JOURNAL OF GEODYNAMICS 12, 137-147 (1990) 137

D E E P S T R U C T U R E O F T H E B E T I C C O R D I L L E R A D E R I V E D F R O M T H E I N T E R P R E T A T I O N O F A C O M P L E T E B O U G U E R A N O M A L Y M A P

ALBERT CASAS 1 and ANDRES CARBO 2

1 Dept. Geoquimica, Petrologia i Prospecci6 Geolbgica. Universitat de Barcelona, Barcelona, Spain. 2 Dept. Geodinamica. Universidad Complutense, Madrid, Spain.

(Accepted May 30, 1990)

ABSTRACT

Casas, A. and Carbo, A., 1990. Deep structure of the Betic Cordillera derived from the interpretation of a complete Bouguer anomaly map. In: J. J. Dafiobeitia and B. Pinet (Editors), Geophysics of the Mediterranean Basin. Journal of Geodynamics, 12: 137-147.

A complete Bouguer-anomaly map of the Betic Cordillera and surrounding areas has been compiled from 2995 land-gravity observations. The measurements used to construct the map come from previous surveys as well as from a survey carried out specially for this project.

A large negative anomaly reaching -145 mGal is centered on the intra-mountain basins of Loja- Granada and Baza-Guadix, and systematically displaced from the region of greatest topograhic relief. This gravity low is interpreted as produced by the thickening of the earth's crust at a principal contact between two domains of continental crust having different origins and geological evolution.

Three gravity profiles selected to investigate the deep structure of the crust in the Betic Cordillera are presented and discussed.

INTRODUCTION

The knowledge of the deep structure of the Betic Cordillera is of great importance in understanding the geological evolution of this zone during Alpine times.

Several geophysical studies have previously been made in this zone to determine their deep structure.

These include: - Long seismic refraction profiles (Banda & Ansorge, 1980). - Land and sea gravity data (Bonini et aL, 1973). - C o m b i n e d gravity and seismic refraction data (Hartzfeld, 1976;

Surifiach & Udias, 1978). - Surface-wave analysis (Marilier & Muller, 1985). The purpose of this set of geophysical surveys was to obtain a more

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138 CASAS AND CARBO

accurate model of the earth's crust in order to help with the geological inter- pretation.

In this paper, we present a gravity study of the Betic Cordillera based on the interpretation of a complete Bouguer-anomaly map.

GEOLOGICAL SETTING

The Betic Cordillera, together with the Rif, are the Western-most Alpine ranges formed under the Tethys. Their history is related to the dis- placements produced by the Atlantic opening and the relative movements between Africa and Europe.

The Betic Cordillera can be divided into two major tectono-paleogeo- graphic zones presenting a different geologic evolution: the external and the internal domains. The external domain (Prebetic and Subbetic zones) is located on the northern border of the chain and constitutes the passive continental margin of the Iberian Plate.

The internal zones, clearly showing an arcuate structure, are formed by the thrusting of allochtonous units (Alborhn domain) grouped onto three main complexes: Nevado-Filabride, Alpujarride and Malaguide, in ascend- ing tectonic order (Egeler & Simon, 1969). All three complexes consist of Paleozoic metasediments and Mesozoic covers (essentially Permo-Triassic) affected by different grades of metamorphism. Particular magmatic phenomena, such as, for instance, ultramafic intrusions, are also well represented.

In addition, other units of controverted paleographic of filiation exist: the Betic Dorsal and the allochtonous units of the Campo de Gibraltar. The Betic Dorsal, a long and narrow outcrop of materials, the so-called Rondaides, is present between the two main domains.

A variety of models has been proposed for the evolution of the Betic Cordillera and surrounding areas (Andrieux etal., 1971; Kampschuur& Rondel, 1975; Torres Rold~m, 1979). Although some important subjects are still debated, it is generally accepted that the present structure of the Gibraltar arc is only a superimposed feature linked to the large westward displacement of the Alborfin domain.

Figure 1, from Fontbot6 & Est6vez (1980), shows a simplified geological map of the Betic Cordilleras which defines the main tectonic units.

GRAVITY SURVEY

The new gravity map of the studied area has been produced from a com- pilation of all pre-existing gravity data (Instituto Geografico Nacional),

DEEP STRUCTURE OF THE BETIC CORDILLERA 139

1 2 3 4

0 100 km

Fig. 1. Geological sketch map of the Betic Cordillera, from Fontbot6 and Est6vez (1980). 1) Prebetic zone 2) Subbetic zone 3) Betic zone 4) Allochtonous units of the Campo de Gibraltar 5) Betic Dorsal 6) Olistostrome of the Guadalquivir Valley 7) Post-orogenic terranes of the Guagalquivir valley 8) Hercynian basement of the Iberian Massif 9) Mesozoic cover of the Iberian Massif

10) Neogene and Quaternary volcanic rocks

conveniently homogenized to the Internation Gravity Standardization Net (I.G.S.N. 1971). New measurements in some areas where the data distribu- tion was sparse were added. Obviously, these areas were those of most difficult of access.

A total of 2995 gravity land-data measurements were used to cover the studied area, which represents an average station observation density of one per 16 square kilometers. The distribution of land-data points is now fairly homogeneous..

The observation sites were located on 1/50,000 scale maps, and latitudes and longitudes were measured directly from the maps. Location errors are estimated to be less than +0.05 Km.

The elevation of the sites was determined either from barometric leveling

140 CASAS AND CARBO

or from benchmarks wherever possible. Uncertainties in elevation account for the largest errors in calculating the Bouguer values. Errors for elevation depend on the source of the elevation data. The determination of elevation by barometric leveling, after temperature and atmospheric pressure change corrections have been made, is accurate to within _ 5 meters.

The Geodetic Reference System (G.R.S. 1967) was used in order to obtain the normal gravity value at sea level, and the standard reduction methods, with a mean density of 2.67 g/cm 3, were applied for the computation of the Bouguer-anomaly values.

Topographic reduction had not been calculated on earlier works, therefore we had to undertake the tiresome task of making topographic reductions of all data. The Betic Cordillera has the greatest elevation of the Iberian Peninsula, reaching a height of 3482 m at the Mulhac6n Peak in the Sierra Nevada, located at only 20 kilometers from the Granada Basin and 35 km from the coastline.

Inner-zone terrain corrections (up to 50 m) were estimated for the new survey in the field and calculated by the method proposed by Klingel6 (1980). These corrections were generally small since stations had been deliberately positioned well away from major changes whenever possible. In general, errors of only 2 or 3 miligals might be introduced by not comput- ing the inner zone, although certain stations on sharp peaks or in deep canyons would reach the greatest values. Outer-zone terrain corrections were calculated using a prism method extending out to 22 km from a synthetic topographic model obtained from the manual digitalization of elevation contour lines a 50-meter intervals.

The maximum topographic correction obtained is of 40 mGal, although 50 % of the data points do not reach 5 mGal.

The combined error in the total reduction process is considered to be less than +0.45 mGal.

Random land data were converted to an equally spaced grid by fitting a local bi-quadratic parabola to the points enclosed within a circle of 10 km radius and subsequently machine contoured within an interval of 5 miligals. Sea-gravity data have been taken from Gantar et al. (1968) and Finetti & Morelli (1973), and processed together with land data; however contour lines are presented at 10 milligal intervals as on the original map.

GRAVITY INTERPRETATION

The Bouguer Anomaly map (Fig. 2) shows five major features; 11 A broad negative anomalous zone reaching -145 mgal. This low is

produced by the superposition of two effects: one regional, the other local.

DEEP STRUCTURE OF THE BETIC CORDILLERA 141

4200

4r~5

4150

4125

4100

4075

a050

~025

4000

:975

3950

3925

3900

38r5

250 275 300 325 ~50 3?5 400 425 450 475 500 525 550 575 600

m UIIU[l lU[ l~ IU~R~

5875

250 275 300 325 350 575 400 425 450 475 500 525 550 575 600

Fig. 2. Bouguer anomaly map of the Betic Cordillera and surrounding areas computed with a 2.67 g/cm3 density reduction. Contour interval is 5 milligals for land data and 10 milligals for the sea. In the areas of steep gradient values only main lines are drawned.

a) Residual local anomalies are clearly produced by the intra-moun- tain basins of Loja-Granada and Guadix-Baza, generated during the exten- sional period existing from the Tortonian age, with the accumulation of a thick series of sediments due to a strong subsidence process (Est6vez & Sanz de Galdeano, 1980). The gravity effect of these basins filled with low-density sediments is of 18 milligals at Granada-Loja. The structural interpretation of such local anomalies will not be discussed here, but will be treated in a separate paper.

b) The regional minimum is produced by the strong lateral variations in crustal thickness, which changes from 17 km under the Alborhn Sea, to 24 km under the coastline and 40 km north of the Sierra Nevada.

The axis of this minimum follows a sigmoidal line and coincides with the position of the Betic Dorsal units. The crustal thickening is interpreted as a result of the collison of two different types of crust along a suture zone masked by the Neogene sediments that fill the basins.

142 CASAS A N D CARBO

2) A coastal positive anomaly trend which correlates in one part with a zone of outcrops of ultramafic and associated metamorphic rocks.

3) The Albor&n Sea central high, produced by the earth's crust thinning, as has been deduced from seismic surveys.

4) The high detected in the Guadalquivir Valley, west of Cordoba, which we believe is due to the combined effect of crustal thinning and the existence of gaboroic rocks outcroping in this area. This anomaly is located in the north-west corner of our map and therefore it is not possible for us to discuss its shape and extension.

5) The trend variation of the isoamalous lines on the Gata Cape area is caused by the existence of a thinned and highly intruded crust by calco- alkaline volcanism on the east side of the Carboneras-Palomares-Alhama fault system.

Three crustal structure profiles across the Betic Cordilleras were con- structed from the gravity data (Fig. 3), and their theoretical gravity anoma- lies for 21/2 dimensional models were computed using a computer program based on the equations of Cady (1980), which allows the user to change the

: . . . . J">c, o,~,~oL , , . ~?j7 '

") w i>,L L£~

SIER~A NEv,XD A .j 0 c:::' ?

40~r~ Ronda G' , ~ "

• I.s~ '~ _ ~ - . ~ j - f L - - - - - . ~ )."-~ <~\ " -L"

I 4075 j

I , Alboran ,sla,:d

/

~90(~ " \ - - f - - " ~ j { ' - \ \

4'?)'

~ga,

Fig. 3. Location of the interpreted gravity profiles.

DEEP STRUCTURE OF THE BETIC CORDILLERA 143

geometry and/or the density of the model structures interactively in order to obtain a good fit between the gravity model and the measured anomaly.

The first profile passes through the Sierra Nevada and the Albor~in island. The geometry of this model is constrained by the depths obtained by Banda & Ansorge (1980) from long refraction seismic profiles. Moreover, Nafe & Drake's (1963) density-velocity relationships were used to obtain the densities adopted for the deeper layers; consequently, the model was con- structed assuming densities of 2.75 g/cm 3 for the Iberian Upper Crust, 2.90 g/cm 3 for the Iberian Lower Crust, 2.80 g/cm 3 for the Alborhn Crust and 3.2 g/cm 3 for the Upper Mantle.

The boundary between the External and Internal Betic Zones has been shown to exibit a characteristic gravity signature and, therefore, this fact has been utilized to delineate the boundary between two different types of crusts belonging to the Iberian and the Albor~m domains.

It is interesting to point out the displacement between the regional gravity low produced, in our opinion, by the thickening of the earth's crust in the collision zone, and the location of the highest peaks at the Sierra Nevada, generated by a later process which affected upper structural levels.

On the other hand, the proposed model can assume the existence of an anomalous mantle of 3.0 g/cm 3 under the Albor~m Sea, with no significant variation of the crustal thickness in this zone.

The introduction of the effects produced by the Baza-Guadix Basin, the Alpujarras Basin and the effects of the water sheet and sea bottom sediments, allows us to fit our model with an error of less than five milligals, which is the maximum error expected in our experimental data.

The second profile runs more to the east, near Almeria, and shows a pat- tern similar to those presented before. This means than the contact between the Iberian crust and the Alborhn domain coincides with the gravity low, and therefore, according to the model proposed, the Cartagena Type Crust (De Larouzi6re et al., 1988) is only a thinner and highly intruded crust of the same Albor~in domain.

The third profile intersects the Ronda massif and is characterized by a positive-negative anomaly couple. The gravity high can be attributed to the mass excess produced by the dense ultramafic rocks, but cannot be satisfied without introducing a deep root into the Ronda peridotitic rocks. The average density of the Ronda ultramafic massif, estimated from measured samples, is in the range 2.8 to 3.0 g/cm 3 (Robertson, 1970). Therefore, the gravity interpretation indicates that if we try to correlate the gravity high with the Ronda massif, the mafic complexes must be deep seated and may only be considered relatively allochtonous, with displacements in the order of ten kilometers. It should be noted that the peridotitic massifs must be so thin that they cannot give rise to a significant gravity anomaly. Further-

144 (‘ASAS AND CARBO

(a)

(b)

N S

Fig. 4. Crustal cross-sections of three north-south gravity profiles, showing model used to explain

Bouguer anomalies:

a) profile across the Sierra Nevada

b) profile across Gata cape (Almeria)

c) profile across the Ronda Massif

Major features are discussed in the text.

DEEP STRUCTURE OF THE BETIC CORDILLERA 145

(c)

N S

I00

8O

60

40

22 - 2 0

- 4 0 °

- 6 0

- 8 0

- I O 0 °

:,'22 85 b 2'5

~ ~ - , ~ , ~ k k' ' ".'.,2.2.;.2.2.;.2.2.2.:.2.2.7.; ~ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ........:.:.;.:.;.: ^ ^ ^ A A A A A A

^ ^

5bKm

N L E G E N D F O R M O D E L S

Fig. 4.

J - ~ N e o g e n e Sediments

E x t e r n a l l ~ Upper Crust

Domain I ~ L o w e r Crust

" b o r a n Domain Indifferenciated Crust

l ultramafic rocks

~ Mantle

(continued)

more, we agree with Hartzfeld (1976) than it is necessary to consider a low- density upper mantle below this sector of the Albor~in Sea to match the observed gravity field with the structural model inferred from seismic measurements.

CONCLUSIONS

The gravity field of the Betic-Albor~in region is characterized by local anomalies produced by Neogene basins, which are superimposed on a wider

146 CASAS AND CARBO

anomaly with positive gradients toward the Albor~in Sea, caused by the presence of a thin crust below this area.

Further, it is interesting to point out the three evident positive axes located one in the Ronda area, the other to the north of M~ilaga, and the third near Motril. The first two maxima must be related with the peridotitic outcrops of the Ronda Massif if we consider that those bodies had been moved some km to the NW before their emplacement.

The interpretation of three selected gravity profiles of the Betic Cordillera, together with the more recent geologic studies, allow us to define a schematic model of the deep structure.

ACKNOWLEDGEM ENTS

The authors are particularly indebted to Prof. J.M. Fontbot6 for his encouragement and fruitful discussions. Dr. Fontbot6 passed away suddenly while we were working on this paper and, therefore, we would expecially like to dedicate it to his memory.

The English version of this paper has benefitted from the useful correc- tions and comments of Frances Luttikhuizen.

Helpful comments of two anonymous reviewers are also gratefully acknowledged.

Funds for the gravity field work were supplied in part by the CAICYT grant number 1151/84.

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DEEP STRUCTURE OF THE BETIC CORDILLERA 147

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