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

Metamorphic core complexes are domal uplifts of metamorphic and plutonic ro cks bounded by shear zones that separate them from unmetamorphosed cover ro cks(1). Interpretations of how these features form are varied and controver sial, and include models involving extension on low-angle normal faults(2), plutonic intrusions(3) and flexural rotation of initially high-angle norma l faults(4). The D'Entrecasteaux islands of Papua New Guinea are actively f orming metamorphic core complexes located within a continental rift that la terally evolves to sea-floor spreading(5). The continental rifting is recen t (since similar to6 Myr ago)(5), seismogenic(6) and occurring at a rapid r ate (similar to 25 mmyr(-1))(5). Here we present evidence-based on isostati c modelling, geological data and heat-flow measurements that the D'Entrecas teaux core complexes accommodate extension through the vertical extrusion o f ductile lower-crust material, driven by a crustal density inversion. Alth ough buoyant extrusion is accentuated in this region by the geological stru cture present which consists of dense ophiolite overlaying less-dense conti nental crust this mechanism may be generally applicable to regions where th ermal expansion lowers crustal density with depth.