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Major structural elements of the Vema fracture zone

The Vema FZ is a rigth-lateral slow-slipping transform fault which displace the North America and African plates at 32 mm/y full rate CANDE1988. The Vema offsets the MAR axis by 320 km (Fig. 1), causing a maximum lithospheric age contrast across the transform fault of about 20 My at the RTI. The transform fault is located in an approximately E-W trending tranform valley. The valley is 25-40 km wide as defined by the crests of the northern and southern walls, while the valley floor (defined by 5000 m contour) is 15-20 km wide (Fig. 2). The transform valley floor is covered by up to 1.5 km thick turbidites transported from the Amazon cone BADER1970, PERCH1977. The transform valley is bounded by 2-4 km high E-W walls. The southern wall is also the northern side of a major transverse ridge (VTR) (Fig. 2) more than 270 km long and up to 4.5 km high relative to the valley floor.

Seismic reflection data give evidence of a narrow (< 2 km wide) zone of deformation in the turbidite deposits of the transform valley EITTREIM1975, KASTENS1986. The disturbed zone has been interpreted as the active trace of the transform fault, or a principal transform displacement zoneĀ» (PTDZ) (Fig. 2).

The transform valley, near the eastern RTI is divided into a northern and a southern trough by a median ridge (Fig. 2 ), which rises up to 1200 m above the valley floor. Seismic data show no active tectonics in the northern trough, but give evidence of at least 800m of undisturbed layered sediments, probably turbidites MACDONALD1986. In contrast, the southern trough is tectonically active, relatively free of sediment (less than 2 m) and covered by pillow basalt and sheet flow MACDONALD1986. Moving west the median ridge becomes buried by sediments; however, between 41:55'W and 41:45'W, it sticks out above the sediments by about 500m (Vema mound) (Fig. 2). It is approximately 8 km wide and 18 km long. There is no evidence that the Vema mound has undergone recent vertical tectonics. The northern side of the Vema mound was dredged and the following lithologies were recovered: gabbros and metagabbros 55%; serpentinized peridotites 25%; volcanoclastic 15%; basalt 5% (E. Bonatti, unpublished data).

At the Vema eastern RTI the MAR axial valley is covered by fresh and glassy pillow lavas MACDONALD1986. The axial neovolcanic zone extends into the nodal basin splitting it in two smaller subsidiary basins (Fig. 2). Two other fossil neovolcanic zones lie east and west of the nodal basin; they have a sediment cover (10-20 cm) and abundant fresh talus on their flanks, indicating that their constructional morphology has been affected by fault activity and mass wasting MACDONALD1986. Submersible observations confirm that nodal deep is sediment-free and covered by fresh pillow basalt flows.

Figure: .
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Morphology and lithology of the southern Vema transverse ridge The multibeam bathymetry, acquired by the R/V Ewing during cruise EW9305 LDEO KASTENS1998, gives a detailed image of the VTR morphology (Fig. 3a and b). The VTR develops 140 km west of the RTI. Ocean floor morphology between the VTR and the eastern RTI is characterized by abyssal hills, which rise, bending few hun- dred meters, toward the fracture zone. The VTR abruptly shoal from 3400 m to 2000 m below sea level through a sharp N-S wall, probably fault controlled. The VTR extends for over 270 km that correspond to 16.8 My (from 6.8 Ma to 23.6 Ma) assuming an averaged half spreading rate of 16 mm/y CANDE1988.

The morphology ofthe VTR is not homogenous (Fig. 3a and b). N-S cross sections are mostly asymmetric, with a steeper northern side and a gentler southern side; however, some portions of the VTR are nearly symmetric. The northern side shows generally a bench or terrace along the middle part of the scarp. The upper slope ofthe VTR is cut by gullies and canyons and is steeper ($>$20°) than the lower slope ($<2$°). Fan shaped deposits, probably resulting from mass wasting and gravitative instability are observed at the base of both the northern and southern slopes, particularly on the western, presumibly older part of the VTR.

The western end of the VTR (Fig. 3a) has the shallowest summit, cresting at 450 m below sea level, Here the transverse ridge is capped by a $\sim$ 500m thick shallow water limestone platform BONATTI1983, BONATTI1993, KASTENS1995, KASTENS1998. To the east ($\sim$ 100 km) another $\sim$ 80m thick carbonate platform has been found on the VTR crest between 1000-1200 m below sea level (Fig, 3a) BONATTI1993, KASTENS1998.

Rocks dredged from the VTR include samples from all the the lithologies that characterize major units of the oceanic crust BONATTI1971, MELSON1971, BONATTI1976, HONNOREZ1984. Nevertheless, the stratigraphic relationships among the different units exposed on the northern side of thc VTR were determined only after direct observation and sampling with the Nautile submersible AUZENDE1989. Two dive transects (Fig. 4), 5 km apart, located around 42:41' W, revealed the exposure of a complete sequence of oceanic lithosphere, consisting from bottom to top: mantle derived ultramafics, gabbros, dyke complex, basaltic upper crust.

Mantle derived ultramafics: serpentinized peridotites (1 km in thickness). The outcrops are massive or in part tectonically disrupted and brecciated. Most of the rocks have porphyroclastic texture, some are mylonitic. Peridotites are highly serpentinized but contain relicts of olivine, orthopyroxene, clinopyroxene and spine], suggesting P-T equilibration under spine] peridotite facies. Some samples contain amphiboles CANNAT1995.

Gabbros: are generally massive and predominantly Fe- and Ti- rich, Their thickness is about 500 m.

Dyke complex: it outcrops almost continuously for a thickness of 700-1100 m, interrupted only by a few sub-horizontal or chaotic lava flows. The dykes are vertical and they are oriented parallel to the strike of the present MAR axis.

Basaltic upper crust: it consists of brecciated rather altered basalt (thickness of up to 800m).

One the objectives of cruise S19 was to sample at close horizontal intervals the lower, ultramafic unit exposed on the northern side of the VFR, as described in the next section.

Figure: .
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2010-05-12