distinct ridges are attached and form a single unit: the St. Peter’s and St. Paul’s
Rocks (SPPR) massif. The two structures are: (1) The North Ridge comprising the
SPPR islets, which is composed essentially of mylonitized peridotite. The lithosphere uplift during strike-slip shearing tectonism is responsible for disrupting the
volcanic stratigraphy and for producing a large amount of mass-wasted material,
and (2) The South Ridge, which consists of a serpentinized diapir of peridotite
(lherzolite) emplaced during plate extension. The few volcanics seen in this area
occur in a graben-like structure, 1 km wide and less than 100 m deep, at 2200 m
depth. This appears to be similar to the volcanism observed on other spreading
ridge segments of the MAR.
Thus the SPPR massif is a fractured, composite structure with localized IntraTransform Ridge (ITR) volcanism in its southern part (South Ridge). The ITR
were all formed in the same way and are found along the St. Peter’s and St. Paul’s
Rocks multiple fracture zones. The ITR were formed within the active transform
zone where plate motion is responsible for enhancing an extensional component
giving rise to intra-transform volcanism. Thus, volcanic rocks such as the basalt
and the intrusions of dolerite dykes found within the SPPR massif have similar
compositions to the northern branch of the present day MAR, and the eruption or
intrusion of these mafic rocks took place during lithospheric extension. A similar
situation where volcanism occurs within a transform fault (called a leaky transform) was seen in the Garrett transform fault in the South Pacific.
References
Arcyana (1975) Transform fault and Rift valley from bathyscaph and diving saucer. Science
190:108–116
Bonatti E (1978) Vertical tectonic in oceanic fracture zons. Earth Planet Sci Lett 37:369–379
Bonatti E, Ligi M, Borsetti AM, Gasperini L, Negri A, Sartori R (1996) Lower cretaceous
deposits trapped near the Mid Atlantic Ridge and the opening of the equatorial Atlantic.
Nature 380:518–520
Campsie J, Bailey JC, Rasmussen M (1973) Chemistry of tholeiites from the Galapagos islands
and adjacent ridges. Nature 245(147):122–124
Cannat M (1993) Emplacement of mantle rocks in the sea floor at mid-ocean ridges. J Geophys
Res Solid Earth 98:1978–2012
Casey JF, Bryan WB, Klitgord K, Dmitriev L, Silantyev S, Smith SE (1991) Long wavelength,
inter-segment, and intra-segment scale geochemical variations in basalts between 30° and
34°N, Mid-Atlantic Ridge. EOS Trans Am Geophys Union 72:471
Constantin M, Hekinian R, Ackermand D, Stoffers P (1995) Mafic and ultramafic intrusions into
upper mantle peridotites from fast spreading centers of the easter microplate (south east
Pacific). In: Vissers RLM, Nicolas A (eds) Mantle and lower crust exposed in ocean ridges
and in ophiolites, Kluwer Academic Publishers, The Netherlands, pp 71–120
DeMets C, Gordon RG, Argus DF, Stein S (1990) Current plate motions. Geophys J Int
101:425–478
Detrick RS, Needham HD, Renard V (1995) Gravity anomalies and crustal thickness variations
along the Mid-Atlantic Ridge between 33EN and 40EN. J Geophys Res 10:3767–3787
298
8 Fracture Zones and Transform Faults
Rocks (SPPR) massif. The two structures are: (1) The North Ridge comprising the
SPPR islets, which is composed essentially of mylonitized peridotite. The lithosphere uplift during strike-slip shearing tectonism is responsible for disrupting the
volcanic stratigraphy and for producing a large amount of mass-wasted material,
and (2) The South Ridge, which consists of a serpentinized diapir of peridotite
(lherzolite) emplaced during plate extension. The few volcanics seen in this area
occur in a graben-like structure, 1 km wide and less than 100 m deep, at 2200 m
depth. This appears to be similar to the volcanism observed on other spreading
ridge segments of the MAR.
Thus the SPPR massif is a fractured, composite structure with localized IntraTransform Ridge (ITR) volcanism in its southern part (South Ridge). The ITR
were all formed in the same way and are found along the St. Peter’s and St. Paul’s
Rocks multiple fracture zones. The ITR were formed within the active transform
zone where plate motion is responsible for enhancing an extensional component
giving rise to intra-transform volcanism. Thus, volcanic rocks such as the basalt
and the intrusions of dolerite dykes found within the SPPR massif have similar
compositions to the northern branch of the present day MAR, and the eruption or
intrusion of these mafic rocks took place during lithospheric extension. A similar
situation where volcanism occurs within a transform fault (called a leaky transform) was seen in the Garrett transform fault in the South Pacific.
References
Arcyana (1975) Transform fault and Rift valley from bathyscaph and diving saucer. Science
190:108–116
Bonatti E (1978) Vertical tectonic in oceanic fracture zons. Earth Planet Sci Lett 37:369–379
Bonatti E, Ligi M, Borsetti AM, Gasperini L, Negri A, Sartori R (1996) Lower cretaceous
deposits trapped near the Mid Atlantic Ridge and the opening of the equatorial Atlantic.
Nature 380:518–520
Campsie J, Bailey JC, Rasmussen M (1973) Chemistry of tholeiites from the Galapagos islands
and adjacent ridges. Nature 245(147):122–124
Cannat M (1993) Emplacement of mantle rocks in the sea floor at mid-ocean ridges. J Geophys
Res Solid Earth 98:1978–2012
Casey JF, Bryan WB, Klitgord K, Dmitriev L, Silantyev S, Smith SE (1991) Long wavelength,
inter-segment, and intra-segment scale geochemical variations in basalts between 30° and
34°N, Mid-Atlantic Ridge. EOS Trans Am Geophys Union 72:471
Constantin M, Hekinian R, Ackermand D, Stoffers P (1995) Mafic and ultramafic intrusions into
upper mantle peridotites from fast spreading centers of the easter microplate (south east
Pacific). In: Vissers RLM, Nicolas A (eds) Mantle and lower crust exposed in ocean ridges
and in ophiolites, Kluwer Academic Publishers, The Netherlands, pp 71–120
DeMets C, Gordon RG, Argus DF, Stein S (1990) Current plate motions. Geophys J Int
101:425–478
Detrick RS, Needham HD, Renard V (1995) Gravity anomalies and crustal thickness variations
along the Mid-Atlantic Ridge between 33EN and 40EN. J Geophys Res 10:3767–3787
298
8 Fracture Zones and Transform Faults