in the Iberian Ranges (Manolo Hoyos, personal communication); 5) Hydrothermal water springs in many areas of the central and eastern AIM (Martín Escorza 1992).

Plate tectonics usually gives a predominant role to horizontal translations, with only minor and plate-independent participation of vertical movements (mantle upwellings and downwellings; e.g., Moores & Twiss 1995; Ishida et al., 1999; Anderson, 2001; Maruyama et al., 2007). In a sense, this occurred  because plate tectonics theory had to fight hard against the previous verticalist geotectonic paradigm (e.g., Beloussov, 1962; Van Bemmelen, 1966).  As stated by Gurnis (1992), “For although plate tectonics has been immensely succesful at providing a kinematic framework of large-scale horizontal motions, it cannot explain vertical motions of continents.”  Several decades after the advent of plate tectonics it is now evident that both horizontal and vertical movements occur on the planet (e.g., Foulger et al., 2005). In fact, one of the proponents of the plate tectonics model (Tuzo Wilson) suggested in his latest work that “a major revision of tectonic theory” should be undertaken to account for the fact that in the southwestern United States “hot spot plumes vertically uplift mountains” (Wilson 1990). In this sense we are witnessing today a revival of two contrasted paradigms: plate and plume tectonics (Smith and Lewis, 1999; Foulger et al., 2005). Vertical motions triggered by mantle upwellings and downwellings are active in a series of  geological processes: the upwelling of mantle plumes and hot-spots, the uplift of continental plateaus and oceanic ridges, the anomalous topography of  some seafloor swells, postglacial crustal rebound, the secular low intensity vertical movements of lithospheric plates, the blanketing effect of supercontinental caps, antipodal geodynamics the downwelling of mantle avalanches, the recycling of oceanic crust, the formation of sedimentary basins, the morphology of subduction zones, the breakup of supercontinents, the evolution of compressional and extensional orogenic belts (e.g., Artyushkov et al. 1980; McGetchin et al. 1980;  Hoffman 1989; Davies, 1998, Kearey & Vine 1990; Wilson 1990; Moores & Twiss 1995; Veevers 1995; Pysklywec & Mitrovica 1997; Liu & Shen 1998; Pavoni, 1999; Maruyama et al., 2007). In fact, subvertical mantle upwellings are presently accepted as major protagonists in the evolution of the Eurasian lithosphere (e.g., Ziegler, 1993; Yegorova et al., 1997; Pavlenkova, 1998; Herrick, 1999; Doblas et al., 2007; Steiner and Conrad, 2007)

Subcircular, arcuate, elliptical, or polygonal crustal patterns associated to deep-seated and vertical upwelling or downwelling phenomena have long been described in many parts of the world in the geological literature (e.g. Beloussov 1962; May 1971; Morgan 1972;  De Jong & Scholten 1973; Saul, 1978;  Ollier 1981; Ramberg 1981;  Byler 1987; Rickard 1987; Zverev & Kats, 1990; Griffiths & Campbell 1991; Coffin & Eldholm, 1994;  Moores & Twiss 1995; Ernst et al. 1995; López et al. 1999; Anguita et al., 2001; Spencer, 2001; Grindrod & Hoogenboom, 2006; Foulger et al., 2005). The origin of most of these circular-type structures which have been localized in different planets of our solar system remains enigmatic (the so-called corona conundrum; Watters and Janes, 1995; Anguita et al., 2001; Grindrod and Hoogenboom, 2006).

Even though, they have been ascribed to a wide variety of processes, such as: verticalist mantle mega-undations or geotumors, gravitational phenomena within the crust, convection currents in the mantle (toroidal mantle flows and rotational drag of the lithosphere; Scoppola et al., 2005; Zandl and Humphreys, 2008), rifting processes, mantle hotspots and plumes, other mantle upwelling phenomena, downwelling mantle avalanches, plutonic or volcanic dome-shaped intrusions, diapiric ascent of  incompetent material, seismic shaking, meteoritic impacts (e.g. Van Bemmelen 1966; Morgan 1972; Wilson 1973; De Jong & Scholten 1973;Grieve 1980; Frey 1980; Ramberg 1981; Byler 1987; Rickard 1987; McNutt & Judge 1990; Griffiths & Campbell 1991; Maltman 1994; Ernst et al. 1995; Moores & Twiss 1995; Pysklywec & Mitrovica 1997; López et al. 1999; López-Ruiz et al., 2001; Doblas et al., 1998c, 2001). Moreover, there is often controversy regarding the origin of a given subcircular crustal pattern: e.g.,  the giant central Atlantic radial dyke swarm is interpreted either as the result of a deep-seated superplume (Oyarzun et al. 1997) or shallow-seated passive rifting phenomena (McHone 2000);  some of the Canadian craters are interpreted either as cryptoexplosion geoblemes (Currie 1965) or as meteoritic impacts astroblemes (Roddy, 1968).

 

Main structural elements of the AIM and  Iberia

             We will now describe the different structural elements that controlled the Cenozoic evolution of the AIM and the rest of Iberia in terms of a two-fold mechanism:

            1) Subcircular to arcuate patterns in the AIM and Iberia that we will interpret in terms of vertical mantle upwelling phenomena.

            2) Other structural elements resulting from horizontal tectonic regimes, such as major dip-slip and strike-slip fault systems, fault-controlled sedimentary troughs, zones of indentation, wedge-shaped escaping blocks, etc...

 

Subcircular to arcuate crustal patterns in the AIM and Iberia resulting from vertical mantle upwelling phenomena.

Circular, subcircular, semicircular, arcuate, oval or elliptical structures have been recognized since a long time in the literature regarding the Iberian Peninsula, and they have been associated to very different phenomena. 1) Deep-seated superplume activity: A Permo-Carboniferous gigantic elliptical structure defined by widespread magmatism in the central Pangean realm (including the Iberian peninsula) has been associated  to the activity of a mantle superplume exhaust valve (Doblas et al. 1998 a,b; Oyarzun et al. 1999) and to the kinematics of earth rotation and equatorial membrane tectonics (Doblas et al. 1999). The Cenozoic alkaline basaltic volcanic subcircular fields in Iberia (Calatrava, Ampurdán, La Selva, Garrotxa) have been associated to ascending mantle diapirs (López-Ruiz et al. 1993; Cebriá et al. 2000) ultimately resulting from long-lived, large-scale, superplume-related NE-directed asymmetric mantle upwelling phenomena occurrying in the eastern central Atlantic, western North Africa, and European realm (Oyarzun et al. 1997; Doblas et al. 1998 a) ultimately leading to the establishment of  an anomalously hot asthenospheric domain (European asthenospheric reservoir, EAR; Cebriá and Wilson, 1995; Goes et al., 1999; Cebriá et al., 2003). 2) Hotspot activity and associated rifting: The Triassic subcircular volcanic field of the SE Iberian Ranges has been related to the activity of a hotspot (Dewey et al. 1973) generating a major rifting event whose last manifestation might be recognized as a series of upper Pliocene to present crustal domes (associated to a mantle diapir; Simón Gómez 1989). 3) Other mantle upwelling phenomena: Unspecified mantle upwelling phenomena are thought to have triggered several Cenozoic elliptical- to arcuate vaults (NW mountains in Galicia and Cantabria and “Dorsal Gallega”; Sierra Morena; Castellano-Extremeña; Extremadura and Toledo) in many sectors of the AIM  (NW, SW, and center; Alía 1976; Martín Escorza 1977; Alía et al. 1980; Portero & Aznar 1984; Martín Serrano 1989; ITGE, 1994; Muñoz Jiménez & Sanz Herraíz 1995). The whole eastern Iberian realm (including part of the western Mediterranean) shows a series of Pliocene to Present crustal domes associated to radial extension phenomena whose origin is not specified by the author ( Sanz de Galdeano 1996). Weijermars (1988) explained the evolution of the Alborán basin in terms of uplift and subsidence movements associated to an ascending diapiric mantle bulge (similar to the verticalists-type scheme; Van Bemmelen 1966; Ritsema 1970). 4) Compressional-related exhumation: The NW sector of the Variscan orogen underwent exhumation of  subcircular deep-seated allochtonous metamorphic complexes (e.g., Cabo Ortegal, Ordenes, Braganca and Morais; e.g. Ribeiro et al. 1990). 5) Oroclinal bending and crustal buckling: Compressional tectonics associated to the Variscan, Pyrenean, and Betic orogenic belts generated arcuate-shaped oroclinal bendings (e.g. Asturian “knee” in the northern Variscan belt; e.g. Martínez García 1997) or a generalized buckling of the lithosphere. 6) Indentation phenomena: Miocene to present NW-directed indentation tectonics in the easternmost Betic cordilleras generated a major subcircular realm in the Iberian foreland (Ruidera uplift; Doblas et al., 1991). 7) Granitic emplacement: The syn to post-tectonic emplacement of granitic bodies of Variscan and late Variscan age within Iberia or Morocco is recognized (as in many other parts of the world) as an effective mechanism to generate detectable subcircular to elliptical crustal features (e.g., Martin Escorza 1977 b; Castro 1986; Doblas 1985; López Plaza & Gonzalo 1986). 8) Extensional detachment tectonics: Late Variscan extensional detachment tectonics gave rise to the generalized gravitational collapse of  central Iberia, large-scale upward arching, and exhumation of  subcircular metamorphic core complexes on the Earth (Doblas et al. 1988, 1994; Doblas 1985, 1991) and in other planets (Spencer, 2003). Similarly, the Neogene extensional collapse of the western Mediterranean Alborán and Valencia basins and the Betic cordilleras triggered the exhumation of subcircular metamorphic and mantle core complexes (Doblas & Oyarzun, 1989 a,b, 1990; Doblas et al., 2007).  9) Volcanism: As already mentioned, Cenozoic volcanism in the Iberian plate gave rise to subcircular or elliptical crustal domains in central Spain (Calatrava) and NE Iberia (Ampurdán, La Selva, Garrotxa; Araña et al. 1983; López-Ruiz et al. 1993; Cebriá et al. 2000). 10) Diapirism and collapse of evaporates or karstic elements  : A climate-related hydration diapirism model has been suggested for the genesis of large evaporite mounds in two Neogene basins of central Spain (Tajo and Calatayud; Hoyos et al. 1996). The plastic deformation of underlying Neogene evaporites triggered diapiric and collapse deformation features in the Tajo trough (Alía 1972). 11) Coastal subsidence collapses:  This mechanism is thought to have triggered a series of oval-shaped features in eastern Iberia and its mediterranean coastal margin (e.g., Valencia and Teruel-Almansa ovals; Alicante and Aguilas ovals; Rey Pastor 1948;  Alía 1972; Goy & Zazo 1974; Martín Escorza 1982; Simón Gómez 1984). 12) Seismic shaking: The large dome-shaped evaporitic mound in the Neogene Tajo trough is though to have been partially triggered by seismic shaking destabilizing events (Hoyos et al., 1996). 13) Neotectonics: Several circular to arcuate crustal features related to neotectonic disturbances have been described within the Neogene Tajo and Gascueña basins in central Iberia (Alía et al. 1980; Martín Escorza 1980, 1983). 14) Meteoritic impacts: a supposedly meteoritic impact origin has been associated with the Azuara circular feature in eastern Iberia (Ernstson et al. 1985).

Surprisingly enough, a series of large-scale elliptical, arcuate to subcircular Iberian crustal features obvious from the simple observation of  satellite images, or topographic, geologic and geophysic maps, have apparently remained undetected. These subcircular features are connecting very different geological elements, and thus they cannot be explained in terms of orogenic-related processes such as oroclinal bending.

The topographic map of the Iberian peninsula (Fig. 2) clearly reveals a major circular feature which is defined by a series of elevations (from N to S León and Cantabrian Mountains, Iberian Ranges, and Betic central mountains) which we will call here the “Cantabrian Mountains, Sierra de la Demanda, Montes Universales, Sierras de Segura/Cazorla, and Sierra Nevada” (CDUSCN) great circle. In fact, the CDUSCN great circle probably constitutes the most important and obvious subcircular element of the Iberian Peninsula, and it is puzzling that it has never been described before (to our knowledge). A smaller elliptical feature is also evident in the topographic map (Fig. 2) defined by the following elements: Spanish Central Range, NW sector of the Iberian Ranges, and León and Cantabrian mountains.

Figure 3. Satellite image of Iberia (EDISAT 1995) and its interpretation (with data from Manolo Hoyos, personal communication). (1): major subcircular to elliptical crustal patterns mostly within the AIM. (2): other minor arcuate to subcircular features. (3): major bounding faults. (4): previous structural trends in the Pyrenees, the Variscan belt, or the Betics. (5): minor fractures.

The satellite image of the Iberian Peninsula (EDISAT 1995)  shows clearly  several subcircular to elliptical crustal patterns within most of the AIM realm (Fig. 3). Particularly interesting are a series of  subconcentric major ellipses that occupy the central part of the Iberian Peninsula (Fig. 3). These are partly defined by some of the topographic elevations previously discussed (e.g. León and Cantabrian mountains, Iberian Ranges, Spanish Central Range), and by the NW-oriented variscan trends in SW Iberia. Some other arcuate shaped features correspond to tectonic lineaments such as the one cross-cutting Iberia from Portugal towards the Duero Basin to the NE. The Duero and Tajo Cenozoic sedimentary basins seem also controlled by subcircular elements. These are also abundant in the SE sector of the Iberian Ranges (as shown by Simón Gómez 1989).

Figure 4. Geological map of Iberia (IGME 1980) and its interpretation. (1): major elliptical to subcircular patterns. (2): arcuate Iberian coasts. (3): other subcircular to arcuate elements related to Variscan or Alpine orogenic events. Inset shows the subdivision of Iberia in two realms: Variscan to the W, and Alpine to the E.

The geological map of the Iberian Peninsula (Fig. 4; IGME 1980) also reveals a series of arcuate, elliptical to subcircular major features within most of the AIM and adjacent areas. The most evident of all is a large-scale deformed subelliptical pattern connecting the following elements (from N to S; Fig. 4): the N boundary of the Duero basin, the Cenozoic sedimentary basins of the Iberian Ranges, the N border of the Prebetic front, and the WNW-oriented variscan trends in SW Iberia. This major subelliptical element grossly corresponds to the boundary between the Variscan Iberia to the W and the Alpine Iberia to the E (see inset in Fig. 4). Subcircular to arcuate patterns are also obvious within the Cenozoic Duero and Tajo basins. Many of the coasts of the Iberian peninsula display also conspicuous arcuate  features.

 

Other structural elements of the AIM and Iberia resulting from horizontal tectonic stresses.

After describing the circular to arcuate crustal to sublithospheric patterns in the AIM and Iberia, in the present section we will review the other major structural elements that defined the Cenozoic evolution of the AIM and Iberia, and resulted from subhorizontal tectonic stresses, i.e. dip-slip and strike-slip fault systems, fault-controlled continental sedimentary troughs, zones of indentation, wedge-shaped escaping blocks, areas of localized gravitational collapse or uplift, etc...

Figure 5. Tectonic sketch of the late Cenozoic Iberian Peninsula showing other major structural elements influencing the evolution of  the Alpine Iberian Meseta (some data from Capote & Carbó 1983; Emery & Uchupi 1984; Moreira 1991; Doblas et al. 1991; Olivet 1996; Martínez García 1997; and  Manolo Hoyos, personal communication). (1): major bounding transcurrent faults (GF: Guadalquivir fault; MF: Messejana fault; PF: Pyrenean fault; SF: Segre fault; VF: Ventaniella fault). (2) other minor transcurrent faults. (3): thrust faults. (4): normal faults. (5): NS magnetic gradient trend in the Iberian platform (Olivet 1996). (6): peridotite ridge in the Iberian platform (Boillot et al. 1989). (7): other minor fault trends. (8): zones of indentation (BI: Betic indenter; PI: Pyrenean indenter). (9): left-lateral drag-effect of the Guadalquivir fault on the Variscan trends. (10): continental sedimentary troughs. (11): Iberian platform highs (GB: Galicia bank; GH: Gorringe high; IB: Iberian bank; TB: Tajo bank). (12): olistostrome nappes in the Gulf of Cádiz (Emery & Uchupi 1984). Other abbreviations: BC: Betic cordilleras; CR: Catalonian ranges; DB: Duero basin; EB: Ebro basin; GT: Guadalquivir trough; HM: Hesperian Massif; IR: Iberian ranges; PY: Pyrenees; TB: Tajo basin. 

Following the main Alpine orogenic events in the Pyrenees (Eocene to early Miocene) and the Betics (late Cretaceous to early Miocene compression and middle to late Miocene extension; Doblas & Oyarzun 1989 a, b), the late Cenozoic evolution of the AIM was controlled by the following elements (Fig. 5):

1) A major wedge-shaped W-directed complex block comprising the whole AIM and most of the Iberian plate (except the Betics and the eastern Iberian realm) whose kinematics is controlled by final indentation phenomena in the eastern Pyrenees (S-directed) and  Betics (NW-directed; Doblas et al. 1991). This block is bounded by the EW dextral Pyrenean fault to the N (triggering large-scale domino-like tectonis in northern Iberia), the ENE sinistral Guadalquivir fault to the S (triggering a large-scale drag-effect on the previous variscan NW trends), and the NE sinistral Segre fault to the E (Fig. 5). This last fault is accompanied by a whole set of parallel faults that seem to have controlled the late Cenozoic tectonomagmatic evolution of  eastern Iberia, the Gulf of Valencia, the Alborán sea, northern Morocco, even reaching the european rift system to the N (Doblas et al. 1991; Sanz de Galdeano 1996; Oyarzun et al. 1997; López Ruiz et al., 2002). 

2) A W-directed wedge-shaped crustal block comprising most of the AIM (included within the previous block; Fig. 5), characterized by a set of conjugate strike-slip faults inherited from the late Variscan  tectonic scenario (grossly NE sinistral and NW dextral). This block is bounded to the N by the NW dextral Ventaniella fault and to the S by the NE sinistral Messejana-Plasencia fault. A whole set of V-shaped conjugate faults parallel to these two major bounding lineaments are found within this block, and occasionally outside of it to the E and SE. In the Spanish Central System the NE-directed fault trends bounding it acted as inverse faults uplifting this mountains in between two sedimentary troughs (Tajo and Duero; Doblas et al. 1991).

3) A set of grossly N-oriented (NNW, N, NNE) normal faults (most of them dipping to the W) which concentrate in the western part of the AIM and are subparallel to the Iberian N-oriented Atlantic margin (Fig. 5). To the E, the Duero Basin seems controlled by a N-S normal fault in its western boundary (dipping to the E). This extensional tectonics scenario continues and increases to the W into the Iberian passive margin, accompanied by the progressive thinning of the lithosphere towards the Atlantic. In the Iberian platform extension is characterized by NS-oriented listric normal faults rooted into low-angle detachment horizons, tilted/uplifted blocks (Fig. 5; Galicia bank, Iberian bank, Tajo bank, and Gorringe high; Emery & Uchupi 1984; Boillot et al. 1989; Wilson et al. 1989; Olivet 1996), and even the exhumation of mantle peridotites to the W of the Galicia bank (Boillot et al. 1989; Brun & Beslier 1996; Vázquez et al., 2008) probably continuying to the S as evidenced by a strong NS magnetic gradient (Olivet 1996). The Gulf of Cádiz displays huge W-directed  gravitationally-induced olistostrome nappes (Fig. 5; Emery & Uchupi 1984).

 

The proposal of a model

             The geodynamic evolution of the late cenozoic AIM might be understood in terms of a mixed-dual model involving the close temporal and spatial interaction between the following elements: 1) Vertical mantle upwelling phenomena related to anomalous sublithospheric hot residuum, that leaves its imprint in the form of subcircular to arcuate crustal patterns. 2) Horizontal tectonic stresses related to the NS convergence of  Africa and Eurasia crushing the Iberian plate between the Pyrenees and the Betic/Rif orogenic systems, and living their imprints in the form of crustal-scale W-escaping wedge-shaped blocks bounded to the N, E, and S by major transcurrent faults, and to the W by NS-oriented normal faults. Thus, Iberia behaves as a major W-escaping wedge-shaped megablock rafting irregularly on top of a long-lived anomalous hot sublithospheric residuum rooted beneath this plate.

 

The late Cenozoic evolution of the Alpine Iberian Meseta (AIM).

The late Cenozoic evolution of the AIM resulted from the close spatial and temporal interaction between horizontal tectonic movements and mantle upwelling phenomena (Fig. 6). 

Figure 6. Proposal of a model for the late Cenozoic evolution of the Alpine Iberian Meseta (AIM). A) Tectonic sketch of  Iberia showing its main structural elements: (1): major bounding transcurrent faults (GF: Guadalquivir fault; MF: Messejana fault; PF: Pyrenean fault; SF: Segre fault; VF: Ventaniella fault). (2): domino-like tectonics triggered by the dextral PF in northern Iberia. (3): drag-effect of the sinistral GF on the Variscan trends in the southern AIM. (4): zones of indentation (BI: Betic indenter; PI: Pyrenean indenter). (5): normal faults. (6): Duero basin. (7): Iberian and northwestern Moroccan continental platforms. (8): uplifted blocks in the Iberian continental platform. (9): NS-trending zone of mantle exhumation in the Iberian platform (including peridotites to the N). (10): W-directed olistostrome nappes in the Gulf of Cádiz. B) Sketch showing the three major W-directed escaping blocks in Iberia and its continental platform (W1, W2, W3). C) Sketch displaying the major subcircular to arcuate features influencing the evolution of the AIM (see also Figs. 3 & 4). D) Subdivision of  western Iberia and its continental platform into 6 major NS-oriented extensional zones: (1): Duero basin. (2): western AIM block. (3): Iberian margin coastal block. (4): Iberian platform block with uplifted banks and highs. (5): NS-trending zone of mantle exhumation in the Iberian continental platform. (6): Olistostromes in the gulf of Cádiz. E) WE schematic cross-section showing the strong crustal thinning towards the Atlantic and  the different extensional zones depicted in D: (aam): anomalously hot asthenospheric mantle. (cc): continental crust. (ml): mantle lithosphere. (oc): oceanic crust.

1) Horizontal tectonic stresses resulted from final indentation phenomena occurrying in the eastern Pyrenees and Betics orogens (S- and NW-directed, respectively; Sibuet, 1989; Doblas et al. 1991; Olivet, 1996; Dèyes et al., 2004), triggering crustal-scale W-escaping lateral extrusion of wedge-shaped blocks bounded by major faults (Figs. 5 & 6 A & B): 1) A huge complex W1 block (Fig. 6 B) comprising the AIM and most of the Iberian plate, bounded by the EW dextral  Pyrenean fault to the N (triggering large-scale domino-like tectonics), the ENE sinistral Guadalquivir fault to the S (triggering a large-scale drag-effect), and the NE sinistral Segre fault to the E;  2) A major W2 block (Fig. 6 B) comprising most of the AIM  and bounded to the N by the NW dextral Ventaniella fault and to the S by the NE sinistral Messejana-Plasencia fault; 3) A minor W3 block (Fig. 6 B) in the western Iberian margin bounded by E-dipping high-angle normal faults. These notions are compatible with the present state of stress of the Iberian peninsula characterized by two compressional sectors in SE and NE Spain as related to the Betics and Pyrenean indentation zones (De Vicente et al., in press).

Similar models of W-directed lateral extrusion of  wedge-shaped blocks have been suggested for the Alpine evolution of the Iberian plate (Saenz de Santa María 1976; Menduiña Fernández 1978;  Fonseca & Long 1991), the Alborán basin (Fonseca & Long 1991; Gómez et al. 2000), and the Moroccan Meseta in northwestern Africa (Fonseca & Long 1991; Carracedo et al. 1998). In fact, the whole peri-Mediterranean Alpine orogenic system related to the NS-convergence between Africa and Eurasia was the site of W- to E-directed escape of crustal blocks (Tapponnier 1977; Jolivet 1997): e.g. the W-directed escape of the Anatolian wedge; the E-directed escape of the eastern Alpine wedge giving rise to the Carpathians curved orogen in its open eastern frontal zone (Moores & Twiss 1995). Classical examples of this type of lateral extrusion of wedge-shaped crustal blocks are the following: the collision between India and Asia along the Himalayas, triggering the SE-escape of a series of triangle-shaped crustal wedges in SE Asia (Jolivet 1997); the E-directed escape of the Caribbean realm compressed between North and South America (Burke et al. 1978); the E-directed escape of the triangle-shaped Mojave crustal wedge bounded by the Garlock and San Andreas strike-slip faults (Cummings, 1976).

 Lateral extrusion of wedge/triangle-shaped crustal blocks compressed between two crustal plates is a common mode of deformation in intraplate domains subjected to collision, indentation, and/or convergence (Moores & Twiss 1995; Jolivet 1997). According to this model, the wedge-shaped crustal block is bounded  by two convergent  deformation zones (strike-slip, subduction, collision, obduction, and/or indentation) and an unconstrained margin in its open side, which allows the lateral escape of the block with a series of extensional features (e.g. Jolivet 1997). The internal deformations of the Caribbean and Mojave wedges have been described in terms of  a Prandtl cell model involving sets of conjugate strike-slip faults and extensional and compressional features (Cummings 1976; Burke et al. 1978).

The Iberian plate might be understood in terms of a modified Prandtl cell model  with internal deformations very similar to the case of the eastern Alpine wedge (Moores & Twiss 1995) involving the following: an extensional zone in its wedge-shaped closed eastern end (the Duero basin); a set of conjugate strike-slip fault within the central parts of the escaping wedge; and a set of NS-oriented extensional zones in its unconstrained open western side (Vázquez et al., 2008).

 2) Sublithospheric mantle upwelling phenomena active within the AIM and the rest of the Iberian Peninsula might be related to different scenarios: 1) the long-lived influence of the Eastern Central Atlantic Western African European anomalous hot sheet residuum resulting from the long-lived effects of the Triassic/Jurassic “Central Atlantic Magmatic Province superplume, CAMP; Oyarzun et al., 1997) triggered a whole series of arcuate to subcircular crustal patterns within the AIM (and the rest of the Iberian plate and its Atlantic margin). Some of the  mantle upwelling cells were probably accomodated along previous structural elements of the Iberian lithosphere that acted as preferential weaker zones for the ascent of the sublithospheric residuum, e.g.: Variscan orogenic trends, late Variscan transcurrent faults, Alpine Spanish Central System and Iberian Ranges,  Cenozoic grabens of the Iberian Ranges. Other upwelling cells were probably decisive in the evolution and present configuration of some geological elements (e.g. the Duero and Tajo Cenozoic basins). It is conceivable that these long-lived asthenospheric mantle upwelling phenomena following the eastern central atlantic asymmetric volcanic margin (Callot et al., 2001) worked closely together with the horizontal tectonic scenario previously described to induce the W-extrusion of the wedge-shaped Iberian blocks, while triggering the generalized uparching of the AIM anomalously high domain. 2) Mantle insulation processes active beneath the long-lived Iberian craton contributing to uprise permanently this domain (e.g., Doblas et al., 2002). 3) Edge-driven convection (King and Anderson, 1998) as related to the asymmetric opening of the eastern central Atlantic volcanic margin (e.g., Etheridge et al., 1988; Wernicke and Tilke, 1988; Malod, 1987) generating an asthenospheric flux between Europe/Africa and the eastern central Atlantic domain accompanied by a strong crustal isostatic and epeirogenic uplift of the eastern upper block of the detachment systems acting as rolling hinges (Iberia and western Africa; e.g., Sahagian, 1988; Wernicke and Axen, 1988; Hartley et al., 1996; Doglioni et al., 2003; Fernandes et al., 2004; Benmohammadi et al., 2007) where high heat fluxes are predominant (Hamoudi et al., 1998). According to Doglioni et al (2003) the depleted lighter asthenosphere generated below the ocean ridge was shifted eastward relative to the lithosphere, thus determining a density deficit below the eastern flanks (Iberia and western Africa). In any case, the western African asthenosphere was the site of continued feeding of plume material coming from the African superplume/superswell giving rise to the characteristic dome/swell topography of the African plate lithosphere (Ollier, 1981; Pavoni, 1992; Nyblade and Robinson, 1994; Ebinger and Sleep, 1998; Grevemeyer, 1999; Gurnis et al., 2000). A similar asymmetric geotectonic scenario was active in the northern Atlantic were the eastern margin was also the site of continued asthenospheric diapirism (Rohrman and van der Beek, 1996; 4) The anticlockwise rotation of the Iberian peninsula during the Mesozoic/Cenozoic contributing to open the bay of Biscay (Malod and Mauffret, 1990; De Jong, 1990; Srivastava et al., 1990).

 3) The set of  extensional-related  NS-oriented blocks bounded by normal faults which concentrate in the western part of the AIM and the Iberian Atlantic margin (Figs. 6 D & E; Vázquez et al., 2008) seem to result from the combined action of several elements:  A) The W-directed Iberian wedge-shaped blocks escaping towards their unconstrained western open side; B) The differential upwelling of the AIM with respect to the Atlantic/Iberian margin according to a W-directed rolling-hinge model proposed  by Oyarzun et al. 1997 (involving the major uplift of the western Iberian margin and a zone of permanent upward dragging of sublithospheric mantle); and, C) The Iberian passive margin W-increasing extensional scenario triggering the progressive thinning of the lithosphere towards the Atlantic till the final exhumation of mantle peridotites (Figs. 6 D & E). The western Iberian thin passive margin constituted the frontal part of the Iberian raft which was highly unstable on top of the sublithospheric anomalous hot residuum. In this zone, the upwelling asthenospheric mantle cells triggered differential collapsed and uplifted domains (e.g. olistostrome nappes of the gulf of Cádiz, Galicia and Iberian banks).

 

Conclusions

The Cenozoic geodynamic evolution of the AIM and the Iberian plate might be understood in terms of the close temporal and spatial interaction between two major factors: mantle upwelling phenomena and horizontal tectonic stresses. The proposed model for the AIM involves a major W-escaping wedge-shaped lithospheric megablock (a Prandtl cell) rafting irregularly on top of a plume-related long-lived hot and upwelling asthenospheric mantle residuum.

The sublithospheric upwelling movements in the AIM are related to several possible geodynamic scenarios: 1) The far-reaching and long-lived effects of a major Triassic/Jurassic thermal superplume event within the gigantic central Atlantic tholeitic magmatic province (CAMP). Plume material was channeled towards the NE as far away as 4000-5000 km (SW Europe). Plume-induced mantle upwelling processes continued till the present along the NE-directed ECAWAE realm defining a long-lived anomalous sublithospheric hot residuum sheet attached to the roots of the moving plates within this gigantic domain. These long-lived mantle upwelling processes left their imprints in the form of large- to small-scale subcircular to arcuate crustal to sublithospheric patterns the Iberian Peninsula, and the AIM (Hoernle et al. 1995; Oyarzun et al. 1997; Maury et al. 2000). 2) Long-lived mantle insulation conditions beneath the Iberian stable craton contributing to the permanent uprising of this domain. 3) Edge-driven convection of the eastern Altantic margin liberating the heat accumulated beneath the European domain towards the Atlantic realm. 4) Anticlockwise rotation accompanying the opening of the bay of Biscay during the Mesozoic/Cenozoic. We favour the first hypothesis as it has been shown that the Eastern Atlantic/Iberian/Moroccan/European domain is underlaying by a common mantle component that contributed to generate the different alkaline volcanic provinces spanning from the Cape Verde islands up to northern Europe (Oyarzun et al., 1999).

Late Cenozoic horizontal tectonic stresses are associated to the NS convergence between Africa and Eurasia crushing the Iberian plate between the eastern Pyrenees and Betic indentation zones. This scenario triggered a series of  W-escaping wedge-shaped blocks bounded to the N, E, and S by major transcurrent faults, and defining in their western open sides a major W-increasing extensional scenario involving NS-oriented high- to low-angle normal faults,  differentially uplifted and collapsed blocks, and the denudation of the asthenospheric mantle in the Iberian-Atlantic passive margin. This would constitute as a whole a Prandtl-cell-type model of intracontinental wedge-shaped deformation. At least three wedge-shaped  Prandtl cells decreasing in size to the W were generated by this  W-directed extrusion model (Figs 6 A & B). Some of the major subelliptical features observed in Iberia are apparently also migrating to the W while decreasing in size (Figs. 2, 3, 4 & 6).

            One may wonder how did this mixed scenario of mantle upwellings and horizontal tectonic stresses work in the practice, as it might seem on a first approach that such contrasted structures as linear megafaults and subcircular patterns are geometrically incompatible. However, it is easy to envision that both elements may have  progressively interacted (Fig. 6 C; at the same time and/or alternatively), according, for example, to the following hypothetical scheme: 1) upwelling of a major subcircular central domain; 2) W-escape of a huge wedge-shaped block bounded by a conjugate transcurrent fault system transecting the previous subcircular pattern; 3) renewed upwelling of a smaller  inner sector bounded by the previous conjugate transcurrent faults; 4) disruption of the western realm by major NS-oriented W-dipping normal faults; 5) arcuate-shaped  upwelling of the eastern footwall blocks of these faults according to a rolling-hinge model; 6) W-escape of a smaller Prandtl-type wedge in the W; 7) generalized upwelling of the western platform extensional domain; etc...

It is conceivable that the mantle upwelling phenomena played an important role in the Neogene extensional collapse of the Betic/Rif orogenic belt, and some of the western Mediterranean basins (Alborán and Valencia; Doblas & Oyarzun 1989 a, b; 1990). In this sense, Weijermars (1988) recognized the relevance of such large-scale mantle upwelling processes in the Alborán basin, even if his model was based on a verticalist-type scenario which is difficult to believe.

The Cretaceous rotation of the Iberian Peninsula seems to be a unique case in the plate tectonics literature (similar scenarios have rarely been described). We suggest that the anomalous hot and upwelling sublithospheric residuum was a key factor governing the behaviour of this rotating and rafting plate.

Acknowledgements: Special thanks and thoughts are dedicated to the late Manolo Hoyos, a unique geologist and friend who was an endless source of inspiration and ideas. I wish to thank J. López Ruiz, C. Martín Escorza, M. Alía and N. Youbi  for constructive discussions and comments. We thank José Arroyo for the drafting of the figures.

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