(1) The first phase was a major tectonic break due to the large scale shearing of
two rigid plates forming the north and south walls, representing the uplifted
transverse ridge of the St. Paul’s Rocks fracture zone. This phase was followed
by an extension or pull-apart within the two walls of the fracture zone giving
rise to the small Intra- Transform spreading Ridges (ITR). During lithospheric
extension, the serpentinized peridotites were emplaced, as has been observed
elsewhere on slow and ultra-slow magma-starved spreading ridge segments
(see chap. 4).
(2) The second phase giving rise to the SPPR massif was the result of the
localized compression of a thrust-faulting and shearing event during plate
readjustment that involved the northern transverse ridge bounding the fracture
zone’s northern wall. During this readjustment and the strike-slip tectonic
motions, the transverse ridge was uplifted and mylonitized (tectonically
metamorphosed).
Thus, the North Ridge, including the St. Peter’s and St. Paul’s Rocks islets,
consists essentially of mylonites and was the site of both compression and thrust
faulting events. The islets of St. Peter’s and St. Paul’s Rocks are part of the summit
of a transverse ridge (Bonatti 1978, Bonatti et al. 1996), which runs parallel to the
F.Z. Grinding and shearing during extreme strike-slip and thrust faulting motions
produce the mylonites with their foliated structures found on the islets and the
North Ridge. These thrust-faulting and shearing events were accompanied by a
small rotational motion of the lithosphere, which gave rise to the ‘‘S’’ shaped
(sigmoidal) structure of the SPPR.
The tectonics of the thrust-faulting phenomenon is an on-going process as has
been suggested by the intense seismic activity of the area. Such seismicity on the
SPPR massif is attributed mainly to strike-slip and thrust faulting motions (Wolfe
et al. 1993). Earthquake analyses indicate that the depth of the focal mechanism of
these tectonic faulting events is located at 7–14 km depth (Wolfe et al. 1993),
which also corresponds to the brittle-ductile boundary (Solomon et al. 1988) of
rock deformation (mylonitization) in the lithosphere. This boundary is believed to
correspond to the region of the mantle where mylonitization takes place during
episodic strike-slip shearing, thrust faulting and uplift, such as what has given rise
to the SPPR massif. Mylonitization must have taken place at a high temperature
(600°–800 °C) in the mantle when the original mantle peridotites were squeezed
upward into the lithosphere and transformed into amphibole bearing rocks as
suggested by their mineral deformation and transformation (Melson et al. 1967,
Jaroslow et al. 1996).
In summary, the main results of the Saint Paul cruise have proven that the
SPPR islets are not volcanic, unlike most other oceanic islands; on the contrary,
these islets were formed during the uplift of deep-seated mantle peridotite which
rose to the surface of the ocean from a depth of more than 7 km within the crust.
The St. Peter’s and St. Paul’s Rocks (SPPR) massif is made up of an ‘‘S’’
shaped sigmoidal structure resulting from the complex shearing and extension of
the lithosphere plate with a small rotational component. The two structurally
Origin of the St. Peter’s and St. Paul’s Rocks (SPPR) Massif
297
two rigid plates forming the north and south walls, representing the uplifted
transverse ridge of the St. Paul’s Rocks fracture zone. This phase was followed
by an extension or pull-apart within the two walls of the fracture zone giving
rise to the small Intra- Transform spreading Ridges (ITR). During lithospheric
extension, the serpentinized peridotites were emplaced, as has been observed
elsewhere on slow and ultra-slow magma-starved spreading ridge segments
(see chap. 4).
(2) The second phase giving rise to the SPPR massif was the result of the
localized compression of a thrust-faulting and shearing event during plate
readjustment that involved the northern transverse ridge bounding the fracture
zone’s northern wall. During this readjustment and the strike-slip tectonic
motions, the transverse ridge was uplifted and mylonitized (tectonically
metamorphosed).
Thus, the North Ridge, including the St. Peter’s and St. Paul’s Rocks islets,
consists essentially of mylonites and was the site of both compression and thrust
faulting events. The islets of St. Peter’s and St. Paul’s Rocks are part of the summit
of a transverse ridge (Bonatti 1978, Bonatti et al. 1996), which runs parallel to the
F.Z. Grinding and shearing during extreme strike-slip and thrust faulting motions
produce the mylonites with their foliated structures found on the islets and the
North Ridge. These thrust-faulting and shearing events were accompanied by a
small rotational motion of the lithosphere, which gave rise to the ‘‘S’’ shaped
(sigmoidal) structure of the SPPR.
The tectonics of the thrust-faulting phenomenon is an on-going process as has
been suggested by the intense seismic activity of the area. Such seismicity on the
SPPR massif is attributed mainly to strike-slip and thrust faulting motions (Wolfe
et al. 1993). Earthquake analyses indicate that the depth of the focal mechanism of
these tectonic faulting events is located at 7–14 km depth (Wolfe et al. 1993),
which also corresponds to the brittle-ductile boundary (Solomon et al. 1988) of
rock deformation (mylonitization) in the lithosphere. This boundary is believed to
correspond to the region of the mantle where mylonitization takes place during
episodic strike-slip shearing, thrust faulting and uplift, such as what has given rise
to the SPPR massif. Mylonitization must have taken place at a high temperature
(600°–800 °C) in the mantle when the original mantle peridotites were squeezed
upward into the lithosphere and transformed into amphibole bearing rocks as
suggested by their mineral deformation and transformation (Melson et al. 1967,
Jaroslow et al. 1996).
In summary, the main results of the Saint Paul cruise have proven that the
SPPR islets are not volcanic, unlike most other oceanic islands; on the contrary,
these islets were formed during the uplift of deep-seated mantle peridotite which
rose to the surface of the ocean from a depth of more than 7 km within the crust.
The St. Peter’s and St. Paul’s Rocks (SPPR) massif is made up of an ‘‘S’’
shaped sigmoidal structure resulting from the complex shearing and extension of
the lithosphere plate with a small rotational component. The two structurally
Origin of the St. Peter’s and St. Paul’s Rocks (SPPR) Massif
297