First results of cruise S19 (PRIMAR Project): petrological and structural investigations of the Vema transverse Ridge (equatorial Atlantic)

Paola Fabretti$^1$, Enrico Bonatti$^1$, Alexander Peyve$^2$,

Daniele Brunelli$^1$, Anna Cipriani$^1$, Xenia Dobrolubova$^2$,

Vladimir Efimov$^2$, SergeJ Erofeev$^2$, Luca Gasperini$^1$,

Jean H. Hanley$^1$, Marco Ligi$^1$, Andrey Perfiliev$^2$,

Vladimir Rastorguyev$^2$, Yuri Raznitzin$^2$, Galina Savelieva$^2$,

Victor Semjonov$^2$, Sergei Sokolov$^2$, Sergey Skolotnev$^2$,

Sara Susini, Ilya Vikentyev$^2$

$^1$ ISMAR-CNR, Bologna

Received: March 2010 / Accepted: May 2010 - PDF FOR PRINT

Abstract:

We carried out in January-March 1998 a geological-geopbysical cruise to the Vema Fracture zone that offsets by 320 km the Mid Atlantic Ridge in the central Atlantic. This expedition (S19) was part of PRIMAR (Russian-ltalian Mid Atlantic Ridge Project). The field work aimed at obtaining geophysical and petrological data from a prominent transverse ridge that runs on the southern side of the transform valley and constitutes a major topographic anomaly relative to the depth/square root of age relationship. Previous work had shown that a relatively undisturbed section of oceanic lithospbere is exposed on the northern side of the transverse ridge for roughly 270 km along a seafloor spreading flow line. Given an average spreading half rate of 16 mm/y, this length corresponds to over 16 My. One of the objectives of our expedition was to sample at close-spaced ($\sim$ 5 km) horizontal intervals the mantle ultramafic basal unit, in order to detect temporal variations of mantle composition and of accretion processes at ridge axis. Preliminary observations on ultramafic rock samples obtained at 35 sites suggest strong temporal variations of mantle structure and composition. Multichannel seismic reflection profiles were carried out in order to understand the processes that uplifted the transverse ridge and exposed the sliver of oceanic lithosphere. Magnetometric proliles were made to better constrain spreading rates.

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