COLLISIONAL DELAMINATION: LITHOSPHERIC RUPTURE AND MAGMATISM

When a continental margin (or lithosphere capped by exceptionally thick oceanic crust or arc) enters and jams a subduction zone, subterranean rupturing of the lithosphere occurs in the process of collisional delamination. Where crust is thinner than 15-20 km, it is welded to lithospheric mantle and subducts. Where continental crust is thicker, the upper crystalline part is cold and strong, but the lower part is weak and only viscously coupled to the underlying mantle. The first response to the downwards bending of positively buoyant lithosphere (initial collisional jamming) is the decapitation and imbrication of strong crystalline crust. As this occurs, the pull force from the negative buoyancy driving subduction continues to transmit updip through the strong upper 30 km or so of the sinking lithospheric mantle; the part that is also the most dense because it is the coldest mantle. Because relatively large differential stresses already exist in the area of bending, the hinge localizes the separation of positively buoyant from negatively buoyant lithosphere. Where convergence is faster than 2 cm/yr, initial ductile necking rapidly leads to rupture because while the subduction pull force remains steady, the local differential stresses increase. This causes the cool, dense, and strong tongue of upper lithospheric mantle to pull out from beneath the imbricated blocks of crust and from above the wedge of hot and weak, nearly neutrally buoyant, lower lithospheric mantle. Stretching and decompression of the lower lithospheric mantle causes small degrees of partial melting. The asthenosphere, which is already at a condition of incipient melting, upwells as fast as the plate tears apart and undergoes adiabatic decompression melting in amounts increasing in proportion to the local amount of upwelling. Heating by intrusion of mantle-sourced magma and conductive heating from below by rapidly upwelled asthenosphere, can cause melting, sometimes extensive, in the lower crust. Consequently, late syn- to post-collisional magmatism can have widely varying compositions depending upon the fraction of melt derived from lithospheric mantle, asthenosphere, and crust. Collisional delamination in New Guinea has caused a magmatic event since 8 Ma centered along the spine of the Central Range that is now in the waning stage.