COLLISIONAL DELAMINATION IN NEW GUINEA: THE GEOTECTONICS OF SUBDUCTING SLAB BREAKOFF

The formation of the island of New Guinea has long been cited as the product of Cenozoic arc-continent collision. Geologic studies in the Central Range of the Papua Province of Indonesia has revealed field and timing relationships which combined with mechanical considerations leads to a refined model for the effects of collisional tectonism. When a continental margin enters and jams a subduction zone, subterranean rupturing of the lithosphere occurs in the process of collisional delamination. Crust thinner than 15-20 km 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 is the decapitation and imbrication of strong crystalline crust. This began in west New Guinea at about 8 Ma and generated a giant fold in the basement. 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. Initial ductile necking rapidly leads to rupture because as 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 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. This has caused a major magmatic event along the spine of the Central Range since 6 Ma as the tear in the plate propagated from west to east.