In modern oceans, ultramafic intrusions are also exposed on active and passive
margin regions where two plates of different density meet, such as the seismically
active margins of the circum-Pacific subduction zone’s deep trenches where
faulting and folding of drifting oceanic plates takes place. Other examples of
ultramafic rocks such as peridotites and dunites were reported from the Tonga
Trench in the southwest Pacific at 9,000–9,400 m deep (Fisher and Engel 1969) and
in the Porto Rico trench in the western Atlantic (Shido et al. 1974). In subduction
zones, down-sliding slabs of serpentinized peridotites will undergo large volume
reduction due to the release of their volatile contents (mainly water, up to 25 %).
Examples of ultramafic complexes found on passive margins at the transition
zone between oceanic and continental lithosphere were reported from the Galicia
Bank during the ODP Ocean Drilling Program, legs 103, 149 and 173 (Boillot
et al. 1988, 1995). Age dating on a drill-core containing both basalt and gabbroic
rocks was determined to be 122–100 Ma (Charpentier et al. 1998). Another area in
the most southern part of the Galicia Bank near the Iberian coast where peridotite
and gabbroic complexes were sampled is found on the Gorringe Bank. The
Gorringe Bank consists of two shallow seamount-like structures called Gettysburg
and Ormond. The first indication of peridotite in this area was reported during
dredging by the R.V. R.D. CONRAD (Gavashi et al. 1973) and the N.O. J. Charcot
(Hekinian and Aumento 1973). The Cyagor diving cruise was organized with
Cyana and its support ship LE SUROIT in May–June 1981. During the Cyagor
cruise (Auzende et al. 1982), geologists observed a lithological sequence of the
peridotite-gabbro-basalt association. The gabbroic succession above the peridotites
was dated as being 143 Ma (Ar/Ar age data, Feraud et al. 1986). On the basis of
the scientists’ observations, it was suggested that an eastern tilt of the oceanic
basement followed by a vertical uplift gave rise to the shallow topography of the
Gorringe Bank. In a larger context, it was believed that the Gorringe Bank was
uplifted during a major tectonic motion when the African plate under-thrusted the
European plate in a southeast-northwesterly direction (Fukao 1972).
The presence of mantle peridotite on the sea floor will depend on the spreading
rates, which are influenced by the extent of volcanic activity.
Slow Spreading Ridges. Peridotites associated with slow spreading centers
(\3 cm/yr. total rate) along the northern MAR are best documented at 45–46°N
(Aumento and Loubat 1971), at 36°15
0 N in the ‘‘Rainbow field’’ (German et al.
1996), at 37°N (DSDP Leg 37, Melson et al. 1977), 35°40
0 N (Gracia et al. 1997),
30°N (Blackman et al. 1998; Blackman et al. 2004), and 24°–24°40
0 N (Allerton
et al. 1996) (Fig. 4.5). Several Russian-French, US, Japanese and German investigations have been carried out in the Cap Verde fracture zone (14°25
0 –14°45
0 N)
and at 12°58
0 N–12°10
0 N in the rift valley (Matsumoto et al. 1998) during the last
15 years. These areas where nearly continuous outcrops of peridotite were
observed are comparable to the Saint Peter’s and St. Paul’s Intra-transform
spreading ridges (see Chap. 8) because they also contain mylonitized peridotite,
pyroxenite and gabbro which suggest that the emplacement of peridotite and
associated rocks must have occurred due to the forceful injection and squeezing of
the upper mantle material during differential compressive and strike-slip plate
94
4 Sea Floor Rocks
margin regions where two plates of different density meet, such as the seismically
active margins of the circum-Pacific subduction zone’s deep trenches where
faulting and folding of drifting oceanic plates takes place. Other examples of
ultramafic rocks such as peridotites and dunites were reported from the Tonga
Trench in the southwest Pacific at 9,000–9,400 m deep (Fisher and Engel 1969) and
in the Porto Rico trench in the western Atlantic (Shido et al. 1974). In subduction
zones, down-sliding slabs of serpentinized peridotites will undergo large volume
reduction due to the release of their volatile contents (mainly water, up to 25 %).
Examples of ultramafic complexes found on passive margins at the transition
zone between oceanic and continental lithosphere were reported from the Galicia
Bank during the ODP Ocean Drilling Program, legs 103, 149 and 173 (Boillot
et al. 1988, 1995). Age dating on a drill-core containing both basalt and gabbroic
rocks was determined to be 122–100 Ma (Charpentier et al. 1998). Another area in
the most southern part of the Galicia Bank near the Iberian coast where peridotite
and gabbroic complexes were sampled is found on the Gorringe Bank. The
Gorringe Bank consists of two shallow seamount-like structures called Gettysburg
and Ormond. The first indication of peridotite in this area was reported during
dredging by the R.V. R.D. CONRAD (Gavashi et al. 1973) and the N.O. J. Charcot
(Hekinian and Aumento 1973). The Cyagor diving cruise was organized with
Cyana and its support ship LE SUROIT in May–June 1981. During the Cyagor
cruise (Auzende et al. 1982), geologists observed a lithological sequence of the
peridotite-gabbro-basalt association. The gabbroic succession above the peridotites
was dated as being 143 Ma (Ar/Ar age data, Feraud et al. 1986). On the basis of
the scientists’ observations, it was suggested that an eastern tilt of the oceanic
basement followed by a vertical uplift gave rise to the shallow topography of the
Gorringe Bank. In a larger context, it was believed that the Gorringe Bank was
uplifted during a major tectonic motion when the African plate under-thrusted the
European plate in a southeast-northwesterly direction (Fukao 1972).
The presence of mantle peridotite on the sea floor will depend on the spreading
rates, which are influenced by the extent of volcanic activity.
Slow Spreading Ridges. Peridotites associated with slow spreading centers
(\3 cm/yr. total rate) along the northern MAR are best documented at 45–46°N
(Aumento and Loubat 1971), at 36°15
0 N in the ‘‘Rainbow field’’ (German et al.
1996), at 37°N (DSDP Leg 37, Melson et al. 1977), 35°40
0 N (Gracia et al. 1997),
30°N (Blackman et al. 1998; Blackman et al. 2004), and 24°–24°40
0 N (Allerton
et al. 1996) (Fig. 4.5). Several Russian-French, US, Japanese and German investigations have been carried out in the Cap Verde fracture zone (14°25
0 –14°45
0 N)
and at 12°58
0 N–12°10
0 N in the rift valley (Matsumoto et al. 1998) during the last
15 years. These areas where nearly continuous outcrops of peridotite were
observed are comparable to the Saint Peter’s and St. Paul’s Intra-transform
spreading ridges (see Chap. 8) because they also contain mylonitized peridotite,
pyroxenite and gabbro which suggest that the emplacement of peridotite and
associated rocks must have occurred due to the forceful injection and squeezing of
the upper mantle material during differential compressive and strike-slip plate
94
4 Sea Floor Rocks