Evaluating upper versus lower crustal extension through structural reconstructions and subsidence analysis of basins adjacent to the D'Entrecasteaux Islands, eastern Papua New Guinea

The D'Entrecasteaux Island (DEI) gneiss domes are fault‐bounded domes with ~2.5 km of relief exposing ultrahigh‐pressure (UHP) and high‐pressure (HP) metamorphic gneisses and migmatites exhumed in an Oligocene‐Miocene arc‐continent collision and subduction zone subject to late Miocene to recent continental extension. Multichannel seismic reflection data and well data show the Trobriand basin formed as a fore‐arc basin caused by southward Miocene subduction at the Trobriand trench. Subduction slowed at ~8 Ma as the margin transitioned to an extensional tectonic environment. Since then, the Trobriand basin has subsided 1–2.5 km as a broad sag basin with few normal faults deforming the basin fill. South of the DEI, the Goodenough rift basin developed after extension began (~8 Ma) as the hanging wall of the north‐dipping Owen‐Stanley normal fault that bounds the basin's southern margin. The lack of upper crustal extension accompanying subsidence in the Trobriand and Goodenough basins suggests depth‐dependent lithospheric extension since 8 Ma has accompanied uplift of the DEI gneiss domes. Structural reconstructions of seismic profiles show 2.3–13.4 km of basin extension in the upper crust, while syn‐rift basin subsidence values indicate at least 20.7–23.6 km of extension occurred in the entire crust since ~8 Ma. Results indicating thinning is preferentially accommodated in the lower crust surrounding the DEI are used to constrain a schematic model of uplift of the DEI domes involving vertical exhumation of buoyant, postorogenic lower crust, far‐field extension from slab rollback, and an inverted two‐layer crustal density structure. Key PointsGneiss exhumed with little upper crust extension in adjacent sedimentary basinsSusbsidence analysis shows lower crust accommodated majority of extensionLower crustal flow and inverted crustal density layers aided gneiss exhumation