encountered brecciated and incoherent looking material that was easily fragmented
when touched by the submersible’s mechanical arm.
Extrapolating from the diving observations made on the two sides of the
southern wall, it was found that the contact between the basalt and the dyke
complex is located between 2700 m (dive PI 19) and 2500 m depth (dive PI 17)
(Fig. 8.14e). This finding permits us to state that the dykes are lower in the crustal
section along the south-facing wall than what was observed on the north-facing
wall inside the fault. Such a discrepancy is the result of the wall’s tilting during the
uplift of the serpentinized peridotite. Indeed, no peridotite-gabbro complexes were
observed on the south-facing wall. These associations are believed to occur deeper
in the crust, and are only visible on the inside of the southern wall but are not
presently exposed in the area of dive PI 19, which is ‘‘outside’’ the transform fault.
Hence, the main results obtained by submersible observations in the Terevaka
Transform are the evidence of a stratigraphic sequence exposing deep-seated
ultramafic (peridotite) and mafic (gabbro, dolerite and basalt) formations. Another
major observation was the evidence of melt circulation that gave rise to gabbroic
and dolerite veins (dykes) \1 m thick crossing the upper mantle-lower crust
peridotite. Such melt circulation and subsequent reaction with the surrounding
peridotite is related to tectonic activity during plate motions, which is responsible
for the fracturing and fissuring of the lithosphere. Also, the presence of hydrothermal circulation and the living biological communities observed within the
dyke complex indicates a warm environment, probably related to the mechanical
stress during the forceful emplacement of the peridotite-gabbro sequences within
the lithosphere.
St. Peter and St. Paul’s Rocks Fracture Zone
(Atlantic Ocean)
The St. Peter and St. Paul’s Rocks (SPPR) Fracture Zone (in the Equatorial
Atlantic located at 0°40’N-25°W) is found in an area where the most prominent
displacement between Africa and South America has taken place since the opening
of the Atlantic Ocean 120 million years ago. The name of the fracture zone is
given after that of the islets that have emerged in the area near 1°N. A large portion
of the St. Peter and St. Paul’s Rocks (SPPR) Fracture Zone is still tectonically
active and the main spreading segments of the Mid-Atlantic Ridge have been
displaced by more than 300 km to the East at about 1°N-30°20
0 W and 0°30
0 N24°W (Fig. 8.15a, b). Along this tectonically active portion of the fracture zone,
the oceanic lithosphere has been torn apart in several directions. The two main
directions are East–West and North–South. The East–West direction corresponds
to that of the major tectonic motions of fracturing and shearing while the North–
South tectonic direction is mainly related to the volcanic events of spreading.
280
8 Fracture Zones and Transform Faults
when touched by the submersible’s mechanical arm.
Extrapolating from the diving observations made on the two sides of the
southern wall, it was found that the contact between the basalt and the dyke
complex is located between 2700 m (dive PI 19) and 2500 m depth (dive PI 17)
(Fig. 8.14e). This finding permits us to state that the dykes are lower in the crustal
section along the south-facing wall than what was observed on the north-facing
wall inside the fault. Such a discrepancy is the result of the wall’s tilting during the
uplift of the serpentinized peridotite. Indeed, no peridotite-gabbro complexes were
observed on the south-facing wall. These associations are believed to occur deeper
in the crust, and are only visible on the inside of the southern wall but are not
presently exposed in the area of dive PI 19, which is ‘‘outside’’ the transform fault.
Hence, the main results obtained by submersible observations in the Terevaka
Transform are the evidence of a stratigraphic sequence exposing deep-seated
ultramafic (peridotite) and mafic (gabbro, dolerite and basalt) formations. Another
major observation was the evidence of melt circulation that gave rise to gabbroic
and dolerite veins (dykes) \1 m thick crossing the upper mantle-lower crust
peridotite. Such melt circulation and subsequent reaction with the surrounding
peridotite is related to tectonic activity during plate motions, which is responsible
for the fracturing and fissuring of the lithosphere. Also, the presence of hydrothermal circulation and the living biological communities observed within the
dyke complex indicates a warm environment, probably related to the mechanical
stress during the forceful emplacement of the peridotite-gabbro sequences within
the lithosphere.
St. Peter and St. Paul’s Rocks Fracture Zone
(Atlantic Ocean)
The St. Peter and St. Paul’s Rocks (SPPR) Fracture Zone (in the Equatorial
Atlantic located at 0°40’N-25°W) is found in an area where the most prominent
displacement between Africa and South America has taken place since the opening
of the Atlantic Ocean 120 million years ago. The name of the fracture zone is
given after that of the islets that have emerged in the area near 1°N. A large portion
of the St. Peter and St. Paul’s Rocks (SPPR) Fracture Zone is still tectonically
active and the main spreading segments of the Mid-Atlantic Ridge have been
displaced by more than 300 km to the East at about 1°N-30°20
0 W and 0°30
0 N24°W (Fig. 8.15a, b). Along this tectonically active portion of the fracture zone,
the oceanic lithosphere has been torn apart in several directions. The two main
directions are East–West and North–South. The East–West direction corresponds
to that of the major tectonic motions of fracturing and shearing while the North–
South tectonic direction is mainly related to the volcanic events of spreading.
280
8 Fracture Zones and Transform Faults