r'-
Cie n ci ~
daTerra (UNL) pp. 9-249 Figs. .Evolution of Iberia during the Cenozoic
with special emphasis on th e formation of th e Betic Cordillera and
its relation with the western Mediterran ean
C. Sa nz de Ga ldeano
lnlil;luto Andaluz deCienciesde la Tierra. 1807 1 Granada (Spain); tel, (34) 958 246267 , fax (34) 958 24338 4; clillll2@goliat.ugr.C$
AB STRA CT
Key-word s. Iberia; Betic Cordillera; Cenozoic; western Mediterranean.
The extensional process affecting Iberia during the Triassic and Jurassic times change from the end of the Cretaceous and, throughout the Palaeocene. the displacement between the African and European plates was clearly convergent and pan of the future Internal Zone of the Betic Cordillera was affected. To the west, the Atlantic continued to open as a passive margin and, to the north, no significant deformation occurred. During the Eocene, the entire Iberian plate was subjectedtocompression. which caused major deformations in the Pyrenees and also in theAlp ujanidc and Nevado-Filabride, Internal Betic, complexes. In the Oligocene continued this situation, but in addition, the new extensional process oceurring in the western Mediterranean area, together with the constant eastward drift oflberia duetoAtlantic opening, compressed the eastern sectoroflbcria, giving rise to the structuring of the Iberian Cordillera. The Neogene was the time when the Betic Cordillera reached its fundamcntal features with the westward d isplacement of the Betic-RifI ntemal Zone, expelled by theprogressiveopening of the Algerian Basin, opening prolonged till the Alboran Sea. From the late Miocene onwards, all Iberia was affected by a N-S to NNW-SSE compression, combined in many points by a near perpendicular extension. Specially in eastern and southern Iberia a radial extension superposed these compression and extension.
RESUMO
Palavras-chave, Iberia; Cordilheira betica; CenozO ico; Meditemineo ocidcntal.
o
processo de extenssc que afectou a Iberia durante 0 Triasico e Jurassicc alterou-se a partir do final do Cretacico. Ao longo do Paleocenico, 0 deslocamento entre as places africana e europeia foi clararnenteconvergente, tendo sido afectada partc da futura zona interne da Cordilheira betica. AOesle, 0 Athintico continuou a abrircomo uma margem passive e, para None, nio houve defbrmacao significativa. No Eocenico, a placa iberica foi submetida a ccmprcssac que originou deformaeces nos Pirineus e nas zonas bences internes das Alpujarridas e Nevado-Filabridas.Estasituar;io menreve-seno Oligocenico; todavia, no Mediteminco ocidental houve novos proeessos distensivos que, juntamente com a deriva para Este da Iberia devidcIiabcrtura do Atli ntico, originou compressiks no sector Bste conduzindoitestruturaeao da Cordilheira iberica. No Neogenico, a Cordilheira berica adquiriu as caracteristicas essenciais, com deslocamento para Oeste da zona interne bcuco-rifenha, progressivamente segregada pela abcrtura da Bacia argelina que se estendcu ate 0 marde Alboran. A partir do Miocenicc superior, a Iberia foi submetida a ccmpressao N-S a NNW-SSE, com extenssc quase perpendicular; no Bste e SuI.alemdesta ccmpressao, bouve distensi o radial.10Cong resso so bre
0Ccnozcico de Portugal
INTROD UC T IO N
Iberia is a lithos pheric (s ub) plate located between the NW of Africa and the SW Euro pea n pla te (Fig. 1). Fro m the beginning o f the Mesozoic until late in the Early Cretaceous the movements of Iberia co incid ed ge nerally with those of Europe . whereas, from that time to the Oligocene. Iberia mo ved toge ther with Africa. Since the Oligocene or early Miocene. the Iberian Peninsu la has been joined to Europe (Malad.
1989).
T he nucleus of the Iberian Peninsula is the
Iberian
(or Hesperian) Massif (Fig.2),
whic h was deformed durin g Hercyn ian orogeny; this massif is surrounded by several domains. To the north lies the Bay of Biscay, the oceanic crust of w hich fo rmed duri ng the Cretaceous. To the no rtheast is the Pyrenean Co rdillera, comprised by a Palaeozo ic nucleus and a Mesozoic and Tertiary co ver. TheIberian
Cordillera is situated to theeastand
the
Catalan Coasta lRange
to thesou theast of the Pyrenees, while theBet ic Cor di llera occup ies the south and southeast border. To the west , the peninsula is bo unded by the Atlantic O cea n, form ing a passive margin.
Wit hin the Iberian Massi f, there are two large Tert iary basins (Duero and
Fagus),
separated bythe
Spanish Central System. The Guadalquivir Bas in is located so uth o f the massif and no rth of the Beti c Cordillera, while the Ebro Basin lies between the Iberian Cordilleraand
the Pyrenees.Numerous
sma ller Tertiary bas ins exist in all the domains mentioned above.REG iONAL S IT UATI O N OF T ilE IBER IA
P E~ I:" S ULA A:'\ D T HE ~ I A I:"oO FE ATUR ESOFTH E BET IC CO R DILLERA
The geologic evol utio n of Iberia during the Meso zoic and the Tertiary has been greatly con tr o lled by the interaction between the Afric an and European p lates.
lVRO PEAS
pun:
[[ill
~ta ssirs
~///
1I1i1i1i I
Alpine Cbai ns and th eir internal zones
r;-- --.
~ I es o z o ic
ocea niccru st
•
Cenozoic oceani c crust
Ctmcias do Terra (UNL). 14
which in tum were related to processes ofocea nic growt h, especially to the opening of the Atlantic and also by the evolution of the we stern end of the Tethys.
At the beg inning of the Mesozoic, the Iberi an Massif wa s loca te d ne ar T unisia and A lge r ia (Fig. 3 A). Throughout this period the West ern Mediterranean (wes tern end of the Tethys Ocean) unde rwent a majo r process ofextension and fonnation o f oceanic crust, which probably extended from the original areas of sedimentation of the Betic-Rif co rd illeras to beyond the bas in of the Internal Zone of the Alps (Ligurian Basin). At the same time, the progres sive o pening o f the South Atlant ic grad ually displaced Africa to the east, so that Iberia moved relatively over 1200Ianwestwards. During the Cretaceous, the opening of the Cen tral and Nort h Atlantic caused the anti-cloc kwise rota tion of the Iberian Massif (some 30 deg rees in all, Fig. 3B), thereby induc ing the opening of the Bay of Biscay. This may have occurred together with some displacement of Iberia towards the south, southwest or southeast, simultaneou sly with the first con vergent
movement between Africa and Europe andwiththe first compressive deforma tions in the Internal Zone ofthe Alps. All these movements, firstly extensional and afterwards compressive, were responsib le forthe
future
differentiation of the diverse complexes of the Bet ic Cordillera. This cordillera is composed of the Internal and External zones, as well as the Flysch units (Campo de Gibraltar units,formed
from the forme r Flysch Basin) and many Neogene bas ins (Fig .4).T he Internal Zone is made up of three complexes, which are from botto m to top: the Filabride, theAlpujarride and the Malaguide. The Nevada-Filabr ide and the Alpujarride, strongly affected by the Alpine metamorphism, present a series ranging from the Palaeozoic to the Mesozoic, specially the Triassic. The Malaguide generally wa s not affected by this metamor phism. It presents a Palaeozoic series and its Jurassic to Tertiary series are much better develo ped than in the other two complexes, but with abundant hiatuses. Another co mplex, the Dorsal, which corresponds partially to the stratigraphic cover of the Malaguide (the Internal200km .
h:aby li;1100
o
( "r!JilIna G ulf of \'iZ('a~:l
Rif Call1:lhriall
.\ I:!'l rl l·
,
'So
~ ,
'~
,
-,
1-r-:
/'c....,..,~Ie Congresso sa bre 0 Cenozcico de Portugal
\
'"
Z
'"
U
o
'"
-e
""
Uo;
Fig. 3 - Interpretative schemes of the evolution of Iberia. A: Situation of Iberia at the beginning o f the Dogger, when the western Med iterranean area was affected by extension. B: During the Late Cretaceous, the weste rn Mediterranean, after the rotat ion o f Iberia, underwent the first compressions. C: During the Palaeocene, the process ofsubduction ofthc western Mediterrane an affected strongly the Betic Internal Zone. D: During the Eocene, the Pyrenees, Cantabrian area and the Betic Internal Zone underw ent importan t compressions. Key of the abbrevia tions: A: Alpujarrid e, D& Pd: Dorsal and Predorsal, Ma: Malaguide, N-F: Nevado-Filabride, Pb: Prebetic , Sb: Subbetic. Taken from Sanz de Galdeano (1997) . The arrows indicate the d irection o f tectonic transport.
Dorsal), formed the passage to the south (the External Dorsal)to the Flysch Basin. The Int ernal Zone of the
Rif
inthenorth of Moroccoisthe same Betic Internal Zone (a lthough there, in the
Rif
the Nevado-Filabride Complex isabsent), and thuswe
will refer to the Betic-Rifinternal Zone.TheExternal Zone of the Betic Cordillera is formed by the Subbe tic (to the south) and the
Preberic
(outcropping to the NE).Bothdo mains have Mesozoic and Tertiary sediments, those of the Prebetic corresponding to a more s ha llow marine basin o r even to continen tal environment The Ex ternal Zone in the Rif is also formed by Mesozoic and Te rtiary sed iments but di ffers fro m the Bet ic Externa l Zone. In fact, this last zone fa nned the cov er of the south a nd so uthe ast bord er of the Iberian Massif, wh ile theRif
Externa l Zone made a progressive passage towards the north of the Mesozoi c and Tertiary cover of the Alias.
Cii"ciasda Terra (UNL). 14
The infi lling of the Neogene basins began with the Early Miocene, but most of the basins, apart from the Guadalquivir Basin, deve loped from the Tortoniao onwards, coveringareasofthe Internal and External zones (Sanz de Galdeaoo& Vera, 1992).
EVOLUTION DURING T H E PALEO CENE AND TilE EOCEl'"E
The displacement between the African and Euro pe an p lat e s was cl earl y con verge nt f rom the e nd of th e Cretaceous throu ghout the Pa laeocene (Tapponier, 1977). In the wes tern M editerranean the Ligurian Bas in, which formed duri ng the Mesozoic, began in the Creta ceou s to undergo a subduction (Fig. 3C),probab ly dipping to the so uth. That is, the oceanic crust ofthe Ligurian Basin sa nk
Fig. 4· General geologic map o f tile Beric Cordillera
I- Congresso sobre 0 Cenoz6ico de Portugal
underAfrica or under the domain situated between Europe and Afri ca, called the South-Sardinian domain (Sanz de Galdeano, 199 0, 1997) 0 Alkapcca (Alboran, Kabylia,
Pelo ritani Mountainsand Calabria) (BouiIlinetal., 1986).
(The name of Alboran in this case re fers to the Beti c-Rif Interna l Zone). At th e en d of th e Cretaceous and the beginning o f the Palaeocene an ap prox imately N -S co mpression weakly affec ted Iberi a. To the west, th e Atla ntic co ntinued to open as a pas sive
margin
and, to the n or th, no signi fica nt defo rm ati o n occ urred a nd sedi me ntation continued in most o f the Pyrenees.The process of subduc tion affecti ng the Ligurian Bas in mo ved progressively to co llis ion towards the end of the Palae ocene or in the Ea rly Eocene. Then , durin g th e Eocene , the entir e Ib erian plate wa s s ubj ecte d to compression (Fig. 3 D), which ca used major deformations, most significantly in the Pyr enee s (Pyrenean Eocene phase ), fonning large nappes (M egias, 1988).Deepseismic pro files show that the Iberian plate is subd ucted be neath the European plate (Roure et a/., 1989 ; Suriiiac het a/.,
199 3) (F ig. 5 A), even though there ma y not previous ly have been a large vo lume of oceaniccrust
Duri ngthese deformations of the Pyrenees, the foreland basin (Ebro Bas in ) (Anad6n et a /., 19 85) m igrated southwards, while the northernmarginof the basin was
partia lly inco rpora ted into the yo unge r thrust s heets (Munozet a/.•1988).
Th e co mpressive movements weretransmi ttedfurther to the west durin g tb is Eocene p hase , so that th e no rthwes tern marg in of Ibe ri a tended to b ecome superposed overtheBay of Biscay; i.e.,part of the oceani c crus t in the Bay of Biscay was subdueted beneath
Iberian
(Bo illotet a/.,1984). Part of the Cantabriancoast(Nand NW Ibe ria) was also deformed.
Some de fonnation also occurred
in
the Cata lan Co astal Range , with approximately N-S co mpressi on (continuing into pa rt ofthe Ol igocene)(Ouimera, 1983), togethe r with strong develop ment ofNE-SW sinis tra l stri ke-slip fau lts (Ana d6net al., 1985). Th e crus t that was to be co me the Iberian Cordillera from the Oli gocene on wards (Viallard, 1989) was also affecte d.The se compressional episodes also had an effect on the Span ish Ce ntra l Sys tem duri ng the Eocene, causing crustal thickening ( Vegaset a/.,1990) and reac tivation of late Hercynian faults.
With regard to the present-day Betic Cordillera, the Be tic-Rif Intemal Zone were located so me 500kmeast of
their
presen t position (Fig. 3)andunderwent a major stag eof tec tonic s truc turing and metamorp his m. The fir st process of formation of nappes
in
the Nevado-FilabrideFig. 5 • Schematic geological cross-sections. A: Interpretation of the geological structure of the Pyrenees. Simplified from Munoz (1992) and modifications inspired in Velasqucet al.,(1989). B: Structure of the Valencia trough. Simplified fromMartiet al. ( 1992 )
and Fontbotcet af.( 1990). Lower crust in points. Their position is marked in Fig. 2.
Complex prob ably occurred from the Late Cre taceo us and continued throughout part ofthe PalaeOCene with tectonic trans port towar ds the NW. T he total shortening of the Neva do-Fi labride Complex was at least 200 kID and, at the same time, the oceanic crust practically disappeared from the SW end of the Ligurian Basin. The Alpujarride and the Malaguide, originally situated more to the so uth and southeast, remainedina position of back-arc and did not unde rgo important deformat ions. In the Ma laguide Complex, the Upper Cretaceous and the Palaeocene are both partially or completely absent, perhaps due to a possible emersion of this area . According to Garrido ( 1995), at this time and in this position of the Alp ujarride and Malaguide complexes , there co uld have been the intrusion of the peri dotites existing at the bottom of some upper units of the Alpuj arride as well as in the Kabylias (which can be considered equ ivalent to the Malaguide Complexinthe N ofAlgeria) .
The length of time of these processes of subd uction and collis ion that especially affected the Nevado-Fi labride as well as perhaps some of the most internal units of the Alpujarride remains unclear. It even remains deba table whether one or more intercalate periods ofrelaxation and ex tension occurred . De Jong (1 9 91) considered the Palaeocene to be an epoch of cooling, relaxat ion and exhumation. Prob ably this relaxation occurre d at the end of the Palaeocene, ex hum ing part of the units which fonnely were deeply sunken during the subduction and su bsequent collision. Monic et aJ.( 199 1)" place this exhumation 48
m.y,
a., during the Early Eocene. In this sense, the conglomerate marbles describe d by Pugaetal.,
( 1996) in the top of several units of the Nevado-Filabride could have formed duri ng this stage of exhumation.
Meanwhile, the Betic Externa l Zone underwent no majo r defo nnations , although in so me places, especially in the Prcbetic, there are well-developed unconformities.
EVOLUTION DURING THE OLIGOCENE (AND PART OF EARLY AQUITANIAN)
Some of the processes active in the Eocene co ntinued in the O ligocene, and consequently Iberia began to move to geth e r w ith Eu ro pe . Ba sic all y, N-S or NW-SE compressions continued in the Pyrenees (Megias, 1988) (Oligocene phase) (Fig . 6 A), ca using more displacements of nappes and southward migration of the foreland basin (Bbro Basin).
The deformation occurringinthe Cantabrian zone (NW Iberia) and the Bay of B iscay was much weaker during the Oligocene (and laterinthe Neogene) thaninthe Eocene (Boillotet al., 1984).
However, a new phenomenon developed during the Oligocene, addi ng to the co nvergence of Afric a and Europe. This was the ap prox imately E-W ex tension affecting Central Europe since the Eocene, and reaching the Western Mediterranean (in the Oligocene) through the Rhone Valley (Boillot et af., 1984). The present Gulf of Lyon opened in NE Iberia and the eastward displacement and gradual anti-clockwise rotation ofCorsica and Sardinia
Cunctas
aa
Terra (UNL). 14began. Further southwards, the Valencia Trough opened (Fig. 6 B), which in tum ca used eastward displace ment and gradual rota tion of the Baleari c Isles .
This exte nsional process occurr ing in the weste rn Medi terranean (with the initial opening of the Valenc ia . Trough , Font bcte et aJ. 1990 ; Maillard et a l. 1992 ),
together with the constant eastwarddriftof Iberia due to Atlantic ope ning, co mpressed the eastern sector o fIber ia (in an approximately E-W dire ction). Particularly during the Late Oligocene [Vial lard, 1985; Simon, 1990) and until the earlies t Miocene, this gave rise to the structur ing of the Iberian Cord illera (Fig. 6 A& B), which is a clearly in tracontine ntal co rdillera and present s a ge nerally moderate degree of deformation.
Within the Iberian Massif the reactivation oforiginally late H ercynian faults de termi ned most of the Alpine structure of the Spanis h Centra l System, and to a large extent also controlled the formation of the Duero and Tagus Bas ins, as well as many other smaller ones (Vegas. 1975). The age of these structures is not accurately known. Alt hough, in gene ral ter ms, we ca n acce pt that some de formation began at the end of the Cretaceous (Vegaset al.;1986), the first well-deve loped episode occurred during the Ol igocene- Earl y M iocene (Capote et al.; 1990 ),
coinciding with the deformation ofthe Iberian Cordi llera. With respect to the present-day Betic Cordillera, during the Late Eocene and the Ea rly O ligocene renewed compressions affec ted strongly the Ne vado-Fila bride Comp lex and, especially in this case, the Alpujarride Complex, forming a great part of their present nappe structure and overthrusted the Nevado-Filabride (Fig. 6 A). The probable direction ofcompression was NN W-SSE to N-S. Meanwhile, the Malaguide Com ple x a nd the Dorsal underwent great tect onic instab ility, forming large deposits of breccias and uncon formiti es (Serrano etal.,
1995).
The overthrusting ofthe Ma laguide on the Alpujarride Complex, could have begun during the Oligocene and part ofthis process ocurred at the end of the Oligocene (Fig. 6 B), perhaps till the begi nnings of the Aquitanian.
At this time, during the Oligocene and the Early Aquitanian, tectonic instability in the Betic External Zone ca used some deformations and unconformities, but on the whole this area continued receiving sedimentation.
EVOLUTIO N FROM T HE AQUITANIAN TO MIDDLE MIOCENE
I- Ccngresso score0Cenozcicc de Portuga l
OUGOCE.'iE
•
--LATE OUGOCE." t:· lARLY AQ{,'lTA.'ilA....
B
LATE AQUITANlAN . BVRDIGAU AN LAT E BURDl GAUAN . l.ANG HlAN
D
LATE TO RTONIAN . M I!.SSINt A.... PLIOCEN E . QUATERNARY
F
•
.'.
i · . <>~
•
•
t.
I~~
~
/
/
/
E
Seo..tb-San:IIaiu Domalll
(lIldudeclllle Bedc . RlrJ..le. ...7.0....)
••
••
PindioII .fC'HIpnaloa
DinctlN.af fllnlioll
Fig. 6 - Interpretative schemes of the evolution of SE Iberia. A: Durin g the Oligocene . cont inued the defo rmations in the Pyrenees and in the Betic-Intema l Zone and bega n the seucmreuo n of the Iberian Cord illera. B: During the Late Oligocene- Early Miocene . theopcning of the Algcro--Provem;:al basin accelerated. Then began the overthrusting ofthe Malaguide Complex over the Alpujanide and, immediately, the extension of lhe Algerian Basin prolonged to the Alboran area. C: During the Early Miocene continued Ihe opening of the Algerian Basin and, especially, during the Burdigalian the Betic-Rif' Inremal Zone was westwards expelled. D: Th is westwards expulsion continued during the Late Burdigalian till the Middle Miocene. The Flysch Basin and the Subbetic were strongly deformed. E: During Ihe Late Miocene. N· S to NNW·SSE compressions affected the entire Iberia, combined in many points with a near E·W extension. F: Continued the same geodynamic situation, but some eastern areas ofI beria underwent radial
in the Bet ic-Rif region is debat able, with oppossing hypotheses on several aspects. One grou p of hypo theses considers the present structure to have fonned due to the existence of a dome of the up per mantl e, located a pproximate ly in the Al boran Sea, movi ng as an intumescence and forming a radial displacemenl of the different complexes. Another grou p explains the presen t positi on of the Internal Zone by its strong wes tward displacement A third gro up consi ders the existence of a subduction in which the Albo ranSeaopen ed as a back -arc. A co mmon feature to those hypotheses is the grea t crust mobi lity necessary in each case.
We will take int o acco un t th e second gro up of hypotheses, although combined with possible subduction. In this sense wc must conside r a doub le process:
a) Th e extension that began in the Oligoce ne in the wes tern Med iterr anean continued to develo p thro ughout the Neogene. Th is process cont inued throughout the Early M ioce ne. formin g the(A lgero-) Prove n~ a1 basin.
b) Around the Oligocene a new subduction began, s inking the oceanic crus t of the N W Afr ica. T his subduction dipped to the N and therefore differed from to the previous ones. AI this point, it was the African plate wh ich
was
sinking, beginn ing wi th the oceanic crus t formed during the Mesozoic and part ofthe Tertiary south o f the Alkapeca domain, tha t is. directely north of the African plate (this area corresponding to the Flysch Basin). AI present, the rest ofthissubduction is to be found in southern Italy,inthe Calabrianart.During the process of fonnation ofthe Provencal Basin the different domains that previously occ upied the western M editerranean began 10be expe lled. Cors ica and Sardin ia moved eastwards and rotated anti-clo ckwise, while to the west fonned the oceanic crus t of the Provencal Basin (Reh ault et al., 1984). Ho wever. the open ing of the Valencia Trough did not signi ficantly co ntinue beyond the Early Miocene. The Provencal Basin ex tended southwards, developing a new
basin
in an approxima tely E-W direct ion (the Algerian Bas in) also with new ocea nic crust (Algero-Pro vencal basin when considered in a whole) (Fig. 6 B). The westward continuation ofthe Algerian Basin opened the Alboran Sea or Basin(Sam de Galdeano, 1990, 1996. 1997). in which oceanic crust was not clearly formed . although the continental crus t noticea bly thinned (Figs. 6 C&D and 7) . Boillotetal.( 1984) considered theAlgerian Basin to be a back-basin related to a poss ible subduction of Africa .This extens ional process caused major compressions on its margins . Towards the sou th, in this case co mbined with the African subduc tion, these compressions caused the formation ofthe Kabylias nappes in the Tell Co rdillera in Algeria; this happe ned approximately at the beginning of the Late Burdigalian. Then, the eastern partofthe Flysch Basin was co mpletely disorgani zed .
The Betic-Rif int em al Zone was expelled to the WSW, orig inally lying some
SOO
Ian
east of its present position, according totheinterpretations of Andrieuxet aJ.
( 1971).Ciellcias da Terra(UNL).14 Durand Delga ( 1980), Durand Delga& Fontbote (1980). Sanz de Galdeano ( 1990, 199 7), Marti n Algarra (1987).
ClC.Inthis
process.
this intemal zone destroyed the western part ofthe Flysc h Basin and dragged it in its front, forming thepresent Flysch units of the north of the Rif and of the Campo de Gibraltar.The process ofex pulsion to the WSW ofthe Betic-Rif Internal Zone lasted for a re latively long time .Itmust have begun during the Late O ligocene, when the collisio n made the Alpujarride Complex overthrust the Nevado- Fila bride . In this moment, the process of extens ion in the western Medi terra nea n an d the Afri can s ubduc tio n b egan to accel era te, and bo th co mplexes bega n tobeexpell ed at the same time. Th is provoked the thru st of the Ma laguide Co mplex over the Alpuj arride complex, thereb y initialy absorbing in this area the effects ofthe expanding Algero-Provencal Basin. Afte rwards. these three complexes. now the whole Betic-Ri f Internal Zone, were j ointly expe lled WSWward, reac hing and disorganizing the most internal sectors o f the Flysch B as in during the end o f the Aquitanian, although the more external part (to the south) co ntinued receiving deposits. This basin
was
destroyed completely at the beginning of the late Burdigali an .During this process of wes tward dis placement, the Betic-Rif Intemal Zone
was
affec ted in a double way. extension in its areas next to the progressively opening Algerian Basin, while its front underwent compression. This extension advanced as far as the present AlboranSea, which infact is the western end of the Algerian Basin, where its conti nenta l crus t greatly thinned.AI the beginning ofits expulsion, the Betic-Rif'Intemal Zone was greatly thickencd and many ofits units, currently outcropping, were situated several tens of kilometres deep . At the same time as the westward d isplac ement of the Internal Zone, many of its units were up li ft in g and undergoin g a rapid exhumation. In this way, there was a combination ofwestward movements (approxima tely 500
Ian
in total) and exhumations on the orde r of 40-6 0Ian
in several units. This occ urred in approximate ly 10 m .a. and the principal movem ent was from the end of the Early Burdigal ian to the Langhian.The external zones of both the Betic Cordillera and the Rif were deformed by the advance of the Betic -Rif Internal Zo ne beginning towards the end of the Early Burdigalian. The Subbetic Basi n was entirely destroyed (Fig. 6 D) and divided into many different units , usually having cl ockwise rotations. T hen formed th e Bet ic foreland basi n (G uada lquivir Basin, forme rly the North Betic Strait). Also, the Gibraltar
arc:
formed, not be ing palaeogeograp hic in origin, but clearly tectonic.10Congresso sobre0Cenoz6ico de Portugal
z
o
Fig. 7 - Cross-sectionfromthe Betic Cordillera to the Rif. Note the assimetTy of the distribution of the Beric-Rif Intemal Zone in both sidesof the Alboran Sea. This feature was caused by the openningand thinnig of'the continental crust oftheAlboranSea.
Its position is marked in Fig. 2. 18
tecto nic inversion occurred and overthrusts formed (Fontbote etal.,1990; Banda& Santanach, 1992 ).
In the Be tic Cordi llera , major exten sional proce sses ha ve been cited durin gthe Early and M iddle Miocene, linked to the progressive exhumation of the Internal Zone . At the same time, occurr ed a tran scurrent tectonics by
E-w
to N70 E faults which also co ntributed to the westward d isp la cem ent.Garc ia-Duenas et al, (1992) de scr ibe ductile-fragile shear zones with low-angle faults, moving the hanging block s to theWNW,
Wand SW. T hese same struc tures appear to have con tributed to the thinningof the Alboran crust.EVOLUTION DURING TH E LATE M IOCENE TO THE PRESENT (THE NEOTE CTONIC PERIOD,
sens u lato )
Some areas prese nt notab le deformations ( Fig. 6 E). In the Spanis h Central Syste m the NW-SE to NNW-SSE com pression conti nued througho ut the Early and Middl e Mio cene , but changed the direction to N-S from the Late Miocene10 the Quaternary (Cap oteet aI., 1990). These compressio ns caused reverse fa ults with imbricate thrusts (de Vice nte et aJ., 1992) and strike -sl ip fa ult s, wh ich contr ibuted to the upli ft of t he syste m a nd clearer separation of the Duero and Tag us Bas ins. as well as the format ion of many sma ll basins. More northe ml y, in the Iberian Chain, the Sierra de Camero s was alsodeformed (Pere z Lorente, 1987) .
From the Midd le M iocene onwards , NE-SW sinistral strike-slip faults (Cabreraet aI.,1988) led to the fonnation of basins such as the Cerdaiia Bas in, cutting the Pyren ean struc tura l trend . Compress ions ofNW· SE to almost N-S dire ction are also de tected in Portu gal, at least from the Torton ian on .
Moreover, at the same time, especially in the eastern part of Iberi a fromthe Middle-Lat e Miocen e on wards, a WNW - ES E e xtensio n (Mois sen e t, 19 89 ) o cc urred . followed by a radi al exte ns ion partic ularl y fro m the Pliocene onwards. as Sim6n G6m ez (l990) cit ed in the Iberian Chain (Fig.6 F).
Ciencias da Terra (UNL). 14
passing the area ofAlboran Isle and reaching the Rif(Fig. 4) 0
More westerly, other fault systems affect the Betic Cordillera. For instance. the faults N ofGranada that form the W border of Sierra Nevada and more to the S pass to the low valley of the Guadalfeo river. These faults mark the western limit of the Nevado-Filabride Complex and. to the Si the western limit of the lower Alpujarride units. The general displacements indicate a tectonic transport to the SW and a sinking of the wes.t block of these faults, while the east block progressively uplifts. This mechanism is repeated in ot her sets of faults. although to a lesser
degree. The general result is that the Betic-Rif Internal Zone is more sunken to the west. This fact is consistent in general with the progressive opening of the Alboran Sea in the same direction. The focal mechanism in theAlboran Sea shows a roughly E-W extension. combined with a N-S to NNW-N-SN-SE compression [Bufornet al., 1995).
The continental crust in Alboran is very thin. with a minimum ofabout 15km(Hatzfeld&Boloix, 1976) (Fig. 7) and there are important volcanic
effussions.
However. perhaps, the more striking fact is the existence of many eart hquakes with focus located at intermediate depth, which roug hly form a westerly convex arc. TheseEssauira
AFRICAN
PLATE
...
...
.
'..'
_
..
_.,
-_.
...
, . .0....0 · 0
IBERIAN
SUBPLATE
./
IV
CCAN
", 0SUBP1A1'E
. / / '
I
I
'"
/
/
. /
-
~
-.... 10 - , - , - 11
1 ~ ~ ~ ~ : 1 12
""""""",
13q
72
A " " 8 9
3 I I I I I I 4 - - 5 6
Fig. 8 - Interpretation of the present tectonic situation of the Iberian-Moroccan area. I: Faults. 2: Probable faults.3: Approximate contact between the Iberian and African plates. 4: Former contact between the African plate ant the South-Sardinian domain.
S:Possible prolongation to the east of the contact between the African and Iberian plates. 6: Position of the arc produced by the sinking of the Iberian andAfrican lithospheres in the WAlboran Sea. 7: Arrows indicating direction of crustal sinking in the W Alboran Sea. 8: Present boundaryof the Bctic-Riflntcmal'Zone. 9: Position. according Blanco& Spackman (1993) of thesunken lithospheric sheet in the Betic-Rifarea. 10: General directionof displacement oft he Betic-Riflnternal Zone. 11: Approximate limit
of theoceanic crust. 12: Oceanic crust in the Algerine Basin. 13: Betic-Rifain internal zone.
l " Cc ngresso sobre0Cenozoico de Portu gal
...
,
,
\
I\
?l
'. \
"
I\
I .." "\
-,l}i
•
/
a
"'h,
-{j
I
Fig. 9· Outline of' the main faults affecting the contact between thc African and Iberian plates between the Azores Islands and the
Gibraltar arc. Drawn from data ofAuzend eet 0/ .(\ 979 ), Baldyet al .( 1977), Bufomet a/.( 1988), Emery&Uchupi ( \ 984), Madeira
&Ribeiro (1990) , Mauffrelet aJ. ( \ 98 8), Moreira ( \ 985) and Udiaset0 /.(\986).
earthquakes have bee n interp reted as posible evidence of subduction (Bufom
et al.,
1995, Moraleset al., 1999),
although possib ly it is only th e result of the collisio n of th e lith os p here of th e Al bo ra n Do mai n agains t th e lithosphere of the Euro pean (Ib erian in this are a) and African plat es, which could bend and sink, but without rea l subduction (or perhaps with only incipient subd uction) (Fi g . 8).
Inthis sense, it is worth mention ing the record offour very deep earthquakes, more than 600 kmina vertical approximately situa ted S ofGr anada . Blanco&Spackman ( 1993) described a sunken lithospheric body in the area of the Betic Cordille ra, and perhaps with the present data, the most accurate interpretation would be to consider these dee p earth qu akes as being linked to this lithospheric body and the origin of this sunken body to the proce ss of the westward displacement ofthe Betic-Riflntema l Zone, the lith ospheric root of which co uld have begun sinking from the Ear ly M iocene.
A fin al pro blem is to det ermine th e present limit between the African and the European plates in this area of th e Albor an. Probably, at the end of the Palaeozo ic the limi t was qu ite clear. Afterwards, with the formati on of the Ligurian Basin a par t of the European plate was sep arated , form ing the A lkapeca or So uth Sa rdi nian domain, but to th e so uth , the Fl ysch basin continued ma r kin g the m a in limit between the tw o plates . Nev ertheless, with the format ion ofthe Algero-Provencal basin and the destruction of the South Sardi nian domai n, the limit between these plates is no t clear. In fact, the old
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limi t has been destroyed with the displacem ent of th e Kabylias to the south and of the Betic -Rif Internal Zo ne to the west.
Ifwe consider all the limit from the Azo res to the Alboran Sea, the contac t betwee n both plates is relatively cle ar till the are a of Go rri nge (Fig. 9), from where the great number of faults mak es it diffic ult to ascertain the real posi tio n of the limit. Probably this limit continues towards the Gibraltar area, but its position it is not clear. From Gibraltar to th e east the same occurs, but this limit prob ably tends to be re-situated in the ax is ofAl boran Sea and the Alge rian Basin.
Finally, menti on should be made of the process of regi onal uplift with radial ex tensio n described by Sim6n Gomez (1 990) in the Ibe rian Cordillera (and which may also have affec ted the Valencia Tro ugh). This process also affected most of the Betic Cordille ra from the Tortonian and more es pe cially the Pliocene onwar ds (Sanz de Galdeano& L6pez Garrido, 1991 ). As this phenomenon affects a wide area ( Fig . 6 F), it is diffi cult to determine the reason for its form ation. Isostatic ex plana tions can be co nsidered over a great part of the Be tic and even Atlas ic areas, but, on the whole, it is prob ably connected with movem ents originating in the mantle.
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