and German (R.V. SONNE - Leg84, P. Stoffers) expeditions (1994) respectively.
The American research vessel ROBERT D. CONRAD had obtained some scarce
bathymetric data from the central portion of the transform fault between 1°N27°40
0 W using conventional methods in 1987 (Courtesy of Jean Guy Shilling,
from the University of Rhode Island, USA). It was in this context, with insufficient
bathymetric coverage, that the diving expedition started. But this was a challenge
and an opportunity for us to improvise in order to reach our scientific targets while
at sea.
Diving in the Equatorial Atlantic
Finally after all the scientific and administrative procedures were overcome, the
Saint Paul cruise in the Equatorial Atlantic was scheduled and I was very glad and
eager to start the action. The Saint Paul cruise with the submersible Nautile and its
support ship NO NADIR began on December 20, 1997 at 8:00 PM from the port of
Recife in Brazil and ended in Fortaleza (Brazil). The total length of the cruise from
port to port was 23 days and we were able to make 13 dives. The Saint Paul cruise
ended on January 6, 1998 after the last dive. We left the site of the St. Peter and
St. Paul Rock’s islets for the next port of call, Fortaleza, about 60 h away.
In addition to the fact that this region of the Atlantic has the deepest and longest
fracture zones going from coast to coast, a major reason for exploring this particular Equatorial region is because it is also among the areas of the World with the
lowest volcanic activity and is therefore an area with a relatively thin crust. This
enhances the possibility of observing deep-seated material from the lower-crust
and upper-mantle. The tectonics of fracturing and shearing are still ongoing processes overriding that of magmatic upwelling during spreading. The localized
volcanic structures, called intra-transform ridges (ITR), were also targets for our
exploration because their location has an important implication in correlating the
Earth’s deep- seated mantle material exposed during fracturing with the upwelling
of magma produced during lithosphere melting.
Thus, the St. Peter’s and St. Paul’s Rocks Fracture Zone, which has displaced
en echelon segments due to several transform faults, is a site with small spreading
ridges, called Intra-Transform Ridges (ITR). These small ridges are less than
20 km long and less than 10 km wide and are gashed by a central graben to depths
of about 4100 m. We were confronted by the fact that we were able to explore two
different geological activities (the emplacement of deep seated mantle and volcanism) within a relatively small area. Thus, it was going to be feasible to fulfill
our double objectives during the limited time allotted to our project (only 15 days
on site).
The geological setting and situation of the SPPR FZ is somewhat similar to
what had been previously found in the Garrett transform fault where small sigmoidal ridges with recent volcanism were found inside a large fracture domain.
The goal of the St. Paul cruise was to investigate two structurally different regions
286
8 Fracture Zones and Transform Faults
The American research vessel ROBERT D. CONRAD had obtained some scarce
bathymetric data from the central portion of the transform fault between 1°N27°40
0 W using conventional methods in 1987 (Courtesy of Jean Guy Shilling,
from the University of Rhode Island, USA). It was in this context, with insufficient
bathymetric coverage, that the diving expedition started. But this was a challenge
and an opportunity for us to improvise in order to reach our scientific targets while
at sea.
Diving in the Equatorial Atlantic
Finally after all the scientific and administrative procedures were overcome, the
Saint Paul cruise in the Equatorial Atlantic was scheduled and I was very glad and
eager to start the action. The Saint Paul cruise with the submersible Nautile and its
support ship NO NADIR began on December 20, 1997 at 8:00 PM from the port of
Recife in Brazil and ended in Fortaleza (Brazil). The total length of the cruise from
port to port was 23 days and we were able to make 13 dives. The Saint Paul cruise
ended on January 6, 1998 after the last dive. We left the site of the St. Peter and
St. Paul Rock’s islets for the next port of call, Fortaleza, about 60 h away.
In addition to the fact that this region of the Atlantic has the deepest and longest
fracture zones going from coast to coast, a major reason for exploring this particular Equatorial region is because it is also among the areas of the World with the
lowest volcanic activity and is therefore an area with a relatively thin crust. This
enhances the possibility of observing deep-seated material from the lower-crust
and upper-mantle. The tectonics of fracturing and shearing are still ongoing processes overriding that of magmatic upwelling during spreading. The localized
volcanic structures, called intra-transform ridges (ITR), were also targets for our
exploration because their location has an important implication in correlating the
Earth’s deep- seated mantle material exposed during fracturing with the upwelling
of magma produced during lithosphere melting.
Thus, the St. Peter’s and St. Paul’s Rocks Fracture Zone, which has displaced
en echelon segments due to several transform faults, is a site with small spreading
ridges, called Intra-Transform Ridges (ITR). These small ridges are less than
20 km long and less than 10 km wide and are gashed by a central graben to depths
of about 4100 m. We were confronted by the fact that we were able to explore two
different geological activities (the emplacement of deep seated mantle and volcanism) within a relatively small area. Thus, it was going to be feasible to fulfill
our double objectives during the limited time allotted to our project (only 15 days
on site).
The geological setting and situation of the SPPR FZ is somewhat similar to
what had been previously found in the Garrett transform fault where small sigmoidal ridges with recent volcanism were found inside a large fracture domain.
The goal of the St. Paul cruise was to investigate two structurally different regions
286
8 Fracture Zones and Transform Faults