1. A slow-spreading segment with a small magmatic budget where the lithosphere
is more heterogeneous than on fast spreading ridges (Hess 1962; Dick and
Natland 1996). This implies a long-lived low-magma budget where the mantle
undergoes serpentinization, uplift and steady state lateral spreading. Such
processes will characterize the entire ridge segment.
2. The presence of deep ([15 km) earthquake epicenters, which are an indication
that fracturing of the rigid lithosphere will enable seawater circulation and the
formation of a low-density hydrated peridotite (Aumento and Loubat 1971;
Bonatti 1976).
3. Low-angle faulting during tectonic extension beneath the rift valley during a
period of low magmatism will enable the alteration and exposure of peridotite
on the ridge axis.
4. A denudation of the oceanic crust, which can take place during a stretching of
the lithospheric plates (Tapponier and Francheteau 1978) or by the viscous
drive of the asthenosphere (Sleep 1969; Lachenbruch 1973) leading to exposure
of peridotite. The mechanism responsible for the tectonic denudation at segment tips is related to an asymmetrical extension (Karson and Dick 1983;
Karson 1990) during spreading. Denudation could influence the temperature
gradient in the lithosphere, which is sensitive to mantle melting processes.
5. The dehydration of down-moving slabs in fore-arc regions is also responsible
for serpentinization (Haggerty 1987; Mottl 1989). For example, the serpentinite
mud found in the Mariana fore-arc in the western Pacific’s mound is due to the
subduction of a plate (Fryer 1992).
The hypothesis of peridotite emplacement on the sea floor has long been
debated and it is most likely that several processes were involved in the
emplacement of these kinds of rocks. Thermal conductivity is sufficient to inhibit
peridotite melting and trigger a local diapir. Diapiric upwelling as well as lowangle faulting mechanisms are both attractive models to explain the emplacement
of mantle peridotites. The occurrence of peridotites in spreading centers fits with
the diapiric model. When sheared, serpentinized peridotite undergoes plastic
deformation under stress and it could incorporate fragments of country rock. This
mixture is an indication of diapiric motion or shearing during low angle faulting.
Also, during spreading in a magma-starved area, serpentinized peridotite
upwelling will be facilitated. A forceful injection of mantle peridotites could take
place in a brittle lithosphere accompanied by tectonic deformation and melt circulation. However when the serpentinized peridotites occur in transform faults and
are associated with only a strike-slip motion, it is more difficult to explain their
presence by simple diapirism, unless a component of crustal opening occurs during
lithospheric readjustment.
The failure to find fresh peridotite outcrops on the ocean floor even during
drilling indicates that serpentinization has been very intense because of olivine’s
instability at shallow depths, under low temperatures and in the presence of water.
Independently from the process of serpentinization, there are two alternative
processes that could drive mantle peridotite to rise within the lithosphere where it
Emplacement and Distribution of Peridotite
97
is more heterogeneous than on fast spreading ridges (Hess 1962; Dick and
Natland 1996). This implies a long-lived low-magma budget where the mantle
undergoes serpentinization, uplift and steady state lateral spreading. Such
processes will characterize the entire ridge segment.
2. The presence of deep ([15 km) earthquake epicenters, which are an indication
that fracturing of the rigid lithosphere will enable seawater circulation and the
formation of a low-density hydrated peridotite (Aumento and Loubat 1971;
Bonatti 1976).
3. Low-angle faulting during tectonic extension beneath the rift valley during a
period of low magmatism will enable the alteration and exposure of peridotite
on the ridge axis.
4. A denudation of the oceanic crust, which can take place during a stretching of
the lithospheric plates (Tapponier and Francheteau 1978) or by the viscous
drive of the asthenosphere (Sleep 1969; Lachenbruch 1973) leading to exposure
of peridotite. The mechanism responsible for the tectonic denudation at segment tips is related to an asymmetrical extension (Karson and Dick 1983;
Karson 1990) during spreading. Denudation could influence the temperature
gradient in the lithosphere, which is sensitive to mantle melting processes.
5. The dehydration of down-moving slabs in fore-arc regions is also responsible
for serpentinization (Haggerty 1987; Mottl 1989). For example, the serpentinite
mud found in the Mariana fore-arc in the western Pacific’s mound is due to the
subduction of a plate (Fryer 1992).
The hypothesis of peridotite emplacement on the sea floor has long been
debated and it is most likely that several processes were involved in the
emplacement of these kinds of rocks. Thermal conductivity is sufficient to inhibit
peridotite melting and trigger a local diapir. Diapiric upwelling as well as lowangle faulting mechanisms are both attractive models to explain the emplacement
of mantle peridotites. The occurrence of peridotites in spreading centers fits with
the diapiric model. When sheared, serpentinized peridotite undergoes plastic
deformation under stress and it could incorporate fragments of country rock. This
mixture is an indication of diapiric motion or shearing during low angle faulting.
Also, during spreading in a magma-starved area, serpentinized peridotite
upwelling will be facilitated. A forceful injection of mantle peridotites could take
place in a brittle lithosphere accompanied by tectonic deformation and melt circulation. However when the serpentinized peridotites occur in transform faults and
are associated with only a strike-slip motion, it is more difficult to explain their
presence by simple diapirism, unless a component of crustal opening occurs during
lithospheric readjustment.
The failure to find fresh peridotite outcrops on the ocean floor even during
drilling indicates that serpentinization has been very intense because of olivine’s
instability at shallow depths, under low temperatures and in the presence of water.
Independently from the process of serpentinization, there are two alternative
processes that could drive mantle peridotite to rise within the lithosphere where it
Emplacement and Distribution of Peridotite
97