3Miocene basins in the Betic fold and thrust zone
The Betic Cordillera of southern Spain togetherwith the Rif in Morocco and the Tell Mountains innorthern Algeria form a morphologically distinct, arc-shaped orogen, which constitutes the western end ofthe Mediterranean Alpine chain (Fig. 3.1). The outerarc of this orogen consists of a fold and thrust belt (theBetic and Rif External Zones), whilst the inner arc ismade up of an allochtonous pile of mostly metamor-phic rocks exposed in the Betic and Rif InternalZones. Part of the Internal Zone is presently sub-merged in the Alboran Sea. The Betic-Rif arc geom-etry developed during the latest Oligocene and earlyto middle Miocene, when a combination of westwardmotion plus extensional deformation of the InternalZone and slow but continuous African Europeanplate convergence resulted in outward thrusting andfolding of the Mesozoic Cenozoic cover in front ofthe migrating Internal Zone (Fig. 3.2). A number ofhypotheses have been proposed in the literature to ex-plain the extensional collapse and westward drift ofthe Internal Zone, and the simultaneous folding andthrusting in the External Zones, in terms of orogenicprocesses at the African European plate boundary(e.g., Platt and Vissers, 1989; Garca Dueas et al.,1992; Royden, 1993; Seber et al., 1996; Vissers et al.,1995; Lonergan and White, 1997; Spakman andWortel, 2004).
During the Miocene, basins developed both on topof the growing fold and thrust belt of the ExternalZone and within the extending Internal Zone. Thischapter focuses on Miocene basins in the fold andthrust belt, in particular on those in the eastern part ofthe Betic External Zone (Fig. 3.1). During theNeogene, the External Zone (Pre- and Subbetics)formed in essence a foreland domain, in which theMiocene is represented by a marine Flysch series(or Tap facies, or Moratalla Formation; e.g.,Hermes, 1978; Garca-Hernndez et al., 1980; OttdEstevou et al., 1988; Sanz de Galdeano and Vera,1992). The flysch deposits pass upwards into lateMiocene molasse type continental deposits (e.g.,Dabrio, 1972; Calvo et al., 1978; Ott dEstevou et al.,
1988; Sanz de Galdeano and Vera, 1992). Both theMiocene and underlying Mesozoic Cenozoic rocksare folded along approximately ENE to NE trendingaxes or are cut by generally northwest directedthrusts. Some of the Miocene basins, e.g., thePontones basin (Figs. 3.2c and 3.3), have (partially)been overridden by thrust units and have been discon-nected from the surrounding basins. But within theBetic External Zone, and in the Prebetics in particular,there are also clearly extensional structures, whichrun parallel to the general ENE-WSW trend of thecompressional structures as indicated on the geologi-cal maps of the Instituto Geologico y Minero deEspaa (IGME). Some of the Miocene basins in thePrebetics are bounded by these extensional structures,such as for example the Santiago de la Espada basin(Figs. 3.2c and 3.3) which is bounded along its north-ern margin by a morphologically distinct normal fault.At first inspection, the geometry of this basin suggestsan extension-related origin with significantdisplacements along the extensional faults, and a di-rect relationship between the compressional struc-tures (the folds and thrusts) and the extensional faultsis not immediately obvious. Except of a study byLujn et al. (2000) and by Crespo-Blanc and Campos(2001), both in the Gibraltar fold and thrust belt, ear-lier structural studies of the External Betics (e.g., DeSmet, 1984; Ott dEstevou et al., 1988; Banks andWarburton, 1991; Platt et al.; 2003) seem to have ei-ther overlooked or ignored the existence and signifi-cance of these extensional structures.
At least four different explanations may accountfor the development of extensional structures such asthose in the Santiago de la Espada basin: (1) collapseof an overthickened fold and thrust belt or orogenicwedge (e.g., Davis et al., 1983; Platt, 1986; Dahlen,1990), (2) displacement of the locus of extension fromthe Internal towards the External Zone (e.g., Crespo-Blanc and Campos, 2001), (3) stratal extension in thefootwalls of thrust faults (Platt and Leggett, 1986), (4)hanging-wall collapse in response to a specific faultplane geometry (a shallowing-upward fault; Coward,1983). The first two explanations imply that theextensional structures should accommodate regional,
Figure 3.1. Geological map of part of the External Zone of the Betic Cordillera in southern Spain, modified from MapaGeologico de la Peninsula Iberica (IGME, 1:1.000.000, 1981). Structural data in the Prebetics after Platt et al. (2003).Rectangle marks area of interest in western Prebetic Miocene basins (Pontones and Santiago de la Espada basins). IEZB- Internal-External Zone Boundary. (1) External Betics profile from Banks and Warburton (1991); (2) Carzola and Nerpio-Fuensante sections from Platt et al., (2003); (3) Pontones-Santiago de la Espada section (this study); for sections, see Figure 3.2.
Iberian Meseta:Variscan basementand Mesozoic cover
Santiago dela Espada
P. de DonFadrique
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Alboran DomainExternal Zone