J. Geodynamics Vol. 13, Nos 2-4, pp. 141-161, 1991 0264-3707/91 $3.(10+0.00 Printed in Great Britain. All rights reserved Copyright (~ 1991 Pergamon Press plc

OPHIOLITES AND THE O C E A N I C CRUST: N E W E V I D E N C E F R O M THE T Y R R H E N I A N SEA AND THE W E S T E R N ALPS

G. MASCLE ~, M. LEMOINE 2, J. MASCLE z, J. P. R E H A U L T 3 and P. TRICART 4

'Institut Dolomieu, Rue M. Gignoux, 38031 Grenoble, France 2Laboratoire de G(odynamique sous-rnarine, B.P. 48, 06230 Villefranche sur Mer, France 3Universit# de Bretagne Occidentale, 6, avenue Le Gorgeu, 29283 Brest, France 4Universit# du Maine, Route de Laval, 72017 Le Mans, France

A B S T R A C T

Mascle, G., Lemoine, M., Mascle, J. , Rehault, J. P. and Tricart P., 1991. Ophiolites and the oceanic crust: new evidence from the Tyrrhenian Sea and the Western Alps. Journal o f Geodynamics, 13:141-161

The succession recovered in ODP hole 107-651 in the young oceanic Vavilov basin (Tyrrhenian Sea) com- prises, beneath a thick Pleistocene to Upper Pliocene sedimentary cover (chiefly volcanoclastics), four basement units: (1) MORB-type basaltic pillows and breccias; (2) a complex succession made of dolerites, albitites, basaltic breccias, metadolerite pebbles (including an intercalated sandy layer with peridotite clasts);

(3) MORB-type basaltic pillows and breccias; (4) highly serpentinized peridotite. Between units 3 and 4, granitoid pebbles occur.

This sequence is surprisingly similar to successions known in the Western Alps' Tethyan ophiolites. There, the sediments (Callovian-Oxfordian radiolarian cherts) lie stratigraphically upon breccias mostly derived from underlying serpentinite, and sometimes gabbroic basement. At some places, thin basaltic (tholeiitic) pillows and breccias occur between the radiolarian cherts and the breccias.

From the comparison between a present day setting (the central Tyrrhenian Sea) and a formerly emplaced basement succession (the Western Alps), we stress the following (a) both the here-discussed ophiolites and oceanic basement are different from classical ophiolite sequences; (b) both occurrences imply unroofing of

mantle rocks that therefore were directly outcropping on the seafloor; (c) such a comparison may indicate a very slow spreading rate for the Alpine Tethyan ocean.

I N T R O D U C T I O N

This paper deals with a brief comparison between two areas where surprisingly similar magmatic basement successions have been described: (a) within the present day Tyrrhenian Sea, a small and very young back-arc-type basin develop- ing in the middle of former orogenic belts; and (b) within the French Western Alps, a Mesozoic oceanic-type basin and its surrounding margins (Fig. 1).

We will firstly review here the magmatic basement succession drilled in the central Tyrrhenian Sea and then outline data from ophiolitic outcrops known in

141

142 M A S C L E ET AL.

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Fig. 1. Location of the Galicia Gorringe Alp ine-Apennine and Tyrrhenian occurrences of ultramafic ocean floor

in western Europa.

the Western Alps. Finally, the strong similarities between both ophiolite succes- sions should help to better understand the Tyrrhenian magmatic basement emplacement events and, conversely, to substantiate that the Mesozoic oceanic space, formerly active between Europe and Apulia, was likely a relatively narrow space, probably characterized by a slow accretionary rate.

I. T H E T Y R R H E N I A N SEA

One of the major goals of ODP Leg 107 in the Mediterranean Sea was to determine the nature of the basement flooring the deep basins of the central Tyrrhenian Sea. During the Leg, three holes were drilled into the magmatic basement of the two deep Vavilov and Marsili basins (Kastens et al., 1987). Pre- viously, hole 356 had been drilled on the eastern side of the Northern Vavilov basin (Hsu et al., 1978).

The four sites drilled in this small back-arc basin have recovered basaltic series; in addition, ODP hole 651 yielded a lithologic succession including serpentinized peridotites.

Geophysics o f the Tyrrhenian Sea

An outline of the structure of the Tyrrhenian Sea has recently been given by Rehault et al. (1987). According to these authors, the Tyrrhenian Sea (Fig. 2), a small basin that has opened behind the overriding edge of a subducting plate boundary (the Calabrian subduction zone) is characterized by: (1) a Benioff zone, dipping to the northwest beneath the toe of Italy (Caputo et al., 1972;

OPHIOLITES AND THE OCEANIC CRUST 143

Fig. 2. Simplified bathymetry of the Tyrrhenian area (200, 1000, 2000, 3000 m bathymetric lines). The deep central basin (dotted) is enclosed by the 3400 m isobath. Locations of ODP 107 sites are shown.

Gasparini et al . , 1982) with an underlying parallel high seismic-wave velocity zone (Spakman, 1986); (2) an active volcanic belt lying on the arcward side of the basin, the Eolian Islands (Barberi et al . , 1974; Beccaluva et al . , 1985; Keller, 1982); (3) a progressive shoaling of the Moho beneath the central deep basin (Recq et al . , 1984; Steinmetz et al . , 1983; Duchesnes et al . , 1985); (4) active tholeiitic volcanism, chiefly located in the center of the basin (Barberi et al . , 1978; Dietrich et al. , 1978); (5) high heat-flow (up to 200 mw/m 2) (Erickson and Von Herzen, 1976; Della Vedova et al , , 1984; Hutchinson et al . , 1985); and (6) more or less oceanic-type magnetic anomalies indicative of a magmatic type basement (Morelli, 1970; Vogt et al . , 1971; AGIP, 1981).

Heat-flow results (Della Vedova et al . , 1984), refraction data (Steinmetz et al . , 1983), as well as the distribution of dredged basalts (Selli et al . , 1977) have shown the existence of two discrete areas of magmatic crust within the Tyrrhenian Sea: a northwestern basin (the Magnaghi-Vavilov basin) and a south-eastern basin (the Marsili basin) (Fig. 2).

The almost-triangular Vavilov basin (Fig. 3a) is a relatively small basin (width of 60 km, length of about 150 km) trending N30°E bounded on its eastern and

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western rims by very steep continental slopes, at the base of the Sardinia and Tuscany margins respectively. A voluminous seamount , the Vavilov volcano, (culminating at 684 m below sea level) bisects the southern part of Vavilov basin, and has been interpreted as a potential spreading center. Within the basin, seis- mic reflection profiles (Fig. 3b) show a diffracting basement covered by a thin sedimentary blanket, locally reaching a thickness on the order of 1 km (Rehault et al., 1987). In the northern sector, where the volcano lava flow structures are not concealed, the basement consists of a succession of 10 to 20 km wide and axial bulges sometimes offset by lineaments trending N100°-120°E which are well ex- pressed on sedimentary isopachs (Fig. 4).

These have been interpreted as possible transform lineaments, truncating an incipient spreading center (Moussat, 1983; Rehault et al. . , 1987).

Finally, two symmetrically arranged, almost-linear, narrow basement ridges lie on both sides of this axial zone (Fig. 4).

Taking these data into account, the Vavilov basin can be viewed as a young oceanic basin of restricted extension, presently floored by MORB type basalts, and bissected by an axial spreading center slightly offset by transform lineaments.

The Vavilov basin basement

The three DSDP and ODP sites have been drilled on the eastern side of the axial bulge (ODP Site 651), and on both western and eastern linear ridges (ODP Site 655; DSDP 373). At all three sites oceanic MORB type tholeites were recovered (Kastens et al., 1987; Bertrand et al., 1990; Beccaluva et al., 1990) with radiometric ages ranging from 2.4 to 2.6 Ma for the axial bulge to 4.3 +_ 0.3 Ma on site 655 (Feraud et al., 1990) and between 3.5 to 4 MY for site 373, according to the recent discussion by Sartori et al. (1987).

At ODP Site 651 (located at 40°09'03N, 12°45'39E by 3578,0 m) (Figs 2, 3 and 4), the total penetrat ion was 550.9 m. Three major formations were drilled and correlate to three main sequences of reflectors as shown on seismic reflection data (Kastens et al., 1987). The uppermost unit, from 0 to 110 m below sea floor (mbsf), is a succession of turbiditic sequences, chiefly made of reworked volcani- clastic materials (cinerites, tufts, pumice, glass) intercalated within hemipelagic foraminiferal and nannofossil oozes. The second sedimentary unit (190 m thick) also consists of volcaniclastic turbidites, but with a downward increasing amount

Fig. 3.a, Detailed bathymetr ic map of the central deep basin and surrounding margins. Contour interval: 100 m.

The main scarps and highs or seamounts have been mapped using a detailed mul t ibeam bathymetric survey in

1082 on board the Jean Charcot. Note the asymmetry of the lower margin seamounts , notably the Monte de

Marchi and Flavio Gioia bathymetr ic highs. Locations of profile 3B and O D P Site 651 are indicated (after

Rehaul t et al., 1987). b, Interpretat ion of a seismic profile across the lower Sardinia continental margins and the

Vavilov basin; hatched: continental basement ; vv: oceanic basement (peridotite and basalts)" dots: Upper Miocene to lower Pliocene sedimentary sequences; in white: Upper Pliocene to recent sedimentary cover. To be

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of hemipelagites. The upper part of the third unit corresponds to a 40 m thick marly dolomitic section resting on an altered basaltic basement. The basement consists of a complex succession drilled on 165 m (Figs 5 and 6). From 386.15 to 464.0 mbsf (cores 42 to 49), (1) we first note a basaltic sequence made of MORB type (Bertrand et al . , 1990; Beccaluva et al. , 1990) altered basaltic pillow lavas; (2) a 10 cm thick dolerite and albitite layer and a 14 cm thick interval of serpen- tinic pebbles representing the second unit; (3) a second dolerite and albitite sequence, about 30 m thick, is interrupted by two dolomitic sedimentary layers, and by two thin intercalations of serpentinite breccias; (4) a basaltic sequence (30 m thick), also made of MORB-type and altered pillow lavas and basaltic breccias, lies upon (5) a 80 cm thick coarse and graded sedimentary sequence (mainly made of serpentinite clasts); this unit is followed by a thin layer of basaltic breccia and by (6) a thin layer of granitic pebbles; finally (7) the last sequence (Fig. 7) consists of highly serpentinized harzburgites and dunites (29 m) including two in- tercalations of granitic pebbles (at 10 m and 20 m). These pebbles are believed to result from drilling pollutions originating from the upper granitic interval, since they occur on the top of each core. In the peridotites, the foliations vary from 11 ° to 62 ° in dip, with an average of 20 ° between 521.9 and 535.5 mbsf and 35 ° below (541.2 to 550 .9 mbsf). The foliation indicates a high-temperature deformation (HT) characterized by spinel and sometimes flattened orthopyroxene (Bonatti et al . , 1990). In sample 107-651 (A) 58 R1 131-133 the presence of chlorite indicates

O P H I O L I T E S A N D TH E O C E A N I C CRUST 149

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ments (PMG); B: general bearing of the HT-foliation dip; C: value of HT-foliation dip as measured on board (dot) and on oriented thin sections (cross); D: down-dip of mineral lineation measured on board (dot) and on oriented thin sections (cross); E: general bearing of the dip of tensional fractures of the three successive generations.

a second foliation event but in a lower thermal regime (Bonatti et a l . , 1990). The dip appears to be parallel to the HT foliation.

Directions of mineral lineations have also been measured on both cores and on oriented thin sections for 9 samples (Fig. 8). The observed angles, between the dip of the foliation and the lineation, show a large range of variation (3-90 degrees). Such an observation suggests that the deformation may have been generated in a dominant strike-slip regime (Girardeau, 1986). Finally, three different and successive generations of extensional microfractures have been characterized using cross-cutting relationships and observations on successive in- fillings. The oldest ones (F1) are common (Fig. 8) and show a dip between 10 ° and vertical, while the pitch of slickensides on them remains large (75°); conju- gate patterns have been observed. All these observations suggest that the deformation was chiefly tensional but with a strike-slip component. F2 fractures

150 MASCLE E T AL.

are less abundant (Fig. 8) and show a dip between 30 ° and vertical, with steeply plunging slickensides (pitch 80°). F3 fractures are also common (Fig. 8); they dip from horizontal to vertical and, as shown by the presence of successive infillings, these fractures have often been using the previous F1 pattern which were reopened. All these observations tend to indicate that the general stress field did not change considerably during the period of brittle deformation, even when the temperature was decreasing (Bonatti et al . , 1990).

Despite uncertainties concerning the continuity of the drilled section, tentative core orientation measurements were a t tempted using paleomagnetic data. The results support that the strike of HT-foliation is nearly NW-SE and that the trend of lineation is nearby NNW-SSE. The dominant fracture directions are NS to NE-SW and NW-SE to EW.

D i s c u s s i o n

On board, in 1986, the party speculated that the drilled peridotites were part of a former ophiolitic alpine (or older) system foundered during the recent disten- sive evolution of the Tyrrhenian Sea domain. If true, the rocks should have exhibited evidences of Alpine-type deformation and metamorphism, but these were not observed. Alpine-type deformations and metamorphism have only been observed in ophiolitic rock fragments drilled on the De Marchi seamount at site 656, at the base of the Sardinia continental margin (Fig. 2).

On board, we also noticed that the magnetic inclination measured on drilled samples was compatible with the recent inclination and not in agreement with the inclination expected for peridotites emplaced during the Alpine event. Alpine emplaced peridotites in such case should have tilted tectonically for the exact in- clination difference between Eocene times and today. As we have no indications of tectonic tilting of the peridotites, we therefore believed that the drilled mantle derived-rocks were samples of a true, and recent, oceanic basement flooring the Vavilov basin, as supported by geochemical and geophysical evidences (Bonatti et al. , 1990; Rehault et al . , 1987).

It should be stressed, however, that the recovered sequence appears quite different from a classic ophiolitic sequence (Penrose Conference, 1972). In parti- cular, the basaltic pillows member appears very thin. Neither evidence of a dyke complex nor cumulates were observed.

In this setting, it is also important to stress that the sediments are very close to the top of the serpentinized peridotites. Fur thermore, serpentinites appear as reworked fragments (pebbles and breccia) along with clasts of continental material in different occurences. This implies that, during the Vavilov basin creation, altered, uplifted material that most probably originated in the upper mantle was locally outcropping directly on the sea floor, not too far from a source of continental material, before and during the basalt emplacement.

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Fig. 10. Location of Queyras ophiolitic massifs. Map A: sketch of the internal zones of the Western Alps Black-- the main oceanic thrust sheets (ophiolites and overlying sediments); hatched---other internal units.

OPHIOLITES AND THE OCEANIC CRUST 153

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II. THE WE ST ERN ALPS OPHIOLITES

Ophiolitic units of the Western Alps

The ophiolites of the Western Alps, as well as those of Corsica and of the Northern Apennines, originated from the Ligurian ocean (Lemoine, 1984; Lemoine et al., 1986). At the latest Middle Jurassic (160 m.y.), the initial break-up of Pangea actually gave birth to two segments of the Mesozoic Tethys, namely the Central Atlantic and the Ligurian Tethys (Fig. 9). The latter ocean was located between two passive continental margins, a European margin to the North and Northwest and an Apulian margin (i.e. African) to the South and Southwest. Most of the Western Alps' structural units derive from the European margin that underwent a Liassic-Middle Jurassic rifting (Lemoine et al., 1986); the initiation of spreading in the Ligurian ocean occurred in the Late Middle Jurassic (160 m.y.), and lasted at least a few tens of Ma later (Late Jurassic, earliest Cretaceous). The Atlantic Ocean still continues to spread, wheras, since 70 Ma, Africa-Europe convergence has led to the disappearance of the Ligurian Ocean and to the continental collision that gave birth to the Alps.

In all the the ophiolite massifs of the Western Alps and of the Northern Apennines, the three major components of an ophiolite suite are present: serpen- tinized peridotites, gabbros and related cumulates, tholeiitic pillow basalts. As a matter of fact, the Northern Apennines are the "locus typicus" of Steinmanns'

154 MASCLE E T AL.

Fig. 12. Photograph of an ophicalcitic sedimentary breccia (Chenaillet)

[1926] ophiolite "trilogy". But the mutual relationships of these ophiolitic rocks are here quite different from those of some "classical" ophiolite massifs (e.g. Troodos, Oman) that were taken as models of the lithological composition and layered structure of the crust of present-day ocean (Penrose Conference, 1972).

The ophiolitic thrust-sheets of the Western Alps comprise remnants both of the ophiolitic ocean-floor, and of its sedimentary cover. During the Alpine (Late Cretaceous-Cenozoic) collision, these ophiolitic nappes underwent several phases of strong synmetamorphic folding (Tricart, 1984). In some ophiolite massifs, however, both the internal structure of the ocean floor and its relation- ships with the overlying sedimentary cover of Jurassic-Cretaceous age were preserved (Tricart et al., 1983, 1986; Lagabrielle et al., 1984, 1985), especially in the Queyras area (Fig. 10).

Some examples from the Queyras ophiolites

In the Western Alps, and especially along the Brianqon-Chenaillet and Queyras transects (Fig. 10), small scatterred ophiolitic massifs, which retained

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s e dimen ts , .e;-*i. ~:. T. i-" :-..-?: 1 ~ ~ " ' " " " : ' ; l l ~ ~ ' ! : : i l Serp. Brecc ia

~ ..- ~ . U N C O N F O R M I T Y

Gabbro ~ v ~ /~ /~f 1 BASEMENT

Serpentinite

Fig. 13. The Rocher Blanc series (Cascavelier massif). Location on Fig. 11.

155

their original, but now metamorphosed, sedimentary cover (Callovian to Kimmeridgian radiolarian cherts up to Upper Cretaceous calcschists: Lemoine and Tricart, 1986) are inferred to represent the overthrust remnants of the Ligurian ocean floor.

As an example, the relationships between various kinds of ophiolitic rocks and their Jurassic-Cretaceous sedimentary cover in two serpentinite massifs (Cascavelier and Roche Noire, location on Fig. 10) have been reconstructed by Tricart and Lemoine (1983) as shown on Fig. 11. There, the oceanic sediments of the Ligurian Tethys rest upon a thin layer (a few centimetres to a few tens of metres) of serpentinite breccias (a part of the "ophicalcites"; Fig. 12).

All these sediments (ophicalcitic breccias and overlying pelagic oozes) were laid down directly upon serpentinized peridotites (mainly lherzolites) and upon minor gabbro bodies which were previously intruded into the lherzolites; at some places in other massifs these sediments rest upon already foliated and metamorphosed (amphibolitized) gabbros.

In addition, in the section of Rocher Blanc in the Cascavelier massif (Figs 11 and 13) a polygenic conglomerate with granitic pebbles is interlayered between the sedimentary ophicalcites and the pillow-basalts, and is cut by basaltic dykes (Caby et al. , 1971). Whether these granitic rocks are "oceanic", or on the contrary originate from a continental block, is a matter of discussion. The last investiga- tions, however, performed on the zircon crystals clearly indicate a continental origin (Piboule et al . , 1988). Whatever the case, this Rocher Blanc section strongly recalls that of ODP Hole 651 in the Vavilov basin (Figs 5 and 13).

156 MASCLE E T A L .

In all the investigated sections, the basaltic pillow-lavas are often lacking, and in any case are not very thick (a few tens of metres to a few hundreds in some rare occurrences, such as the Chenaillet).

Therefore, when compared with the "classical" ocean crust (Penrose Confer- ence, 1972), or with the ophiolites suites of Cyprus or Oman, the Ligurian ocean floor was quite different (Tricart and Lemoine, 1991): thin and above all discon- tinuous basaltic lavas, no dyke complex, neither true gabbroic rocks. The first ocean floor that appeared at the very opening of the ocean was made up of ser- pentinized peridotites with minor gabbro bodies; these rocks, at least the gabbros, bear the imprint of earlier tectonization and metamorphism, e.g. the foliated and amphibolitized gabbros of the Chenaillet massif (E. of Brian~on; see Fig. 10) which are cut by basalt dykes (Mevel et al., 1978) that, very probably, are the feeders of the overlying pillow lava flows. The upper surface of the serpen- tinized peridotite and associated gabbros is a major unconformity (Figs 11 and 12); above this unconformity, ophicalcite breccias were deposited, then, locally, pillow-lava flows were emplaced; all these events occurred before the deposition of the first pelagic sediments (Fig. 14).

Discuss ion

Numerous, if not all, other ophiolite massifs of the Alps, Corsica and the Apennines display comparable features.

Therefore, the reconstructed Jurassic structure of the Ligurian ocean floor does not display the standard ophiolitic suite (ultramafites, gabbros, dyke complex, thick pillowed basalts; Penrose Conference, 1972): it is not a true "oceanic crust" in the commonly accepted sense. In the Queyras serpentinite massifs, the following points should be emphasized: (a) the poorly depleted

Deposition of pelagic sediment 1 / I v i i , , ' /~ l . , " '~ t I M ~ Basaltic flows (2nd magmatic evellt )

:-:-:.:.:...~-:-,-_,-r~..-.:.~,--~ B ~ ~ . . ~ . = ~ : ' T - ~ : " ' ~ f - ' - ~ . ' . ' . ' _ .3 . .~ r - . . ._ ' ~.,;-:.- Rapid deposition of 0C2 brecclas >~ ~ . + ~ ® ~ ; ~.~+' .~; - . Uncovering of serpentinite and

gabbroic sea floor *'5

' " ~ ~ Brittle deformation (0C1) * . . . . .~ . . . . . .~_~ - ~.. Ductile deformation and metamorphism o =

. . . . : . . . . ~ Mantle partial melting and mtmsion of u~ B Basalls M Upper Jurassic marbles qabbros (first magmatic event ) '~ G Gabbros R Upper 3"urassic radiolarian Gf Foliated gabbros cherts S Serpentinites OC1 , 0C2 , ophicalcites 1 and 2 ~D 9~

Fig. 14. Sketch of the mutual relationships of the Ligurian ophiolites and of the overlying Late Jurasic

sediments with inferred major geological events (after Lemoine et al., 1987).

OPHIOLITES AND THE OCEANIC CRUST

. $tretch/nQ and subsidence

• s/retching and subsidence

----...._

A c t i v e fau l t

\

\ \ Rehcl fau l t

'..... Future fault

-~-----=.-=- D u c t i l e • '¢'- s h e a r zone

157

- 1 - - -

( subsTdence ~ (~ $1re/ch/n# t

• O°o° . ° ° o °

~ , , ~ ° " . . " o .° o o° o

t 3 load of upper p . ~ . = ~ -- ~---'---'q is removed ~ ' ~ . . . .

seolevel slretch/nq and substdence

Fig. 15. Model of asymmetrical opening of a continental rift above detachment faults leading to mantle unroofing in the central Vavilov basin (after Kastens e t al., 1988).

character of the peridotites (mainly lherzolites) indicating a low degree of partial melting; (b) the absence of a sheeted dyke complex; (c) the very small thickness of the basaltic lavas; (d) the peculiar chronology of events involving the denuda- tion of the mantle derived peridotites, their appearance on the sea floor, and their erosion before the local emplacement of basaltic lava flows which predates the beginning of pelagic sedimentation.

In order to explain these peculiarities, several models have been proposed. The first ones (Decandia and Elter, 1969; Lombardo and Pognante, 1982) suggest a decoupling of the already thinned continental crust along the Moho, leading to a denudation of the underlying mantle. Another model has been proposed after

158 MASCLE E T A L .

the drilling (ODP Leg 103, hole 637) of the peridotite ridge located at the foot of the Galicia margin (Fig. 9) of the Eastern North-Atlantic (Boillot, Winterer et al . , 1987); this latter model has been adapted to the Ligurian Tethys (Lemoine et al . , 1987).

This model, which is indeed provisional, requires the occurrence of a large in- clined detachment zone that cuts across the whole lithosphere, successively leading to (i) the thinning of the continental crust, and then (ii) the unroofing of the underlying mantle. On the other hand, Karson (1987) and Fox (1987) have discussed a very complex internal structure of oceanic crust produced at slow- spreading ridge segments; both suggest that, in these areas, lower crustal and even upper mantle rocks could be directly exposed on the sea floor.

CONCLUSIONS

The strong similarities between the sequence drilled at hole 651 in the Tyrrhen- ian Sea and the ophiolitic sequences of the Western Alps allow a similar model to be proposed for both. The unroofing of the Tyrrhenian peridotites may result from considerable offset along a detachment fault system cutting across the continental crust and the upper mantle at the emplacement of the future Vavilov basin in Late Miocene times (Fig. 15), as proposed by Kastens et al. (1988) and by Mascle and Rehault (1990). They may also result from slow spreading centers as noted by Karson (1987) or Fox (1987).

Both analyses (Vavilov and Queyras) are mutually enlightening. The observa- tion of outcrops in the highly variable and complex ophiolitic sequences of the Alps allows the section at hole ODP 651 to be better explained. In the latter case, the basalts build a thin succession of flows interlayered with peridotic breccias; this sequence lies directly upon the serpentinized mantle peridotites that under- went a complex structural history before their unroofing to become the sea floor. Their reworking in the breccias suggests a rugged topography of the top of the sea floor peridotites. All these features are observed in the Western Alps.

Conversely, we know that the Vavilov basin is a small oceanic realm that opened recently, and was characterized by a very slow rate of accretion. Very likely, the ophiolites of the Apennines and of the Western Alps are remnants of the floor of a narrow oceanic basin, as shown by kinematic reconstruction (Lemoine, 1984), whose accretion rate was also very slow.

Contribution N°491 of the G E M C O (Groupe d 'Etude de la Marge Continentale et de l 'Ocean U R A CNRS 718) and No. 1055 of LGCA (Laboratoire de G6odynamique des Chaines Alpines U R A CNRS 69).

A C K N O W L E D G E M E N T S

We thank the Captain and the crew of Joides Resolution and ODP scientific staff. Discussions with J. Girardeau and G. Boillot were helpful, whilst Jeffrey

OPHIOLITES AND THE OCEANIC CRUST 159

Karson provided constructive comments and many suggestions to improve the manuscript.

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