the observed brittle/ductile deformation observed in the schistose or foliated,
serpentinized peridotite-mylonite/cumulates (dunite), which must have occurred at
the 750 °C isotherm, thought to represent the brittle/ductile transition (Phipps
Morgan and Chen 1993).
In summary, our results were more than we had hoped for. Not only had we
found that the oblique ridges (Alpha, Beta and Gamma) were the sites of recent
volcanic activity, such as what could be observed on normal spreading ridge
segments, but in addition, we were surprised to observe that other volcanic events
had taken place along the deepest parts of the transform fault domain. This volcanism gave rise to smaller mounds (\20 m high), and to sporadic lava flows
invading the older terrain observed in the deepest part of the transform troughs and
walls (3500–5000 m), where upper-mantle and lower-crustal sequences were
exposed. These observations indicated that conjugated extensional motions were
responsible for the emplacement of mantle peridotite as well as for the eruption of
lava.
Based on the detailed study of the geological setting in relation to the composition of the extrusive flows and the intrusive peridotite, it was possible to make
comprehensive inferences on their origin. The presence of melt impregnation
forming patches, dykelets and dykes throughout the exposed peridotite suggested
the circulation of basaltic liquids. Some of theses impregnations forming dykelets
are crossed by later examples of serpentinization (chrysotile), which suggested that
there were late intrusions, which occurred after a main, plastic deformation had
emplaced the ultramafic formation.
The example of the study performed in the Garrett Transform was followed and
extended to other large fracture zones and to amagmatic spreading segments
associated with slower spreading ridge systems. A comparative study of the
Garrett FZ’s volcanism and its associated deep-seated mantle rocks, when compared to other Pacific and Atlantic provinces, has been important in helping scientists create a global picture of the magmatic processes taking place in the
World’s oceans. Several scientific papers have been written retracing the implications of the peridotite composition and its emplacement in the Garrett transform
(Edwards et al. 1996). Our cruise ended on January 31, 1991, with the last dive
(GN19), which carried Jean Francheteau, Dubois and Edmond, to observe the
western intersection zone of the transform fault and the western branch of the EPR.
When they returned from their dive, we all enjoyed a glass of champagne and a
barbecue dinner served on the main deck of the ship to mark the end of a very
successful cruise.
Terevaka Transform Fault
After the Pito cruise of 1993, which was aimed at studying the Easter Microplate, I
had hoped to publish a paper on the Terevaka transform fault. However, up to now,
the results have not yet been published and that is why I want to dedicate this
272
8 Fracture Zones and Transform Faults
serpentinized peridotite-mylonite/cumulates (dunite), which must have occurred at
the 750 °C isotherm, thought to represent the brittle/ductile transition (Phipps
Morgan and Chen 1993).
In summary, our results were more than we had hoped for. Not only had we
found that the oblique ridges (Alpha, Beta and Gamma) were the sites of recent
volcanic activity, such as what could be observed on normal spreading ridge
segments, but in addition, we were surprised to observe that other volcanic events
had taken place along the deepest parts of the transform fault domain. This volcanism gave rise to smaller mounds (\20 m high), and to sporadic lava flows
invading the older terrain observed in the deepest part of the transform troughs and
walls (3500–5000 m), where upper-mantle and lower-crustal sequences were
exposed. These observations indicated that conjugated extensional motions were
responsible for the emplacement of mantle peridotite as well as for the eruption of
lava.
Based on the detailed study of the geological setting in relation to the composition of the extrusive flows and the intrusive peridotite, it was possible to make
comprehensive inferences on their origin. The presence of melt impregnation
forming patches, dykelets and dykes throughout the exposed peridotite suggested
the circulation of basaltic liquids. Some of theses impregnations forming dykelets
are crossed by later examples of serpentinization (chrysotile), which suggested that
there were late intrusions, which occurred after a main, plastic deformation had
emplaced the ultramafic formation.
The example of the study performed in the Garrett Transform was followed and
extended to other large fracture zones and to amagmatic spreading segments
associated with slower spreading ridge systems. A comparative study of the
Garrett FZ’s volcanism and its associated deep-seated mantle rocks, when compared to other Pacific and Atlantic provinces, has been important in helping scientists create a global picture of the magmatic processes taking place in the
World’s oceans. Several scientific papers have been written retracing the implications of the peridotite composition and its emplacement in the Garrett transform
(Edwards et al. 1996). Our cruise ended on January 31, 1991, with the last dive
(GN19), which carried Jean Francheteau, Dubois and Edmond, to observe the
western intersection zone of the transform fault and the western branch of the EPR.
When they returned from their dive, we all enjoyed a glass of champagne and a
barbecue dinner served on the main deck of the ship to mark the end of a very
successful cruise.
Terevaka Transform Fault
After the Pito cruise of 1993, which was aimed at studying the Easter Microplate, I
had hoped to publish a paper on the Terevaka transform fault. However, up to now,
the results have not yet been published and that is why I want to dedicate this
272
8 Fracture Zones and Transform Faults