Crustal structure of the Balearic sea

Tectonophysics, 20 (1973) 295-302 0 Elsevier Scientific Publishing Company,

CRUSTAL

STRUCTURE

Amsterdam

OF THE

- Printed

BALEARIC

in The Netherlands

SEA

K. HINZ Bundesanstalt fiir Bodenforschung, (Received

November

Hannover (Germany)

1, 1971)

ABSTRACT Hinz, K., 1973. Crustal structure of the Balearic Sea. In: S. Mueller Earth’s Crust, based on Seismic Data. Tectonophysics, 20 (l-4): Within the frame vestigated north and For the southern ing of a 4.0 km thick Vp = 2.35 (km/set)

(Editor), The Structure of the 295-302.

of the German-French project ANNA-1970, two long refraction profiles were insouth of the island of Majorca. Balearic Basin an oceanic crust can be derived from the travel-time curves consistCenozoic sedimentary layer with: + 0.35 (set-‘)

X Z (km)

and a 5 km thick layer with: VP = 4.0 (km/set)

+ 0.28 (set-‘)

X Z (km)

The transition to the upper mantle takes place at a depth of 12 km. Directly south of Majorca thickening was measured which may be caused by the process of crustal shortening. In the northern Balearic Basin a faulted transitional type of crust has been observed indicating ably an embryonic and juvenile ocean expansion.

a crustal prob-

INTRODUCTION

Within the framework of the France-German geophysical research project ANNA, two seismic refraction profiles were investigated in the northern and southern part of the Balearic Sea in 1970. In the following, only a brief summary of the marine seismic results obtained by the Bundesanstalt fur Bodenforschung, Hannover, is given. A comprehensive consideration of all seismic records of the project ANNA will be given at a later date.

TECHNIQUE

AND INTERPRETATION

In the seismic recording floating telemetry buoys were used. The seismic records set up as reduced seismogram sections were interpreted according to the Wiechert-Herglotz method after the seismic arrivals had been fixed and correlated, assuming that the velocity is only a function of depth. These velocity-depth values were then

I(. HINZ

296

used to calculate a seismic model by the trapezoid method (Stein, 1968). The seismic model was varied so that the theoretical

travel-time curve fitted the measured one well enough.

RESULTS OF PROFILE ANNA III

The seismic refraction

investigations

were carried out along two N-S running lines: the

line ANNA III, a split profile lying south of the isle of Majorca. and line ANNA II, a reversed profile lying north of Majorca (Fig.’ 1). Fig. 2 shows the seismic crustal mode1 developed for the refraction profile ANNA III. The calculated travel-time branches correspond well with the measured data. Below the water a thin sedimentary cover follows: layer 2 of about 0.5 km thickness. The underlying rock complex (layer 3 ) is, south of position A, 4 km thick. Within this layer the velocity increases

I

d oftjtmmwich

Fig. 1. Location map of the Balearic Sea.

CRUSTAL

STRUCTURE

OF THE BALEARIC

297

SEA

T-$ [set]

S

N

--5

,4

ANNA

profile

/,/7~

III

- - -

--3

B-

Fig. 2. Crustal

section

pos;i On

position 20

10

of the southern

with depth: V = 2.35 (km/set)

Balearic

ralcu~d~vdiime curve obsem?dimml time cuwe

Basin (Profile

+ 0.35 (set-‘)

3l

4okrn

ANNA III). Seismic velocities

C

arc indicated.

XZ (km). North of position A, the thickness

of layer 3 varies considerably. The marked anomaly in the section can be well explained by a thickening of the underlying layer 4. From Glomar Challenger results (“Summary of Deep Sea Drilling Project - Leg XIII”, University of California, Report, 1970) it seems justified to assume that layer 3 is predominantly composed of Tertiary sediments. The thickness of the underlying

layer 4 is about 5 km, south of position A. Within this

layer the velocity increases with depth: V= 4.0 (kmisec) + 0.28 (set-‘) x 2 (km). The tranjition to ultrabasic mantle rocks of V, > 8 km/set takes place at a depth of 12 km in the southern part of profile ANNA III. North of position A, layer 4 seems to pass continuously into the upper mantle. A discontinuity has not yet been observed. Discussion

Within the region of the Balearic Isles, little is known about the interrelation of velocity. rock type, and stratigraphy, respectively. So there is a wide scope for a geological-petrographical interpretation of the seismically developed model: Fig. 3 shows an attempt of a schematic geological-petrographical interpretation. In the southern part of profile ANNA [II

K. HlNZ model B

S

I

deformed sediments

seismic layer 6; a

Fig. 3. Schematic

and partial1 metamorphosed fvr, * v) - 6. I kmhl

geological-petrographical

a crustal structure

interpretation

of section

ANNA III.

close to an oceanic one was found, consisting of a 3.5 km thick presum-

ably Cenozoic sedimentary series, a 5 km thick oceanic crust with velocities of 6-7.4 km/set, and ultrabasic mantIe material lying at a depth of about 12 km. Near the island of Majorca, i.e., north of position A, the thickness of the Cenozoic sediments varies considerably. Below the Tertiary here velocities of 4.6-6 km/set were observed. As even Paleozoic (Hollister, 1934) occurs in the Balearic island of Menorca, this part of layer 4 has been interpreted as deformed, partly metamorphosed Mesozoic to Paleozoic rocks. The deeper part, the velocities of which are increasing with depth from 6.2 to 7.8 km/see, is assumed to represent relicts of oceanic crust, with basic intrusives and extrusives. In terms of plate tectonics (Dewey and Bird, 1970) the following mechanism may be set up for the geological model: Associated with the opening of the Atlantic Ocean, the oceanic lithosphere descended, causing compression of the oceanic crust, formation of a belt composed of basic rocks and compression of sediments of the former continental rise. This process finally leads to the formation of an orogene within the region of the Balearic Isles.

CRUSTAL

STRUCTURE

s

OF THE BALEARIC

ANNA profile

8-A

299

SEA

II

__.__._. “_. “._. he

lOOhm

a0

15-

-15 v=4.0+0.794z

6.6 km/s 22

t

-

2oi

ZQkm

Fig. 4. Crustal

RESULTS

section

of the northern

OF PROFILE

Balearic

Basin (Profile

II). Seismic velocities

are indicated.

ANNA II

Fig. 4 shows in the lower part the seismic crustal model developed for profile ANNA II. In the upper part of this figure, both the travel-time curves observed and those calculated for the model are given. In the northern Balearic Basin, within the region of refraction profile ANNA II, the water layer is underlain by a presumably Pliocene-Quaternary sedimentary layer (layer 2)*. The underlying layer 3 has a thickness of 4 km and velocities of 4.3-S km/set. At the northern end of the profile, layer 3, which according to Montadert Tertiary sediments, has become reduced in thickness.

et al. (1970) is mainly built up of

As granites are exposed in the coastal region east of Barcelona, the upper part of layer 4 with velocities of 5.9-6.3 km/set could be interpreted as acid crystalline and metamorphic rocks. According to Ringwood and Green (1966), intermediate to acid rocks (typical for continental shields) have velocities within this range, indicating that here the crust is con-

* Confirmed

by deep-sea

drilling.

See next paper by Hsii and Ryan,

pp. 303-306

(Editor)

300

tinental.

K. IfIN

In the central part of the northern

Balearic Basin, granitoid rocks constitute

a

much smaller part of the crust. f_Iere the thickness of layer 4 is reduced by at least 5 km. Below a depth of 14 km, rocks already occur with velocities of 6.7-7.5 according to investigations

by Engel and Engel (1969), Christensen

and Green (1966) represent basic to ultrabasic So the seismic crustal model determined

km/set. which

(1970) and Ringwood

rocks.

for the central part of the northern

Balearic

Basin can be neither designated as continental, owing to the small thickness of the crust, nor as oceanic due to the high portion of granitoid rocks. At the northern end of the profile, the surface of layer 4 lies at a depth of 4 km; at 90 km it is depressed by 4 km. Within layer 4 the velocity increases with depth from 5.9 to 6.6 km/set. For the first 90 km of the profile, layer 4 has an average thickness of 8 km and is slightly upwarped. Between 90 and 110 km layer 4 has a thickness of at least 13 km. The underlying rock complex has velocities of more than 6.7 km/set. The seismic crustal model derived for the northern Balearic Basin within the region of refraction

profile ANNA II, considerably

deviates from the model set up for the southern

Balearic Basin. A striking feature is that in the northern Balearic Basin rocks of velocities of 5.9-6.3 km/set (typical for granitoid rocks) form a greater part of the crust than in the southern

Balearic Basin.

Discussion An attempt of a schematic geological interpretation northern Balearic Basin is shown in Fig. 5.

Fig. 5. Schematic geological-petrographical

interpretation

of the seismic crustal model for the

of section ANNA II.

CRUSTAL

STRUCTURE

OF THE BALEARIC

301

SEA

The seismic layer 2 may be classified as Pliocene-Quaternary

and layer 3 as mainly Ter-

tiary. It is an abnormal to intermediate

crust, which, compared with continents,

upper crust and comparatively

is marked by a thinned

high upwelled basic to ultrabasic

acid

rocks. It

is therefore suggested that in former times the Balearic Isles lay nearer to the Spanish mainland, thus forming a direct link between the Betic Cordillera of South Spain and the eastern Pyrenees. The formation of the abnormal type of crust is attributed to the Balearic block’s separation and drifting off from the Spanish continent. This process in our opinion led to the development of a field of tension within the range of the Betic-Balearic orogene (Stille. 1934; Wunderlich,

1969) and to a fracturing

of the continental

crust along with upwelling

of ultrabasic mantle material, from which basaltic melts have both intruded and extruded into the fractured upper crust. With the gradual cooling of the ultrabasic mantle material, fractional crystallization took place, including the formation of gabbroic rocks and the production of granite and diorite differentiates. The abnormal seismic crust under the present northern Balearic Basin is interpreted as a mixed crust composed of juvenile rocks from the earth’s mantle, and continental fragments.

crustal

ACKNOWLEDGEMENTS

The author is gratefully indebted

to Corn. Alinat, Prof. Dr. H. Closs, Dr. 0. Leenhardt,

Prof. Dr. H. Menzel and to the Deutsche Forschungsgemeinschaft for promoting and supporting this project. Thanks are also due to participating colleagues from IPE (Institut fiir die Physik des ErdkGrpers, Hamburg), MOM (Mu&e ocCanographique Monaco) and the Bundesanstalt fiir Bodenforschung (Hannover), especially to the shooting crew.

REFERENCES

Christensen. N.I., 1970. Composition

and evolution of the crust. Mar. Geol., 8(2): 139- 155. Dewey, SF. and Bird, M.S., 1970. Mountain belts and the new global tectonics. J. Geophys. Rex, 75(19): 2625-2647.

Dewey, S.F. and Horsfield, B., 1970. Plate tectonics, orogeny and continental growth. Nature, 225: 521-52s. Engel, A.E.S. and Engel, C.G., 1969. The rocks of the ocean floor. Morning Review Lectures, Int. Oceafiogr. Congr., UNESCO, 2nd, Moscow, 1961, pp. 161-187. Fahlquist, D.A., 1963. Seismic Refraction Measurements in the Western Mediterranean Sea. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 173 pp. Hollister, F.S., 1934. Die Stellung der Balearen im variscischen und alpinen Orogen. Abh. Ges. Wiss. GGttingen, Math.-Phys. Kl.. 3, Heft 10: 208 pp. Montadert, L., Sancho, .I., Fall, F.P., Debyser, J. and Winnock, E., 1970. De I’Pge tertiaire de la s&ie salifgre responsable des structures diapiriques en Me’diterranke Occidentale. C.R. Acad. Sci.. Paris. SekieD, 271: 812-815.

302

I<. HINZ

Ringwood, A.E. and Green, D.H., 1966. Petrological nature of the stable continental crust. In: J.S. Steinhart, T.J. Smith (Editors), The Earth beneath the Continents. Am. Geophys. Union, Monogr., 10: 611-619. Ryan, W.B.F., 1969. The Floor of’ the Mediterranean Sea, I and 2. Thesis, Columbia University, New York, N.Y., 135 pp. and 58 pp. Stein, A., 1968. Consideration about D.S.S. Paper presented at the Ass. Eur. Seismol. Comm.. IOth, Leningrad,

1968.

Stille, H., 1934. Bemerkungen zur perimesetischen Faltung in ihrem sudpyreniisch-balearischen Anteile. Abh. Ges. Wiss. Giittingen, III, H. 10: 1477-1490. Wunderlich, H.G., 1969. Aufgaben und Ziele vergleichender aktuotektonischer Forschung: Schwereverteilung und rezente Orogene im Mediterrangebiet. 2. Deursch. Geol. Ges., 118: 266-284.