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2 A. Nicolas and F. Boudier

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those they were studying. Among their reasons was the difcultyof accepting the existence of a magma chamber at shallow depth

below an active ridge. Their reluctance was increased by thefact that most geologists were claiming that this chamber had avery wide top, a view that admittedly is untenable from both amechanical and a thermal point of view. Ultimately, the melt lens

discovery helped to bridge the gap between these two communi-ties. Another development helped to establish a more condentand constructive relation between the ophiolite community andthe marine geology and geophysics communitythe collectiveresults of the Ocean Drilling Program (ODP) Leg 147 (to theHess Deep) in 1993 (Gillis et al., 1993). The foliated-layeredgabbros of the Hess Deep originated at the East Pacic Rise,and displayed an astonishing petrological and structural analogywith the Oman ophiolite gabbros observed at the same depth. Formany geophysicists, this analogy has been a strong incentive tointegrate the ophiolite data into their ridge studies.

Where do we stand now regarding the origin of ophiolites?What are the present questions about their genesis and the ques-

tions we foresee in the near future? How do we address them?We need rst to clarify our motivation in studying ophiolites.Widespread at Earths surface, ophiolites can be studied fortheir record of regional paleogeographic and geodynamicevolution. Here, we are more interested in their contributionto the origin and evolution of the oceanic lithosphere that theyrepresent and in using them to better understand the dynam-ics of oceanic spreading centers. Slow-spreading ridges arecharacterized by major topographic gradients, along which theinternal structure of in situ oceanic lithosphere can be directlystudied and sampled by submersibles and by dredging. In fast-spreading ridges, however, (and with the exception of a fewdeeps), the internal structure of oceanic lithosphere is masked

by a uniform blanket of basalts. The contribution of ophiolites,where the deep structure of fossil oceanic lithosphere is directlyexposed, thus is mainly interesting in the latter situation. Unlikeslow-spreading activity, fast-spreading activity also tends to betime-independent on the scale of at least a few million years andto generate ridges that are homogeneous over lengths of 50100 km. To better understand the dynamics of seaoor spread-ing, it is thus reasonable to consider primarily fast-spreadingridges, mainly the East Pacic Rise, which is the most studiedof all. In this respect, structural and petrological studies inophiolites that originated at fast-spreading centers complementremarkably well the geophysical studies conducted on active

fast-spreading ridges.Our main goal here is to illustrate by a few examples thiscombined approach of structural studies in selected ophiolitesand marine geophysical studies conducted mainly at the EastPacic Rise. Karson (1998) presents an important contributionon this topic that is based on a detailed review of the tectonicwindows in ocean oor through which some insight on the struc-ture of the oceanic lithosphere is obtained. Karson emphasizesthe contrast between the stratiform architecture of the ophio-lites and these tectonic windows in the oceans. On the basis of

such premises, this contrast is not a surprise. Our mapping over500 km in the OmanUnited Arab Emirates ophiolite (Nicolaset al., 2000) conrms both the general stratiform architecture ofthis huge ophiolite and the existence of a locally intense ridgetectonic activity. For instance, steepening of the Moho to thevertical at the propagator tips is described below. The associated

attenuated ophiolite crust suggests that this situation compareswith that of the East Pacic Rise deeps.

Before returning to this topic, we wish to address two con-troversial issues, and to present viewpoints that are based ondetailed structural mapping of 15 ophiolite massifs and on morelimited studies in a similar number of other massifs throughoutthe world. Finally, we call attention to similarities and differencesin ophiolites that may help to better situate them with respect tooceanic lithosphere and to enlighten their origin.

TWO CONTROVERSIAL ISSUES

We discuss here two subjects that so far have been too

loosely accepted as ground truth by most geoscientists interestedin ophiolites. In so doing, we update but also substantially repeatearlier discussions (Nicolas, 1989, p. 199201 and 258).

Ophiolites Originate Dominantly in SSZ Settings

From a very general point of view, the inferred SSZ originis difcult to sustain for all ophiolites, simply because the ~70ophiolites for which sufcient information of chemical afnityis available exhibit too much diversity to be so narrowly con-strained. For example, a few ophiolites display a felsic extru-sive section, such as the Canyon Mountain in Oregon (Misseriand Boudier, 1985) or the Guana Gato in Mexico (Lapierre etal., 1992), tting well with an island arc environment origin,

but most do not. A few ophiolites, such as Karmoy in Norway(Pedersen and Hertogen, 1990), are covered by island-arc relatedsediments, which points to an island arc environment or backarc

basin origin, but many have a deep ocean sedimentary coverthat is more compatible with an MOR origin. Incidentally, a neanalysis of the associated marine sediments may turn out to bethe best criterion for unraveling the environment of origin of anophiolite (Pesagno et al., 2000).

The generalized interpretation of SSZ origin of ophiol-ites has been largely spread out in literature since Pearce andcollaborators (Pearce and Cann, 1973; Pearce et al., 1981)

pointed out that the Troodos and Oman ophiolite lavas exhibitgeochemical characteristics that are transitional between mid-ocean ridge basalt (MORB) and island arc tholeiite (IAT). InTroodos, lower pillow basalts are similar to ocean oor basalts,although upper pillow lavas and subsequent Arakapas ultra-mac volcanics have high-Mg, low-Ti, and other high-strengtheld element (HSFE) concentrations that point to afnities withIAT (Pearce, 1975), besides a large variability of these charac-teristics (Cameron, 1985). In Oman, the range from MORB toIAT signatures suggests a succession of volcanic episodes (Ala-


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Where ophiolites come from and what they tell us 3


baster et al., 1982): the Geotimes ridge-axis volcanics (V1 forErnewein et al., 1988) are very close to MORB, and subsequentseamount-type lavas (V2), Lasail, Alley, Cpx-phyric trace anevolution toward an arc signature. The chronology of thisevolution is well constrained to 12 Ma on the basis of biostra-tigraphy (Tippit and Pessagno, 1981), and is consistent with the

time difference between V1 and the initial detachment of theophiolite from its igneous setting (e.g., Hacker, 1994).

Actually, the arc signature covers a large spectrum of geo-chemical characters, variably represented in present forearc andarc volcanic compositions, and whose end member, boninite, iswell dened (e.g., Juteau and Maury, 1997, p. 3335) by (1) anextremely depleted character, i.e., high silica and magnesiumcontent and high depletion in rare earth elements (REE) andHFSE (Zr, Ti, Nb, Ta), interpreted as resulting from a high degreeof partial melting in the mantle wedge, and (2) an enrichmentin highly incompatible large ion lithospheric elements (LILE),interpreted as a subsequent contamination by hydrous uids. It isthe occurrence of one or the other of these characters in volcanics

overlying many ophiolites that supported the common thinkingthat ophiolites originated in a SSZ environment.

A major problem concerning this model is how to explainthe rapid shift from an MOR to an SSZ signature, either intime, as in Oman (~12 Ma; Hacker, 1994), or in space, as inAlbania (20 km, Shallo et al., 1995). Keeping in mind that theSSZ signature reects uid contamination during melting of adepleted mantle source, can we suggest possibilities to producea SSZ signature other than the arc-related uid ux in the mantlewedge overlying a subduction zone? Early in the study of theOman ophiolite, we related the V2 lavas to the initial stage ofoceanic thrusting of the ophiolite (Boudier et al., 1988) andexplained its occurrence by a water contamination of the meltsource due to the dehydration of the underthrust oceanic crust, a

process akin to a subduction-related melting. Other hypothesesmay be envisaged as well, such as contamination by seawater ofthe still hot mantle at a distance of a few tens of kilometers fromthe ridge, in keeping with the recent discovery of deep and hothydrothermalism near ridge axis (Manning et al., 2000; Nicolaset al., 2003). This second hypothesis has been further developed

by Godard et al. (2003) to explain the transitional trace elementsand isotopic strontium signatures of V1 Geotimes and V2 Lasailin Oman volcanics. In compilations that include recent data onseamounts from the East Pacic Rise (Niu and Batiza, 1997) and

North Fiji Basin (Fleutelot, 1996), the authors point to the fact

that the V2 Lasail lavas, which had been assigned to SSZ originon the basis of incompatible elements ratios, were lying in theextended eld of seamounts. We wish to defend an open-mindedattitude on the basis of the fact that the great variability of arcsignature documented in volcanics from ophiolites suggests thatdifferent processes (not necessarily understood yet) may rule thisdiversity. When the geochemical approach does not meet withdata from other origin, an SSZ setting should not be regarded asdecisive on the basis only of geochemical signature of the off-axis volcanics.

Abyssal and Ophiolitic Peridotites

Many studies have been devoted to the specimens of mantleperidotites collected in the oceans, the most extensive study beingthat of Dick et al. (1984). Their data are still largely used, remain-ing the most extensive source for petrological and geochemical

modeling of oceanic lithosphere. Comparing their data on abyssalperidotites to those available on peridotites from ophiolites, Dickand Bullen (1984) conclude that there are signicant discrepan-cies. They observe that, on average, the ophiolite mantle is moredepleted than the oceanic one and conclude that ophiolites shouldresult from a higher degree of melting, and thus should be goodcandidates for the hydrous mantle from SSZ environments. Wewish to reiterate our doubts on the relevance of the data on abys-sal peridotites, recalling that they represent only slow-spreadingoceanic lithosphere and associated transform faults, where mantlespecimens are readily available on the seaoor. With no basaltsattached to them, they may not represent constructional oceaniclithosphere. Mantle specimens from fast-spreading environments

are exceptional, being collected along a few transform faults anddeeps, which probably are not representative of the normal fast-spreading lithosphere. Since the Dick et al. study, more analyseshave been published on the East Pacic Rise mantle. As anexample, in Figure 1, we have plotted the characteristic chro-mium number index measured in peridotites from the elds ofabyssal domains, East Pacic Rise, island arcs, and three selectedophiolites. The biased sampling of abyssal peridotites in favorof slow-spreading lithosphere is clearly reected in this gure.The Bay of Islands ophiolite, thought to represent moderatespreading rate conditions (5 cm/yr; Suhr, 1992) plots in this eld,and the Oman ophiolite attributed to fast-spreading conditions

plots in the East Pacic Rise eld. The Josephine ophiolite plotsin the SSZ domain, in agreement with the geodynamic evolutionof this Jurassic ophiolite in the Mesozoic California. Altogether,abyssal peridotites have a more fertile signature than most ophio-lites. This is not surprising, knowing that the majority of ophiol-ites were derived from moderate- to fast-spreading environments(see below). From this discussion, we draw two conclusions:(1) that the high chromium number of many ophiolites does notnecessarily mean that they originated in SSZ environments, and(2) that petrological and geochemical modeling of fast-spreadingridges should rather turn toward ophiolites and stay away fromthe abyssal peridotites reference.

MARINE GEOPHYSICAL AND OPHIOLITICSTRUCTURAL DATA: THE NECESSARY

CONFRONTATION

As mentioned above and stressed by Karson (1998), the atrelief of fast-spreading ridges masks all internal structures. Thegeophysical, mainly seismological, tools provide only indirectinformation. This is where structural data from ophiolites witha clear fast-spreading center origin can provide the missinginformation; however, matching the results from both sources


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is difcult for two reasons. The main one deals with the dif-

ference in the scale; the resolution of seismology is poor below~10 km, below which the scale of structural resolution usuallyoperates. We notice here that the reason why most structuralreconstructions are limited is not intrinsic, but imposed by theeld conditionsthe size of the massifs, and their internal dis-membering. In this respect, the OmanUnited Arab Emiratesophiolite constitutes an outstanding exception, with continuityin the eld over many tens of kilometers. The second difcultyin matching oceanic and ophiolitic results is more apparent thanreal. It derives from the fact that ophiolites represent remnants offossil lithospheres, whereas most geophysical studies deal withactive spreading centers. As discussed below, many ophiolites,though fossil, have sampled spreading centers, thus keeping thestructural record of their activity; this phenomenon makes thetwo sources complementary.

We now use a few examples drawn from our own experi-ence to illustrate the difculty and wealth of confronting marinegeophysical and ophiolitic data.

Diapir Controversy

From the mapping of steep and concentric high temperaturemantle ow structures in the Zambales (Philippines) and theCyprus ophiolites, Nicolas and Violette (1982) have proposedthat the asthenospheric mantle ow beneath oceanic ridges was

diapiric (three-dimensional). The East Pacic Rise segmentationwas discovered at the same time (MacDonald and Fox, 1983;Lonsdale, 1983; Whitehead et al., 1984) and its scale of 50100 km, comparable to the spacing of transform faults along theMid Atlantic Ridge, prompted the idea that the uprising mantlecould be partitioned accordingly. This resulted in a number ofthree-dimensional models of mantle upwelling (Whitehead et al.,1984; Rabinovicz et al., 1984, 1987; Scott and Stevenson, 1989).The increasing connement for lower spreading rates predicted

by the last model has been conrmed by subsequent analog

(Magde et al., 1996) and numerical models (Lin and Morgan,

1992; Sparks and Parmentier, 1994; Magde et al., 1997) and bythe discovery of a strong mantle Bouguer negative anomaly inthe middle of the Mid Atlantic Ridge segments (Detrick et al.,1995), suggesting that a hot and melting asthenosphere is risingfrom below. In contrast, the East Pacic Rise has no clear mantleBouguer anomaly, suggesting a two-dimensional mantle upris-ing. The remarkable MELT experiment was run in the fastest andmost linear part of the East Pacic Rise to test the mantle owand melting below through a combination of seismology, grav-ity, and electromagnetism. With the resolution of the experimentlimited to a depth of 100 km, the mantle uprising is clearly two-dimensional (Melt Seismic Team, 1998). Finer scale geophysicalinvestigations conducted at the 9 N East Pacic Rise naturallaboratory do not rule out that, on the scale of 10 km, mantleupwelling may have a three-dimensional structure (Barth andMutter, 1996; Dunn et al., 2000; Canales et al., 2003) (Fig. 2).

In the meantime, increasingly detailed structural studies inthe Oman ophiolite (Ceuleneer et al., 1988; Nicolas and Boudier,1995; Jousselin et al., 1998) (Fig. 3) conrmed the occurrence ofseveral frozen diapirs. Because the Oman ophiolite is regarded

by all authors as being issued from a fast, possibly a super fast(Nicolas et al., 2000) spreading center, a comparison is pos-sible with the East Pacic Rise. The most apparent explanationregarding the discrepancy between two- and three-dimensionalmantle uprising is that the resolution of most East Pacic Rise

geophysical data is not yet sufcient to image small diapirs,the size of those mapped in Oman. Another explanation is thatthe small mantle uprising diapirs in Oman, breaking just belowMoho where they are mapped, may be local heads of largerthree-dimensional mantle upwellings whose ow would rotateat a greater depth, below the 510 km depth sampled by theophiolite. Following this idea, Nicolas and Boudier (2000) havemapped in this ophiolite the domains of intense melt circulationthat should be related to mantle uprising. Such domains arecharacterized by the development of thick Moho transition zones

Figure 1. Fields of the chromium number inspinels from abyssal plain, East Pacic Rise(EPR), arc-related and selected ophiolite peri-dotites, mainly harzburgites. Data are present-ed in fractions of the total number of analyses

bracketed in the gure. The data for abyssal peridotites are from Dick and Bullen (1984);for EPR, from Dick and Natland (1996), Nico-las et al. (1971); for arcs, from Parkinson andPearce (1998), and for ophiolites, from Dickand Bullen (1984) regarding Bay of Islandsand Josephine, and from Kelemen (1995) re-garding Oman.


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(MTZ) where the mantle harzburgites are replaced by dunitesand by a similarly large development of olivine gabbros and weh-rlitic intrusions into the lower crust. This mapping has revealedthe existence of a few areas, some 50 km across, incorporatingthe small mapped structural diapirs of Figure 3. These largerdomains, in which the MTZ is thicker on average and the gabbrounit is richer in olivine, could be the petrological signature oflarger mantle diapirs whose size seems more compatible with thescale of segmentation expected below oceanic ridges.

Seismic Anisotropy And Mantle Tectonics

In Figure 2, P-wave anisotropy, deduced from crossed seis-mic lines, shows a fast-velocity axis remarkably parallel to thespreading direction at a large distance from the ridge, which iscomparable to the ndings of the MELT experiment. This fast

seismic velocity is now interpreted as being parallel to the plasticow direction in the mantle. At the ner scale of the ellipse ofattenuated seismic velocities seen in Figure 2, which is that ofthe diapirs mapped in Oman, the expected mantle ow shouldnot be so regularly oriented, but rather should be diverging. Themeasurements necessary to check this point seem now in viewand this question may serve as an entry into a new and promisingresearch domain.

The interest for new seismic waves, such as the SKS shearwaves, which rise vertically from the core mantle boundary and

whose splitting can be used to know the local seismic anisotropyin the horizontal plane (Silver, 1996), has paved the way for newstudies on what has been called mantle tectonics. Because it is

possible to obtain a large number of such data on a regional scale,it becomes possible to relate the mantleow pattern deduced fromthem to the regional tectonics deduced from surface studies. Thishas been illustrated by beautiful examples incorporating the uppermantle into continental crust tectonics (Barruol and Souriau,1995). It should not be long before such techniques are com-monly applied to the oceanic lithosphere, revealing the patterns ofupper mantle ow. This will make a direct comparison possible,at the same 10 km scale, with similarow patterns deduced fromstructural studies in ophiolites. Spectacular progress in betterunderstanding of mantle ow below oceanic ridges is in view.

What is a Magma Chamber?

The magma chamber in which the layered gabbros of ophio-lites were generated traditionally has been regarded as a largemelt pouch in which crystals would be sorted by gravity afternucleation at the colder ceiling and the heavier crystal mushsettled on the oor. This view was shattered by the discovery ofthe perched melt lens at fast-spreading ridges


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seismic attenuation (Fig. 2). This attenuation has been ascribedto the presence of hot gabbros with trapped melt pockets. Formarine geologists and geophysicists, the magma chamber thusis reduced to a melt lens. The vast attenuated domain underneathwould have been lled with the gabbros that started crystalliz-

ing within the melt lens before subsiding as a gabbro mush andeventually evolving into a deforming solid (Quick and Dellinger,1993; Phipps Morgan and Chen, 1993). This view is incompat-ible, however, with the ophiolite record showing that the gabbro

pile still displays magmatic textures and not solid-state deforma-tion textures. Structural studies conducted in the gabbro unitof the Oman ophiolite show that these gabbros were intenselydeformed during their crystallization by a process akin to suspen-sion ow, with no trace of plastic deformation, except locally atthe Moho level (Nicolas, 1992). The critical melt fraction above

which suspension ow is possible is ~40%, far above the fractioninitially accepted for the attenuated domain below ridges, whichwas 10% and possibly ashigh as 24% (Lamoureux et al., 1999) (Fig.4).

Figure 3. Trajectories of mineral linea-tions in the mantle equated with mantleow trajectories in and around a mantlediapir in Oman (Maqsad in Semail mas-sif). The Moho is nearly horizontal (10SE dip). The dashed lines visualize the

45 and 60 plunge contours and the ar-rows indicate the dominant shear sense(motion of upper block/lower block).Based on 408 eld stations (Jousselinet al., 1998).

RIDGE

AXIS

PROPAGATOR

45

3 km

45

60

60

Maqsad

Mantle harzburgite

Lineation trajectories

& shear motion (relative motion of top)

Lineation plunge contourMoho transition zone (including gabbro sills)

Crustal unit

408 measurements


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In the meantime, analog experiments have shown that thecritical liquid fraction could be reduced to ~25%30% whenat particles were submitted to a large shearow, because oftheir strong alignment compared to their random initial assem-

blage (Nicolas et al., 1993). In these experiments, it was alsoobserved that the at particles would slip one on top of the other

until blocked by disoriented laths. The discovery in the gabbrosof microtextures of impingement of one plagioclase lath into

another, indicative of pressure solution, has been interpreted asnatures response to the problem of blockage. A model has been

proposed in which crystals slip by suspension ow until blockageand activation of pressure solution (Nicolas and Ildefonse, 1996).With this model, the melt fraction can be reduced drastically, theabsolute limit being the few percent of melt where pressure solu-

tion would operate alone. The mush viscosity can be estimatedfrom the model, and can be as high as 1015 Pa sl, a value indepen-

Figure 4. A: Estimations of melt frac-tions in the crustal low-velocity zone(LVZ) below East Pacic Rise on the

basis of the East Pacic Rise seismolog-ical data and structural measurementsin the Oman gabbros. B: These dataare integrated with petrological model-ing of the seismic anisotropy induced

by both the crystallographic fabricsand the geometry of melt lenses in thegabbros. Because the lower gabbrosare dominantly horizontal, the seismicvelocity is close to the modeled VpX;

because the upper gabbros are domi-nantly vertical, the seismic velocity isclose to the modeled VpZ. The modeledVpX and VpZ correspond to melt frac-tions of 12%24% (light gray area) and20%25% (dark gray area), respectively(in Mainprice, 1997).


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dently derived from a physical numerical modeling of the Omanmagma chamber (Chenevez et al., 1998)

We conclude that the attenuated domain below fast-spread-ing ridges should be considered to be the magma chamber asthe continuation of the melt lens above; however, it is lledwith a mush nearly as viscous as a plastically deforming body.

Comparing the structures in the Oman gabbros with those of theSkaergaard Complex in Greenland, the best-documented crustalmagma chamber, we observed no signicant differences betweenthem and suggested that the crystallizing gabbros deposited

below the free surface in Skaergaard were rapidly forming athick mush during their gravity-driven ow, much like the Omangabbros (McBirney and Nicolas, 1997).

Tectonic Activity Below the Basalt Blanket of Fast-

Spreading Ridges

Except for special areas such as fractures zones or microplates, the seaoor of fast-spreading ridges is remarkably at

with only few tens of meters of throw associated with normalfaults parallel to the ridge, the small nodal basins between over-lapping spreading centers and, of course, seamounts. The gravitysignal also is very weak. These features suggest that the deepcrustal and uppermost mantle structures are also simple and pre-sumably are dominantly horizontal. Turning to Oman again, thisis exactly what has been mapped there, particularly within theAswad massif in the northern United Arab Emirates. In the west-ern part of this massif, the Moho is absolutely horizontal with thelower gabbros dening cuestas and buttes over distances of tensof kilometers. In other parts of the belt, however, the Moho can

be vertical more than 1020 km along-strike, where it is com-monly underlain by shear zones that are a few tens of metersthick, reecting conditions of ~1000 C in the mantle. Above,the lower crust has suffered an intense hydrous recrystallization;it is also strongly deformed and cut by amphibolite facies shearzones that are branching on the steep Moho shear zones. Such

peculiar areas are also invaded by mac dikes that are mostlynoritic and that evolve to websterites in the mantle section. Allthese features point to an activity occurring very close to theridge axis. In contrast to these vertical structures observed inthe middle of the ophiolite section, both the basal contact andthe basalt ows and sediments on top are moderately inclined

and nearly parallel to each other (Fig. 5). These areas character-ize the limits of segments and are most developed at the tips of

propagators, as typical in the western part of the Nakhl massif,the eastern part of the Haylayn massif, and the center of the Fizhmassif. The Mansah area in the Semail massif where an off-axisdiapir has been mapped (Jousselin and Nicolas, 2000a) displays

similar features. They are similarly interpreted to be a result ofthe uprising mantle punching a soft lithosphere, not older than1 Ma. The asthenospheric pressure would tilt the Moho, fracturethe crust, allow penetration of seawater to this depth, and thusfavor the development of hydrous magmatism, as proposed byBoudier et al. (2000). To conclude, we suggest that the deep crustand uppermost upper mantle of fast-spreading ridges might belocally intensely tectonized, as observed in the Oman ophiolite,

but invisible from the seaoor because of the very efcient duc-tile relaxation in this hot lithosphere.

OPHIOLITES AND SPREADING RATES

Ishiwatari (1985a) and Boudier and Nicolas (1985) inde-pendently proposed to classify ophiolites according to how thenature of their mantle constituents reects the degree of partialmelting in the mantle of origin. Ishiwatari considered petrologi-cal and chemical criteria in gabbros and basalts, whereas Boudierand Nicolas introduced more general criteria, largely structural.Going a step further, these authors related mantle depletion tospreading rate of the ridge of origin, a point that is discussed

below. Boudier and Nicolas gave the names LOT (for lherzoliteophiolite type) to ophiolites whose mantle was lherzolitic andless depleted, and HOT (for harzburgite ophiolite type) to ophio-lites whose mantle was harzburgitic and more depleted. Ishiwa-tari proposed three types: his Liguria type coincided with LOTand his Papua type with HOT, and his Yakuno type was in

between. Several criteria are attached to the LOT and HOT types,the most prominent being the reduction or even the absence ofthe gabbro unit in LOT compared to HOT. The reduction affectsmainly the layered gabbros that, when present, are usually poorlystructured compared to those in HOT (Table 1).

Boudier and Nicolas (1985) related the degree of partial melt-ing to the spreading rate, a position that has been and is still disputed

because excess or reduced melting with respect to an average asthe-nospheric mantle uprising below an oceanic ridge can be explained

w w w

w ww

W metamorphic aureolebasal LT peridotites

HT mantle unit lower gabbrosupper gabbros

sheeted dike

basalt & sedimentsMOHO

E

0 2 km

Figure 5. Cross-section through the Oman ophiolite nappe in the northern Fizh massif (modied from Boudier et al., 1988). The Moho has beensheared and tilted to the vertical, whereas the lower and upper contacts of the thrust nappe are gently east-dipping. The revealed internal tectonicactivity has no visible consequence on the seaoor. LTlow temperature; HThigh temperature.


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TABLE 1. OPHIOLITES TYPES

Harzburgite ophiolite type(HOT)

Harzburgite lherzolite ophiolite type(LHOT)

Lherzolite ophiolite type(LOT)

MASSIFS CONSIDERED Oman-UAE (1), Papua (2),New Caledonia, southern

massif (3)

Bay of Islands (4), Yakuno (5), Mirdita (6),Troodos (7), Antalya (8), Kizildag (9), Aladag

(10), Xigatse (11), Muslim Bagh (12),Josephine (13), Vourinos (14), Zambales (15)

Lanzo (16), Liguria (17), Trinity(18), Othris (19)

CRUSTAL SECTION Thickness: 46 km Thickness: 23 km Thickness: 01 km

Volcanics overlyingsheeted dikes

Low-alumina tholeiite High-alumina tholeiite Alkali basalt

Sheeted dikes Well expressed and steeplydipping

Steeply to moderately dipping (4, 7, 9, 12, 13,14, 15), horizontal (11, 15), absent or poorlyorganized (5, 6, 8)

Absent to poorly organized sillsor dipping dikes

Gabbros

exposure Thick, continuous (1, 2) Thin, continuous (4, 12, 13, 14, 15), nestled (7,9), locally absent (5, 6, 9, 11)

Absent or restricted to 100-m-sized bodies

lithology Lower: Ol-gabbro (1, 3)

Upper: Ol-gabbro andgabbronorite

Dominant gabbronorite (4, 5, 8, 10, 13, 14, 15),

Ol-gabbros (6, 7, 9, 11, 14) and ferrogabbro

Ol-gabbro (16, 17, 18)

structure Lower layered gabbroUpper foliated gabbro(1,2)

Well layered (8, 9, 10, 13, 14, 15),poorly layered-foliated gabbro (4, 6, 7, 11, 12,

13), isotropic gabbro

Isotropic gabbro, foliated gabbro

penetrativedeformation

Magmatic deformation Magmatic to plastic (flasergabbro) deformation

shear bands Amphibolite facies, steep,horizontal motion

Granulite to greenschist facies, listric faults (6, 9,12, 13)

Common

wehrlite intrusions Abundant (1, 3) Present (4, 6, 7, 12, 14) Absent

MANTLE SECTION

lithology Harzburgite and dunite Cpx-harzburgite, locally Sp-,Plag-lherzolite anddunite (5, 6, 11, 15)

Plag- (Sp-) lherzolite

temperature ofdeformation

Very high to high temperature High temperature to moderate temperature Moderate temperature to lowtemperature

internal structures Flat foliation (except diapirs) Flat (4, 6, 11, 12, 13, 14) to steep (7, 8, 14, 15)foliation

Steep foliation (14, 16, 17)

shear zones Uncommon, vertical with flatlineation

Common, steep with steep lineation (8, 10, 11,14)

Very common, steep

diabaseoccurrence

Uncommon Common (8, 10, 11, 12, 14, 15) Common dikes and sills in uppersection

moho transitionzone

Interlayering of dunite, andgabbro, websterite lenses,variable thickness


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equally well by thermal or compositional differences in the risingasthenosphere. For example, because it is under the inuence ofthe Iceland hot spot, the Reykjanes Ridge in the northern AtlanticOcean has a fast-spreading ridge structure for a slow spreading rate;The hydrous melting of the mantle slab located above a subductionzone illustrates the role of composition, readily generating depleted

mantle rocks whatever the spreading rate is. The discussion devel-oped above on SSZ or MOR origin of ophiolites shows that it isnot easy to take into account the compositional factor. The situationis more difcult with the thermal factor. We are not aware of anystudy of ophiolites that discusses the relation of ophiolites with hotspot or cold spot environments. Another difculty relating ophiolitefacies to spreading rate, also emphasized by Karson (1998), appearswith the heterogeneous structure of slow-spreading ridges, wherethere are major differences between the segment centers and thevicinity of fracture zones. As documented by Karson and Rona(1990) and Tucholke et al. (1998), tectonically denudated mantlecore complexes may occur in fractures zones that contrast withnormal crustal structure in the middle of ridge segments. This dif-culty is overcome in the Mirdita ophiolite in Albania (see below),which is large enough to encompass both a core complex and anormal lithosphere; but in a smaller ophiolite displaying either ofthese components, a wrong diagnosis of LOT and HOT is possible.This being acknowledged, differences between oceanic lithosphereaccreted at slow- and fast-spreading ridges are so great (Karson,1998) that Boudier and Nicolas (1985) early classication of ophi-olites in terms of their inferred spreading rates, referring mainly totheir mantle signature, was largely valid. In Table 1, we comparethe revisited HOT and LOT classication , and in Table 2, we sum-marize the new and rapidly expanding database derived from thevarious oceanic oors.

With more data and experience, however, we need to reap-praise the meaning of LOT in terms of spreading rate. The abys-sal peridotites, which are issued from slow-spreading ridges, arenormally composed of harzburgites, and strictly speaking thecorresponding ophiolites should be considered as HOT. A closerexamination shows, however, that the abyssal peridotites areless depleted than the harzburgites that correspond to true HOT.They commonly contain a small percentage (


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TABLE 2. OCEANIC LITHOSPHERE

Fast-spreading Slow-spreading Very slow-spreadingoceanic rifting

OCEANIC RIDGES East Pacific Rise: Garrett FZ (1),Hess Deep (2), Costa Rica Rift (3)Easter microplate (4)

Mid-Atlantic Ridge: Fracturezones(5)

SW Indian Ridge (6); Gakkel-Lena Ridge (7); Zabargad (8);Galicia bank (9)

Spreading rate 10 cm/yr 3 cm/yr


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chromite would be dispersed within crystallizing gabbros.

Metamorphic Aureole, Signature of Ophiolites Origin

Moores (1982) and Coleman (1984) proposed to dividethe ophiolites into two families depending on whether they had

been incorporated into the continental crust at a passive margin(their Tethyan ophiolites) or at an active margin (their Cordille-ran ophiolites) (Fig.7). In the former case, ophiolites rest as giantthrust sheets upon a continental substrate; in the latter case, theyare incorporated, often dismembered as a mlange, into accretionterranes mainly issued from trench sediments and seamount frag-ments (Coleman, 2000). From an earlier review (Nicolas, 1989, p.289309), we retain that more than 60 ophiolite massifs that com-

prise most, if not all, of the least dismembered ophiolites belongto the Tethyan group, with which they share a fascinating feature,the presence of a metamorphic aureole at their base. This aureolegrades upsection over a few hundred meters from the underlyingundeformed basalts and sediments to intensely strained amphibo-

lites and granulites that are equilibrated above 800 C. Theseamphibolites and granulites are overlain by concordant mylonitic

peridotites deformed at 9001000 C that are grading over 12 kminto normal mantle peridotites. Tectonic reconstructions (Boudieret al., 1988) and models (Hacker et al., 1996) show that this meta-morphism was produced by the ironing effect of a ~10-km-thicklithospheric slab that represents the future ophiolite. Followingthermal models of oceanic ridges (Phipps Morgan and Chen, 1993;Henstock et al., 1993; Cormier et al., 1995), we conclude that theage of detachment of the lithospheric slab cannot be older than 2Ma. Accordingly, in Oman, the age of the metamorphic aureole is12 m.y. younger than the ridge of origin. Young ages in metamor-

phic aureoles are the rule, with the few exceptions of 1015 m.y.younger ages (review in Wakabayashi and Dilek, 2001). Because

both the geological and the thermal modeling information arecompelling, we assume that the few datations with 1015 Ma dif-ferences are dubious (Hacker et al., 1996).

Major consequences can be drawn from the most commonexistence of basal aureoles below ophiolites. These ophiolites weredetached at no more than 10100 km from their ridge of origin,depending on the spreading rate and the age difference betweenigneous accretion and metamorphism in the basal aureole. At theirtime of detachment, they were thin and hot pieces of oceanic litho-sphere. This extraordinary feature raises as many questions as ithelps to solve. Is it not possible that the SSZ secondary signature

of many ophiolites would be due to water penetration into the over-riding future ophiolite? (Boudier et al., 1988) Are there such activesites in present day oceans, or do they belong to peculiar periods inearth history? Paleomagnetic studies in Oman (Perrin et al., 1993)show 30 rotations within 1 Ma, suggesting that this ophiolitemight have originated as a microplate, possibly comparable to theEaster or Juan Fernandez microplates along the East Pacic Rise(Boudier et al., 1997). This model has a few appealing features:a microplate is a pre-cut piece of oceanic lithosphere, conned

by ridge segments and compressive boundaries. With these active

boundaries, it might be more easily detached from its site of originthan a normal piece of lithosphere. This future ophiolite also con-tains internally active ridge segments, a feature that accounts forthe presence of frozen diapirs in some ophiolites. In addition, tipof propagating segments, such as Pito or Endeavour Deeps fromEaster and Juan Fernandez, respectively, create locally very slow-spreading conditions in the overall fast-spreading environment;such contrasted situation might have its equivalent in the differentcharacteristics pointed out in the Oman ophiolite between activesegments and their propagating tip (Nicolas and Boudier, 1995).In an ophiolite as small as Troodos in Cyprus, the slow-spreadingsituation documented on the base of evidence for a rifted valley(Varga and Moores, 1985) could be reconsidered if the ophiolitewere originated from the tip of a propagator. There are, however,

many pending questions. Can the compressive zones in micro-plates generate the large thrusts responsible for the metamorphicaureoles of ophiolites? Why and how is the microplate-ophiolitedetached and overthrust?

CONCLUSION: CALL FOR AN INTEGRATED

APPROACH IN OPHIOLITE STUDIES

As studies of ophiolites turn toward a better understandingof the functioning of oceanic spreading centers, the rst problem

Figure 7. The two main modes of ophiolite emplacement. A: The Teth-yan-type, related to obduction onto a passive margin. B: The Cordille-ran-type, related to upheaval of oceanic lithosphere in an accretionary

prism. After Moores (1982) and Coleman (1984).

mantle

continental crust

oceanic crust

B


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is to identify the type of center where the considered ophiolitehas formed. We have recalled a number of criteria that can beused to estimate the spreading rate of this center (Tables 1 and 2).Spreading rate is clearly the most important parameter to accountfor the diversity of the oceanic lithosphere (Karson, 1998), andit is also clearly reected in the structure and composition of

ophiolites. In the study of slow-spreading oceanic ridges, theophiolite reference is of lesser importance because, thanks to thesubmarine reliefs that can be >4 km, direct observation of deepcrust and mantle is possible. This reference becomes compulsoryin fast-spreading situations where the subdued reliefs allow onlyexceptionally deep structures to be exposed. We have concludedabove that many ophiolites were likely originated from spread-ing centers corresponding to intermediate to fast rates and, by afew examples showing the interplay between structural studiesin ophiolites and geophysical studies in oceanic ridges, we havetried to illustrate the potential of this cross-fertilizing approach.

Spreading rate is not the unique parameter, however, and acloser analysis should consider the effects of different tempera-

tures or compositions in the mantle sources. The environmentof origin (MOR or SSZ) that is discussed above and that hasmuchtoo muchmobilized the ophiolite community, belongsto this category of problems. It has been approached from thestandpoint of geochemistry, but with disputable results and moreelementary pieces of information, e.g., the associated sedimen-tary record often having been overlooked.

This drives us to our main conclusion, which is a call forintegrated studies. Through a few examples, we have illustratedthe fruitful cooperation between structural geologists in ophiolitesand marine geophysicists. A similarly rich cooperation is possible

between geochemists and petrologists working in ophiolites andmarine environment. Dealing with the relative roles of structuraland geochemical studies in ophiolites, we insist on the commonsense idea that these approaches are complementary, but that geo-chemistry should be applied within the framework of a thoroughunderstanding of the structures, or at the very least, of the generalgeology. Indeed, many ophiolites have been dismembered exten-sively by collision-related tectonic deformation, making it difcultto conduct ridge-related structural studies. One might question theresults of geochemical studies that are not supported by structuralinformation in such ophiolites. In contrast, in an ophiolite whoseinternal structure has been preserved and decrypted, the solution tofurther problems depends largely on focused geochemical studies.Typically, the answer to the question evoked above regarding a

mantle source outside the thermally or chemically average mantlesource can be addressed by geochemistry.As a concluding remark, we would like to encourage young

scientists to undertake more structural studies in selected ophiol-ites, in association with and opening the way for geochemists. Evi-dently, they should keep an eye on corresponding marine research,

but when their results are at odds with marine studies, as has hap-pened in the past, they should remember that careful observationsmade on relevant outcrops in the eld and the complementarylaboratory studies provide the most compelling evidence.

ACKNOWLEDGMENTS

We deeply acknowledge the involvement of a large numberof students and colleagues in our studies, as well as the ofcialsupport received from the French research institutes (CNRS,MAE) and the friendly cooperation established in several

countries, particularly Oman. The manuscript has been largelyimproved thanks to constructive reviews from Paul Robinson,Robert Coleman, and Yildirim Dilek. The editorial contributionof Yildirim Dilek is particularly acknowledged.

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