In the early phases of the rift-to-drift transition, extension is expressed first through normal faults and then through detachment faults that dip towards the rift axis and cut through pre-rift sedimentary units and continental crust before soling into the subcontinental mantle. Lithospheric scale shear zones then develop in the mantle lithosphere which accommodate asthenospheric and subcontinental mantle upwelling (Fig. 8, bottom panel). These shear zones are the result of grain size reduction and DRX which enhance dislocation creep in olivine (Bickert et al., 2020; Ruh et al., 2022; Warren & Hirth, 2006). Decompression melting accelerates the upwelling of the asthenosphere until that the initial system of detachment faults couples with the mantle shear zones to exhume sub-continental mantle. As the melt region grows and shallows, it changes the stress regime of the rifting lithosphere and a system of anastomosing, out-of-sequence detachment faults with opposite dip-directions forms (Fig. 8, lower middle panel). The shallowness of this melt producing regions connects the melt triangle to shallow crustal depths via dikes, shear zones, and dunite channels (Kaczmarek & Müntener, 2008; Liang et al., 2010; Liu & Buck, 2021; Müntener & Piccardo, 2003). This leads to increasing volcanism and magmatism seaward, including the development of layered oceanic crust via sill and dike intrusions at the rift axis (which later becomes the footwall of the out-of-sequence detachment system). The new detachment fault system further exhumes the mantle, creating core-complex-like domes of peridotite that form the boundary between the hyperextended continental lithosphere from the newly created oceanic lithosphere (Fig. 8, upper middle & top panels).
This model for magma-poor rifted margin evolution shows the importance of melt production in determining the large-scale structures. Decompression melting accelerates the process of mantle upwelling, which in turn drives the exhumation phase of rifting and the transition in stress regime from gravity-driven (top-down, with faults dipping towards the rift axis) to melt-driven (bottom-up, with faults dipping away from the rift axis) during the rift-to-drift transition. Additionally, the anastomosing high temperature shear zones may provide conduits for melt to pool, to source a dike and sill network, or to follow fracture networks which initiate volcanism and then seafloor spreading as the volume of melt increases.
This framework for understanding magma-poor rifts allows us to put different rifted margins into context (Fig. 9). Comparing the Ivorian margin, the Err-Platta Nappes, and the Lanzo Massif to the conceptual model (from Fig. 8, top panel) show how building blocks or rifted margins exposed at Earth surface are homologous to sections of the simulated rifted margins. Err-Platta (outlined in magenta) is a homology of the shallower region of hyperextension, where the mantle domes upwards in contact with syn-rift sedimentary rocks (Manatschal et al., 2007). Down section and deeper, Lanzo (outlined in orange) is homologous with the region where formerly asthenospheric mantle is brought up against sub-continental mantle by anastomosing faults to form an extensional duplex. This region is also where melt intrusion has created proto-oceanic lithosphere (Kaczmarek & Müntener, 2008). The homologous equivalent of the Ivorian margin (outlined in blue) overlaps both regions, but also extends to the first appearance of oceanic crust.