Subduction Initiation
One big question is how do subduction zones even begin? An pertinent query as the ridge push and slab pull with minor mantle convection input are believed to drive plate tectonics ( Conrad and Berteloni 2002). There are two current modesof subduction initiation:
1) Induced
2) Spontaneous
Induced
Induced subduction is caused by the initial movements of plates before the subduction zone forms. This movement will force plates together, causing compression, uplift and then subduction. There are 2 known types:
Transference
Oceanic crust is subducting beneath another piece of oceanic/ continental crust. The machine is jammed in an area, where an super buoyant piece of crust begins to subduct . However, Subduction continues along the length of the boundary and drags the oceanic lithosphere, sitting behind the buoyant crust, down. Thus, as illustrated in FIG Y , an new subduction zone forms ridge-ward from the old.
Polarity Reversal
This is also caused by a buoyant crust entering the subduction zone, instead this time the overriding plate begins to subduct beneath the previously subducting plate and the subduction dip reverses ( FIG Y). The compression is still occurring, breaks at the soft young crust in the back arc and begins to subduct underneath the light buoyant blockage crust.
Why Polarity reversal would occur instead of Transference has not been modelled. One problem with the transference model is that the cold oceanic crust has been shown too be to strong to rupture without previous weaknesses. Since the Eocene, India has been colliding with Eurasia and rift-ward on the Indian plate perfect example of Induced rifting should exist. Instead Eurasia has continued to be uplifted. This means that it is easier to compress the continental plate, than rupture the cold and dense lithosphere. Models have shown that pre existing weaknesses would make a transference subduction more likely.
Spontaneous
Spontaneous Initiation is where plate movement plays no role in the initiation of subduction. Instead it is the gravitational instability of a plate. There are two types; Passive margin collapse and transform collapse. I shall only explain transform collapse as passive margin collapse scenario is not relevant to intra-oceanic subduction.
Transform collapses occurs if an transform fault is extensive enough to place a really old, dense piece of crust next to and young new piece
of MOR crust ( see FIG 2). This causes a gravitational instability and the old crust begins to sink down into the asthenosphere.
1) Induced
2) Spontaneous
Induced
Induced subduction is caused by the initial movements of plates before the subduction zone forms. This movement will force plates together, causing compression, uplift and then subduction. There are 2 known types:
Transference
Oceanic crust is subducting beneath another piece of oceanic/ continental crust. The machine is jammed in an area, where an super buoyant piece of crust begins to subduct . However, Subduction continues along the length of the boundary and drags the oceanic lithosphere, sitting behind the buoyant crust, down. Thus, as illustrated in FIG Y , an new subduction zone forms ridge-ward from the old.
Polarity Reversal
This is also caused by a buoyant crust entering the subduction zone, instead this time the overriding plate begins to subduct beneath the previously subducting plate and the subduction dip reverses ( FIG Y). The compression is still occurring, breaks at the soft young crust in the back arc and begins to subduct underneath the light buoyant blockage crust.
Why Polarity reversal would occur instead of Transference has not been modelled. One problem with the transference model is that the cold oceanic crust has been shown too be to strong to rupture without previous weaknesses. Since the Eocene, India has been colliding with Eurasia and rift-ward on the Indian plate perfect example of Induced rifting should exist. Instead Eurasia has continued to be uplifted. This means that it is easier to compress the continental plate, than rupture the cold and dense lithosphere. Models have shown that pre existing weaknesses would make a transference subduction more likely.
Spontaneous
Spontaneous Initiation is where plate movement plays no role in the initiation of subduction. Instead it is the gravitational instability of a plate. There are two types; Passive margin collapse and transform collapse. I shall only explain transform collapse as passive margin collapse scenario is not relevant to intra-oceanic subduction.
Transform collapses occurs if an transform fault is extensive enough to place a really old, dense piece of crust next to and young new piece
of MOR crust ( see FIG 2). This causes a gravitational instability and the old crust begins to sink down into the asthenosphere.