interpretation of an aeromagnetic survey of the spanish mainland

Earth and Planetary Science Letters, 105 (1991) 55-64 55 Elsevier Science Publishers B.V., Amsterdam

[XLeP]

Interpretation of an aeromagnetic survey of the Spanish mainland

I sabe l Socias a, Jul io M e z c u a a, J o h n L y n a m b and R e c a r e d o Del P o t r o c

a Instituto Geografico Nacional, Madrid, C/General Ibahez de Ibero 3, 28003 Madrid, Spain b Geophysics Consulting~ UK

c Aurensa, Madrid, Spain

Received May 3, 1990; revision accepted April 15, 1991

ABSTRACT

An aeromagnetic survey of the Spanish mainland was carried out in 1987. A first interpretation of this survey is given here, with particular reference to the structure and tectonic development of the Spanish part of the Iberian Peninsula. This involves the integration of the geophysical interpretation, where possible, with the current understanding of the tectonic framework as determined from previous geological and geophysical investigations. Of particular interest is the apparent structural symmetry and the possible manner in which this might be associated with the generally acknowledged structural units of Spain; this gross structural symmetry even seems to extend to the northern and southern coastlines where the magnetic responses are somewhat similar in character. From a magnetic point of view, the Spanish mainland appears to be dominated by a series of sub-circular structures about an E S E - W N W axis, centered on the Gredos Mountains and remaining open in the axial direction into Portugal.

1. Introduction

A high-sensitivity aeromagnetic survey was carried out over the whole of the Spanish mainland. The data were originally acquired under contract between C o m p a ~ a Espahola de Trabajos Foto- gram&icos Arreos S.A. (CETFA), Hunting Geol- ogy and Geophysics Limited and the Instituto Geografico Nacional ( IGN), during the period September 1986 to June 1987. The objective of this project was to acquire airborne magnetometer data which would map the variations in the Earth's total magnetic field intensity over Spain at a barometric elevation of 3000 m. The datum for the whole survey was the field intensity at the survey height directly above the San Pablo de los Montes Observatory (Toledo, 39°32 '50"N, 4°20 '55"W) on 1 January 1987.

2. Survey characteristics

In this section we will give a brief account of these aspects; full details are given in Ardizone et al. [1]. The survey network consisted of N - S flight

lines with a 10 km spacing and E - W control lines spaced 40 km apart, and was flown at a barometric elevation of 3000 m above sea level. A magnetometer of the C E N G double resonance Over- hauser type with a sensitivity of 0.01 nT was used, sampling the field every half second. Accurate navigation was achieved by using a combination of digital Doppler and visual observation with 1:50,000 scale topographic maps. After correc- tions and reductions, the data were gridded at a 2.5 km interval, the final RMS difference was 0.33 nT. Figure 1 shows the residual map after remov- ing the I .G.R.F. of the Spanish mainland.

3. Qualitative interpretation

The residual map has been subdivided into different magnetic zones by simple visual inspect- ion (Fig. 2), ascribed to variation in the magnetic texture within the area. This subdivision was carried out, initially on magnetic grounds alone so that the resulting boundaries and trends either agree, disagree or only partially agree with mapped geology and structure.

0012-821X/91/$03.50 © 1991 - Elsevier Science Publishers B.V.

56 I. SOCIAS ET AL.

4 2

4 0 4 0

3 8 ~38

3 6 - - ND - ! 150

Q IOOKm

8 6 4 2 0 2

Fig. 1. Residual field map of the Spanish mainland. Barometric flight altitude, 3000 m. Reference model I.G.R.F-85.

3.1. Magnetic lineaments

Following a detailed study a large number of magnetic lineaments is observed at a scale of 1 : 1,000,000, but only the most distinctive, usually in terms of strike length and visibility, are discussed in this description. Many of these lineaments are undoubtedly faults, detected in the data because of the way they distort the magnetic contours in their vicinity and, sometimes because of the presence of magnetic material in the fault plane. These generally fall into one of three groups:

(1) N E - S W striking l ineaments - -To this group belong the strongest lineaments, and is best char- acterized by L1 which is the expression of the Plasencia fault. L2 lying to the north and L3, L4 to the south are almost certainly structurally related to L1. A second subgroup, comprised of L5, L6 and L23, fan outward from the southernmost

tip of Spain in the vicinity of Algeciras (AI). The two first ones are particularly extensive, crossing the whole of the peninsula to terminate at the extreme northeast (Ebro Valley). All these sub- groups extend long distances across the country and intersect and cross many of the main structural elements of Spain.

(2) E N E - W S W striking l i neamen t s - -Th i s group is most strongly represented by L16 in the south, which represents a major discontinuity and produces a breakdown in the gross structural symmetry recognised earlier. L17 and L18 extend completely across central Spain, L19, L20 and L21 have similar strike directions in the northwest, crossing the Leon-Wes t Asturian zone and in the case of L21, curving northwards to terminate between Pamplona (P) and San Sebastian (SS) on the northern coastline.

(3) N W - S E striking l ineaments - -This group is

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

-10 ^ 44 ~ - - ~

57

3

36 ~ 36 -10

-0 -6 -4 -2 0

Fig. 2. Qualitative interpretation map from the residual field map shown in Fig. 1. The thick lines correspond to magnetic lineaments, the thin ones represent magnetic zones boundaries.

typified by L9, L10, L l l and L12. In many cases these lineaments form natural zone boundaries, i.e., they separate areas having distinctly different magnetic characteristics.

3.2. Magnetic zones

On the basis of the differences in magnetic character, Spain has been divided into several zones. Zones M1, M2 and M3 are to be found at the south of Spain and each continues across the Spanish-Portuguese border to the WNW. The first zone has been recognised by Julivert et al. [2] as the Ossa-Morena (O-M) zone (Fig. 3) and is one of several areas of longitudinal zonation which comprise the main elements of Spanish structure. It extends from beyond the border, in an ESE direction, by some 300 kin, terminating near the north of Granada (GR). The zone has one of the most distinctive magnetic signatures of the whole

survey, it is defined by a strong magnetic banding which takes up the regional strike. The short response wavelengths coupled with the large amplitudes indicate that the magnetic sources are relatively near-surface. The northern boundary, between this zone and the main body of the Central Iberian (C-I) zone is defined by a strong magnetic depression and it is suspected to be a feature of crustal dimensions. In the south and west the zone boundary is defined by a series of strong magnetic bands which define a sharp contact with zone M2. Further to the east, the zone boundary is more diffuse and constitutes the southern boundary of the Meseta, with the Guadalquivir basin lying to the south. This segment of the boundary is defined by L16, and is well observed in the magnetic data. Some of the lineaments that transect the zone produce offsets of zone boundaries which is taken to indicate that the faulting postdates the zone formation. An important feature of zone M1 is its

58 I. sOCiAS ET AL.

apparent extension to the ESE by some 60 km beyond the currently accepted southern limit of the Meseta, the whole of this boundary lies beneath the sedimentary cover of the Guadalquivir basin. In addition the line of this contact, usually taken as the Guadalquivir fault, is rotated clock- wise.

Zone M2 lies immediately to the south of M1 in the western part of the latter. This is the South-Portuguese (S-P) zone in which a pyrite belt is located. Compared toM1, the gross magnetic characteristics are of a relatively depressed and somewhat subdued magnetic response. The sharp northern boundary with M1 probably represents a faulted contact. In the south the boundary is more diffuse, and is probably related with L24.

Zone M3 is situated immediately to the north

of the western segment of M1 and it terminates against a magnetic lineament at its eastern limit. Magnetic responses within the zone are generally elevated and the major anomaly appears to be caused by a single magnetic unit. It is suspected that this anomaly may correspond to a local uplift of magnetic basement, possibly controlled by the strike fault lying along the boundary between M1 and M3. Structurally M3 forms part of the C-I zone.

Zone M4 is characterised by smooth, open contours and it may be considered as a transition zone between the O-M and the C-I zones. A number of weakly magnetic granites, of which the Pedroches batholith is the most representative, are found within M4, together with sequences of Cambrian metasediments.

8 ° 6 ° 4 ° 2 ° 0 o 2 °

H E R C Y N I A N

El BETICS

P Y R E N E E S

IBERIAN C H A I N 8 ° 6 ° 4 ° 2 ° 0 o 2 °

Fig. 3. M a i n geological d o m a i n s in the Ibe r i an Peninsula . Legend : ! = C a n t a b r i a n zone; 2 = N a r c e a a n t i f o r m ; 3 = Wes t A s t u r i a n -

Leon zone; 4 = Ollo de Sapo; 5 = Cen t ra l Ibe r ian zone; 6 = Pedroches bathol i th ; 7 = O s s a - M o r e n a zone; 8 = South Po r tugue se

zone; 9 = Ter t i a ry basins; 10 = external uni ts o f the Betics; l l = in ternal uni ts o f the Betics; 12 = Sier ra N e v a d a and Sier ra de los

Fi labres ; 13 = uni ts o f the G i b r a l t a r arc; 14 = Pyrenees ; 15 = Ibe r i an Chain .

I N T E R P R E T A T I O N O F A N A E R O M A O N E T I C S U R V E Y O F T H E S P A N I S H M A I N L A N D 59

Zone M5 is considered as the southern branch of the C-I zone. It is characterised by magnetic units and foliation, which trend W N W - E S E in the west and central part of the zone, whereas in the east they swing towards the north. Magnetic banding is particularly strong in the southeast, in the central region a series of basic volcanics has been mapped corresponding with a region of elevated magnetic responses. In the extreme west, the zone curves around the western limit of the Madrid basin to form the northern branch of the C-I zone.

It is considered that MS, M7 and M12 are structurally linked despite their different magnetic character, it is supposed that these three zones constitute an area where granitic basement outcrops, or is close to the surface. In particular, M7 is the magnetic signature of the Sierra de Gredos, mapped as a post-Hercynian granite, and part of the Central System. The three zones appear to be the central Hercynian core o f Spain.

Zone M6 in its southern limit represents the geophysical expression of the Madrid basin, as can be expected in an area covered by sediments. The magnetic contours are smooth and open indicating both the weakly magnetic character of the sediments and the considerable depth to the basement.

Magnetically M6 remains open to the north and northwest and corresponds to a part of the Duero basin; here, the magnetic field is not as smooth as over the Madrid basin, and this may indicate a thinning of the sedimentary section.

The nature of the anomalies of M l l suggests that the magnetic sources lie at depth beneath relatively non-magnetic sediments.

Zone M13 is a large area having a flat unvary- ing magnetic response that includes sedimentary basins. The zone bifurcates around a more magnetic area which is taken as the southern branch of the Iberian chain (M14).

The Ebro basin, M15, has perhaps the most subdued magnetic signature of the whole survey and confirms the non-magnetic nature of the sedimentary infill. In addition the flat, open nature of the responses suggests either that the magnetic basement here is extremely deep a n d / o r relatively weakly magnetic.

The Precambrian core of the Ollo de Sapo antiform (M16) is one of the strongest and most

extensive magnetically active area of the survey. The zone is intersected by a number of magnetic lineaments with a dominant E - W trend and which have produced an offset in the zone boundaries, presumably indicating late-stage faulting. The magnetic data indicate a degree of continuity and uniformity that is not evident in the geology and further suggest that it truncates abruptly in the south. It is suspected that it may represent Hercynian granites which crop out extensively.

Zone M17 represents the inner zone of the West Astur ian-Leon complex, it remains magnetically active but is much more subdued in response amplitude than M16.

Zone M18 is closely related to M17 in magnetic character but exhibits a greater degree of magnetic differentiation which is probably related to the proximity of the Narcea antiform immediately to the east (M19). This core has a very strong curva- ture about an E - W axis. The southern limb continues to the east beyond its mapped limits and beneath Tertiary cover. It is suspected that this trend continues across country, swinging toward the ESE, and may be structurally related to the formation of the C-I chain.

M20 is the main core of the Asturian arc, named the Cantabrian zone by Julivert et al. [2]. Its magnetic signature is one of a smooth magnetic gradient with values decreasing to the south as the Narcea antiform is approached, which suggests an extensive thickness of weakly magnetic sediments overlying the basement.

Zone M22 is the strongest and most extensive magnetic anomaly mapped. The broad similarity w i t h M 2 0 (and its suspected extension offshore) gives rise to the possibility that the two zones are structurally linked to the west in the Bay of Bi- scay. The only reasonable interpretation of this discrete magnetic response is that it may be due to a very large, deeply buried basic intrusion, probably of basaltic origin. The smooth nature of the contours suggests that the overlying strata are only weakly magnetic and probably represent the sedimentary infill of the Cantabrian basin.

M8 may be considered a continuation of M4 and is characterised by an open magnetic signature with relatively few magnetic trends. The outline of this zone corresponds closely with the pre-Betic unit of the Betic Cordillera. If magnetic crystalline basement exists here, then its depth is

60 L sOCiAS ET AL.

probably several kilometers beneath weakly magnetic sediments. In the southeast, L7 (the Alhama de Murcia fault) forms the boundary with M9.

Zone M9 exhibits a distinct increase in magnetic activity. It is correlated with the Betic zone, and is comprised of metamorphic Hercynian basement incorporated into the Alpine structures. The dominant trends are E -W, associated with Sierra Nevada and Sierra de los Filabres, to change abruptly across L8 to NE-S W. Magnetic signatures are particularly strong in the south and occur on the other side of the Gibral tar Strait (Rift Cordillera).

M10 is an area of relatively smooth, large wave- length, high amplitude magnetic anomalies. This zone encloses three distinct geological structures: the Guadalquivir basin, the sub-Betic zone and the Tertiary rocks of the Campo de Gibraltar. The shape of these units suggests deep-seated structures. The gravimetric study of Bonini et al. [3] indicates that M10 produces a strong negative Bouguer anomaly.

4. Quantitative interpretation

The modelling philosophy is based upon the assumption that Spain is underlain by a magnetic basement with uniform magnetic properties and which outcrops at the Hercynian granite core around Gredos Mountains. This is certainly a broad assumption since it has only been estab- lished that magnetic granite outcrops in west- central Spain. There are, however, other investigations which support this assumption: the results of seismic refraction profiles by Banda et al. [4-6], Surinach and Vegas [7], Cordoba et al. [8] and Payo [9], and the data obtained from a series of boreholes which has been drilled in various parts of Spain. However, the depth to magnetic basement is not necessarily synonymous with the depth to crystalline basement. It is in fact speculated that the deeper-sourced magnetic response may derive from below the surface of the Paleozoic crystalline basement and that the low-velocity (granitic) layer, as recognised by Banda et al. [4-6] and Payo [9], is the more probable cause of the anomalies.

There are many restrictive factors which are sources of uncertainty in the quantitative interpretation: the availability of geophysical control,

magnetic sources that occur at intermediate depths, the assumption that the deep magnetic basement has a uniform induced magnetization and that the anomalies results from the topographic relief of this basement: the implicit assumption of the two-dimensional modelling program used in this paper (MAGIX, Interpex Limited, Golden, USA, 1988). All these factors may reduce the certainty of the modelling and it is therefore better to consider the results in a statistical manner, or to be relative to one another on an arbitrary depth scale.

A total of 24 sections have been modelled, each of them selected on the basis of the questions and hypotheses raised during the qualitative interpretation to obtain further information of the main structural elements. Here, only two of the modelled sections will be discussed.

Section A-A' is a segment of a N - S flight line (Fig. 4) and B-B' a segment of an E - W tie-line (Fig. 5), see Fig. 3 for their location. Section A-A' was the calibration one, the start point being Sierra de Gredos as the outcropping basement. The model indicates that magnetic granite plunges from an outcrop elevation of 2000 m above sea level (a.s.1.) beneath the Madrid basin to a depth of 4500 m below sea level (b.s.1.). The southern boundary of the basin corresponds to a localised uplift of about 1000 m, beyond which the level drops again to 5000 m across the C-I zone. As the junction of the C-I zone and the O-M zone is approached, the magnetic basement rises sharply to 2000 m b.s.1, and the body of the latter is modelled as a block of higher magnetic susceptibility sited on top of the basement. To the north of Gredos the magnetic basement plunges beneath the sediments of the Duero basin to a depth of 500 m b.s.1. Section B-B' begins over outcropping magnetic basement (zone M5), crosses L1, traverses the Madrid basin and terminates beyond the eastern zone boundary of M6. The section crosses A-A' at 335 km mark. In the west, the model suggests that the magnetic basement outcrops at around 500 m a.s.1, and plunges abruptly to 4000 m b.s.1, toward the east. L1 response is seen clearly in the data and is modelled as a thin vertical dyke (representing the basic material of the fault plane) of higher magnetic susceptibility than the basement. Thereafter, the basement grad- ually deepens to a maximum of 6000 m b.s.1, and

I N T E R P R E T A T I O N O F A N A E R O M A G N E T 1 C S U R V E Y OF T H E S P A N I S H M A I N L A N D 61

A - A' 1 3 0 0 ' ' ' ' '

<

":~ 1 2 5 0

~" 1 2 0 0 o . . , J ~ - ' ~ ° 3 Z < 1 1 5 0 ~r~,, z o . . a M o . e ,b~ri . . Mo.r,,, s '~7" D . . . .

. zooe L ~ o . ~ i zo.o ,~..,o o.e.,o. ,~.. , . 1 1 0 0 0 r , , - - -

1 - B I B' 2.

I

4- 0.002

5-

0. 7:

6.

C~ 9-

10 . . . . , . . . . r . . . . i . . . . , . . . . i • • •

4 1 0 0 4 2 0 0 4 3 0 0 4 4 0 0 4 5 0 0 4 6 0 0 4 7 0 0

D I S T A N C E ( K i n )

Fig. 4. Modelled N-S section (A-A'). The susceptibility contrast for the magnetic basement was 0.016 SI units. In this figure and Fig. 5, all interpreted depths are related to depth below the magnetic sensor, thus requiering the subtraction of 3000 m to relate them to

depth relative to sea level.

begins to rise again toward the end of the section. At the po in t of in tersect ion with A-A', both sec- t ions indicate a basement dep th of a round 4500 m b.s.1.

4.1. Results

The main results presented below const i tu te bo th in te rpola t ions and ex t rapo la t ions d rawn from an in tegra ted s tudy of the indiv idual magnet ic

sections. (1) In the most general terms, the model l ing

lends conf i rmat ion to the s t ructural overview, in terms of the existence of a l imited central core to the Meseta and s t ructura l closure to the north, south and southeast . Certainly, the hypothesis that the b road-sca le magnet ic features relate to deep- seated s tructural var ia t ions is val idated.

(2) It is conf i rmed that the magnet ic basement ou tc rops extensively at the centra l core of the Meseta . F u r t h e r m o r e ou tc rops are suspected within the core of the Ollo de Sapo an t i fo rm and benea th the most westerly par ts of the Narcea ant i form.

(3) Southward and southeas tward from the Sierra de G r e d o s the magnet ic basement deepens

to 2500 5000 m b.s.1, benea th the M a d r i d basin. The southern b o u n d a r y of the basin, which corresponds app rox ima te ly with the To ledo M o u n t a i n range in its centra l par t , coincides with a fur ther d rop in basement level into the body of the C-I zone.

(4) Magnet ic basement benea th the C-I zone lies between 5000 and 6000 m b.s.1. F o r much of this zone the basement is over la in by shor t -wave- length magnet ic responses which der ive f rom shallow C a m b r i a n metasediments .

(5) Between the C-I and the O - M zones lies an in te rmedia te zone, cal led the Trans i t ion zone, which has no dis t inct ive express ion ref lect ion in previous studies of gross s tructure. Qual i ta t ive ly the zone is charac ter i sed by a smooth re turn to background magnet ics levels f rom the O-M zone to the south. I t is loca ted over a series of weakly magnet ic grani tes and gran i to ids inc luding the P e d r o c h e s b a t h o l i t h , the M e r i d a a n d the Montanchez granites. Mode l l ed dep th to magnet ic basement benea th M4 is be tween 5000 and 7000 m b.s.1, and it paral le ls the O-M zone a long its entire strike.

(6) The physical b o u n d a r y be tween the t ransi- t ion and the O-M zones is mode l l ed as an ab rup t

62 1. sOCiAS ET AL.

uplift in magnetic basement from about 5000-6000 m to 2500-3000 m b.s.1. The O-M zone conforms with a complex series of supra-basement structures with enhanced magnetic susceptibility that are close to the present-day surface.

(7) Beyond the O-M zone, to the south and southeast of the central core of the Meseta, there is a very distinct hiatus in the deep structural trends that appear, at least in part, to be controlled by lineament L16. This represents perhaps the most complex and interesting structural aspect of the Peninsula. The qualitative study indicates, in the west, a normal strike junction with the S-P zone. In its central part, the junction between M1 and M10 is sharply discontinuous, reflecting the O-M zone to the north and the Guadalquivir basin to the south. The magnetic boundary here lies well to the south of its geological and structural coun- terparts indicating a termination of the O-M zone beneath the Guadalquivir basin. In addition, the boundary is considerably rotated to the south, in direct disagreement with structural relationships in this area.

The junction between the O-M and the S-P zones corresponds to a (faulted) depression of the magnetic basement down to about 7000 m from

3000 m b.s.1, beneath the O-M zone. This is partially confirmed by Strauss et al. [10]. Further eastward, where the boundary is coincident with the Guadalquivir basin margin, there is not a distinctive increase in basement depth. However, the magnetic texture and the forward modelling both suggest very considerable depths to magnetic sources, probably well below the crystalline basement level. The Sevilla anomaly is modelled as having a depth of around 7000 m b.s.1.

Towards the southeastern limits of the O-M zone, magnetic lineament L16 corresponds to an abrupt change in magnetic strike from W N W - E S E in zone M1, to E W in zone M9. Despite the strike rotation, magnetic similarities exist across L16. The Sierra Nevada models as a basement upthrust from 5000 m to 1000 m b.s.1. In the extreme east of zone M9, magnetic trends undergo yet a further rotation to the NE SW corresponding to the Carboneras, Palomares and Murcia fault systems that are important structural elements of the eastern Betic transcurrent shear zone. This southeastern part of Spain, to the south of L16 and to the east of the S-P zone, is of very considerable structural significance, corresponding to the convergence of three distinct magnetic trend

1240 <

1230 m

1220 0 Z < 1210 Z

1200 0

10

15

100

B - g ' I I I l I

, " V - s - ' - granite v L/ Madrid Basin

w ] i I . . . . . I I l E

l i / \ ~ /t

o

200 300 4 0 0 500 600

D I S T A N C E ( K i n )

Fig. 5. Model led section ( B - B ' ) is a segment of the E - W tie line (for details see capt ion top Fig. 4).

700

I N T E R P R E T A T I O N OF A N A E R O M A G N E T 1 C S U R V E Y OF T H E SPANISH M A I N L A N D 63

directions. This has been historically recognised as evidenced by the number of deep seismic investigations and earthquake statistical studies.

(8) To the north of the central core of the Meseta, forward modelling generally confirms the shallow nature of the Duero basin (500-1000 m b.s.1.) to the north of Madrid. Structures across the Duero basin are generally in a WNW ESE direction, forming the northern limbs of gross structural symmetry. These trends generally reflect faulted variations in the depth to magnetic basement.

(9) Magnetic zone M l l is of particular interest because it appears to reflect substantial variations in the depth to magnetic basement about a N S axis (the same trend direction as the Altamira hills). Modelling shows that multiple faulting in M l l steps the magnetic basement down from 1500 m to 5500 m to 4000 m b.s.1., giving maximum depths exceeding those beneath the Madrid basin.

(10) The Iberian chain in central Spain (M19) corresponds to an upward-faulted block, with a second block fault related to zone M14 to the southwest. Zone M13, which encompasses both of these zones, corresponds in its southern parts to the Almazan basin and magnetic basement here lies at 3000-4000 m b.s.l. M19 is interpreted to continue to the west-northwest where it links structurally with the Narcea antiform.

(11) Depths to magnetic basement beneath the Ebro basin range from 3000 m to 6000 m b.s.l., approaching 1000-2000 m towards the Pyrenees and 2500-3000 m beneath the Iberian chain. In the eastern parts of the basin, depth to basement reduces to around 2500 m b.s.1, in the area of Lerida.

(12) The area around Bilbao is dominated by a large dipolar magnetic anomaly whose causative body is modelled at a depth of about 7000 m b.s.l. The smoothness of the response, particularly along the southern flank of the anomaly, indicates that the overlying section, whether it be sediment, crystalline basement or a combination of these, is both weakly magnetic and magnetically homoge- neous.

(13) Beneath the Cantabrian zone extension, depths to magnetic basement reach 2500 m b.s.l., rising to 800 m a.s.1, over the Narcea antiform before falling again to 1000 m b.s.1, over the northern part of the Duero basin.

5. Conclusions

The aeromagnetic survey provides a new geophysical perspective of the geology of the Spanish mainland and its interpretation gives additional insights to the main structural elements. The study makes no claim to be definite. The size of the dataset, coupled with the large area of investiga- tion and the structural complexity, have necessi- tated a certain degree of superficial treatment.

It is evident from the above discussions and comments that much work remains to be done if the maximum possible information is to be ex- tracted from the aeromagnetic data. In many aspects the present interpretation must be considered as a starting point for more detailed studies in the future.

The main conclusions of this study are: (1) Most immediately apparent are a series of

relatively isolated magnetic anomalies with large amplitudes, they are typified by smooth magnetic contours which suggest a considerably large depth to the magnetic source.

(2) A belt of high and variable magnetic activity extends across much of southern Spain, from the Portuguese border in the vicinity of Badajoz (BA) in an east-southeast direction to Jaen (J).

(3) In direct contrast to the trend in (2), in the extreme southeast of the country (Almeria (Al)/Alicante (A)), the magnetic response is strongly depressed in a N E - S W direction, i.e., almost orthogonal to that in (2).

(4) Along the southern coastline (with exception to the extreme east), magnetic trends have a strong E - W component. Furthermore, in the extreme south and southwest the magnetic character is markedly different and suggests a considerably increased depth to magnetic source. Similar comments also apply along parts of the northern coastline.

(5) Magnetic activity increases markedly in the extreme northeast and southeast, corresponding respectively to the Pyrenean and Betic mountain chains.

(6) In west-central Spain, two strong, sub-paral- lel magnetic lineaments cut across the regional structures, trending approximately N E - S W .

(7) In general terms, with exception of the south of the country, magnetic activity decreases in an easterly direction.

64 1. SOCIAS E T A L

(8) There are four main areas subdued to magnetic activity: the Ebro valley, the Maestrazgo

basin, the Madrid basin and the area west of

Alicante. (9) There appears to be a certain symmetry

about the aeromagnet ic responses in the nor th and

south of the country, possibly associated with the generally acknowledged structural uni ts of Spain.

Arcuate magnet ic trends, often persisting over many hundreds of line kilometers, indicate a gross formal symmetry about an axis which trends approximately NG0°W through the Gredos Moun- tains in west-central Spain. The persistence of these trends and the m a n n e r in which they often cut across mapped geology, substant ia te the belief that, for the most part, they represent the responses of deep-seated structures which are essen- tially of tectonic origin. This structure is recog-

nised in the structural in terpre ta t ion advanced by Julivert et al. [2], the major difference between the

two interpreta t ions is the apparent structural closure of these tectonic pat terns towards the southeast of Spain. Thus, from the magnet ic point of view, the Spanish part of the Iber ian peninsula appears to be domina ted by a series of sub-circular structures about an ESE W N W axis,

centered upon the Gredos Moun ta ins and remaining open in the axial direction into Portugal. At the outer margins of this major structural circula- tion, the symmetry is broken on several fronts. In the extreme northwest, the strongly curved aspect of the Leon West Astur ian zone, with the Can tab r i an zone at its core, is strongly reflected in the aeromagnet ic data. Julivert et al. [2] suggest that this structural componen t links directly with

the Armor ican massif in northwestern France, the break between the two being a direct consequence of the separation of Spain from the Eurasian plate accompanied by the opening of the Bay of Biscay.

In the south of Spain the aeromagnet ic data suggest the presence of a major tectonic unconfor- mity, a very dist inctive dislocation separates the ESE-t rending O-M and S-P zones from the E - W - t rending Betic zone to the south. In the extreme west, this d iscont inui ty locates approximately with the nor thern bounda ry of the Guada lqu iv i r basin.

However, the direct ion of its t rend across Spain has a much stronger E - W componen t than the mapped location of the bounda ry and it is evident

that the O-M zone in par t icular cont inues considerably beyond its mapped limits to the east-southeast. It is inferred that the d iscont inui ty lies at

depth, beneath the sedimentary infill of the Guada lqu iv i r basin and, in part, beneath the pre-

sently mapped posi t ion of the sub-Betic zone. It is interest ing to note that the gross s tructural symmetry of Spain, as suggested before, even extends to the nor thern and southern coastlines where the magnet ic responses are somewhat similar in character and these appear to be related to strongly arcuate tectonic fold pat terns, both of which lie along strike from major Alpine fronts.

Acknowledgements

We wish to thank the reviewers for their con- structive critical comments .

References

1 J.A. Ardizone, J. Mezcua and I. Socias, Mapa AeromagnGtico de la Espafia Peninsular, 29 pp., Instituto Geografico Nacional (MOPU), Madrid, 1989.

2 M. Julivert, F.J. Martinez and A. Ribeiro, The Iberian segment of the European Hercynian Foltbelt, Colloq. C6, 26th Int. Geol. Congr., Geology of Europe from Pre- cambrian to the Post-Hercynian Sedimentary Basins, Mem. B.R.G.M. 108, 132-158, 1980.

3 W.E. Bonini, T.P. Loomis and J.D. Robertson, Gravity anomalies, uhramafic intrusions and the tectonics of the region around the Strait of Gibraltar, J. Geophys. Res. 78, 1372-1382, 1973.

4 E. Banda and J. Ansorge, Crustal structure under the central and eastern part of the Betic Cordillera, Geophys. J.R. Astron. Soc. 63, 515-532, 1980.

5 E. Banda, E. Surinach, A. Aparicio, J. Sierra and E. Ruiz de la Parte, Crustal and upper mantle structure of the central Iberian Meseta, Geophys. J.R. Astron. Soc. 67, 779-789, 1981.

6 E. Banda, Crustal parameters in the Iberian Peninsula, Phys. Earth Planet. Inter. 5l, 226-234, 1988.

7 E. Surinach and R. Vegas, Lateral inhomogeneities of the Hercynian crust in Central Spain, Phys. Earth Planet. Inter. 51, 226-234, 1988.

8 D. Cordoba, E. Banda and J. Ansorge, P-wave velocity depth distribution in the Hercynian crust of northwest Spain, Phys. Earth Planet. Inter. 51, 235-248, 1988

9 G. Payo, Structure of the Crust and Upper Mantle in the Iberian Shield by means of a long period triangular array, Geophys. J.R. Astron. Soc. 20, 493 508, 1977.

10 G.K. Strauss and J. Madel, Geology of massive sulphide deposits in the Spanish-Potuguese Pyrite Belt, Geol. Rundsch. 63, 191-211, 1974.

interpretation of an aeromagnetic survey of the spanish mainland

Documents