taken place during differential vertical motions of a partially serpentinized body
forming the rift walls. The dyke and gabbroic intrusions that did not reach the sea
floor represent the conduits that have produced the thin veneer of lava flows
topping the residual peridotite. The sporadic occurrence of basalt associated with
peridotite along the faulted walls of the rift valley suggests a successive stretching
and uplift of the lithosphere in a magma-starved ridge segment (see chap. 4). This
is comparable to the denudation hypothesis (Cannat 1993) suggested for spreading
ridge segments with low magma supply.
Diving on St. Peter and Paul’s Massif
The St. Peter’s and St. Paul’s Rocks (SPPR) massif on which the islets are built is
about 90 km in length, 21 km wide at the 3000 m contour line, and has a sigmoidal (‘‘S’’) shape according to the bathymetric studies of the area (Fig. 8.15a, b).
This SPPR massif located between the northern MAR and the 27°42’W ITR, is
associated with a seismically active 8–9 Ma old lithosphere (Wolfe et al. 1993).
A compilation (Wolfe et al. 1993) of earthquake locations and their mechanisms of
propagation along the SPPR massif indicates that the main seismic events have
taken place within a band less than 2 km wide extending in an East–West direction. These seismic epicenters pass through a graben where basal-dolerite rocks
(characterizing a spreading center) were observed during the submersible dives.
Based on the distribution of the seismic epicenters and submersible observations,
the SPPR massif was divided into two ridge systems: the North and South Ridges
(Fig. 8.15a). Thus the SPPR massif is composed of two distinct structures: one
trending N-E (North Ridge) and the other trending in a S–W direction (South
Ridge) separated by an active fault coinciding with the concentration of the
seismic epicenters (Fig. 8.15a, b). A near complete N–S geological section of the
SPPR was conducted during six submersible dives.
Dives on the North Ridge of St. Peter and Paul’s Massif
The North Ridge includes the islets of St. Peter and Paul’s Rock’s and, as mentioned before, they consist of extremely fractured and brecciated mylonites of
mainly peridotite protoliths, tectonic breccias and mylonityzed conglomerates. The
dives programmed on this ridge were aimed at determining the extension of such
complex material at depth on the ocean floor and were intended to correlate the
geology with that of the subaerial exposed formations on the Islets. How deep do
the roots of the islets reach into the lithosphere?
Roger Hekinian (dive # SP08), Susanna Sichel (dive #SP09) and Gleb Udintsev
(dive #SP10) did the dives on the North Ridge of the St. Peter’s and St. Paul’s
Rocks Massif. Dive SP08 took place with the pilot Patrick Cheilan, co-pilot Patrice
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0 W ITR
293
forming the rift walls. The dyke and gabbroic intrusions that did not reach the sea
floor represent the conduits that have produced the thin veneer of lava flows
topping the residual peridotite. The sporadic occurrence of basalt associated with
peridotite along the faulted walls of the rift valley suggests a successive stretching
and uplift of the lithosphere in a magma-starved ridge segment (see chap. 4). This
is comparable to the denudation hypothesis (Cannat 1993) suggested for spreading
ridge segments with low magma supply.
Diving on St. Peter and Paul’s Massif
The St. Peter’s and St. Paul’s Rocks (SPPR) massif on which the islets are built is
about 90 km in length, 21 km wide at the 3000 m contour line, and has a sigmoidal (‘‘S’’) shape according to the bathymetric studies of the area (Fig. 8.15a, b).
This SPPR massif located between the northern MAR and the 27°42’W ITR, is
associated with a seismically active 8–9 Ma old lithosphere (Wolfe et al. 1993).
A compilation (Wolfe et al. 1993) of earthquake locations and their mechanisms of
propagation along the SPPR massif indicates that the main seismic events have
taken place within a band less than 2 km wide extending in an East–West direction. These seismic epicenters pass through a graben where basal-dolerite rocks
(characterizing a spreading center) were observed during the submersible dives.
Based on the distribution of the seismic epicenters and submersible observations,
the SPPR massif was divided into two ridge systems: the North and South Ridges
(Fig. 8.15a). Thus the SPPR massif is composed of two distinct structures: one
trending N-E (North Ridge) and the other trending in a S–W direction (South
Ridge) separated by an active fault coinciding with the concentration of the
seismic epicenters (Fig. 8.15a, b). A near complete N–S geological section of the
SPPR was conducted during six submersible dives.
Dives on the North Ridge of St. Peter and Paul’s Massif
The North Ridge includes the islets of St. Peter and Paul’s Rock’s and, as mentioned before, they consist of extremely fractured and brecciated mylonites of
mainly peridotite protoliths, tectonic breccias and mylonityzed conglomerates. The
dives programmed on this ridge were aimed at determining the extension of such
complex material at depth on the ocean floor and were intended to correlate the
geology with that of the subaerial exposed formations on the Islets. How deep do
the roots of the islets reach into the lithosphere?
Roger Hekinian (dive # SP08), Susanna Sichel (dive #SP09) and Gleb Udintsev
(dive #SP10) did the dives on the North Ridge of the St. Peter’s and St. Paul’s
Rocks Massif. Dive SP08 took place with the pilot Patrick Cheilan, co-pilot Patrice
Volcanism and Mode of Formation for the 25°27
0 W ITR
293