DeMets et al. 1990). This microplate includes a major transform fault called the
Terevaka Transform.
The Terevaka transform fault is made up of two parallel valleys separated by a
horst, which is a type of ridge, having a total offset of about 90 km. A total offset
of 100 km and a maximum depth of about 5000 m separates the EPR spreading
center to the north from the West Rift spreading ridge propagator of the Easter
Microplate. The spreading rates of the ridge segments, which are located to the
north and to the south of the transform, were estimated to be about 118 mm/yr
(118 km/million years) (Naar and Hey 1991). One of the reasons for exploring and
collecting samples from the Terevaka transform is because it represents one of the
deepest troughs along the East Pacific Ridge system and it is therefore likely to be
a site where lower-crust/upper-mantle material has been exposed.
First Samples from Terevaka Transform
The first sampling of the transform, which suggested the presence of deep-seated
peridotite, turned out to be very influential for conducting further exploration on
this structure. The earliest samples were collected during the ‘‘Midplate’’ expedition (SO80, June–September 1992) on the FS SONNE with Captain Kull and
chief scientist Peter Stoffers. The transit was from Valparaiso to Easter Island, and
while transiting along the Easter Microplate, we could not resist the urge to
explore one of the deepest depressions, reaching nearly 5900 m depth on the East
Pacific Rise, called the Pito Deep. The Pito Deep and the Easter Microplate are
associated with a Transform Fault called Terevaka.
The transform was completely mapped using the multibeam Hydrosweep system of the FS SONNE. As soon as the map was ready, we chose a dredging target
in order to have an idea about the composition of the crust in this particular region.
We were all secretly hoping to be able to sample some deep-seated mantle peridotite. This would permit us to add new information to our data set, previously
obtained in the Garrett FZ and from the Hess Deep. If we could find more peridotite here, this would enable us to speculate on the emplacement and composition
of the lower crust-lithosphere boundary of the Pacific Ocean. The mapping was
done on July 21, 1992, and the dredging operation started the next day.
The first dredges took a long time, more than 6 h, because there were no bites.
Colin Devey was in charge of the operation and he was frustrated because of the
time he was spending, with no sign of success. Finally, at around 10 PM we
decided to bring the dredge up and we all hoped that the dredge would contain a
few rock specimens. Peter Stoffers and I left the scientific control room and went
to bed leaving Colin in charge and hoping for the best.
Early in the morning, at around 6 AM when we got up, the first thing I did was
to go out on the back deck and on to the sample storage room, to see if the dredge
was successful. To my great disappointment, I learned from the watch that we had
been unsuccessful in recovering any samples from the Transform Fault. In the
274
8 Fracture Zones and Transform Faults
Terevaka Transform.
The Terevaka transform fault is made up of two parallel valleys separated by a
horst, which is a type of ridge, having a total offset of about 90 km. A total offset
of 100 km and a maximum depth of about 5000 m separates the EPR spreading
center to the north from the West Rift spreading ridge propagator of the Easter
Microplate. The spreading rates of the ridge segments, which are located to the
north and to the south of the transform, were estimated to be about 118 mm/yr
(118 km/million years) (Naar and Hey 1991). One of the reasons for exploring and
collecting samples from the Terevaka transform is because it represents one of the
deepest troughs along the East Pacific Ridge system and it is therefore likely to be
a site where lower-crust/upper-mantle material has been exposed.
First Samples from Terevaka Transform
The first sampling of the transform, which suggested the presence of deep-seated
peridotite, turned out to be very influential for conducting further exploration on
this structure. The earliest samples were collected during the ‘‘Midplate’’ expedition (SO80, June–September 1992) on the FS SONNE with Captain Kull and
chief scientist Peter Stoffers. The transit was from Valparaiso to Easter Island, and
while transiting along the Easter Microplate, we could not resist the urge to
explore one of the deepest depressions, reaching nearly 5900 m depth on the East
Pacific Rise, called the Pito Deep. The Pito Deep and the Easter Microplate are
associated with a Transform Fault called Terevaka.
The transform was completely mapped using the multibeam Hydrosweep system of the FS SONNE. As soon as the map was ready, we chose a dredging target
in order to have an idea about the composition of the crust in this particular region.
We were all secretly hoping to be able to sample some deep-seated mantle peridotite. This would permit us to add new information to our data set, previously
obtained in the Garrett FZ and from the Hess Deep. If we could find more peridotite here, this would enable us to speculate on the emplacement and composition
of the lower crust-lithosphere boundary of the Pacific Ocean. The mapping was
done on July 21, 1992, and the dredging operation started the next day.
The first dredges took a long time, more than 6 h, because there were no bites.
Colin Devey was in charge of the operation and he was frustrated because of the
time he was spending, with no sign of success. Finally, at around 10 PM we
decided to bring the dredge up and we all hoped that the dredge would contain a
few rock specimens. Peter Stoffers and I left the scientific control room and went
to bed leaving Colin in charge and hoping for the best.
Early in the morning, at around 6 AM when we got up, the first thing I did was
to go out on the back deck and on to the sample storage room, to see if the dredge
was successful. To my great disappointment, I learned from the watch that we had
been unsuccessful in recovering any samples from the Transform Fault. In the
274
8 Fracture Zones and Transform Faults