found in the upper amphibolite facies (Cannat and Casey 1995). Brittle-ductile
deformation is also observed when plagioclase has recrystallized so hornblende
has been transformed into actinolite, which is seen as small (\0.04 mm) irregularshaped neoblast crystals in the lower amphibolite-greenschist facies. The decrease
of the neoblast sizes suggests an increase in the flow stress, which is consistent
with strength increase as temperature decreases in the brittle domain.
Fractures and micro-fractures are often filled with lower amphibolite to
greenschist facies indicating that the plutonic sequences have undergone brittle
deformation. These veins and veinlets are also filled with hydrothermal precipitates suggesting that circulating fluids were present after deformation. The temperature of mineral transformation is obtained from some critical mineral
compounds such chlorite, which equilibrates at 150–299 °C (Chatelineua and Niva
1985). Also the presence of actinolite in gabbroic rocks suggests a fluid temperature of about 300 °C during brittle deformation accompanied by vein development (Liou et al. 1974).
In peridotite, the preferential orientation for the flattened orthopyroxene surfaces gives an indication of crystal-plastic deformation, while new generations of
olivine crystals display a well-developed euhedral shape. Crystal-plastic deformation is also seen during the formation of the fine-grained matrix and in the
closely spaced sub-grain boundary between olivine and pyroxene (i.e. porphyroclastic texture). The term ‘‘porphyroclast’’ is used to indicate a pseudoporphyritic
(coarse grained) texture showing foliation and the term ‘‘granoblast’’ is used when
the minerals are sub-equigranular without any foliation.
Often the peridotite rocks are called tectonites when the plastic deformation and
mineral recrystallization that took place at high temperature is recognized. Kinky
structures (deformed lattices showing foliation) and small-recrystallized minerals
are observed as replacement products of orthopyroxene, clinopyroxene and olivine. The most frequent textures are porphyroclast and granoblast. Porphyroclasts
are set in a mosaic-like matrix of the recrystallized minerals. The most extreme
case of plastic deformation is that observed in mylonite (metamorphic gabbro)
where peridotite has been transformed into a fine-grained granulated mosaic-like
texture. These types of deformation are believed to take place in the upper mantle
(Mercier and Nicolas 1975), probably in the lithosphere. More insight into the
plastic deformation of the Earth’s lithosphere can be found in a publication by
A. Nicolas (1990).
The rigidity of the plate thickness increases with the age of the lithosphere.
Thus a particular depth above which the upper mantle has negligible strength will
determine the location of the elastic layer. Extrapolating from the cooling plate
model of Parsons and Sclater (1977), the isotherm (thermal gradient) will also
correspond to the lower limit of the elastic thickness. On average, the rigid lithosphere underneath the sea floor is less than 100 km thick (see Chap. 2).
The Brittle and Ductile transition zone
103
deformation is also observed when plagioclase has recrystallized so hornblende
has been transformed into actinolite, which is seen as small (\0.04 mm) irregularshaped neoblast crystals in the lower amphibolite-greenschist facies. The decrease
of the neoblast sizes suggests an increase in the flow stress, which is consistent
with strength increase as temperature decreases in the brittle domain.
Fractures and micro-fractures are often filled with lower amphibolite to
greenschist facies indicating that the plutonic sequences have undergone brittle
deformation. These veins and veinlets are also filled with hydrothermal precipitates suggesting that circulating fluids were present after deformation. The temperature of mineral transformation is obtained from some critical mineral
compounds such chlorite, which equilibrates at 150–299 °C (Chatelineua and Niva
1985). Also the presence of actinolite in gabbroic rocks suggests a fluid temperature of about 300 °C during brittle deformation accompanied by vein development (Liou et al. 1974).
In peridotite, the preferential orientation for the flattened orthopyroxene surfaces gives an indication of crystal-plastic deformation, while new generations of
olivine crystals display a well-developed euhedral shape. Crystal-plastic deformation is also seen during the formation of the fine-grained matrix and in the
closely spaced sub-grain boundary between olivine and pyroxene (i.e. porphyroclastic texture). The term ‘‘porphyroclast’’ is used to indicate a pseudoporphyritic
(coarse grained) texture showing foliation and the term ‘‘granoblast’’ is used when
the minerals are sub-equigranular without any foliation.
Often the peridotite rocks are called tectonites when the plastic deformation and
mineral recrystallization that took place at high temperature is recognized. Kinky
structures (deformed lattices showing foliation) and small-recrystallized minerals
are observed as replacement products of orthopyroxene, clinopyroxene and olivine. The most frequent textures are porphyroclast and granoblast. Porphyroclasts
are set in a mosaic-like matrix of the recrystallized minerals. The most extreme
case of plastic deformation is that observed in mylonite (metamorphic gabbro)
where peridotite has been transformed into a fine-grained granulated mosaic-like
texture. These types of deformation are believed to take place in the upper mantle
(Mercier and Nicolas 1975), probably in the lithosphere. More insight into the
plastic deformation of the Earth’s lithosphere can be found in a publication by
A. Nicolas (1990).
The rigidity of the plate thickness increases with the age of the lithosphere.
Thus a particular depth above which the upper mantle has negligible strength will
determine the location of the elastic layer. Extrapolating from the cooling plate
model of Parsons and Sclater (1977), the isotherm (thermal gradient) will also
correspond to the lower limit of the elastic thickness. On average, the rigid lithosphere underneath the sea floor is less than 100 km thick (see Chap. 2).
The Brittle and Ductile transition zone
103