Fig. 7: Within zones of high gravitational body forces, the relaxation of the crustal root generates a flow within the middle and lower crust.

a–f Illustration of the interfacing of deformation between layers of the experiment with a crustal root at 30, 24, 18, 12, 6, and 0 Ma, respectively. Vectors represent the flow field through time. Blue dashed line represents the zoomed-in area for panels to the right. g–l Illustration of the rotation of principal axes of deviatoric stresses within the upper crust through high- and low-angle normal faulting. Red vectors represent tensional and black vectors represent compressional principal axes of deviatoric stresses. Black arrows show sense of slip offset accumulation on the detachment. Dashed line below detachment shows depth zone of mylonitic gneiss. Thicker blue and green lines denote where main and conjugate detachments, respectively, remain active and thinner blue and green lines show where detachment has become dormant. Note that the gravitational body forces generated by the topography and crustal root cause an upward flow pattern of the ductile lower and middle crust, which is facilitated by a dominant low-angle detachment surface. 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 geometry across the extended region relaxes to a flat geometry, in accord with seismic constraints. In total, there is a lateral extension of ~75 km so that ~40 km of extension is within the metamorphic core complex zone. Experiments start at 30 Ma and evolve to 0 Ma. Stages of evolution are given in Ma.