At 1.0 Myr distributed extension along high-angle normal faults (stretching phase) begins, as does the attenuation of the mantle lithosphere. This is particularly visible in figure 4b where zones of high strain rate (>10-13 s-1) mark the localization of high-angle normal faults in the crust, the initiation of H-block formation and mantle thinning (Lavier and Manatschal, 2006; Huismans and Beaumont, 2011; Lavier et al., 2019). After 2.1 Myr crustal deformation becomes localized in an H-block with a 5 km thick root and sub- continental mantle shear zones (red shear zone dipping at ~ 20º) are initiated and accelerate mantle lithosphere attenuation. These shear zones form as a result of DRX being activated at 2 Myr. Subsequent deformation is characterized by the formation detachment faults rooted in low angle ductile shear zones in the middle crust. At 4.0 Myr coupling between crustal detachment faults and the sub-continental mantle shear zones begins and produces of lithospheric scale detachment faults dipping at < 20 and upwelling of the asthenosphere to within < 30 km of the surface. At this stage the extension becomes highly asymmetric as is clearly seen in the strain rate and localized to the conjugate located on the right of the model. Rapid necking of the lithosphere is characteristic of magma- poor margins and observed in many models (Huismans and Beaumont, 2001, Svartman Dias et al., 2015; Ros et al., 2017; Lavier et al., 2019).
Starting at 5.7 Myr melt production begins and the 1300ºC isotherm ceases to rise through transport because heat dissipation by latent heat of melting depresses the geotherms beneath the melt region. Between 4 and 5.7 Myr, out-of-sequence landward dipping detachment faults initiate in the crust and mantle with a tendency to dip landward and away from the rift axis. This lithospheric scale detachments are causing the formation of concave upward domes in the mantle underlying and thinning the crust locally down to 5 km. The high-temperature mantle shear zone forming by DRX generate a network of anastomosing shear zones. While deformation is more intense at the rift axis, a large 100 km wide horizontal detachment system connecting crustal normal faults and landward dipping high-temperature mantle shear zones is still active at the right flank of the rift. Reaching 6.5 Myr, the mantle lithosphere continues to form domes and lithospheric boudins under the influence of the out-of-sequence detachment faults. Melt production reaches 6% and is focused on the rift axis. The asymmetry is active and deformation persists off axis.
At 8.0 Myr the melt area (reaching 14% melt) displays some skewness as it is transported to the right of rift axis in contact with the crust (thinned to < 5km) and focuses deformation and uplift beneath one of the crustal boudins that was previously formed. High temperature shear zones at the Moho depth laterally transport the subcontinental mantle to make space for the asthenospheric mantle that was initially located beneath the continent. The intensity of the deformation (strain rate) correspondingly increases at this site but remains distributed. The temperature structure at and below this crustal block (originating from the H-block) is following a pure shear model of deformation (McKenzie, 1978). The top of the thermal structure is at the seafloor at 10ºC while the base is located at the 1100ºC isotherm and Moho remains near 800ºC. The area of asthenospheric mantle bounded by the 800ºC and the 1100ºC isotherm (Min temperature for the presence of active melt) below the former rift axis delineates a new mantle lithosphere layer that expands through extensional processes occurring in the crust and denuded former mantle lithosphere.
At 10.0 Myr: The crustal detachment system shuts off except for the rift axis; percentage melt reaches highest value of 19%. The crustal boudin is extruded by detachment structures originating in the high-temperature mantle and the mantle lithosphere thins by pure shear. Transport of melt across the solidus in the new mantle lithosphere layer generates pods of crystalized melt that are trapped in a new lithosphere formed by mechanical processes. 300 kyr later, at 10.3 Myr, new high temperature mantle shear zones form with opposite vergence in the new asthenospheric layer; asthenosphere-derived material reaches the surface; pods of recrystallized melt continue forming in the asthenosphere and seafloor spreading initiated as deformation and melt production are fully localized. When the model reaches 12.0 Myr oceanic lithosphere is formed by mantle core complexes and flip flop detachments rooted in high temperature mantle shear and weak recrystallized melt pods as in a very slow spreading environment (Bickert et al., 2020), homologous to what is seen as slow-spreading centers and seafloor spreading as described by Hess (Hess, 1962; Reston, 2018). However, the melt production area is expanding instead of being extruded by processes such as diking. A situation like that in our model may be possible only if DRX high temperature mantle shear zones become a barrier to melt migration.
4.2 All model results
The following are brief descriptions of each case with initial conditions outlined in Table 2 and shown in Figs. 5-7. Models are grouped by mantle potential temperature, then by extension rate, then by surface heat flux.