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Vema Fracture Zone 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
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