different from that giving rise to the East Pacific Rise basalt. Niu and Hekinian
(1997) calculated that in order to derive the depleted type of lava that would have
been responsible for the origin of samples collected from the Garrett, it would be
necessary to have extensive ([20 up to 25 %) melting of residual peridotite
(harzburgite type). Indeed, the harzburgites sampled in the Garrett transform fault
are depleted in their more fusible mineral components such as clinopyroxene and
enriched in their more refractory olivine-spinel minerals, similar to the samples
that are found in the Terevaka and Hess Deep regions. This suggests that the extent
of melting beneath the EPR is similar to, or even higher than, the melting beneath
spreading ridges influenced by hotspots (e.g. The Azores hotspot in the Atlantic
Ocean).
Median Ridge Ultramafic and Gabbroic Rocks
Within the Garrett transform domain, there is a topographic high about 1,000 m
tall, 3.5 km wide and 10 km in length located between two oblique ridges (Beta
and Gamma). This is called the Median Ridge. Since the first dive in the transform
revealed the presence of peridotite at the base of the Median Ridge’s northern
slope, we decided to further investigate the whole structure.
Mathilde Cannat and Rejean Hébert, both experts in the study of ultramafic
mantle rocks, were designated for the three dives (dives GN03, -13 and -14), which
were scheduled on the eastern part of the Median Ridge (Figs. 8.1 and 8.2). The
presence of peridotite mantle material was quickly confirmed during radio communication between the ship and the submersible. As soon as the submersible
reached the slope, the divers observed steep scarps with a relief of 5–20 m. The
scarps were extensively fractured, leading to in situ fragmentation and landslides
Fig. 8.9 Pillow lava erupted
in central basin within the
Garrett transform fault’s
ATZ. Photograph taken at
4444 m depth. An octopus
going upslope shows the
location of pillow lava tubes.
(Copyright IFREMER,
Garrett cruise 1991, Nautile
dive GN 01)
Transform Fault Volcanism
269
(1997) calculated that in order to derive the depleted type of lava that would have
been responsible for the origin of samples collected from the Garrett, it would be
necessary to have extensive ([20 up to 25 %) melting of residual peridotite
(harzburgite type). Indeed, the harzburgites sampled in the Garrett transform fault
are depleted in their more fusible mineral components such as clinopyroxene and
enriched in their more refractory olivine-spinel minerals, similar to the samples
that are found in the Terevaka and Hess Deep regions. This suggests that the extent
of melting beneath the EPR is similar to, or even higher than, the melting beneath
spreading ridges influenced by hotspots (e.g. The Azores hotspot in the Atlantic
Ocean).
Median Ridge Ultramafic and Gabbroic Rocks
Within the Garrett transform domain, there is a topographic high about 1,000 m
tall, 3.5 km wide and 10 km in length located between two oblique ridges (Beta
and Gamma). This is called the Median Ridge. Since the first dive in the transform
revealed the presence of peridotite at the base of the Median Ridge’s northern
slope, we decided to further investigate the whole structure.
Mathilde Cannat and Rejean Hébert, both experts in the study of ultramafic
mantle rocks, were designated for the three dives (dives GN03, -13 and -14), which
were scheduled on the eastern part of the Median Ridge (Figs. 8.1 and 8.2). The
presence of peridotite mantle material was quickly confirmed during radio communication between the ship and the submersible. As soon as the submersible
reached the slope, the divers observed steep scarps with a relief of 5–20 m. The
scarps were extensively fractured, leading to in situ fragmentation and landslides
Fig. 8.9 Pillow lava erupted
in central basin within the
Garrett transform fault’s
ATZ. Photograph taken at
4444 m depth. An octopus
going upslope shows the
location of pillow lava tubes.
(Copyright IFREMER,
Garrett cruise 1991, Nautile
dive GN 01)
Transform Fault Volcanism
269