made on the rift valley floor going from the west to the east indicates a relatively
thin volcanic unit (\150–200 m thick) made up of dolerite dykes and basalt
overlying mantle peridotite. The adjacent East and West marginal walls consist of
serpentinized mantle peridotite intruded by gabbro and dolerite. The extrusive
lavas are found in the axial rift and on the top of the wall. The top of the East wall
was an ancient spreading center less than 1 million years old, with a thin volcanic
crust, which is the same as what was found in the recent axial valley; however, this
ancient spreading center has been uplifted during the emplacement of a serpentinized diapir.
We left the first site on the evening of December 30, 1997, and stopped midway across the SPPR massif to make one more dive (SP07) on another ITR at
27°42
0 W. The next morning on December 31, Thierry Juteau was scheduled to
make this dive on the ITR segment at 27°42
0 . The dive revealed the presence of
fresh glassy pillow lava flows at about 4500 m depth along the eastern part of the
rift valley. Unfortunately, because of technical problems encountered with the
Nautile, this dive was short and lasted only 38 min on the sea floor. The problem
was due to the main engine of the Nautile. It was not operational and they had to
return to the surface for repairs. Nevertheless, we had acquired enough information
on the ITR to satisfy our questions. Thus, we decided to leave this site in order to
go on to the SPPR islets, another important priority. It was clear from our
observations and sampling that these short ITR segments had erupted a limited
amount of basaltic lava, which was mainly confined to their rift valleys. They are
comparable to other magma-starved MAR segments found elsewhere.
Volcanism and Mode of Formation for the 25°27
0 W ITR
From the dive data and based on the samples collected, we were able to calculate
the volume and extent of volcanism as well as make a solid hypothesis on the
mode of emplacement of deep mantle material in the context of short-lived or
magma starved spreading ridge segments. The volume of volcanic activity was
estimated on the basis of the presence of dykes and lava flows outcropping on the
East and West walls of the 25°27
0 W IRT. Our observations suggest intermittent,
short-lived magmatic events.
The emplacement of the different lithology encountered on the ITR is inferred
from the information gathered during both surface ship operations and dives. The
difference between the rift valley depths at 4700 m and the shallow (\2500 m)
topography of the rift valley walls at the 25°27
0 W ITR suggests an important uplift
([1000 m). A possible explanation for this uplift is attributed to the buoyancy
effect of the loaded lithosphere and its rheology, which implies deformation and
the flow of matter. Serpentinization weakens the oceanic lithosphere and enhances
faulting during uplift. A colder lithosphere with heterogeneous rock types, such as
serpentinized peridotite-gabbro with small amounts basaltic flow, will increase the
amplitude of uplift. The emplacement of the residual peridotite is believed to have
292
8 Fracture Zones and Transform Faults
thin volcanic unit (\150–200 m thick) made up of dolerite dykes and basalt
overlying mantle peridotite. The adjacent East and West marginal walls consist of
serpentinized mantle peridotite intruded by gabbro and dolerite. The extrusive
lavas are found in the axial rift and on the top of the wall. The top of the East wall
was an ancient spreading center less than 1 million years old, with a thin volcanic
crust, which is the same as what was found in the recent axial valley; however, this
ancient spreading center has been uplifted during the emplacement of a serpentinized diapir.
We left the first site on the evening of December 30, 1997, and stopped midway across the SPPR massif to make one more dive (SP07) on another ITR at
27°42
0 W. The next morning on December 31, Thierry Juteau was scheduled to
make this dive on the ITR segment at 27°42
0 . The dive revealed the presence of
fresh glassy pillow lava flows at about 4500 m depth along the eastern part of the
rift valley. Unfortunately, because of technical problems encountered with the
Nautile, this dive was short and lasted only 38 min on the sea floor. The problem
was due to the main engine of the Nautile. It was not operational and they had to
return to the surface for repairs. Nevertheless, we had acquired enough information
on the ITR to satisfy our questions. Thus, we decided to leave this site in order to
go on to the SPPR islets, another important priority. It was clear from our
observations and sampling that these short ITR segments had erupted a limited
amount of basaltic lava, which was mainly confined to their rift valleys. They are
comparable to other magma-starved MAR segments found elsewhere.
Volcanism and Mode of Formation for the 25°27
0 W ITR
From the dive data and based on the samples collected, we were able to calculate
the volume and extent of volcanism as well as make a solid hypothesis on the
mode of emplacement of deep mantle material in the context of short-lived or
magma starved spreading ridge segments. The volume of volcanic activity was
estimated on the basis of the presence of dykes and lava flows outcropping on the
East and West walls of the 25°27
0 W IRT. Our observations suggest intermittent,
short-lived magmatic events.
The emplacement of the different lithology encountered on the ITR is inferred
from the information gathered during both surface ship operations and dives. The
difference between the rift valley depths at 4700 m and the shallow (\2500 m)
topography of the rift valley walls at the 25°27
0 W ITR suggests an important uplift
([1000 m). A possible explanation for this uplift is attributed to the buoyancy
effect of the loaded lithosphere and its rheology, which implies deformation and
the flow of matter. Serpentinization weakens the oceanic lithosphere and enhances
faulting during uplift. A colder lithosphere with heterogeneous rock types, such as
serpentinized peridotite-gabbro with small amounts basaltic flow, will increase the
amplitude of uplift. The emplacement of the residual peridotite is believed to have
292
8 Fracture Zones and Transform Faults