bakker & dejong_eclogites: p-t-deformation path mulhacen complex, betics, spain_journal of...

8/6/2019 Bakker & DeJong_Eclogites: P-T-deformation Path Mulhacen Complex, Betics, Spain_Journal of Metamorphic Geology

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1. metamorphic Geol., 1989, 7 359-381

The geodynamic evolution of the Internal Zone of theBetic Cordilleras (south-east Spain): a model based onstructural analysis and geothermobarometryH. E . BAKKE R'Ceologisch Instituut, Universiteit van Amsterdam, Nieowe Prinsengracht 130, 1018 VZ Amsterdam,The NetherlandsK. DE JONC,' H. H E L M E R S AND C. B I E R M A N Nlnstituut voor Aardwetenschappen, Vrije Universiteit, Postbus 7161, 1007 MC Amsterdam, TheNetherlands

ABSTRACT The Internal Zone of the Betic Cordilleras consists of several superimposed major thrust sheets withdifferent P-T-t evolutions. On the basis of an integrated field, miCroscopic and laboratory study, thetectono-metamorphic history of the Mulhacen Complex and Almanzora Unit has been reconstructed indetail. The Mulhacen Complex has been afected by at least five phases of penetrative deformation, whichhave been labelled Dx-,, D., DX+,,D,,, and DX++D,-, and D, are related to continent-continentcollision, which is indicated by high pressure-low temperature ( WILT) and subsequent intermediatePIT metamorphic conditions. D,,, is related to crustal thinningand heterogeneous extension. During thisevent the ALmanzora Unit was juxtaposed against the Mulhacen Complex. This phase was succeeded bythe establishment of low pressure-high temperature (LPIHT) conditions and at least two phases offolding and overthrusting. The Almanzora Unit shows a comparable tectono-metamorphic evolution p tD,,,. However, the P IT conditions prior to D.,, indicate a higher crustal position with respect to theMulhacen Complex during the collisional event.Key w& Alpine orogeny; Betic Cordilleras, Spain; continent-continent collision; crustal thickening;crustal thinning; deformation structures; geothermobarometry; P-T-t path.

.

INTRODUCTIONThe Betic Cordilleras of southern Spain represent thewestemmost part of the pen-mediterranean alpineorogenic belt of southern Europe. It has a northernExternal Zone of essentially non-metamorphic Triassic toMiddle Miocene sediments, which were deposited onto thesouthern continental margin of the Iberian plate (Hemes,1978;Garcia-Hernandez, Lopez-Garrido, Rivas, Sanz deGaldeano & Vera, 1980) and a southern Internal Zone,comprising mainly intensely deformed and metamorphosedsedimentary rocks of dominantly Triassic and Palaeomicage. The depositional realm of these metasediments is notwell known (Bourrouilh & Gorsline, 1979;M2ke1, 1985).

The regional structure of the Internal Zone originatesfrom large-scale overthrusting. The individual thrust unitshave been organized in four thrust sheet complexes on thebasis of differences in tectono-metamorphic evolution.The lowermost thrust complex is the Veleta Complex,

Present addrtss:%atec Geo-Exploration Consultants, Noor-dcrstraat 76"'. 1017TW Amsterdam, The Netherlands.* Reprint requests to K. de Jong.

characterized by low pressure/low temperature (LPILT)metamorphism (Puga & Diaz de Federico, 1978;Diaz deFederico, Gomez-Pugnaire, Puga & Sassi, 1979). It isoverlain by the Mulhacen Complex, a series of thrustsheets with early high pressure/low temperature (HPILT)metamorphism, subsequently overprinted by medium-grade metamorphism of intermediate and low pressuretype (Nijhuis, 1964;Puga & Diaz de Federico, 1978;Diazde Federico et uf., 1979;Gomez-Pugnaire & Fernandez-Soler, 1987). Both complexes are commonly included intothe ill-defined Nevada-Fiabride Complex (Egeler, 1964).In the Internal Zone the thrust sheets of the Veleta andMulhacen Complex have been overthrusted by a largenumber of thrust slices of mainly the Alpujarride Complexand the overlying Malaguide Complex. The thrust units ofthe Alpujarride Complex show a variable metamorphicgrade indicating intermediate to low P IT ratios and locallyW / H T conditions (Westerhof, 1975; Torres-Roldan,1979;Akkerman, Maier & Simon, 1980). The Malaguideunits consist of w n - or very low-grade metamorphic rocks(Roep & MacGillavry, 1962;Egeler & Simon, 1969).In the northeastern part of the Internal Zone Alpujarridethrust units are overlying rocks of the Almagride Complex,

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sz H. E . B A K K E R Er A L .which show strong stratigraphic affinities with Triassicrocks of the (Subbetic) External Zone (Kozur, Mulder-Blanken & Simon, 1985). Therefore these Almagriderocks can be interpreted aswindows of the External Zone.

The lowermost of these thrust units is the AlmanzoraUnit. On the basis of lithostratigraphy and geochemicalcharacteristics this unit shows close affinities with thethrust sheets of the Mulhacen Complex. However, becauseit has not suffered the same deformational and metamor-ph ~c istory during the alpine orogeny, it is regarded as aseparate tectonic element.

The purpose of the present study is to establish thetectono-metamorphic history of the Mulhacen Complexand the Almanzora Unit and to discuss its implications forthe geodynamic evolution of the Internal Zone of the BeticCordilleras. For this reason detailed structural and meta-morphic analyses have been carried out in the easternSierra de 10s Filabres (Figs 1 and 2). The tectonic units inthis area show a complicated small-scale structural ge-ometry produced by the superposition of several gener-ations of folds and foliations. The deformation phases arelabelled D,-,, D,, D,,, etc. with the main deformationphase D. coinciding with a phase of regional metamorph-ism. In order to establish the physical conditions prevailingduring the successive phases of deformation, extensiveelectron microprobe analyses were performed and themetamorphic reactions and time relations between mineralgrowth and deformation have been studied, using criteriadeveloped by Zwart (1%2), Spry (1969) and Misch (1%9).GEOLOGICAL SETTING A N DLITHOLOGY OF THE THRUST UNITSThe Sierra de 10s Fdabres forms the eastern extension ofthe Sierra Nevada. The topography of the mountain rangeis controlled by an east-west trending antiformal structure,dipping eastwards below essentially post-orogenic sedi-ments of Late Miocene and younger age (Volk, 1967;Voet, 1%7; Weijermars, Roep, Van den Eeckhout,Postma & Kleverlaan, 1985; Montenat, Ott dEstevou &Masse, 1987).

In the investigated area the Mulhacen Complexcomprises three thrust sheets. In ascending order they arethe Nevado-Lubrin Unit (Nijhuis, 1964). the Macael-ChiveUnit (Kampschuur, 1W5) and the Huertecicas Altas-Almacaizar Unit (Helmers & Voet, 1%7).The lower part of the Nevado-Lubrin Unit consists of analternation of dark-coloured mica schists and quartzites. Itis covered by a Triassic sequence of schists and overlyingmarbles with intercalated schists, quartzites and locallygypsum. This sequence contains extrusive and effusivemafic and (minor) ultramafic rocks. During the alpinedeformation and metamorphism most mafic rocks weretransformed into amphibolites with a markeddifferentiated layering. Only locally the original igneoustextures are preserved. Both the igneous and volcanicrocks show WPB-affinities (Pearce & Cann, 1973).

Age diagnostic fossils have not been preserved.However, based on striking lithostratigraphical and

geochemical similarities, the carbonate sequence has beencorrelated with Late Triassic carbonate series of theAlmanzora Unit.

A similar lithostratigraphical sumssion is present in theoverlying Macael-Chive Unit and the Huertecicas Altas-Almocaizar Unit. In these units the Triassic succession ofclastics and overlying carbonates rests upon a suite ofPalaeozoic rocks. In the Macael-Chive Unit this suitecomists of a monotonous sequence of graphite-bearingmica schists with quartzite and marble intercalations,which has been intruded by granite. The intrusion wasassociated with skarn formation in adjacent carbonaterocks (Helmers, 1982). Due to severe alpine deformationthe granite has been transformed into augen andeven-grained gneisses. The 269Ma isochron age (Priem,Boelrijk. Hebeda & Verschure, 1966) for the metagraniteindicates a pre-Early Permian age for the country rock.

The Palaeozoic sequence of the Huertecicas Altas-Almocaizar Unit is largely composed of darkcolouredbiotite, epidote and amphibolite-rich gneisses, often withaugen structures. The gneisses are associated withgraphitic mica schists and quartzites and locally withmarbles and calc-silicate rocks.

The thrust units of the Mulhacen Complex represent theoldest alpine thrust sheets which have been recognized inthe area. The original nature of the contacts has beenlargely obliterated during subsequent defomation whenthe contacts were isoclinally folded and sheared. On theoutcrop scale the contacts are parallel to the transposedfoliation. Regional mapping, however, showed that thebasal contact of the Macael-Chive Unit truncateslithological units of the underlying Nevado-Lubrin Unit.

The thrust units of the Mulhacen Complex are overlainby the Almanzora Unit. The contact is essentially a ductileshear zone, which transects the transposed lithologicallayering of the Mulhacen Complex on a regional scale.This contact has been reactivated during later deformationphases, especially in the eastern part of the area (Fig. 2).

The lower part of the Almanzora Unit consists of analternation of quartzites and phyllites. The higher parts arecharacterized by carbonate rocks with locally intercalatedphyllites and abundant gypsum with massive carbonaterocks at the top of the sequence. Metabasic rocks arepresent within the gypsum horizons. Pre-Triassic rocks arenot present in the Almanzora Unit. An Early Kamian agecan be attributed to the upper part of the carbonatesequence (Kozur, Mulder-Blanken & Simon, 1985).DEFO RMAT ION STRUCTURESIn the M ul hG n Complex the main deformation structureis a D, transposition foliation (Sx). The foliation is adifferentiated quartz-mica layering with a spacing of onemillimetre or less. S, foliations enclose intrafolial folds,budins, augen structures and contain a pronouncedstriping and mineral lineation. Boudinaged crystalsindicate that the lineation was formed parallel to aneast-west to north-west-southeast trending stretchingdirection (Fig. 3b). Quartz c-axis fabrics demonstrate


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364 H. E. BAKKER ET A L .dominant noncoaxial strain with west to north-westdirected shear during D,. Locally S, is parallel to the axialplane of centimetre to metre scale, tight to isoclinal similarfolds. The widespread Occurrence of folded quartz veinsindicates that D, was preceded and accompanied by theformation of tension gashes and solution transferprocesses.S . is penetratively developed in all lithologies with theexception of some mafic bodies, which have eitherundeformed cores or contain a penetrative older S . _ ,foliation. In gneiss bodies becomes progressivelyfolded towards the contact zone with the metasedimentarycountry rock, where the gneisses contain a penetrative Sxfoliation.

Across the contacts between the Mulhacen thrust slicesno D, strain or metamorphic gradient is present. It istherefore concluded that the main thrust contacts withinthe Mulhacen Complex were formed prior to D,.

Gneissic, platy quartzite and mica foliations which arefolded in the hinge zones of D, folds indicate that D. waspreceded by D,_, deformation (Fig. 4a). Boudins, wrappedby S, contain an S,_, fabric and are characterized byminerals from the high-pressure assemblage. In amphibol-ites D,-I structures appear to be only locally preserved inrelics. They comprise platy glaucophane-epidote-micafoliations, intrafolial folds, mineral and striping lineations,extensional crenulation cleavages (Platt & Vissers, 1980),boudins and tension gashes filled with quartz andaragonite. During formation of S , _ , randomly orientatedglaucophane sheaves were progressively rotated into apronounced WNW-ESE trending mineral lineation (Fig.3a).

The central part of the Macael-Chive gneiss body southof Lubrin (Fig. 2) contains a penetrative S . - , foliation witha WNW-ESE directed stretching lineation (Fig. 3a).Asymmetric tails around feldspar porphyroclasts indicateWNW-directed shear (Fig. 4b). In mica schists evidencefor pre-D. deformation is, apart from local micafoliations within microlithons, usually restricted to theinternal fabrics within pre- and synkinematic D, minerals.In these minerals internal S,_, oliations are constituted byboudinaged epidote crystals, graphite trails or shapefabrics of opaque minerals.

Dx-, structures have been observed at different levels inall three tectonic units of the Mulhacen Complex. It istherefore concluded that D,-, was also a phase ofpenetrative deformation. Differences in intensity of Dx-,deformation along and across the thrust sheet contactswithin the Mulhacen Complex can not be establishedbecause D,-, structures were severely overprinted by thelater phases of deformation.Main phase D, structures were overprinted by a groupof D,, , structures including isoclinal similar folds,mylonites, ultramylonites (in carbonate rocks) withapproximately east-west trending mineral and stretchinglineations, extensional crenulation cleavages (ECCs) andfoliation boudinage structures (Fig. 4c). In general theECCs form conjugate sets and show a variable spacingfrom one to several centimetres. They are present mainly

within the mica schists.On a regional scale the ECCs are prominently

developed in distinct zones. The most pronounced zone islocated just below the contact with the Almanzora Unit.At this thrust contact the ECCs are developed in conjugatesets which define the most prominent planes in the rocks.Downwards, away from the contact, the ECCs graduallybecome broader spaced and less prominent.

On a regional scale the basal thrust contact of theAlmanzora Unit truncates the S . foliation within theunderlying Mulhacen Complex. This observation and theincreasing intensity of D,,, ECCs towards the thrustcontact indicates that the contact Mulhacen Complex-Almanzora Unit was formed during D,,,. The orientationof mineral and stretching lineations indicate east-westdirected shear along this contact (Fig. 3d). Quartz c-axisfabrics demonstrate Edirected shear during D,,,.

Within the Almanzora Unit Dx+, structures have onlybeen observed directly above the contact with theMulhacen Complex. The structures in this zone are opento isoclinal folds, transposition foliations, intrafolial andsheath folds, mylonites and associated east-west trendingmineral and stretching lineations, ECCs and foliationboudinage structures. The D,,, structures have over-printed the main phase structures in the Almanzora Unitwhich comprise transposition foliations (S,) with associatednorth-west-southeast trending striping lineations, shearedworm tubes (Figs 3c & 4d), boudinaged bedding planesand open to isoclinal folds. Penetrative amphibole-epidoteS, fabrics have been locally developed in thin mafic bodiesand locally in the contact zone with the metapelitic countryrock. Folded quartz veins indicate that the main deforma-tion phase was preceded and accompanied by the forma-tion of tension gashes and solution transfer processes.

The main deformation structures indicate penetrativenon-coaxial deformation of the Almanzora Unit with anorth-west-southeast stretching direction. Based on theirpre-D.,, age, comparable style, similar orientatedstretching direction and metamorphic characteristics thesestructures are correlated with D. structures in theMulhacen Complex (Fig. 5). Evidence for pre-D,deformation is only derived from microscopic observa-tions. Blue-green amphibole crystals, which constitute S.,contain cores of crossite and glaucophane with an internalfabric of epidote s l . , rutile and opaques. This implies theexistence of D,-, deformation.

During D,,, the previously formed transposition folia-tions were folded and the rocks were affected by south tosouth-west directed large-scale reverse faulting and over-thrusting. These structures are typically observed in theMulhacen Complex.The character of the Dx+Z eformation is stronglyassociated with the lithostratigraphic position. In theclastic sequence of the Nevado-Lubrin Unit D,,,deformation was penetrative and exclusively ductile. Thestructures here comprise inclined to upright open to verytight flexural slip folds, controlled by a broad to closelyspaced crenulation cleavage (S,,,). In the overlyingsequences ductile Dx+, deformation was restricted to thrust


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GEODYNAMIC EVOLUTION OF THE BETICS 365


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GEODYNAMIC EVOLUTION OF THE BETICS J7


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3E) H. E . BAKKER ET AL.

zones, in which a sharp increase 'in prominence of Dx+2occurs. Open south to southwest vergent flexural slip folds(Figs 3d and 6a) become flattened and concurrently thespacing of the axial plane crenulation cleavage is stronglyreduced. Near the thrusts the folds are tight to isoclinaland a closely spaced crenulation cleavage or differentiatedlayering is present. The thickness of these zones is a fewtens of metres.The sharp contrasting amount of flattening between theclastic sequence of the Nevado-Lubrin Unit and theoverlying imbricated calcareous series suggests that animportant detachment zone was present between bothsequences during Dx+,. This interpretation is supported bythe Occurrence of prominent south to south-west vergentD,+* folding just above and below this contact (Fig. 6b).Metre-thick chlorite-bearing quartz and carbonate veinsalong the thrusts ndicate pronounced solution transfer at amacroscopic scale.During D,+, the previously formed structures wereoverprinted by a group of structures including open totight folds with associated axial plane cleavages, brecciazones, low and high angle extensional faults, reverse andtear faults, overthrusts and locally brittle-ductile shearzones with associated ultramylonites. Stretching lineationsand asymmetric strain shadows in carbonate ultramylon-ites, slickensides, the orientation of SX+, cleavages androtated older structures suggest a north to north-eastdirected tectonic transport (Fig. 30.The folds are characteristically disharmonic with wave-lengths varying from one to one hundred metres. Theyhave been preferentially formed within the contact zone ofthe clastic and carbonate sequences. S,+, cleavages areonly prominently developed in the top of the clasticsequence of the Nevado-Lubrin Unit where they arelocally developed as penetrative closely spaced crenulationcleavages.The breccia zones are mainly developed in carbonatehorizons, in particular in the basal part of the Nevado-Lubrin carbonate sequence. The zones originate as thinhorizons parallel to the main foliation. During progressivedeformation they extended and transected the main folia-tion. Large breccia wnes commonly show a lens shapewhich may attain lateral magnitudes of several kilometres.Brittle-ductile shear zones have been observed in micaschists and carbonate rocks. n the carbonate rocks ductileand brittle deformation occurred intermittently.The conspicuous D,+, strain gradient in the top of theclastic sequence of the Nevado-Lubrin Unit indicates thatthe D,+, detachment zone at this contact was reactivatedduring D,,,. In the Almanzora Unit Dx+, structures aremost prominently developed in a zone in the northeasternpart of the area where locally the Mulhacen Complex hasoverthrusted this unit.

METAM0 R P H I S MIn 1% Nijhuis presented the first detailed description ofthe plurifacial metamorphism of the rocks of the Mulhacen

Complex for a small area south of Lubrin in the easternSierra de 10s Filabres. Since then his metamorphic schemehas been demonstrated to be valid, with minor additions,in most of the central and eastern Sierra de 10s Filabres(Langenberg, 1972; Kampschuur, 1975; Vissers, 1981).In this section the most significant metamorphicparageneses are described in chronological order. Becausethe stackingof the tectonic units of the Mulhacen Complexhas occurred before D, and no significantly differentpre-D. mineral parageneses have been recognized betweenthe three units, the metamorphic evolutions of thedifferent units will be described together. The relationshipbetween metamorphism and deformation is shown in Fig.7a and b.So far only limited geothermobarometry has beenpublished (Helmers, 1983). For this study over 500electron microprobe mineral-pair analyses were used toestablish the P-T-t path of the Mulhacen Complex andthe Almanzora Unit (Fig. 8). Electron microprobeanalyses were performed with a Cambridge InstrumentCo. Microscan-9 with an accelerating voltage of 20kV.Analysed mineral samples were used from standards andapparent concentrations were corrected using the onlineZAFcorrection program. For further details of themicroprobe data H. Helmers can be contacted at the FreeUniversity in Amsterdam.Pre D,-, metamorphismThe first phase of alpine metamorphism is characterized byincipient eclogitization of dolerites and garnet-hedenbergite skams, by crystallization of quartz-jadeite,quartz-aragonite pairs and by the formation of albite- andepidote-bearing omphacitites at the contacts of maficigneous rocks and carbonate layers.In slightly deformed massive dolerite bodies incipienteclogitization is indicated by the development of garnetbetween magmatic plagioclase and clinopyroxene and bythe formation of omphacite and rutile from augiticpyroxene. Eclogitization of skarn bodies mainly affectedthe outer zones. The central parts are usually stillcomposed of the original skarn mineralogy: acmitichedenbergite, intermediate grossular-almandine, titaniteand apatite (Helmers, 1982). In boudinaged quartz-jadeiteveins jadeite (Jdn-) is present as unorientated crystals,which are always separated from quartz by a latersymplectitic rim of acmitic pyroxene and albite withinclusions of hematite and paragonite. Metagranites andgneisses locally contain omphacite relics enclosed in garnetand jadeite-aanite, low-albite, Si-rich phengite (3.52)andAl-rich titanite. In mica sch is t s only pre-D.-, epidotecrystallization has been recognized.Temperature conditions prior to D.-, are indicated bygarnet-omphaate K,, values (Rheim & Green, 1974;Ellis& Green, 1979), ranging from 41 to 24 and K,, values ofgarnet-phengite pairs (Krogh & Raheim, 1978; Green &Hellman, 1982) varying between 27 and 21.5. For thedetermination only compositions of unorientated inclusionsof omphacite and phengite are used with compositions in


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CEODYNAMIC EVOLUTION OF THE BETICS 311

the garnet core adjacent to the inclusions. Equilibration isindicated by the limited spread of values.Pressure indicators are derived from jadeite(J&=)-albite pairs (Currie & Curtis, 1976; Kushiro, 1969;Holland, 1980), jadeite-quartz pairs (Newton & Kennedy,l W ) , rutile-almandine (Bohlen, Wall & Boettcher, 1983)and the presence of aragonite (Boettcher & Wyllie, 1%7).These data indicate (Fig. 8) conditions of 350-400"C at9.5-10.5 kbar. Taking into account the Cacontents ofgarnet (usually approximately Gr,) a slightly highertemperature range of 4oo-46o"C is obtained. This isappropriate, due to the non-ideal mixing character ofCa-substitution in garnet. Si-values of phengites (Mas-sonne & Schreyer, 1983) from undeformed metagranite(which contains K-feldspar) cluster around 3.52. Theirstable occurrence against jadeite and jadeite-acmite and/orlow-albite, in absence of garnet, suggests initial tempera-tures below 350C in approximately the same pressurerange. The P-T box based on these data is separatelyindicated in Fig. 8 at the beginning of the metamorphicpath.Complete absence of lawsonite crystals or pseudo-morphs in this early high pressure assemblage indicates thepresence of a partial CO, pressure (Nitsch, 1972). Thisconclusion is supported by the presence of carbonate veinswith aragonite relics. A low P is indicated by abundantdevelopment of anhydrous mnerals and absence ofglaucophane s.1. and phengite in mafic rocks during thisstage.

"P.

SynkinematicD,-, metamorphismIn mafic rocks synkinematic D.-l metamorphism is ingeneral characterized by an increasing Ph0. Growth ofglaucophane s . f . , garnet, epidote s . f . , pure low-albite,paragonite and locally omphacite has occurred. Inboudinaged carbonate veins aragonite pseudomorphs havebeen found.

Glaucophane has been formed in necks of boudinagedomphacite crystals (Fig. 6c).Locally glaucophane has beencrystallized to omphacite. At other localities howevereclogitic parageneses have remained stable. Zonalomphacite prisms growing from wall to wall indicate rapidgrowth in open 0uid-Ned veins discordant to s-,.hisindicates varying and BuctuatingPw conditions.

The epidote crystals are commonly zoned with agradually decreasing Fecontent towards the rim, locallyreaching a clinozoisite composition. The Fe-rich(b)cores contain equidimensional, random orientated opaques.Towards the Fe-poorer rims a progressively strongerinternal fabric is defined by stretched and folded opaques.

This observation indicates that the epidote compositionchanged during Dx-l. Synkinematically formed clinomisitelocally fills the necks of Dx-l garnet boudins.In tourmaline gneisses stretched phengite-epidote-garnet aggregates crystaked at the expense of magmaticbiotite. In mica schists synkmematic formation of glauco-phane (Fig. a), hengite, epidote, chloritoid, Ti-hematite and locally garnet. has been observed. In micaschists and amphibolites chloritoid-glaucopbane pairs arepresent as arrnoured relics in garnet.

The K,,-values of garnet-clinopyroxene pairs (18 to 14)of schistose eclogites, garnet-phengite pairs (14.5 to 12) ingneisses and the Jd-values of omphacites (45 to 35) pointtowards temperatures of 475-525" C at 9-11 kbar. Takinginto account the Cacontents of the garnets (up to Grm) atemperature range of 485-540"C is indicated (Fig. 8).However, we question the uppermost temperature becauseit is calculated using the outer rim of the garnetporphyroblast, which may have grown post-Dx-l. Anincrease in temperature during D,-l is indicated by thedecreasing Fecontent in epidote s.1. towards the rim,locally reaching clinozoisite composition.In the Miyashiro-F'RGM-diagram glaucophane andcrossite compositions near to the glaucophane S.S.end-member indicate high pressure (Carman & Gilbert,1983) and a temperature below 550C (Maresch, 1977).The relative large gap in Jdcontent of clinojyroxenes(Fig. 9) is in accordance with the pressure indicated by thegeobarometers (Mottana, 1983). Further high-pressureindicators are Mg-rich chloritoid (Fig. 10; Bearth, 1963;Ganguly, 1972; Chopin & Schreyer, 1983), almandine-rutile intergrowth, aragonite stability and the Si-values(clustering around 3.37) and b,,-values of phengites (Fig.11).

SynkinematicD, metamorphismSynlunematic D. reactions in ma6c rocks are characterizedby a further increase in P H p . Omphacite, glaucophane andcrossite have recrystallized to blue-green amphibole,usually accompanied by albite growth. This recrystalliza-tion process has been nearly complete in rocks whichcontain a penetrative S, foliation. S. is further constitutedby clinozoisite, zoisite and paragonite. Garnet andplagioclase (albite and oligoclase) OCCUT as synkinematicporphyroblasts. For the plagioclase both increasing anddecreasing Ancontents towards the rim have beenobserved (Ans,). Aragonite, occurring in carbonate veinsin mafic rocks, has recrystallized to calcite together withblue-green amphibole growth at the expense of omphacite.In mica schists the D, mineral assemblage consists of

Fig. 8. (Continued.)19nk 6) staurolite-in Hoschek. 1969): (7 ) antieorite= forsterite+ talc +H,O (Evans etd.. 976):18) aoorthite+H,O =& t C + misite +q& (New& & K e k d y , i963); 9) barroisite stability-(E&t, 1979); (10) Ah&tc triple poini(Holdaway,1971); (11) stilpnomelane+phengite= biotite+chlorite+ quartz (Nitsch, 1970;) (12) pyrophyllite=Al-silicate+quartz+H,O(Chatterjeeet d., 984). Si-valuesaccording to MWMC & Schreyer (1983). Jd-percentages according to Currie & Curtis (1976).Boxes indicate the established P-Tconditions for each deformation phase D. Pre-D,-,-A of the Mulhacen P-T-t path points tothe precclogite stability box, prc-D,-,-B to the early dogite stability box. Rc-D,-A of the AlmanzoraP-T-t path indicates themlourttssamphibole pressure box en route to pn-Dx-B, the blue amphibolestability.


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372 H. E. BAKKER ET AL .

AI

., .+ . * \

\' / -

-6-4-2

Fig. 9. Compositions of diopside, acmite-jadcite, omphacite and jadeite in central and eastern Sierra de 10s Filabres. Calculation ofend-members from electron microprobc a n a l p by method of Papike et ul. (1974). Augite end-members always adiopside-hedenbergite solid solutionwith less than 1% wollastonite. Ca-tschcrmakite or cnstatite. Arrows indicate core and rimcompositions n one crystal. Below: determination of Jd-contents by method of Esscne & Fyfe (1967) using the value of (221)re&ction ofX-ray photographs. Both methods agree within 5% and frequently within 3%.

phengite, quartz, chloritoid. kyanite, staurolite,almandine-rich garnet and (clino)zoisite. Locally blue-green amphibole or taramite (Linthout & Kieft, 1970)replaces glaucophane. Staurolite (Fig. 10) contains aconsiderable amount of Z n and Mg. In the absence ofbiotite the staurolite is probably formed from kyanite+chloritoid. This reaction indicates a decreasing H,Ocontent. Its occurrence is restricted to graphite-bearingmica schists surrounding metabasite bodies or ortho-

gneisses. The reason for this distribution pattern canprobably be found in a lower PHI,, and Ps in theimmediate surroundings of the former igneous bodies.In calcitic and dolomitic marbles the paragenesestremolite + misite and colourless mica +quartz+Mg-richchlorite (penninite) have been formed. This indicates a lowP in these rocks. I n ultramafic rocks chlorite+anbgorite are stable. Neither forsterite nor pseudomorphsafter this mineral have been observed. In gneisses taramite


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Tzn+++Mn++

I I Mg++0 20 50 80Psg. 10. Compositionof chloritoid (circles from mica schists; trianglesfrom amphmolltes;crosses from Almanzora mica schists) all lowin Mn2' and with up to 4% Fe3+in Mg-rich members, and of staurolite (points) a l l low inMI*+.rrows indicate changeincomposition to the rim, tie-lines are dram betweencoexisting dorit oid and staurolite.

was locally formed from jadeite-acmite, concurrently withthe recrystallization of K-feldspar. High Si-phengite hasremained stable.

According to the Brown (1977)diagram the compositionof taramite, in an ill-defined part of the diagram, and ofblue-green hornblende indicate a pressure of 7 bar (Fig.

gar, 9.Ox) 9.020 9.030 9.042 9050 *cooFig. 11. Cumulativefrequency ines of b,-values of phengiteaccordingto methad of Sassi & Scolari (1974). (1) mica schistsofthe Mulhac.cn Complex in the Sierra de 10s Fiabres (S,,,); (2)mica schistsof the Mulhaccn Complexin eastern Sierrade 10sFiiabres (S,); (3 ) micaschistsof the Mulhaccn Complex in theCentral Sierra de 10s Filabres(SA; 4) mica schists of theAlmanzora Unit (Sx); (5) phyllitesof the ALmanzora Unit (S,); (6)tourmaline gneisscs (Sx-,) of the Mulhacen Complex, casternSierra de 10s F'iiabres.

12). Kelyphitic rims of blue-green amphibole and albitearound omphacite (Jd-) point to a maximum pressure of9 bar. In the marbles relatively high pressures areindicated by the presence of tremolite+zoisite (Franz &Spear, 1983). In the mica schists high pressures areindicated by the moderate Mg-content of chloritoid(Bearth, 1963;Ganguly, 1972;Chopin & Schreyer, 1983)and by the association kyanite+zoisite (Newton &Kennedy, 1963) in absence of margarite. The occurrenceof kyanite+ilmenite+quartz (Bohlen et al., 1983)indicates a lower pressure and a higher temperature thanduring D,- ,The garnet-hornblende geothermometer (Graham &Powell, 1984) indicates temperatures of 535-595"C,usingthe rim of synlunematically grown garnet porphyroblasts.The formation of staurolite (Hoschek, 1969) indicates aminimum temperature of 560C at 8kbar pressure,disregarding its Mgcontent of 12-27%. A maximumtemperature of about 580C can be inferred from theabsence of forsterite in ultramatic rocks (Evans, Johanna,Oterdoom & Trommsdorff, 1976), never containingbrucite.

Zoning profiles of syn-D, garnet (Lmthout & Westra.1968), showingan increasing Mg/Fe ratio and a decreasingMn/Fe ratio from core to rim, suggest an increasingtemperature (Thompson, 1976) during D,. The b,,-valuesof phengite (Guidotti & Sassi, 1976) constituting S, (Fig.11) indicate a change in gradient to intermediatetemperature conditions. The Sicontent of these phengitesranges from 3.24 to 3.32.The P-T conditions during D, are indicated in Fig. 8 at8.25 kbar at about 570"C.

Metapelitic rocks containing chloritoid or pseudomorphsafter this mineral intercalate at the decimetre scale withnonchloritoid-bearing rocks. This intercalation is believedto reflect chemical differences in the on@ sedimentary


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374 H. E . B A K K E R Er A L .2.0

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I I I I2 0.2 0:s ole 1o A l X 1.4 1.6 1.8 2.0Fig. 12. Composition of over Z M electron microprobe analysesof amphiboles, plotted in the diagramof Brown ( 1 9 n ) . Analysesrecalculatedon 0, to an uncharged formulae manipulating Fe2+/Fe3+.Shaded areas contain analyses of blue-green amphibolesin kelyphitic intergrowths around omphacite. Arrows point to outerrims of crystals. Double lines indicate amphibolecompositions in Ahanzora Unit metabasites.

layering. Minerals like chloritoid and kyanite are unable to Na-poor, chloritoid-bearing layers from neighbouringform in rocks with a high Nacontent (Hoschek, 1969). Na-rich layers, where the element has been stored withinAfter D, but prior to Dr+,, chloritoid, kyanite and the mica lattice. This process reflects a decreasingstaurolite have become unstable and have been altered to temp erature (Cipriani, Sassi & Scolari, 1971; Guidotti &colourless mica (usually a mixture of paragonite and Sassi, 1976). In amphibolites Na-liberation by re-phengite), with or without chlorite. This non-isochemical crystallization of glaucophane to blue-green amphibolebreakdown shows the introduction of Na into previously causes the breakdown of chloritoid.


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SynkinematicDx+,metamorphismDuring this phase no pronounced mineral blastesis hastaken place. The metamorphism is characterized bynon-isochemical retrograde reactions, which are spatiallyassociated with D,+, extensional crenulation cleavages.In mica schists chlorite was formed at the expense ofphengite and garnet. In tourmaline gneisses biotite hasformed during dephengitization. Locally D,+, tensiongashes filled with albite have been formed in these rocks.In m a k rocks the most s ipficant reactions include partialcrystallization of blue-green hornblende to colourless mica,chlorite and locally actinolite and biotite.

The composition of rims of some amphiboles indicates adecrease in pressure to about 5-6 kbar (Fig. 12). accordingto the calibration of Brown (1977). A few outermost rimsindicate a pressure decrease to about 4 kbar. The mutualpresence of blue-green hornblende and, in other rocks,ripidolitic chlorite (locally pseudomorphing amphibole)indicates a temperature path that approximates the marginof the barroisite stability field (Ernst, 1979), to about4 kbar and a temperature below 450" C. The combinationof the hornblende-plagioclase (@) and garnet-phengitethermometers indicate 5.5 kbar and 505"C along this path.Thepressure drop isalso indicated by dephengitization.Thistrajectory reflects a post-D, cooling and decompression.The D,+, event is located at the lower P-T end of thispath (Fig. 8).Synkinematic metamorphismDuring D,+,ocal blastesis of garnet, kyanite, chloritoid,biotite and stilpnomelane has occurred. The compositionof the garnet has become richer in Mn and Ca and poorerin Fe and Mg (Linthout & Westra, 1968). The outer r i msof a few garnet porphyroblasts show a renewed increase inalmandine content, with decreasing Ca and Mn (Lmthout& Westra, 1968). In the waning stages of t h i s metamorphicphase however, all garnet have started to form chlorite(and quartz). Albite growth is striking in mica schists.

In mica schists pre- to syn-D, grown kyanite andchloritoid porphyroblasts have been partially crystallizedinto masses of paragonite, muscovite and chlorite.Colourless mica or albite pseudomorphs have locally beenformed after these minerals. These reactions together withthe local occurrence of albite coronas around chloritoidcrystals indicate non-isochemical conditions. Albite andchlorite porphyroblasts have also grown at the expense ofthe main foliation mica. X-ray data indicate lower&-values for the &+, mica (Fig. 11).In Fe-rich mafic rocks blue-green hornblende hasremained stable. Locally major transformation intointergrowths of chlorite and albite together with minorepidote or d a t e is observed in Mg-rich types. Inultramafic rocks tremolite was decomposed intoantigorite+ carbonate and/or talc.Locally stilpnomelane was formed (Nijhuis, 1%4),whereas normally chlorite and biotite growth has occurred.This indicates a temperature close to the upper stability

temperature of stilpnomelane (Nitsch, 1970). In combina-tion with the local kyanite growth of this stage, the P-Tconditions are estimated at 425C and 3-4 kbar (Fig. 8).The zoning profiles of garnet indicate that during D,,, atemperature decrease changed into an increase.

Synkinematic Dx+3metamorphismDuring D,+3 earlier grown albite porphyroblasts oraggregates have been surrounded by an anhedral rim ofplagioclase (up to An,), suggesting a solid-fluid reactiontype!.Small euhedral prisms of staurolite have nucleated inthe colourless micachlorite reaction rims around syn-D,grown chloritoid, kyanite and staurolite or in chlorite-mica pseudomorphs after these minerals. Chlorite masseshave partially been transformed into oxychlorite, alongthe cleavage or at the margins. Locally chlorite has beenrecrystallized into colourless mica or phlogopitic biotite.The Si-content of these micas is 3.07-3.10. In carbonatebreccias and marbles euhedral plagioclase porphyroblastsshow an increasing Ancontent towards the rim,with amaximum of An,. Locally sodic plagioclase-Na-richscapolite pairs (b -Mei ,) are present. A temperature of510-525C is indicated by the presence of Fe-richstaurolite (Hoschek, 1%9). Absence of forsterite limits thetemperature to about 550C (Evans et 1,976). Thestrongly varying plagioclase composition in combinationwith the absence of late zoisite, grossular and wollastoniteindicates a comparable temperature range, in combinationwith a partial CO, pressure (Storre & Nitsch, 1972).The stability of staurolite in the absence of cordieriteindicates pressures above 2-3kbar (Richardson, 1968).This is also suggested by the stability of chlorite(clinochlore and ripidolite) against quartz in the absence ofgarnet (Hirschberg & Winkler, 1%) together with thedecomposition of spessartine-grossularite-richgarnet dur-ing the waning stages of the preceding phase. A similarpressure is indicated by the Sicontent of Sx+3 icas (Fig.8).During Dx+2 and D.+3 mineral parageneses aredeveloped locally and incompletely. Mineral reactions arerestricted to zones of pronounced deformation. Deforma-tion may have offered the required activation energy tostart up metamorphic reactions, but the distribution canalso reflect local heating by hydrothermal processes. Theactivity of fluid migration during D,,, is indeed suggestedby a significant Fe- and Mg-enrichment in fracture zones.So, n' our opinion, the maximum recorded temperaturesfor D,+, might have been reached in distinct zones only.

The metamorphic evolution of theAlmanzora UnitThe Almanzora Unit shows its own distinct structural andmetamorphic development. This paper presents the 6rstdata on the relationship between deformation andmetamorphism (Fig. 7c) and the geothermobarometry(Fig. 12) of the unit.Bodies of blastophytic metabasites, wrapped by themain foliation S, in the metapelites, show hardly any


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376 H. E . BAKKER ET AL.

deformation structures. The metamorphic minerals of themafic rocks are albite, amphibole s . l . , biotite, chlorite,epidote s . l . , garnet and phengite. These minerals generallyOCCUT in a random fabric. Penetrative amphibole-epidoteS, fabrics have been developed only locally. Blue-greenamphibole crystals, which constitute this fabric, containcores of glaucophane and crossite with an internal planarfabric correlated with S,-,.

The mafic rocks contain relics of magmatic minerals likesodic plagioclase, augitic pyroxene, and brown horn-blende. Blue and green amphibole crystals and aggregatesare mainly formed from former plagioclase crystals.Locally they surround brown magmatic hornblende. Theamphiboles display the following zoning pattern:magnesioriebeckite-crossite-blue-green amphibole. Sepa-rate aggregates of actinolite inside magmatic augite showan increasing content of Na and A1 towards the rims (Fig.12, double arrow). According to the Brown (1977)diagram this composition points towards a pressure up to5 kbar. Their presence inside magmatic augite, which islocally surrounded by blue and blue-green amphibole,suggests an early origin of the actinolite.

In the Miyashiro diagram for glaucophane s.1. themagnedoriebeckite shows a volume change value (MuirWood, 1980) of about -2, and the crossite a value of-2.5. This is explained by an increase in pressure. Thecrossite, containing limited tetragonal Al, plot at 7 kbar(Fig. 12) in the diagram of Brown (1977). Blue-greenamphibole indicates the same pressure, although their CIcontent up to 3.6wt% may influence this figure. Garnetcontains up to 30% grossular. Its stability against the zonalblue amphibole in absence of omphacitic pyroxeneindicates a temperature below W C , the minimumtemperature for eclogite equilibrium. The garnet-phengitethermometer (Green & Hellman, 1982) indicates 440-450" C at 6-8 kbar. The garnet-hornblende thermometer(Graham & Powell, 1984), applied to the rim of the garnetcrystals and C1-poor amphibole, indicates 400C. However,this value may still be affected by the Fe-enrichment effectof C1-content of the amphibole.

In the metapelites phengite, chloritoid, kyanite (ex-tremely rare) and epidote s.1. have been formed pre- tosyn-D, (Fig. 7c). The Mg-content of chloritoid ( 4 4 4 5 % )points to a relatively high pressure (Fig. 10; Bearth, 1%3;Ganguly. 1972; Chopin & Schreyer, 1983). This issupported by the b,-values of S,-phengites (Fig. 11) andtheir Si-content of 3.27. This value closely coincides withthe values for S, micas in the Mulhacen Complex. Epidotes.1. crystals lying with a body preferred orientation withinS, become less Fe-rich towards the rim. This indicates anincreasing temperature during D.. These characteristics,together with local growth of kyanite and biotite in theabsence of stilpnomelane (Nitsch, 1970) are in accordancewith the P-T conditions for D, derived from thegeothermobarometry of the mafic rocks (Fig. 8).

After D, blastesis of chlorite and colourless micaoccurred at the expense of kyanite and chloritoid. Oftenonly pseudomorphs were left. The Occurrence of post-D.albite indicates Na-homogenitization and non-isochemical

conditions, as in the Mulhacen Complex. The Si-content(3.15-3.11) of phengites cross-cutting S. coincides with thelowest Si-values within the Mulhacen Complex duringDx+3. In mica schists and marbles late growth ofplagioclase (max. An,,,, in part rimmirag albite) and biotiteindicates a similar late temperature increase as ex-perienced by the Mulhacen Complex (Fig. 8).DISCUSSIONThe reconstructed P-T-t paths of the Mulhacen Complexand the Almanzora Unit are shown in Fig. 8. The HPIL Tconditions prior to D,-l indicate that the MulhacenComplex has been subjected to a phase of rapid tectonicburial, to a depth of approximately 37km. This process isconsidered to represent a phase of crustal underthrustingof relatively cold continental crustal material, whichcaused deformation of the original pattern of subhorizontalisotherms in the upper part of the lithosphere (e.g.England & Thompson, 1984). No deformation structureshave been observed that document this tectonic process.

On the trajectory towards D,-I the P-T-r pathindicates that the rocks were subjected to an increase intemperature at approximately constant pressure condi-tions. This pattern clearly illustrates that the Mulhacenrocks were heated while they were kept at constant depth.

These conditions continued during and after Dx-,, theoldest recognized deformation phase. This isobaric heatingpattern is caused by the cessation of the rapid crustal-scaleunderthrusting, enabling the start of the restoration of thedisturbed thermal structure of the lithosphere. Thecessation of large-scale underthrusting is probably bestexplained by the property of the continental crust,provided that a crust-mantle detachment has beenestablished, that it is only allowed to descend to a limiteddepth during a process of crustal doubling, because theforces associated with the underthrusting are balanced bythe buoyancy of the underthrust segment.The relationship between the penetrative D,-l deforma-tion and formation of thrust sheets within the MulhacenComplex is completely obliterated due to severe D,overprinting. Therefore, it is not possible to establishwhether the thrust sheets were formed prior to or duringD,-l. However, we consider it likely that the formation ofthe thrust sheet contacts has been associated withpenetrative deformation. Hence we propose that thecontacts were formed during D x - l and that the originaltectonic transport direction has been parallel to theglaucophane lineation and stretching lineation in gneissesduring west-north-west shearing. This D,-, pattern ofimbrication within the Mulhacen Complex, with presentthrust sheet thicknesses of about one kilometre, isexplained by continued collision. Further crustal-scaleunderthrusting is hampered by buoyancy SO that theoriginal underthrust segment starts to imbricate.

The temperature increase lasted up to D,. This progradetrajectory is characterized by the recrystallization ofseveral HP minerals to intermediate pressure parageneseswith a decrease in density of several per cent. Locally


19/23


eclogites hydrated to glaucophane schists, but most oftenglaucophane has grown at the expense of magmaticminerals. This indicates that the underthrusted crustalsegment, containing the Mulhacen Complex, was nottransformed to a high-density eclogite segment. Subse-quently glaucophane recrystallized to intermediatepressure blue-green amphiboles. The main effect of thiscompositionalchange isthe reduction of the HzOwt. % of theamphiboles, only giving rise to a small increase in density.However, liberation of HzO, also indicated by extensivequartz veining in mica schists before and during D,, leads toan increase in volume and therefore to a decrease in densityof a unit rock volume. This density reversion might be a pre-requisite for uplift (cf. Richardson & England, 979).The P-T-r path for the Mulhacen Complex indicatesthat after D,-, the rocks were consistently brought to ahigher level in the crust. From D,-, to D. the rocks wereuplifted some 7 km while the temperature increased by anamount of 70C. The P -T conditions indicate that D, tookplace a depth of 2&34km (Fig. 8). D, itself has beenassociated with an upward movement of the affectedcrustal segment towards a higher level in the crust.Because this phase proceeded under peak temperatureconditions it has resulted in the most penetrative andhomogeneous deformation. Structural data point todominant noncoaxial deformation and suggest animportant component of lateral transport. Quartz c-axisfabrics indicate west to north-west directed shear during D,.

Recently, several models have been put forward concern-ing the processes which may be responsible for the uplift ofhigh-pressuremetamorphic rocks (e.g. Draper & Bone, 1980;Davy & Gillet, 1986; Platt, 1986, 1987). Platthas presented a model in which uplift is essentially theeffect of extension in the upper rear part of a mechanicallycontinuous wedge above an active subduction zone.An alternative model is that the rocks are activelytransported over the footwall. of a previously formedcrustal-scale overthrust zone during D,. The rocks arebrought to the surface by progressive crustal-scale footwallcollapse combined with erosion during continued collision(Fig. 13a). Thus D, is considered as a continuedimbrication of a crustal segment unable to descend becauseof buoyancy.

A similar process has been described by Davy & Gillet(1986) in the western Alps. They described a process ofdiscontinuous crustal-scale stacking and transport towardshigher crustal levels of previously underthrusted sheets.We suggest that D,-l and D, are related to a similarprocess and represent distinct pulses during progressivecontinent-continent collision.

The initial part of the P-T- t path of the AlmanzoraUnit indicates that this unit has also been involved in aprocess of continent-continent collision. This unitexperienced lower maximum pressure and temperatureconditions than the Mulhacen Complex. This indicates thatthe Almanzora Unit has been buried to a lesser degree andconsequently has been located at a higher crustal level thanthe Mulhacen Complex before and during D,. The mostpenetrative structures were formed during D, under peak

temperature conditions. Because of the lower thermalconditions recrystallization has been less pervasive, com-pared with the Mulhacen Complex. Becauseof thisa part ofthe burial process isstillvisible in the mineralogy of the malicrocks (Fig. 8).

Given the striking stratigraphic similarities of the lowerpart of the Almanzora Unit with the top of theNevado-Lubrin Unit of the Mulhacen Complex, theAlmanzora Unit might have formed part of the samemajor thrust sheet but has merely been buried to a lesserdegree (Fig. 13a; cleaved arrow). However, the apparentthermal gradient for the Almanzora Unit is approximatelytwice the value for the Mulhacen Complex. This is morereadily explained by assuming that the Almanzora Unithas initially been included into a higher thrust sheet (Fig.13a; cleaved arrow). In any case the Almanzora Unit alsoformed part of a crustal-scale hanging wall during D,. Thelower maximum P-T conditions, but higher thermalgradient and the heating after the initial burial can beexplained by a combination of a screening effect of theunderlying rocks (the Mulhacen Complex) and a heatingby an overlying (hypothetical) thrust sheet (Fig. 13a),according to the models of Davy & Gillet (1986).

The subsequent post-D, trajectory in both crustalsegments is characterized by a marked decompressionunder decreasing temperature conditions (Fig. 8). At theend of this retrograde trajectory the Almanzora Unit hasbeen placed on top of the Mulhacen Complex. Since theAlmanzora Unit was previously located at a higher crustallevel compared to the Mulhacen Complex, this juxtaposi-tion must have involved heterogeneous extension andcrustal thinning. The thickness of the excized crustamounts to approximately 6km. The very low angle oftruncation of the S . in the underlying Mulhacen Complex,the orientation of stretching lineations and quartz c-axisfabrics demonstrate an eastward slip during D,+, extension(Fig. 13b).

The metamorphic data indicate that during and afterD.+* a significant temperature increase took place (Fig. 8).The data show that locally a geothermal gradient ofapproximately 7"C/km has been established during DX+,.T h i s high geothermal gradient might have been the conse-quence of the crustal thinning process initiated during

In the western part of the Internal Zone, metasedimentsof the Alpujarride Complex have been intruded byultramafic rocks, for which an K/Ar age of 22 Ma has beenderived (Priem, Boelrijk, Hebeda, Oen, Verdurmen &Verschure, 1979). It is envisaged that this intrusion initiallytook place during a phase of crustal thinning, although theactual contacts indicate that the ultramafic rocks wereinvolved in a later overthrusting event (Westerhof, 1975;Tubia & Cuevas, 1986). Correlation of the establishmentof the high geothermal gradient in the eastern Sierra de 10sFilabres with this intrusion event might suggest anOligocene to Early Miocene age for the D,,, to D,,,trajectory. A phase of crustal stretching followed byoceanic spreading during this period is well documented inthe western Mediterranean (Alvarez, Cocozza & Wezel,

Dx+,.


20/23

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21/23

GEODYNAMIC EVOLUTION OF THE BETICS 379

1974; Bellon, Coulon & Edel, 1977; Rehault, Boillot &hhufiet, 1984). It is obvious that this process afected thewhole continental crust and underlying mantle. Thisregional phase of extension and oceanic spreading alsoaffected parts of the crust which have not been thickenedconsiderably during alpine orogeny (e.g. the BalearicIslands and Sardinia). Therefore this extension is notrelated to the geometry and dynamics of a crustal wedge(Platt, 1986; 1987) but to processes within the mantle.

D,+, and D,+, structures, which reflect south tosouth-west vergent and north to northeast vergentlarge-scale overthrusting respectively, postdate the phaseof extensional deformation in the Internal Zone. Thesestructures have been imprinted on a crust that had locallybeen thinned wnsiderably and that was obtaining anirregular thermal structure. The simultaneous Occurrenceof a locally developing high geothermal gradient, whileextensional deformation has ceased, has been predicted byconductive thennomechanical models of rifting (Moretti &Froideveaux, 1986).Thesemodels show that once a thermalanomaly is generated, for instance by regional extensionand crustal thinning, the further creation of a highgeothermal gradient becomes a self-propagating process.So while the compressional D,+* and Dx+3 structuressuggest crustal thickening the metamorphic data mightreflect that crustal thinning has still been active duringthese phases.

The established pressure conditions (Fig. 8) indicate thatthe structures have been formed at a depth of about10-12 km (D.+,) and 5-8 km (D,+J. In comparison to theolder deformation structures these structures reflect morelocalized and more heterogeneous deformation. Thesestructures largely determine the present morphology of theeastern Sierra de 10s Fdabres.ACKNOWLEDGEMENTSElectron microprobe analyses were performed at theelectron microprobe laboratory of the Institute for EarthSciences of the Free University, Amsterdam, with finanicaland personnel support by NWO-WACOM (research groupfor analytical chemistry of minerals and rocks subsidizedby the Dutch Organization for the Advancement of PureResearch). We thank Dr P. Maaskant, Dr C. Kieft and W.J. Lustenhouwer for performing the analyses.

We also thank the technical staffs of the Free Universityand the University of Amsterdam for the preparation ofmany hundreds of thin sections,Alwine Prinsen for typingthe manuscript and Fred Kievits for drawing the figures.One of us (HEB) benefited from a grant from HetVakgroepfonds Structurele Geologie van de Universiteitvan Amsterdam. Professor Dr B. R. Frost and Dr J. P.Platt have reviewed this paper thoroughly and quickly.Their reviews and subsequent discussions with Dr J. P.Platt helped to improve the text and clarified our way ofthinking.Last but not least we would like to thank Dr 0.J. Simonand K. Linthout for their criticism and many stimulatingdiscussions on the geology of the Betic Cordilleras.

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350pp.

Received 15 February 1988; eviswn accepted 6 uly 1988.

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