Section of the Oceanic Lithosphere
The association of dolerite (dykes)-gabbro-peridotite is not easily found on the
ocean floor. For many years, geologists have used land-based ophiolite sections to
infer the lithology of the oceanic lithosphere. In subaerial regions, tectonic events
such as folding, faulting and diapiric ascent are responsible for exposing deep
sections of the lithosphere-crust. Deep-seated formations on the sea floor are
essentially exposed in transform faults (fracture zones), on spreading ridges and in
subduction zones when a slice of the lithosphere has been uplifted and overrides
other structures.
A geophysical approach for defining the crust, lithosphere and mantle boundaries is based on the physical characteristics of the rocks related to the transmission of sound waves. Geophysical information about crustal thickness is directly
related to rock density and will reveal variability with depth, which suggests a
layering of the lithosphere. The concept of a layered lithosphere model based on
seismic experimental data suggests that the axial asthenosphere—lithosphere
boundary coincides with the roof of axial magma reservoirs under spreading ridge
axes, or we may also be seeing a mass of solidified magma, which percolated from
the asthenosphere towards the sea floor.
If the lithosphere has been affected by mantle magma injection and alteration
(serpentinization), this will hide any original layering of the lithosphere. Thus, it is
important to reconsider our view of seismic layering and take into account that the
various suggested geological models based on seismic observations may simply be
a general approximation. In order to obtain a stratigraphic section of the Earth’s
outer shell, which is as close as possible to reality, we need to redefine the rigid
lithosphere-crust boundary for each specific area of the world’s oceans.
Sea Floor Exposed Through Time
Prior to the 1960’s, our knowledge about deep-layer sections which sliced into
Earth’s outer shell leading to exposed peridotites, gabbros and dykes (dolerite) was
mainly inferred from our understanding of the land-based geology concerning
peridotites and associated volcanic rocks exposed on mountain chains and called
ophiolites. The term ophiolite means ‘‘snake rock’’ (from the Greek ophis = snake
and lithos = rock). Also commonly called ‘‘greenstones’’, ophiolite complexes are
composed of greenish and dark colored veined rocks made up of serpentinized
peridotite overlaid by dykes and basaltic lava, which are usually associated with
folded mountain belts and therefore deformed and altered. Ophiolite complexes
were exposed during the collision and subduction of the oceanic lithosphere at
contact with continental landmasses.
As spreading and plate motion takes place, peridotites and associated formations are transported across the oceanic basins and are eventually pushed against
Section of the Oceanic Lithosphere
99
The association of dolerite (dykes)-gabbro-peridotite is not easily found on the
ocean floor. For many years, geologists have used land-based ophiolite sections to
infer the lithology of the oceanic lithosphere. In subaerial regions, tectonic events
such as folding, faulting and diapiric ascent are responsible for exposing deep
sections of the lithosphere-crust. Deep-seated formations on the sea floor are
essentially exposed in transform faults (fracture zones), on spreading ridges and in
subduction zones when a slice of the lithosphere has been uplifted and overrides
other structures.
A geophysical approach for defining the crust, lithosphere and mantle boundaries is based on the physical characteristics of the rocks related to the transmission of sound waves. Geophysical information about crustal thickness is directly
related to rock density and will reveal variability with depth, which suggests a
layering of the lithosphere. The concept of a layered lithosphere model based on
seismic experimental data suggests that the axial asthenosphere—lithosphere
boundary coincides with the roof of axial magma reservoirs under spreading ridge
axes, or we may also be seeing a mass of solidified magma, which percolated from
the asthenosphere towards the sea floor.
If the lithosphere has been affected by mantle magma injection and alteration
(serpentinization), this will hide any original layering of the lithosphere. Thus, it is
important to reconsider our view of seismic layering and take into account that the
various suggested geological models based on seismic observations may simply be
a general approximation. In order to obtain a stratigraphic section of the Earth’s
outer shell, which is as close as possible to reality, we need to redefine the rigid
lithosphere-crust boundary for each specific area of the world’s oceans.
Sea Floor Exposed Through Time
Prior to the 1960’s, our knowledge about deep-layer sections which sliced into
Earth’s outer shell leading to exposed peridotites, gabbros and dykes (dolerite) was
mainly inferred from our understanding of the land-based geology concerning
peridotites and associated volcanic rocks exposed on mountain chains and called
ophiolites. The term ophiolite means ‘‘snake rock’’ (from the Greek ophis = snake
and lithos = rock). Also commonly called ‘‘greenstones’’, ophiolite complexes are
composed of greenish and dark colored veined rocks made up of serpentinized
peridotite overlaid by dykes and basaltic lava, which are usually associated with
folded mountain belts and therefore deformed and altered. Ophiolite complexes
were exposed during the collision and subduction of the oceanic lithosphere at
contact with continental landmasses.
As spreading and plate motion takes place, peridotites and associated formations are transported across the oceanic basins and are eventually pushed against
Section of the Oceanic Lithosphere
99