Supercontinent break up and the mantle
Super continents are large aggregates of plates, surrounded by subduction zones,which have formed and disintegrated throughout the earth's
history.Over the course of millions of years these break apart. However the normal mechanisms for plate break up - mantle shear traction, ridge push and slab pull -are not enough to drive the separation of these plates!
There are three main current mechanism that have been suggested:
1) Mantle plumes
How do mantle plumes contribute to splitting?
Characteristic continental flood basalts ( Figure.1) , a sign of mantle plume activity, have been associated both temporally and spatially with
super continent break up. One current model is that a supercontinent is bounded by active subduction zones. The slab break from the oceanic lithospheric plate, sinks through the upper mantle and accumulate on the 660km density level ( Fig.2). The weight of the slab, causes an instability and the slabs all fall into the lower mantle. As an result, the thermal disturbance causes a mantle plume to rise from the core mantle boundary, heat and lift
the continental crust and cause rifting. (Rogers and Santosh 2004)
history.Over the course of millions of years these break apart. However the normal mechanisms for plate break up - mantle shear traction, ridge push and slab pull -are not enough to drive the separation of these plates!
There are three main current mechanism that have been suggested:
1) Mantle plumes
How do mantle plumes contribute to splitting?
Characteristic continental flood basalts ( Figure.1) , a sign of mantle plume activity, have been associated both temporally and spatially with
super continent break up. One current model is that a supercontinent is bounded by active subduction zones. The slab break from the oceanic lithospheric plate, sinks through the upper mantle and accumulate on the 660km density level ( Fig.2). The weight of the slab, causes an instability and the slabs all fall into the lower mantle. As an result, the thermal disturbance causes a mantle plume to rise from the core mantle boundary, heat and lift
the continental crust and cause rifting. (Rogers and Santosh 2004)
FIG.1(Left) The Finger Mountain Sill of Jurassic Ferrar Dolerite, Upper Taylor Glacier, Antarctica is about 600ft thick and is intruded into Beacon Sandstone. This is a small example of an continental flood basalt Taken from.http://www.geokem.com/flood-basalts-2.html.
FIG.2 (Right) As the slab is subducted into the mantle, the more dense material will sink break off and sink to the lower mantle creating a discontinuity elsewhere known as a slab avalanche.(Roger and Santosh, 2004)
FIG.2 (Right) As the slab is subducted into the mantle, the more dense material will sink break off and sink to the lower mantle creating a discontinuity elsewhere known as a slab avalanche.(Roger and Santosh, 2004)
2. Continental Insulation
Continental aggregates over geiod lows form supercontinents rimmed by subduction zones. The Continental crust is so large that the cold down-welling plates do not cool the mantle under the central continental lithosphere. Mantle under the thickened supercontinent lithosphere is thought to cease normal convection, as the heat cannot be lost through the thick crust. Thus the mantle continues to heat under the continental lithosphere, until a geiod high so large occurs that it rifts apart the supercontinent. Over time all the continents aggregate over a geiod low, another supercontinent forms and the cycle begins again. See Fig.3 for summary of process.
Continental aggregates over geiod lows form supercontinents rimmed by subduction zones. The Continental crust is so large that the cold down-welling plates do not cool the mantle under the central continental lithosphere. Mantle under the thickened supercontinent lithosphere is thought to cease normal convection, as the heat cannot be lost through the thick crust. Thus the mantle continues to heat under the continental lithosphere, until a geiod high so large occurs that it rifts apart the supercontinent. Over time all the continents aggregate over a geiod low, another supercontinent forms and the cycle begins again. See Fig.3 for summary of process.
Note that for insulation to occur the continent must be stable and any shear traction can dispel the specific lateral convection
which occur below thick continent. See http://www.mantleplumes.org/EDGE.html for more detail.
which occur below thick continent. See http://www.mantleplumes.org/EDGE.html for more detail.
3. Subduction
Subduction on either side of the supercontinent could lead to extensive rifting when:
-Oceanic plate shearing along the edge of the continental plate resulting in extensive fracture zones.
-Subducting plate rolling back to an shallow dipping slab and having high shear traction on the base of the plate to an steeply dipping slab. If this happens on opposite sides of continent, in opposite directions, rifting can occur.
(Storey, 1995)
Subduction on either side of the supercontinent could lead to extensive rifting when:
-Oceanic plate shearing along the edge of the continental plate resulting in extensive fracture zones.
-Subducting plate rolling back to an shallow dipping slab and having high shear traction on the base of the plate to an steeply dipping slab. If this happens on opposite sides of continent, in opposite directions, rifting can occur.
(Storey, 1995)