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Preliminary seismic studies in the eastern Mediterranean
Earth and Planetary Science L etterss, 21 ( 1974) 355- 366 © North-Holland Publishing Company, Amsterdam - Printed in The Netherlands
PRELIMINARY
S E I S M I C S T U D I E S IN T H E E A S T E R N
MEDITERRANEAN
J.M. LORT*, W.Q. L1MOND and F. GRAY Department of Geodesy and Geophysics, University of Cambridge, Cambridge (Great Britain]
Received November 19, 1973 Revised version received February 4, 1974
Seismic reflection and refraction data are described from four locations in the eastern Mediterranean. Over the Mediterranean Ridge the profiles reveal a great thickness of material in the velocity range 3.6 - 5.5 km/sec. The crust is very disturbed and so penetration is limited, but at the foot of the Nile Cone an unreversed line gives a depth to the mantle of 27 kin. This suggests that the crust in the eastern Mediterranean is more likely to be continental than oceanic in structure.
1. Introduction
2. Experimental details
Much geophysical information has previously been collected in the eastern Mediterranean including bathymetric soundings, gravity, magnetics, heat flow and seismic reflection [ 1]. However, a marked gap in the knowledge of the area lies in the absence of seismic refraction studies. Therefore, in September and October 1971, the Department of Geodesy and Geophysics, University of Cambridge, undertook a cruise on board M .V."Researcher" to investigate by seismic methods the nature and thickness of the crust in the Levantine Sea. Fig. 1 shows the locations of seismic reflection and refraction profiles in relation to the major physiographic provinces. The lines R1, R4, R6 and R7 were first profiled using the air-gun reflection equipment to ascertain their suitability as sites for refraction experiments, and all file refraction lines except R1 were subsequently reversed as R4 R, R6 R, R7 R. A variable angle reflection/refraction experiment was also executed along line R4A, adjacent to profile R 4 - R 4 R .
The reflection system comprised a 500-cm 3 air gun operated at 100 atm and fired every 10 sec into a G6omecanique hydrophone array with three active sections [2]. A Mufax wet-paper recorder was used for immediate display of the records and the signals were also recorded on frequency modulated analogue magnetic tapes. These recordings were replayed through various analogue band-pass filters. The refraction experiments used the Cambridge radio telemetric sonobuoys [3] with an internal recording system [4]. The refraction lines were first shot over a range of 120 km using Geophex charges of up to 136 kg. The results were carefully studied before reversing three out of the four profiles, reducing their length and increasing the charge sizes to 3 6 0 450 kg at the furthest range, as it was obvious that the penetration achieved using small charges was limited. A new system of firing was devised, where two or three separate charges of 1 3 6 - 180 kg were linked by 15 m lengths of Aquaflex cord explosive and a 65 m length was connected to the fuse end. They were all discharged from a tilting platform after the fuse had been lit. This method proved more successful than either floating the charges or firing a single
* Now at : Institut de Physique du Globe, Observatoire G6ophysique du Parc St. Maur, 94 St. Maur des Foss4s, France
356
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Fig. 1. Location of refraction and reflection profiles in the eastern Mediterranean, M.V. "Researcher", 1971. The main physiographic provinces are drawn from collated information: Cambridge (this paper), Marine Geophysical Survey, Watson and Johnson [8], Allan and Morelli [9], Emery et al. [10], lnstitut Franqais du Pgtrole, Sancho et al. [5].
large shot, when incomplete detonation or total misfires were experienced due to the charges breaking up. Generally three buoys were laid on each forward line, at 2.5- and 4-km spacing, and on the reverse line four buoys were used at 8-kin spacing to increase the spread. The profiles were oriented parallel to the strike of the structural trends of the Hellenic arc and Mediterranean Ridge for ease in interpretation (Fig. 1). Line R~ lies on the southern flank of the Mediterranean Ridge on the portion described as the Upper Plateau [5]. Line R 4 lies in the Herodotus Baisn, a relatively deep water area in a region of basin sedimentation between the Nile Cone and the southern flank of the Mediterranean Ridge. R6 lies to the northeast of the Herodotus Basin and on the southern side of the Mediterranean Ridge in an area of coarse-grained texture [6, 7]. R7 lies in only 1.5-km water at the foot of the Nile Cone. The physiographic provinces of Fig. 1 were drawn from collated bathymetric and reflection data available for the eastern Mediterranean.
3. Reflection profiles The reflection data confirm the sedimentary features of the Mediterranean Ridge already described in the literature [11, 12]. Line diagrams of R1 and R6 on the Ridge are included in Fig. 2. Rx displays a very irregular relief and coarse texture, formerly called cobblestone topography [ 10]. Diffraction hyperbolae are numerous on both echo-sounder and reflection records, and penetration is restricted to only 0.3 sec (two-way time), occasional small pockets of sediment occur, filling hollows to 200 m thickness. The lateral continuity of the strata is disturbed by immediate sub-surface faulting. In places on R6 a more continuous undulating reflector can be recognised, at 2.0 sec depth, having a wavelength of about 15 km and amplitude of 6 0 0 - 1000 m (this is represented by a thickened line in Fig. 2). The upper sediment velocity, measured by matching diffraction curves to the hyperbolae on the record is 2.1 km/sec. Faulting reduces the penetration locally along the profile. The prominent
PRELIMINARY SEISMIC STUDIES IN THE EASTERN MEDITERRANEAN
357
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;
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Fig. 2. Reflection profiles of R 1, R 6, R4 and R 7. The vertical scale is in seconds of two-way time. were numerous and masked some of the events, but reflector, the acoustic basement, is again seen on R4 penetration was generally 1.2 sec. (Fig. 2) where basin sedimentation presents a thick sequence of conformable sediments, 1.3 km thick. The wavelength of folding is similarly about 15 km and the amplitude increases towards the northeast. The 4. Refraction r e s u l t s basement rises towards the southwest, where it is also fragmented by minor faulting. The refraction Few refraction data existed in the eastern Mediterresults later proved that this boundary, of high reranean before 1971. Short profiles made by Gaskell flectivity, marks the top of the consolidated sediet al. [13] showed maximum penetration to a 4 . 3 ments, which have a velocity of 3.8 km/sec. R4 in the 4.7-km/sec layer. Moskalenko [ 14] detected, on a northHerodotus Basin thus showed the most promising south profile between Crete and Libya, velocities of features for refraction studies. 3.7 and 4.7 km/sec overlying material with velocities Line R7 displays characteristic delta sedimentation of 6.1 and 7.0 km/sec which he correlated with granitwith a series of distributary channels on the generally ic basement. These observations do not seem to be flat surface, and deposition occurs as small lenses and well-founded and thus there was no significant inforslumped bodies. Intense folding and faulting are indicatmation available for the deep structure. ed by the abundance of diffraction hyperbolae (Fig. 2). The reduction of data from "Researcher" refraction Due to the shallow water depth of only 1.5 km, multiples
358
J.M. LORT, W.Q. LIMOND AND F. GRAY
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Time in s
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Fig. 3. Travel-time graphs of refraction profiles (a) R1, (b) R6, (c) R 4 and (d) R 7. Equations to the straight lines are in the form: intercept time (sec) + x/velocity (km/sec).
PRELIMINARY SEISMIC STUDIES IN THE EASTERN MEDITERRANEAN
359
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60 Distance
in
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90
105
360
J.M. LORT, W.Q. LIMOND AND F. GRAY
TABLE 1 Structure and location of seismic profiles Line
R1
R6
R6R
R4
R4A
R7
Lat. and long. (1)*
33°50'N 22° 30'E
33° 54:N 29° 00'E
34° 03'N 30° 27'E
32°42'N 27°44'E
33° 33'N 28°42'E
32° 54'N 32° 00'E
Lat. and long. (2)
33° 35'N 24° 00'E
34° 03'N 30° 27'E
33° 54'N 29° 00'E
33° 33'N 28° 40'E
32° 34'N 28° 09'E
32° 59'N 33° 12'E
v (velocity, km/sec) d (layer thickness, km)
1.5 1.4
1.5 3.0
1.5 3.1
1.5 3.1
1.5 2.9
1.5 1.6
o unconsol. d sediments
1.8, 2.3 0.3, 7.5
2.8 1.3
2.0, 3.1 0.5, 1.0
2.6 2.7
2.1, 2.3, 2.6 0.3, 0.6, 0.9
2.0 0.5
o (3.5 -4.1) d
3.5 2.0
3.7 11.3
3.6 9.5
3.8 0.3
3.8 1.0
4.1 5.9
o (4.5 -5.8) d
5.8
4.8
5.5
4.5, 5.4 9.0, 1.9
4.5, 5.4 4.0, 0.9
5.1 7.5
6.5
6.4
v (6.4 -6.5) d o mantle d
6.4 12.9 8.4
* Position (1): buoy position; position (2): last shot. d (layer thickness) is calculated under buoys for reversed lines. R 4 and R 7 are reversed profiles; R1, R4A, R6and R 6 and R6R are unreversed. profiles involved the application of a series of corrections to the travel times: one for clock drift of the buoys, one to the shot break for the slant range between the shot and the hull geophone m o u n t e d on the ship, one for the depth of the shot and the hydrophone, and a topographic correction, compensating at the level of the first sub-bottom refractor which was assumed to provide the relief. Traveltime plots were produced for each profile, the lines fitted by a least-squares method, and dipping-layer solutions computed from the apparent velocities and time intercepts to give the depths, true velocities and dips of the layers. Distances between shot and receiver were measured using the water travel time, though it was not always easy to recognise the direct water wave arrival D, which was much smaller in amplitude than the reflected wave R'. In this case the value of D could be extrapolated at farthest ranges as (D-R') approaches a constant value [15]. Profile RI is an unreversed line, the travel-time plot is shown in Fig. 3a and the deduced structure is summarised in Table 1. The profile, shot from west to east, gives first arrivals out to 65 kin, and velocities
of 1.8, 2.3, 3.5 and 5.8 km/sec. Discontinuities in the lines suggest the presence of at least two faults, the first affecting the 2.3-km/sec and 3.5-km/sec layers with downthrow of 1.5 km to the east. The layer of 5.8 km/sec has probably experienced faulting also as early arrivals are seen only beyond a range of 45 km (D in Fig. 3a). R6 is a reversed profile, also situated on the southern flank of the Ridge and faulting again complicates the structure. The travel-time plot is shown in Fig. 3b. The velocities in the forward direction, 2.8, 3.7 and 4.8/sec, do not agree with those of 2.0, 3.6 and 5.5 km/sec, probably because of local inhomogeneities or dipping layers. Further, an anomalous thin layer with a velocity of 3.1 km/sec occurs on Re R as second arrivals. A multiple with velocity 3.9 km/sec, which is omitted from Fig. 3b, results from the 2 . 0 - 3 . 6 - k m / s e c boundary after internal reflection in the upper layer. A depth of about 1.5 km from the sea bottom to the 3 . 6 - 3 . 7 - k m / s e c layer corresponds to the prominent undulating basal horizon seen at 1.5 sec on reflection records. A dipping-layer solution is hard to justify in this profile as the computed water depth does not fit the true depth, so
PRELIMINARY SEISMIC STUDIES IN THE EASTERN MEDITERRANEAN possibly the relocation of the line for reversal was not exact. Separate structures for both R 6 and R 6 R are therefore listed in Table 1. At R4 thick sediments occur and numerous multiples are observed, which can be attributed both to ringing within the sediments and also to reflection between the surface and bottom of the sea, before layer refraction. The arrivals seem markedly attenuated over the range 4 5 - 6 5 km but return out to 90 km (Fig. 3c). Even at this distance, however, there is no evidence of deep penetration to the Moho. Discontinuity in the travel-time graph may be explained either by a fault with a large downthrow to the southwest, or to an intermediate thin high-velocity layer which may prevent a greater part of the seismic energy from travelling to depth and thus weaken the arrivals [16, 17]. Shooting from the southwest along R4 velocities of 2.5, 4.0, 4.5 and 6.5 km/sec were recognised, which reverse with those of 3.8, 4.5 and 6.6 km/sec to give "true" velocities of 2.6, 3.8, 4.5 and 6.5 km/ sec. The structure for the reversed line R4 is presented in Table 1. A thin layer of velocity 5.4 km/sec is recognised only on R4 possibly due to its local occur-
rence, or it may not be detected on R4R because of sparse data. First to third order multiple reflections are recognised from the 3.8, 4.5 and 6.5-km/sec layers, and the layer at the base of the consolidated sediments recognised on reflection records as the acoustic basement, produces several multiples by internal reflection in the overlying sediments. To adjust the results to the depth of the sea measured by a PDR it is necessary to postulate an upper sedimentary layer which is not detected by refraction. An average velocity of 2.0 km/sec was assumed for the unconsolidated sediments after fitting curves to the hyperbolae seen on reflection records. This velocity agrees with Gaskell et al. [13] and Ryan et al. [12]. Fig. 4 represents a structure profile along the Ridge from R1 eastwards, showing that the crustal column broadly thickens in this direction. The arrivals at RT, a reversed profile at the foot of the Nile Cone in a shallow water depth of 1.5 km, were again swamped by multiples. However, first arrivals were stronger than on any other line, and were received out to 90 km even using small charges on RT, while in the reverse direction, arrivals were obtained at 112 km (Fig. 3d). Velocities of 4.0, 5.3 and 6.6 km/sec were
t
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t ,
4 8
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Fig. 4. Structure profiles for R1, R4, R 6 and R 7. The figures adjacent to the columns are thicknesses in km and the scale shows depth in km.
362
J.M. LORT, W.Q. LIMOND AND F. GRAY
.
.
.
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Fig. 5. Jet-pen display of vertical reflection profile R4 A, filter: 10- 100 Hz. Distance in km is shown along the horizontal axis and the vertical axis is the two-way time in seconds. obtained along R7, shooting from the west end, and these reverse with velocities of 4.2, 4.9 and 6.2 km/sec. The structure for the reversed line R7 appears in Table 1, with layer velocities of 2.0, 4.1, 5.1 and 6.4 km/sec above a mantle velocity of 8.4 km/sec occurring at 27 km depth, which was detected only on RTR when the large charges were used. At R4A a variable angle reflection/refraction experiment was also conducted in the Herodotus Basin using the explosive line sources Aquaflex and Geoflex. Four sonobuoys were laid at the start of the line and the ship then steamed past the buoys, towing a G6omecanique array. 200 shots of 3 0 - 1 0 0 m lengths were fired at 2-rain intervals; vertical reflections were received over a range of 40 km and variable angle reflections and refractions were received by the sonobuoys over 50 km range. The results from the array were replayed from tape recordings, broad band-pass filtering was applied, and the correlations were clearly seen on a jet-pen display of the traces side by side (Fig, 5). The prominent undulating acoustic basement at 1.5 sec depth again corresponds to the top of a layer of velocity 3.8 km/ sec. At 8.0 sec depth there appears to be a real event,
which cannot be explained as a multiple. Owing to the fact that during the variable angle experiment the shot breaks were not recorded on the same magnetic tapes as the seismic variable angle reflection and refraction data, it was difficult to replay the traces side by side to obtain correlations of seismic signals. Therefore, the data on the buoy tapes were digitised, replayed, and the necessary time corrections applied to give correctly aligned traces on a time-distance diagram. The results from one buoy only are presented here. The final plot was obtained after low pass filtering at 40 Hz, and gain scaling (Fig. 6). The display shows a slightly asymmetrical direct wave reflection hyperbola, due to gently dipping bottom topography. The offset, i.e. the distance at which the ship passed closest to the buoys, was accurately measured for each buoy for use in a 3-D computing model, and the profile was then interpreted as a flat, multi-layered system. A greater amplitude is observed on the left-hand limb of the hyperbolae than on the right. This indicates the directionality of Aquaflex as the source moves away from the receiver [18].
PRELIMINARY SEISMIC STUDIES IN THE EASTERN MEDITERRANEAN
Fig. 6. Final aligned plot of R4A variable angle reflection data for a single buoy. Filter: 40 Hz. Horizontal axis is distance m kin, vertical axis is two-way time in seconds. Events are marked by letters which are described in the text. The display reveals the following characteristics (Fig. 6): A,A' - prominent sea-bottom multiples up to the 4th order. B - a region of low sediment velocity arrivals immediately below the sea bed, whose amplitude decreases rapidly with range. C - a prominent higher velocity reflection, 0.8 sec belowA. D - the refraction arrival tangential to the reflector C emerging from a high energy, large amplitude area.
363
Fig. 7. Correlations visible in the final plot. Solid lines mark well-defined events. E - a complex of small amplitude events, 9.0 sec from the origin. F - a large amplitude, high frequency multiple, possibly consisting of more than one event, similar to C. B - A ' - higher velocity events, which cannot be correlated satisfactorily over a large enough range to distinguish them. The basic structure was deduced by fitting reflection and refraction curves to the prominent events (Fig. 7). The multiples were then used in conjunction with the first arrivals on both limbs of the hyperbolae to obtain a compatible model.
364
J.M. LORT, W.Q. LIMOND AND F. GRAY
25
20
15
the layer may be dipping. The vertical reflection profile supports this, for the layer is seen to be undulating. There is possibly a great density contrast at the 2 . 6 - 3 . 8 - k m / s e c interface between the unconsolidated sediments and the consolidated sediments below. Models were also fitted to the multiples which were derived from two different processes: (a) the reflection of sound between the sea surface and the sea bottom; (b) the reflection of sound internally in the sediment layers specifically between the sea bed and the top of the 3.8-km/sec layer. Thus at the event F described previously, the energy increase corresponds to multiples from the 3.8- and 4.5-km/sec layers, from reflection both within the water and the sediment layers. The sedimentary reflectors are thus clearly defined, as the correlation between the analogue vertical reflection and the digital variable angle reflection data is good. Below the 4.5-km/sec layer, however, there remains some ambiguity, as correlations can only be seen over a limited range, and so the higher velocity events are not well determined. The velocities determined from the Aquaflex experiment include additional sedimentary velocities of 2.1 and 2.3 km/sec to those determined from fire refraction results, otherwise the figures are similar (Table 2). However, if the events have been picked
TABLE 2 Comparison of structures R4A and R4
50
Fig. 8. Basic structure of R4A, velocities of 2.1, 2.3, 2.6, 3.8, 4.5 and 5.4 km/sec are recognised. Squares mark reflection curves, triangles mark refractions, solid triangles mark direct wave.
Depth (km) R4 R4R
Depth (km) R4A
Interval velocity in layer (km/sec) 1.5
3.1
2.7
2.9 2.1 3.2 2.3 3.8
The structure obtained is shown in Table 1 and Fig. 8. Sedimentary velocities of 2.1,2.3 and 2.6 km/ sec were recognised above a prominent 3.8-km/sec layer at C. D represents refraction arrivals with velocity 3.8 km/sec. The arrivals in this area have larger amplitudes because one is approaching the theoretical critical distance for the reflectors. A slight misfit of the model to the curve for 3.8 km/sec indicates that
2.6 5.8
5.6
4.7
6.1
10.7
5.7
3.8 4.5 15.1
9.7 5.4
17.0
17.1
10.6 6.4
PRELIMINARY SEISMIC STUDIES IN THE EASTERN MEDITERRANEAN
vel. km/s
2 way time secs
15
39
;~ I 2.3
Q';] O5
2.6
07
38
05
45
Fig. 9. Sketch of the multiple-producing event F in the variable angle reflection profile of R4A. correctly for R4 A it seems that the deeper layers occur at a much shallower depth on R4 A than o n R 4 . It may be that the first arrivals have not been detected using conventional refraction techniques. Here the difference between the structure for the two profiles is equivalent to the additional time required for a multiple between the 2.6-3.8-km/sec interface and the sea surface (Fig. 9). Unfortunately, the experiments at R4A did not allow sufficient penetration for the crustal depth to the Moho to be determined. Extremely long profiles are required to obtain arrivals from depths as great as 27 km, as on R7 R, and further difficulties are encountered in a fractured crust which does not transmit energy easily.
365
anism for the thickening process as the African plate plunges below the relatively south-moving Aegean plate [21, 22]. The Mediterranean Ridge would then be a zone of adjustment for great thicknesses of material and intense deformation of recent sediments would occur. However, Sancho et al. [5] believe that there has been no recent crustal thickening in this region as their deep reflection profile, located on the Lower Plateau of the Mediterranean Ridge, southwest of Crete (Fig. 1) reveals a relatively undisturbed thick sedimentary series, probably of Tertiary age. Our data in this area on the Upper Plateau ( R , ) reveal intense faulting with little penetration. Further to the east on profiles R4 and R 6 the folding of an acoustic basement reflector suggest that, in fact deformation has occurred prior to the deposition of the upper series of sediments, which are 1 0 0 0 15 000 m in thickness. Therefore the Ridge does not seem to be formed from thickening of recent sediments, as previously believed. Refraction results from German workers (Hinz, Closs, Weigel) indicate that in the Ionian Sea a 6.1km/sec layer occurs at about 10 km depth, and an 8.1-km/sec layer at 17 km depth [23, 24]. Here, the Moho is significantly shallower than our results suggest for the far eastern Mediterranean.
Acknowledgements 5. Conclusions The combined results of these seismic experiments indicate that the crust of the eastern Mediterranean can neither be ascribed a simple oceanic nor a continental character. Over the Mediterranean Ridge material of velocity 3 . 6 - 5 . 5 km/sec is extremely thick, the depth to the 4.8-km/sec layer on R6 being 15 km. This is much greater than material of equivalent velocity in the west, where the depth to a lowvelocity mantle is only about 12 km. If the unreversed velocity of 8.4 km/sec obtained on the line RTR is accepted then the total crustal thickness, at least at the foot of the Nile Cone, is 27 kin. In this case it seems that the crust is more likely to be continental [19, 201. Gravity results are consistent with this deduction, and crustal shortening has been suggested as a mech-
This work was supported by grants from the Natural Environment Research Council. We would like to thank Dr. D.H. Matthews, Mr. D.I. Sewart and Dr. A.P. Stacey, and Professor Sir Edward Bullard for reading the manuscript.
References 1 J.M. Lort, The tectonics of the eastern Mediterranean : a geophysical review, Rev. Geophys. Space Phys. 9 (1971) 189. 2 J. Cassand, J.P. Fail and L. Montadert, Sismique r6flexion en eau profonde (Flexotir), Geophys. Prosp. 18 (1970) 600. 3 M.N. Hill, Single ship seismic refraction shooting, in : The Sea, 3, ed. M.N. Hill (lnterscience, New York, 1963) 39.
366 4 F. Gray, T.R.E. Owen, A recording sono-radio buoy for seismic refraction work., Oceanol. Int. 1969 Conf. Brighton (1969). 5 J. Sancho, J. Letouzey, B. Bijou-Duval, P. Courrier, L. Montadert and E. Winnock, New data on the structure of the eastern Mediterranean Basin from seismic reflection data, Earth Planet. Sci. Lett. 18 (1973) 189. 6 W.B.F. Ryan, The stratigraphy of the eastern Mediterranean, I, II, Ph.D. Thesis, Columbia University, New York (1969). 7 H.K. Wong, E.F.K. Zarudski, J.D. Phillips and G.K.F. Giermann, Some geophysical profiles in the eastern Mediterranean, Bull. Geol. Soc. Am. 82 (1971) 91. 8 J.A. Watson and G.L. Johnson, The marine geophysical survey in the Mediterranean, Int. Hydrogr. Rev. 46 (1969) 81. 9 T.D. Allan and C. Morelli, A geophysical study of the Mediterranean Sea, Boll. Geofys. Teor. Appl. 13 (1971) 99. 10 K.O. Emery, B.C. Heezen and T.D. Allan, Bathymetry of the eastern Mediterranean Sea, Deep-Sea Res. 13 (1966) 173. 11 J.B. Hersey, Sedimentary basins of the Mediterranean Sea, in : Submarine Geology and Geophysics, ed. W.F. Whittard and R. Bradshaw, Proc. 17th Symp. Colston Res. Soc. (1965) 75. 12 W.B.F. Ryan, D.J. Stanley, J.B. Hersey, D.A. Fahlquist and T,D. Allan, The tectonics and geology of the Mediterranean Sea, in: The Sea, 4, ed. A.E. Maxwell (Wiley, New York, 1970) part 2, 387. 13 T.F. Gaskell, M.N. Hill and J.C. Swallow, Seismic measurements made by H.M.S. "Challenger" in the Atlantic, Pacific and Indian Oceans and in the Mediterranean Sea, 1950 - 1953, Phil. Trans. R.Soc. Lond. Set. A 251 (1958) 23.
J.M. LORT, W.Q. LIMOND AND F. GRAY 14 V.N. Moskalenko, New data on the structure of sedimentary strata and basement in the Levant Sea, Oceanology 6 (1966) 828. 15 J.M. Lort, The crustal structure of the eastern Mediterranean, Ph.D. Thesis, University of Cambridge, Cambridge (1972). 16 M. Lavergne, Model study of the thin bed problem in seismic refraction, Geophys. Prosp. 9 (1961) 60. 17 J.P.H. Poley and J.J. Nooteboom, Seismic refraction and screening by thin high-velocity layers, Geophys. Prosp. 14 (1966) 184. 18 W.Q. Limond, Deep seismic reflection profiling at sea, Ph.D. Thesis, University of Cambridge, Cambridge (1973). 19 J.S. Steinhart and R.P. Meyer, Explosion studies of continental structure, Carnegie Inst. Washington Publ. 62 (1961). 20 D.E. James and J.S. Steinhart, Structure beneath the continents: a critical review of explosion studies 1960 - 1965, in: The Earth Beneath the Continents, ed. Steinhart and Smith, Geophysical Mongraph 10 (1966) 291. 21 P.D. Rabinowitz and W.B.F. Ryan, Gravity anomalies and crustal shortening in the eastern Mediterranean, Tectonophysics 10 (1970) 585. 22 J. Woodside and C. Bowin, Gravity anomalies and inferred crustal structure in the eastern Mediterranean Sea, Bull. Geol. Soc. Am. 81 (1970) 1107. 23 W. Weigel and K. Hinz, Seismic measurements in the Ionian Sea and on the Malta Shelf, Comm. Seism. Europ. 15, 5 (1971). 24 H. Closs and K. Hinz, Seismische und bathymetrische Ergebnisse yon Mediterranean Rucken und Hellenischen Graben, in: Aegean Symposium, Hannover (1972).
PRELIMINARY
S E I S M I C S T U D I E S IN T H E E A S T E R N
MEDITERRANEAN
J.M. LORT*, W.Q. L1MOND and F. GRAY Department of Geodesy and Geophysics, University of Cambridge, Cambridge (Great Britain]
Received November 19, 1973 Revised version received February 4, 1974
Seismic reflection and refraction data are described from four locations in the eastern Mediterranean. Over the Mediterranean Ridge the profiles reveal a great thickness of material in the velocity range 3.6 - 5.5 km/sec. The crust is very disturbed and so penetration is limited, but at the foot of the Nile Cone an unreversed line gives a depth to the mantle of 27 kin. This suggests that the crust in the eastern Mediterranean is more likely to be continental than oceanic in structure.
1. Introduction
2. Experimental details
Much geophysical information has previously been collected in the eastern Mediterranean including bathymetric soundings, gravity, magnetics, heat flow and seismic reflection [ 1]. However, a marked gap in the knowledge of the area lies in the absence of seismic refraction studies. Therefore, in September and October 1971, the Department of Geodesy and Geophysics, University of Cambridge, undertook a cruise on board M .V."Researcher" to investigate by seismic methods the nature and thickness of the crust in the Levantine Sea. Fig. 1 shows the locations of seismic reflection and refraction profiles in relation to the major physiographic provinces. The lines R1, R4, R6 and R7 were first profiled using the air-gun reflection equipment to ascertain their suitability as sites for refraction experiments, and all file refraction lines except R1 were subsequently reversed as R4 R, R6 R, R7 R. A variable angle reflection/refraction experiment was also executed along line R4A, adjacent to profile R 4 - R 4 R .
The reflection system comprised a 500-cm 3 air gun operated at 100 atm and fired every 10 sec into a G6omecanique hydrophone array with three active sections [2]. A Mufax wet-paper recorder was used for immediate display of the records and the signals were also recorded on frequency modulated analogue magnetic tapes. These recordings were replayed through various analogue band-pass filters. The refraction experiments used the Cambridge radio telemetric sonobuoys [3] with an internal recording system [4]. The refraction lines were first shot over a range of 120 km using Geophex charges of up to 136 kg. The results were carefully studied before reversing three out of the four profiles, reducing their length and increasing the charge sizes to 3 6 0 450 kg at the furthest range, as it was obvious that the penetration achieved using small charges was limited. A new system of firing was devised, where two or three separate charges of 1 3 6 - 180 kg were linked by 15 m lengths of Aquaflex cord explosive and a 65 m length was connected to the fuse end. They were all discharged from a tilting platform after the fuse had been lit. This method proved more successful than either floating the charges or firing a single
* Now at : Institut de Physique du Globe, Observatoire G6ophysique du Parc St. Maur, 94 St. Maur des Foss4s, France
356
J.M. L O R T , W.Q. L I M O N D A N D F. G R A Y
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'
.--
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Fig. 1. Location of refraction and reflection profiles in the eastern Mediterranean, M.V. "Researcher", 1971. The main physiographic provinces are drawn from collated information: Cambridge (this paper), Marine Geophysical Survey, Watson and Johnson [8], Allan and Morelli [9], Emery et al. [10], lnstitut Franqais du Pgtrole, Sancho et al. [5].
large shot, when incomplete detonation or total misfires were experienced due to the charges breaking up. Generally three buoys were laid on each forward line, at 2.5- and 4-km spacing, and on the reverse line four buoys were used at 8-kin spacing to increase the spread. The profiles were oriented parallel to the strike of the structural trends of the Hellenic arc and Mediterranean Ridge for ease in interpretation (Fig. 1). Line R~ lies on the southern flank of the Mediterranean Ridge on the portion described as the Upper Plateau [5]. Line R 4 lies in the Herodotus Baisn, a relatively deep water area in a region of basin sedimentation between the Nile Cone and the southern flank of the Mediterranean Ridge. R6 lies to the northeast of the Herodotus Basin and on the southern side of the Mediterranean Ridge in an area of coarse-grained texture [6, 7]. R7 lies in only 1.5-km water at the foot of the Nile Cone. The physiographic provinces of Fig. 1 were drawn from collated bathymetric and reflection data available for the eastern Mediterranean.
3. Reflection profiles The reflection data confirm the sedimentary features of the Mediterranean Ridge already described in the literature [11, 12]. Line diagrams of R1 and R6 on the Ridge are included in Fig. 2. Rx displays a very irregular relief and coarse texture, formerly called cobblestone topography [ 10]. Diffraction hyperbolae are numerous on both echo-sounder and reflection records, and penetration is restricted to only 0.3 sec (two-way time), occasional small pockets of sediment occur, filling hollows to 200 m thickness. The lateral continuity of the strata is disturbed by immediate sub-surface faulting. In places on R6 a more continuous undulating reflector can be recognised, at 2.0 sec depth, having a wavelength of about 15 km and amplitude of 6 0 0 - 1000 m (this is represented by a thickened line in Fig. 2). The upper sediment velocity, measured by matching diffraction curves to the hyperbolae on the record is 2.1 km/sec. Faulting reduces the penetration locally along the profile. The prominent
PRELIMINARY SEISMIC STUDIES IN THE EASTERN MEDITERRANEAN
357
0 I
seconds
10
20
I
I
30 km I
R1
W
E 2 / x
v
4R0
l: w
E ---
R4
3-NE
SW
4-
;
6-
i
;
:
i
R7
I-E
W
i
Fig. 2. Reflection profiles of R 1, R 6, R4 and R 7. The vertical scale is in seconds of two-way time. were numerous and masked some of the events, but reflector, the acoustic basement, is again seen on R4 penetration was generally 1.2 sec. (Fig. 2) where basin sedimentation presents a thick sequence of conformable sediments, 1.3 km thick. The wavelength of folding is similarly about 15 km and the amplitude increases towards the northeast. The 4. Refraction r e s u l t s basement rises towards the southwest, where it is also fragmented by minor faulting. The refraction Few refraction data existed in the eastern Mediterresults later proved that this boundary, of high reranean before 1971. Short profiles made by Gaskell flectivity, marks the top of the consolidated sediet al. [13] showed maximum penetration to a 4 . 3 ments, which have a velocity of 3.8 km/sec. R4 in the 4.7-km/sec layer. Moskalenko [ 14] detected, on a northHerodotus Basin thus showed the most promising south profile between Crete and Libya, velocities of features for refraction studies. 3.7 and 4.7 km/sec overlying material with velocities Line R7 displays characteristic delta sedimentation of 6.1 and 7.0 km/sec which he correlated with granitwith a series of distributary channels on the generally ic basement. These observations do not seem to be flat surface, and deposition occurs as small lenses and well-founded and thus there was no significant inforslumped bodies. Intense folding and faulting are indicatmation available for the deep structure. ed by the abundance of diffraction hyperbolae (Fig. 2). The reduction of data from "Researcher" refraction Due to the shallow water depth of only 1.5 km, multiples
358
J.M. LORT, W.Q. LIMOND AND F. GRAY
Ca) vv
Time in s
25
20
15
R1
10
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15 I (b) Time in s
W
0-6+xj1"8 1-4+xl2. 3 5'3 +xJ3.5 8.9+xj5.8
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45 I
Distance
in km
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~,.'i5
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15
10
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i
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2.1+xl2.0 3"5+xf3.1 4.2+xl3.6 8-7+xl5"5
60 i
,
75 I
,
Fig. 3. Travel-time graphs of refraction profiles (a) R1, (b) R6, (c) R 4 and (d) R 7. Equations to the straight lines are in the form: intercept time (sec) + x/velocity (km/sec).
PRELIMINARY SEISMIC STUDIES IN THE EASTERN MEDITERRANEAN
359
(c) SW
Time L
NE
in s 30
25
20
15
R4R
R4
10
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15
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75
60
Distance
in
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105
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(d) .
Time in s
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0
10
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30
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45
60 Distance
in
75 km
90
105
360
J.M. LORT, W.Q. LIMOND AND F. GRAY
TABLE 1 Structure and location of seismic profiles Line
R1
R6
R6R
R4
R4A
R7
Lat. and long. (1)*
33°50'N 22° 30'E
33° 54:N 29° 00'E
34° 03'N 30° 27'E
32°42'N 27°44'E
33° 33'N 28°42'E
32° 54'N 32° 00'E
Lat. and long. (2)
33° 35'N 24° 00'E
34° 03'N 30° 27'E
33° 54'N 29° 00'E
33° 33'N 28° 40'E
32° 34'N 28° 09'E
32° 59'N 33° 12'E
v (velocity, km/sec) d (layer thickness, km)
1.5 1.4
1.5 3.0
1.5 3.1
1.5 3.1
1.5 2.9
1.5 1.6
o unconsol. d sediments
1.8, 2.3 0.3, 7.5
2.8 1.3
2.0, 3.1 0.5, 1.0
2.6 2.7
2.1, 2.3, 2.6 0.3, 0.6, 0.9
2.0 0.5
o (3.5 -4.1) d
3.5 2.0
3.7 11.3
3.6 9.5
3.8 0.3
3.8 1.0
4.1 5.9
o (4.5 -5.8) d
5.8
4.8
5.5
4.5, 5.4 9.0, 1.9
4.5, 5.4 4.0, 0.9
5.1 7.5
6.5
6.4
v (6.4 -6.5) d o mantle d
6.4 12.9 8.4
* Position (1): buoy position; position (2): last shot. d (layer thickness) is calculated under buoys for reversed lines. R 4 and R 7 are reversed profiles; R1, R4A, R6and R 6 and R6R are unreversed. profiles involved the application of a series of corrections to the travel times: one for clock drift of the buoys, one to the shot break for the slant range between the shot and the hull geophone m o u n t e d on the ship, one for the depth of the shot and the hydrophone, and a topographic correction, compensating at the level of the first sub-bottom refractor which was assumed to provide the relief. Traveltime plots were produced for each profile, the lines fitted by a least-squares method, and dipping-layer solutions computed from the apparent velocities and time intercepts to give the depths, true velocities and dips of the layers. Distances between shot and receiver were measured using the water travel time, though it was not always easy to recognise the direct water wave arrival D, which was much smaller in amplitude than the reflected wave R'. In this case the value of D could be extrapolated at farthest ranges as (D-R') approaches a constant value [15]. Profile RI is an unreversed line, the travel-time plot is shown in Fig. 3a and the deduced structure is summarised in Table 1. The profile, shot from west to east, gives first arrivals out to 65 kin, and velocities
of 1.8, 2.3, 3.5 and 5.8 km/sec. Discontinuities in the lines suggest the presence of at least two faults, the first affecting the 2.3-km/sec and 3.5-km/sec layers with downthrow of 1.5 km to the east. The layer of 5.8 km/sec has probably experienced faulting also as early arrivals are seen only beyond a range of 45 km (D in Fig. 3a). R6 is a reversed profile, also situated on the southern flank of the Ridge and faulting again complicates the structure. The travel-time plot is shown in Fig. 3b. The velocities in the forward direction, 2.8, 3.7 and 4.8/sec, do not agree with those of 2.0, 3.6 and 5.5 km/sec, probably because of local inhomogeneities or dipping layers. Further, an anomalous thin layer with a velocity of 3.1 km/sec occurs on Re R as second arrivals. A multiple with velocity 3.9 km/sec, which is omitted from Fig. 3b, results from the 2 . 0 - 3 . 6 - k m / s e c boundary after internal reflection in the upper layer. A depth of about 1.5 km from the sea bottom to the 3 . 6 - 3 . 7 - k m / s e c layer corresponds to the prominent undulating basal horizon seen at 1.5 sec on reflection records. A dipping-layer solution is hard to justify in this profile as the computed water depth does not fit the true depth, so
PRELIMINARY SEISMIC STUDIES IN THE EASTERN MEDITERRANEAN possibly the relocation of the line for reversal was not exact. Separate structures for both R 6 and R 6 R are therefore listed in Table 1. At R4 thick sediments occur and numerous multiples are observed, which can be attributed both to ringing within the sediments and also to reflection between the surface and bottom of the sea, before layer refraction. The arrivals seem markedly attenuated over the range 4 5 - 6 5 km but return out to 90 km (Fig. 3c). Even at this distance, however, there is no evidence of deep penetration to the Moho. Discontinuity in the travel-time graph may be explained either by a fault with a large downthrow to the southwest, or to an intermediate thin high-velocity layer which may prevent a greater part of the seismic energy from travelling to depth and thus weaken the arrivals [16, 17]. Shooting from the southwest along R4 velocities of 2.5, 4.0, 4.5 and 6.5 km/sec were recognised, which reverse with those of 3.8, 4.5 and 6.6 km/sec to give "true" velocities of 2.6, 3.8, 4.5 and 6.5 km/ sec. The structure for the reversed line R4 is presented in Table 1. A thin layer of velocity 5.4 km/sec is recognised only on R4 possibly due to its local occur-
rence, or it may not be detected on R4R because of sparse data. First to third order multiple reflections are recognised from the 3.8, 4.5 and 6.5-km/sec layers, and the layer at the base of the consolidated sediments recognised on reflection records as the acoustic basement, produces several multiples by internal reflection in the overlying sediments. To adjust the results to the depth of the sea measured by a PDR it is necessary to postulate an upper sedimentary layer which is not detected by refraction. An average velocity of 2.0 km/sec was assumed for the unconsolidated sediments after fitting curves to the hyperbolae seen on reflection records. This velocity agrees with Gaskell et al. [13] and Ryan et al. [12]. Fig. 4 represents a structure profile along the Ridge from R1 eastwards, showing that the crustal column broadly thickens in this direction. The arrivals at RT, a reversed profile at the foot of the Nile Cone in a shallow water depth of 1.5 km, were again swamped by multiples. However, first arrivals were stronger than on any other line, and were received out to 90 km even using small charges on RT, while in the reverse direction, arrivals were obtained at 112 km (Fig. 3d). Velocities of 4.0, 5.3 and 6.6 km/sec were
t
I
I
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0"3~'4
361
~
~
-°
3.11--175-1
t ,
4 8
/
25
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I
I
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Fig. 4. Structure profiles for R1, R4, R 6 and R 7. The figures adjacent to the columns are thicknesses in km and the scale shows depth in km.
362
J.M. LORT, W.Q. LIMOND AND F. GRAY
.
.
.
.
.
#
:
i
¸
#
iiiiii
iiii
i
i
i
i
.......
Fig. 5. Jet-pen display of vertical reflection profile R4 A, filter: 10- 100 Hz. Distance in km is shown along the horizontal axis and the vertical axis is the two-way time in seconds. obtained along R7, shooting from the west end, and these reverse with velocities of 4.2, 4.9 and 6.2 km/sec. The structure for the reversed line R7 appears in Table 1, with layer velocities of 2.0, 4.1, 5.1 and 6.4 km/sec above a mantle velocity of 8.4 km/sec occurring at 27 km depth, which was detected only on RTR when the large charges were used. At R4A a variable angle reflection/refraction experiment was also conducted in the Herodotus Basin using the explosive line sources Aquaflex and Geoflex. Four sonobuoys were laid at the start of the line and the ship then steamed past the buoys, towing a G6omecanique array. 200 shots of 3 0 - 1 0 0 m lengths were fired at 2-rain intervals; vertical reflections were received over a range of 40 km and variable angle reflections and refractions were received by the sonobuoys over 50 km range. The results from the array were replayed from tape recordings, broad band-pass filtering was applied, and the correlations were clearly seen on a jet-pen display of the traces side by side (Fig, 5). The prominent undulating acoustic basement at 1.5 sec depth again corresponds to the top of a layer of velocity 3.8 km/ sec. At 8.0 sec depth there appears to be a real event,
which cannot be explained as a multiple. Owing to the fact that during the variable angle experiment the shot breaks were not recorded on the same magnetic tapes as the seismic variable angle reflection and refraction data, it was difficult to replay the traces side by side to obtain correlations of seismic signals. Therefore, the data on the buoy tapes were digitised, replayed, and the necessary time corrections applied to give correctly aligned traces on a time-distance diagram. The results from one buoy only are presented here. The final plot was obtained after low pass filtering at 40 Hz, and gain scaling (Fig. 6). The display shows a slightly asymmetrical direct wave reflection hyperbola, due to gently dipping bottom topography. The offset, i.e. the distance at which the ship passed closest to the buoys, was accurately measured for each buoy for use in a 3-D computing model, and the profile was then interpreted as a flat, multi-layered system. A greater amplitude is observed on the left-hand limb of the hyperbolae than on the right. This indicates the directionality of Aquaflex as the source moves away from the receiver [18].
PRELIMINARY SEISMIC STUDIES IN THE EASTERN MEDITERRANEAN
Fig. 6. Final aligned plot of R4A variable angle reflection data for a single buoy. Filter: 40 Hz. Horizontal axis is distance m kin, vertical axis is two-way time in seconds. Events are marked by letters which are described in the text. The display reveals the following characteristics (Fig. 6): A,A' - prominent sea-bottom multiples up to the 4th order. B - a region of low sediment velocity arrivals immediately below the sea bed, whose amplitude decreases rapidly with range. C - a prominent higher velocity reflection, 0.8 sec belowA. D - the refraction arrival tangential to the reflector C emerging from a high energy, large amplitude area.
363
Fig. 7. Correlations visible in the final plot. Solid lines mark well-defined events. E - a complex of small amplitude events, 9.0 sec from the origin. F - a large amplitude, high frequency multiple, possibly consisting of more than one event, similar to C. B - A ' - higher velocity events, which cannot be correlated satisfactorily over a large enough range to distinguish them. The basic structure was deduced by fitting reflection and refraction curves to the prominent events (Fig. 7). The multiples were then used in conjunction with the first arrivals on both limbs of the hyperbolae to obtain a compatible model.
364
J.M. LORT, W.Q. LIMOND AND F. GRAY
25
20
15
the layer may be dipping. The vertical reflection profile supports this, for the layer is seen to be undulating. There is possibly a great density contrast at the 2 . 6 - 3 . 8 - k m / s e c interface between the unconsolidated sediments and the consolidated sediments below. Models were also fitted to the multiples which were derived from two different processes: (a) the reflection of sound between the sea surface and the sea bottom; (b) the reflection of sound internally in the sediment layers specifically between the sea bed and the top of the 3.8-km/sec layer. Thus at the event F described previously, the energy increase corresponds to multiples from the 3.8- and 4.5-km/sec layers, from reflection both within the water and the sediment layers. The sedimentary reflectors are thus clearly defined, as the correlation between the analogue vertical reflection and the digital variable angle reflection data is good. Below the 4.5-km/sec layer, however, there remains some ambiguity, as correlations can only be seen over a limited range, and so the higher velocity events are not well determined. The velocities determined from the Aquaflex experiment include additional sedimentary velocities of 2.1 and 2.3 km/sec to those determined from fire refraction results, otherwise the figures are similar (Table 2). However, if the events have been picked
TABLE 2 Comparison of structures R4A and R4
50
Fig. 8. Basic structure of R4A, velocities of 2.1, 2.3, 2.6, 3.8, 4.5 and 5.4 km/sec are recognised. Squares mark reflection curves, triangles mark refractions, solid triangles mark direct wave.
Depth (km) R4 R4R
Depth (km) R4A
Interval velocity in layer (km/sec) 1.5
3.1
2.7
2.9 2.1 3.2 2.3 3.8
The structure obtained is shown in Table 1 and Fig. 8. Sedimentary velocities of 2.1,2.3 and 2.6 km/ sec were recognised above a prominent 3.8-km/sec layer at C. D represents refraction arrivals with velocity 3.8 km/sec. The arrivals in this area have larger amplitudes because one is approaching the theoretical critical distance for the reflectors. A slight misfit of the model to the curve for 3.8 km/sec indicates that
2.6 5.8
5.6
4.7
6.1
10.7
5.7
3.8 4.5 15.1
9.7 5.4
17.0
17.1
10.6 6.4
PRELIMINARY SEISMIC STUDIES IN THE EASTERN MEDITERRANEAN
vel. km/s
2 way time secs
15
39
;~ I 2.3
Q';] O5
2.6
07
38
05
45
Fig. 9. Sketch of the multiple-producing event F in the variable angle reflection profile of R4A. correctly for R4 A it seems that the deeper layers occur at a much shallower depth on R4 A than o n R 4 . It may be that the first arrivals have not been detected using conventional refraction techniques. Here the difference between the structure for the two profiles is equivalent to the additional time required for a multiple between the 2.6-3.8-km/sec interface and the sea surface (Fig. 9). Unfortunately, the experiments at R4A did not allow sufficient penetration for the crustal depth to the Moho to be determined. Extremely long profiles are required to obtain arrivals from depths as great as 27 km, as on R7 R, and further difficulties are encountered in a fractured crust which does not transmit energy easily.
365
anism for the thickening process as the African plate plunges below the relatively south-moving Aegean plate [21, 22]. The Mediterranean Ridge would then be a zone of adjustment for great thicknesses of material and intense deformation of recent sediments would occur. However, Sancho et al. [5] believe that there has been no recent crustal thickening in this region as their deep reflection profile, located on the Lower Plateau of the Mediterranean Ridge, southwest of Crete (Fig. 1) reveals a relatively undisturbed thick sedimentary series, probably of Tertiary age. Our data in this area on the Upper Plateau ( R , ) reveal intense faulting with little penetration. Further to the east on profiles R4 and R 6 the folding of an acoustic basement reflector suggest that, in fact deformation has occurred prior to the deposition of the upper series of sediments, which are 1 0 0 0 15 000 m in thickness. Therefore the Ridge does not seem to be formed from thickening of recent sediments, as previously believed. Refraction results from German workers (Hinz, Closs, Weigel) indicate that in the Ionian Sea a 6.1km/sec layer occurs at about 10 km depth, and an 8.1-km/sec layer at 17 km depth [23, 24]. Here, the Moho is significantly shallower than our results suggest for the far eastern Mediterranean.
Acknowledgements 5. Conclusions The combined results of these seismic experiments indicate that the crust of the eastern Mediterranean can neither be ascribed a simple oceanic nor a continental character. Over the Mediterranean Ridge material of velocity 3 . 6 - 5 . 5 km/sec is extremely thick, the depth to the 4.8-km/sec layer on R6 being 15 km. This is much greater than material of equivalent velocity in the west, where the depth to a lowvelocity mantle is only about 12 km. If the unreversed velocity of 8.4 km/sec obtained on the line RTR is accepted then the total crustal thickness, at least at the foot of the Nile Cone, is 27 kin. In this case it seems that the crust is more likely to be continental [19, 201. Gravity results are consistent with this deduction, and crustal shortening has been suggested as a mech-
This work was supported by grants from the Natural Environment Research Council. We would like to thank Dr. D.H. Matthews, Mr. D.I. Sewart and Dr. A.P. Stacey, and Professor Sir Edward Bullard for reading the manuscript.
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