Key frames in the evolution of Model 1, with focus on lithology (left) or phase (right) strain-rate. 2.1 Myr: localized crustal deformation and initiation of sub-continental mantle shear zones (necking phase). 4.0 Myr: coupling between crustal detachment faults and the sub-continental mantle shear zones produces asymmetry in detachment faults and upwelling of the asthenosphere (exhumation phase begins). 6.5 Myr: formation of mantle lithosphere boudins and doming; melt production matures and the 1300ºC isotherm ceases rising because of the latent heat of melting; out-of-sequence detachment faults initiate.. 8.0 Myr: melt focuses underneath rift axis, "choosing" the site of future seafloor spreading. 10.0 Myr: crustal detachment system shuts off except for the rift axis; percentage melt reaches highest value. 10.3 Myr: new faults form with the opposite vergence; asthenosphere-derived material reaches the surface; seafloor spreading initiated. 12.0 Myr: end-state of the model, producing oceanic crust. The waviness of the lower boundary is an artifact how our data visualization software extracts subsets and doesn’t indicate anything meaningful. Isotherms for 400, 600, 800, 1000, 1200, and 1300 ºC are plotted from yellow to red.