motions. The short intra-transform spreading ridge segments located within the
Saint Paul’s multiple transform fault investigated by the Nautile in 1997–1998
showed the presence of a thin crust (probably less than 300 m thick) associated
with large peridotite outcrops (Hekinian et al. 2000).
The presence of topographic highs at the intersection of the MAR and Atlantis
transform fault near 30°N indicated that localized peridotite outcrops could be
exposed during detachment faulting. This topographic high of peridotite is called
the ‘‘Atlantis Massif’’ and is found at 1,800 m depth, 15–20 km west of the MAR
axis on crust that is 1–2 million years old. Gravity anomaly measurements indicate
that the lower crust and the upper mantle forming this outcrop are not symmetrically distributed (Blackman et al. 1998). This asymmetrical distribution could be
the result of a detachment faulting of the serpentinized massif. From the deep tow
survey (TOBI) it was shown that the massif is about 1.5 km in diameter (Blackman et al. 2004).
Geophysical studies such as magnetic, gravimetric and bathymetric surveys
were undertaken during Russian cruises during the ‘‘Equaridge Project’’ by the
R.V. AKADEMIK NIKOLAI STRAKHOV in 1988–1991 (Udintsev et al. 1996) in
the equatorial Atlantic. The results of these cruises have shown that where peridotites are exposed, there is very little sign of volcanism. For example, at the MAR
bordering the axial valley near 14°45
0 N, the rift mountain region shows almost
continuous outcrops of serpentinized peridotite which suggests the presence of a
thin volcanic crust and abundant mafic and ultramafic intrusions. This is also
suggested by the presence of ultramafic rocks found within the rift valley, which are
also associated with localized volcanic constructions. In this case, it is believed that
localized (‘‘puntiform’’) volcanic activities have created part of the oceanic crust
exposed in the rift valley floor deeper than 3850 m, while other parts have been
affected by the intrusion of ultramafics. Also, the presence of basaltic constructions
on the rift mountain area of the MAR at various depths (3,000–1,600 m depths)
indicates that alternate volcanic activities have taken place with the intrusion of
ultramafic-gabbro associations during at least the past 2 million years (when
assuming a half spreading rate of about 10 mm/yr). In addition, mass-wasted
material consisting essentially of ultramafics and basaltic debris were derived from
localized volcanic constructions bounding the rift valley to the east between 3,800
and 3,000 m at 14°45
0 N on the MAR. This area shows a thin sediment cover as well
as fresh un-sedimented talus suggesting that tectonic activity is an important ongoing process with respect to the more ephemeral volcanic events.
Serpentinized peridotites are found along the wall of the axial graben as well as
on the inside corner of the individual ridge segments, either associated with a
transform or a non-transform discontinuity in slow spreading ridge segments. The
fact that the rift mountain region bordering the axial valley shows almost continuous outcrops of serpentinized peridotite suggests that these rocks have been
intruded at 12°40
0 N on the MAR. This intrusion is related to the cold lithosphere
and its lack of magmatism. Although the exposure of peridotite is unrelated to
depth, nevertheless it would make sense to look for more peridotite outcrops on
magma starved ridge segments that are deeper than 3,500 m along the MAR.
Emplacement and Distribution of Peridotite
95
Saint Paul’s multiple transform fault investigated by the Nautile in 1997–1998
showed the presence of a thin crust (probably less than 300 m thick) associated
with large peridotite outcrops (Hekinian et al. 2000).
The presence of topographic highs at the intersection of the MAR and Atlantis
transform fault near 30°N indicated that localized peridotite outcrops could be
exposed during detachment faulting. This topographic high of peridotite is called
the ‘‘Atlantis Massif’’ and is found at 1,800 m depth, 15–20 km west of the MAR
axis on crust that is 1–2 million years old. Gravity anomaly measurements indicate
that the lower crust and the upper mantle forming this outcrop are not symmetrically distributed (Blackman et al. 1998). This asymmetrical distribution could be
the result of a detachment faulting of the serpentinized massif. From the deep tow
survey (TOBI) it was shown that the massif is about 1.5 km in diameter (Blackman et al. 2004).
Geophysical studies such as magnetic, gravimetric and bathymetric surveys
were undertaken during Russian cruises during the ‘‘Equaridge Project’’ by the
R.V. AKADEMIK NIKOLAI STRAKHOV in 1988–1991 (Udintsev et al. 1996) in
the equatorial Atlantic. The results of these cruises have shown that where peridotites are exposed, there is very little sign of volcanism. For example, at the MAR
bordering the axial valley near 14°45
0 N, the rift mountain region shows almost
continuous outcrops of serpentinized peridotite which suggests the presence of a
thin volcanic crust and abundant mafic and ultramafic intrusions. This is also
suggested by the presence of ultramafic rocks found within the rift valley, which are
also associated with localized volcanic constructions. In this case, it is believed that
localized (‘‘puntiform’’) volcanic activities have created part of the oceanic crust
exposed in the rift valley floor deeper than 3850 m, while other parts have been
affected by the intrusion of ultramafics. Also, the presence of basaltic constructions
on the rift mountain area of the MAR at various depths (3,000–1,600 m depths)
indicates that alternate volcanic activities have taken place with the intrusion of
ultramafic-gabbro associations during at least the past 2 million years (when
assuming a half spreading rate of about 10 mm/yr). In addition, mass-wasted
material consisting essentially of ultramafics and basaltic debris were derived from
localized volcanic constructions bounding the rift valley to the east between 3,800
and 3,000 m at 14°45
0 N on the MAR. This area shows a thin sediment cover as well
as fresh un-sedimented talus suggesting that tectonic activity is an important ongoing process with respect to the more ephemeral volcanic events.
Serpentinized peridotites are found along the wall of the axial graben as well as
on the inside corner of the individual ridge segments, either associated with a
transform or a non-transform discontinuity in slow spreading ridge segments. The
fact that the rift mountain region bordering the axial valley shows almost continuous outcrops of serpentinized peridotite suggests that these rocks have been
intruded at 12°40
0 N on the MAR. This intrusion is related to the cold lithosphere
and its lack of magmatism. Although the exposure of peridotite is unrelated to
depth, nevertheless it would make sense to look for more peridotite outcrops on
magma starved ridge segments that are deeper than 3,500 m along the MAR.
Emplacement and Distribution of Peridotite
95