DeJong_Geodynamic Evolution western Mediterranean & Betics (Spain)_Proceedings-Kon.Ned.Akad.Wetensch 1993

Proc. Kon . Ned. Akad. v. Wetensch. 96 (3) , 295- 333 September 27, 1993

The tectono-metamorphic and chronologic development of the Betic Zone {SE Spain) with implications for the geodynamic evolution of the western Mediterranean area

Koen de Jong

Jnstituut voor Aardwetenschappen, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands

Communicated by Prof. W.P. de Roever at the meeting of November 30, 1992

ABSTRACT

An integrated (micro)structural , petrological and geochronological study of the M ulhacen and Al­pujarride Complexes in the eastern Betic Zone has resu lted in well constrained Pressure-Temperature­time paths. These P-T-t paths display the essential features of the geodynamic evolution of the Betic Zone.

Early Alpine High-Pressure metamorphism is due to subduction of the Mulhacen and Alpujarride Complexes. It is argued that subduction took place below the lower crust of the Malaguide Domain during the Early Cretaceous. Subduction to the West implies that the Malaguide Domain was located closer to the External Zone than the metamorphic nappe complexes of the Betic Zone. Subduction resulted from ESE-ward motion oflberia from 119 to 80 M a, which resulted in break-up of the Jurassic transtensional Africa-Eurasia plate boundary. Exhumation and cooling of the HP metamorphic rocks occurred by sequential underthrusting of deeper nappe complexes and by concomitant extension of the hanging wall of the subduction system, where the Malaguide Complex was located. Ductile de­fo rmation of the metamorphic rocks resulted in their elongation and extreme thinning.

Advanced cooling of the thinned metamorphic nappe pile occurred during northward thrusting of the Betic Zone over the southernmost External Zone in Early to Middle Eocene times, thus coeval with the climax of crusta! shortening in the Pyrenees at the northern margin of Iberia. Overthrusting resulted in HP / LT metamorphism in the overthrust part of the External Zone (A lmagride Complex) and flexural bulging in its northern part where sedimentation continued.

During the Late Oligocene and younger tectonic evolution extension and crusta! shortening fol­lowed each other rapidly during continuing Africa-Eurasia convergence, pointing to roll-back, stee­pening and detachment of a subduction slab. Phases of reheating and resetting of isotope systems correlate with extension and upwarping of hot mantle material and associated magmatism, whereas phases of cooling are due to thrusting of (re)heated rocks over less extended, cooler lithosphere. Early Miocene inversion of the extended area and concentration of overthrusting in the most thinned area resulted from slab detachment enabling transmission of compression in the shallow remainder of the

295

slab. Slab steepening and detachment can further explain concentration of Miocene and younger magmatism into a narrow zone, its deep source and deep earthquakes, which can not be due to steady state subduction as Africa-Eurasia convergence falls short during this period. The structural response of the Mulhacen and Alpujarride Complexes during the younger Alpine evolution was different due to their location in the footwall and hanging wall of the ex tensional system, respectively.

INTRODUCTION

From the early days of geological investigations in the Betic Cordilleras it was firmly established that the regional structure of the Betic Zone is the result of large scale subhorizontal nappe movements , which were explained by thrusting of Africa over sediments of the Tethys and over Europe (Brouwer, 1926). However, the origin of the chain was almost immediately explained alternatively as a result of nappe shedding from a central high under the influence of gravity (Van Bem­melen, 1933). At present there is still no unique geodynamic model for this ora­genic belt, which forms the western end of the European Alpine orogen (fig. 1), and the importance of extension and shortening during the tectonic evolution of the belt are hotly debated (cf. Doblas & Oyarzun, 1990 and Frizon de Lamotte et

IBERIAN MESETA

~ E3J CJ

Jr/' m:m:m:m:m:1

TELL

Alp1ne deformed Mesozoic and Tertiary rocks (with m1nor reworked Paleozo1c rocks)

Flysch Uni ts

Internal Zones of the Betic Cordil leras and Rif (Alpine metamorphic rocks )

Kabyllan Ma ssifs

00

00

GULF DE LION

Fig. I. Tectonic sketch map of the western most Mediterranean area including the Betic Cordilleras. The regional geology of the eastern Betic Cordilleras is depicted in Fig. 2, corresponding to the out­lined a rea.

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al., 1990; Zeck et al., 1992a and De Jong 1992). Widely different models have been proposed, which generally highlight only one aspect of the tectonic evolu­tion. This seems principally due to the fact that most models have no sound basis of structural and Pressure-Temperature-time data. Generally, the metamorphic part of the orogen, the Internal or Betic Zone, is considered as an allochtonous tectonic element or micro plate (Alboran Micro Plate, Alboran Domain or Al­boran Block, e.g. Andrieux et al., 1971; Bouillin et al., 1986; Dercourt et al., 1986; Comas et al., 1990; Sanz de Galdeano, 1990; Vegas, 1992; Geel et al., 1992), which was juxtaposed to the External Zone, the former Mesozoic and Early Tertiary rifted margin of SE Iberia (Hermes, 1978; Peper & Cloetingh, 1992; De Ruig, 1992), along ENE-WSW trending wrench faults during the Tertiary (Hermes, 1978; De Smet, 1984; Bouillin et al., 1986). Deformation in the Betic Zone has been discussed within this concept (Frizon de Lamotte et al. , 1989; Vauchez & Nicolas, 1991). However, the validity of this model is questioned by the occur­rence of HP / LT metamorphic rocks of the Almagride Complex as the structu­rally deepest unit in the northeastern Betic Zone. The Almagride Complex con­sists of (very-)low-grade Triassic rocks with a stratigraphy which strongly resembles that of the Subbetic in parts of the eastern External Zone in the pro­vince ofMurcia (Simon, 1987). This author proposed that this complex represents the southern continuation of the Subbetic below the nappe complexes of the Betic Zone, implying overthrusting of the External Zone by the Betic Zone (Simon, 1987; De Jong, 1990, 1991). The outcrop of the Almagride Complex in windows 50 km south of the present-day boundary between the External and Internal Zones (fig. 2) demonstrates that the tectonic evolution of the Betic Cordilleras cannot be explained merely by juxtaposition of two different tectonic domains by wrenching.

Moreover, investigations in the boundary zone between the External and In­ternal Zones point to presence of only minor strike-slip movements during the Middle Miocene, taking place after overthrusting (De Ruig et al. , 1987; Martin­Algarra et al., 1988; Van der Straaten, 1990; Lonergan, 1991; De Ruig, 1992).

In the stack of nappe complexes in the Betic Zone the Malaguide Complex is underlajn by three metamorphic nappe complexes (Egeler & Si m on, 1969; Puga & Diaz de Federico, 1978; De Jong, 1990, 1991, fig. 2), from top to bottom:

3) the A1pujarride Complex, 2) the Mulhacen Complex, 1) the Veleta Complex. In contrast to these nappe complexes, the Malaguide Complex has largely es­

caped metamorphism during the Alpine tectonic evolution. The Mesozoic to the Early Miocene stratigraphic column of the Malaguide Complex (Make!, 1985) is a record of its sedimentary history and the surface expression of deformation of the metamorphic rocks at depth (De Jong, 1990, 1991). The model presented here is based on a detailed integrated (micro)structural, petrological and geochrono­logical study of the eastern Betic Zone (De Jong, 1991) in order to get insight into the changing P-T conditions in the course of the tectonic evolution, which eluci­date the principal crusta! scale tectonism in the belt. In the first part of this article the relationship of mineral growth with respect to deformation phases will be

297

MOTRIL

S.d.E. = S.d.IE = S.d.A.::: S.d. I.F.::: S. N, =

INTERNAL

OR

BETIC ZONE

Sierra de Espufla Sierra de !as Estancias Sierra de Almagro Sierra de Ios Filabres Sierra Nevada

Neogene and Quaternary sediments and volcanics

lGuadalquivir Units

Prebetic Zone

Subbetic Zone

J Malaguide Complex

Alpujarride Complex

Mulhacen Complex

Veleta Complex

Almagride Complex

Mesozoic cover of Variscan Basement

Variscan Basement of Spanish Meseta

0 50 100krn L---------~,_----~

Fig. 2. Tectonic map of the eastern Betic Cordilleras; the main areas of investigation are outlined by boxes A to E. The regional structure of the Internal Zone is formed by a stack of four nappe complexes, which is thrust over the External Zone; equivalents of this zone crop out in windows as the (very-)low­grade metamorphic Almagride Complex.

discussed. This analysis shows that the tectono-metamorphic evolution involved two stages. During the first stage HP conditions prevailed, whereas temperatures increased; the most penetrative deformation in the Mulhacen Complex (D2mulh)

was initiated at peak temperature conditions of 550- 570oC and progressed during decompression coeval with uplift from 37 to 30 km, accompanied by cooling, which continued during and after the subsequent deformation phase, D 3 mulh . The second stage started with important reheating which culminated during D 4mulh

and was in turn followed by a second phase of cooling during D 5mulh and D 6muih _

The tectono-metamorphic evolution of the Alpujarride Complex was similar to that of the Mulhacen Complexes: initial HP conditions (0.7 GPa) followed by decompression and cooling, succeeded by reheating.

During early Alpine tectonism deformation structures with similar kinematic significance were formed in the Alpujarride and Mulhacen Complexes. The late Alpine structural evolution, however, shows important differences. In the second part of the article the Pressure-Temperature-Deformation-time paths of both complexes are used to constrain a geodynamic model for the Betic Cordilleras. Early Alpine HP metamorphism is probably due to Early Cretaceous subduction

298

©Miilll ~ L::::J L:J ~

lliilliD mill

suggested by 40 Ar I 39 Ar tourmaline ages up to about 89 M a . Subduction resulted from ESE-ward movement of Iberia, which SE leading edge, formed by the Ma­laguide Domain, overrode the Betic Zone to the SE of it. This tectonic model implies a paleogeography which differs fundamentally from the generally adop­ted paleographic reconstruction, as will be discussed at the end of the paper. Late stage reheating is the result of latest Oligocene to earlymost Miocene extension, which resulted in mantle upwarping introducing a transient heat source into the crust . Extension and subsequent crusta! shortening are discussed within the con­cept of slab roll-back and detachment. As during reheating the temperature do­main for diffusion of radiogenic isotopes was re-entered, significant resetting of metamorphic ages has occurred shown by (very) young mica K-Ar and Rb-Sr ages of 12.5- 15.5 Ma (Priem et al., 1966; Andriessen et al., 1991) and mica 40 Ar / 39 Ar plateau ages between 14.4 and 17.6 Ma in the Mulhacen Complex (Monie et al. , 1991; De Jong, 1991 ; De Jong et al., 1992).

Tectono-metamorphic evolution of the Alpujarride and Mulhacen Complexes

It is well established that the Alpine metamorphism in the Mulhacen and Al­pujarride Complexes was plurifacial (De Roever & Nijhuis, 1963; Nijhuis, 1964; Langenberg, 1972; Vissers, 1981; Bakker et al., 1989). Overprinting relations in the field showed that the Mulhacen Complex experienced six phases of penetrative deformation D 1rnulh to D 6rnui\ whereas the Alpujarride Complex was influenced by four deformation phases D 1 alpu to D/1

pu (De Jong, 1991, 1993b). A micro­scopic study provided insight into the microstructural evolution and paragenetic relations of mineral assemblages during the polyphase deformation (fig. 3). It appeared that all three tectonic units of the Mulhacen Complex experienced a similar tectono-metamorphic evolution; the Alpujarride tectonic units on the other hand went each through a similar tectonic evolution, but they experienced a slightly different metamorphic history. The chronology of the Alpine tectonic evolution was studied by well selected samples from the Mulhacen Complex characterised by the main phase tectono-metamorphic foliation D 2mulh.

Early Alpine evolution

Pre-D I and D I tectono-metamorphic phase

Early Alpine pressures in the Mulhacen Complex fall in the range of 1.0- 1.2 GPa, during which temperature increased from about 350 to 525°C; coeval with a change from static to synkinematic conditions (figs. 3a, 4; Bakker et al. , 1989). P-T estimates are based on geomthermometry ofRaheim & Green (1974) and Ellis & Green (1979) for garnet-omphacite pairs in eclogites and Krogh & Raheim (1978) and Green & Hellman (1982) for garnet-phengite pairs in gneisses; pressures are determined from the jadeite content of omphacite using the calibration of Currie & Curtis (1976). Such P-T estimates agree with the stability of glaucophane and aragonite (fig. 3a) . P-T conditions in the central Sierra de Ios Filabres (Sierra de Baza, Gomez-Pugnaire et al., 1989) and the western Sierra Nevada (Velilla &

299

a

OMPHACITE

GARNET

GLAUCOPHANE s.l.

EPIDOTE

CUNOZOISITE

AMPHIBOLE (bl. gr.)

ACTINOLITE

ALBITE

OUGOCLASE

CHLORITE

ARAGONITE

CALCITE

b

300

GARNET

EPIDOTE

CLINOZOISITE

GLAUCOPHANE s.l.

CHLORITOID

KYANITE

STAUROLITE

AMPHIBOLE (bl. gr. )

BIOTITE

CHLORITE

ALBITE

OLIGOCLASE

OXYCHLORITE

MICA (colourless)

Neoformation

D ""'" PRE 1

D MUl H

1 D ""'"

2 D ""'"

3

- Ill! I! 0 0 00 • . DD

DD • . - -Ill! !iilllll!l lil!!ll!ll

-= -. . . D ""'"

1 D ""'"

2 D M UlH

3 D ""'"

4 D ""'"

5

--------- D- DDDDDDODD =•

...... ~ .. moD .. mmm==EEmm=~mm

~ 0 D---=== ========

------m DD - DDD = DDDD=•

- DD ---==:JI- D 0 =. - DDDDDODDD===-DO =• ----=DD DD ======

= ---- DD _____ __

=---=--DD

11111!11111!1111 !111 iilil!lllil!lllllll! - iii!l!!il!l!i!!liiii ]!11111111 lll!!lllll l!l! lbllll

Deformation Complete breakdown

llllilllllllllllllllllll!llllll 1 !llllllllllll 0 0 D 0 0 ~·~·~·~·~·======

Syntectonic and/or annealing recrystallization

Partial breakdown No recognizable response

c DALPU

1 D ALPU

2 D ALPU

4

CLINOZOISITE 1111- -

CHLORITOID -----~=== 0 0 0 0 0

AMPHIBOLE (bl. gr.)

GARNET ======------· 0 0 0 0

BIOTITE

OXYCHLORITE

CHLORITE ----==~-•oo=====------=~rnrrc=

ALBITE

OLIGOCLASE .. KYANITE

ANDALUSITE

STAUROLITE

MICA (colourless) ill!lililliiliiil!li !l!!l!!!ll!!!!!!l!lll !!!!l!l!llll!!lll!! Ill! !!lll!l!!l

Fig. 3. Relationship between mineral growth and deformation phases in a) mafic rocks and b) mica schists of the Mulhacen Complex in the Sierra de los Filabres and Sierra Alhamilla and c) metapelites of the Alpujarride Complex (Partaloa-Variegato and Oria units) in the Sierra de las Estancias and Sierra de los Filabres.

Fenoll Hach-All, 1986) are comparable. A number of tectonic units of the Alpu­jarride Complex of the eastern Betic Zone metamorphosed under pressures around 0.6- 0.7 GPa at temperatures between 300°-400°C (fig. 4; Bakker et al., 1989; Goffe et al., 1989; De Jong, 1991). Pressure estimates are based on the oc­currence of glaucophane and crossite in the Almanzora Unit, for which the Brown (1977) geobarometer indicated pressures around 0.7 GPa (Bakker et al., 1989) and the occurrence of magnesio-carpholite in the Oria Unit (Goffe et al., 1989). Temperatures in the Almanzora Unit were estimated to have been below 400oC on account of stability of grossular-rich garnet (30%) against zonal mag­nesioriebeckite-crossite in absence of omphacite (Bakker et al., 1989), whereas occurrence of Fe-rich chloritoid in the Oria Unit implies similar maximum tem­peratures (De Jong, 1991).

~ D,mulh structures were formed at the end of the isobaric heating trajectory (fig. 4) and are mainly left unaffected by subsequent deformation in glaucophane schists, eclogites and in the core of a gneiss body in the eastern Sierra de Ios Fi­labres. D,mulh structures demonstrate important E-W to ESE-WNW stretching during top-to-the-west shear. Due to lack of resistant rocks no D 1 alpu structures other than inclusions in porphyroblasts and microlithons have been left.

301

1.1 D MUL H

PRE 1

1.0

0.9

GLAU-in

0 .8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

300 400 500 600

Temperature (°C)

Fig. 4. P-T-t path of the Mulhacen Complex (light shading) and the Alpujarride Complex (dark shad­ing) based on P-T-t determinations and thermo-geochronologic data from Bakker et al. , 1989; De Jong, 1991 ; De Jong et al., 1992. Glaucophane-in (Maresch, 1977); Al-silicate triple point (Holdaway, 1971).

D2 tectono-metamorphic phase

The D 2 tectono-metamorphic phase in the Alpujarride and Mulhacen Com­plexes started at the culmination of the heating stage and continued during de­compression to 0.7- 0.6 GPa concomitant with cooling to about 550°C- 500oC in the Mulhacen Complex and to 0.5 GPa and 450- 400°C in the Alpujarride Corn-

302

plex (fig. 4). The main metamorphic change in mafic rocks are the instability of omphacite, glaucophane and crossite, which recrystallised to blue-green amphi­bole, with or without albite; aragonite has inverted to calcite (fig. 3a) . Such reac­tions have gone to near completion in rocks with a penetrative S2• Pressures are derived from the Brown (1977) method for blue-green amphiboles and from ty­pical stoichiometric Si-values of 3.32 and 3.27 in S2 phengites of the Mulhacen and Alpujarride Complexes, respectively (Bakker et al., 1989; De Jong, 1991). Temperatures in the Mulhacen Complex are given by the stable assemblage kya­nite-chloritoid-staurolite and the garnet-hornblende thermometer of Graham & Powell (1984) (Bakker et al., 1989). Maximum metamorphic temperatures in the Alpujarride Complex are derived from the garnet-phengite thermometer of Green & Hell man (1982) and from the stability of chloritoid and kyanite and ab­sence ofstaurolite (fig. 3c; Bakker et al., 1989; De Jong, 1991).

In both nappe complexes D 2 resulted in the most penetrative deformation at all scales. Recumbent D 2 folds are tight to isoclinical, resulting in transposition of bedding (So) and S1 parallel to the main foliation S2• In the Mulhacen Complex this transposition stratigraphy is continuous along strike over at least 25 km (De long & Bakker, 1991, encl. I) . Strain determinations in both complexes point to a layer-normal shortening of 75% and an elongation parallel to the E-W trending D 2 stretching lineation of 380% on average (fig. 5; triangles) .

Asymmetric K-feldspar phenoclasts, fish-shaped phengite crystals, quartz-fil­led pressure shadows and rotated garnets point to top-to-the-west shear in the Mulhacen Complex. Symmetric quartz c-axes preferred orientations, however, point to dominant coaxial deformation in quartz-rich lithologies. This implies partitioning of D 2 mulh strain into dominant rotational deformation in layered lithologies, like gneisses and mica schists, and a more non-rotational deformation in quartzites. Weakly developed but systematically symmetrical, quartz lattice

a

1.5 1.0 Log Z/Y

1.0

Log XIY

0.5

b

1.5 'i.o Log ZIY

Fig. 5. Log-strain diagram (Wood, 1974) for a) feldspar phenoclasts in amphibolites of the Mulhacen Complex, eastern Sierra de Ios Filabres and b) quartz pebbles in Triassic phyllite-quartzite series of the Oria Unit, Alpujarride Complex, eastern Sierra de las Estancias (box C' in Fig. 2).

303

preferred orientations in the Alpujarride Complex also point to coaxial de­formation . The intensity of D 2mulh increases upwards as documented by para­crystalline rotated garnets and fold structures. In the lower part of the Mulhacen Complex rotation angles of garnets vary between 65° and 170°, whereas in the upper part rotation angles between 120o and 270° were measured. D 2mulh folds in deeper levels of the complex are less tight than in higher parts and S2 refracts on folded bedding. D 2mulh folds at this structural level are curvi-linear; fold axes commonly make a high angle to the stretching lineation, which has a constant ESE-WNW trend. In the uppermost 1-1.5 km of the complex, however, axes of isoclinal D 2 mulh folds are parallel to the ESE-WNW trending stretching lineation. This points to rotation of folds axes into parallelism with the shear direction during upwards increasing D 2mulh strain, demonstrating an increasing displace­ment going to the top in the Mulhacen Complex, as in shear zones strain is related to displacement (Ramsay, 1980). The strain gradient is probably due to the translation of the overlying crusta! segment with the Alpujarride Complex during this phase, although the present-day nappe contact was formed later. In the overlying Alpujarride Complex, absence of kinematic indicators precluded es­tablishment of the dominant shear sense and deformation partitioning during D

2alpu.

The contact between the Mulhacen Complex and the underlying Veleta Com­plex is parallel to S2 in both complexes and folded by D 3 folds in both complexes (De Jong, 1991, 1993a). There is no downward increasing D 2mulh strain, but D 2 vei

strain the suppermost 400 m. of the Veleta Complex sharply increases towards the Mulhacen Complex, culminating in a mylonite zone at the nappe contact; these tectonites yielded quartz lattice preferred orientations indicative of top-to-the­west displacement during D 2mulh and D 2 vel (De Jong, 1991, 1993a).

Timing of cooling during and after D2muth

Cooling during and especially after D 2mulh is manifest by widespread retro­gression. In mafic rocks blue-green amphibole reacted into Na-rich plagioclase and chlorite often accompanied by calcite growth (fig. 3a); in mica schists chlor­itoid, kyanite, and staurolite were replaced by phengite ( ± paragonite) often to­gether with chlorite (fig. 3b). Consequently, radiometric ages of the Mulhacen Complex should be considered as cooling ages.

Despite thermal resetting, discussed below, an older isotopic system seems locally preserved. Monie et al. (1991) obtained a 40 Ar I 39 Ar plateau age of 48.4 ± 2.2 Ma from a barroisitic blue-green amphibole, which characteristically grows at the expense of glaucophane during D 2mulh (fig. 3). Andriessen et al. (1991) reported K-Ar tourmaline ages between 115 and 80 Ma. 40 Ar / 39 Ar dating of tourmaline, obtained from gneisses with a D 2 mulh fabric, resulted in reference lines with ages between 89.1 ± 0.9 and 52 ± 1 Ma (De Jong, 1991). In addition, D 2mulh phengites yielded Rb-Sr ages of: 65.7 ± 10.1 Ma and 41.1 ± 4.6 Ma (De Jong, 1991). These ages being obtained from syn-D 2mulh minerals are inter-

304

preted to date the passing through the closure temperature of the various isotopic systems, during cooling following D 2mulh.

Late Alpine evolution

Mineralogical changes show that subsequent to the D 2 cooling the Mulhacen and Alpujarride Complexes experienced reheating, which was followed by rapid cooling (figs. 3 and 4). Structural response of both complexes during the late stage evolution was entirely different, indicated by the difference in deformation pha­ses. The Mulhacen Complex was influenced by four deformation phases, D 3mulh to D 6mulh' and the Alpujarride Complex by two phases D 3alpu and D 4alpu.

Mineralogical changes

D 3mulh occurred during advanced retrogression shown by widespread decom­position of blue-green amphibole, kyanite, garnet, chloritoid and phengite (figs. 3a,b) . Important growth of albite points to Na-metasomatic reactions. Local synkinematic kyanite growth (fig. 3b) implies temperatures around 425°C, whereas stoichiometric Si-values of S3 phengite between 3.18 and 3.20 (De Jong, 1991) point to pressures in the order of0.35- 0.45 GPa. In the Mulhacen Complex of the Sierra de los Filabres temperature increase is shown by local syn-D4mulh growth of tiny crystals of staurolite (fig. 3b) in phengite-chlorite decomposition mantles around D 2mulh staurolite, chloritoid and (partly syn-D3mulh)kyanite. In the Sierra Alhamilla a larger grain size and much more frequent occurrence of late stage staurolite point to a stronger reheating in the southern part of the Mulhacen Complex. The Fe-rich composition of the staurolite points to tem­peratures upto 525°C (Bakker et al., 1989). Growth of oxy-chlorite and biotite at the expense of chlorite and formation of oligoclase-andesine rims around albite in mica schists (figs. 3a,b) are consistent with temperature increase. Extreme Ca enrichment shown by the growth of Ca-rich bytownite in some carbonates (Nijhuis, 1964) is probably controlled by the ea-content of the host rock.

In the Alpujarride Complex increase in temperature is also shown by growth of staurolite (fig. 3c) in the graphite-rich basal series of a number of tectonic units. In the Triassic series staurolite growth has not taken place, presumably due to un­suitable chemistry implied by rarety of chloritoid in these rocks . However, local growth of cordierite accompanying andalusite blastesis points to similar tem­peratures as staurolite formation in the basal series, i.e. in excess of 550°C.

~ Widespread andalusite growth (fig. 3c) points to pressures below 0.4 GPa. D 5mulh and D 3alpu occurred during falling temperatures as indicated by wide­

spread growth of chlorite partly at the expense of staurolite. Continued growth of andalusite post-D3alpu (fig. 3c) implies that temperatures in the Alpujarride Complex did not drop below about 400°C.

Timing of reheating and subsequent cooling

Isotopic dating of reheating in the Betic Zone is difficult as its concerns dating

305

of a partially reset older isotope system, like the one which was closed during cooling after D 2muih_ Reheating in the Mulhacen Complex is tentatively dated at around 25 M a. This is based on 40 Ar I 39 Ar laser probe dating of a D 2 mulh phengite single grain, which shows evidence for argon loss at around 25 M a (De Jong, 1991; De Jong et al. , 1992). In addition, modelling of a 40 Ar 139 Ar tourmaline age spec­trum, showing indications for partial Ar-loss, resulted in a 23 .5 Ma model age for this event (De Jong, 1991). Thus, reheating in the Betic Zone can be considered as an earlymost Miocene event.

Cooling of the Alpujarride Complex is well dated at around 19 Ma by various isotopic dating methods. Biotite K-Ar ages from the leucogranites associated with ultramafic rocks near Ronda have an average of 19.7 ± 0.6 Ma (Priem et al., 1979); muscovite from comparable rocks in the Sierra Cabrera yielded K-Ar ages between 18.9 ± 1.0 and 19.8 ± 0.5 Ma (Andriessen et al. , 1991). High-grade me­tamorphic rocks have an average 21 ± 2 Ma Rb-Sr muscovite whole-rock age (Zeck et al. , 1992b) and biotite and muscovite 40 Ar I 39 Ar ages between 18.4 ± 0.6 and 20.3 ± 0.3 M a (Monie et al., 1991; Zeck et al., 1992b). Radiometric dating in the Mulhacen Complex has, however, not resulted in a tight cluster of cooling ages. Integrated 40 Ar 139 Ar ages of phengite vary between 25.9 and 14.3 Ma (De Jong, 1991; De Jong et al., 1992). Modelling of the age spectra implied that they were the result of repeated thermal resetting, which will be discussed in a later section.

Structural response

The D 2mulh shear planes were influenced by important S-vergent folding and associated thrusting during D 3mulh at the end of the retrograde trajectory (fig. 4). Mesoscopic D4mulh structures are generally N-vergent lower order folds of km­scale folds . In contrast, in the Alpujarride Complex the first important deforma­tion phase subsequent to the D 2a lpu main phase occurred during the waning stages of the second thermal peak (fig. 4), when D 3 alpu folds were formed, which are the most conspicuous fold structures in the Alpujarride Complex. The in­tensity of D 3a lpu increases structurally downwards in the Alpujarride overthrust mass: tight N-vergent E-W trending folds with overturned limbs grade into tight to isoclinal recumbent, often strongly curvi-linear similar folds , directly above the Mulhacen Complex. D 3alpu structures were coeval with D 5mulh in the underlying Mulhacen Complex as shown by a number of observations. Firstly, the intensity of D 5mulh increases upwards towards the overlying Alpujarride Complex. Sec­ondly, the intensity of D 5mulh deformation increases from north to south in the contact zone, similarly as D 3alpu structures. Thirdly, commonly NNE-SSW trending axes of D 3a lpu folds in the deeper structural level approach the orienta­tion of D 5m u lh stretching lineations in the mylonite zone in the top of the Mulha­cen Complex in the southern part of the contact zone. In this part of the nappe contact dm-spaced D 5mulh extensional crenulation cleavages, a few hundred metres below the thrust contact, grade into mylonites directly below the Alpu­jarride basal thrust. Along the D 5mulh strain gradient E-W trending D 2mulh

306

lineations rotate progressively to the NNW-SSE orientation of D 5mulh stretching lineations in the mylonites. The asymmetry of pressure shadows and re­crystallization tails of porphyroclasts and micro faults in them, secondary grain shape fabrics and lattice preferred orientations in quartz mylonites systematically indicate top-to-the-north shear. Hence, the Alpujarride Complex was translated northward with respect to the underlying Mulhacen Complex during D 3alpu_

D 5mui\ resulting in truncation of all older structures in the footwall. Deformation of the contact between the Alpujarride and Mulhacen Complexes

and of D 5mulh mylonites by large scale D/1pu and D 6mulh folds and thrusting of lithologies of the upper part of the Mulhacen Complex over the Alpujarride Complex shows reactivation of the contact between these complexes. In both nappe complexes chevron folds and associated younger brittle-ductile shears and cataclasites were formed, pointing to P-T conditions in the field of brittle de­formation. Extensional structures of this generation are systematically superimposed on folds, showing late stage extension after or during advanced thrusting.

Dynamics of metamorphism and tectonism: geodynamics of the Betic Cor­dilleras

In this section the tectonic and metamorphic data will be combined to establish the P-T-t paths of the Alpujarride and Mulhacen Complexes in order to gain in­sight into the crusta] scale tectonism and the geodynamic evolution of the Betic Cordilleras.

Early Cretaceous subduction in the Betic Zone

Early Alpine metamorphic pressures in the Alpujarride and Mulhacen Com­plexes indicate burial to depths of about 27 and 37 km, respectively. Such depths point to subduction below a segment with a crusta] thickness. It is argued that this segment is represented by the crystalline basement below the Paleozoic and younger sediments of the Malaguide Domain (fig. 6a). Although the Mala guide Complex in the Betic Zone is actually extremely thin, Late Paleozoic rocks document important clastic influx of granites, gneisses and medium-grade meta­morphic rocks (Soediono, 1971; Gee!, 1973; Make!, 1985; Herbig & Stattegger, 1989). One gneiss pebble yielded a Rb-Sr whole-rock age of 535 ± 75 Ma (Soe­diono, 1971, recalculated at 528 ± 25 Ma, Institute of Isotope Geology, Am­sterdam, unpubl. data), pointing to the presence of a Cambrian basement. The Dorsale Betique, which is considered as the margin of the Malaguide depositional domain (Bouillin et al., 1986), has a stratigraphy comparable to that of the Ma­laguide Complex (Make!, 1985) and has experienced a similar clastic influx in the Late Paleozoic (Peucat & Bossiere, 1981). These clastic rocks are strikingly similar to the Precambrian basement of the Dorsale Kabyle, formed by the Great Kabylian Massif in coastal Algeria (fig. 1; Peucat & Bossiere, 1981). Thus, in this north African massif the Malaguide Complex might presently still be attached to its crystalline basement. Influx of fresh detrital muscovite, biotite and K-feldspar

307

WNW

a

b

c

308

r-

~ rl ~ ESE

Mulhacen Complex

r- r-

~rr r~

Fig. 6. Nappe stacking in the Betic Zone by sequential detachment of upper crusta! rock sequences, in order of underthrusting: a) the Alpujarride Complex, b) the Mulhacen Complex and c) the Veleta Complex. Detached upper crusta! slices are added to the hanging wall of the subduction system formed by the Malaguide Complex (MAL) with a Kabylian-type of crystalline basement (KAB) while the lower crust (crosses) and mantle (random striping) continue to subduct to the west. Sequential underthrusting results in dramatic cooling in the overlying earlier subducted nappe complexes shown by the insert P-T-t paths for the Alpujarride and Mulhacen Complexes, panels b) and c), respectively. P-T conditions of the Veleta Complex (square in the insert P-T graph of panel c) imply that it is un­derthrust by an upper crusta! unit (coarse stipple, panel c), which is, however, not exposed at the pre­sent erosion level. Bars for horizontal and vertical scale: 15 km.

into Jurassic carbonate rocks of the Malaguide Complex (Gee!, 1973; Roep, 1980) and the internal Dorsale Rifaine (Wildi, 1983) shows erosion of crystalline rocks at this time. This indicates that the Malaguide Complex had ·a normal crustal thickness before subduction was initiated.

Individual nappes of the Betic tectono-metamorphic complexes have a thick­ness in the order of several kilometers and essentially consist of metasedimentary rocks of Triassic and/ or Paleozoic age (Egeler & Simon, 1969; De Jong & Bakker, 1991). Minimum shortening values of about 75% perpendicular to the transposed bedding in the Alpujarride and Mulhacen Complexes (fig. 5) point to pre-colli­sional thickness of the sedimentary sequences of around 4 times the present structural thickness. This implies that early Alpine nappe stacking in the Betic Zone was probably the result of sequential detachment of upper crusta! segments with thicknesses in excess of 10 km, which are added to the overriding plate, while the deeper part of the lithosphere continuous to subduct (fig. 6). The higher early Alpine metamorphic pressure experienced by the Mulhacen Complex, indicating about 10 km deeper burial than the Alpujarride Complex, can consequently be explained by underthrusting of the Mulhacen Complex below the crustal segment with the Alpujarride Complex (fig. 6b).

The P-T-t paths of the different tectono-metamorphic complexes display the thermal consequences of sequential stacking of cool upper crusta! segments. Thermal modelling of stacking of crustal scale segments showed that cooling or reduced heating of a plate may result from underthrusting by cooler crust (Eng­land & Thompson, 1984; Davy & Gillet, 1986; Van den Beukel, 1990; Van Wees et al. , 1992). Taking into account the results of such modelling studies, cooling of the Alpujarride Complex can be explained by underthrusting by a relatively cool crusta! segment, containing the Mulhacen Complex (fig. 6b). Similarly, cessation of isobaric heating of the Mulhacen Complex may be due to underthrusting by the cool crusta! slab with the Veleta Complex (fig. 6c). The interpretation of cooling of crusta! segments by means of underthrusting by other cool upper crusta! slabs constrains the amount of plate consumption. For this purpose the geometry of the upper surface of the descending slab has been adopted from Van den Beukel & Wortel (1988) (fig. 6). Pressures in the Alpujarride Complex imply 100 km of movement on such a subduction slab. Subsequent underthrusting of the Mulhacen Complex to about 37 km requires a movement of about 130 km.

309

w ~

0

North America Eurasia

Fig. 7. Plate reconstructions at a) the onset of W-ward subduction in the Betic Zone at about 115 M a (An MO) and b) at the end of subduction at about 85 M a (An 33). Sub­duction is initiated due to compression resulting from northward propagating North Atlantic oceanic spreading to the west ofiberia (a). Subduction of the Betic Zone below the leading edge of Iberia, where the Malaguide Complex (MAL) was located, resulted from 400 km ESE-ward movement of Iberia as part of Africa arising from oceanic spreading in the Atlantic Ocean and Bay of Biscay (b). The subduction zone (triangles) is continuous to the Alps and probably initiated in small oceanic basins (dark shading) with ftysch sedimentation (f) between the Malaguide Complex and the Alpujarride Complex (ALPU), forming the western margin of the subducting Betic Zone. The position of the Alpujarride, Mulhacen (MULH) and Veleta (VEL) Complexes according to the stacking model (fig. 6). Movement of spreading centres (double lines) and rift axes (single lines) indicated by arrows, on strike-slip and transform faults by harpoons. Position of Atlantic plate boundaries and continents after Malod & Mauffret (1990).

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311

Cooling of the Mulhacen Complex at a depth of 37 km as a result of its under­thrusting by a slab containing the Veleta Complex indicates another 130 km of crusta! shortening. Plate consumption by subduction of upper crusta! segments may thus have amounted to about 360 km.

Age estimates of cooling after D 2 mulh upto about 90 Ma indicate that subduc­tion has occurred earlier. The core of a gabbroic mass in the M ulhacen Complex, the margins of which are transformed into eclogite (Morten et al. , 1987; Bakker et al. , 1989), yielded a 146 ± 4 Ma Rb-Sr mineral isochron age (Hebeda et al., 1980), pointing to intrusion in the latest Jurassic. Consequently, subduction was prob­ably of Early Cretaceous age. Subduction in the Betic Zone is likely to have been caused by the ESE-ward movement of Iberia due to oceanic spreading in the Atlantic Ocean to the west of Iberia and in the Bay of Biscay (fig. 7). Oceanic spreading in these areas was in full swing between anomalies MO and 33 (about 119-80 Ma, using the time scale of Kent & Gradstein, 1986), during which Iberia was displaced about 400 km to the ESE as part of Africa (Srivastava et al. , 1990; Malod & Mauffret, 1990; cf. figs. 7a and b) . The amount of spreading shows that the envisaged 360 km of subducted lithospheric slab in the Be tic Zone is feasible .

Mafic rock suites including small bodies of olivine-bearing gabbro, locally as­sociated with pillow basalts (Puga et al. , 1989), are widespread in the Mulhacen Complex (De Jong & Bakker, 1991). The sample which yielded a latest Jurassic radiometric age has a 87Sr I 86Sr ratio of 0.702 (Hebeda et al., 1980), close to the primitive mantle composition, pointing to limited crusta! contamination of the magmas. The geochemistry of the mafic rocks suggest formation in a not evolved oceanic basin (Bodinier et al. , 1987), whereas incorporation of continental xeno­liths implies a continental setting (Gomez-Pugnaire & Munoz, 1990). This makes it likely that the mafic rock suite was formed in local oceanic pull-aparts, or leaky transform faults, in an array of strike-slip faults that transsected the entire con­tinental crust that formed the Late Jurrasic trans-tensional African-Eurasian plate boundary, which was continuous with the Ligurian Ocean (fig. 7a). The plate boundary was strongly weakened due to the presence of young and weak oceanic crust, which may have been loaded by flysch sedimentation. Such margins are the most suited for transformation into a subduction zone at the onset of compres­sion on account of their low strength and stressed state due to loading (Cloetingh et al. , 1982; Vlaar & Cloetingh, 1984). Eclogitization of the Late Jurassic mafic rocks indicates that subduction was initiated in the former Africa-Eurasia plate boundary (fig. 7a).

Cooling and exhumation of high pressure rocks

The P-T boxes of D,mulh and D2mulh overlap (De Jong, 1991), which may in­dicate that the later part of D 1 muih is coeval with initiation of decompression of the Mulhacen Complex. Hence, D 1 mulh is likely to be related to subduction of the slab with the Veleta Complex. The important decompression of the Mulhacen Complex during D 2muih implies that this deformation phase is approximately coeval with detachment of the Veleta Complex from the subduction slab. Simi-

312

larly, D 2 alpu can be explained by underthrusting of the Alpujarride Complex by the Mulhacen Complex; detachment of the latter and its addition to the over­riding plate may have resulted in D 2 alpu during decompression of the Alpujarride Complex. Thus, the main tectono-metamorphic D 2 structures in the Alpujarride and Mulhacen Complexes are related to exhumation of the high-pressure meta­morphic rocks. The Mulhacen Complex experienced a stronger decompression than the Alpujarride Complex pointing to differential exhumation and, hence, differential movements of the two nappe complexes. The upwards increasing in­tensity and rotational component of D 2mulh, discussed earlier, is probably due to this movement. Rotation sense ofsyn-D2mulh garnets demonstrate that the Mul­hacen Complex moved eastwards with respect to the overlying Alpujarride Complex during decompression. Such kinematic and P-T constraints indicate movements on a westward dipping shear zone (fig. 8). The important coaxial stretching component of D 2mulh and D 2alpu implies elongation of both nappe

complexes, pointing to an extension of the collision belt in an East-West direction (fig. 8). The dominant rotational deformation in the top of the Veleta Complex and in the mylonite zone at the contact with the overlying Mulhacen Complex also indicates a top-to-the-west movement. However, such a movement on a westward dipping plane, as implied by the sequential detachment model (fig. 6), would not place lower pressure metamorphic rocks on top of higher pressure rocks, but the reverse. This seems to indicate that the coaxial component of D 2mul\ which results in elongation of the Mulhacen Complex moving it east­wards updip, outweighed the westward movement in the mylonite zone and in the top of the Veleta Complex. An important implication of this kinematic analysis is that nappe emplacement was not due to a directed force ('push from behind') but stems from a body force, originating from the buoyancy of the subducted and detached upper crusta] segments, which provides a constant upward force (B in fig. 8). This force is resolved into shear parallel to the anisotropy provided by the detached crusta] segments and leads to extension of the upper plate where the

Fig. 8. Exhumation of HP/ LT metamorphic rocks of the Betic Zone due to extension of the overlying Malaguide Complex (MAL) and partial excision of its Kabylian-type crystalline basement (KAB). Extension results from resolution of buoyancy (B) of the detached underthrust upper crusta] se­quences into elongation of the metamorphic nappe complexes and top-to-the-west shear component along the westward dipping contacts between the hanging wall and the Alpujarride Complex and between the stacked nappe complexes during D2.

313

Malaguide Complex is located and to removal of part of its pre-Silurian basement (fig. 8). Widespread erosion in the Malaguide Complex during Aptian-Aibian and Cenomanian times and local pre-Albian tilting and submarine faulting and, in addition, rapid vertical motions of tens of metres during the Senonian (Roep, 1980) are explained by extension of the upper plate. Occurrence of Late Cretac­eous rocks, with fragments of Jurassic rocks on either Early Cretaceous or Jur­assic rocks (Durand-Delga & Foucault, 1968) in the Dorsale Betique north of the westernmost Sierra Nevada and lack of sedimentation of middle Cretaceous age (125-90 Ma) in this tectonic element in the western Betic Zone (Martin-Aigarra & Vera, 1982) can be explained similarly. In the Great Kabylian Massif, considered as the crystalline basement of the Malaguide-Dorsale depositional domain, a middle Cretaceous thermo-tectonic event is well documented by 40 Ar I 39 Ar dat­ing. Mica separates from rocks in vertical shear zones with ESE-WNW trending subhorizontal stretching lineations yielded plateau ages in the range of95-80 Ma (Monie et al. , 1984, 1988). The stretching direction agrees with that of D 1 and D 2

in the tectono-metamorphic complexes in the Betic zone, implying transfer of extension in the detached nappe complexes to the basement of the overlying plate.

Advanced cooling during Eocene overthrusting of the External Zone

The P-T-t paths of the Alpujarride and Mulhacen Complexes demonstrate in­creased cooling during advanced exhumation (fig. 4) , which is explained by thrusting of the Betic Zone over cooler crust. A number of features show that this crust is formed by the southernmost part of the External zone. Mafic rocks of the Almagride Complex, an inlier of the External Zone in the Betic Zone, contain an early mineral assemblage indicative of metamorphic pressures in the order of 0.35- 0.55 GPa at temperatures around 350- 400oC (Puga & Torres-Roldan, 1989). The Antequera-Osuna nappe in the western Betic Zone has Triassic rocks which partly resemble those of the Almagride Complex (Simon, 1987); its mafic rocks record pressures of 0.3 GPa (Puga et al. , 1988). The southern part of the Subbetic, which has been overthrust by this nappe (Cruz-Sanjulian, 1976; Pineda Velasco, 1987), contains a mineral assemblage pointing to pressures of about 0.3 GPa (Puga et al. , 1988). The metamorphic data, hence, indicate burial of these rocks to depths of 10- 20 km, which are envisaged by their overthrusting by the Betic Zone (fig. 9) , resulting in cooling of the hanging wall. A thrust load of 10- 20 km thickness can result in the flexure in the External Zone around 50 Ma that emerged from tectonic subsidence analysis of the more northern parts of the Ex­ternal Zone (Peper & Cloetingh, 1992) where pelagic sedimentation persisted into the Middle Miocene (Geel, 1973; Hermes, 1978; De Smet, 1984). Timing of over­thrusting in the Eocene agrees with coeval vertical movements implied by strati­graphical and sedimentological analysis (Kenter et al. , 1990; De Ruig et al. , 1991; De Ruig, 1992), by non-calcareous influx into carbonates of Ypresian and Lu­tetian age in both the Subbetic and the Malaguide Complex (Geel, 1973) and furthermore with the presence of thrusts in the Malaguide Complex which are sealed by Oligocene conglomerates (Lonergan, 1991). Overthrusting in the

314

s

Subbetic Prebetic

. ' .. " .. . .. . . . . . . . . . " . . . . . ' . . . ' . . . . . . . . . . . . . . . . . . . . . . . . ' . ..... . " . . . . . . . . . . . . . . . . . . . . . .

N

Fig. 9. Early to Middle Eocene overthrusting of the southernmost External Zone by the partially structured Betic Zone. Burial and loading resulted in HP / LT metamorphism in the overthrust part (Almagride Complex) and upward and downward flexure of the Subbetic and Prebetic, respectively (arrows), where sedimentation continued.

southern Betic Cordilleras has a similar timing as the climax of crus tal shortening in the Pyrenees (De Jong, 1990; De Ruig, 1992). Both may therefore be the result of initiation of oceanic spreading in the Norwegian-Greenland Sea around 55 Ma, causing an additional NW-SE compressional component in the African­Eurasian collision (Srivastava et al., 1990). Taking a southward dip of the thrust plane of 15° the minimum amount of thrusting of the Betic Zone over the Ex­ternal Zone is around 40 km (fig. 9) , showing part of this plate convergence is re­solved in the Betic Cordilleras.

The main deformation phases in the Almagride Complex (D 1alrn and D 2a1rn)

occurred after the HP I LT metamorphism in this complex . D 2 aim folds show si­milar features as D 3 folds in the Mulhacen and Veleta Complexes (De Jong, 1991) that are related to a late stage extensional phase, as will be discussed below. This indicates that overthrusting of the southernmost part of the External Zone took place before this extensional phase.

Late Oligocene to Early Miocene extensional tectonics

Southvergent D 2 folding in the Almagride Complex and D 3 folding in the Ve­leta and Mulhacen Complexes have no equivalent in the overlying Alpujarride Complex, indicating a translation of the latter complex with respect to the three former complexes. The vergence of these folds implies a southward movement of the Alpujarride Complex (fig. 10) . Progressive southward thinning has led to complete excision of the Mulhacen Complex and to the development of a rider in the Sierra Alhamilla (figs. 10, 11), indicating large scale ductile normal faulting. Southvergent D 3 mulh and D 3 vel folding is probably due to back-rotation of the swell domain between two principal normal faults separating the Mulhacen and Veleta Complexes from the underlying Almagride Complex and the overlying Alpujarride Complex (fig. 10). Ductile accommodation of the shape of this back­rotated domain is probably the reason for development of D 3mulh and D 3 vel folds during extension. Back-rotation of a footwall is a mechanical feature resulting

315

+ ~---ALPU MULH ~Di V Fu V a

s

b c

Sierra Alhamllla Southern Sierra de Ios Fllabres Northern Slerl'll de Ios Filabres

4

Almagride Complex

Fig. 10. Southvergent 0 3 folding resulting from form adaption of the M ulhacen and Veleta Complexes between two major curved extensional faults F Land F u due to ex tensional unloading of the hanging wall (Alpujarride Complex). Extension has resulted in progressive southward thinning of the Mulha­cen Complex and excision of its lithologic units; (I =higher Mulhacen un its, 2 =La Yedra Marbles and Schists, 3 = Tahal Schists, 4 = Velefique Schists) extreme extension produced a completely isolated slice of the Mulhacen Complex (Sierra Alhamilla). Extension gave ri se to back-rotation of S2 in the Alpu­jarride Complex (insert b); during subsequent inversion of the extensional structure, S2 was located in the compressional sector of the flow field (shaded areas, insert c) giving rise to north vergent D3alpu

folds.

from upward flexure of the lithosphere during extensional unloading (Lister et al. , 1986).

Reheating in the Mulhacen Complex can be explained by excision of the lower sections of the crust, giving rise to mantle upwarping (fig. 11), emplacing a tran­sient heat source at crusta! levels. Maximum temperatures during extension-re­lated reheating were reached after D 3mulh, demonstrating that reheating at a particular crusta! level occurred after extension itself, which is shown by 2D modelling of the part of the P-T-t path pertinent to extension (Van Wees et al., 1992). Stronger reheating in the rider, which characterises the southernmost Mulhacen Complex agrees with a more advanced extension in this part of the complex.

In the Alpujarride Complex reheating-induced staurolite growth over un­deformed S2 (fig. 3c) shows that extensional tectonics did not result in folding of the main schistosity as occurred in the underlying complexes. This may be ex-

316

N

s r- r-

p A~' p -j ! •

Solana Fm

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Fig. 11. Cartoon of the Betic Zone during Late Oligocene to Early Miocene ex tensional tectonism. The Mulhacen and Veleta Complexes are pinched out southwards towards an extensional mantle uplift. The main ex tensional fault dips southwards resulting in reheating at progressively higher pressure in the Alpujarride units (insert P-T paths; P-T conditions at Ronda (square) , after Westerhof, 1977), schematically from north to south: 5) Almanzora Unit, 4) Variegato-Partaloa Unit, 3) Oria Unit, 2) Adra Unit, I) Almijara group of units. The Malaguide Complex is the site of coarse clastic sedi­mentation, viz. the early deposits of the Solana Formation, which occurs in an ex tensional basin be­tween the Internal and External Zones.

plained by the location of S2 in the Alpujarride Complex in the extensional sector of the flow field, in agreement with the position of this complex in the hanging wall of a low-angle extensional system (figs. 10, 11). During extension the thermal gradient in a crusta! segment is elevated, which is likely to be the reason that a number of Alpujarride tectonic units have a downward increasing metamorphic grade (Aldaya et al., 1979; Torres-Roldan, 1979; Akkerman et al. , 1980). Differ­ences in P-T conditions during reheating in different Alpujarride tectonic units is due to a southward down cutting of the upper main ex tensional fault below the Alpujarride Complex into the mantle (fig. 11). The Almijara group of units (fig. 11) experienced high-grade metamorphism and local anatexis in association with emplacement of ultramafic rocks in the western Betic Zone at high pressures (Westerhof, 1977; Tubia & Ibarguchi, 1991). The less dramatic P-Tevolution of the other Alpujarride tectonic units is due to their location above less thinned crust formed by the Almagride, Veleta and Mulhacen Complexes (fig. 11). In the Al­manzora Unit- the lowermost tectonic unit of the Alpujarride Complex (fig. 11)­reheating produced only biotite, whereas in the Oria and Partaloa units (Sierra de

317

N

footwall formed by the Mulhacen, Veleta and Almagride Complexes (fig. 12). Deformation structures formed during this event show that the stronger reheated Alpujarride Complex reacted differently from the Mulhacen Complex. Rocks of the Alpujarride Complex were strongly affected by northvergent D 3 aipu folds, in­dicating that S2 was located in the compressional sector of the flow field (fig. 10, insert c). D 3a lpu folds have the same characteristics as the most conspicuous folds in other parts of the Betic Zone, viz. Ds folds of Platt et al. (1983) in the Sierra Alhamilla and D 3 folds of Cuevas (1991) south of the Sierra Nevada. In contrast, the Mulhacen Complex was less severely affected. The large scale D 4rnulh struc­ture is formed by open antiforms with sizes of a half to several km's, probably marking crusta! shortening during an early stage of inversion. During advanced inversion the Mulhacen Complex was overthrust by the Alpujarride Complex and deformation was mainly concentrated in the D 5rnulh mylonite zone in the top of the complex at the contact with the Alpujarride Complex and in a number of shear zones at deeper level. 2D thermal modelling using a depth dependant rheology, showed that inversion and concentration of deformation in the former extensional structure is the result of a pronounced drop in strength of the lower part of the upper crust and the lower crust as a result of extension-related re­heating (Van Wees et al., 1992).

s N

Fig. 12. Inversion of the extensional structure around 21 Ma resulting in substantial cooling of the Alpujarride and Mulhacen Complexes (insert P-T paths). The regional structure of the Alpujarride Complex is characterised by thrusting of progressively higher metamorphic units over low grade units. The Almagride Complex and the Antequera-Osuna nappe (A-0) represent overthrust of metamorphic equivalents of the Subbetic. The extensional flysch basin between the Internal and External Zones is closed; the Early Miocene Espejos formation is deposited after northward thrusting.

319

The regional structure of the Alpujarride Complex is the result of inversion of the extensional structure during which part of the extensional mantle uplift in the western Betic Zone was decapitated (De Jong, 1992). Geophysical data imply that the large peridotite massif of Ronda does not root in the mantle but re­presents a thrust sheet (Barranco et al. , 1990; Torne et al. , 1992). In the stack of Alpujarride nappes higher grade metamorphic units generally occur above lower grade units (Aldaya et al. , 1979), pointing to thrusting of stronger reheated rocks over less reheated rocks (fig. 12). Inversion resulted in thrusting of slices of ultra­mafic rocks of several km's thickness over high-grade metasediments of the Al­mijara group along a mylonite zone (Westerhof; 1977; Tubia & Cuevas, 1986). This provoked local anatexis of the high-grade metapelites resulting in formation ofleucogranites (Westerhof, 1977) with Early Miocene radiometric ages (Priem et al. , 1979).

Overthrusting of hotter Alpujarride rocks has locally caused heating of the underlying Mulhacen and Veleta Complexes in the southernmost part of the overthrust zone. This is inferred from the data of Van den Eeckhout & Konert (1983) , that show an upward increasing An% of syn-overthrusting plagioclase towards the overlying Alpujarride Complex. The heat dissipation from the Al­pujarride rocks into the footwall during thrusting over cooler rocks (De Jong, 1992; Van Wees et al., 1992; fig. 12) and extension and thinning of the Alpu­jarride overthrust mass (Platt et al., 1983; Cuevas & Tubia, 1990, De Jong, 1991) during thrusting, bringing hot rocks closer to the surface, both led to its extremely rapid cooling. Based on a combination of strain rate and finite strain estimates, Behrmann (1984) concluded that mylonitization in the top of the Mulhacen Complex, D 5mulh of the present paper, lasted about 4 Ma. D 5mulh mylonization progressed under falling temperatures pointing to cooling of the overthrust sys­tem (De Jong, 1992). Clustering of cooling ages in the Alpujarride Complex around 19 Ma, as discussed before, hence document the swiftness of the cooling and the end of ductile overthrusting.

Ages of the oldest post-nappe sediments deposited in the Betic Zone itself and at the border with the External Zone are in agreement with rapid cooling and document the end of ductile thrusting. The oldest post-nappe deposits bordering the Sierra de Ios Filabres, the Alamo formation, which contains metamorphic detritus of the Alpujarride Complex (Volk & Rondeel, 1967), has a N5 age (Gee! et al. , 1992), placing an upper age limit to these rocks of about 19 Ma, using the Hag et al. (1989) time scale. The rocks are consequently at least Early Burdigalian in age. The equivalent of the Alamo formation in the contact zone between the Internal and External Zones, the Espejos formation, which is much less disturbed than the underlying Solana formation (Gee!, 1973), has a similar age (Gee! et al., 1992). The occurrence of pebbles with D 3alpu folds in the Espejos formation north of the Sierra Espufia (Meyboom, 1985; De Jong, 1991) points to a completion of overthrusting of the Alpujarride Complex and deep erosion of the nappe pile during the Early Burdigalian. Sealing of thrust planes between high-grade meta­morphic Alpujarride units (including ultramafic rocks) and the Malaguide Com­plex by the Vifiuela formation (Torres-Roldim et al., 1986; Zeck et al. , 1992b), an

320

equivalent of the Espejos formation in the western Betic Zone (Gee! et al. , 1992), also indicates that thrusting in the Internal Zone ended before 19 Ma. This timing indicates that the period of extension, followed by inversion took place between 25 and 19 M a, thus within a very short period of only 6 Ma. Early Miocene folding and thrusting also affected the External Zone (Martln-Algarra et al., 1988; Gee! et al. , 1992; Beets & De Ruig, 1992), after this zone was transformed into a foreland fold and thrust belt in late Aquitanian - Early Burdigalian time (De Ruig, 1992). Beets & De Ruig (1992) relate this tectonic development to loading of the Iberian lithosphere by a collison wedge formed by the nappes of the Betic Zone.

Renewed Middle Miocene extension

Mica ages in the Mulhacen Complex are generally very young, down to about 12.5 Ma (Priem et al., 1966; Andriessen et al., 1991 ; Monie et al., 1991; De Jong, 1991 ; De Jong et al., 1992). Modelling of 40 Ar / 39 Ar phengite age spectra pointed out that this may be due to repeated thermal resetting, which has a similar timing as the main episode ofvolcanism in the basins bordering the metamorphic ranges in the eastern Betic Zone (De Jong, 1991; De Jong et al., 1992). Epigenetic ore deposits and hydrothermal alteration in rocks are related to adjacent Late Mio­cene volcanism (Oen et al. , 1975). Such fluids associoated with volcanism might well be responsible for the isotope resetting in the Mulhacen Complex. Occur­rence of Middle and Late Miocene magmatism (Bellon et al. , 1983; Di Battistini et al. , 1987; De Larouziere et al., 1988; Serrano, 1992) points to renewal of exten­sion as is clearly shown by locally occurring calc-alkaline dykes in the Alpujarride and Malaguide Complexes. The dykes are not folded and at least a part intruded after cooling in the Alpujarride Complex was completed (Torres-Roldtm et al., 1986), indicating that intrusion took place after D 3alpu and D/1

pu thrusting pha­ses in the Alpujarride Complex. A relation between extension and magmatism clearly appears from the concentration of Miocene volcanic rocks in the thinnest crust of the western Mediterranean area (fig. 13). Interpretation of reflection profiles and borehole data from the Alboran Basin, south of the Betic ranges, demonstrated an important Middle to Late Miocene extension (Comas et al., 1990). A second phase of extension superimposed on the Late Oligocene to Early Miocene event equally emerged from modelling of the gravity field of the Betic Cordilleras (Van der Beek & Cloetingh, 1992).

Oligocene and Miocene plate convergence and slab roll-back

The rapid shifting of extension to compression and renewal of extension during Oligocene and Miocene times occurred during continuing convergence of the African and Eurasian plates, implied by plate kinematic data (Srivastava et al., 1990). The first phase of extension during the Late Oligocene to Early Miocene is thus probably due to slab roll-back. Detachment of a (rolled-back) slab results in substantial decrease of the slab-pull force, leading to a diminishing of the flexural bulge of the slab, which gives rise to a better coupling of the shallow remainder of the slab (Spakman, 1990). Such a relatively rapid process might underlie the ob-

321

Fig. 13. Map of Bouguer anomalies (in m gal) in the western most Mediterranean area (after Van den Bosch , 1974). The Betic-Rif arc is underlain by an arcuate pattern of negative anomalies, correspond­ing to the thickest crust (diamonds, after Banda & Ansorge, 1980 and Barranco et al. , 1990), which diminishes progressively towards the Alboran Basin. Miocene and younger strike-slip faults and vol­canism (dots, after: De Larouziere et al., 1988) are concentrated in the thinnest crust.

served fast and dramatic inversion of the Late Oligocene to Early Miocene ex­tensional structure in the Betic Zone. Recent seismic tomographic studies point to the existence of a detached slab below the Betic Cordilleras (Wortel & Spakman, 1992), which is supported by earthquakes occurring as deep as 600 km (Grimison & Chen, 1986). Reprise of extension during the Middle and Late Miocene may thus result from renewed roll-back, steepening and to sinking of the subduction slab beneath the Betic Cordilleras (Platt & Vissers, 1989; De Jong, 1991). Miocene and younger magmatism has been explained by partial melting of subducted li­thosphere (Araiia & Vegas, 1974; De Roever, 1975; Torres-Roldtm et al., 1986). Concentration of magmatism into a NNE-SSW trending narrow zone (fig. 13), in which a clear chronological and chemical zonation is absent (Bellon et al., 1983; Di Battistini et al., 1987; De Larouziere et al., 1988), agrees with melting of a steep, detached slab. The chemistry of Late Miocene lamproites of the eastern Betic Cordilleras points to derivation from the mantle at a maximum depth of 100 km (Venturelli et al., 1988). The isotopic composition of these rocks indicate

322

mixing of the mantle with a component which has the characteristics of con­tinental crust or sediments derived from such a crust (Nelson et al., 1986). Sub­duction of continental material to such depths is likely to be due to slab detach­ment because plate convergence during this time falls short. Focal mechanisms of earthquakes in the most western Mediterranean area agree with the presence of a detached slab as they point to decoupling of tectonics at mantle and crusta! level (Grimison & Chen, 1986). Decoupling is implied by E-W compressive stresses of earthquakes deeper than 100 km, whereas intermediate quakes demonstrate NNW-SSE compressive stresses (Grimison & Chen, 1986). The latter direction agrees with the NW-SE to NNW-SSE compression in the Tortonian to Recent stress system in the eastern Betic Cordilleras (Montenat et al., 1987; De Ruig, 1990, 1992; Buforn & Udias, 1991), which is related to the final stages of the Africa-Eurasia collision (Letouzey, 1986; Bergerat, 1987). The complex Late Oli­gocene to Recent tectonic evolution of the Betic Cordilleras thus probably arose from interference of crusta! shortening related to Africa-Eurasia collision and extension due to mantle tectonics stemming from roll-back, steepening and de­tachment of a subducted slab below the collision zone. This dualism is probably best demonstrated by the persistence of a compressive stress regime in the Lan­ghian to Serravallian (ea. 16-10 Ma), which gave rise to folding and strike-slip deformation in the External Zone (De Ruig et al., 1987; De Ruig, 1990, 1992; Gee] et al., 1992) and in the Betic Zone (Montenat et al., 1987; Bon et al., 1989), coeval with outpouring volcanism (Serrano, 1992) from the anomalous mantle below the collision zone. The interference of crusta! shortening due to collision and exten­sion due to slab roll back can explain rapid changes of extension and crusta! shortening, which are well documented in the External Zone (De Ruig, 1992), when one force became predominant over the other.

Paleogeographic implications

The proposed tectonic model , a prolonged tectonic history of phases of inter­mittent crusta! shortening and extension from the Early Cretaceous on, differs strongly from all current tectonic scenarios in which the tectonism in the Betic Zone is related to collision of Iberia and Africa with the Alboran micoplate, as outlined in the introduction of this article. In a large number of these models thrusting essentially occurred from South to North (Egeler & Simon, 1969; Wildi , 1983; Make!, 1985; Gee! et al., 1992). Hence, the uppermost nappe complex, the Malaguide Complex, had the most southern, i.e. 'African' provenance, whereas the underlying Alpujarride Complex is also of southern derivation, but is gen­erally thought to have been originally located to the north of the Mala guide Do­main . The present article has, however, clearly demonstrated that these north­ward directed nappe movements occurred as the last major thrusting phase in the tectonic evolution of the Betic Zone during the Early Miocene. Hence, they can not be used for paleogeographic reconstructions. In the foregoing it was argued that the stack of nappe complexes in the Betic Zone essentially stems from the earliest deformation in Cretaceous times, although the stack was strongly mod-

323

ified during Eocene thrusting and considerably thinned during the latest Oligo­cene to earlymost Miocene extension. This shows that paleogeographic re­constructions should be made applying the kinematic constraints offered by the earliest, that is Cretaceous, deformation. It was argued that underthrusting of the metamorphic nappe complexes occurred westwards below the Malaguide Domain. Sequential underthrusting of the Alpujarride Complex, followed by the Mulhacen Complex and the Veleta Complex implies that originally the Alpujarride Complex was situated to the east of the Mala guide depositional do­main and to the west of the Mulhacen Domain (fig. 7a). Therefore, the paleogeo­graphic reconstruction on the basis of the early Alpine kinematics resulted in a somewhat uncommon reconstruction in the sense that it placed the Malaguide Complex to the Eurasian side of the future collision belts and not to the African side as in the more classic reconstructions (Wildi, 1983; Make!, 1985; Geel et al., 1992). Therefore, it should be tested on the basis of comparison of lithologic se­quences in the area. Testing of the proposed paleogeographic reconstruction is, however, hampered by a number of uncertainties. The only Mesozoic sediments present in all nappe complexes are Triassic rocks; post-Triassic deposits are nearly exclusively developed in the Malaguide Complex. However, direct com­parison of Triassic and older rocks of the different nappe complexes on the one hand and the External Zone on the other is precluded by the presence of a strike­slip fault zone between Eurasia and Africa at the latitude of the Betic Cordilleras in Jurassic times, with a total offset of about 750 km (Srivastava et al., 1990). Re­constructions of the paleogeography based on a comparison of the Paleozoic se­quences of the nappe complexes compared to the different zones of the Hesperian Massif in Iberia (Make!, 1988) are even more uncertain (De Jong, 1991) as also during the Late Variscan history the area between Africa and Iberia was the site of important wrench faulting (Ziegler, 1989; Matte, 1991). Consequently, the proposed paleogeography in the present study can be tested to a certain degree only, using the Triassic sequences present in the area. The Triassic sequences of the Alpujarride and Mulhacen Complexes show a number of similarities not shared by the Malaguide Complexes and those of the External Zone, viz. thick sequences (taking into account the considerable Alpine flattening, fig . 5), which are rich in carbonates. On the other hand the sedimentary sequences of the Malaguide Complex are relatively thin, rich in coarse grained clastic rocks and relatively poor in carbonates. The Triassic sequences of the Alpujarride and the Mulhacen Complexes have an 'Alpine' facies development, whereas the Triassic of the Malaguide Complex is developed in a 'Germanic' facies like the External Zone. However, a detailed comparison of stratigraphic columns, which show a different stratigraphic development between the different Malaguide tectonic units, also reveals important differences with the Subbetic (Makel, 1985). Such differences are not surprising given the deposition of the Triassic rocks in a tee­tonically active environment (De Jong & Bakker, 1991; De Jong, 1991). A south­ern sediment supply in the Malaguide Complex (Make!, 1985), thus towards the Hesperian Massif, implies that the Malaguide depositional domain was not in direct contact with the External Zone. This is compatible with deposition on

324

opposite sides of a rift structure or the development of a Malaguide ribbon con­tinent adjacent to the External Zone (De Jong, 1991). The post-Triassic strati­graphy of the Subbetic has a number of characteristics common to that of the Malaguide Complex (MacGillavry, 1964; Pineda Velasco, 1985), although im­portant differences, which become increasingly more pronounced from the Early Jurassic on, exist (MacGillavry, 1964; Hermes, 1978; Make!, 1985). Such differ­ences are, however, to be expected in a tectonically active environment. Similarly, the marked differences in stratigraphic development of the External Zones of the Rif and the Betic Cordilleras (Bouillin et al., 1986) can also be accounted for by the existence of a strike-slip fault between Iberia and Africa (fig. 7; De Jong, 1990). Consequently, the proposed paleogeography cannot be definitively approved by comparison of lithologies. At the same time this shows that tectonic reconstruction of the western Mediterranean based on comparison of strati­graphy (Wildi, 1983; Make!, 1985, 1988) are faced with fundamental difficulties.

Conclusions

Study of the relationship between polyphase deformation and mineral growth during plurifacial metamorphism in the Alpujarride and Mulhacen Complexes has resulted in well constrained P-T paths, which reflect the essential features of the Alpine tectonic evolution of the Betic Zone, summarised in Table I. Early Al­pine HP metamorphism points so subduction below the Kabylian-type lower crust of the Malaguide Complex: the Alpujarride Complex was subducted first, followed by the Mulhacen Complex, which was in turn underthrust by the Veleta Complex. Nappe stacking thus occurred by sequential underthrusting and de­tachment of upper crusta! segments. Subsequent decompression shows exhuma­tion of the HP metamorphic rocks, during which ductile flattening and exten­sional tectonism were dominant. Radiometric dating suggests initiation of cooling after HP metamorphism in the Mulhacen Complex at about 90 Ma, im­plying an Early Cretaceous age for subduction. Trusting of the Internal ( = Betic) Zone over the relatively cool External Zone during the Eocene, at about 50 Ma resulted in advanced cooling of the former. Overthrusting caused HP I LT meta­morphism in the overthrust part of the External Zone (Almagride Complex) and resulted in flexural bulging of the not overthrust northern part of the passive margin of SE Iberia. Important late stage reheating is tentatively dated at about 25 Ma and resulted from upwarping of hot mantle material during extension. Subsequent compression resulted in crusta! shortening concentrated into the strongly weakened Late Oligocene to Early Miocene extensional structure. Thrusting of heated rocks over cooler crust, concomitant with thinning of the overthrust mass during inversion of this structure induced rapid cooling. Radio­metric and paleontologic data show that ductile thrusting was completed at about 19 Ma. Resetting of isotope systems in the Mulhacen Complex, important vol­canism and dyke intrusion point to renewed extension after this time, which in­terfered with episodes of crusta! shortening.

The tectono-metamorphic evolution of the Betic Zone reflects the dynamics of

325

w N 0\ Table I. Summa ry of the tectono-metamorphic evolution of the Betic Zone. The length of the boxes schematically shows the relative duration of tectono-meta­

morphic phases

Malaguide Complex

Alpujarride Complex 11 ~,;jpu 1 D 2 ·~

Mulhacen Complex

Veleta Complex

Almagride Complex

Events

Timing

Radiometric ages

I DJ mulh I D2

mulh . J

ID vel D vel I

1 _ _L~

Subduction

Westward sequential stacking

of metamorphic nappe complexes

ID~ ~!_j

Northward overthrusting

Betic Zone over southernmost

External Zone

Cooling and decompression

Aptian - Campanian (115-SO Ma) I E. - M. Eocene (55-50 Ma)

81 Ma

ID3mulh l

ID3vel I

lD/'m]

Extension

ID1mal -Jl)~

I D/PU ID/'PU I

ID4mulhj D

5mulh ID

6mulh I

ID/e' I

I D3alm --[D/'m J

Compression

Overthrusting

Ductile Brittle

Reheating Cooling and decompression

L. Oligocene- E. Miocene (30-17 Ma)

31 Ma 25 Ma 21 Ma

Sealing of thrust sheets

by post-nappe deposits

17 Ma

shortening and extension of the three main stages in the Africa-Eurasia collision in the western Mediterranean: 1) ESE-ward movement of Iberia between 119 and 80 Ma, due to spreading in the Atlantic Ocean to the west oflberia, resulted in westward subduction of the Betic Zone below the leading edge oflberia, where the Malaguide Domain was located. 2) Overthrusting of the most southern part of the External Zone by the partially structured Betic Zone (around 50 Ma) is coeval with collision in the Pyrenees which results from an additional NW-SE compressional component into the Africa-Eurasia collision due to oceanic spreading in the Norwegian-Greenland Sea. 3) Late Oligocene to Early Miocene crustal and subcrustal extension and sub­sequent inversion of the extensional structure, which was completed at about 19 Ma, occurred during continuing Africa-Eurasia convergence. This points to roll-back, steepening and detachment of the subduction slab. Detachment of the deeper part of the slab caused a better coupling of its shallow remainder with the overlying plate enabling transfer of compression due to plate convergence. Deep sources of Middle Miocene and younger volcanism and earthquakes agree with the presence of a detached slab as plate convergence during the Miocene is not sufficient for steady state subduction to such depths.

ACKNOWLEDGEMENTS

Prof.Dr. W.P. De Roever and Dr. O.J. Simon are thanked for their thorough reviews of the typescript and many valuable suggestions that improved the text considerably.

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