ELSEVIER Marine Geology 155 (1999) 943
The Betic orogen and the IberianAfrican boundary in the Gulf ofCadiz: geological evolution (central North Atlantic)
Andres Maldonado a,, Lus Somoza b, Lorenzo Pallares c
a Instituto Andaluz de Ciencias de la Tierra, C.S.I.C.=Universidad de Granada, Campas Fuentenueva s=n,E-18002 Granada, Spain
b Geologa Marina, Instituto Tecnologico Geominero de Espana, Rios Rosas, 23, 28003 Madrid, Spainc Departamento de Geodinamica, Universidad de Granada, 18071 Granada, Spain
Received 20 November 1996; revised version received 3 November 1997
The study of the Gulf of Cadiz on the basis of multichannel seismic profiles and wells illustrates the stratigraphy andtectonics. The evolution of the southern Iberian margins was more complex than in most North Atlantic margins sinceit entailed several phases of rifting, convergence and strike-slip motions. Three main tectonic provinces surround theinternal zones of the Gibraltar Arc orogenic belt. These include in the Iberian margin of the Gulf of Cadiz the flyschunits of the Campo de Gibraltar complex, the Betic External Zones, and the Neogene basins of the Guadalquivir Valley.Fault-bounded blocks of flysch and Subbetic units crops out over large areas of the southeastern Iberian shelf. Thebasement of the northwestern area, in contrast, is represented by the Paleozoic rocks of the Hercynian massif of Iberia.Half-graben structures determined the main structural trends of the margin during the Mesozoic, which were affected byinversion structures during the Neogene compressional stages. The Mesozoic and lower Cenozoic units are best observedin wells and seismic profiles from the northern area. These units are either obscure below a thick olistostrome depositor are absent in most of the rest of the Gulf of Cadiz. Seven lithoseismic units from Triassic to Upper Oligocene andanother seven Neogene and Quaternary units are identified based on the relationship to the depositional sequence and theemplacement of the olistostrome. The first tectonic phase was characterized by a passive margin, which was controlled bythe development of half-graben extensional structures and carbonate platforms. This evolution comprises the Mesozoicand early Cenozoic. Ocean-spreading in the North Atlantic induced extensional tectonics, which deformed the Cretaceoussyn-tectonic post-Aptian deposits. Increased amounts of terrigenous materials were supplied to the margin from Aptianto Albian times, controlling depositional patterns, while terrigenous siliciclastic facies replaced the Jurassic carbonateplatforms. From Middle Eocene to Early Miocene the margin was influenced by the relative motions of Iberia and Africaand the development of the Alpine orogeny. The IberianAfrican boundary in the Gulf of Cadiz experienced transpressionand the Mesozoic basins probably underwent subduction. The emplacement of an olistostrome took place in the Gulf ofCadiz towards the Central Atlantic basin plains during the Tortonian. The end of the olistostrome emplacement during theLate Miocene coincides with accelerated tectonic subsidence, while thick progradational and aggradational depositionalsequences were developed. The occurrence of closely juxtaposed regions of compression and extension during the Miocenemay reflect the westwards progression of the Gibraltar Arc mountain front over a subducting thinned Tethys crust. Whenthe motion between Iberian and Africa was NS to NNWSSE oriented, the migration of the arcuate mountain front intothe eastern Gulf of Cadiz is attributed to a mechanism of collision induced delamination of the mantle lithosphere. In
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0025-3227/99/$ see front matter 1999 Published by Elsevier Science B.V. All rights reserved.PII: S 0 0 2 5 - 3 2 2 7 ( 9 8 ) 0 0 1 3 9 - X
10 A. Maldonado et al. / Marine Geology 155 (1999) 943
contrast to the BeticRif belts, however, there is no geophysical evidence to postulate that continental collision took placein the Gulf of Cadiz during the Cenozoic. 1999 Published by Elsevier Science B.V. All rights reserved.
Keywords: continental margin evolution; plate boundary tectonics; Gulf of Cadiz; central North Atlantic; Gibraltar Arc
The Atlantic margins of Iberia have receivedmuch attention during the last two decades, thusproviding significant geological information to en-large the present understanding of the evolution ofthe North Atlantic and the development of passivemargins (Tankard and Balkwill, 1989; Lister et al.,1991). Atlantic margins evolved over a hundred mil-lion years, starting with an early Mesozoic periodof rapid rift propagation into the North Atlantic andthe spreading of Iberia and the Grand Banks inthe Early Cretaceous (Srivastava et al., 1990a). Themargins along North America became recognized asAtlantic-type passive margins and they underwentsignificant postrift thermal subsidence that resultedin the deposition of a seaward-thickening terracewedge, which in places is more than 10 km thick(e.g., margins of eastern Canada; Keen et al., 1990).The conjugate passive margins of the eastern NorthAtlantic, however, indicate the presence of an ap-parent asymmetry in their structure and evolutionarypaths and the comparatively thin postrift cover re-veals a wide variety of tectonic types (e.g., Atlanticmargins of Iberia; Boillot et al., 1989; Wilson et al.,1989).
Active and transcurrent margins in the North At-lantic are rarer and the best examples are locatedalong the plate boundaries of Iberia, where pri-mary horizontal motions occurred since the Meso-zoic (Dewey et al., 1989). Horizontal motions alongstrike-slip systems were important in the northernIberia margin of the Bay of Biscay, which also be-came an active margin during the Paleogene due tothe relative motions of Iberia and Eurasia (Boillot etal., 1989; Garca-Mondejar, 1989; Srivastava et al.,1990b).
The tectonic evolution in the Gulf of Cadiz wasalso more complex than in most North Atlantic mar-gins since it entailed several phases of extension,convergence and strike-slip motions. Understandingthe geological evolution of the Gulf of Cadiz mar-
gin is, moreover, important because it occupies afocal position between the westernmost segment ofthe Mediterranean realm and the IberianAfricanboundary (Fig. 1). The opening of the North Atlanticduring the Late Cretaceous and Tertiary induced therotational divergence of North America and Eurasia,which was paralleled by the counterclockwise con-vergence of Africa and Eurasia. The northwards driftof Africa caused the progressive closure of oceanicbasins of the Tethys and the rapid westwards prop-agation of the Alpine, BeticRif orogenic collisionfront in the Gulf of Cadiz, in parallel with the devel-opment of the western Mediterranean basins (Deweyet al., 1989; Garca-Duenas et al., 1992; Jabaloy etal., 1992; Maldonado et al., 1992). As a consequencethe margins of the Gulf of Cadiz preserve the dif-ferent phases of extension and collision along theIberianAfrican boundary and also the evolution ofthe westernmost segment of the Alpine orogeny.
The tectonics, stratigraphy and chronology ofevents for the development of the Gulf of Cadiz havebeen discussed in several studies (Roberts, 1970; La-jat et al., 1975; Malod and Mougenot, 1979; Martnezdel Olmo et al., 1984; Mougenot, 1988; Flinch et al.,1996). In spite of much recent work on southern Iberiaand northern Africa, however, several conflicting geo-dynamic hypotheses have been proposed and a gen-eral consensus has not been reached (cf. Dewey, 1988;Doblas and Oyarzun, 1989; Platt and Vissers, 1989;Maldonado et al., 1992; Docherty and Banda, 1995;Flinch et al., 1996; Seber et al., 1996a,b). Moreover,the precise growth patterns of the margins and their re-lationships with the major tectonic events occurring inthe plate boundary, the principal structural elements,and the evolution of the area have not previously beendescribed in detail. In this study, we analyse seismicreflection profiles from commercial and academic in-vestigations that allow the identification of growthpatterns and the history of development to be con-strained. In particular, we characterize the morpho-logical and structural provinces, which are discussedwithin the framework of the surrounding lithospheric
A. Maldonado et al. / Marine Geology 155 (1999) 943 11
Fig. 1. Geological setting and simplified bathymetry of the Gulf of Cadiz and surrounding areas. Legend: 1 D Hercynian Massif; 2D BeticRifean Internal Zones; 3 D dorsal complex; 4 D flysch units; 5 D Meso- and Intrarifean units; 6 D Prerifean units; 7 DBetic External Zones; 8 D Mesozoic paleomargins of IberiaAfrica; 9 D Guadalquivir olistostrome units; 10 D Neogene basins; 11 Dolistostrome front; 12 D seismic foci; COB D oceancontinent boundary. Bathymetry in meters.
elements. These data allow the geometry of the struc-tures and the tectonic processes that occur around theboundary between Iberia and Africa to be identified.The expression of the westernmost segment of theAlpine orogenic belts and its influence on the devel-opment of the Gulf of Cadiz is also analysed. Theobjective of this study is to provide new informationfor a better understanding of the evolution of conti-nental margins in a complex tectonic setting locatednear plate boundaries.
The Spanish continental margin of the Gulf ofCadiz has been intensively explored by oil compa-nies, thus providing a dense network of multichannelseismic (MCS) profiles (Delaplanche et al., 1982;
Martnez del Olmo et al., 1984; Riaza and Martnezdel Olmo, 1996). Additional MCS profiles werealso obtained with the B=O Hesperides during theHE-91-3 cruise (Fig. 2). These MCS profiles havebeen postprocessed to common depth point, stackand time migrated using standard procedures. Infor-mation is also available from 26 commercial wellsdrilled offshore in the Gulf of Cadiz (IGME, 1987).
The seismic units recognized in the MCS profileshave been characterized on the basis of the bore-hole data (thicknesses, lithology, facies, age) from 8representative wells (B-1, B-3, C-1, D-1, E-1, G-1,6Y-1bis, and ATLANTIDA-2) of several sectors of theGulf of Cadiz (Fig. 2). In order to better understandthe history of lithospheric subsidence, decompactionand backstripping techniques were applied in 5 se-lected wells, using the methods described by Stecklerand Watts (1978) and Stam et al. (1987).
12 A. Maldonado et al. / Marine Geology 155 (1999) 943
Fig. 2. Simplified bathymetric chart of the Gulf of Cadiz. Contour interval is 100 m with additional contours at 50 m. The grid ofmultichannel seismic (MCS) profiles and location of commercial wells analysed for this study is shown. Solid lines with numbersindicate MCS profiles displayed in Figs. 5 and 6.
3. Geological setting
3.1. Breakup of Pangea and the Mesozoic margins
Plate kinematic solutions indicate that during theTriassic the area of the Gulf of Cadiz was part of theTethys-related rift systems, when Permo-Carbonif-erous fracture zones became tensionally reactivated(Ziegler, 1989). The breakup of Pangea resulted inTriassic rifting and the development of the marginsof southern Iberia and northern Africa (Heymann,1989). Late Triassic basic volcanism was associatedwith this rifting and is represented by hydroclas-tic eruptions of basic volcanic rocks in the Cadizmargin. Post-volcanism tectonic extension initially
occurred in a SSW direction, changing later to a SEtrend (Garca-Navarro et al., 1994).
The Central Atlantic ridge ended in the GloriaTransform Fault, which was a major tectonic bound-ary throughout the early Mesozoic. This transformfault was probably connected with the extensional,deep oceanic basins of the Tethys. The Late Juras-sic and Early Cretaceous progressive rifting of theCentral Atlantic caused a major sinistral translationbetween Africa and Laurasia, which probably oc-curred along wrench-induced deep-crustal fractures,where the relative motion between the Central andNorth Atlantic rift systems was accommodated (Kl-itgord and Schouten, 1986; Ziegler, 1989). Jurassicspreading of the Tethys ocean took place through a
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major transcurrent fault, separating northern Africanfrom Iberian margins (Dercourt et al., 1986; Mauffretet al., 1989). This fault zone is thought to be locatedat the present area of the Alboran Sea (Andrieuxet al., 1971), but it could has extended westwardsinto the Gorringe Bank region, attributed to the latestJurassic (Feraud et al., 1986). Northwards, the ageof the older oceanic crust of the Tagus Plain, show-ing the continentocean boundary, corresponds tomagnetic anomaly M21 (150 Myr, early Tithonian).Mauffret et al. (1989) suggest that during the Creta-ceous occurred a northwards migration of the riftingprocess and a westwards jumping of the ridge, whichimplies an Early Cretaceous (anomaly MO, Aptian)abandoned spreading centre in the Tagus Plain. Seafloor magnetic anomaly reconstructions show thatthe separation of Eurasia from North America initi-ated 118 Myr ago north of the NewfoundlandAGFZ(AzoresGibraltar Fracture Zone) (Srivastava et al.,1990a). A spreading axis and associated transformfaults were in existence during this time in the Gulfof Cadiz area, while a triple junction occurred be-tween the AfricanIberiaNorth American plates inthe vicinity of the Gorringe Bank (Fig. 3).
3.2. Cretaceous and early Tertiary North Atlanticspreading
During the Cretaceous and Tertiary, the openingof the North Atlantic induced the rotational diver-gence of North America (Srivastava et al., 1990a).Iberia acted as an independent plate for most ofthe middle Cretaceous, until it became attached toAfrica sometime before chron 34 (Late Cretaceous,84 Myr), when the plate boundary was located in theBay of Biscay (Srivastava et al., 1990b). The MCSprofiles and the magnetic lineations from the easternHorseshoe Plain indicate Upper Jurassic deposits be-low a pre-Aptian reflector which represents a majorangular unconformity (Mauffret et al., 1989; Sartoriet al., 1994). Lower Aptian deposits were drilled atthe southern Gorringe Bank in DSDP Site 135 (Ryanet al., 1973), which also indicates that sedimenta-tion related to North Atlantic ocean spreading beganduring the Aptian in the area of the AGFZ, whereasthe Jurassic deposits were largely controlled by theNorth AfricanIberian ocean spreading.
During the Middle Eocene to Middle Miocene
(chron 18 to chron 6c), Iberia became once againan independent plate. Strike slips along the Glo-ria Transform Fault passed into the compressionalarea of the Gorringe Bank, while the AGFZ wasan active plate boundary between Africa and Iberia(Roest and Srivastava, 1991). The relative motionbetween the two plates in the Gulf of Cadiz wassmall between chrons 18 and 13, but since then com-pression has existed east of Gorringe Bank, with asignificant amount of shortening in the...