Late exhumation stages of the Alpujarride Complex (western Betic Cordilleras, Spain): new thermochronological and structural data on Los Reales and Ojen nappes

TECTONOPHYSICS ELSEVIER

Tectonophysics 285 (1998) 253-273

Late exhumation stages of the Alpujarride Complex (western Betic Cordilleras, Spain): new thermochronological and structural data on Los Reales and Ojen nappes Marc Sosson a,*, Anne-Claire Morillon a, Jacques Bourgois b, Gilbert Frraud c, Grrard Poupeau d, Pierre Saint-Marc a " Giosciences Azur,, UMR 6526, CNRS-UNSA, 250 rue Albert Einstein, Sophia-Antopolis, 06560 Valbonne, France h Laboratoire de G(ocl~'namique, Tectonique et Environnement, CNRS-UPMC, Bo~te 119, 4 Place Jussieu, 75252 Paris cedex 05, France Gdosciences Azur, UMR 6526, CNRS-UNSA, Pan: Valrose, 96 Av. de Valrose, 06100 Nice, France ~lLaboratoire de G~odynamique des Cha~nes Alpines, UPRES-A 5025, CNRS, Universit( Joseph Fourier, 15 rue Maurice Gignoux, 38031 Grenoble, France

Received 1 October 1995; accepted 15 June 1996

Abstract New thermochronological and structural studies were conducted to quantify the cooling and late exhumation histories of the Internal Zone of the western Betic Cordilleras. The study was carried out in the Ojen-Marbella region where the Ojen and Los Reales nappes of the Alpujarride Complex outcrop. 4°Ar/39Ar single-grain analyses of muscovite and biotite display plateau ages of 19 Ma, on the same rocks. Apatite fission-track datings give ages of 18-16 Ma for the last step of cooling below 110°C, and confined track measurements indicate a very rapid cooling between 110° and <60°C. The combination of both data suggests a fast cooling phase at 19-16 Ma with a gradient higher than 100°C/Ma between 500 ± 50°C and <60°(;. This fast cooling was associated with extensional tectonics accommodating thinning of the metamorphic alpine-type pile of nappes. The exhumation rate ranges from 1 to 3 km/Ma during the 19-18 Ma interval and is close to 1 km/Ma in the 18-16 Ma interval. Moreover an uplift of the Ojen and Los Reales units began after the Early Pliocene (5 Ma). The sediments sealing the strike-slip contact forming the boundary between the Alpujarride Complex and the Neomumidian Formation of Early Miocene age contain Early Pliocene planktonic foraminiferal assemblages (Zone N18). Benthic foraminiferal assemblages of these sediments, actually outcropping at 150 m above sea level, indicate a deposition in the upper bathyal zone ranging from 200 to 600 m water depth. Our conclusions confirm and precise the exhumation processes during the 19 to 16 Ma interval, and uplift of the Internal Zones since 5 Ma. © 1998 Elsevier Science B.V. All rights reserved. Keywords: exhumation; thermochronology; Betic Cordillera; Ar/Ar; fission track

* Corresponding author. Tel.: +33-4-9395-4255; Fax: +33-4-9365-2717: E-mail: [email protected] 0040-1951/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PIl S 0 0 4 0 - 1 9 5 1 ( 9 7 ) 0 0 2 7 4 - 6

254

M. Sosson et al./Tectonophysics 285 (1998) 253 273

1. Introduction

Delga, 1966; Julivert et al., 1974). The Internal Zone consists of an alpine-type pile of nappes (DurandDelga, 1966; Egeler and Simon, 1969a,b) comprising high-grade metamorphic rocks and peridotites. The nappe accumulation includes from bottom to top (Durand-Delga, 1966; Torres-Rold~in, 1979): the Nevado-Filabride Complex which has undergone high-pressure-low-temperature metamorphism (De Roever and Niijhuis, 1964; Morten et al., 1987), the Alpujarride Complex consisting of high-grade metamorphic rocks (Goff6 et al., 1989; TuNa et al., 1992; Azafion et al., 1992), which include the Ronda peridotites (Kornprobst, 1976; Priem et al., 1979; Davies et al., 1993; TuNa, 1994) and the Malaguide Complex mainly composed of weakly to non-metamorphic rocks of Palaeozoic to Early Miocene age (Paquet, 1969; M~ikel, 1985; Lonergan, 1993). In the western Cordilleras these units are unconformably overlain by the Las Millanas Formation of Lower Miocene age (Bourgois et al., 1972a,b; Bourgois,

Late orogenic extension in mountain belts previously thickened during collision stages has been one of the main topics of earth sciences researches over the past decade. The alpine evolution of the Betic Cordilleras has been reinterpreted in the light of new concepts concerning the collapse stages of the belt using new methods of investigation. In an attempt to clarify the exhumation processes and the late orogenic evolution of this belt, thermochronological studies were recently performed in the Internal Zone of the Betic Cordilleras comprising high-grade metamorphic rocks (De Jong, 1991; Moni6 et al., 1991, 1994; Zeck et al., 1992; Johnson, 1993). Nevertheless, if extensional deformation was demonstrated in the eastern Internal Zone, the age and mechanism of the late orogenic tectonics is less well-documented westward. The Betic Cordilleras (Fig. 1) are separated into External and Internal Zones (Fallot, 1948; Durand-

)

~al U)

~',,~ Figure 11

10 Km j1

/

~J

Estepona

L_

Pliocene to Quaternary sediments External zone

'TU

|

Mediterranean Sea

Lower Miocene Alozaina-Las Mitlanas Fm

Neonumidian Fm and "Argiles & blocs"

Malaguide Complex

Rondai"des

Alpujarride Complex

Fig. 1. Simplified structural map showing the location of the studied area (modified from Bourgois, 1978). Location of Figs. 2 and I 1 is shown.

ll4. Sosson et al./Tectonophysics 285 (1998) 253-273

1978). Recently, Durand-Delga et al. (1993) have proposed that the age of the base of the Las Millanas Formation is Aquitanian (23.7-20 Ma). The Las Millanas Formation is covered by the Lower Miocene of the Neonumidian Formation (Bourgois et al., 1973) which exhibits olistolithes originating from the External Zone (Bourgois, 1973, 1978). Consequently, the emplacement of the nappes of Internal Zone occurred before the Late Oligocene. Nevertheless, the tectono-metamorphic history of the Internal Zone is complex as shown by the wide range of the proposed interpretations. Following Durand-Delga (1966), Bourgois (1977), TuNa and Cuevas (1986, 1987), Cuevas et al. (1986), Moni6 et al. (1991) and TuNa et al. (1992), a major alpine compressive phase occurred before 25-20 Ma. Platt (1986), Galindo-Zaldfvar et al. (1989), Platt and Vissers (1989) and Garcfa-Duefias et al. (1992) emphasized the importance of extensional tectonics in the building of the Betic Cordilleras. Doblas and Oyarzum (1989) considered that extension alone could be responsible for the emplacement of the Ronda peridotites at 18 Ma, a still unsatisfactory result not taking into account thrusting of the Ronda peridotites at 22-20 Ma (Loomis, 1975; Kornprobst, 1976; Priem et al., 1979; TuNa and Cuevas, 1986, 1987; TuNa et al., 1992; Tubfa, 1994) and their existence as detrital material in the Lower Miocene Las Millanas Formation (Bourgois et al., 1972a; Bourgois, 1978; Durand-Delga et al., 1993). Presently a number of works including structural and geochronological data (Garcfa-Duefias et al., 1988, 1992; Zeck et al., 1992; Johnson, 1993; Moni6 et al., 1994) suggest that the extensional tectonics began around 2322 Ma. Other works as those of De Jong et al. (1992) and Van Wees et al. (1992) document a major extensive phase from 26 to 23 Ma followed by contractional tectonics from 21 to 18 Ma. Based on geochronological studies, Moni6 et al. (1994) showed that the last step of crustal thinning is marked by a fast cooling of the Alpujarride Complex for the temperature domain 600-300°C, in the 20-19 Ma range. In order to clarify the late steps of the Internal Zone exhumation in the western Betic Cordilleras, we have focused our work on thermochronological and structural constraints. Our aims were to obtain a cooling path and the rate of exhumation for the lowest high-grade metamorphic rocks (Ojen and Los

255

Reales units) of the western Alpujarride Complex. We used two geochronological methods (4°Ar/39Ar and apatite fission-track analyses) partly on the same samples to extend the temperature range from 350 ° to less than 60°C. Moreover, a structural analysis on the ductile to brittle structures bounding the western Alpujarride Complex was made in order to establish the relation between the extensional deformations and the cooling evolution. In an attempt to constrain the tectonic evolution we used stratigraphic data on sedimentary rocks sealing the main late- to post-orogenic structures.

2. Geological setting A field study was carried out in the OjenMarbella area (Fig. 1) in the western Betic Cordilleras, southeast of the Ronda massif, where deep structures of the Alpujarride Complex (the Ojen and Los Reales nappes) outcrop. A section near Ojen exposes the Ojen nappe overthrusted by the Los Reales nappe which includes peridotite sheets (Fig. 2). The Ojen nappe consists of marbles and metapelites interlayered with anatectic gneisses and garnet micaschists. The rock exhibits a mylonitic fabric. A SE-dipping ultramylonitic zone is at the base of the Los Reales nappe. Boudins of eclogitic relics as well as leucogranite are elongated in foliation planes. The Los Reales nappe is characterized upward by peridotites, mostly serpentinized along shear planes and locally intruded by granodiorite dykes (TuNa and Cuevas, 1986, 1987; TuNa et al., 1992; Tubia, 1994). Metamorphic mineral assemblages show that the Ojen and Los Reales nappes have suffered a polyphased metamorphic alpine-type evolution (Tubfa and Gil-Ibarguchi, 1991; Tubfa, 1994). Three main tectono-metamorphic events were recognized. Crustal rocks of the Los Reales nappe have undergone a high-pressure (at least 11-12 kbar), high-temperature (at least 700-750°C) metamorphic grade followed by a decompression stage (5-7 kbar) associated with increasing temperature (800-900°C). The first stage is related to a crustal thickening culminating in the Late Oligocene (Moni6 et al., 1991). According to Moni6 et al. (1991, 1994) the second stage of metamorphism marks the beginning of the extensional deformation, with the development of the

256

M. Sosson et al./Tectonophysics 285 (1998) 253-273 W 5 ° 00'

SIERRA BERMEJA

..Q-

_

j

- =~

.

....=,.....

=.

' ~- 2 -t

J

'v cJ> Istan

•

I

_

7

i!!!!:i ,-,

Figure 3

20 ~q~,~" ) B'43

, 7..,,

.....

.................

~ -

" igure 9 ~@X~: L _ ~

~-lstanfaul

.]/

,

,,__

" :-!;:iiiil~::-Marbeiia

7:.i:. ..... =================5== N 3 5 ° 30'

F

5 km 4

MALAGU1DE COMPLEX

ALPUJARRIDE COMPLEX i

[ LOS REALES NAPPE

Pliocene to Quaternary sediments strike slip fault thrust reactivated as detachment fault thrust ,~, -

schists and gneisses

OJEN NAPPE metapelites [ ~--1_2 and gneisses

i~: ~ =

!

(black, peridotite slices)

normal fault (arrows indicate the sliding strike)

mylonites

I< : ? ( , , ,

I

marbles

,,

Fig. 2. Structural sketch map of the Ojen area (see Fig. 1). Location of Figs. 3 and 9 is shown. main foliation at depth. Following TuNa and Cuevas (1986) and Tubfa (1994) an ENE direction of motion is indicated by kinematic markers of ductile deformation which was coeval with the development of low-pressure-high-temperature mineral assemblages. Within the Los Reales nappe, the late stage of the tectonic evolution is suggested by ductile to brittle shear zones (TuNa, 1994) associated with cooling. Considering the Ojen nappe, the decompression stage is marked by a decrease of the thermal gradient from 800 ° to 600°C (TuNa and Gil-Ibarguchi, 1991 ).

3. Geochronological results Apatite fission-track and 4°Ar/>Ar analyses were performed on Ojen and Los Reales nappe samples (Figs. 2 and 3). Both methods were used on the same samples (B7, B20, B43) and two different minerals were analysed using the 4°Ar/39Ar method. Sample B7 is a granite injected in the foliation plane of metapelites of the Ojen nappe along the eastern flank of the Sierra Blanca. Rock B20 sampled at Ojen pass (710 m) consists of migmatite included in an

M. Sosson et al./Tectonophysics 285 (1998) 253 273

257

WSW

ENE Sierra Bermeja

Fi9. 10 , I ,

2_ km[i

_ .~. ~i:~!!__.,~,~;~ i~

2km

~"~ ~'-'.

J

COMPLEX

ALPUJARR1DE COMPLEX

I:~i.::.~!-:!:!

? i -i

I

...." ~:' "

Ojen pass

Sierra Alpujata Los Reales nappe

~-

~

i ~ "Ojen ! ~ : nappe i - ~

~ ~P

"

/

MALAGUIDE

.~ .i

Sierra Blanca

Los Reales

~_~.~]

Nappe

schists and gneisses

Ojen Nappe

~"

'2,~

lj'

~'~'

peridotites (black : peridotite slices)

~ rnetapelites and gneisses

:

_2 f

~m

' B-Zl

¢B~20!

~B 4 3

mylonites

--~7 7 marbles

Fig. 3. SW-NE-trending cross-section through Ojen and Los Reales nappes (location in Fig. 2).

ultramylonitic zone at the bottom of the Los Reales nappe. Sample B43, collected a few kilometres east of Ojen, is a muscovite and biotite granodiorite exhibiting plagioclase phenocrysts, that intrudes the peridotites of the Los Reales nappe.

3.1. 4°Ar/39Ar data Single grains of biotite, muscovite and hornblende were separated using standard procedures of hand-crushing, magnetic separation and hand-picking under a binocular microscope. Single grains were chosen in 200-500 ttm or 250-500 # m separates. Samples wrapped in Al-foils were irradiated in the McMaster University Reactor in Canada (Hamilton) using the hornblende standard Hb3Gr as a monitor (age: 1072 Ma). The laser probe in Nice consists of a Coherent 70-4 continuous argon ion laser, in combination with a low-volume high-vacuum inlet system and a noble gas spectrometer (VG 3600). Details of this technique are described by Ruffet et al. (1995). Heating of the grain was achieved with a defocused beam of about twice the grain diameter, which ascertains a homogeneous energy distribution over the grain. Fusion was achieved by further focusing the beam. Gas purification during 3 min was followed by a static measurement of the Ar isotopes. Typical blank values were in the range from M/e 4° = 5.3×10 13 cm 3 STP to M/e 36 = 3.9x10 -14 cm 3 STP.

Ages of each individual step are corrected for irradiation-induced components from Ca and K in the sample. Ages, which were calculated using the decay constants of Steiger and J~ger (1977), are given with lc~ error estimates and do not include the errors of the ratio 4°Ar*/39ArK and age of the monitor. Plateau ages are calculated for at least three l c~ concordant steps corresponding to at least 70% of 3 9 A r degassed, the error bar includes the error of the 4°Ar*/39ArK ratio of the monitor. The B7 muscovite gives a plateau age of 19.0i0.2 Ma. However, the slight but regular increase in age vs. temperature (Table 1; Fig. 4) could be the result of an older formation age for this mineral, not totally reset by the thermal event leading to a final closure at 350 4- 50°C at 19 Ma, This seems a frequent occurrence with white micas of older formation reset by a younger thermal event as in the Sierra de Los Filabres (De Jong et al., 1992). Biotites of B20 and B43 samples belonging to two different units (Fig. 3) yield concordant plateau ages of 18.9 4- 0.5 Ma and 18.9 4- 0.4 Ma, respectively (Table 1; Fig. 4). Our results are in the same age range as those obtained by De Jong et al. (1992) and Moni6 et al. (1994) for the Internal Zone.

3.2. Apatite fission-track data Apatite grains of the same samples were included in an epoxy resin, polished and etched in a HNO3 molar solution for fossil tracks. We used the 'ex-

258

M. Sosson el al./Tectonophysics 285 (1998) 253-273

Table 1 4°Ar/39Ar step-heating data obtained with the laser probe Step

Atmosph. contain. (%)

B7, Muscovite single-grain, J = 0.02997126 52 mV 100 100 mV 56.70 150 mV 72.79 230 mV 64.97 232 mV 55.03 134 mV 21.88 260 mV 8.45 281 mV 4.24 300 mV 3.77 339 mV 2.73 380 mV 4.36 424 mV 8.41 570 mV 2.77 FUSE 6.95

39Ar (%)

37Arca/39ArK

4°Ar*/39ArK

Age 4- lcr (Ma)

0.02 0.04 0,13 1.59 3.35 5.35 12.79 21.01 11.96 9.77 21.16 4.68 4.78 3.37

0.000 0.000 0.003 0.029 0.006 0.007 0.004 0.004 0.006 0.003 0.003 0.007 0.006 0.007

-0.03 4.81 0.93 0.65 0.67 0.63 0.63 0.64 0.64 0.66 0.65 0.63 0.67 0.64

0.00 5- 0.00 137.51 i 49.83 27.47 ± 19.32 19.41 4- 2.38 19.74 ± 0.86 18.60 4- 0.49 l 8.68 5- 0.14 18.84 5- 0.11 18.85 ± 0.23 19.41 4- 0.41 19.23 ± 0.17 18.54 4- 0.68 19.75 5- 0.45 19.02 5- 0.81

Total age B20, Biotite single-grain, J = 0.02997126 30 mV 67.54 50 mV 17.47 80 mV 15.10 140 mV 0.00 300 mV 3.29 460 mV 2.90 FUSE 36.22

19.47 4- 0.53 8.25 21.15 15.31 11.92 36.69 2.49 4.20

0.158 0.024 0.083 0.043 0.029 I).084 0.160

0.55 0.61 0.59 0.74 0.67 0.67 0.45

Total age B43, Biotite single-grain, J = 0.02997126 53 mV 34.56 100 mV 10.96 170 mV 4.60 300 mV 11.76 460 mV 16.70 FUSE 28.98

16.22 4- 2.47 17.84 5- 1.04 17.45 5- 1.32 21.76 ± 2.23 19.82 4-0.53 19.84 5- 8.91 13.36 5- 5.73 18.86 5- .43

6.5 23.51 18.33 38.65 7.25 5.77

Total age

ternal detector' method. Induced tracks were thus r e c o r d e d o n a p l a s t i c foil ( K a p t o n ) d u r i n g i r r a d i a tion, p e r f o r m e d at t h e O r p h d e r e a c t o r o f the C e n t r e d ' l ~ t u d e s N u c l r a i r e s o f Saclay, F r a n c e . S a m p l e s B 4 3 a n d B 2 0 m o u n t s w e r e i r r a d i a t e d twice. T h e r e s u l t s are r e p o r t e d in T a b l e 2. F o r e a c h s a m p l e t w o to t h r e e o b s e r v e r s c o n t r i b u t e d to the m e a s u r e m e n t s . A n h o m o g e n e i t y test (X 2 : G a l b r a i t h , 1981) p e r f o r m e d s h o w e d t h a t all c r y s t a l s for a g i v e n s a m p l e h a v e c o n c o r d a n t ages. M o r e o v e r , the r e s u l t s o f all o b s e r v e r s are c o n c o r d a n t . T h e r e f o r e , for e a c h sample, a weighted average was calculated. The o b t a i n e d a p p a r e n t a g e s are as f o l l o w s : 17.3 4- 1.1 M a

0.162 0.047 0.048 0.034 0.132 0.184

0.76 0.66 0.65 0.62 0.59 0.50

22.54 4- 2.91 19.59 4- 0.70 19.48 4- 1.06 18.40 ~ 0.50 17.33 4- 1.54 14.93 4- 3.72 18.96 ± 1.07

for B 4 3 , 17.8 4- 1.1 M a for B20, a n d 15.6 ± 1.8 M a for B7. M e a n c o n f i n e d t r a c k l e n g t h s are as l o l l o w s : 14.53 4- 0.83 # m for B 2 0 , 15.1 4- 0.55 t~m for B 4 3 a n d 15.06 4- 1.06 # m for B7. T h i s is o n l y s l i g h t l y s h o r t e r t h a n for t h e F i s h C a n y o n T u f f v o l c a n i c apatites s t a n d a r d , for w h i c h t h e m e a n l e n g t h r e a c h e s 15.31 4- 0 . 8 9 t~m (Fig. 5). A m e a n c o n f i n e d t r a c k l e n g t h b e t w e e n 14 a n d 15 # m w i t h a s t a n d a r d d e v i a tion < 1 # m for o u r s a m p l e s i n d i c a t e s a fast c o o l i n g p h a s e b e l o w 110 4- 10°C to less t h a n 60°C a b o u t 17 M a ago. S e v e r a l m o d e l s u s i n g the a n n e a l i n g m o d e l o f

259

M. Sosson et al. / Tectonophysics 285 (1998) 253-273

30

B43

18.9+0.4 Ma

B20

18.9±0.5 Ma

A

15 ¢-.

Single grain biotites OI

0

20

40

60

%Ar 39 Released

25 B7

100

Single grain muscovite

19.0±0.2 Ma

< ¢-

80

2O

<

151 0

20

40

60

80

100

%At 39 Released Fig. 4. 4°Ar/39Ar age spectra obtained by the laser step-heating method. The plateau ages and apparent ages for each temperature step are given at the l~r level.

Laslett et al. (1987) were proposed to predict the confined track lengths distribution and fission-track ages resulting from a given thermal history (Gleadow et al., 1986). We used the Monte Trax model of Gallagher, based on the use of a genetic algorithm allowing to converge towards a 'most probable' solution (Gallagher et al., 1994; Gallagher, 1995). Repeats of Monte Trax experiments on B20, the sample on which the larger number of track lengths could be measured, gave rather reproducible results. The en-

velope of the most probable histories on six of these experiences is given in Fig. 6: a fast cooling from more than l l0°C to about 40°C in the 18-16 Ma interval is followed by a quasi-isothermal step up to about 5 Ma, then by a slight reheating. The thermal history is ending with a last cooling phase starting some 2-3 Ma ago, to the present time. Although samples B7 and B43 have much less track lengths measured, Monte Trax experiments on these samples arrive essentially at similar results.

260

M. Sosson et al./Tectonophysics 285 (1998) 253 273 £'5

B20 14.53 _+0.83 ~m

B20 Los Reales

Age = 16.2 ± 1.5 Ma

2

21.3 18.9 16.5 14.1

1 '~, , , , ~ : : ........... %

0 -1

i!il il

¢~

ii!i

-2

10

11.7 9.4 o

~ 2 4

f

B43 LOS R e a l e s

6 8 10 1," 14 16 18

Age = 16.4 + 2.2 Ma i

j

20.4 18.8 17.1 15.5 13.8 12.2

2 4. 6 8 10 12 14 16 18 15

B7 Ojen 2 -

B7 15.06 _+1.06 pm

Age = 14.5 + 3.2 Ma ~ 23.6 21,5

.

....

1-

10

171193

a

....

0-

wll

mn

t5.0

::

12.8

'L II#

_2 ~

o

2 4

15

6 8 10 12 14 16 18

Standard FishCanyonTuff 15.31 -+0.89 ~m

~o

Age=t+ 1 6Ma

l

ti_t

2-/~J 1 -I ~

..... ¸¸¸

= :

ti, ¸¸¸

~=

~:

•

¸¸

Ma "~ z

5

ff'llO~rIs

Fig. 5. Fission-track age radial plots and confined track length distributions, t. fission-track ages as measured by A.-C. Morillon (Table 2 ti and o-i, respectively, fission-track age and precision for the ith crystal in a sample.

M. Sosson et al. / Tectonophysics 285 (1998) 253-273

261

Table 2 Apatite fission track data Sample

n

Los Reales 18 B 20 21 7

P~ ( 105 tr-/cm2) (N~)

Pi ( 105 tr./c m2) (N~)

P (X 2) (%)

,0d (105 tr./cm 2) (N~)

Age 4- 2or (Ma)

Obs

3.60 (561) 3.46 (669) 1.10 (67)

21.66 (3372) 16.31 (3154) 3.60 (220)

77

6.088 (15700) 6.088 (15700) 3.324 (13188)

16.2 -4- 1.5

1

20.44- 1.8

2

16.3 4- 4.6

1

16 40

Ni

Lm ::E lcr

47

14.53 4- 0.83

14

15.10 + 0.55

14

15.06 4- 1.06

17.8 4- 1.1 B43

20 9 22 17

1.78 (266) 2.85 (222) 2.58 (491) 1.82 (267)

10.63 (1588) 14.34 (1117) 8.18 (1558) 5.99 (882)

96

6.088 (15700) 6.088 (15700) 3.324 (13188) 3.324 (13188)

19 8 89

16.4 4- 2.2

1

19.1 ± 2.8

2

17.8 4- 1.9

3

16.2 4- 2.3

1

17.3 4- 1.1

Ojen B7

15 7

2.55 (314) 1.70 (103)

8.94 (1101) 6.26 (379)

60

3.324 (13188) 3.324 (13188)

20

16.1 4- 2.1

3

14.5 -4- 3.2

1

15.6 4- 1.8 n: number of crystals counted; P~ and Pi, respectively density of spontaneous and induced tracks (in tracks/cm2); Ns and Ni, total number of spontaneous tracks (apatite) and induced tracks (kapton); P (X2), probability of X 2 value; Pd and Nd, density of tracks on the kapton associated to NBS monitor glass wafers (in tracks/cm 2) and total number of tracks counted. As all P (X 2) are >5%, the ages were calculated using pooled statistics. Bold characters, weighed ages. Obs, observers: 1 = A.-C. Morillon, zeta = 321; 2 = A. Azdimousa, zeta = 316; 3 = E. Labrin, zeta = 339. Confined track length measurements: NI, number of tracks measured; Lm and
T(°C)

B20

20 40

60 80 100 120

Time (Ma) --

25

I'

i

20

]

I

15

I

F

10

[

I

5

0

Fig. 6. Envelope of the ten 'better' cooling histories obtained in each of six runs of the Gallagher (1995) Monte Trax optimization program (see text).

262

M. S o s s o n et a l . / T e c t o n o p h y s i c s

4. Cooling history The different isotopic systems used in this study have various closure temperatures. The 4°Ar/39Ar system closure temperatures are estimated at 350 i 50°C and 300 -4- 50°C for muscovite and biotite, respectively (e.g. McDougall and Harrison, 1988). In apatite, with fission-track dating, it is the thermal history since the last cooling below about l l0°C which is accessible. Considering the overall temperature range accessible by the thermochronometers we used, cooling histories are more constrained than those previously suggested, especially because both 4°Ar/39Ar and fission-track methods were applied to the same samples. An age vs. closure temperature diagram (Fig. 7) which includes the 18.7 -4- 0.6 Ma age (at the 2or level) obtained by Moni6 et al. (1994) on an amphibole from the same unit as the B7 sample shows that the studied region suffered a high cooling gradient in the Lower Miocene. This gradient is well constrained between 500 ± 50°C and 300-4-50°C, where it is higher than 90°C/Ma between 20 and 19 Ma (if one takes into account the error bars on the ages and on the temperatures). The existence of a high thermal gradient in this temperature range is strengthened by the fact that concordant data were obtained on samples from different nappes (Los Reales and Ojen), arguing for a general exhumation of the Alpujarride Complex. These results are in agreement with the previously suggested thermal gradient of 100-350°C/Ma during the 20-19 Ma interval in the 600-300°C temperature domain (Moni6 et al., 1994). The apatite fission-track ages are in the 18-16 Ma interval. Two hypotheses may account for this dispersion of ages which are nevertheless concordant within 2or. (1) Either, the apatite fission-track age differences result only from counting statistics and thus are not significant and the age of the three samples is 17 =L 1 Ma. From Fig. 7a, a minimum mean cooling gradient of about 100°C/Ma may be derived between 500°C and 40°C in the 19-16 Ma interval. However, the figure suggests that the cooling rate may have been higher some 19-18 Ma ago and somewhat lower from 18 to 16 Ma. (2) Or, the age difference between the Los Reales and Ojen samples is real. This discordancy might be

2 8 5 (1998) 2 5 3 - 2 7 3

Temperature (°C) 550 Amphibole 500 40/~r/39Ar

M o n i 6 et aL. 1994 ~

45O 4OO

a

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Age (Ma) 30

25

20

15

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Temperature (°C) (: :{ : ]! I:~:~1 i 17 : :2 I

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Fig. 7. Possible thermal gradients of the Alpujarride Complex in the studied area, inferred from 4°Ar/39Ar and fission-track thermochronometries. Error bars on the ages are given at the 2~r level, Moni6 et al. (1994) and our samples B7, from Ojen nappe; B20 and B43, from Los Reales nappe. (a) Admitting same cooling history for the Ojen and Los Reales nappes. (b) Admitting different cooling histories for these units below about 280°C (see text).

due to differences in apatite chemical composition or in thermal histories. In the latter, the lower age of 15.6 4- 1.8 Ma (B7 from the Ojen nappe) might be explained by a reactivation as a cataclastic normal fault of the contact between the Los Reales and Ojen nappes, between 18 and 16 Ma (Fig. 7b). In Sierra Alpujata, Tubfa (1994) has shown that such a detachment fault was active in the last step of the extensional deformation within the Los Reales nappe. The youngest sample B7 is located some hundred metres below the tectonic contact of the

M. Sosson et al. / Tectonophysics 285 (1998) 253-273

nappes (Fig. 3). This age could result from the tectonic denudation of the Ojen nappe in the late stage of exhumation along the nappe contact, later than the Los Reales nappe which gives apatite fission-track ages of 17.3 ± 1.1 Ma and 17.8 + 1.1 Ma for samples B20 and B43, respectively. In this hypothesis, the cooling history of the Los Reales nappe is about the same as previously, whereas the mean cooling rate of the Ojen nappe drops to some 50°C/Ma between 18 Ma and 16 Ma. Fission-track dating of zircons, whose partial track retention zone is probably between about 260°C and 175°C (Brandon and Vance, 1992) would usefully constrain the cooling history, as is the case in the Beni Bousera peridotite unit (Morocco) where a coupling of 4°Ar/39Ar biotite dating as well as zircon and apatite fission-track dating has outlined a slight slowing of the cooling gradient at the end of exhumation (A. Azdimousa, pers. commun.).

263

the Alpujarride nappes. A consistent left-lateral displacement along the shear zone is obtained from the asymmetry of S - C microstructures and shear bands. The southern block comprises marbles tilted to the southwest along high-angle N 120°E trending normal faults. In this area, the structural data suggest a N E SW-directed extension in the Alpujarride nappes and strike-slip motion at the ductile to brittle transition. North of Monda, Permian and Silurian terranes of the Malaguide nappe are disrupted by the same shear zone which borders the Alpujarride marble units (Fig. 2). Between Monda and Istan, the shear zone clearly shows ductile to brittle fabrics such as shear bands and strike-slip faults. Kinematic indicators as shear bands (Fig. 8) and slickensides on fault planes indicate a left-lateral displacement and

NW

SE

5. Late- to post-metamorphic structures The Sierra de Alpujata and the Sierra Blanca are bounded to the north by a fault zone which was previously interpreted as a thrust fault between the Malaguide and Alpujarride Complexes (Didon et al., 1973). Our structural analysis allows us to conclude that this thrust was reactivated as a vertical shear zone to the north (the Cartama-Istan fault) and by a high-angle southwestward facing normal fault to the southwest (the Istan fault) (Fig. 2). North of Sierras Blanca and Alpujata, a vertical to 70°NW dipping and N45 ° to 70°E trending shear zone, 40 km long, outcrops at Cartama and between Coin and Istan (Fig. 1). It separates high-grade metamorphic rocks of the deep Alpujarride units from the low-grade metamorphic Malaguide rocks and the overlying Neonumidian Formation of Burdigalian age. Consequently, a large jump in metamorphic grade occurs across the shear zone. 5.1. North o f Sierra Blanca and Sierra de Alpujata

Southwest of Coin, the Malaguide nappe is bordered southward by a vertical mylonitic to cataclastic zone trending N70°E (Figs. 1 and 2). The cataclastic zone includes peridotites locally serpentinized, gneiss and marble slivers originating from

Fig. 8. Strike-slip and normal components along the shear zone (northeast of Monda). Photograph shows a subhorizontal plane.

264

M. Sosson el ell./Tectonophysics 285 (1998) 253-2Z~

a normal component. The N70°E strike of the shear zone merges with the N45°E trend. The left-lateral strike-slip faults are associated with N10 ° to 30°E trending dextral conjugated planes. In some places, slices of peridotite are elongated within the shear zone (Fig. 2). 5.2. I s t a n a r e a

In the southwestern part of the Sierra Blanca, the vertical strike-slip Cartama-Istan fault merges with a normal fault zone (Istan fault) (Figs. I and 2). The hanging wall of the normal fault is made of Malaguide and Alpujarride (Los Reales nappe) series. The footwall is composed of high-grade metamorphic rocks of the Ojen nappe. Near lstan, marbles dipping 40 ° to the west-southwest, outcrop in the footwall of the normal fault. The fault plane is close to vertical and crosscuts the metamorphic rocks and the Malaguide/Alpujarride contact (Figs. 9 and 10), In the hanging wall, tens of metres from the normal fault, decollement planes responsible for the northeastward tilting of the Malaguide series are tilted up to vertical by the displacement of the hanging wall near the southwestward dipping normal fault

NE

(Figs. 3, 9 and 10). These structural features indicate that the extension was firstly guided by a southwestward-facing low-dipping detachment which was tilted up to vertical in a late stage of the deformation along a high-angle normal fault (Figs. 3, 9 and 10). Such a structural pattern is known in the Alpujarride Complex of the Sierra Nevada where high-angle normal faults crosscut low-angle normal faults (CrespoBlanc et al., 1993; Crespo-Blanc, 1995).

6. Age of the late- to post-metamorphic structures Near Cartama, a N70°E trending vertical shear zone outcrops (Fig. 1), between the marbles of the Alpujarride nappes and the Lower Miocene nonmetamorphic series (Fig. l 1). The contact is characterized by slices of metamorphic rocks and shear bands associated with S - C fabrics which indicate a left-lateral motion along the shear zone. Sedimentary deposits including clasts of metamorphic Los Reales and Ojen nappes unconformably overlie the left-lateral shear zone (Fig. 11). Series are made of sandy clays and marls, containing Pelecypods and Echinids debris. In order to date and obtain

SW

Fig. 9. The normal fault of Istan, showing the contact of the Ojen marble unit with the Malaguide Complex.

M. Sosson et al. / Tectonophysics 285 (1998) 253-273

265

NE

SW

0 I

50 I cm NE

SW serpentinized peridotites

marbles Ojen Nappe

I

schists and sandstones of Malaguide Complex

~t lineation : 12~NW 2Q I

I

~Lj J

Fig, 10. Cross-section through Istan fault (south of Istan), Photograph showing the Malaguide series up to vertical tilted along the normal fault of Istan. Note that the faults are connected to a decollement plane (see text for explanations).

266

M. Sosson et al./ Tectonophysics 285 (1998) 253-273

Early Pliocene marine detrital sed;ments" .(~ / ~ ~--~J' z. /

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1841,<

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0 Neonumidian Formation of Burdigalian age

Cartama-lstan Fault left-lateral strike-slip fault

,

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Fig. 11. The cross-section located north of Cartama exposes the Early Pliocene unconformity (location is shown in Fig. l). The Early Pliocene deposits seal the left-lateral strike-slip displacement along the Cartama-Istan fault.

palaeobathymetric constrains, one sample (B41) was selected for the study of its microfaunal content. Planktonic foraminifers, constituting 60 to 70% of the foraminiferal assemblage enable us to date the Early Pliocene (Globorotalia margaritae margaritae zone, Zone N18; Bolli and Saunders, 1985; Iaccarino, 1985). The complete list of fossils is given in Appendix A. Benthic foraminifers are also numerous in the sample (see Appendix A). These microfaunal assemblages indicate an environment directly connected with open sea, well oxygenated (occurrence of epifaunic and endofaunic benthic foraminifers; Corliss, 1985, 1991). Bolivinids and species like Siphonina planoconvexa, Melonis padanum, Uvigerina cf. rutila, Anomalinoides cf. flinti, Sigmoilinopsis schlumbergeri, suggest a deposition in the upper bathyal zone (200 to 600 m water depth; Van Morkhoven et al., 1986). The presence of zone taxa (Amphistegina, Asterigerinata, Cibicidoides lobatulus) usually developing in a coastal environment, is linked to a syn-sedimentary reworking and indicates the proximity of a shallow and well-oxygenated carbonate platform. The earliest Pliocene sediments were deposited at depths between 200 and 600 m, and now outcrop at 150 m above sea level. This implies a relative change of base level ranging from 350 to 750 m. The stratigraphic constraints of the Cartama area imply that the strike-slip fault was active during

the Burdigalian to Early Pliocene. The tectonic features and kinematic indicators observed along the Cartama-Istan left-lateral shear zone and the Istan normal fault show a displacement toward the southwest which was active in ductile to brittle conditions. The left-lateral shear zone occurred throughout the Alpujarride nappes including peridotite, gneiss and marble slivers from base to top. As documented by our geochronological results, the Alpujarride nappes were in the 300 5: 50°C to about 40°C temperature range during the 18-16 Ma interval (Burdigalian). The deformations of the continental crust in this temperature interval are basically characterized by the transition from ductile to brittle. Consequently we assume that the left-lateral shear zone and normal fault system were at least active during this 18-16 Ma interval in ductile to brittle conditions. Furthermore such extensional tectonics along normal faults are well-constrained during the same period in the Alpujarride Complex of the Sierra Nevada area (Crespo-Blanc, 1995). 7. Exhumation evolution Our results imply a uniform and fast cooling from 500±50°C to 250±50°C higher than 90°C/Ma during the 19-18 Ma interval. Considering a 'normal' continental geothermal gradient of 30°C/kin, the apparent exhumation rate would be at least 3 km/Ma. The P - T trajectories of the Alpujarride Complex rocks

267

M. Sosson et al. I Tectonophysics 285 (1998) 253-273

cooling + exhumation 16Ma

18Ma

Fiss. track

0

I

Temperature

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-

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_

70

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Fig. 12. P - T trajectories of the Ojen and Los Reales nappes. The P - T data are compiled from Tubfa and Gil-lbarguchi (1991) for the Ojen nappe, and from Tubfa (1994) for the Los Reales nappe. The age of the end of the decompression is deduced from 4°Ar/39Ar data of Moni6 et al. (1994). Different significant geotherms have been drawn (10°C, 30°C and 100°C/km). Our 4°Ar/39Ar and fission-track thermochronological data imply a quasi-isobaric cooling during the 19-16 Ma interval for a range of temperature of 600°C to <60°C.

imply that the near-isobaric cooling from 500 4- 50°C to 300 4-50°C during 1 Ma (19-18 Ma interval) follows a quasi-adiabatic decompression (Fig. 12) (Tubfa and Gil-Ibarguchi, 1991; Zeck et al., 1992; Balany~i et al., 1993; Moni6 et al., 1994). As suggested by Moni6 et al. (1994) this thermal evolution would correspond to a fast thermal relaxation from an abnormally steep geotherm toward a more stable one during a short period of time. As shown in Fig. 12, the high geothermal gradient of 100°C/Ma (which fits with the P - T data after the decompression) involves a cooling rate of the same order and consequently an exhumation rate of 1 km/Ma. Accordingly, we may estimate a mean exhumation rate ranging from 1 to at least 3 km/Ma during the 19-18 Ma interval. Such a result is in agreement with the values proposed by Moni6 et al. (1994) for the 20-19

Ma interval. Additional information is given by the fission-track data which suggest that the decreasing rate of cooling during the 18-16 Ma interval can be linked to the transition in thermal regime from a high geotherm gradient to a shallower stable geotherm (from 100°C to 25°C/Ma) (Fig. 7). In this case the last step of exhumation in the 120° to about 40°C temperature range should be close to 1 km/Ma. Combination of both thermochronological methods suggests that during the 20-16 Ma interval the exhumation rate could be ranging from 1 to at least 3 km/Ma. Such values of the exhumation rates during a short interval of time are in agreement with a quasiisobaric cooling (from 20 to 16 Ma) succeeding to a near-adiabatic decompression (from 25 to 22 Ma as proposed by Moni6 et al., 1994). If such a decompression reflects a tectonic denudation (Albar~de,

268

M. Sosson et al./Tectonophysics 285 (1998) 253-273

1976; Ruppel et al., 1988), such a fast cooling also reflects the importance of erosion and tectonic processes (Ruppel et al., 1988; Davy et al., 1989). The tectonic evolution of the studied area can be constrained as follows (Fig. 13). The decompression stage is associated with motion under ductile conditions along low-angle shear zones (Moni6 et al.,

1994; Tub/a, 1994), which likely reactivated some thrusts. The steep P - T path of the decompression is consistent with a pure-shear extension model of Ruppel et al. (1988) (Fig. 12). In this case, the reactivated thrusts accommodate the extension and the exhumation (Fig. 13). The following fast cooling is associated with the southwestward displacement of

A 25 Ma

Alpujarride nappes

B

Thickened crust

20 - 18 Ma

C Exhumation + cooling cooling gradient : 80 to 25°C/km exhumation rate : 1 km/Ma

Fig. 13. Three-dimensional synthetic evolution which illustrates the exhumation processes along left-lateral strike-slip to normal fault, of the Ojen and Los Reales nappes during the 20-16 Ma interval.

M. Sosson et al. / Tectonophysics 285 (1998) 253-273

the uppermost nappes of the Alpujarride Complex along normal faults and transfer strike-slip faults. This event of Burdigalian age (19-16 Ma) is coeval with subsidence in the Lower Miocene basin of 'flysch' (i.e. the Neonumidian and the 'Argile blocs' formations; Bourgois, 1973, 1978). Extension and wrenching characterize this tectonics. During the Middle and Late Miocene the left-lateral strike-slip faults and normal faults must still be active as documented in the Cartama area where the Burdigalian flysch is faulted by the strike-slip fault. The end of this Miocene exhumation is marked by the unconformity of the Early Pliocene marine deposits over the Cartama-Istan strike-slip fault. 8. Discussion and conclusions Succeeding prograde alpine-type metamorphism, thrusting and near-isothermal decompression, the late orogenic evolution of the deep Alpujarride nappes, are documented from our thermochronological and structural studies: a fast 19-16-Ma-old cooling related to exhumation, and a <5 Ma to Recent uplift. (1) Combination of 4°Ar/39Ar (including amphibole 4°Ar/39Ar data of Moni6 et al., 1994) and fission-track geochronological data indicates a fast cooling mean gradient of more than 100°C/Ma between 500 + 50°C and 90 4- 30°C and from 19 to 17 Ma (Fig. 7). Moni6 et al. (1994) suggested that the main ductile fabric and mineral assemblages from the Alpujarride Complex were related to a stage of decompression, probably associated with extension and the collapse event of the thickened crust. Balanyfi et al. (1993) documented extensional shear zones associated with an isothermal decompression in the Jubrique Alpujarride unit (Sierra Bermeja, Fig. 2). Tubfa (1994) mentioned ductile to brittle shear zones related to a late extensional deformation within the Los Reales nappe. Our new thermochronological 4°Ar/39Ar data are in good agreement with a cooling phase associated with ductile deformation in the 500 4- 50°C to 300 + 50°C temperature interval and fit well with the pressuretemperature paths of the western Alpujarride Complex (Fig. 12), which show that cooling below 600°C occurred after the main decompression event (Balany~i et al., 1993; Moni6 et al., 1994; Tubfa, 1994).

269

(2) Our new structural data suggest that late- to post-metamorphic structural features document an extensional event responsible for a late stage of unroofing of the deep western Alpujarride nappes. As described by Garcfa-Duefias et al. (1992), CrespoBlanc et al. (1994), Lonergan and Mange (1994), Lonergan and Platt (1995) and Crespo-Blanc (1995), such a late stage of exhumation is known in the eastern parts of the Internal Zone. Tectonic features within the left-lateral Cartama-Istan shear zone indicate that deformations were active under conditions that evolved from ductile to brittle. These late- to post-metamorphic structures explain the late step of the deep Alpujarride nappes exhumation during the 18-16 Ma interval in the 120° to 60°C temperature range and the large jump in metamorphic grade across the Malaguide/Alpujarride contact. We assume that ductile to brittle fault zones which crosscut the metamorphic rocks of Los Reales and Ojen nappes and bound the Malaguide Complex are responsible for the southwestward displacement and the dismembering of the Malaguide and Alpujarride Complexes (Fig. 13). Moreover, our apatite fission-track data suggest a possible brittle differential motion on the contact of nappe which could be reactivated as a detachment fault. Considering this structural pattern the contacts between the various Alpujarride nappes which are included in the Istan fault hanging wall and footwall are possible low-angle normal faults which reactivated thrusts (Figs. 2 and 3). This was previously suggested in the Alpujarride Complex of Sierra Nevada (GarcfaDuefias et al., 1992; Crespo-Blanc et al., 1994). This conclusion is in agreement with the recent study of S~inchez-G6mez et al. (1995) which reinterpreted the basal contact of Ronda peridotites unit as a low-angle normal fault (Fig. 2). (3) In the structural pattern we suggest that the Istan fault is one of the normal faults responsible for the latest stage of the Ojen nappe exhumation. To the north, the strike-slip fault corresponds to a transfer fault accommodating the southwestward motion of the Istan fault hanging wall. The present-day morphology of the Istan fault footwall (Sierra Blanca) appears as a dome shape (Fig. 3). Based on structural analyses of the Sierra Nevada located northeastward of our study area, Crespo-Blanc et al. (1994) suggested that a large-scale anticline folded the previous

270

M. Sosson et al./Tectonophysics 285 (1998) 253 273

Internal Zone exhumation structures during the Tortonian. By comparison, this tectonic phase could be responsible for the arch-shaped present-day geometry of the Istan fault footwall. Another interpretation is that this large-scale structure might at least be partly related to the Lower Miocene extension. As shown in other cases of late orogenic extension the warping shape of exhumed high-grade metamorphic rocks could be explained by a flexural unroofing along a high-angle normal fault (Buck, 1988; Brun and Van den Driessche, 1994). The fact that the brittle high-angle Istan fault crosscut and tilted up to vertical previous low-dipping detachment favours the second hypothesis. (4) Our geochronological data show that the cooling is coeval with the rifting of the Alboran domain to the south (Bourgois et al., 1992; Comas et al., 1992). Moreover, the extensional tectonics and related exhumation of the Internal Zone are also contemporaneous with contractional tectonics in the External Betic Zone (Bourgois, 1978). These conclusions imply that the western Betic Cordilleras have suffered a coeval thrusting on the External Zone and an exhumation related to extension in the Internal Zone. The structural data show that the Lower Miocene exhumation was mainly related to extension and to a left-lateral strike-slip motion. Vissers et al. (1995) suggest that during the Miocene some exhumed internal units were involved in the thrusting of the External Zone. Accordingly, we demonstrate that during the Early Miocene the western Alpujarride Complex was southwestward transported along NE-SW-trending strike-slip faults, such as the Cartama-Istan fault system (Fig. 1). (5) Since <5 Ma, in the Cartama area, a 70 to 150 n f M a rate of change of base level is deduced from the palaeo-environment data. In the hypothesis of a sea-level change, our data would document a relative 350-700 m lowering of the sea level. Nevertheless, if lowering of the sea level by eustatic changes is welldocumented during the Late Miocene in the Mediterranean coasts, the Early Pliocene, on the contrary, is mostly characterized by a transgressional event. Consequently we assume that the relative change of the base level is due to tectonic movements. The Cartama area has since 5 Ma undergone an uplift not just limited to the Ojen and Los Reales units, but probably encompassing all of the western Alpujar-

ride units. The uplift is probably related to a N-S to N W - S E oriented compression affecting the Alboran domain since early Tortonian times as described for the central Betics (Crespo-Blanc et al., 1993). Furthermore, the uplift of the Internal Zone could be due to a combination of compressional tectonics since the Tortonian and related erosional processes, causing an isostatic rebound.

Acknowledgements The authors wish to thank Ali Azdimousa and Erika Labrin who cross-counted the apatite samples. We also thank Dr. A.J. Hurford for providing the Monte Trax program, Dr. K. Gallagher for allowing its use, and Dr. J. Talbot for his radial plot software. Thanks are also due to the referees Drs. L. Lonergan and A. Michard for their helpful criticisms. This study was funded under grant PICS No. 100 (Alboran program) by the Centre National de la Recherche Scientifique (CNRS) and the Minist~re des Affaires Etrangbres (MAE), we gratefully acknowledge this support. It is a publication of the CNRS-UNSA Geosciences Azur No. 166.

Appendix A. List of Early Pliocene foraminifers contained in the B41 sample of the sediments which unconformably overlie the left-lateral strike-slip fault (Cartama area) The assemblage consists of the followingspecies: Globorotalia margaritae margaritae (Bolli and Bermudez), Globorotalia margaritae primitiva (Cita), Orbulina universa (d'Orb.), Globigerinoides extremus (Bolli and Bermudez), G. tuber (d'Orb.), Globigerina decoruperta (Takayanagi and Saito), G. bulloides (d'Orb.), G. cf. apertura (Cushman), Hastigerina siphomfera (d'Orb.). Benthic foraminifers are also numerousand diversified: Siphonina planoconvexa (Silv.), Melonis padanum (Perconig), Bri=alina aft. alata (Segu.), B. cf. dilatata (Reuss), Bolivina cf. placentina (Zanm.), Trifarina angulosa (Will.), Uvigerina cf. rutila (Cush and Todd.), Anomalinoides cf. flinti (Cushman), Hanzawaia boueana (d'Orb.), Elphidium crispum (Linn6), Pullenia cf. bulloides (d'Orb.), Sigmoilinopsis schlumbergeri (Silvestri), Cibicidoides lobatulus (Walk. and Jac.), C. pseudoungerianus (Cushman), Amphistegina lesonnii (d'Orb.), Asterigerinata cf. planorbis (d'Orb.), Marginulina gr. costata (Batsch), Lenticulina rotulata (LMK), Lenticulina ssp., Dentalina sp., Nodosaria sp.

M. Sosson et al. / Tectonophysics 285 (1998) 253-2 73

References Albar6de, F., 1976. Thermal models of post-tectonic decompression as exemplified by the Haut-Allier granulites (Massif Central, France). Bull. Soc. G6ol. Fr. XVIII 4, 1023-1032. Azafion, J.M., Garcfa-Duefias, V., Goff6, B., 1992. High pressure mineral assemblages in the Trevenque Unit (Central Alpujarride, Andalucia). Geogaceta 11, 81-84. Balany& J.C., Azafion, J.M., Sfinchez-G6mez, M., GarcfaDuefias, V., 1993. Pervasive ductile extension, isothermal decompression and thinning of the Jubrique unit in the Paleogene (Alpujarride Complex, western Betics, Spain). C.R. Acad. Sci. Paris 316, 1595-1601. Bolli, H.M., Saunders, J.B., 1985. Oligocene to Holocene low latitude planktonic foraminifera. Cambridge University Press, pp. 155-262. Bourgois, J., 1973. Pr6sence et d6finition dans la r~gion de Cafiete la Real et de Grazalema d'une formation d'argiles ~t blocs (Province de Sdville, cadix et Malaga, Espagne). C.R. Acad. Sci. Paris 276, 2939-2942. Bourgois, J., 1977. D'une 6tape gdodynamique majeure dans la gen~se de l'arc de Gibraltar: l'hispanisation des flyschs rifains au Mioc6ne inf6rieur. Bull. Soc. Geol. Fr. 19, 1115-1119. Bourgois, J., 1978. La transversale de Ronda (Cordill6res bEtiques, Espagne): donn6es g6ologiques pour un module d'dvolution de I'arc de Gibraltar. Ann. Sci. Univ. Besanqon 30, 445 pp. Bourgois, J., Chauve, R, Magn6, J., Monnot, J., Peyre, Y., Rigo, E., Riviere, M., 1972a. La formation de Las Millanas: sdrie burdigalienne transgressive sur les zones internes des Cordill6res b6tiques occidentales. C.R. Acad. Sci. Paris 275, 169-172. Bourgois, J., Chauve, E, Lorenz, C., Monnot, J., Peyre, Y., Rigo, E., Riviere, M., 1972b. La formation d'Alozaina: S6rie d'gge oligoc6ne et aquitanien transgressive sur le B6tique de Malaga (R6gion d'Alozaina-Tolox, province de Malaga, Espagne). C.R. Acad. Sci. Paris 275, 531-535. Bourgois, J., Chauve, E, Peyre, Y., 1973. Trame de l'histoire post-aquitanienne des Cordill6res bdtiques occidentales. C.R. Acad. Sci. Paris 276, 1393-1396. Bourgois, J., Mauffret, A., Ammar, A., Demnati, A., 1992. Multichannel seismic data imaging of inversion tectonics of the Alboran Ridge (Western Mediterranean Sea). Geo-Mar. Lett. 12, 117-122. Brandon, M.T., Vance, J.A., 1992. Tectonic evolution of the Cenozoic Olympic subduction complex, Washington State, as deduced from fission-track ages for detrital zircons. Am. J. Sci. 292, 565-636. Buck, W.R., 1988. Flexural rotation of normal faults. Tectonics 7, 959-973. Brun, J.E, Van den Driessche, J., 1994. Extensional gneiss domes and detachment fault systems: structure and kinematics. Bull. Soc. G6ol. Fr. 165 (6), 519-530. Comas, M.C., Garcfa-Duefias, V., Jurado, M.J., 1992. Neogene tectonic evolution of the Alboran Sea from MCS data. GeoMar. Lett. 12, 157-164.

271

Corliss, B.H., 1985. Microhabitats for benthic foraminifera within deep-sea sediments. Nature 314, 435-438. Corliss, B.H., 1991. Morphology and microhabitat preferences of benthic foraminifera from the northwest Atlantic Ocean. Mar. Micropaleontol. 17, 195-236. Crespo-Blanc, A., 1995. Interference pattern of extensional fault systems: A case study of the Miocene rifting of the Alboran basement (North of Sierra Nevada, Betic Chain). J. Struct. Geol. 17, 1559. Crespo-Blanc, A., Garcfa-Duefias, V., Orozco, M., 1993. Syst~me en extension dans la Cha~ne BEtique Centrale: que reste-t-il de la structure en nappes du complexe Alpujarride?. C.R. Acad. Sci. Paris 317, 971-977. Crespo-Blanc, A., Orozco, M., Garcfa-Duefias, V., 1994. Extension versus compression during the Miocene tectonic evolution of the Betic Chain - - late folding of normal fault systems. Tectonics 13, 78-88. Cuevas, J., Andaya, E, Navarro-Vila, E, Tubfa, J.M., 1986. Caract6risation de deux 6tapes de charriage principales dans les nappes Alpujarrides centrales (Cordill~res b6tiques, Espagne). C.R. Acad. Sci. Paris 302, 1177-1180. Davies, G.R., Nixon, EH., Pearson, D.G., Obata, M., 1993. Tectonic implications of graphitised diamonds in the Ronda peridotite massif, Southern Spain. Geology 21,471-474. Davy, E, Guerin, G., Brun, J.-E, 1989. Thermal constraints on the tectonic evolution of a metamorphic core complex (Santa Catalina Mtns, Arizona). Earth Planet. Sci. Lett. 94, 425-440. De Jong, K., 1991. Tectono-Metamorphic Studies and Radiometric Dating in the Betic Cordilleras (SE Spain) - - with Implications for the Dynamics of Extension and Compression in the Western Mediterranean Area. Ph.D Thesis, Vrije Universiteit, Amsterdam, 204 pp. De Jong, K., Wijbrans, J.R., F6raud, G., 1992. Repeated thermal resetting of phengites in the Mulhacen Complex (Betic Zone, southern Spain) shown by 4°Ar/39Ar heating and single grain laser probe dating. Earth Planet. Sci. Lett. 110, 173-191. De Roever, W.E, Niijhuis, H.J., 1964. Plurifacial alpine metamorphism in the Eastern Betic Cordilleras (Southern Spain). Cuad. Geol. Univ. Granada 8, 37-60. Didon, J., Durand-Delga, M., Kornprobst, J., 1973. Homologies g6ologiques entre les deux rives du d6troit de Gibraltar. Bull. Soc. G6ol. Fr. 7, 77-105. Doblas, M., Oyarzum, R., 1989. 'Mantle Core Complexes' and Neogene extensional detachment tectonics in the Western Betics Cordilleras, Spain: an alternative model for the emplacement of the Ronda peridotite. Earth Planet. Sci. Lett. 93, 7684. Durand-Delga, M., 1966. Titres et travaux scientifiques. Imp. Priester, 43 pp. Durand-Delga, M., Feinberg, H., Magn6, J., Olivier, E, Anglada, R., 1993. Les formations oligo-mioc~nes supra-Malaguides (Cordill~res b~tiques, Espagne) dans l'6volution g6odynamique de la M6diterran6e. C.R. Acad. Sci. Paris 317, 679687. Egeler, C.G., Simon, O.J., 1969a. Sur la tectonique de la zone b6tique (Cordillbres b6tiques, Espagne). Verh. K. Ned. Akad. Wet. 25, 1-90.

272

M. Sosson et al./Tectonophysics 285 (1998) 253 273

Egeler, C.G., Simon. O.J., 1969b. Orogenic evolution of the Betic Zone (Betic Cordilleras, Spain) with emphasis on the nappe structures. Geol. Mijnbouw 48, 296-305. Fallot. R, 1948. Les Cordillbres b6tiques. Estud. Geol. 4, 83 172. Galbraith, R.F.. 1981. On statistical models for fission-track counts. Math. Geol. 13 (6), 471-478. Galindo-Zaldfvar. J., Gonzalez-Lodefro, F.. Jabaloy, A., 1989. Progressive extensional shear structures in a detachment contact in the Western Sierra Nevada (Betic Cordilleras, Spain). Geodyn. Acta 3 (1), 73-85. Galtagher, K., 1995. Evolving temperature histories fi'om apatite fission-track data. Earth Planet. Sci. Lett. 136, 421-435. Gallagher, K., Hawkesworth. C.J.. Mantovani. M.S.M., 1994. The denudation history of the onshore continental margin of SE Brazil inferred from fission track data. J. Geophys. Res. 89 (18), 117-145. Garcfa-Duefias, V., Martfnez-Martfnez, J.M., Orozco, M., Soto, J.I.. 1988. Plis-nappes, cisaillements syn- 'a post-m&amorphiques et cisaillements ductiles fragiles en distension dans les N6vado-filabrides (Cordillbres bdtiques, Espagne). C.R. Acad. Sci. Paris 307, 1389. Garcfa-Duefias, V., Balanyfi, J.C., Mart/nez-Martfnez, J.M., 1992. Miocene extensional detachments in the outcropping basement of the Northern Alboran Basin and their tectonic implications. Geo-Mar. Lett. 12, 157-164. Gleadow, A.J.W., Duddy, I.R., Green, EL., Lovering, J.F., 1986. Confined fission-track lengths in apatite: a diagnostic tool for thermal history analysis. Contrib. Mineral. Petrol. 94, 405 415. Goff6, B., Michard. A., Garcfa-Duefias, V., Gonzales-Lodeiro, F., Moni& R, Campos, J.. Galindo-Zaldfvar, J., Jabaloy, A.. Martfnez-Martfnez, J.M., Simancas. J.F., 1989. First evidence of high-pressure, low-temperature metamorphism in the Alpujarride nappes, Betic Cordilleras (SE Spain). Eur. J. Mineral. 1, 139-142. Iaccarino. S., 1985. Mediterranean Miocene and Pliocene planktic foraminifera. Cmnbridge University Press, pp. 238-314. Johnson. C., 1993. Contrasted thermal histories of different nappe complexes in S.E. Spain evidence for complex crustal extension. In: Seranne. M.. Malavieille, J. (Eds.). Late Orogenic Extension in Mountain Belt (abstr.). BRGM 219. p. 103. Julivert, M., Fontbote, J.M., Ribeiro, A., Conde, L.. 1974. Mapa tectonico de la Peninsula Ibdrica y Baleares y memoria adjunto. Escala I : 1.000,000. Instituto Geologico Min. Espafia, Madrid, Kornprobst, J.. 1976. Signification structurale des pdridotites dans l'orogbne bdtico-rifain: arguments tirds de I'dtude des ddtritus observds dans les sddiments paldozo~.'ques. B.S.G.F. 7. XVIII 3, pp. 607-618. Laslett, G.M.. Green, RE, Dud@. RE, Gleadow. A.J.W., 1987. Thermal annealing of fission tracks in apatite 2. A quantitative analysis. Chem. Geol. (lsot. Geosci. Sect.) 65. 1 13. Lonergan, L., 1993. Timing and kinematics of deformation in the Malaguide Complex. Internal Zone of the Betic Cordillera, SE Spain. Tectonics 12. 460-476.

Lonergan, L., Mange, M.A., 1994. Evidence for Internal Zone unroofing from foreland basin sediments. Betic Cordillera, SE Spain. J. Geol. Soc. London 151,515 529. Lonergan, L., Plait, J.R, 1995. The Malaguide-Alpujarride boundary: a major extensional contact in the Internal Zone of the eastern Befic Cordillera, SE Spain. J. Struct. Geol. 17, 1655-1671. Loom(s, T.R, 1975. Tertiary mantle diapirism, orogeny and plate tectonics east of the Strait of Gibraltar. Am. J. Sci. 275, 1-30. M~ikel, G.H., 1985. The geology of the Malaguide complex and its bearing on the geodynamic evolution of the Betic-Rif orogen (southern Spain and northern Morocco). Geol. GUA Pap. 22, 263 pp. McDougall, I.. Harrison, T.M., 1988. Geochronology and Thermochronology by the 4°Ar/39Ar Method. Clarendon Press, Oxlbrd, 212 pp. Monid, R, Galindo-Zaldfvar, J., Gonzalez-Lodefro, F.. Goffd, B., Jabaloy, A., 1991. First report on 4°Ar/39Ar geochronology of alpine tectonics in the Betic Cordilleras (Southern Spain). J. Geol. Soc. London 148, 289-297. Moni6, R, Torres-Rold~in, R.L., Garcfa-Casco. A., 1994. Cooling and exhumation of the western Betic Cordilleras, 4°Ar/-~gAr thermochronological constraints on a collapsed terrane. Tectonophysics 238, 353-379. Morten, L., Bargossi, G.M., Martfnez-Martfnez, J.M.. Puga, E., Diaz de Federico, A., 1987. Metagabbro and associated eclogites in the Lubrin area, Nevado-filabride Complex, Spain. J. Metamorph. Geol. 5, 155-174. Paquet, J.. 1969. Etude gdologique de I'Ouest de la Province de Murcia. M6m. Soc. Gdol. Ft. 48. Platt, J.R. 1986. Dynamics of orogenic wedges and the uplift of high-pressure metamorphic rocks. Bull. Geol. Soc. Am. 97, 1037-1053. Platt, J.R. Vissers, R.L.M.. 1989. Extensional collapse of thickened continental lithosphere. Geology 17, 540-543. Priem. H.N.A., Hebeda, E.H., Boelrijk, N.A.I.M., Verdurmen. Th.E.A., Oen, I.S., 1979. Isotopic dating of the emplacement of the ultramalic masses in the Serrania de Ronda, Southern Spain. Contrib. Mineral. Petrol. 61, 103-109. Ruffet, G.. Fdraud. G., Balavre, M., Kienast. J.-R., 1995. Plateau ages and excess argon in phengites: an 4°Ar/39Ar laser probe study of Alpine micas (Sesia Zone. Western Alps, northern Italy). Chem. Geol. (lsot. Geosci. Sect.) 121, 327-343. Ruppel, C., Royden, L., Hodges, K.V., 1988. Thermal modeling of extensional tectonics: application to pressure-temperaturetime histories of metamorphic rocks. Tectonics 7 (5), 947957. Sfinchez-G6mcz, M., Garcia-Duefias, V., Mufioz. M., 1995. Relations structurales entre les pdridotites de la Sierra Bermeja et les unit6s alpujarrides sous-jacentes (Benhav(s, Ronda, Espagne). C.R. Acad. Sci. Paris 321,885-892. Steiger. R.H., JS.ger, E., 1977. Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36. 359-362. Torres-Roldfin. R.. 1979. The tectonic subdivision of the Betic Zone (Betic Cordilleras, Southern Spain): its significance and

M. Sosson et al./Tectonophysics 285 (1998) 253-273

one possible geotectonic scenario for the westernmost Alpine belt. Am. J. Sci. 279, 19-51. TuNa, J.M., 1994. The Ronda peridotites (Los Reales nappe): an example of the relationship between lithospheric thickening by oblique tectonics and late extensional deformation within Betic Cordillera (Spain). Tectonophysics 238, 381-398. TuNa, J.M., Cuevas, J., 1986. High-temperature emplacement of the Los Reales peridotite nappe (Betic Cordillera, Spain). J. Struct. Geol. 8, 473-482. Tubfa, J.M., Cuevas, J., 1987. Structures et cin6matiques li6es la mise en place des p6ridotites de Ronda (Cordill~res b~tiques, Espagne). Geodyn. Acta 1, 59-69. TuNa, J.M., Gil-Ibarguchi, I., 1991. Eclogites of the Ojen nappe: a record of a subduction in the Alpujarride Complex (Betic Cordilleras, southern Spain). J. Geol, Soc. London 148, 801804.

273

Tub/a, J.M., Cuevas, J., Navarro-Vilfi, F., Alvarez, F. and Aldaya, F., 1992. Tectonic evolution of the Alpujarride Complex (Betic Cordillera, southern Spain). J. Struct. Geol. 14(2), 193-203. Van Morkhoven, F.C.RM., Berggren, W.A., Edwards, A.S., 1986. Cenozoic cosmopolitan deep-water benthic foraminifera. Bull. Cent. Rech. Explor.-Prod. Elf-Aquitaine, M6m. 11,421 pp. Van Wees, J.D., De Jong, K., Cloething, S., 1992. Two-dimensional P - T - t modelling and the dynamics of extension and inversion in the Betic Zone (SE Spain). Tectonophysics 203, 305-324. Vissers, R.L.M., Platt, J.R, van der Wal, D., 1995. Late orogenic extension of the Betic Cordillera and the Alboran Domain: a lithospheric view. Tectonics 14 (4), 786-803. Zeck, H.R, Moni6, R, Villa, I.M., Hansen, B.T., 1992. Very high rates of cooling and uplift in the Alpine belt of the Betic Cordilleras, Southern Spain. Geology 20, 79-82.