Geophysical evidence for the subsurface distribution and mode of emplacement of ophiolites in the Eastern Rhodope region, N. Greece

355

Tecto~p~ysics, 218 (1993) 355-365 Elsevier Science Publishers B.V., Amsterdam

Geophysical evidence for the subsurface distribution and mode of emplacement of ophiolites in the Eastern Rhodope region, N. Greece F. Maltezou and M. ~u#yanna~s P&k

Petroleum ~o~rat~n

(DEP-EKYI,

199 ~~ssias Avenue, 15124 Maroon

Athens, Greece

(Received June 24,199l; revised version accepted June 24,1992)

ABSTRACT Maltezou, F. and Loucoyannakis, M., 1993. Geophysical evidence for the subsurface distribution and mode of emplacement of ophiolites in the Eastern Rhodope region, N. Greece. Tectonophysics, 218: 355-365. Geophysical techniques have been used to investigate the subsurface distribution and likely mode of emplacement of outcropping ultramafic bodies of ophiolitic affinity in the Greek part of the Rhodope Massif. In addition, a geophysical study in the area of the Orestias basin, in the no~heaste~ part of the Greek Rhodope region, shows ultramafic rocks to occur under a cover of about 2 km of sediments. The interpreted ~~bu~on of ultramafic rocks is used to assess the mode of ophiolite emplacement. It is concluded that the outcropping and concealed ophiolites have a similar geometry. These are thin bodies south dipping, compatible with the regional model of tectonic emplacement of oceanic lithosphere from the

Introduction The Rhodope Massif occupies a large part of northeastern Greece, northwestern Turkey and southern Bulgaria (Fig. 1). Although the age of the Massif is uncertain, most authors regard it as Precambrian in age, overprinted by later tectonic events (Makarov and Spiridonov, 1982; Billett and Nesbitt, 1986). The Massif is flanked by the Balkanide and Hellenide erogenic belts of the Alpine mountain chain. In a study of the geotectonic evolution of the Balkanides in Bulgaria, Ivanov (1988) distinguished a number of separate nappes within the Rhodope Massif, from top to bottom: North Rhodope, Asenica, Madan, Arda and Thrace units. Other authors have simply distinguish~d two separate units: the eastern and western Rhodope Massif (Makarov and Spiridonov, 1982; Maltezou, 1987).

Correspondence to: F. Maltezou, Public Petroleum Corporation, 199 Kifissias Avenue, 151 24 Maroussi, Athens, Greece.

The eastern Rhodope region differs significantly from the western Rhodope region in neotectonic style, geophysical characteristics and overall geological history (Makarov and Spiridonov, 1982). The metamo~hic basement comprises a lower unit of leucocratic and mafic gneisses of the Rhodope Massif and an upper unit of amphibolites and ultramafic rocks (The Amphibolite-Sepntinite Unit of Billett and Nesbitt, 1986). This unit of ultramafic rocks, which separates the basement from the younger nonmetamorphic rocks, has been thrust on to the gneisses and subsequently metamorphosed. Ultramafic rocks are not known to occur in the western part of the Rhodope Massif. The ~phi~lite-Serpentinite Unit is a fundamental element of the geology of the eastern Rhodope region and may belong to the Mesozoic Tethyan ophiolite belt (Billett and Nesbitt, 1986). In this case, the original sequential association of ultramafic rocks with gabbros, dolerite dykes, pillow lavas and cherts, that constitutes an ophiolite appears to be dismembered. Amphibolites and

0040-1951/93/%06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

356

serpentinites are intimately associated with disseminated and massive chalcopyrite-pyritet k galena) mineralization prospects and these bodies can therefore be used as “path finders” in the exploration for copper deposits. The ultramafic rocks in the Eastern Rhodope region are often partially or completely serpentinized. A serpentinite body southwest of Soufli (Fig. 21, at the southern margin of a unit of amphibolites on to which it is overthrust, is highly altered, sheared and brecciated at the margins whilst its interior is more massive (M.F. Billett, pers. commun., 1986). The Soufli ophiolite may form part of a larger ultramafic complex in this area. Billett and Nesbitt (1986) assigned a number of other smaller serpentinite outcrops, such as those in the areas Baiko, Aberdeen and Pessani (Fig. 21, to this complex. The Esohi-Kimi ultramafic body, close to the Bulgarian border, represents the largest ultramafic outcrop (approximately 80 km21 in the eastern Rhodope region (Zachos and Dimadis, 1987).

It comprises peridotites, dunites and scrpentinites and is surrounded by leucocratic gneisses. Although geological information suggests southward dipping margins (Schmitt. 1986) it is not clear whether in the thrusting lines appearing on the geological map (Zachos and Dimadis. 1987) the northern contacts of the Esohi-Kimi body are involved. Other important elements of the geology of the eastern Rhodope region are the Tertiary sedimentary basins, intrusions and volcanic rocks. Their association with pronounced geophysical features has been used to provide information about Tertiary evolution in the area (Maltezou, 1987). In this study we use available gravity, magnetic and seismic data to investigate outcropping ultramafic rocks in eastern Rhodope and concealed bodies predicted to occur beneath the sediments of the Orestias basin in the northeastern corner of the Rhodope region. New data are presented for the Esohi-Kimi and Soufli areas and for the Orestias basin.

The Esohi-Kimi

200km

Fig. 1. The position tonic

frame

MEDITERRANEAN

of the Rhodope

of Greece

and south Nesbitt,

SEA

Massif within the geotecBalkans

1986).

(after

Billett

and

aeromagnetic

anomaly

A high-amplitude aeromagnetic anomaly (ABEM, 1967; Fig. 3) is associated with the Esohi-Kimi ultramafic rocks (Fig. 2) in the northwestern part of the eastern Rhodope region. This well defined anomaly consists of a positive component of 1300 nT with an associated negative component of -400 nT to the north. It is centred over the southwestern part of the outcrop of the body (Figs. 2, 3) whilst the anomaly over the northeastern part of the outcrop is highly disturbed. The northeastern extent of the body is unknown due to the termination of the survey at the Bulgarian border. The non-linear optimization method (also referred as minimization method) was used to produce possible two-dimensional models of the body giving rise to the anomaly along profile line MI (Figs. 2, 3). The minimization approach WChalabi, 1970, 1971; James, 1972, 1978; James and Roos, 1985) allows for uncertainties in some of the modelling parameters. Minimization was performed by accessing the programme MINUIT (James and Roos, 1985)

SUBSURFACE

DISTRIBUTION

AND

MODE

OF EMPLACEMENT

OF OPHIOLITES

installed on the CRAY computer of the University of London Computer Centre. Some knowledge of the physical properties of the body is available from laboratory measurements of surface samples but this information is inadequate to characterize the body at depth. Susceptibility values derived from surface samples range from 2 to 50 X lop3 SI. In addition, the NRMcomponent is significant (Q-values ranging from 0.7 to 2.5 for dunites and peridotites from the Esohi-Kimi area) and needs to be taken into account in the interpretation. One body (Fig. 4) within the range of models generated combines a good fit to the aeromagnetic anomaly (rms error of fit less than 10% of the maximum amplitude of the anomaly) with a subsurface shape in agreement with the prevailing geological struc-

IN EASTERN

RHODOPE

REGION

357

ture (Schmitt, 1986). The only constraint used in this case was a maximum body thickness of 3 km, all other modelling parameters (susceptibility, NRM amplitude and direction) being allowed to vary. The interpreted susceptibility value falls within the range of measured values for this body but it is smaller by a factor of five than the mean susceptibility value of 64 x 10e3 SI of the Soufli serpentinites (Maltezou, 1987). Assuming that the two bodies had a similar original composition, the susceptibility and NRM intensity within each of them would be expected to be a function of the degree of serpentinization. An exponential increase is usually expected in both susceptibility and NRM intensity with increasing degree of serpentinization (Saad, 1969). During the serpentinization process iron atoms released from the

r

BULGARIA

OUFLI

TURKEY

Fig. 2. Geological map of the northeastern Rhodope region. Gravity (G), magnetic CM) and seismic (S) profiles in areas of ophiolite occurences are shown. The location of three boreholes (4, 9, 3) are also shown.

/ i.

,-. ‘. -,--

4

,.‘-\

I

~~--_,‘-‘\ L._.-,i

i

I’_.‘. ‘___

\

\

\_’

\

‘,

SUBSURFACE

DISTRIBUTION

AND

MODE

OF EMPLACEMENT

OF OPHIOLITES

S

Fig. 4. Interpretation model along aeromagnetic profile Ml in the area of the Esohi-Kimi ultramafic rocks. The values obtained as a result of the minimization are: K = 12.7X 10m3 SI, NRM = 2830 X 10m3 SI, NRMinclination = 45.8”, NRM declination = 169.1”.

IN EASTERN

RHODOPE

REGION

359

netic anomaly in terms of a single body is unlikely to represent the detailed geometry of the serpentinites involved in the imbricate zone, some information has been obtained from these data using simple 2-D aeromagnetic modelling along profile line M2 (Figs. 2, 3; Maltezou, 1987). Modelling suggests a south-dipping serpentinite body occupying a thin-skinned tectonic setting compatible with the regional geological model of thrusting (Maltezou, 1987). Thrusting in this area must postdate the Early Tertiary, based on the incorporation of Lower Tertiary sediments into the Amphibolite-Serpentinite Unit (Billett and Nesbitt, 1986). The Orestias Basin Local geology

silicate structure of the paramagnetic olivine and pyroxene are oxidized to form ferrimagnetic magnetite. The intensity of magnetization depends also on the “mode of occurrence and state of oxidation of the magnetite” as well as on the original rock composition (Saad, 1969). The NRM intensity of 2830 X lop3 SI suggested by the minimization interpretation for the Esohi-Kimi ultramafic body, does not differ substantially from the mean value derived from samples from the Soufli serpentinites (3300 X lop3 SI) despite the different degree of serpentinization of the two bodies. This might result from differences in original rock composition. The Soufli aeromagnetic anomaly The Soufli serpentinites (Fig. 2) occur in the north eastern part of the Pessani imbricate zone, which also contains altered amphibolites, gneisses and Lower Tertiary sediments. The serpentinites at Pessani (Fig. 2) crop out as a series of 30-400m-thick slices (Billett and Nesbitt, 1986). The aeromagnetic anomaly in this area delineates the general northeastern trend of serpentinite outcrops (Figs. 2, 3). The aeromagnetic information has been collected by ABEM (1967) along flight lines 0.8 km apart and contoured at 20-nT intervals. Although the interpretation of the aeromag-

The Orestias Basin (Fig. 2) is part of the larger Thracian basin which extends eastward beyond the Greek border (Noussinanos, 1988). The sedimentary basins in the eastern Rhodope region have been grouped together according to their similarities in geological and geophysical characteristics, namely: initiation during the Eocene, absence of Miocene sediments, type of sediment fill and associated volcanicity (Maltezou, 1987; Maltezou and Brooks, 1989). This group of basins has been distinguished from the basins in western Rhodope and interpreted to reflect an earlier phase of extension in the region (Maltezou, 1987; Maltezou and Brooks, 1989; Mercier et al., 1989). The deposits of the Orestias Basin consist of an Upper Eocene unit of lacustrine sediments and an Oligocene marine sequence of limestones, marls and sandstones. Miocene sediments are absent within the basin. The Plio-Pleistocene to Quaternary sequence is represented mainly by conglomerates, sands and clays of terrestrial origin. The Oligocene sedimentation has been interpreted (Andronopoulos, 1977) to indicate deposition in a tectonically undisturbed environment (Gonstantinides et al., 1983; Noussinanos, 1988). The E-W-trending Orestias basin is bounded to the north by the crystalline rocks of the Rhodope Massif and to the south by the amphibolites of the Amphibolite-Serpentinite Unit.

360

TmLE

1

Acquisition

parameters Northern

Date

part

Southern

1983

Source

Dynamite

Source depth

Variable:

Cable geometry

60 channels

Dynamite 20-40

m

off-end

25 m 48 channels metrical

Receiver

array

24 geophones

offset

150-2950

Sampling

rate

2 ms

2 ms

5s

5s

time

m

symsplit

24 geophones

Min-max Maximum

part

1981

lSO-1300

m

Geophysical data

A migrated seismic section S (Maltezou et al., 1990) and a gravity profile G, both running approximately N-S, have been used in the present study (Fig. 2). The length of each profile is approximately 28 km. Well log data and regional gravity maps (Maltezou, 1987; Lagios et al., 1989) have been used to provide additional information. Seismic section S (Fig. 2) consists of two separate sections acquired during different reflection seismic surveys carried out by DEP. The main acquisition parameters are given in Table 1. Both parts of section S (Fig. 5) are the result of a standard processing sequence but they have different display parameters. The main structural features are consistent and key reflectors can be followed through the common boundary of the merged section.

N

The gravity profile consists of 33 gravity stations located along the low-lying areas of the Evros River near the Greek-Turkish border (Fig. 3). Additional information is obtained from a Bouguer anomaly map (Lagios et al., 1989) that accounts for terrain effects and outlines the main features of the gravity field in the Orestias area: a local minimum of 2.5 mgal is bounded by two maxima of approximately 45 mgal to the north and south, respectively. This pattern of anomaly has been interpreted by Maltezou (1987) to reflect the main volume of basin sediments bounded by crystalline rocks of the Rhodope Massif and amphibolites of the Amphibolite-Serpentinite Unit to the north and south. Borehole information from three wells (4, 9. 3 in Fig. 2) has been used to extract accurate velocity and density estimates in the area of the two profiles and substantiate the seismic and gravity interpretations. Given the overall elongate shape of the basin, a two-dimensional modelling approach (Talwani et al., 1975) was used for gravity interpretation. The gravity effect of the sediments represented on seismic section S (Fig. 2) was estimated using ray theory inversion (Hubral, 1977): image rays were used to invert to depth (Fig. 6) the original reflection times on the migrated section (Fig. 5). Velocity analysis of the reflection data, based on moveout times, was combined with independent information from the sonic logs to obtain accurate velocity estimates. The main reflectors on the seismic section and information from the

2 km

S

SUBSURFACE

DISTRIBUTION

AND

MODE

OF EMPLACEMENT

OF OPHIOLITES

wells define three sedimentary sequences corresponding to Eocene, Oligocene and OligocenePlio-Quaternary formations (Fig. 6). Densities were derived from a bulk density log available for borehole OR3 (Fig. 7). Drilling at this borehole encountered a sediment column of 3742 m above a basement consisting of serpentinized gabbro with iron pyrites (Stylianou and Papakyriakou, 1984). Densities from this log were grouped into three depth intervals corresponding to the sedimentary sequences defined above (Fig. 6). Mean density values of 2.37, 2.46 and 2.65 Mg/m3 have been assigned to each of these sequences. Crystalline basement rocks have been assigned a density of 2.75 Mg/m3 based on the density logs and measurements on samples of outcropping gneisses from the area north of Leptokarya (Maltezou, 1987). The calculated effect of the three-layer sedimentary sequence (Fig. 8b) is dramatically different from the observed Bouguer anomaly along profile G (Fig. 8a) from which a constant level of regional field has been removed. The cause of this gravity misfit is assumed to be an independent source of gravity anomaly in the subsurface. In an attempt to produce a more complete geological model along the seismic section, we have investigated a number of reflectors which appear on the seismic section below the main volume of sediments (see Fig. 5). A line interpretation is shown in Figure 9a and a depth con-

CDP 300

6Ccl

361

REGION

ORESTIAS BULK 2.2

0

3.

DENSITYi”!-3) 2.L

28

2.6

1000

E

I 2000 I-

CL

w

0 300-J

i

LOO0

Fig. 7. Bulk density

log from the Orestias

3 borehole.

900

1110

20

DISTANCE

DISTANCE

(km)

Fig. 6. Image rays used to invert to depth selected 9) c&responding

RHODOPE

NUMBER

10

2, 3 in Fig.

IN EASTERN

to the three

model.

horizons

layer

(1,

sediment

Fig. 8. (a) Observed

Bouguer

(Fig. 2). (b) Calculated

gravity

gravity

effect

along seismic section

(km)

anomaly

along profile

of sediments

S (Fig. 2).

(Fig.

G 6)

k. MALTELOIJ

DIST

AN

C

E

(km)

Vertical

Fig. 9. (a) Time model Vertical indicated

exaggeration by

X.

along

seismic

section

S in two-way

= 2 : 1. Body 1 = amphibohte.

Estimates

have been

extracted

traveltime

O’WT)

Bodies 2, 3 = serpentinites.

verted model in Figure 9b. Reflectors 4, 5, 6 (Fig. 9a) define thin bodies 2 and 3 (Fig. 9b) after inversion. The adoption of reflector 7 is based on the combined criteria of a good fit to the observed gravity anomaly (this is obtained when a density of 3.00 Mg/m3 is used in modelling body 1) and compatibility with the geological model of mafic-ultramafic basement in the area. The latter consists of a sequence of amphibolites 1.5-2 km thick overlain by tectonic slices of serpentinites 30-400 m thick (Billett and Nesbitt, 1986). Drilling at borehole OR4 verified the presence of serpentinites at this location at a depth of 1670 m and borehole 3 also encountered serpentinized gabbro at its base 3742 m.

exaggeration

2

1

inverted

to (b) depth

using image

Velocity

values changing

laterally

from the sonic logs and the seismic move-out

AND M. I_OCI(‘OY41\INAKI\

data (interval

velocities

ray processing.

and vertically

calculated

are

from the

times).

Density values of 3.00 Mg/m3 and 2.66 Mg/m3 have been assigned to the amphibolite (body 1) and serpentinite bodies (2, 3) respectively (Figs. 9, IO), as derived from laboratory measurements on samples of amphibolites and serpentinites in the area north and west of Soufli (Maltezou, 1987). The shape of the observed gravity anomaly is closely approached (Fig. 10) after the calculation of the combined effects of the sediments and ultramafic bodies (1, 2, 3; Fig. 9). Discussion

The interpreted geological model for the Orestias area involves thin slices of serpentinites dip-

SUBSURFACE DISTRIBUnON

363

AND MODE OF EMPLACEMENT OF OPHIOLITES IN EASTERN RHODOPE REGION

S

N

CALCULATED SE1

SMIC

ALONG

GRAVITY SECTION

S

K

OBSERVED ANOMALY

I 0

PROFILE

G

1

10

DISTANCE

GRAVITY ALONG

20

Ikm

30

I

AMPHIBOLITE

10

15

1

Fig, 10. Calculated gravity effect of sediments and ultramafic bodies along seismic section S when compared to the observed gravity anomaly along profile G (after the subtraction of a constant level of regional gravity field). The density values of 2.37,2.46 and 2.65 Mg/m’ (Fig. 7) correspond to the three-layer sediment model (Figs. 6, 9b) from top to bottom, according to the density log. Density values of 2.66 and 3.00 Mg/m’ correspond to serpentinites and amphibolites respectively.

to the south and apparently emplaced on top of a more massive but relatively thin (3 km) unit of amphibolites. This model closely resembles the structure in the area of Soufli described earlier.‘Geological mapping in the Pessani area (Fig. 2; Billett and Nesbitt, 1986) assigned a vertical thickness of 1.5 to 2.0 km to the Amphibolite-Serpentinite Unit and described it as being thrust from the south on to the crystalline gneisses to the northwest. Aeromagnetic modelling in the Soufli area has revealed the shape and thickness of a serpentinite body indentified as a south-dipping body occupying a thin-skinned tectonic setting (Maltezou, 1987). This is consistent with a local geological structure in the area of ultramafic rocks consisting of thin tectonic slices emplaced from the south (Billett and Nesbitt, 1986). Minimization modelling suggests that the Esohi-Kimi ultramafic body similarly dips south although it occupies a larger volume (Fig. 4). ping

The overall results of the geophysical interpretation, in conjunction with the geological evidence, suggest that the ophiolite bodies of the eastern Rhodope are thin tectonic slices emplaced along thrusts with a northerly direction of transport. Independent information from a geological study in southern Bulgaria suggests a regional rather than a local significance of these features, In a N-S schematic section of the Balkanides (Ivanov, 1988) the Thrace unit of the Rhodope Massif is shown to contain fragments of ophiolites emplaced from the south. Further geological work is needed in the Rhodope region to investigate the presence of other parts of the possibly dismembered ophiolite sequence. Pillow lavas have recently been found to crop out in the area southwest of Soufli (Fig. 2; M.F. Billet& pers. commun., 1986). $engCir et al. (1984) have pointed out that Rhodopian “mafic and ultramafic rocks of uncertain origin” may

364

F MALTE%OU

very well turn out to be dismembered ophiolites, based on experience in Greece and Turkey. The term ophiolite has already been apopted by some researchers in the Rhodope region (Billett and Nesbitt, 1986; Ivanov, 1988). Ophiolite complexes are considered to be fragments of ancient oceanic lithosphere formed at a spreading plate boundary. As a consequence ophiolites are considered to be records of the oceanic history from the time they are formed until final thrusting over a continental margin. Therefore they can be used to identify main oceanic’ events and constrain general models for the geotectonic evolution of a region. The exact configuration of microplates and oceanic tracts that were operating during ophiolite emplacement in the Rhodope area is not yet clear. Northward subduction of the northern strands of the Neotethys Ocean is assumed to have been taking place in Turkey in Late Cretaceous times along an Andean-type margin until continental collision in the late Eocene. This subduction event has been linked further to the development of fore-arc basins of latest Cretaceous to late Eocene age in central Anatolia (Gorur et al., cited in Robertson and Dixon, 1984). Assuming a similar setting, Eocene basins in the eastern Rhodope region may also be related to an early subduction event recorded by the emplacement of ophiolites and preceding a later subduction event responsible for the development of the Miocene basins in western Rhodope. By adding new information and increasing the number of known ophiolite occurrences in the Rhodope region, this work sets new interpretation targets in the study of the geological evolution of the area.

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SUBSURFACE

DISTRIBUTION

AND

MODE

OF EMPLACEMENT

OF OPHIOLITES

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