Metamorphic core complex formation by density inversion and lower-crust extrusion

Metamorphic core complexes are domal uplifts of metamorphic and plutonic rocks bounded by shear zones that separate them from unmetamorphosed cover rocks1. Interpretations of how these features form are varied and controversial, and include models involving extension on low-angle normal faults2, plutonic intrusions3 and flexural rotation of initially high-angle normal faults4. The D'Entrecasteaux islands of Papua New Guinea are actively forming metamorphic core complexes located within a continental rift that laterally evolves to sea-floor spreading5. The continental rifting is recent (since ∼6 Myr ago)5, seismogenic6 and occurring at a rapid rate (∼25 mm yr-1)5. Here we present evidence—based on isostatic modelling, geological data and heat-flow measurements—that the D'Entrecasteaux core complexes accommodate extension through the vertical extrusion of ductile lower-crust material, driven by a crustal density inversion. Although buoyant extrusion is accentuated in this region by the geological structure present—which consists of dense ophiolite overlaying less-dense continental crust—this mechanism may be generally applicable to regions where thermal expansion lowers crustal density with depth.