Melt-present deformation at the Entia Dome, Central Australia: a metamorphic core complex formed during lower crustal tectonic extrusion

Core complexes are domal structures that exhume crust and mantle during extension. They occur in tectonic settings such as mid-ocean-ridges and continental rifted margins and provide important windows into deep Earth processes. The Entia Dome, central Australia, is an enigmatic core complex since it formed during the contractional Alice Springs Orogeny. Here, we provide field, petrographic, geochemical, and experimental evidence that demonstrates melt-present deformation in the lower crust formed weak high strain gneisses at the Entia Dome. Syn-tectonic melt migration within the high strain zones induced melt-mediated reaction softening, grain size reduction, phase mixing and thermal softening, enhancing the dominant rheological weakening caused by the physical presence of melt. We combine our recognition of melt weakened lower crust with the structural architecture of the dome to suggest that NE- and SW-verging, upper crustal thrusting during the Alice Springs Orogeny was kinematically decoupled from the SE-directed extrusion of rheologically weak lower crust. For the first time, we place the evolution of the Entia Dome into the context of a tectonic extrusion hypothesis proposed for the Alice Springs Orogeny, where lower crustal extrusion provided the extensional setting necessary for core complex formation during a contractional orogeny.