Interpretation of magnetic anomalies in the volcanic area of northeastern Spain

201

~ecr~no~~ysics, 192 (1991) 201-210 Elsevier Science Publishers B.V., Amsterdam

Interpretation of magnetic anomalies in the volcanic area of northeaster Spain H.J. Zeyen a**, E. Banda b and E. KlingelC ’ a Servei Geoldgic de Catalunya, Diputaci6 92, 08015 Barcelona, Spain b Institut Jaume Almera, C.S. I. C., Marti i Franquss s/n, 08028 Barcelona, Spain ’ Institut ftir Geophysik, ETH Hiinggerberg, 8093 Zurich, Switzerland (Received by publisher July 24,1989)

ABSTRACT Zeyen, H.J., Banda, E. and Klingele, E., 1991. Interpretation of magnetic anomalies in the volcanic area of northeastem Spain. In: P. Wasilewski and P.Hood (Editors), Magnetic Anomalies-Land and Sea. Tectonophysics, 192: 201-210. Data from an aeromagnetic survey carried out in northeastern Spain are used for the interpretation of magnetic anomalies in an area of Neogene and Quatemary volcanism. Two, tweed-a-h~f and three Dimensions models lead to the hypothesis that a larger area than that visible at the surface is affected by magmatic activity. In particuiar in the western part of the study a significant amount of basaltic rock seems to be located between the Paleozoic basement and about 2000-2500 m of Tertiary molasse sediments. Further east, basaltic rocks do not seem to be restricted to the areas where they outcrop but could have also fiied extensional fault planes active during the Neogene without having reached the surface. Gt!OlOgy

Intmduetion Between 1984 and 1986 the Geological Survey of Catalonia acquired aeromagnetic data over the area of northeastern Spain, which have been published in the form of a map of residual magnetic anomalies (Zeyen and Banda, 1988). Some of the most important anomalies are located in the eastern part of the area between 2”E, 41’40’N and 3”15’E,

42”15’N

in an area with outcropping

basaltic rocks (Figs. 1 and 2). With the interpretation of the magnetic data of this area we intend to define the area affected by magmatism. In this paper, a short description of the geology and the data acquisition is followed by 2; and 2D interpretation of three profiles and a 3D interpretation of the entire area. The results are briefly discussed and a geological interpretation is proposed.

* Now at:

Geophysics

Institute,

University

of Kadsruhe,

Hertzstrasse 16, Bau 42, 7500 Karlsruhe 21, FRG 0040-1951/91/$03.50

Q 1991 - Elsevier Science Publishers B.V.

The studied area is located in the southern foreland of the eastern Pyrenees, which consists of Hercynian basement rocks unconformably overlain by the Paleogene foreland basin depositional sequences. Neogene deposits occur as basin filling formed during an extensional regime (Fig. 1). In its northern part, the area covers the southernmost part of the Pyrenees, which are bounded to the south by the southern Pyrenean frontal thrust. Most of this thrust is obscured by Neogene deposits, although it is visible at the surface west of the town of Olot, where it separates Paleogene rocks, and in the east, near the Mediterranean coast, where Cretaceous deposits are in contact with Paleogene deposits (Fig. 1, at 515,000, 4,655,OOO m U.T.M). The Hercynian basement of the foreland is represented by a series of gneisses, schists and limestones of Cambro-Ordovician to Devonian age which are intruded by late Hercynian plutonic rocks of granitic to granodioritic composition. These basement rocks, together with the Paleo-

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H.J. ZEYEN

ET AL.

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Fig. 1. Tectonic map of the volcanic area of Catalonia. I = Paleozoic schists; 2 = Paieozoic acidic intrusions; :’ = Mesozoic and Tertiary sediments; 4 = Neogene and Quaternary graben and basin sediment filling; 5 = Neogene and Quaternary basaltic rocks;

4blOOOO

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460000 44OOOO

500000 480000

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Fig. 2. Residual magnetic anomaly map of the studied area. night level = 2500 m above sea level. Contours every 5 nT. Dashed contours = negative anomalies; continous contours = positive anomalies. Profiles I, II and III are interpreted using 2; and 2D methods.

MAGNETK

ANOMALIES

1N THE VOLCANIC

AREA

OF PJGRTH~ASTERN

gene series of the Ebro Basin, were deformed during the Pyrenean tectonic event by NE-SW strike-slip faults, thrusts and folds and now constitute the Catalan Coastal Range along the Mediterranean coast (southeastern part of Fig. 1) (Anadon et al., 1979). Two systems of Neogene extensional faults are superimposed on the Pyrenean compressional structures. NE-SW striking faults delimit grabens in the Catalan Coastal Range (VallCs Graben, Fig. 1) filled by continental Miocene deposits. This system is well documented all along the Mediterranean coast of the Ibexian peninsula and is probably related to the Western European Rift System (Rhine-Rhone graben). The predominant system in the study area is formed by NW-SE striking faults. These faults controlled the formation of Neogene basins mainly filled by Miocene, Pliocene and Quaternary deposits (La Selva and Empordh basins, Fig. 1). These faults facilitated intraplate basdtic volcanism between 10 m.y. and lO,ooO years (Araiia et al., 1983). The basalts, which have an alkaline to nephelinitic composition, outcrop at various locations in the eastern half of the area in form of dykes, dense lava flows and pyroclastic sediments (Fig. 1).

Aeromagnetic suruey The aeromagnetic data used in this paper were acquired during the summers of 1984 and 1985. Because of the rough and irregular topography, with altitudes ranging between sea level and 2100 m, the survey was flown at 2500 m above sea level, with N-S lines every 2.5 km and E-W tie lines every 10 km. We were aiming at a 1: 250,000 mapping scale. The data were reduced at epoch 1984.5 and a residual magnetic anomaly map was obtained following the techniques described by Klingele (1986) by substracting a plane whose expression is:

F=0.535*X+3.25*(Y-4000)+42,552 where X and Y are in kilometres (U.T.M.) and F is in nanoTeslas.

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203

Fig. 2 shows the area of interest for this study. The estimated accuracy of the map is +2.5 nT. Further details of the survey have been published by Zeyen and Banda (1988). Magnetic susceptibility measurements

In conjunction with the aeromagnetic survey the susceptibilities of a number of diferent lithologies were measured in the fieId using a SCINTREX CTU-2 magnetic susceptibility meter. Most of the rocks have susceptibilities below 300 * 10m6 SI. Only the basaltic volcanic rocks have considerable susceptibility: 5~-73~* lob6 SI for pyroelastic rocks and lO~~-I2,~ * 10s6 SI for compact basaltic rocks. These values have been used as a first approach in the models, but it should be noted that they might be too low, mainly because of weathering.

Only one of the major anomalies, near the town of Olot, can be associated with outcropping basaltic rocks. We therefore began the interpretation with 25 and 2D modelling of three profiles covering the other three strongest anomalies (Fig. 2). In order to make the 3D interpretation easier we reduced the data to the pole (Baranov, 1957) using an FFT algorithm. Then the field was continued downwards to a level of 1500 m above sea level (Fig. 3) using 2D filtering with the coefficients given by Henderson (1960). The 24 and 3D models were calculated using the formulas given by Plouff (1975). All models assume only induced magnetization, which seems to be justified by the asymmetry of the anomalies and, taking into account the results, by the geological interpretation which gives young volcanic rocks as the magnetic source. A probable remanent magnetization parallel to the present local field has not been considered because no data are available_

Profile I The shape of the magnetic anomaly indicates a

fairly deep source. Two dimensional

semi-auto-

H.J.ZEYEN ET AL.

204

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-7-J

427000

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500000

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Fig. 3. Residual

magnetic Contours

matic

interpretation

(Hartman

anomaly

4~0000

map of the studied

every 10 nT. Dashed

with

Werner

contours

area reduced = negative

deconvolution

et al., 1971) and power spectrum

analy-

sis (Treitel et al., 1971) gives a minimum depth about 1500 m below sea level (about 2000-2500

of m

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to the pole and continued anomalies;

continous

contours

downwards

to 1500 m above sea level.

= positive

anomalses.

extent of 5 km to both sides of the profile embedded in a larger body of lower susceptibility (7300 * 10e6 SI) extending 20 km to both sides. In addition, a weak but extended source (7800 * 10e6

below terrain surface). Moreover, Werner deconvolution indicates a dip of the top of the disturbing body to the south. The results of the 2:D interpretation are shown in Fig. 4. The body has a lateral extent of 4 km to both sides of the profile and a susceptibility of lOOO* lop6 SI with a

SI) is necessary linear source.

minimum

6). The two bodies have the same susceptibility (12,000 * 10e6 SI) extending 10 km to both sides of the profile. In order to conform to the mea-

depth of 1400 m below sea level.

Profile II This profile passes through a series of sharply peaked anomalies and crosses the marked linear E-W striking source in the north (Fig. 5). In order to explain the negative gradient to the north we had to introduce four small bodies with susceptibilities of 12,000-14,500* 10e6 SI and a lateral

to explain

the data

north

of the

Profile III This profile is intended as a model of the source of the linear anomalies in the north (Fig.

sured anomaly the northern edge of the source has been made to dip to the north at an angle of about 45”. North of the main anomaly, 2D interpretation using the analytical signal method (Nabighian, 1972) indicates the top of the magnetic basement at about 3000-3500 m below sea level.

205

(nT)

-10.00

4620000

-4000

4610000

4640000

4650000

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4640000

4650000

-

-6000 4620000

Fig. 4.2tD

model of profile I (see Fig. 2). Crosses = observed anomaly;

30 interpretation Because. the magnetic anomaly pattern is quite complicated and the execution time of computer programs is limited, the 3D model presented (Fig. 7) can only be a semi-quantitative one. Nevertheless, some conclusions can be drawn: (1) The strongest anomaly on the transformed map (Fig. 3), near Olot, is exactly coincident with the outcropping basaltic and pyroclastic rocks, which are concentrated in the central and eastern parts of the anomaly. On the basis of the measured susceptibility of 12,000* 10m6 SI, the volcanic rocks should have a m~mum thickness

continuous

7 cm) 4660000

line = calculated anomaly.

of about 4500 m in the centre of the area (body 4, Table 1 and Fig. 7b). (2) The smaller positive anomalies nearly all correspond to zones of outcropping basaltic and pyroclastic rocks (bodies 5-8, Table 1 and Fig. 7b). (3) The results of the 2+ D analysis along the three profiles are confirmed by the 3D model (bodies 1 and 9-12, Table 1 and Fig. 7b). (4) A fairly strong WNW-ESE striking anomaly in the eastern part of the area, at about 4,650,OOO m U.T.M. (bodies 13A and B, Table 1 and Fig. 7b), can be explained by a body similar to the northern linear anomaly.

-20.00

-I

462oooa

46~0000

466QOOO

-----m 4 TOOQOO

4680000

4060

flight

2000

tevet

topography

0 -2005 -4056 462Qr?QO Fig. 5.2$D

4610000

4660000

---ZZOSmf

model of profile II (see Fig. 2). Crosses = cibserved anomaly; ~untinttous line 3 calculated anomaly. Suwzptibilities of the bodies in 10-6 SI: I-7300; 2---13,200, 3--15,ooO: 4-13,200; 5-8200; 6--K!,OM%

the remairrder of ~at~o~a the entire area exhibits a positive magnetic background (see ayen and Banda, 1988) which can be .modelled by a fairly thin sheet of varying susceptibility in the subsurface t~ou~~~t the area (bodies 14-f6, Table I and Fig 7b)_ (5)

~(1~0000

~ornp~~with

me to the iderent properties of the ~~t~t~ field there is no unique inversion of the anomalies without use of additional i~o~ati~u. For example, the top and the sus~~b~~ of the source can be rn~~~ed for some of the bodies. Mowever, even this information may not be reliable because the form of the source might vary with depth and, above all, because the ~u~~tib~ty measured at the surface might have been influenced by weathering (and therefore not be representative of

the bas& at depth) and strong remarxent rna~e~zation parallel to the present Ioeal field might be present. Therefore the differences between the 2iD and 3D interpretations, as far as depth to the bottom of the bodies and their ass&md susceptibilities are concerned, are not con&lered to be ~~i~~t. There is some evidence for sus~tib~ties higher than 12,000 + lO+ SI. (An oil exploration drillhole (total depth 550 m) near profile III at m U.T.M. (I.G.M.E., 1987) penetrated basaltie rocks between 200 and 500 m below sea level.) It is possible to r~~nably adjust profile III, ~thou~ not as close as in the model presented in Fig. 6, using a sus~~~b~ty of 48,WO* IV6 SI and the depths indicated in the drillhole. Although there co&d be several basaltic: sheets below the bottom of the hole, in this case the susceptibility would be higher than the ap

MAGNETIC

ANOMALIES IN THE VOLCANIC AREA OF NORTHEASTERN

207

SPAIN

(nT)

-40.00 4b40000

7 cm) 410000Q

4680000

4660000

(ml 4000

.

2000

--

flight

level

topograpt

y

-

(m)

4700000 Fig. 6. 2;D model of profile III (see Fig. 2). Crosses = observed anomaly; continuous line = calculated anomaly.

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46iOOOO

parent susceptibi~ty. For the same reasons the thickness of the other bodies should not be taken as unique solutions. The thin sheet introduced to explain the magnetic background (bodies 14-16, Table 1) should not be interpreted as a homogen~us layer. It seems more reasonable to think of a large number of basaltic dykes, only a few of which having reached the surface. Several small isolated outcropping dykes in different tectonic units outside the grabens might strengthen this hypothesis. The most surprising anomaly is the one situated south of the town of Vie in the Ebro Basin (profile I, Fig. 4). Its location is quite distant from the known volcanic areas and the surface geology does not show any feature which could be related to the magnetic anomaly. Nevertheless, hydrogeological studies in irrigation wells (Fern?mdez et al., 1989) indicate a block structure of the basement

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near the southern edge of the Ebro Basin with strong vertical relative movements. Therefore, the easiest interpretation is to assume that basaltic sheets are embedded between the Paleozoic basement and the molasse sediments. The magma must have ascended along a fault zone separating two basement blocks. An alternative interpretation could be that the anomaly is produced by older rocks. However, the magnetic signature of the anomaly is identical to that produced by younger rocks exposed in the area, although it does differ from that of anomalies in other areas of the Ebro Basin associated with the Triassic rocks (Zeyen and Klingelt, 1988). Conclusions

The magnetic anomalies observed in the study area seem to be caused mainly by outcropping and

208

H ._I. ZEY

EN

ET AL.

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i

(a)

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I

7-

4 20000

460000 440000

503000 480000

!120000

4670000

4650000

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(b)

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J 420000

4 60000 440000

500000 480000

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Fig. 7. (a) Calculated anomalies corresponding to the 3D model shown in (b) at 1500 m above sea level, reduced to the pole. Contour interval is 10 nT. The bodies are right vertical prisms. For susceptibility

values and surface levels, see Table 1.

MAGNETIC

ANOMALIES

IN THE

VOLCANIC

AREA

OF NORTHEASTERN

TABLE 1 Vertical position and magnetic susceptibility making up the 3D model shown in Fig. 7b Body

Top (m)

Bottom (m)

IA 1B 2 3 4A 4B 4c 4D 5 6A 6B 7 8 9 10A IOB 11A 1lB 1lC 12A 12B 13A 13B 14A 14B 14C 14D 15A 15B 16

-1400

-2000 -4000 -loo0 -1000 -4000 0 -500 0 -400 -500 -500 -500 -300 -1000 - 1700 -1700 -1500 -1500 -1500 -500 -1000 - 1500 -1600 -1000 -1000 -1000 -1000 -1000 -IO&l -1500

-200a -500 -500 500 500 500 500 200 200 200 0 200 -500 0 0 200 100 0 200 -500 -100 200 -400 -300 -500 -600 -500 -200 -1000

of the bodies

Susceptibility (lo-6 SI) 12000 12000 lOf@o 8800 13800 12000 12OW 12000 12000 7500 7500 12000 7500 7500 12ax) 12000 15000 15OOi3 15000 11300 11300 7500 7500 15000 15000 15000 15000 7500 7500 7500

buried basaltic rocks. In the west a major anomaly is interpreted as a basaltic layer located between the Paleozoic basement and 2~-25~ m of Tertiary molasse sediments of the Ebro Basin, forming the western boundary of the area affected by the Neogene and Quaternary magmatic activity. In the north, this boundary seems to be marked by the volcanic area of Olot and some long dykes striking roughly E-W and dipping about 45’ to the north. The magnetic basement further north appears to occur below about 3000-3500 m of Tertiary and Quaternary sediments. In the east, magmatic rocks seem to occur at depth down to the coast. The southern boundary is marked by the surface volcanism of La Selva depression south

SPAIN

209

of the town of Girona. A positive magnetic background in the area leads to the assumption that the Paleozoic basement of the entire zone, and especially of the Neogene depressions and the southern edge of the Ebro Basin, is severely faulted. Basaltic magma has probably ascended along these faults, although it has not always reached the surface and has often been buried under younger sediments. Therefore, the Neogene and Quaternary volcanism seems to have a considerably greater extent and volume than that expected from surface geological observations. Acknowledgements We thank our colleagues of the Geological Survey of Catalonia, especially M. Barber&, T. Freixes and J.A. Muiioz, for their help in the geological interpretation, and M. Fernndez and M. Tome for useful discussions. This is contribution 585 of the Institute of Geophysics, ETH Zurich. References Anad6n, P., Colombo, F., Esteban, M., Matzo, M., Robles, S., Santanach, P. and Sole Sugraies, Ll., 1979. Evolution tectonoestratigrfifica de 10s Catalanides. Acta Geol. Hisp., 14: 242-270. Am&a, V., Aparicio, A., Martin Escorza, C., Garcia Cache, L., Ortiz, R., Vaquer, R., Barberi, F., Ferrara, G., Albert, J. and Gassiot, X., 1983. El volcanismo ner5genoxuaternario de Catalunya: Caracteres estructurales, petrologicos y geodin&rnicos. Acta Geol. Hisp., 18: 1-17. Baranov, V., 1957. A new method for interpretation of aeromagnetic maps: Pseudo-gravimetric anomalies. Geophysics, 22: 359-383. FemZmdez, M., Freixes, A., Bosch, X., Berastegui, X. and Banda, E., 1989. Thermometry and hydrog~he~s~ of the southern border of the South Pyrenean foreland basin. In: K. Louwrier et al. (Editors), European Geothermal Update. Kluwer, Dordrecht, pp. 522-531. Hartman, R.R., Teskey, D.J. and Friedberg, J.L., 1971. A system for rapid digital aeromagnetic interpretation. Geophysics, 36: 891-918. Henderson, R.G., 1960. A comprehensive system of automatic imputation in magnetic and gravimetry inte~retation. Geophysics, 25: 569-585. I.G.M.E., 1987. Contribution de la Exploraci6n Petrollfera al Conocimiento de la Gwlogla de Espaha. I.G.M.E., Madrid, 465 PP. Klingelt, E., 1986. Les levb dromagnttiques Geod.-Gwphys. Arb. Schweiz, 37: 69 pp.

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Nabighian, M.N., 1972. The analytic signal of two dimensional magnetic bodies with polygonal cross section: Its properties and use for automated anomaly interpretation.

Geophysics,

grams to compute

determination of depths to buried magnetic basement rocks. Geophys. J. R. Astron. Sot.. 24: 415-428 Zeyen, H.J. and Banda, E., 1988. Aeromagnetic reconnaissance of Catalonia, Spain. Fist

37: 507-517. Plouff, D., 1975. Derivation of formulas and FORTRAN magnetic

anomalies

of prisms.

proU.S.

Geol. Surv. Rep. GD 75-014. Treitel, S., Clement, W.G. and KnauI, R.K., 1971. The spectral

ET AL.

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Break, 6: 217-222. E., 1988. Application

of different

techniques to the magnetic anomaly of Lleida,

NE Spain. Ann. Geophys., 6 (Abstr.).