The Brittle and Ductile Transition Zone
The brittle/ductile transition zone marks the boundary of material instability and
material exchange between the upper mantle (asthenosphere) and the lithospherecrust. This boundary is related more to the physical nature of the oceanic lithosphere than to compositional changes in the material (Fig. 4.6).
Earthquake analyses indicate that knowing about the centroid depth (focal
mechanism and depth source of an earthquake) has helped to constrain the
thickness of the ridge axis and the thickness of the lithosphere. Earthquake epicenters, micro-earthquake observations, and focal depth mechanisms best retrace
this boundary in rupture-depth (brittle nature), which is 7–8 km below the axial rift
valley in slow spreading segments (Toomey et al. 1990; Perfit and Chadwick Jr
1998) (Fig. 4.6). The depth of brittle lithosphere as a function of spreading rate
shows that faulting of the oceanic lithosphere reaches up to about 20 km deep,
which also corresponds to the vertical limit of faulting (Wolfe et al. 1995) in
amagmatic regions associated with slow spreading ridge segments (Fig. 4.6).
Intermediate ([25 mm/yr half spreading rate) and fast spreading centers
(50–80 mm/yr half rate) do not show prominent seismic activity but instead they
are associated with steady-state magma chambers located at less than 4 km
underneath the ridge axis (Perfit and Chadwick Jr 1998). Also, the calculated
(750–800 °C temperature (Perfit and Chadwick Jr 1998) derived from the standard
plate-cooling model of Parsons and Sclater (1977) corresponds to the brittleductile boundary in the lithosphere. Similarly, the temperature conditions associated with the brittle-ductile transition zone were also estimated to coincide with
the formation and stability of key minerals such as olivine at 700–1000 °C (Kirby
1985) and to be above the temperature of formation of metamorphic chrysotile
(Evans et al. 1976).
A model of brittle/ductile transition for the sub-solidus temperature of spinelperidotite was also proposed to occur at 750 °C based on both the pyroxene and
the olivine-spinel equilibrium. Other temperature calculations for the brittle/ductile boundary were obtained from the amphibole-chlorite geothermometry
(determination of the temperature of a given mineral) and based on observations of
the metamorphic mineral assemblages in peridotites. The average temperature of
metamorphism was calculated to be at 550–600 °C (Bazylev and Silantiev 2000).
The brittle/ductile transition zone is attributed to the high to low re-equilibration of
the temperature conditions of minerals forming rocks during tectonic uplift, which
could affect the physical behavior of the lithosphere.
Examples of deformation are observed in the minerals, which compose intruded
crystalline rocks such as gabbros and peridotites.
In gabbroic rocks, such as those studied near 15°N on the MAR, for example,
there are indications that successive magmatic and tectonic events took place in
the brittle-ductile transition zone. This ductile deformation is marked by the recrystallization of magmatic minerals including orthopyroxene, which is seen as
large (0.2–0.3 mm) neoblast (newly formed) polygonal minerals in the granulite,
102
4 Sea Floor Rocks
The brittle/ductile transition zone marks the boundary of material instability and
material exchange between the upper mantle (asthenosphere) and the lithospherecrust. This boundary is related more to the physical nature of the oceanic lithosphere than to compositional changes in the material (Fig. 4.6).
Earthquake analyses indicate that knowing about the centroid depth (focal
mechanism and depth source of an earthquake) has helped to constrain the
thickness of the ridge axis and the thickness of the lithosphere. Earthquake epicenters, micro-earthquake observations, and focal depth mechanisms best retrace
this boundary in rupture-depth (brittle nature), which is 7–8 km below the axial rift
valley in slow spreading segments (Toomey et al. 1990; Perfit and Chadwick Jr
1998) (Fig. 4.6). The depth of brittle lithosphere as a function of spreading rate
shows that faulting of the oceanic lithosphere reaches up to about 20 km deep,
which also corresponds to the vertical limit of faulting (Wolfe et al. 1995) in
amagmatic regions associated with slow spreading ridge segments (Fig. 4.6).
Intermediate ([25 mm/yr half spreading rate) and fast spreading centers
(50–80 mm/yr half rate) do not show prominent seismic activity but instead they
are associated with steady-state magma chambers located at less than 4 km
underneath the ridge axis (Perfit and Chadwick Jr 1998). Also, the calculated
(750–800 °C temperature (Perfit and Chadwick Jr 1998) derived from the standard
plate-cooling model of Parsons and Sclater (1977) corresponds to the brittleductile boundary in the lithosphere. Similarly, the temperature conditions associated with the brittle-ductile transition zone were also estimated to coincide with
the formation and stability of key minerals such as olivine at 700–1000 °C (Kirby
1985) and to be above the temperature of formation of metamorphic chrysotile
(Evans et al. 1976).
A model of brittle/ductile transition for the sub-solidus temperature of spinelperidotite was also proposed to occur at 750 °C based on both the pyroxene and
the olivine-spinel equilibrium. Other temperature calculations for the brittle/ductile boundary were obtained from the amphibole-chlorite geothermometry
(determination of the temperature of a given mineral) and based on observations of
the metamorphic mineral assemblages in peridotites. The average temperature of
metamorphism was calculated to be at 550–600 °C (Bazylev and Silantiev 2000).
The brittle/ductile transition zone is attributed to the high to low re-equilibration of
the temperature conditions of minerals forming rocks during tectonic uplift, which
could affect the physical behavior of the lithosphere.
Examples of deformation are observed in the minerals, which compose intruded
crystalline rocks such as gabbros and peridotites.
In gabbroic rocks, such as those studied near 15°N on the MAR, for example,
there are indications that successive magmatic and tectonic events took place in
the brittle-ductile transition zone. This ductile deformation is marked by the recrystallization of magmatic minerals including orthopyroxene, which is seen as
large (0.2–0.3 mm) neoblast (newly formed) polygonal minerals in the granulite,
102
4 Sea Floor Rocks