Within extreme continental extension areas, ductile middle crust is exhumed at the surface as metamorphic core complexes. Sophisticated quantitative models of extreme extension predicted upward transport of ductile middle-lower crust through time. Here we develop a general model for metamorphic core complexes formation and demonstrate that they result from the collapse of a mountain belt supported by a thickened crustal root. We show that gravitational body forces generated by topography and crustal root cause an upward flow pattern of the ductile lower-middle crust, facilitated by a detachment surface evolving into low-angle normal fault. This detachment surface acquires large amounts of finite strain, consistent with thick mylonite zones found in metamorphic core complexes. Isostatic rebound exposes the detachment in a domed upwarp, while the final Moho discontinuity across the extended region relaxes to a flat geometry. This work suggests that belts of metamorphic core complexes are a fossil signature of collapsed highlands.
© 2022. The Author(s).