Divergent double subduction

[11][12] Note that the term divergent is used to describe one oceanic plate subducting in different directions on two opposite sides.

[2] The melts flow upward and fill the gap and intrude the oceanic plate and welded crust as mafic dykes intrusion.

At the same time, the sinking oceanic plate starts to dewater and release the fluids upward to aid the partial melting of mantle and the crust above.

[2] On the other hand, partial melting of the lower crust (accretionary complex) leads to S-type granitoid intrusions with enriched aluminium oxide throughout the evolution of divergent double subduction.

[2][6] When the oceanic plate detaches from the overlying crust, intense decompressional melting of mantle is induced.

Most of the sedimentary strata and volcanics in the accretionary wedge experience low to medium grade metamorphism up to greenschist or amphibolite facies only.

Sinking of the oceanic plate drag down the welded crust to form a basin that allows continued sedimentation.

[2][6][7] After the oceanic plate completely detaches from the crust above, isostatic rebound occurs, leaving a significant unconformity in the sedimentary sections.

[2][6] In nature, the inverted "U" shape of the oceanic plate in divergent double subduction should not be always perfectly symmetrical like the idealized model.

An asymmetrical form is preferred like the real example in Molucca Sea where the length of the subducted slab is longer on its western side beneath the Sangihe Arc while a shorter slab on its eastern side beneath the Halmahera Arc.

[9] 3D numerical modelling had been done to simulate divergent double subduction, to evaluate different factors that can affect the evolution and geometry of the system discerned below.

[15] The inverted "U" shape of the oceanic plate is not an effective geometry for it to sink because of the mantle materials beneath.

[15] The slab pull, amount of poloidal flow and the rate of convergence on the side with shorter length will be reduced.

This is because it's difficult to break a moving oceanic plate (i.e., acting as a trailing edge, which moving in the reverse direction of the ongoing, earlier-initiated subduction) due to lack of compression required for forced (induced) subduction initiation.

[17] The state of motion of overriding plates control the geometry of divergent doubled subduction and the position of collision.

Schematic diagram showing subduction system in conventional plate tectonics theory and divergent double subduction
Initial stage: The oceanic plate subducts on both side, forming two parallel arcs and accretionary wedges with opposing direction. [ 2 ]
Second stage: Closure of ocean basin and the soft collision of two overriding plates [ 2 ] [ 6 ]
Third stage: Detachment of oceanic plate resulting in partial melting of mantle and lower crust [ 2 ] [ 6 ]
Final stage: Continued sinking of the oceanic crust. Partial melting of mantle and lower crust continue to drive intrusion and volcanism. The volcanic and sedimentary rocks deposit unconformably on the accretionary complex. [ 2 ] [ 6 ] [ 7 ] Dashed lines with arrow show poloidal mantle flow induced by slab rollback . [ 2 ]
Schematic cross section showing modern example of divergent double subduction system in Molucca Sea Collision Zone, Indonesia. [ 10 ] The Sangihe arc is overriding the Halmahera Arc and accretionary complex is formed on forearc of Halmahera Arc [ 10 ]
Toroidal flow of slab trapped mantle at the edge of the oceanic plate