Vema Fracture Zone (central Atlantic): Tectonic and Magmatic Evolution of the Median Ridge and the Eastern Ridge-Transform Intersection Domain Yves Lagabrielle, Vassilios Mamaloukas-Frangoulis, Mathilde Cannat, Jean-Marie Auzende, Jose Honnorez, Catherine Mevel, Enrico Bonatrl
To cite this version:
Yves Lagabrielle, Vassilios Mamaloukas-Frangoulis, Mathilde Cannat, Jean-Marie Auzende, Jose Hon- norez, et al.. Vema Fracture Zone (central Atlantic): Tectonic and Magmatic Evolution of the Median Ridge and the Eastern Ridge-Transform Intersection Domain. Journal of Geophysical Research, Amer- ican Geophysical Union, 1992, 97 (B12), pp.331-348. 10.1029/92JB01086. insu-01894655
HAL Id: insu-01894655 https://hal-insu.archives-ouvertes.fr/insu-01894655 Submitted on 12 Oct 2018
HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. B12, PAGES 17,331-17,351,NOVEMBER 10, 1992
Vema Fracture Zone (Central Atlantic): Tectonicand MagmaticEvolution of the Median Ridge and the Eastern Ridge-TransformIntersection Domain
YVESLAGABRIELLE, 1VASSILIOS MAMALOUKAS-•••, 1MATH]LDE CANNAT, 2 JEAN-MARIEAUZENDE, 3JOSE HONNORF• 4CATHERINE MEVEL, 5 AND ENRICOBONATrl 6
The easternVerna fracture zone intersectionwith the Mid-Atlantic Ridge (MAR) axis was surveyed with the French submersibleNautile during the Vemanaute cruise in 1989. At this ridge-transform intersection(RTI), an elongated,E-W ridge, more than 50 km long, is presentin the transformvalley. This medianridge risesup to 1000 m abovethe surroundingseafloor. The crestof the median ridge is lower and presentsan arcuateshape at the tip of the MAR axis. Dive observationson both the southern and northernflanks of the medianridge as well as samplestudies suggest that this morphologicalfeature is not a serpentinizedmantle protrusionor a recentvolcanic constructionalridge but representsa sliver of uplifted oceaniclithosphere covered by a sedimentarybreccia formation. This detrital cover consists of polymictic sedimentarybreccias, sandstonesand siltstones,composed of basaltic, doleritic, and gabbroic clasts, with less frequent serpentinite and spinel fragments which originated from the disaggregationof shallowto deeplevels of tectonicallyuplifted oceaniccrust and uppermantle. Most of theseclasts have undergonegreenschist facies metamorphismprior to their incorporationin the detrital formation. Disaggregation,mass wasting and rapid emplacementof detrital formations on the valley floor by gravity flows are likely to be relatedto a major tectonicepisode that affectedone or both the fracturevalley walls. This eventcould be relatedto the uplift of the southernwall of the fracturezone (the "transverseridge") which took place probablybetween 10 and 3 Ma ago. Since this uplift episode,the transverseridge (which is now undergoingsubsidence) and the detritus covered transform valley floor, separatedby the transform fault zone, have migrated westward and eastward respectively. Vertical tectonicsof the medianridge at the approachof the RTI can not be explainedsolely by the hypothesisof a diapiric intrusionof serpentiniteas proposedby earlier authors.A possibleinterpretation follows the suggestionsthat the anomalouscrust of the fracturevalley nearthe westernRTI, is more than 1 km out of isostaticequilibrium. Recent tectonic and magmaticevents including subsidence and lava emplacement which occurredat the tip of the MAR axis have been recordedon the southernflank of the medianridge. Severalstages in the very recenttectonic-volcanic history of the easternRTI, that is, roughlyduring the last 300,000 years, can thus be defined. The lower elevation and narrow, arcuate shapeof the median ridge east of 40ø57'W are inferred to have resultedfrom tectonicextension during the creationof the nodal basin.
1. INTRODUCTION Biehler, 1970 ] . The bestknown of the intersectionsbetween Ridge axis/transform fault intersections(RTIs) are wide the Mid-Atlantic Ridge (MAR) axis and its major fxacturezones (offset greater than 30 km) are both RTIs of the Kane fracture areas where an accretingplate boundaryintersects a boundary along which lithosphereis theoretically neither created nor zone (FZ), and the easternRTIs of both the Oceanographerand destroyed.RTI domains are the sites of complex interactions Vema fracturezones. All of theseRTIs have beenthe targetsof between magmatic and tectonic processesrelated to seafloor conventional bathymetric surveys, geophysical studies, Seabeammapping and diving cruises[Karson and Dick, 1983; spreadingand to transformfaulting, respectively[Sleep and Fox and Gallo, 1984; Oceanographer Tectonic Research (OTTER) Team, 1984; Williams et al., 1984; Macdonald et al., 1CentreNational dela Recherche Scientifique, Unit6de Recherche 1986; Pockalnyet al., 1988; Mdvel eta!., 1989]. Associ6e:Domaines Oc6aniques, Universit6 de BretagneOccidentale, Brest, France. Atlantic RTIs often display comparable morphological 2CentreNational dela Recherche Scientifique, Unit6de Recherche features. The MAR axis broadens and deepens when Associ6e:Domaines Oc6aniques, Universit6 de BretagneOccidentale, approachingthe transformfault. In the intersectiondomain, it Brest,France; now at Laboratoirede P6trologieM6tamorphique, plunges into a circular or triangular-shapeddepression, the Universit6Pierre et Made Curie, Pads, France. nodal basin.The depth of nodal basinsvaries fxom 5000 m up 3InstitutFran•ais deRecherche pourl'Exploitation dela Met, to more than 6000 m, at the Kane eastern RTI. One or several D6partementG6osciences Marines, Centrede Brest, France. neovolcanic constructionalridges crosscutthe floor of some 4InstitutdeG6ologie, Universit6 Louis Pasteur, Strasbourg, France. 5CentreNational dela Recherche Scientifique, Laboratoire de nodal basins, indicating recent propagation of magmatic P6trologieM6tamorphique, Universit6 Pierre et Marie Curie, Paris, activity into the intersectiondomain. France. Despite such morphological similarities, the Atlantic RTIs 6Lamont-DohertyGeological Observatory ofColumbia University, exhibit different geological characteristicssuggesting various Palisades,New York, USA. stages in their tectonic or magmatic evolution. Dredging results as well as direct sampling during submersibledives Copyright1992 by the AmericanGeophysical Union. indicatethat a great variety of rocks are exposedin the nodal Paper number 92JB01086, basins. The Vema eastern nodal basin is floored with fresh 0148-0227/92/9 2JB-01086505.00 basalticlavas only, while gabbrosand serpentiniteshave been
17,331 17,332 LAGABRIELLEETAL.: VEMA FRACTURE ZONE'S MEDIAN RIDGE
westernRTI •: 4 4 C4
11' • -- •.,v ",,/""s'• MEDIANRIDGE rI •"J' - SF O R M VALLEY =-- •- :.".-•..•_.-.2_-_-..-__-.::"..'..:..:•. • • , _
45' 44' 43' 42' 41' 40'W
Fig. 1. Schematicbathymetric map of the Vema fracturezone. Hatched area emphasizes the transverseridge; shaded area emphasizesthe medianridge and its westernextension, the "Vernamound". sampled in the Kane and OceanographerRTIs, respectively, mound, the western extension of the Vema median ridge at showingthat deep crustal levels as well as mantle rocks may be about 41o50' (E. Bonatti, unpublisheddata, 1984). Dredge exposed at RTIs. A number of models accounting for the hauls collectedfrom the medianridge of the RomancheFZ at topographicand geologicalcharacteristics of the best studied 18ø12'W, 0ø10'S consisted of all the usual oceanic basic and RTIs have been proposed[Karson and Dick, 1983; Fox and ultrabasic rocks and their metamorphic equivalents and Gallo; 1984, Mdvel et al., 1991]. All of them emphasizethat a hydrothermalpyrite concretions[Bonatti and Honnorez, combination of alternating periods of magmatic construction 1976]. The Atlantis II FZ median ridge (southwestIndian and tectonic stretching is responsible for their peculiar Ocean) was dredged, drilled, and explored with a towed patterns. The authors generally conclude that tectonic televiewerduring OceanDrilling Program(ODP) leg 118, and processesare dominant in the lithosphere exposed at nodal during the preliminarysurvey cruise [Robinsonet al., 1989; basinsof RTIs with respectto magmaticprocesses. Dick et al., 1991]. It appearsto have a thick cover of sandand Topographicfeatures such as ridges locatedin the transform gravel, and occasional large blocks of gabbro, basalts, valley are found only in some RTIs. In particular, the Vema serpentinizedperidotites, and amphibolites. eastern RTI domain is characterized by the presence of a A geologicalstudy of the Vema FZ wascarried out in 1988, transformparallel topographicfeature, the median ridge, that duringthe Vemanautecruise by the Frenchsubmersible Nautile. bisectsthe transformvalley east of 41ø30'W (Figure 1) rising Two zones were surveyed:the first one is located along the up to 1000 m above the valley floor. Similar ridges have been southernwall of the fracture zone where a section including describedin other major Atlantic fracture zones (Gibbs FZ mantle serpentinites,gabbros, sheeted dykes and basaltic [Olivet et al., 1974; Searle, 1981]; Romanche FZ [Bonatti et pillowshas beenobserved and sampled[Auzende et al., 1988; al., 1974; Bonatti and Honnorez, 1976]; Doldrums FZ Cannat et al., 1991]; the secondone correspondsto the eastern (Mazarovich et al., unpublished)),but the ridges may also be RTI domain.This paperfollows two previouspapers [Auzende reduced (Kane FZ [Pockalny et al., 1988]) or totally missing et al., 1989; Mamaloukas-Frangouliset al., 1991] where dive (OceanographerFZ [07TER Team, 1985]). Prominentmedian observations on the morphological expression of the ridges have also been mapped in some fracture zones of the transformactivity at the easternRTI and on the nature of the Indian Ocean (Atlantis II FZ [Dick et al., 1991]), and the Pacific nodal basin and neovolcanicridge are presented. (ClippertonFZ [Gallo et al., 1986]; TamayoFZ [Kastenset al., In this paper, we present additional data obtained from 1979; Macdonald et al., 1979]). Little is known about the submersible observations across the Vema transform median geologyand the deep structureof thesemedian ridges. Despite a ridge,between 41øW and the RTI nodalbasin (Figures 1, 2 and scarcity of direct geological data, median ridges have been 4) and petrologicalinformation. Based on theseobservations, interpretedby most authorsas resultingfrom the diapiric rise we proposeinterpretations regarding the geologyof the Vema of serpentinized upper mantle in the transform valley medianridge, andwe discuss(1) the originof the medianridge; [Macdonald et al., 1986]. Median ridges, however, are not (2) the possiblecauses of its uplift as one approachesthe RTI; universally present, and are usually not continuousalong the (3) a simple model of the very recentmagmatic and tectonic entire length of the transform.Moreover, previousdredging or evolutionof the median ridge-MAR axis junction. drilling results from median ridges have revealed that they do not consistexclusively of ultramafic rocks. Rocks dredgedat 2. MAIN STRUCTURAL UN1TS OF TIlE VEMA about 41ø00W, 10ø45N along the southernflank of the Vema Fe,n• ZONE median ridge include undeformed and cataclastic basalt The Vema FZ, located near liøN latitude, is the fragments [Mdvel , 1981] and sedimentarybasaltic breccias. northernmostof a set of major equatorialtransform faults that Gabbro, basalt and volcaniclastic material in addition to offset the MAR. The total offset along the Vema FZ is 320 km peridotiteswere dredgedfrom the northernslope of the Vema (Figure 1). Given a slip rate of 32 mm/yr [Cande et al., 1988], LAGABRIEII•F ET AL.: VEMA FRACIXJREZONE'S MEDIAN RIDGE 17,333
41020 41ø10 4lO0•0 {• 40050
4-.. NORTHERN , WALL lOøSO
.----.. r.
SOUTHERN i i • TROUGH••••'•E. nodal
10ø40 SOUTHERNI I • • I • I - Western•p T4000 misobath •MARaxis 1ø21 41007.7 40o52.5 51 km
,• •, 27km 24km
Fig. 2. Main morphomctricfeatures of the Vcmamedian ridge. Hatched areas represent zones deeper than 5 km. Note thatthe higherportion of the medianridge is not locatedat the tip of the MAR axis. Depth in kilometers. the age of the oceaniccrust facing eachMAR axis is about20 processesaccounting for the creationof the ridge have been Ma. The active segmentof the fracture zone is marked by a proposed,including volume increase due to serpentinizationof 5100 m deepand 15-20 km wide, flat-flooredvalley boundedby upper mantle rocks, changes in plate motion and related two walls. The valley is filled with late Tertiary turbidires oscillatoryspreading, and thermaleffects due to spreadingaxis originatingfrom the Amazon cone, which have been emplaced intersection,its origin remains controversial. across the Demerara abyssal plain [Van Andel et a/.,1967; Seismic profiles indicate the presence in the transform 1971; Bader et al., 1970; Perch-Nielsen et al., 1977; Kastens et valley of one or more partiallyburied basement highs [Eittreim al., 1986]. This turbiditic infill is much thicker in the western and Ewing, 1975; Bowen and White, 1986; Kastens et al., part of the transformvalley and is up to 1.5 km thick at the 1986]. Due to the lack of accurate data, it remains unclear western RTI [Eittreim and Ewing, 1975; Bowen and White, whether the buried basementhighs observedin the westernhalf 1986]. By contrast,the easternRTI has a very thin to absent of the transformfault are part of a continuouselongated feature. pelagic cover [Macdonald et al., 1986; Kastenset al., 1986; A basementhigh emergeslocally from the sedimentaryinfill of Auzende et al., 1989]. A narrow zone of disturbed sediments the valley floor at 41o50' forming the Vema mound(Figure 1). identified on seismic lines recorded by Eittreim and Ewing The basementemerges again in the easternmostpart of the [1975] and interpreted as the trace of the transform fault, active segment.It forms a continuousridge, the medianridge, connects the western and eastern intersection basins. running parallel to the valley axis and culminatingat about The southernwall of the transform valley is defined by a 1000 m abovethe valley floor (Figures2 and 3). major topographicfeature, the transverseridge (Figure 1), The 3.5-kHz reflection records obtained near the eastern RTI which culminates at 600 m below sea level along the western at about41o30 ' (profile B in Figure 5 of Eittreim and Ewing inactive segmentof the fracture zone [Van Andel et al., 1967, [1975]) show that the flat floor of the valley located to the 1971]. This ridge constitutes a major topographic anomaly north of the transformfault zone is uplifted. This is consistent relativeto the predicteddepth-age relationsh!p. Geophysical with the interpretationof Deep Tow's 4-kHz seismicprofiles data, dredgedsample studies and submersibleobservations have suggestingthat the western tip of the median ridge has shown that the transverseridge can be interpretedas an uplifted undergonerecent, i.e., Pleistocene,uplift at a rate possiblyas and tilted section of mature oceanic crust [Thompson and high as lmm/yr [Kastenset al., 1986]. Melson, 1972; Prinz et al., 1976; Bonatti and Honnorez, Available geophysicaldata on the Vema FZ concernonly 1976; Ludwig and Rabinowitz, 1980; Honnorez et al., 1984, the westernportion of the transformvalley (west of the Vema Louden et al., 1986; Auzende et al., 1989; Cannat et al., 1991]. mound; Figure 1). There is unfortunatelyno constrainton the Paleontological and stable isotope data from dredged crustal structureof the easternpart of the transformfault and limestones indicate that its summit was above sea level from more particularlyon the deep structureof the median ridge. 10 to 3 Ma and subsequentlysank to its present position at a Seismicexperiments [Ludwig and Rabinowitz, 1980; Detrick et rate of about 0.3 mm/yr [Honnorez et al., 1975; Bonatti and al., 1982; Louden et al., 1986] indicate that the crust in the Crane, 1982; Bonatti et al., 1983]. Although numerous westernpart of the transformvalley is 4 to 5 km thick, and is 17,334 LAGABRIELLEETAL.: VEMA FRACTURE ZONE'S MEDIAN RIflE
SOUTHERN TROUGH NORTHERN TROUGH
TFZ' MEDIAN RIDGE CREST
3 ======.... , •
6 • •km km no v.e.
3
km
km
NB 'Nodal Basin NVR : NeovolcanicRidge TFZ : Transform Fault Zone
Fig. 3. ThreeN-S sectionsof the Vemamedian ridge (see Figure 2 for location).Hatched area in section3 representsthe medianridge topography according to section2. This suggeststhat the southernflank of the medianridge has undergone local subsidencein responseto the creationof the nodalbasin. There is no verticalexaggeration.
characterizedby low velocitiesand thus by a low bulk density. Macdonald et al. [1986] have shown that the axial Derrick et al. [1982] proposedthat these anomalouslylow neovolcanic zone and the normal faults located on the western velocities and densities resulted from intense fracturing and edgeof the RTI are trending200-40 ø obliqueto thenorth-south hydrothermalalteration of thetransform valley crust and upper spreadingaxis orientation,as a resultof the transmissionof manfie.An ,importantconsequence of suchlow crustaldensities shear stressesfrom the transformsystem to the rift system. is that the transform valley is not in local isostatic They alsoindicate that the shearzone is 2-4 km wide nearthe equilibrium:the western part of theVema transform valley may easternRTI and narrows to a 20-100 m wide single tectonized be about1 km deeperthan its equilibriumdepth [Derrick et al., furrow20 km west of the RTI. They interpretthe medianridge 1982].SeiSmic and gravity profiles across the transform valley as a serpentinizedultramafic intrusionrising up as a screen [Pottset al., 1986;Louden et al., 1986;Robb and Kane, 1975; throughthe highly fractured crust of the transformfault zone. Princeand Forsyth, 1988] •suggest that the crust is locally Resultsconcerning the recentand present-daytectonic and significantlythinner beneath the transformvalley walls. volcanic activities of the nodal basinsand the southerntrough Alongthe southernwall, thisthin geophysicallydefined crust have been publishedin two previouspapers [Auzende et al., appearsto coincidewith outcropsof serpentinizedmantle- 1990; Mamaloukas-Frangoulis et al., 1991]. The most derivedperidofites [Auzende et al., 1989;Cannat et al., 1991]. importantresults of our Nautile dive observationscan be summarized as follows: 3. MORPHOSTRUCrURALUNITS OF THE EASTERN 1. The present-daydisplacement zone (PDDZ, as definedby V•-MX RTI Mamaloukas-Frangouliset al. [1991]; Figure 4) along which Before the Nautile diving cruisein 1988, the easternVerna plate motion is focusedcan be traced within the southern RTI had been the subjectof a Seabeamsurvey (D. Needham, troughof the transformvalley. It correspondsto an extremely unpublishedmap, 1978) and a high resolutiondeep tow study narrow belt, less than 300 m wide, showingevidence of active [Macdonald et al., 1986]. As shown in Figure 4 where dive deformation,mainly closely spacedparallel fault scarpsand tracksare reported,our geologicalsurvey with the submersible openfissures within talusand soft sediments. Nautile exploredall the distinctmorphostructural provinces of 2. Closer to the intersection area, the present-day the RTI: the Mid-Atlantic Ridge valley axis which deepens displacementzone runs E-W andthus crosscuts the neovolcanic progressivelytoward the north, the nodal basin at the RTI ridge. Consequently,the northernpart of the nodal basin is which comprisesa westernand an easternbasin separatedby connectedto the median ridge. the neovolcanicridge, the southernand northernwalls of the 3. Both eastern and western nodal subbasinsare entirely transformvalley, and the southernand northerntroughs of the floored with fresh basalts, contrastingwith some other RTIs transformvalley, separatedby the medianridge. suchas the Kane and OceanographerRTIs wheredeep crustal or LAGABRIELLEET AL.: VEMAFRACTURE ZONE'S MEDIAN RIDGE 17,335
i
NORTHERN TROUGH
ß
10ø45
' asln
10ø40
I I 41000 40o55 ß hematite-richmetabasaltic sandstone c] metabasalt breccia with hematite cement 0 polymictvolcanic-plutonlc breccia, sandstone or siltstone I serpentinizedperidotite or spinelfragment within detrital rock /• basalt,metabasalt, dolerite A gabbro,metagabbro
• _ areawhere freshto relatively freshbasalts havebeen recovered
Fig.4. Bathymetricmap of the Verna eastem RTI and location ofthe samples collected from the median ridge and from the northernwall of the eastern Verna transform fault intersection domain during the Nautile dives (encircled numbers). The line marked"p.d.d.z." represents thepresent-day displacement zone according toMamaloukas-Frangoulis et al., [1991].
upper mantle rocks are exposed.This indicatesthat the 4. THEVE•MEDI•RI•EAND•JA••AS: intersectionarea has recently been the site of abundant DIVEOBSERVATIONSANDSAMPLESTUDiES magmaticactivity. Previousto ourdives in theRTI region,data regarding the Themain morphological feature of theVema eastern RTI is lithologyof theVema median ridge were obtained at twosites. thatthe transform valley is dividedinto a northernand a Onesite (R/V Charcot, 1977, about 41ø00W, 10ø45N, dredge 9) southerntrough by themedian ridge (Figures 2 and 3). ThisE- releasedundeformed basalt fragments, cataclastic brecciated W trendingfeature is continuousfrom 41ø21'W to the basaltsand sedimentary basaltic breccias [Mdvel, 1981]. easternmostpartof the mapped area, at 40ø40'W. It reaches its However,this location was wrongly believed tobe along the maximumelevation (3400 m) at41ø00'W. Immediately to the southernwall of the fracture valley. Improved navigation of the northof theMAR axis,the crest of themedian ridge is bathymetrymap has now revealed that this previous dredge shallowerand exhibits an arcuate shape. This bend in theridge haul was located along the southern flank of themedian ridge. crestcontrasts with the linear trace of theaxis of thenorthern Most of thebasaltic fragments show extensive greenschist trough.The width of themedian ridge (7 km) is relativelyalteration with numerous chlorite veins, pumpellyite, growth constantto the westand to theeast of the intersectionarea, of whitemicas within albitized plagioclases, etc. Another site while it decreasesto lessthan 5 km at the northerntip of the is on the northernflank of the "Vemamound" between 41ø50'W MAR axis. and41ø55'W (Figure 1), wherethe R/V Conrad(cruise RC 21) 17,336 LAG^BRIELLEET AL.: VEMA FRACI'URE ZONE'S MEDIAN RIDGE
,•, • '-•'?,• • ,•-•'•-•.,;-•C•-,' .,_,,•,•'•;•:?:::{;•, •,•. • ': •' -" 4000
.... c'?,'•,C' •• I•:4 •C,; :, ;• •,'••,: ?" ' 1,2 3
•** •'ß•.':"':•'•":'•-: .....• -o:':'8'•:' '" d 45 o o
.;..,...... No vertical exaggeration • i 1000 2000 3000
Horizontal distance m DIVE N ø 13
Fig. 5. Geologicreconstruction of Nautile dive 13 (northernflank of the median ridge; without vertical exaggeration). Numbers(1 to 10) indicatelocations of samplescollected along the dive. See Figure 4 for locationof dive track.
dredgedperidotites (25% by weight);gabbros and metagabbros it is not possibleto assesswhether the massive rocks observed (55%); volcaniclasticmaterial (15%) and basalt (5%) (E. hererepresent exposures of theridge basement or wideblocks Bonatti, unpublisheddata, 1984). within a very coarsedetrital formation. Nautile dives13 and 12 wereentirely devoted to thestudy of Observationsmade during dive 13 showthat the northern the northernand southernflanks, respectively,of the median flank of the Vema median ridge lacks evidenceof recent ridge (Figure 4, 5, and 6). The resultsallow the reconstructiontectonic activity. This contrastsstrongly with the southern of a speculativeNS transectof theridge (Figure 7). Additionalflank of theridge where evidence of recentfaulting is abundant informationconcerning the lithology of the ridge and its (seebelow). Nevertheless, the scarpsexposing the breccias are geologicalboundaries with the southernvalley and the nodal probablythe trace of nowinactive major normal faults. basin has been obtained from dives 8, 9, 10, 11, 17 and 18 Samplescollected during the dive are eitherclasts recovered (Figure 4). in situ from the breccias and from talus at the base of the scarps,or pieces of poorly consolidatedbreccias. The 4.1. The NorthernFlank of the MedianRidge (Dive 13) lithology of the largest clasts is variable. Metagabbros, Dive 13 beginsat 4658 m, that is, 500 m higherthan the metabasaltsand metadolerites,showing a stronggreenschist deepestpart of the northerntrough, in a flat area flooredwith metamorphismoverprint, are the most abundantrocktypes. recentpelagic ooze (Figure 5). The dive ends at the summitof Small fragmentsof serpentinizedperidotites and detrital the median ridge (3750 m), less than 1 km to the northwestof spinelswere found locally in onemicrobreccia sample (VE 13- the endpointof dive 12. The slopeof the ridge is regularand 3; Table 1). The protolith of theseserpentinized peridotite moderate and is covered with abundant, white pelagic fragments appears to have been a clinopyroxene-rich sediments.Ripple marks resulting from very active bottom harzburgite,or a lherzolite, with a porphyroclastictexture currentshave been observedat the foot of the ridge. The typicalof uppermantle rocks. The fine-grainedmatrix of the smoothsurface of sedimentsis cut from place to place by brecciasis composedof smalllithie fragmentsand of detrital moderatelysteep scarps, 1 m to a few metershigh, locally more minerals such as pyroxenes, more or less albitized (up to 30 m at 3810 m), that exposeonly poorlyconsolidated, plagioclases, green to pale greenamphiboles, epidote, chlorite subhorizontalpolymictic breccias showing rounded to angular, and hematite.Fragments of microfossilsand micritegrains are decimeter-sizedclasts of metagabbrosand metabasaltsin a also found within the matrix of the breccias. fine-grainedmatrix. Large blocks, a few decimeterswide, are found either isolatedon the pelagicsediments or partially 4.2 The SouthernFlank of the MedianRidge buriedwithin the breccias.A thin ferromanganesecrust can be to the Westof the Nodal Basin: observedon the outcropsas well as on the samplescollected, ObservationsDuring Dives indicatingthat rock exposuredid not occur very recently. 8, 9, 10, 11 and 12 Locally, alongthe highestscarps, graded bedding within the Dive 12. Three successive formations have been observed brecciascan be observedsuggesting rapid emplacementby duringdive 12 alongthe sectionacross the southernflank of gravityflows. The crestof the medianridge shows relatively the medianridge (Figure6). continuousexposures of angularrocks and isolatedblocks with 1. The baseof the ridge flank consistsof semiconsolidated characteristicstypical of doleritesand gabbros.No pillow to consolidatedbasaltic and gabbroicsedimentary breccias. fragmentsor lavatubes were observed here and the only sample These detrital formations have been observed and sampled recoveredis a gabbro(sample VE 13-10,Figure 9c). However, alongthe basalslopes of the ridgebut alsoalong a very recent LAGABRIELLEET AL.: VEMA FRACTURE ZONE'S MEDIAN RIDGE 17,337
10 N
Depth
4000
i I I
4500
5000
No vertical exaggeration
! I I I 1000 2000 Ho•zonteld•tance rn DIVE Nø12
, Fig. 6. Geologicreconstruction of Nautile dive 12 (southemflank of the median ridge; without vertical exaggeration). Numbers(1 to 10) indicatelocations of samplescollected along the dive. See Figure4 for locationof dive trackand Figure5 for key to symbolsused. Blocks with triangle representbreccia blocks. Heavy lines representactive or inactive faults; the dip of faults is hypothetical. fault scarp located in the valley floor at 5149 m and blocks of massiverocks, more than 1 m wide, were commonly representingan active splay of the present-daydisplacement observedduring the dive. Thin sectionobservation reveals that zone [Mamaloukas-Frangouliset al., 1991]. The brecciasare some microbrecciasare affected by pervasivecleavages, shear exposed from 5100 to 4500 m along scarps separatedby bandsand recumbentfolds (Figure 10b). These microstructures gentle, south dipping slopes covered with white pelagic are linked to displacementsalong faults which crosscut the sediments(Figure 6). All exposuresare free of ferromanganesebreccias. Numerous gabbroic fragments show cataclastic crust and fresh debris slides are abundantsuggesting recent deformation. However, it is unclear whether these rocks were exposure and active tectonism. It is rare that the criteria deformedprior to or after their depositionwithin the breccias. allowing good resolutionof the generalattitude of the bedding A few samplesshow a dark red color relatedto high hematite of the breccias are met. At one place, however (4863 m), a content. Hematite is present both within the metabasaltic horizontalcontact between white and red breccia layers, some clasts and within the fine-grained sedimentarymatrix of the decimeters thick, has been observed. In other places, breccias. As discussed below (section 5), the abundance of horizontalbedding is definedby the attitudeof red microbreccia hematite is probably due to extended low temperature layers interbeddedwithin coatsetbreccias. hydrothermal circulation within major fractures that crosscut Submersibleobservations as well as thin sectionstudies of the ridge. It must be pointed out that such hematite-bearing samples (Figure 10 and Table 1) indicate that the material breccias have not been found on the northern flank of the reworkedwithin the brecciasobserved during the first part of medianridge and thus appearto be restrictedto this part of the dive 12 is highly heterogeneous.Fragments of metagabbros, section. Hematite-rich metabasaltic breccias were also metadolerites, metabasalts and isolated detrital minerals such recoveredfarther west, along the southernflank of the ridge and as weathered plagioclase, more or less amphibolitized along the present-daydisplacement zone during dives 8, 9, 11, pyroxene,green amphibole,chlorite, and epidote are the usual 12, 17 and 18 (Figure 12). components of the breccias. The nature of the clasts At 5156 m, a small escarpment,a few metershigh, exposes demonstratesthat the sourceof this material correspondsto massive rocks, showing regularly spacedvertical cleavages, different levels of oceanic crust that underwent extensive representingprobably sheeted dykes. This exposure may greenschistmetamorphism. Less abundantfragments of upper representthe crustalbasement of the medianridge ratherthan a mantle material suchas serpentineand spinel are also found in voluminous block enclosed within the detrital formation. the breccias (Figure 10 and Table 1). The size of the mafic 2. The secondformation consists of pillow basalts,massive fragmentsusually rangesfrom 1 mm to somecentimeters, but lavas and basalticbreccias exposed from 4400 to 4000 m depth 17,338 LAGABRIELLEETAL.: VEMA FRACTURE ZONE'S MEDIAN RIDGE
SOUTHERN TROUGH MEDIAN RIDGE NORTHERN TROUGH TRANSFORMFAULTZONE • ctivefaulting.nomanganese crust noactive faulting. tiickmanganese crust
7 km •:•...... 0 i 2no v.e.3 detritalcoverunit I I I •km ...... •• uppercrust(basalts, dikes,..) lowercrustand mantle Fig.7. SyntheticN-S geologicalsection of thetransform walls and the median ridge of theVema FZ at 40ø59'W,based mainlyon geological observations during dives 10, 12, 13, 14.Section is shownwithout vertical exaggeration. along numerousvertical escarpmentsseparated by steps valleyfloor and on thefoot of themedian ridge [Auzende et al., coveredwith unconsolidatedpelagic sediments (Figure 6). Very 1989; Marnaloukas-Frangoulis et al., 1991]. The rocks recentfaulting in this part of the sectionis evidencedby the exposedalong these scarps are mainlysedimentary breccias abundanceof freshfault scarpsprincipally striking E-W, N140 showinglocally clear horizontal bedding outlined by variably andN50 andby thelocal occurrence of exposedfault planes. At coloredlayers (Figure 9e). In someplaces, the brecciasare 4089 m, a vertical, N140 trending, fault plane shows clear deformedin fault gaugesand exhibittectonic cleavages and striae with a moderatesouthward pitch (0ø to 20ø), indicating slickensides.Thin sectionsof samplescollected in situ or from both vertical and lateral displacementalong the fault. talus at the base of the scarps show a well developed Due to the fact that the secondformation is exposedalmost schistosity.In two placesonly basaltictubes and pillows are continuouslyfrom 4400 m to 4000 m withoutany intervalof exposedalong scarps located in thevalley floor (dive 11) or at polymicticbreccia or anyoutcrop of gabbroand peridotire, we the foot of the medianridge (dive 10). This indicatesthat the tentativelysuggest that the basaltsrepresent exposure of the detrital cover is locally thin or absentdue either to initial crustalbasement of the median ridge. Such an interpretation reducedthickness or to further faulting. It is unclearhowever shouldbe confirmedby furtherlateral submersible explorations whether the detrital formations observed on the valley floor or drilling. representdebris flows emplaced recently in relationto tectonic 3. Dive 12 ends in the third formation which consists of eventswhich affected the southemflank of the median ridge, or gabbroicand basaltic sedimentary breccias (sample VE 12-10, olderdetrital deposits corresponding to the initial coverof the Figure 10d). The brecciassparsely crop out along gentle, median ridge. muddyslopes from 4000 m to thetop of theridge at 3800m. In In contrast to results of dives 10 and 11, observationsmade contrastto the regionsexplored earlier during the dive, the last duringdives 9 and 8, to the eastof 40ø58'W,indicate that partof the transectis characterizedby gentlerelief andby the activefaults of the present-daydisplacement zone cut through lack of steepscarps and recent talus (Figure 6). This indicates freshpillows belonging to thebase of the southemwall of the that this area is tectonicallyinactive. This third formationis transform.Therefore, the boundarybetween the medianridge probablythe southwardextension of the detritalformation and the volcanic formations of the southern wall of the exposedalong the northernflank of the medianridge and transform tends to trend NE-SW between 40o58 ' and 40o55 '. sampledduring dive 13. Thus,the activefaults of the present-daydisplacement zone do Dives 8, 9, 10 and 11. Poorly consolidatedto consolidated not alwayscorrespond to the southerngeological boundary of polymicticmafic brecciasand microbrecciaswith hematitic the median ridge. matrix as well as mafic graded-beddedsandstones containing abundant greenschistdetrital minerals (albite, chlorite, 4.3. The Median Ridge to the North of the RTI and Its actinolite, amphibolitizedpyroxenes) were observedand BoundaryWith the Nodal Basin(dives 17 and 18) sampledat the foot of the southemflank of themedian ridge The major part of dive 18 observationsconcern the western duringdives 8, 9, 10 and 11 (Table 1 andFigure 10e). The nodal basin of the RTI, and the recent volcanic constructions active faults which characterizethe present-daydisplacement locatedalong the neovolcanicridge. From 5200 m to 5100 m, zone crosscutthese detrital formationsonly in the westernmost in a relatively flat volcanicarea locatedon the northernedge of part of the surveyedarea, i.e., westof 40ø58'W(dives 10, 11 the western nodal basin, we crosseda set of parallel, closely and 12, Figures9c and9d). Very freshscarps, V-shaped valleys spaced fault traces marked by V-shaped furrows, funnel with active debrisslides were commonlyobserved both on the alignmentsin the sedimentsand fresh scarpscutting through LAGABRIELLEETAL.: VEMA FRACTURE ZONE'S MEDIAN RIDGE 17,339
TABLE 1. Brief Descriptionof SamplesCollected During Nautile Dives Along the VernaMedian Ridge
Depth, Brief description Numbers mbsl
Dive 8 VE08-1 4896 Sedimentarybasaltic breccia with red matrix (Fe-oxides).Clasts from rims and core of weatheredpillows. Radiolaria and Foraminiferaare presentin the matrix. VE08-2 4986 Cataclastic chloritized basalt.
Dive 9 VE09-1 4695 Sedimentarybasaltic breccia with red matrix (Fe-oxides). VE09-2 5066 Sedimentarybasaltic breccia with red matrix (Fe-oxides). Dive 10 VE10-1 4810 Red (hematite rich), mafic greenschist,graded-bedded sandstone. Contains 1 mm-sized lithie dasts (basalts, dolerites and weathered basaltic glass) and smaller fragments of isolated minerals (plagioclases,pyroxenes, amphibolitized pyroxenes, chlorites, amphiboles, hematite) and micrite grains.
Dive 11 VEll-1 4921 Deformed,mafic greenschistmicrobreccia with red matrix (hematiteand Fe-oxides).Contains millimeter- sized clasts of metadolerite, and isolated fragments of minerals (plagioclases, pyroxenes, amphibolitizedpyroxenes, green amphiboles,actinolite, chlorite, hematite) and micrite grains. VE 11-2 4856 Sedimentarybasalt breccia (clasts up to a few centimetersin diameter)in a matrix of red basalticsandstone showinggraded beddingand local schistosity.Chlorite-calcite veins crosscutthe breccia. VE 11-3 4848 Polymictic greenschistfacies breccia. Clasts are dominantly basalts showing various textural types. Fragmentsof sedimentarybasaltic breccias are common.The rock also containsrare detrital spinel as well as fragments of pyroxene-rich sandstones. VE11-4 4848 Greenschist facies dolerite with cataclastic deformation. VE11-5 4772 Brownishpelagic sediment.
Dive 12 VE12-1 5149 Gabbro. VE12-3 5168 Talc-chloritemylonite with hydrogamet,probably a stronglydeformed roddingite. VE12-4 5138 Deformedmafic layered sandstone.The rock is composedof altematingred and green layers, 1 mm thick, showingisoclinal folds. Red layers containhematite grains and small fragmentsof chlorite, epidote and acfinolite. Green layers containdetrital pyroxenesand needlesof green amphibolein a chlorite- rich matrix. No fresh plagioclaseshave been observed. VE12-5 4944 Aphyric basalt. VE12-6 4873 Greenschistfacies gabbrobreccia. The brecciacontains large roundedclasts of greenschistmetagabbro in a matrix of gabbroicsandstone with abundantfragments of clinopyroxenes,altered plagioclases, green amphibolesand chlorite. Micrite grainsare present.Serpentinite fragments have been observed. VE12-7 4661 Pillow breccia with hematite-rich matrix. VE12-8 4515 Chloritized metadolerite VE12-9 4406 Cataclasticmetabasalt with quartz-chloriteveins. VE12-10 3832 Greenschistmafic polymicfic breccia with metagabbro,metadolerite and metabasaltfragments. Isolated detrital mineralssuch as pyroxenes,amphiboles and plagioclasesare abundantwithin the matrix. The rock containsrare detrital spinel. Clastsare matrix-supported.A prehnite-quartzvein crosscutsthe breccia.
Dive 13 VE13-1-1 4626 Gabbro. VE13-1-2 4626 Dolerite. VE13-1-3 4626 Dolerite. VE13-2 4626 Metagabbroblock from an unconsolidatedpolymictic breccia. VE13-3-1 4585 Poorlyconsolidated polymictic breccia with a matrixof mafic sandstoneand siltstonecontaining isolated plagioclases,pyroxenes, large amphiboles,epidote, chlorite, hematiteand spinel. Micrite grains and microfossilsfragments are also present. VE13-3-2 4585 A centimeter-sized clast from breccia VE13-3-1: serpentinized, undeformed lherzolite with fresh orthopyroxeneand clinopyroxene. VE13-4 4545 Fragmentsof basalt and gabbrofrom an unconsolidatedpolymictic breccia. VE13-5 4323 Dolerite. VE13-6 4044 Poorly consolidatedgreenschist polymictic breccia with angular to rounded clasts of metabasalt and metagabbroin a fine-grainedmatrix of mafic greenschistsandstone. VE13-7 4000 Metabasalt. VE13-8 3850 Poorly consolidatedgreenschist plutonic microbreccia.Lithic and crystal fragments derive principally from gabbroic rocks. Chlorite and amphibolitizedpyroxenes are abundantwithin the matrix of the microbreccia. VE13-9 3826 Metadoleriteblock set in a mafic greenschistsandstone. VE13-10 3753 Gabbro.
Dive 17 VE17-2-1 4902 Greenschistpolymictic coarseto fine-grainedsandstone. Both samplesshow gradedbeddings and cross- and VE 17-2-2 beddings.They containlithic fragmentsof basalt,dolerite and microgabbroand minor serpentinite, and relateddetrital minerals(clinopyroxene, chlorite, hematite, spinel, serpentine). 17,340 LAGABRIELLEET AL.: VEMA FRACWURE ZONE'S MEDIAN RIDGE
TABLE 1. (continued)
Sample Depth, Brief description Numbers mbsl
VE17-3 4790 Greenschistbedded siltstone. Same elements as samplesVE17-2. VE17-4 4654 Sedimentarymarie breccia.It consistsof angularfragments of dolerite and isolatedminerals showing a greenschistfacies overprint set in a red, fine-grainedmatrix (Fe-oxides). VE17-5 4529 Gabbro. VE17-7 4481 Fine-grained dolerite. VE17-8 4219 Gabbro.
Dive 18 VE18-6 4769 Cataclastic metadolerite breccia. VE18-7 4725 Sedimentarybasalt breccia with red matrix (Fe-oxides).
Here, mbsl denotesmeters below sea level. pillows. These features are inferred to be the surficial neovolcaniczone. Furthermore,this confirmsthe resultsof expressionof thepresent-day displacement zone [Mamaloukas- dive 8 showingthat the present-daydisplacement zone is not Frangouliset al., 1991].At the endof thedive, i.e., from4800 the geologicalboundary between the medianridge and the m to 4700 m, we reachedthe southernflank of the median recentvolcanics emplaced in the RTI domain. ridge. This area is characterizedby tectonically inactive, Dive 17 providesa completesection of the southernflank of highly sedimentedsmooth slopes, contrasting with the rough the medianridge to the eastof the neovolcanicridge and to the topographyof the neovolcaniczone previouslyobserved northof the easternnodal basin (Figure 4). The divebegins on during this dive. Locally, a chaotic mixture of meter sized, the westernflank of the neovolcanicridge (4958 m) in an area angular blocks is partially buried in the sediments.Samples showing fresh volcanic formations, mainly basaltic pillows collected include a cataclastic metadolerite breccia (VE 18-6) and lava tubes, and fresh talus, and continues to the floor of the and a fragment of mafic sedimentarybreccia with hematite-rich easternnodal basin(5172 m; Figure 8). The sedimentaryinfill matrix (VE 18-7). Both samples show a well-developed appearsto be thick and no mark of recenttectonic activity was ferromanganesecrust, more than 1 cm thick on sample VE 18- observed on the western wall and the flat floor of the basin. 7, indicating old exposure.The clasts of breccia VE 18-7 Both observationssuggest that the faults responsiblefor the consistof centimetersized, angular fragmentsof greenschist subsidenceof the nodal basin are now inactive. Farther faciesmetabasalts set in a fine-grainedmatrix containinglithie upslope,at 5071 m, on the northernwall of the easternnodal fragmentsand isolatedminerals such as plagioclases,hematite, basin we observeda vertical fault scarp exposingsections of chlorite and actinolite. pillows slightly dustedwith pelagic sediments(Figure 8). No No mark of any recenttectonic activity was observedat the debriswas observedat the foot of the scarp,because of a thick boundarybetween the neovolcanicridge and the medianridge. sedimentarycover, suggestingthat the fault is inactive.This Consequently,the median ridge appears now to be weldedto the scarp probably representsone of the major normal and/or
N $
8
Depth 7
ß:5:':'::-;':'.: •:•-"...':•:•/•'.:; •t•-,,•,• :-...... 3 4500 .....?o....
\ 5000 / '?:'::i%t:??:::.::'
No vertical exaggeration
1000I I 20100 I 30100 I 4000 Horizontal distance rn
DIVE N ø 17 Fig. 8. Geologicreconstruction of Nautile dive 17 (southernflank of the medianridge at the tip of the MAR axis;without verticalexaggeration). Numbers (1 to 8) indicatelocations of samplescollected along the dive. See Figure4 for locationof dive track and Figure 5 for key to symbolsused. Heavy lines representactive or inactive faults; the dip of faults is hypothetical. LAGABRIELLEET AL.: VEMA FRACTUREZONE'S MEDIAN RIr3GE 17,341
:•":"::'::?,?..•:::.:•:•:-:•:::-•: ?:3::: :I. .::.> . :'. '? • --'6 :':':..C :G..b• ..... •:: ...... :• .::'•:?:?' ....::-':; ...... 7•:,'.
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Fig. 9. (captionon followingpagO 17,342 LAGABRIELLEET AL.: VEMA FRACIIJREZONE'S MEDIAN RIDGE strike-slipfaults that accommodatedthe formationof the nodal However,no definitive observationconfirms this hypothesis. basin. Here again, the gabbro blocks could be large fragments Farther north, the dive track crossesa narrow E-W trending, includedin a very coarsedetrital formationemplaced on the sediment-coveredvalley and enters the median ridge domain basementof the medianridge. where no fresh voltanits were observed(Figure 8). Rock types similar to the detrital formations capping the median ridge 5. SUMMARY OF MAIN RESULTS fartherwest were observedand sampledat 5000 m, immediately after the narrow valley. These are isolated blocks of massive, 5.1. The Detrital Cover of the Median Ridge tabular graded-beddedsandstones and siltstonesa few meters Submersibleobservations show that the Vema medianridge wide. Thin section analysesof samplesVE 17-2 collected in is cappedby a detrital cover (Figures7 and 11) which consists this area reveal that the coarser sandstonesare made up of of a successionof polymicticsedimentary breccias, sandstones variously altered millimeter sized fragments of dolerite and and siltstonesmade up of metabasaltic,doleritic, and gabbroic basalt, palagonitized basaltic glass, and isolated mineral clasts, with less frequent serpentiniteand spinel fragments. grains such as large clinopyroxenederived most likely from Most of the rocks have been subjectedto greenschistfacies plutonit rocks, amphibolitizedpyroxene and minor spinel and metamorphismprior to, their incorporationwithin the detrital orthopyroxene. Minor lithie components are fragments of cover. The compositionof the clastssuggests that the detrital serpentiniteand microgabbro.Hematite is commonin most of cover originatedfrom the disaggregationof outcropsof various the lithie fragments. Associatedsiltstones (sample VE 17-3) crustal and upper mantle oceanic rocks. The occurrenceof are composedof isolated mineral debris including plagioclase, sandstone and siltstone horizons indicates that the detrital amphibole, ohiorite, pyroxene, hematite, spinel, and material was significantlytransported prior to deposition;and palagonitized glass shards. the gradedbedding occasionallyobserved within detrital beds The nature of the reworked material in the breccias and suggestsemplacement by gravity flows. The northernflank of sandstones is similar to that of clasts found in breccias the ridge is entirely covered with such a polygenic detrital recoveredall along the southernwall of the median ridge. The formation and shows no evidence of recent tectonics. In occurrenceof fine-grainedsediments such as greenschistfacies contrast,the southernflank of the median ridge to the east of sandstonesand siltstones suggeststhat the detrital material 40ø56'W was affectedby recent faulting and is locally covered suffered significant transportbefore deposition. by hematite-rich monogenicbasaltic breccias and sandstones The sandstonesare better exposedhigher up, at 4755 m, (Figure 7). Such monogenicformations resulted probably from along a N110-N90 trending, 80 m high, vertical scarp (Figure the in situ brecciationof the medianridge basement. 8) correspondingto an inactive major fault. Relict fault planes with striae dipping 45 ø to 30 ø to the west are still visible 5.2. The Basementof the Median Ridge locally on this scarp (Figure 9f). We interpretthis fault surface It is clear now that the Vema median ridge is not a very as a now inactive dextral strike-slip fault with a vertical recent constructionalvolcanic feature and can no longer be component,allowing the southernflank of the median ridge to interpretedas a serpentinizedmantle ridge. However, the nature subsidetoward the nodal basin.The averagepresent-day dip of of the basementof the Verna medianridge, beneaththe detrital the sandstones is 30 ø to the west. These structural observations cover, is still a matter for speculation. In our interpretation, confirm that the medianridge was subjectedto local tilting in the basaltic pillows exposed along the southern flank of the responseto normal and strike-slip faulting. ridge (dive 12) are thoughtto representthe crustalbasement of After this scarp, the slope becomes more gentle. We the ridge. Additional explorationis neededin order to test the observedfrequent chaotic blocks of mafic brecciascomposed of lateral continuity of these outcropsalthough our interpretation angular, matrix-supported clasts of greenschistmetabasalts, is supportedby the observationthat the pillow formation is dolerites and microgabbroand isolated minerals (sample VE exposedalmost continuouslyfrom 4400 m to 4000 m without 17-4; Table 1 and Figure 10a). The matrix of the brecciasshows any interval of polymictic breccia or without any outcrop of the high hematite content that characterizes the samples gabbro and peridotite. Other smaller outcrops of dykes and collectedalong the southernboundary of the medianridge. The gabbrosfound respectively at the base of the southernflank dive ended at 4216 m in an area of complexmorphology with (dive 12) and at the summit of the ridge (dive 13) remain chaos of blocks and talus of various lithology, mainly mafic questionable. Since they are associated with more or less brecciasand gabbros.The abundanceof gabbroicrocks might abundant polymictic breccias they cannot be regarded suggestthat this rock type forms the basementof the ridge. confidently as basementoutcrops.
Fig. 9. Nautile bottomphotographs of the Vema medianridge and northerntransform wall. (a) Typical aspectof the tectonicallyinactive northern transform wall (dive 14, depth:4429 m; the field of view is approximately10 m wide). Outcropsare cappedby a thickmanganese crust and partially covered by pelagicsediments. (b) Very freshscarp at the foot of the southernflank of the medianridge (dive 11, depth:4853 m) exposingpoorly consolidatedpolymictic sedimentary breccias.Dark patch (approximately2 m long) in the centerof the photographcorresponds to hematite-richbreccias. (c) Samplingof a gabbroblock from the summitof the medianridge (sample 10, dive 13, depth:3753 m; the field of view is approximately4 m wide). The massiverocks observed here represent either exposures of the ridgebasement or wide blocks withina very coarsedetrital unit belongingto the medianridge cover formation. (d) Poorlyconsolidated polymictic sedimentarybreccias exposed along an activefault scarpat the foot of the southernflank of the medianridge (dive 11, depth:4922 m; widthof field: 5 m). (e) Detritalformation showing horizontal bedding outlined by variablycolored breccia layers and alternatingpelagic sediments (whim layers)from the foot of the southernflank of the medianridge (dive 9, depth:5038 m; thebreccia layer in the middleof thephotograph is about1 m thick).(f) Detailedview of a 80 m highfault scarpwith obliquestriations exposing mafic sandstones at the southernflank of the medianridge (dive 17, depth:4722 m; the field of view is about 1 m wide). LAGABRIELLEET AL.: VEMA FRACTURE ZONE'S MEDIAN RIDGE 17,343
Fig. 10a. Photomicrographof SampleVE 17-4 (4654 m): sedimentarymafic brecciacomposed of angulardasts of dolerite (d), chloritizeddolerite (chl) and isolatedmineral fragmentssuch as amphibolitizedpyroxene and plagioclase(pl). The fine- grainedmatrix is a flour of greenschistfacies dolerific rocks with abundantiron oxidesgrains (plane polarized light).
Fig. 10b. Photomicrographof SampleVE 12-4 (5138 m): deformed,mafic, layered sandstone from a fault gaugeon the southernflank of the medianridge. The rock is composedof alternatinglayers, l-ram thick, showingisoclinal folds and smallscale shear bands. The dark (red) layerscontain hematite grains and smallfragments of chlorite,epidote and actinolite whereaslight (green)layers containdetrital pyroxenesand needlesof greenamphibole in a chlorite-richmatrix. No fresh plagioclaseshave beenobserved (plane polarized ligh0.
Fig. 10c.Photomicrograph of Sample VE 12-6 (4873m): greenschistfacies gabbro breccia containing rounded clasts of greenschistmetagabbro in a matrixof gabbroicsandstone with fragmentsof clinopyroxenes(CPX), alteredplagiodases (pl), greenamphiboles and chlorite (chl). A largeserpentinite fragment (s) is presentin thecentral part of thephotograph (plane polarized light). 17,344 LAGABRIELI•E-• AL.: VEMA FRACTUREZONE'S MEDIAN RIDGE
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Fig. 10d. Photomicrographof SampleVE 12-10 (3832 m): greenschistmafic polymicticbreccia with metagabbro, metadoleriteand metabasalt fragments. Isolated detrital minerals such as pyroxenes(CPX), amphibolesand plagioclases (pl) are abundantwithin the frae-grained,mafic matrix. Clasts are matrix-supported (plane polarized light).
Fig. 10e. Photomicrographof SampleVE 10-1 (4810 m): hematiterich, mafic, graded-beddedsandstone. The rock consists of millimetersized lithic clastsof moreor lessaltered basalts, dolerites and weatheredbasaltic glass and smallerfragments of isolated minerals (plagioclases,pyroxenes, amphibolitized pyroxenes, chlorites, amphiboles).Abundant hematite grainsgive the dark-redcolor to this rock. Micrite grainsare presentlocally (planepolarized light).
Fig. 10f.Photomicrograph of Sample VE 12-?(4661 m): monomictic pillow breccia. The dark color of thebasaltic clast is mainlydue to developmentof hematite. The glassy matrix of thebreccia is nowreplaced by chlorite(plane polarized light). LAGABRIEI.LE ETAL.: VEMA FRACTURE ZONE'S MEDIAN RIIXlE 17,345
S detritalcover unit: N greenschist volcanic-plutonic - 4ooo breccias and sandstones
. - •' ..•..'0...:•" --r- •"'• neovolcanicridgebasalts ...... -soooI nodalbasinbasa
0 1000 I Im
Fig. 11. SyntheticN-S geologicalsection of the southernflank of themedian ridge of theFZ at thetip of theMAR axis,at 40ø53'Walong track of dive 17.
Assumingthat gabbrosand dykes are largeblocks within a outcroppingalong the southerntransform trough. To the north detrital formation,and that the basaltsrepresent basement of thenodal basin, this boundary can be clearlydefined as the outcrops,the medianridge can be regardedas a portionof northernmostextension of fresh volcanics.This geological relativelyundisrupted oceanic lithosphere covered with a thick boundarycoincides with the PDDZ only in the westernmost sequenceof coarse to fine-grained detrital formations, as part of the surveyedarea, to the westof 40ø56'W(Figure 4). representedin Figure7. The natureof therocks sampled from Between40ø56'W and 40ø54'W, in the areaof dives8 and 18, the westernextension of the medianridge, i.e., the "Verna thisboundary diverges from the PDDZ, and trends NE, roughly mound" at about 41ø50'W, is consistent with this parallel to the 4800 m isobath.Farther east of 40ø54'W, the interpretation. boundaryruns approximativelyE-W, followingthe 5000 m The medianridge is now linkedto the AfricanPlate, since it isobath.It appearsfinally that the northernmostextension of is located to the north of the PDDZ. If we assume that the the neovolcaniczone is now linked to the northern(Africa) transformfault zone (TFZ, asdeftned by Fox andGallo [1984]) Plate. alwaysremained south of the medianridge, the crustof the 5.4. The Hematite.Rich Breccias From the Southern Flank ridge was createdat the westernRTI. In this case,the northern troughof the valley shouldbe consideredonly as the northern of the Median Ridge tectoniclimit of the uplifted medianridge and not as the Metabasaltic breccias with hematitic cement were collected topographicexpression of a fossil transform-relatedfault. duringdives 8, 9, 10, 11, 12, 17 and18 fromthe lower slopes Alternatively,it is possiblethat the TFZ jumpedto different of the southernflank of the medianridge (Figures 4, 10 and successivepositions within the fracture valley, from one edge 12). Thesebreccias are madeup of subangularto sub-rounded of thefracture to theother. In suchconditions, the median ridge rock andmineral fragments from various parts of pillow lavas, basementshould be regardedas the tectonicjuxtaposition of andfrom the innerpart of thickflows or dykes.They do not verticalslices of oceaniccrust created nonpreferentially at the include gabbroor serpentinefragments and thereforediffer westernor easternRTIs, and separatedby fossil tracesof the from the polymicticbreccias recovered along the northern successive TFZs. This hypothesis is consistent with flank of the medianridge. geophysicaldata obtainedat the westernpart of the Verna The natureof the cementhas been determined by X ray transformwhere the crust of thefracture valley has been shown diffractionanalysis. It consistsof a mixtureof clay minerals to havean anomalousstructure interpreted as resultingfrom and hematite which gives a dark red color to the collected intensefracturing and tectonicbrecciation [Derrick et al., breccias.The basaltic elementshave undergonevarious 1982]. alterationswhich demonstratethe polygeniccharacter of the One can alsoimagine that dip-slip normal or reversefossil breccias. For instance, the microlitic basalt fragments faults related to previousnontransform-related tectonic events sometimesdisplay centripetalcolor zonationwith light suchas the uplift of the transverseridge crosscut the basement colored margins around dark coresindicating that the rock of themedian ridge. fragmentswere altered after or whilebeing deposited. Basaltic minerals,except augitc, are generallypartly altered,e.g., 5.3. TheGeological Boundaries of theMedian Ridge plagioclases,or completelyreplaced, e.g., olivine, by Dive observationsand sample analysis lead us to definethe relativelylow temperaturesecondary minerals such as clay southerngeological limit of themedian ridge as the boundary minerals, chlorite, actinolite or zeolites. The basaltic elements betweenthe polymicticdetrital formations exposed on the may includeepidote veinlets but the wholerock samplesare southern flank of the ridge, and the recent volcanics criss-crossed by later zeolite veins. These observations 17,346 LAGABRIELLEETAL.: VEMA FRA• ZONE'SMEDIAN RIDGE
NORTHERN
•ovolcanic zone
SOUTHERN
SOUTHERN •-. WALL
hematite-richmetabasaltic breccias and sandstones r/4 3 km NB ßnodal basin
41ø 10 41ø ..... ::40• 50
Fig.12. Simplified structural map of the eastern part of the Vema transform fault and its intersection withthe MAR axis, and locationof siteswhere hematite-rich metabasaltic sandstones and breccias have been sampled during the Nautile dives. Crustcreated during the last 300,000 years (according to a 3.2 cm/yrspreading rate) is indicated. suggesta polymetamorphicevolution of thebreccias and their Stromboli or Santorini, Mediterranean Sea [Bonatti and elements.Hematite is observedas opaqueinfillings of fissures Honnorez,1972; Puchelt, 1973]. Hematite-cementedbasaltic throughthe rockfragments or as thin light coatingsof cracks breccias have also been recovered from Cretaceouscentral and cleavageswithin plagioclasesand augire phenocrysts. Pacificseamounts where oxygen isotope analysis of hematite Hematite also coats and fills in microfossils such as indicatestemperatures ranging from 30 ø to 90øCfor formation foraminiferas and radiolarians in the matrix of the breccias.The of thecement (J. Hein andH.W. Yeh,personal communication, monogeniccomposition of the brecciasand their location in a 1990).Mafic-ultramafic breccias with red, hematitic cement are restrictedbelt along the tectonized,southern flank of the also abundantin ophioliteswhere evidencesof oceanic medianridge suggest that they originatedfrom the basaltic tectonismare present.In the ligurian ophiolitesof the basementof the medianridge. Therefore,occurrence of this northern Apenninespolymictict breccias with gabbro typeof brecciacan be interpretedas exposureof themedian fragmentsand subordinate basaltic debris are cementedby ridge basement. chlorite,with hematiteand minor carbonates[Gianelli and In summary,this preliminary study of basalticbreccias with Principi, 1974;Barret and Spooner,1977]. Some of these hematitic cement collectedalong the southernflank of the brecciasrest stratigraphicallyover serpentinizedmantle and medianridge indicates a multistageevolution comprising (1) ophicalcitesand are conformably overlain by pillowbasalts. the transportationof already altered basaltic fragments, Occurrence of numerous epidote veins indicates that includingpillow lavasand dolerites' (2) theircementation by hydrothermalcirculation took place within the breccia. clayminerals and hematite and additional alteration by Fe-rich Locally,doleritic dykes cut throughthe breccias.Mafic solutions;(3) their in situ deformation,along subvertical brecciaswith iron-rich cement and hematitic, red sandstones strike-slipor dip-slipfaults related to transcurrentmovements arealso present in Cyprus,within the Arakapas fault belt which alongthe fracturezone and/or to verticalmovements on the is thoughtto representa fossil transform fault [SimonJan and edgesof thenodal basin. Development of hematite results from Gass, 1978]. low temperature(<100øC) hydrothermal circulations probably 5.5 StructuralResults and Implications indicatingthe presenceof deep active crustalfractures. Occurrence of iron-rich basaltic breccias is an additional Two striated,almost vertical fault planes were observed on indicationthat the southernboundary of theVema medianridge thesouthern median ridge's flank: one at 40ø59'W(dive 12) is (west of 40ø56'W), or has been (east of 40ø56'W), strikesa N140direction and exhibits striations with a moderate tectonicallycontrolled by faulting(Figure 12) . pitchto the south (0ø to 20ø), the other one, at 40ø51'W (dive Similar depositshave beenobserved in activesubmarine 17), trendsN90-110 and shows striations with a 30-40ø volcanoes such as Banu Wuhu, Indonesia [Zelenov, 1964]; westwardpitch. The geometry of thestriations indicates that LAGABRIETI .R E'F AL.: VEMA FRACTUREZONE'S MEDIAN RII•E 17,347 these faults accommodateda combination of strike-slip and fracturevalley walls. An importantuplift episodeis known to vertical displacements. have affected the southern wall of the transform valley, Dives 18 and 17 clearly show that volcanicsfrom the MAR resulting in the creation of the transverse ridge. neovolcaniczone have erupted far to the north of the PDDZ, Paleontologicaldata suggestthat this eventmay have occurred and have partly coveredthe southemflank of the median ridge. some 10 m.y. ago [Bonatti and Honnorez, 1971; Honnorez et The median ridge has an arcuateshape in this area, and is less al., 1975; Bonatti et al., 1983]. Bonatti and Crane [1982] elevatedand narrowerthan further to the west (Figure 2). This suggested that this event might have been related to a arcuateshape was first tentativelyinterpreted as a result of the contemporaneous change in plate motion in the central northward propagation of the MAR neovolcanic segment Atlantic [Van Andel et al., 1971; Roest and Collette, 1986]. causingthe bendingof the medianridge and local compression Hence, we proposethat the detrital cover of the Vema median [Auzende eta/., 1989]. The reduced width and elevation of the ridge might representthe sedimentaryrecord of the transverse ridge in the RTI area suggest, however, an alternative ridge uplift (Figure 13). Since this uplift episode, the explanation:we proposethat the southernpart of the median transverseridge and the detritus-coveredtransform valley floor, ridge in the intersection domain has undergone subsidence separatedby the transform fault zone, would have migrated during the deepeningof the nodal basin (Figure 3), prior to the westward and eastward respectively (Figure 13). This model eruption of the lavas which form the floor of the nodal basin assumesthat no significant migration of the transform fault and the neovolcanicridge. took place since about 10 m.y. ago. The 1.5 km thick ponded turbidires of Pleistoceneage that constitutethe sedimentary 6. GENERAL DISCUSSION AND CONCLUSIONS infill of the centralpart of the valley are thoughtto have buried the rough topography inherited from these complex tectonic 6.1. Origin of the Median Ridge Detrital Cover Formation and sedimentaryevents. Considerable volumes of mafic and ultramafic breccias are 6.2. Uplift of the Median Ridge Near the RTI Domain currently being depositedin tectonically active regions of the The mafic detrital formations exposed along the median ocean floor. Thick detrital formations containing mafic and ridge crest culminate at about 1000 m above the floor of the ultramafic fragmentshave been observedand studied in detail transform valley. It seems unlikely that the deposition of a on the flanks of the Gorringebank in the Atlantic [Lagabrielle thick horizontally layered detrital cover would have been and Auzende, 1982], in elongated basins of the Clipperton possible on such a topographic high. Consequently, we fracture zone [Barany and Karson, 1989], at the foot of the concludethat the medianridge is currentlybeing uplifted from northern wall of the Blanco trough (T. Juteau, personal the transform valley floor. This conclusionis confirmed by communication, 1991), and in other low- or fast-slipping seismicdata obtainednear the easternRTI (refer to profile B, transform faults: Romanche FZ [Bonatti et al., 1974; Bonatti Figure 5 from Eittreim and Ewing [1975], and Deep Tow's 4- and Honnorez, 1976]; Atlantis FZ [Dick et al., 1991]; Garret FZ kHz seismic profiles by Kastens et al. [1986]) showing [Hdbert et al., 1983], etc. As emphazisedby Barany and Karson evidencefor vertical tectonicsin the sedimentaryinfill of the [1989], this confirms that wrench tectonics and vertical transform valley. According to the latter authors, the Vema movements of crustal blocks are the two main processes median ridge may be a recent topographic feature, whose causing disaggregationof the oceanic basement and gravity westerntip is currently being uplifted at rates possiblyas high movements of debris. Such processes are both active in as lmm/yr. transform fault settings. Our data provide further constraintsto the mechanismof this The nature of the clasts recovered from the detrital cover uplift. Submersibleobservations demonstrate that the northern formation of the Vema median ridge suggeststhat the breccias flank of the ridge suffered no recent deformationand that the originated from the disaggregation of outcrops of various very recent tectonic activity is only concentrated on the levels of oceanic crust and of upper mantle-derivedperidotires. southernflank and at the summit of the median ridge, where Dredging and diving results show that all these rocks are basementrocks are exposed.This suggeststhat, at least in the frequentlyexposed along both walls of the Vema FractureZone surveyedarea, the uplift of the medianridge is accompaniedby [Bonatti and Honnorez, 1971; Melson and Thompson,1971; a progressive tilting. As shown by results of dive 14, the Thompson and Melson, 1972; Bonatti et al., 1974; Bonatti northernwall of the fracturevalley is cappedby a very thick and Honnorez, 1976; Prinz et al., 1976; Honnorez et al., 1984; manganese crust and is tectonically inactive (Figure 9a) Mdvel, 1981; Auzende et al., 1989, 1990]. Diving studies, indicatingthat the tectonic activity related to the rejuvenation however,also suggestthat thesewalls are not currentlyaffected of the median ridge has been essentially restricted to the by active mass wasting: serpentinites,gabbros and basalts southernpart of the fracturevalley [Auzendeet al., 1989]. exposed along the north wall of the fracture zone explored Severalpossible mechanisms may accountfor the uplift of during Nautile dive 14 at 41øW, are covered by a thick the Vema median ridge near the eastern RTI. Two of these manganesecrust. Moreover, the gabbrosobserved along the mechanisms are discussedhere: (1) serpentinizationof the north flank of the transverseridge around 42ø42'W (Nautile upper mantle beneath the median ridge; and (2) return of the dives 1 to 5 [Auzende et al., 1989; Cannat et al., 1991]) are transform valley to isostaticequilibrium, following reheating conformably covered by pelagic sediments which yielded of the old plate near the tip of the easternMAR segment.The fossilsranging from Miocene to Pleistocenein age [Aubry et second model suggestsa direct relationship between RTI al., 1992]. We thereforepropose that extensivedisaggregation processes and the development of a median ridge in the of deep crustaland uppermantle outcrops,and emplacementof transform valley. the detritus in the median valley of the fracture zone, are not 1. Serpentinizationof peridotites involves up to 30-40% present-dayprocesses but occurred mostly during a previous expansion [Thayer, 1966; Page, 1967; Nicolas, 1969; tectonic episode, involving significant uplift of one or both Coleman, 1971; Komor et al., 1985] and a correlativedensity 17,348 LAGABRIELLEE'rAL.: VEMA FRACrURE ZONE'S MEDIAN RIDGE
10Ma crust ½..
TFZ detrital cover unit TFZ
Fig.13. A speculativemodel for the evolution ofthe Vema median ridge between 10and 0 m.y.Panel 1depicts thesituation 10m.y. ago. The transverse ridge is beinguplifted; the detrital cover is emplacedin the transform valley. Panel 2 depicts thepresent-day situation. The transverse ridge and the valley floor migrate westward and eastward respectively. Thecrust in theeastemmost portion of theactive transform valley undergoes uplift near the tip of theMAR axis resulting in the partial exposureofthe detrital cover formation and the median ridge basement. Thickness of the median ridge detrital cover has beenconsiderably enlarged. Note the strong vertical exaggeration in lower cartoons. TR: transverse ridge; NW northern wall; MR: medianridge; TFZ: transformfault zone. andseismic velocity decrease [Christensen, 1972]. Extensive 2.Detrick et al. [1982]proposed that the Vema transform hydrothermalalteration of the upper mantle in thetransform valley floor lies more than 1 km deeperthan its isostatic domainmay therefore cause uplift through two mechanisms: equilibrium depth calculated with respect to a normaloceanic overpressureresulting from volume increase; and serpentinite crustal section. This interpretation isbased on modeling ofthe diapirismresulting from the density contrast between the crustalstructure from seismic experiments in the western part serpentinitesandthe surrounding upper mantle and lower of theVema transform valley. They further propose that crustalrocks. Nautile dive observations show that the Vema FZ isostaticrebound of the valleyfloor is probablyprevented by medianridge is not composed oflarge volumes ofserpentinite. the rigidity of thecooling lithosphere. We proposethat Basementexposures aremade up of basaltsor possiblyof reheatingof thislithosphere asit approachestheopposite gabbros,and serpentinite clasts are relatively rare in the ridgesegment [Forsyth and Wilson, 1984; Phipps Morgan and detritalcover. Our observationsdo not preclude,however, Forsyth, 1988] shouldlower this rigidityand allow this extensiveserpentinization of the mantle beneath the ridge, isostatic rebound to occur.In addition,reheating of old cold whichmay cause uplift from the density contrast. Because lithosphere asit approachestheRTI shouldcause thermal hydrationofthe upper mantle in thetransform domain may be expansionandconsequently anuplift which could be as large as anongoing process, it could cause the inferred present-day several hundred meters [Louden and Forsyth, 1976; Bonatti, upliftof the western part of the median ridge [Kastens etal., 1978;Forsyth and Wilson, 1984]. In thishypothesis, the 1986].However, wemay ask why median ridges are not present Vema median ridge would represent a portion ofthe transform in alllarge transforms, andwhy do these ridges, when present, valley floor, attached to theAfrican Plate, that underwent oftendevelop only in onepart of thetransform valley? A recentuplift in the western RTI domain. According toDetrick et possibleanswer tothese questions is,as pointed out by Dick et al.'s[1982] calculations, the3900 m averagedepth of the crest al. [1991]in thecase of theAtlantis II FZ,that hydrothermal of the medianridge between 40ø56.7'W and 41ø07.3'W alterationof the upper mantle may be enhanced inportions of correspondsto isostatic equilibrium, implying that the deep transformvalley lithosphere that have experienced thinning, structure ofthe ridge is similar to that modeled further west for or extensiveslicing, during phases of tectonicreorganization the transform valley floor. The lower elevation and narrow, of the transformdomain. This answerremains, however, not arcuateshape of themedian ridge east of 40ø57'Wis inferredto entirelysatisfying inthe case of the Vema FZ, because seismic have resulted from tectonic extension during the subsidence of experimentsinthe western part of the transform valley, where the nodal basin. This may explain why the highest elevation of no bathymetricmedian ridge exists, suggest extensive the median ridge is not found at the tip of the MAR axis. hydrothermalalteration ofthe crust and upper mantle [Detrick A steadystate process could simply explain the isostatic et al., 1982].Other major transform valleys with no rebound,assuming that reheating and softening of the edge of bathymetricmedian ridges also have anomalous seismic crustal the African plate are constant processes. Isostatic rebound of structures,suggesting extensive hydration and themedian ridge is assumed tostart some 50-55 km to the west serpentinization[Whiteet al., 1984; Calvert and Potts, 1985]. of the MAR segment where the median ridge emerges from the Onthe other hand, peridotires recovered from the median ridge sedimentary infill of thefracture valley at 5000 m (Figure2). of theDoldrums FZ arerelatively unserpentinized [Bonatti et Theequilibrium depth of 3900 m isreached about 25 km east of al., 1992].It appearsthat extensive serpentinization of the thewestern tip of theridge. Given an average half slip rate of uppermantle is probablynot by itself sufficient tocause the 16mm/yr, this equilibrium depth isreached after 1.5 Ma. This formationof a medianridge in thetransform valley. correspondstoan uplift rate of theorder of 0.5to 1 mm/yr. LAGABRIET.I.FET AL.: VEMA FRACTUREZONE'S MEDIAN RIDGE 17,349
6.3. A Model of the RecentTectonomagrnatic Evolution 32 mm/yr asproposed by Candeet al. [1988], we may consider of the EasternRTI Domain that the rift valley floor has been created during the last Dives18 and 17 clearly show that relatively fresh volcanics 300,000 years. This may be alsothe ageof the volcanic fromthe MAR neovolcanic zone have erupted far to thenorth basementof the nodalbasin north of the present-day of thePDDZ, up to thefoot of thesouthern flank of themedian displacement zone (Figure 12). ridge.Therefore, for a recentperiod, the RTI wascharacterized Accordingto the resultspresented above, we may define by a pulseof extensivevolcanic activity. Furthermore, a phase severalstages in the very recenttectonic-volcanic history of of localextension occurred probably prior to or duringthe lava the easternRTI, that is, roughlyduring the last300,000 years emplacementresulting in the subsidenceof the southernflank (Figure 14). Thesestages which involvealternatively tectonic of the medianridge and in the retreatof the crestof the median or magmaticprocesses are almost continuousand probably ridge to the north of its previousposition. Consequently, the overlap each other. At this scale of time, it is not possible to basement of the median ridge is thought to underlie the define precisely the duration of each processinvolved. volcanicsof the northernpart of the nodal basin, as shownon Stage1. The RTI is characterizedby an extensionalregime a syntheticcrosssection (Figure 11). The arcuateshape of the anda low magmasupply. It is nowsuggested that in periodsof medianridge crestwas createdat that time. restrictedmagmatic activity, the deformingzones at RTI's are The 9 km width of the rift valley 10 km southof the nodal broaderthan during periods of highmagma supply [Karson and basinis definedby the distancebetween the 4000 m isobathof Dick, 1983; Fox and Gallo, 1984; OTTER Team, 1984; bothwalls of the valley(Figure 12). Givena spreadingrate of Macdonaldet al., 1986;Harper, 1985;Pockalny et al., 1988;
MEDIAN RIDGE
deforming zone MEDIAN RIDGE deforrning zone
--->nodalbasin
= 10 krn 1 3 new crust new crust • •
MEDIAN RIDGE MEDIANRIDGE inactivefault activefault / nodalbasin p.d.d.z. • ....,,,.....-..-:•- •. / •neovolcanic ..:.•i:::!i'"••:.:.• ridge "- •' ••"•'"" i.''•'•':"•::..:.'i:i,•[I •......
------• •:".•:.:.';::•:.::• '/- •2 ::•:• -- :::: .:.:.' / x t .:.:. :•:•:•:•:-, .. ::::: •.:. ::::. • / :•:•::::. L / :•:•: :• :::::• • ::::: •:• ::::, - t :.:-:
:[:[::•:• • , :::: ß.';' :.;. • • • :.5 ¾:•':': • • ::5 ::•:•:::: / , .:.: ß::•; :::: ,, • •:•: [• ::•:.• •, :::: :::::::':' • • :5: 2 new crust 3•o.oooyr. crust
Fig. 14.A modelfor theevolution of theVema eastem RTI duringthe last 300,000 years. The five successivestages are described in detail in the text. 17,350 LAGABRIELLEEr AL.: VEMA FRACTURE ZONE'S MEDIAN RIDGE
Allerton,1989]. As a consequenceof crustal stretching, the M6vel,Direct observation of a section through slow-spreading flankof the median ridge located immediately tothe north of Auzende,oceanicJ.M.,crust. D.Nature, Bideau, 337E. (6209), Bonatti,726-729,M. Cannat,1989. J. Honnorez, Y. theridge axis will suffer local subsidence. Thearcuate shape of Lagabrielle,J.Malavieille, V. Mamaloukas-Frangoulis, andC. thecrest of themedian ridge could be parflyinherited from this M6vel,The MAR-Vema fracture zone intersection surveyed by deep event. submersibleNautile, Terra Nova, 2, 68-73,1990. Stage2. Extensioninthe RTI domain during stage 1 was Bader, R.G., etal., Site 26, Initial Rep. Deep Sea Drill. Proj., 4,77- immediatelyfollowed byextrusion ofabundant lavaflows Barany, 92,1970. I., and J.A. Karson,Basaltic breccias of the Clipperton emplacedboth along the MAR axisand on the collapsed south fracturezone (east Pacific): Sedimentation andtectonics in a fast- flankof themedian ridge. Most of thepresent-day nodal basin slippingoceanic transform, Geol. Soc. Am. Bull., 101, 204-220, floor wascreated during this high magma supply stage. The 1989. relativelyfresh basalts observed during theend of dive 17 to Barter,allochtonousT.J.,and oceanic E.T.C. crustal Spooner, rocksOphiolitic in the eastbreccias ligurian associated Apennines, with thenorth of theeastern nodal basin (Figure 6) wereprobably Italy:A comparisonwithobservations fromrifted oceanic ridges, eruptedduring this period.The deformingzone was narrower EarthPlanet. Sci. Lett., 35, 79-91, 1977. andprogressively migrated farther north. Consequently, oneof Bonatti,E.,Vertical tectonism inoceanic fracture zones, Earth Planet. themain branches of the displacement zonemay have crosscut Sci. Lett., 37, 369-379, 1978. thesouthernmost edgeofthe median ridge. The major fault Bonatti, anomalouslyE.,and old K. uplifted Crane, crust Oscillatorynear oceanic spreadingtransforms, explanationNature, 300 of markingthe geologicalboundary between the medianridge (5890),343-345, 1982. formationsand the neovolcanicproducts erupted in the RTI Bonatti,E., andJ. Honnorez,Non-spreading crustal blocks at the domainwas the siteof hydrothermalcirculation. The mafic MAR,Science, 174, 1329-1331, 1971. brecciaswith hematite and iron oxides sampled along this now Bonatti,MediterraneanE., and Sea, J. Honnorez,in The MediterraneanSubmarine Sea, iron editeddeposits by D.J.from Stanley, the inactiveboundary are the remnantsof this activity. pp. 701-710, Dowden,Hutchinson and Ross,Stroudsburg, Pa., Stage3. The volcanicactivity decreased and the RTI was 1972. subjected to renewed amagmatic stretching.The final Bonatti,E., andJ. Honnorez,Sections of the Earth'scrust in the morphologythat characterizes thepresent-day nodal basin is equatorialAriantic, J.Geophys. Res.,81, 4104-4116, 1976. assumedtohave been created during this stage. The deforming Bonatti,Ultramarie E.,C.carbonate Emiliani, breccias G.Ferrara, from theJ. equatorialHonnorez, MAR, and Mar.H. Rydell, Geol., zonebroadened and a largeset of normaland strike-slip faults 17,83-102, 1974. were active within the entire RTI domain. One of the main Bonatti,E., R. Sartori,and A. Boerstoa,Vertical crustal movements at faults of this set, now inactive,runs probably along the theVerna fracture zone in theAtlantic: Evidence from dredged northernboundary ofthe nodal basin. limestones,Tectonophysics, 91,213-232, 1983. Stage4ß Subsequent volcanic extrusions restricted tothe Bonatti, Skolotnev,E.,A.and Peqve, G. Udintsev,P.Kepezhinskas, Upper mantle N.Kurentsora, heterogeneity M.belowSeyler, the S. centerof the RTI domainoccurred forming the neovolcanic Mid-AtlanticRidge, 0ø-15øN, Earth Planet. Sci. Lett., in press, ridgewhich crosscuts the nodal basin. Volcanics were extruded 1992. tothe north of the present-day displacement zoneup to the foot Bowen,A.N., and R.S. White, Deep tow profiles from the Verna transformand RTI, J. Geol. Soc.London, 143, 807-817, 1986. of themedian ridge. The relatively fresh basalts observed on Calvert,A.J., and C.G. Potts, Seismic evidence forhydrothermally the flank of a conicaledifice at the northerntip of the alteredmantle beneath old crust in the Tydeman fracture zone, Earth neovolcaniczone during dive 18 were probably emplaced Planet.Sci. Lett., 75, 433-449,1985. duringthis period. The deforming zone became progressively Cande, S.C., J.L. Labrecque, andW.F. Haxby, Plate kinematics ofthe morerestricted. SouthAtlantic: Chron C34 to present, J. Geophys. Res., 93(B11), 13479-13492, 1988. Stage5. Thelast stage corresponds to the present-day Cannat, M.,V. Mamaloukas-Frangoulis, J.M.Auzende, D. Bideau, E. situation.The deformingzone is now restrictedto a narrow Bonatti,J. Honnorez,Y. Lagabrielle,J. 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