DeJong_Geodynamic Model, Betics (Spain)_Fisica de la Tierra 1992

8/6/2019 DeJong_Geodynamic Model, Betics (Spain)_Fisica de la Tierra 1992

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Anew GeodynamicModel

for titeBetie Cordilleras basedoitP-T-tpaths

and Structural datafrom theEasternBede

Nuevo modelo geodinmicopara las Cordilleras Bticas basado en curvas P-T-t

y datos estructurales de las Bticas Orientales

KOEN D E JONO

Structural Geology aud Tectonics Department.

Institute of Earth Sciences, Vrije IJniversiteitDe Boelelaan 1085, 1081 Hv Amsterdam. The Netherlands*

ABSTRA CT

This studypresents a newgeodynamie model for the teetonie evolution ofthe Betie

Cordilleras from the Cretaceous to the Miocenebasedon an integrated(micro) structural,petrological aud geochronological study of the eastern l3etic Zone.

Early Cretaceous subduction in the Betic Zone was initiated in thc Jurassie

transtensional Africa-Eurasiaplate boundary, whichwas strongly weakened by extensionaltectonics and strike-slip fautting, whieh persisted until the onset of subduction.Compression in the Africa-Eurasia plate boundary and initation of subduetion in theBetic Zone resulted from ESE-ward motion ofIberia due to oceanie spreading in theAtianticOcean and the Bay ofBiscay to theWand NWofIberia during the 119-80 Maperiod. Following subduetion the high pressure metamorphie Bede nappeswere partiallyexhumed during extension and extreme duetile thinning, whieh also affected theMalaguide Complex in the upper pinte.

Early to Middle Eocene northward thrusting of the entire and partially strueturedBetie Zone over the southernmost part of the External Zone, the former rifted Betie

margin, resulted in HP/LT metamorphism in the overthrust and buried part and flexuralbulgingin the part ofthe ExternalZone which wasnotoverthrust andwhere sedimentationcontinued.

During the Late Oligocene and younger tectonie evolution extension and crustalshortening followed eaeh olber rapidly during continuing Africa-Eurasia convergence,

* Presentaddress:InstitiitdeGodynamiqiie, Universit de Nice, URA. CNRS 1279, ParcValrosee, 06034 Mce Cdex. France.

Fsica de la Tierra, nY 4, 77-107, Editorial Complutense, Madrid, 1992


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78 Koen dciong

pointing to roll-back, steepening and detachment ofthe subduction slab. Early Mioceneinversion ofthe extended area and concentration ofoverthrusting iii the most thinned arcaresulted from slab detachment enabling transmission of comprcssion li x the shallow

remainder ofthc slab. SIab steepening and detachmcnt can further explain concentrationofMiocene and youngermagmatism hito a narrow zone and lis deepsource, wbich cannotbe due to steady state subduction as Africa-Eurasia convergence fals short during thisperiod.

1 . INTRODUCTION

In most geodynamie modeis for tbe Betie Cordilleras the metamorphic partof the orogen, the Internal or Hetie Zone, is considered as an allochtonous

tectonie element or miGro plate (Alboran Domain or Alboran Micro Plate,Andrieux etaL, 1971; Dereourt et aL, 1986; Comas ct al., 1990). Ibis elementis generally envisaged to havebeen juxtaposed to the External Zone, the formerMesozoic and Early Tertiary rifted margin of SE Iberia, along ENE-WSWtrending wrench faults during the Tertiary (Hermes, 1978; De Smet, 1984).Recently, deformation in the Betic Zone has been discussed witbin this coneept(Frizon Delamotte et al., 1989; Vauchez and Nicolas, 1991). However, theoccurrence of flie HP/LI metamorphic Almagride Complex, consisting ofMiddle am i Late Iriassic rocks, which resemble parts ofthe southcrn External

Zone (Simon, 1987), as the structurally deepestunit in the Betie Zone demonstratesthat such modeis cannot explain the tectonie evolution ofthe Betic Cordilleras.The outcrop ofthis complex in windows 50km south ofthe presentdayExternal-Internal boundary (fig. 1) implies that the External Zone is overthrust by the BcticZone (De Jong, 1990), rather than that te two are juxtaposed by wrcnching. Ibisinterpretation agrees with results ofinvestigations in the boundary zone betweenthe External-Internal zones pointing to presence of only minor strike-slipmovements during the Langhian-Serravallian, taking place during and aftermajor overtbrusting (De Ruig et aL, 987: Martn-Algarra etaL, 1988; Van derStraaten, 1990; Lonergan, 1991). In addition, radiometric dating implied that theearly-tectono-metamorphic evolution of the Betic Zone occurred during dx cCretaceous (De Jong, 1991a).Ihe model presented here is based on a detailed integrated (m icro) structural,

petrological and geochronologieal study of the eastern Betie Zone, whichconsists offour staked nappe complexes, from top to bottom: 4) teMalaguideComplex, 3) the Alpujarride Complex, 2) the Muihacen Complex, 1) the VeletaComplex (Egeler and Simon, 1969; Puga and Daz de Federico, 1978; De Jong,1990, 1991a, fig. 1). Ihe objective is to construct Pressure-Temperature-Tecto-


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nc-timepathsfor tbcAlpujarride and Mulbacen complexes, by tying deformationstructures to mineral reaetions, outlined in the first part of this article. lo theseeond paranew geodynamic modelbr dxc Betie Cordilleras wilI be elaboraledusing Ibis approacb.

2. TECTONO-METAMORPHIC EVOLUTION

OF THE ALPUJARRIDE AND MULBACEN COMPLEXES

Ihe relation ofmineral growth witb respect ro deformation phases showedthat the tectono-metamorphic evolution of the Alpujarride and Mulbacencomplexes are raiher similar lo eaeh olber (Bakker tal., 1989; De long, 1991 a).Both nappe complexes experienced HP conditions during earlyAlpine tectonies

when deformation structures with similar kinematic significance were formed.HP metamorphism gaye way to LP conditions concomitant with cooling, at itsturn followed by important reheating during youriger Alpine movements. Ihelate Alpine structural evolution in both nappe complexes, however, showsimportant differences. The deformation scheme used in this article is modifiedwith respect to the original scheme ofBakkeruaL (1989) according to Dc Jong(1991a, b).

2.1. Early Alpine evolution

D, econo-metamorphi phase

Early Alpine pressnres in the Muihacen Complex fal in the range of 1.0-1.2OPa (velilla and Fenoil Hach-AII, 1986; Bakker el aL, 1989; Gmez-Pugnaireeta!. D

1muh struetures were formed at the end ofthe isobaric heating trajectory(fig. 2) aud are mainly left untouched subsequent deformation in gneisses am iglaucophane schists. D 1 mulil structures demonstrate important E-W to ESE-WNWstretehing during top-to-the-west shear.

EarlyAlpine pressures in the Alpujarride Complex ofthe eastern Betie Zonewere lower than in the Muihacen Complex: pressures aroung 0.6-0.7 GPa attemperatures between about 300o~4Oflc C are characteristic for a number oftectonie units (Baker iaL, 1989; Goff taL, 1989, De Jong, 1 991a). Recently,indications for minimun pressures around 1.0 GPa have been reported in dx ccentral Betie Zone (Azan and Goff, 1991). Due to lack ofresistant rock noD alpu struetures olber Iban inelusions in porphyroblasts aud microlithons havebeen left.


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4. A new Geodynamic Modelfor h e Betie Cordilleras~.. 8 1

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D2 etono-rnetamorphcpirase

Tlie 02 teetono-nxetamorphie pliase in tbe Alpujarride and Mulliacencomplexes oceurred uf h e culmination ofdxc heating sage and coincides withslrong deeompression lo 02-0.6 OPa coneomitan with eooling lo about 550~~5000 progressivc evolution offolds: in deeper leveis of

tite comptex fl,mU>h folds are less tight titan in higiter paris and, it t addition, 8.refractis on folded bedding. p~mUa folds at this sructural level are carvi-linear;Ibid axes commoniy make a high angle lo tite sretehing lineation, whieh has aconstant ESE-WNW trend. Ir tite uppermost 1-1.5 km oftite eomplex, ca titeotiher baud, axes ofisoclinal D < ~ ~ < ~ folds are parellel lo tite ESE-WNW trendingstretching lineation. This points to rotatien offold axes lino parallelism with titeshear direction during upwards increasing 02o,,a srain. Because itsitear zonessirain is refated t displacement (cg. Ramsay, 1980), tite 02WU1h .strain gradienidemonstrates un upwards increasing displacenwnt in tite Mulbacen Gomplex.


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4. A new Geodynamic Modelfor h e Betie Cordillerasi.. 83

Cooling during and especially after D2 in both complexes is manifest by

widespread retrogression shown by replaeement ofchloritoid and kyanite, and

in the Muihacen Complex also staurolite, by phengite (paragonite)oftenaccompanied by chiorite.

2.2. Late Alpine evolution

Mineralogical changes show that the Late Alpine tectonic evolution bothcomplexes following the 02 eooling occurred during reheating, again followedby eooling. In 4w Mulhacen Complex increase in temperature is locally weIlestablished by growth of tiny crystals of staurolite in phengite-chloritedecomposition manties around D2muIh staurolite,chloritoid and (partly syn~D3muIh)kyanite. Growtb ofoxy-chlorite and locally biotite al Ihe expense ofchiorile andfurhermore formation of oligoclase-andesine rims around albite probably alsoreflect temperalure increase. In the Alpujarride Complex temperalure increase isshown by growth of slaurolile and sillimanile in the graphite-rich basal series andby andalusile growth locally accompained by cordierite blasteis in lite Triassicseries ofa number ofteclonie unjs.

Structural response ofbothcomplexes during reheating wasenlirely differenl.In h e Mulbacen Complex Ihe 02mIh shear plane was influenced by importanS-vergent folding and associated lhrusling during D3mIh. Ibis even took placetowards tite end oftite retrograde lrajectory oftite P-T-t palh (fig. 2). Anotiterphase, 04muIh ocdurring during tite second titermal peak in this complex (fig. 2),produced km-seae folds and only locally penetrative small seale structures,which are generally N-vergenl. In contrast, in tite Alpujarride Complex the firstimporlant deformalion pitase subsequent to tite D 2aP U main pitase tookplace duringtite waningstages oftite second Ihermal peak(fig. 2), when D 3aIP u fodswere formed.Folds oftitis generation form the mosimportanfold struetures inlite AlpujarrideComplex. Ihe intensily OfD 3~

1 P ~ increasessoutitwards and struclurally downwardsin tite Alpujarride overbrus mass. Going downwards, tight N-vergent E-Wirending folds witit overturned limbs citange mo tight to isoclinal recumbentsimilar folds direclly aboye lite Mulitacen Complex, witicit may be slronglycurvi-linear. D

31~ structures were coeval with

05muh in tite overlying MulitacenComplexas shown by a number ofobservations. Firstly, tite intensily ofduclile05muIb deformation inereases from north lo south in lite contactzone, similarly asD3P~ struclures. Secondly, Ihe inlensity of05muIb increases upwards lowards titeoverlying Alpujarride Complex. Titirdly, commonly NNE-SSW trending axes offlapu folds in tite deeper struetura level approacit the orientation of 05muIhstretciting linealions in the mylonite zone in lite topoftite Muihacen Complex in


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84 Koen de Jong

tite southern pan ofthe contad between botit nappecomplexes. In titis part of[henappe contactdm-spaced D

5muh extensional erenulation cleavages a few hundred

metros beJow h e tbmsteontactgrade mo mylonites directly below Ihe Alpujarridebasal titrust. Concurrenwitit titis upwards increasing 05u,ulh sirain E-W trending0muh Iineaions pregressively rotate o the NNW-SSE trend ofD5mulh stretchinglineations in Ihe mylonites. Movement in Ihe sitear zone was top-to-the-northindicated by tite asymmetry ofa number offabnie elements like pressure shadowsand recrystallization taus of porpityroelasts and micro fauls it t them, andfurlitermore by Ihe asymmetry of secondary grain shape frabrics and Iatticepreferred onientations in quarz mylonites.

05n,h and DQLPU occurred duning falling temperatures shown by widespreadchlonite growtit, partly at the expense ofstaurolite. Post~D3aflu growth ofandalusiteimplies thaI temperatures did nol drop below about 400v C in 1 1 w AlpujarrideComplex. During lJnuI and D4~ P-T conditions reached tite field of britiledeformation, giving nise to ehevron folds and associated younger extensionalbriuk-dueile shears and cataclasites were formed.Large scale 06,,uLh folds deforrn0~muLl1 mylonites aud the contad with Ihe Alpujarnide Complex.

3 . CONTACT OF THE MULHACEN COMPLEXWITII THE VELETA COMPLEX

In contraslo tite basal titrust of tite Alpujarride Complex, witich cuis D


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4. A new Geoynamc Modelfor h e lice Cordilleras... 85

Rb-Sr systems(PurdyandJger, 1976; Jger, 1973, respeetively). Consequently,radiometrieages oflite MulbacenComplex sitould be considered as cooing ages,

witicit date tite passing titrought tite closure temperature during cooling after titepeak of meamorpitism during D{ulb. However, tite P-T evolution clearly

demonstrales lite imporlance oflate sage rehealirxg locally elevatingmetamorphiclemperatures close loor aboye lite mica closure temperatures for difussion of Sram i Ar, respectively. As during reheating tite temperature domain for diffusionofradiogenic isotopes was re-entered, significan resettingofmelamorphic agescan be expected. Tite very young K-Ar and Rb-Sr mica ages of 10-15.5 Ma(Andriessen u aL, 1991) and 4 0 A r/ 3 9 Ar mica ages between 14.3 and 25.9 Ma(Moni ce aL, 1991; De Jong, 1991a, De Jong, e aL, 1992) can be explainedaccordingly.

Despite titermal resetting indications for local preservalion of an olderisotopie systenx are presen. Motil e al. (1991) obtaineda 40ArP9Ar age ofabou48.42.2Ma from a baroisilic ampitibole, witicit eharacteristically grows al titeexpense ofglaucopitane during D

2T~lUlh (D e Jong, 1991a). ~ArP940Ar/39Ar dating

oftourmaline, oblained from gneisses wilh a D2muh fabrie and whicit yieldedK-

Ar ages between 115 and 80 Ma (Andriessen e aL, 1991), resulted in referencelines with ages between 89.10.9and521Ma (D e long, 1 991a; De Jong e aL,1991). In addiion, D2nuLh pitengites yielded Rb-Sr ages of: 65.710.1Ma and41.14.6Ma (De long, 1991a). Titese ages, being obained from syn~D2muLhminerais, are interpreted lo reflect cooling afler D,mub. Tite spread in 40Ar/39Ar andRb-Sr mica ages, witicit may be as young as 14 Ma, probably resulted from(partial) rejuvenalion due to late stage rehaling, as wil be discussed below.

5. DYNAMICS OF METAMORPHLSM AND TECTONLCS:GEODYNAMICSOF THE BETIC CORDILLERAS

In titis section tite leetonie and metamorpitie dala wil be combined loestablisit tite P-T-t paths oflite Alpujarride and Mulitacencomplexes. Such P-T-

palits reflec firsorder tecloniemovemens al Ih e scale ofIh e crust and, hence,elucidate dxc stacking history itt tite Betie Zone and tite subsequentexhumationoftite higit pressure metamorpitic rocks. Titey are furtitermore used lo eslablisita geodynamic model for tite teclonic evolulion oftite Betic Cordilleras.

5.1. Subducton

Early AJpine metamorpitic pressures in tite Alpujarride and Mulitacencomplexes poin t burial deptit in tite order of 27 and 37 km, respectively,


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86 Koen deiong

implying subduetion below a crustal segment witit a Iower crustal basemen.Subduction took place below a eruslal segment that contained Ih e MalaguideComplex (fig. 3a), witicit has an essentially continuous sedimentary record from

lite Early Paleozoje to tite middle Tertiary (Aquitanian), (Egeler and Simon,1969; Geel, 1973). Altitougit lite Malaguide Complex in tite Betie Zone isactually extremely titin, clastie influx ofgranites, gneisses an medium grademetamorpitie rocks in tite Late Paleozoje (Geel, 1973; Herbig and Stattegger,1989) points t tite presence ofa crystalline basemcnt at thai time. Influx offresit

detrital muscovite, biotite and K-feldspar in Jurassic carbonates (Geel, 1973)poins lo erosion ofcrystalline roeks, showing titat tite Malaguide Complex itada nonnai crustal thiekness before subduction was initiated. Simijarities inMesozole stratigrapity ofthe Malaguide Comp lex, tite Subbetic and tite Flyscit

Unils between lite two domains (MacGuillavry, 1964; Martn-Algarra and Vera,1982, Pineda Velasco, 1985) imply titat Ihe Malaguide Complex formed part of

tite same piale as tite Exernal Zope.Tite nappe complexes oftite Betie Zope consist ofmetasedimentary rocks of

Triassic and/or Paleozoic age; individual nappes ave thicknesses of severalkilometres, lite different early Alpine metamorphc pressure sitows that theburial dc pth oftite Alpujarride Complex in tite eastern Betic Zone was aboel 10km less titan tite Muihacen Comp lex. Ibiscan be explained by underthrusting oftite Mulbacen Compex below lite crustal segmenwitit tite Alpujarride Complex

(fig. 3b). Minimum sitortening values in the order of70-75 % perpendicuar to Ih etransposed bedding show titat pre-collisional titicknesses of h e Paleozole andIriassic sedimentary sequences of tite Alpujarride and Mulitacen complexeswere probab> about 4 times tite present thickness. Titese strain values incombination witit tite presence ofmetasedimentary rocks imply titat early Alpinenappe stacking in tite Betic Zone was probably tite result ofsequential dctacitment

ofupper segments with titicknesses itt exeess of lO km. Detached segmens areadded t tite overridingplate, while tite deepper part of tite lititospitere continuest subduct (figure 3).

Figure 3.Nappe stacking la the Betie Zone by seqaenhial detachment ofupper crustal rockseqixences, which are added to the hanging walI ofthe subduchion system formed by the MalguideCornplex (MAL)withacrystallinebascnient (KAS). Thelowercrust (crosses) aud mantte(randornstriping) continue to subduct to the west. Sequential underthrushing resuls iii dramatic cooling u ntite overlying earlier subducted nappe complcxes siiown by tite inser P-T-t paths for titeAlpujarride and Molliacen coraplexes. P-T conditions oftite Veleta Complex (square in tite insertP-T graph ofpanel e) imply that it is underthrust by an upper crustal unit (coarse stipple, panel e)not exposed at tite present erosion level. Seale bars: 5 km.


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Sequential stacking of cool erustal segments also has dramatie titermalconsequences, witicit are expressed by tite P-T-t path ofthe nappes of the Betic

Zone. Thermal modelling of stacking of crustal seale segments showed titat

cooling or reduced heating ofa piale may result from underthrusting by coolercrust (Daxy and Gillet, 1986). Along titese Enes of evidence cooling of titeAlpujarride Complex is explained by underthrusting by a relatively cool crustalsegment, containing h e Muihacen Coinpjex (fig. 3b). Isoharie henting of titeMulitacen Complex may itave ceased fis a resulof underthrusting by anothercool crustal slab with Ihe veletaComplex (fig. 3c). Pressures in Ihe AlpujarrideComplex imp> 100km ofsubhorizontal movement on a low-angle subduetionslab with a dipofabout 150 in tite upper part (fig. 3a). Subsequen underthrustingofIhe Muihacen Complex t about 37km requires a movemen ofabout 130 km.

Cooling oftite Mulitacen Complex al a deph of37 km as a result of its under-titrusing y a slab containing (he Veleta Gomplex indicates anotiter 130km ofpIale consumption. The totalamount ofcrustal shortening by subduetion ofuppererustal segmens ma> titus itave reacited about 360 km.

Ocanie spreading in tite Atlantic Ocean t lite west ofIberia and in tite BayofBiscay was in fuIl swing between anomalies MO and 33(119-80 Ma on tite timescale of Kent and Gradstein, 1 986), during whicit Iberia was displaced about400 kmESE-wards as part ofAfrica (Srivastava eta!., 1990;Malod and Mauffret,1990). Tite amount ofspreading shows thaI tite envisaged 360kmofsubducted

lithospiteric slab in the Betic zope is feasible. Age estimatesofcooli ng atter Di~up t about 90 Ma, indicate that subduction has occurred earlier. Subduetion intibe Betic Zone is Ihus Iikely tobe caused by tibe ESE-ward movemenofIberiadueto oceanic spreading it t tite Atlantic Ocean (fig. 4). Subduction was initiatedin the former Late Jurassic trans-lensional Africa-Eurasia plate boundary, witicitwas continuous witit the Ligurian Ocean (fig. 4). Titis boundarywas characterizedby small-oeeanic basins (fig. 4) with young and weak oceanie crust, witich wasloaded by flyscit sedimenlation, hence, forming tite most suited type ofmargint be transferred into an active plate boundary (Cloetingh et al., 1982).

5.2. Exhumation history

Tite main teclono-metamorpitie pitase D, in tite Alpujarride and Mulitacencomplexes occurred during deeompression, sitowing that U, structures and kine-maties are related lo exitumation of tite itigit pressure metamorpitie roeks.Decompression in tite Alpujarride Complex was less titan in the MuihacenComplex, implying differential exhumalion and, hence, moxements of tite two


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nappe complexes witit respect lo eaeit otiter. Tite upwards increasing intensity ofD,tnuh and jIs augmenting rotational eomponent are probably due t titis movement.Rotation sense ofsyn~D

2muh garnets demonstrates tital tite Mulitacen Complexmoved lo tite east witit respec t tite overlying Alpujarride Complex and wasconcurrently exhumed. Sueit kinematic and P-T constrains poins t movementson a west-ward dipping sitear zone (fig. 5).lite coaxialstretching componenoffln,uIh and D2~~ and tite transation oftite Alpujarride and Mulitacen complexespoin L o extension ofibe collision bel(hg. 5). Tite topoftite Veleta Complex alsoexperienced deformation with a dominant rotational component, irnplying awestward movement of tite overlying Mulbacen Complex. However, sucit amovemen on a westward dipping plane as implied by sequential detacitmentmodel (fig. 3), would fol place itigiter pressure metamorpitie roeks on top of

lower pressure rocks, but Ihe reverse. Ibis seems t indicate titat tite coaxialcomponent ofDt~, resulting in elongation of tite Mulitacen Complex movingit eastwards updip, outweighed Ibe westward movement li i lite contactsitear zonewith tite underlying veleta Complex. lite proposed model shows titat nappeemplaceinenwas not tite resulofa directed force (pusit from beitindo) but wasdue lo a body force. Titis force stems from tite buoynancy oftite subducted anddetached upper crustal segmens, providing a constan upward force, wbich isresolved mo components parallel lo lite anisotropy provided by tite detaciteduppercrustal segmens (fig.5). Elongation and decompression oftite metamorpitie

rocks imply extension of tite upperplate where tite Malaguide Complex is lcaled

Figure 5.Resolution of buoyancy (B)of the detached underthrust upper crustal sequences intoelongation of tite nietamorphic nappe complexes and a shear component along tite contact oftitetanging wall an d lite Alpujnrride Comple.x and between tite stacked nappc complexes during 1 7 ),.

Top-to-the-wessitear on westward dipping planes implies exhumation of metamorphic rocks andextensionofthe metamorphicaappe pile,whichis transerred to tite overlying Malaguide Complex

causing extensional fanlting and partial excision of its crystalline basement (KAn).

B


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4. A newfikodynamic Modelfor h e Betic Cordilleras... 91

anuo removal ofpan ofits Cambnian basemen. Erosion, tilling and submarinefaulting and associated rapid vertical motions in tite Malaguide depositionaldomain from the Early Cretaceous on (Roep, 1980) are explained by extension

of lite upper piale (fig. 5).

5.3. Advanced cooling during Eocene overthrustngof th e External Zone

Tite P-T-t paths of lite Alpujarride and Mulitacen complexes demonstrateincreased cooling during advanced exitumation (fig. 2), which is explained bythrusting oflite Belic Zaneayercoolerernst.A number offealuresshaw thai Ibis

ernst is formed by Ih e soutiternmosl pan oftite External Zone. The AlmagrideComplex, witicit is regarded as an inlier of lite External Zone, contains earlyAlpinemineral assemblages in mafie rocks witich poinlo melamorpitie pressuresof0.35-0.55 GPa andtemperalures around 350~4000 C (Puga andTorres-Roldn,1989). Tite Antequera-Osuna nappe in tite western Belic CordillerashasTriassicrocks witieit partly resemble titose ofIh e Almagride Complex (Simon, 1987); its

mafie rocks record pressures of 0.3 OPa (Puga e aL, 1988). Tite nappe hasovenlitrustite Subbetic (Cruz-Sanjulin, 1976; Pinedavelasco, 1987), ofwitichlite souitern pan also contains mineral assemblages poinling to pressures of

about 0.3 GPa (Puga e aL, 1988). Bunial deptits of 10-20 km, implied by litemetamorpitie dala, can only be envisaged by burial duelo overtitrusting oftiteserocks by Ih e Belie Zone (fig. 6), resulting in eooling oftite itanging wall.A titrusload of10-20 1 c m litickness can explain tite flexure in Ih e External Zone around50 Ma tital follows from lectonie subsidence analysis by Peper and Cloelitigit(1992)oflite more nontiternparts oftite External Zone, witerepelagiesedimentationpersisted mo lite middle Miocene (Geel, 1973; Hermes, 1978; De Smet, 1984).

Timing of overthrusting in tite Eocene agrees wilh coeval vertical movemensimplied by resuls of tectonie subsidence analyses (Kenter etaL, 1990; De Ruig

a L , 1991), by non-calcareous influx m o carbonates ofYpresian and Lulelianage inbotit tite Subbetic and tite Malaguide Complex (Geel, 1973) and furtitermorewith lite presence of litrus in the Malaguide Complex witieit are sealed by

Oligocene conglomerales (Lonergan, 1991). Taking a dip of tite titrus plane of150 tite minimun amonnofovertitrusting is 40km (fig. 6). Overtitrusting iii titesoutitern Betie Cordilleras itas a similar liming as Ihe cimax ofsitoniening in titePyrenees (De Jong, 1990), Botit may Ihus be tite resul ofiniliation ofoceanicspreading in tite Norwegian-Greenland Sea around 55 Ma, causing an additionalNW-SE componen in tite African-Eurasian collision (Srivastava e aL, 1990).


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92 Koen deiong

Figure 6.Thrusting oftite partially structured Betic Zone over the southernmost External Zonein tite Early t Middle Eocene. Burial aud Ioading resulted iii HP/LT metaniorphisni in the

Almagride Complex an d upward and downward flexure oftite Suhbetic and Prebetic, respectively(arrows).

5.4. Late Oligocene to Early Miocene extension

D3 folding in tite Veleta and Mulitacen complexes itas no equivalenin tite

Alpujarride Cornplex, pointing lo translalions oftitese two structural domainswith respec t each otiter. Tite S-SW vergence ofD3m~lh and D< ~

1 folds imply asoutitward movement oftite overlying Alpujarride Complex(fig. 7). Progressive

soutitward thinning leading t complete excision of tite Muihacen Coniplex(figs. 7, 8) indicates large seale normal faulting. Southvergent D,muh and D

3~CL

folding is probably due lo back-rotation oftite swell domain nortit oftite normalfaul (fig. 7). Folding is envisaged as resulting from accomodation oftite sitapeoftite baek-rotated domain below an upward bowed extensiotta! fault. Maximumtemperaturesduringextension-relatedreiteatingwerereacited after D,mulh, sitowingthat reiteating al a particular crustal level occured afler extension itself, inagreemenwith 2D modelling ofIhe pan of tite P-T-t patit pertinent extension

(Van Wees el aL, 1992). Slronger reiteating in tite soutitern pan of tite MulitacenComplex is indicalive of more advanced extension in Ihis part of lite complex.In tite Alpujarride Complex reiteating-induced staurolite growth over

undeformed ~2 sitows titat extension teclonies didnot resulin foldingofthe mainscitistosity as was tite case in tite Mulitacen and Veleta complexes. Hence ~2 intite Alpujarride Complex was located in tite extensional sector oftite flow ficid,itt agreement witit tite position oftitis complex ir tite itanging wall ofa low-angleextensional system (figs. 7, 8). Differences in maximun temperatures atained bytite different Alpujarride tectonic unjs point t a non-uniform reiteating that

-Maiaguide Com p l ex

___________________ ji


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4. A new Geodynamic Modelfor h e Betie Cordilleras... 93

b o

Noamo..na.o N

a

S n o..rNa...

1 1

Figure 7South vergent D, folding re.sulting from forin adaption ofthe Muihacen and Veletacomplexes between two major curved extensional faults FL and F

1 ~ due to extensional unloadingof the hanging walI (Alpujarride Complex). Extension has resulted in progressive southward

thinning ofthe Mulbacen Complex and excision of its lilbologie units. Extreme extension pro-duced a rider ofrocks of the Mulhacen Complex (Sierra Alhamilla). Extensin gaye rise to back-rotation of 5> in tite Alpujarride Coniplex (iasert b), during subsequent inversion of tite extensio-sal structure was located jathe compressionat sector of the flow ficid (shaded arcas, insert c)producing northvergent D,

1~ folds.

oceured, in addition, at different pressures. In Ihe Almanzora Unu rcitealingproduced only biotile, witereas in tite Oria and Partaloa unils (Sierra de lasEstancias, Sierra de los Filabres) slaurolite-andalusite growth occurred (Akkerman e aL, 1981; De Jong, 1991a). Ihe association staurolite-sillimanite in lite Adranappe (soutit ofIh e Sierra Nevada, e.g. Cuevas and Tuba, 1990) implies itigiter

pressures at similar temperatures. The Almijara group ofmus experienced highgrade metamorpitism and ocal analexis in assoeiaton wilit emplacement ofultramafie rocks in tite western Belic Zone at itigit pressures (Westeritof, 1977;Tuba and Ibarguciti, 1991). Different P-T conditions during extension are tite

resul of a southward dip oftite main extensional fault,cutting southwards downmio lite mantle (fig. 8). Tite less dramatie P-T evolution oftite otiter Alpujarridelectonic unlis is due t titeir location aboye less litinned and Ihus less reiteatedcrus, witicit include tite veleta aud Mulitacen complexes (fig. 8).

MiJwldS co.ow n

ANOi.IId 000wI.

Ir


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94 Koen de Jong

Figure 8.Cartoon of the LateOligocene to Early Miocene extensional structurc. Tite Muihacenand Veleta complexes are pinched out southwards towards a r x extensional mantie uplift, resultingin substantial reheating [insertP-Tpaths; P-Tconditions at Ronda(sqixare), afta Westerhof, 977].The mala extensional fauldips southwardsresulting in rehcating at progressively higiter pressurein the Alpujarride units; schematically from north to south: 5) Almanzora Unit, 4) Variegato-Partaloa Unit, 3) Oria Unit, 2) Adra Unit, 1) Ainiijara group. Tite Malaguide Complex is tite site

of coarse clastic sedimentation, tite early deposits oftite Solana Formation occur in an extensionalbasin between the Internal and External zones.

Tite P-T jump al contaet between lite Alpujarride Complex and tite non-metamorphic Malaguide Complex is also due t extension, during witicit titeremainder oftite crystaline basement oftite Malaguide Complex is cut out. Titeextensional basin between tite Internal and External zones (fig. 8) results from titeoutcrop oftite basal detaclimenof tite extensional system (De Jong, 1991a; VanWees ce aL, 1992).

Timing ofxtension

Reheating in tite Muihacen Complex is tentativel> dated al around 25 Ma.Titis is based on 4ttAr/>9Ar laser probe dating of a D

2 m u lh pitengile single grainpointing lo argon loss al around 25 Ma (D e Jong, 199 la; DeJong aL, ] 992).In addition, modelling of a

404r/39Ar tourmaline age spectrum with indicationsfor Ar-loss, resulted in a 23.5 Ma model age for titis even(D e Jong, 1 991a; De

s 7 N

1MC

Sotana Fn,

MAL

KAN

A t~ tJ .. -

apAn Ronda ment~. k4**tt

25Mo;AFRICA


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4 . A newGeodynamic Modelfor h e BetieCordilleras... 95

Jong taL, 1991). Titus extension in tite Betic Zone can be considered as a LateOligocene lo Early Miocene even.

5.5. Inversion ofthe extensional structure

Inversion of tite Early Miocene extensional siructure and northwardoverlitrusling oftite hanging wall oftite extensional system (fig. 9) is manifesby many observations discussed in titis paragrapit.

Figure 9.Inversionoftite extensional structure around 20 Ma resulting in substantial c ooling ofthe Alpujarride and Mulhaceri complexes (insert P-T paths). Tite regional structure of tite

AlpujarrideComplex is citaracterized by superposition of highermetamorphic units on lowergradeunits. The Almagride Complez and the Antequera-Osuna nappe (A-o) represent thrust slicesofrnetamorphic equivalentsof the Subbetic. The extensional flysch basin between tite Internal andExternal zones is closed; tite Espejos Formation is deposited after northward thrusting.

Kinematics ofsmall-scale sinictures in mylonites in tite top oftite MulitacenComplex poinloa NNE-lo NNW-ward movemenoflite overlying AlpujarrideComplex (Behrmann and Plal, 1982; De Jong, i991a), titus in an oppositedirection as during previous extensional tectonies. Tite regional struelure oftite

5 N

> C A B

AFRICA 2OMa

E.0i05 Fm

IBERIA


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9 6 Koen de .Jong

stackofAlpujarride nappes is generallyeitaracterizedby highergrade metamorpiticunits aboye Iower grade units (Aldaya e aL, 1979), pointing t thrusting ofstronger reiteated rocks on top of less reheated unlis due t inversitt of tite

extensional structure (fig. 9). Pan ofthe extensional mantie uplifin the westernBetie Zone was decapitated duringinverston as indicated by tite presence ofuplo 1.5km titick slices ofultramafie rocks witicit itavebeen titrusalong a mylonitezone over itigit-grade metasediments ofthe Almijara group (Westerhf, 1977;Tuba and Cuevas, 1986). Seismie refraction data imply titat tite large peridotitemassif of Ronda does no l root in tite mantle but also represenis a titrus siteel(Barranco e aL, 1990). Thrust emplacement of ultramafie rocks resulted in aninverted gradient and in formation ofanatectie leucogranites due t melting ofitigit-grade m etamorpitic pelites of tite Almijara group (Westerhof, 1977; Tuba

and Cuevas, 1986). Overthrusting ofitolter Alpujarride rocks haslocally causediteating of tite underlying Mulitacen and Veleta complexes in tite soutiternmostpan oftite overtitrus zone. Titis is inferred from tite data ofVan den Eeckitout

and Konert (1983), witich sitow an upward mercase in An% ofsyn-overthrustingplagioclase lowards tite overlyittg Alpujarride Complex.

Two dimensional titermal modelling, using a depth dependant rheology,sitowed tital inversion and eoncentratiott ofdeformation in tite former extensionalsiructure is tite resul ofa pronounced drop in strength oftite lower pan oftiteupper crusand tite lower crust as a result ofextension-related reiteating (Van

Wess e aL, 1992). Deformation structures sitow titat during inversion titeMulitacen Complex in tite footwall and tite stronger reiteated AlpujarrideComplex in tite itanging walI reacted differently. Roeks of the AlpujarrideComplex were strongly folded by norittvergent D ,aI n U structures, indicating titat5. was located in tite compressional sector oftite flow ficid (fig. 7). In contras,tite Mulitacen Complexwas Iess severely affected during inversion; deformationrelated t overlhrustingwas mainly concentrated into a D

5 m U h mylonite zone belowtite Alpujarride Complex. Open D$uIb folds, forming antiforms with italfwavelengths ofa italft several kms, are tite early expression ofinversion.

Daeing ofinversion Cconics

D>apufoldi ng aud D5n~llh mylonitization occurred during falling temperaturesindicated by widespread retrograde mineral reactions associaled witit it . Radio-metnie datingofcling associated with titis event titus constrains tite timing ofinversion oftite extensional sructure. Rb-Sr, K-Ar and

4 < > A r/3 9 A r cooling ages intite Alpujarride Complex oftite entire Betie Zone cLister around 19 Ma (Prieme aL, 1979; Zecke aL, 1989; Andriessen e aL, 1991; Monit aL, 1991).


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4. A new GeodynamicModelbr(h eBeticCordilleras... 97

Deposilion ofundisturbed sediments witit biozone N6-N7 ages afier tite maintitrnsling (fig. 9) poins lo a completion of inversion oftite extensional sructurebefore 17-18 Ma, using tite Haq taL (1989) lime seale. Titis is shown by Espejos

Formation in tite easlern Helies (Geel, 1973), witich conlains pebbles witit D 3 ~ 1 ~folds (DeJong, 1991a)and by tite Viuelas Formalion in tite western Betic Zone,witich seals thrust planes between itigh-grademetamorpiticAlpujarride uniis andtite Malaguide Complex (Gonzlez Donoso c e aL, 1983; Torres-Roldn e aL,1986; Zeck e aL, 1989). Titus, formation of Ihe extensional sruclure and itsinversion were complete within a very short period of about 7-8 Ma. Titeconcomitan extremely rapidcooling is explained by titrtsling oftite AlpujarrideComplex over Iess extended and litus cooler crus (De Jong, 1991a; De Jong ceaL, 1992; Van Wees ea!., 1992, fig. 9).Extension during advanced D(P furher

conributed t cooling.

5.6. Renewed MiddleMiocene extenson

Radiometric dating in tite Mulitacen Complex itas not resulled in a tigitcluster ofcooling ages as istite case for tite Alpujarride Complex. Integrated 40Ar/39Ar agesofpitengile vary between 25.9 and 14.3 Ma (De Jong, 1991a; De Jongce aL, 1992). Modelling ofthe age spectra implied tital lhey were lite result of

repealed litermal resetting. Resetting has a similar timing fis tite main episode ofvolcanism in tite basins bordering tite melamorphic ranges. Late Miocenevolcanism resulted in epigenetie ore deposils and hydrotitermal alteralion in titecounlry rocks (Oen c e aL, 1975). Hence, isotope reselting in tite MulitacenComplex migit similarly be tite resuloffluids associaled wiiit volcanism (DeLong e aL, 1992).

Calc-alkaline dykes occurlocally inIhe Alpujarride and Malaguide complexes.Al leaspan oftite dykes are intruded afler cooling ofthe Alpujarride Complexwas completed (Torres-Roldn e aL, 1986). Tite dykes are nol folded indicalingthai intrusion look place afler D

3

1 P overtitrusting ofthe Alpujarride Complex,pointing lo renewal ofextension. This isalso clearlyexpressed by concentralionofMiocene volcanism in tite moslitinned cmst in tite weslern Mediterranen area(fig. 10). Interpretation ofreflection profiles and boreitole dala from tite AlboranBasin, south oflite Belie ranges, also demonstrated an important Middle lo LateMiocene extension (Comas e al., 1990). A second phase of extensionsuperimposed on the Late Oligocene lo Early Miocene evenalso emerged frommodelling of the gravity field of the Belic Cordilleras (Van der Beek andCloelingh, 1992).


22/31

98 Koen deJong

5.7. SIab roII-back, steepenng and detachment

Rapid sitifting ofextension [o compression and renewal ofextension occurred

during continuingplate convergence, implied by pIalekinematic dala (Srivastava e aL, 1990). Titis poins t slab roll-back as likely mecitanism for LateOligocene lo Early Miocene extension. Detacitment of a (rolled-back) slabresults in subsantial decrease oftite slab-pull force leading t a diminishing oftite flexural bulge ofthe s]ab, giving risc t a belter coupling ofthe shallowremainder oftite slab (Spakman, 1990). Sueit a relativel> rapid process migitt belite reason for lite observed fas and dramatie inversion of tite Early Mioceneexensiona] structure L i lite Betic Zone. Reprise ofextension during the Middleand Late Middle Miocene is either tite resulofrenewed roll-back and steepening

ofa subretion slab or, alternatively, due t sinkittg ofa slab (Platami Vissers,1989; De Jong, 1991 a). Recenseismie tomograpitic studies point lo tite exstenceof a detacited slab below tite Belic Cordilleras (Wortel and Spakman, ] 992),witich is supported by eartitquakes occurring as deep fis 600 km (Grimison andCiten, 1986). Miocene and younger magmalism itas heen explained by partialmelting of subducted lilitospitere (Araa and Vegas, 1974; De Roever, 1975;Torres-Roldn, e aL, 1986). Concenration of magmatism mo a NNE-SSWtrending narrow zone (fig. 10), in whicit a clear citronological and chemicalzonation is absent (De Larouzire e al., 1988), agrees with melting of a steep,detacited slab. Tite eitemislry ofLate Miocene lamproites oftite eastern BetieCordilleras points lo derivation from tite mantle al a maximum depth of100km(Venturelli e aL, 1988). Tite isotopie composition oftitese rocks indicates mixingof lite mantle wilit a componenwitich itas the eharacteristies ofcontinental crusor sediments derived from sucit a crus (Nelson ce al., 1986). litis sitows intro-duction of sucit crust mio tite manle, witicit, however, cannot be tite result ofsteady state subduction, as Miocene plate convergence was too slow, tituspointing lo slab delacitment.

Focal mechanisms ofearthquakes in tite rrxost western Mediteranean areapoint lo decoupling oftectonies al manile and crustal Jevel (Crimison aud Citen,

1986), consistent witit tite presence of a detached slab. Titis is clearly by E-Wcompression of earthquakes deeper titat 1 0 0 km, whereas intermediate quakesdemonstrale NNW-SSE compressive stresses (Grirnison aud Citen, 1986). Titelalter direction agrees witit tite NW-SE lo NNW-SSE compression in liteTorlonian t Recen stress syslem ir tite eastern Betic Cordilleras (Montenal ceaL, 1987; De Ruig, 1990; Buforn and Udas, 1991), whicb isrelated lo tite AfricaEurasia collision (Bergeral, 1987). lite approximate N-S directed erustal sitor-lening in tite Betie Cordilleras itasresulted in strike-slip deformalion (Montenal


23/31

4. A new Geo dyna,nicModelfor h eBetieCordilleras... 99

ea!., 1987; Sanz de Galdeano e aL, 1990), closureofexensional and slrike-slipbasius (Bon e aL, 1989; Coppier ee aL, 1989) and overtitrusting in tite ExternalZone (D e Ruig e aL, 1987; De Ruig, 1990).

Figure 10.Map ofBougeranomalies (inmgal) in the most weslern Mediterranean area(afterVanden Bosch, 1974). The Betic-Rif are is underlain by fin arcuate pattern of negative anomalies;

crustal thickness (diamonds, after Banda andAnsorge, 1980 and Barranco etaL, 1990) diminishpregressively towards the Alboran Basin. Miocene and younger strike-slip faults and volcanism

(dots) (after: De Laroazire e aL, 1988) are concertrated in the thinnest crust.

Tite eomplex Miocene lo Recent tectonie evolutionoflite Betie Cordillerasis litus probably duelo crustal sitortening resulting from Africa-Eurasia collisioninterfering wilh extension related t manle lectonies arising from roll-back,sleepening and detacitment ofa subdueted slab below tite collision zone.

1

38

36


24/31

loo Koendelong

CONCLUSIONS

Sludy of lite relationsitip belween polypitase deformation and mineralgrowtit in tite Alpujanide and Muihacen complexes itasresulted inwell constrainedP-T patits, wl-iicit reflect lite essential fealuresoftite Alpine tectonic evolution oftite Betic Zone. Early Alpine HP conditions poinlo subduction below tite lowerernst oftite Malaguide Complex: lite Alpujarride Complex was subdueled first,foflowed by h e Muibacen Complex, witicit was in turn undertitrusby tite VeletaComplex. Nappe stacking titus occurred by sequential underthrusting attddetacitment of upper cruslal segmens. Subsequen decompression showsexitumation oftite HP melamorphic rocks, during witich duetile flattening andextensional lectonies were dominan. Radiometrie dating points lo initiation ofcooling after 1 -IP metamorpitism in tite Mulitacen Complex at aboul 89 Ma,xmplying an Early Cretaeeous age for subduction. Titrusting ofthe Betie Zoneover tite relatively cool External Zone during tite Eocene resulted in advancedcooling oflite itanging wall. Importanlate stage reitealing is tentatively daled atabout 25 Ma, showing titat erustal and suberustal extension and tite assoeiatedmanle upwaring in tite Betic Zone are Late Oligocene lo Early Miocene features.Subsequenrapid cooling is due lo inversion oftite extensional srneture.

Tite tectonie evolulion of tite Belie Zonereflects tite dynamics ofeompressionand extension oflite lhree main stages in lite Africa-Eurasia collision in titewestern Mediterranean:

1. ESE-ward movemenofIberia between 119 and SOMa, due t spreadingin tite Atlantic Ocean t the west of Iberia, resulted in westwardsubduetionoftite Belic Zone below lite leading edge ofIberia, witere theMalaguide Complex was located.

2. Overthrusting of the mos southern part of tite Exernal Zone by litepartially sructured Betie Zone in lite Eocene (around 5 < ) Ma) caused HP!LTmetamorphism in lite overtitrust partoftite External Zone(AlmagrideComplex) and flexural bulging oflite pan which was not overthrust andwiteresedimentation continued. Titis pitase, witich is coeval witit coll-

sion in lite Pyreness, is caused by an additional NW-SE compressionalcomponent into tite Africa-Eurasia collision due t oceanie spreading inthe Norwegian-Greenland Sea.

3. Late Oligocene to Early Miocene crustal and subcrustal extension andsubsequen inversion oftibe extensional structure, wbich was comp!etedat about 18 Ma, ocurred during continuingAfrica-Eurasia convergence.Titis points lo roll-back, sleepening and detacitment oftite subductionslab. Detacitment ofthe deeper part oftite slab caused a belter coupling


25/31

4 . A new Ce odynamicModelfor theBeticCordilleras... 101

of Ihe sitallow remainder oflite slab witit lite overlying pIale enablinglransfer ofcompression due lo pIale convergence. Reselting ofisotopesystems in tite Mulitacen Complex, imporlan volcanism and dyke

intrusion point lo renewed extension afler 18 Ma. A deep source ofvolcanism agreeswititiitepresenceofadetacitedslab asplaleconvergenceduring tite Miocene is nol sufficient forsleady subduction to sucit deplits.

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DeJong_Geodynamic Model, Betics (Spain)_Fisica de la Tierra 1992