Publikationsdatum:
2019-02-26
Beschreibung:
We carried out in January-March 1998 a geological-geophysical 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-Italian 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 lithosphere 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 (~ 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 profiles were made to better constrain spreading rates.
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