The formation of high temperature (>800oC) ductile shear zones in the mantle by dynamic recrystallization (DRX) is confirmed by petrological observations in oceanic core complexes (Warren and Hirth 2006, Bickert et al., 2021), in rifts in the Lanzo peridotite (Kaczmarek & Müntener, 2008), and potentially by the observation of earthquakes in the East African rift system and the southwest Indian ridge (SWIR) (Lindenfeld & Rümpker, 2011; Zhao et al., 2013). The onset of DRX and diffusion creep follows the approach discussed in the introduction. The recrystallized mantle (mantle2) is modeled as dry olivine and we use the diffusion creep, grain size-dependent rheology of (Hirth & Kohlstedt, 2003) (Table 1). When conditions of temperature and energy are appropriate, we use the Van der Wal et al. piezometer (Van der Wal et al., 1993) to calculate a recrystallized grain size from the local stress value (Bickert et al., 2020). We then calculate a new viscosity using the diffusion creep law. We delay the onset of DRX until 2 Myr to enhance localization of deformation in the center of the model space during the stretching phase of rifting. Erosion and sedimentation transport are modeled with a diffusive formulation and deposition is modeled a source term in the diffusion equation for sediments. Some transport of sediments occurs by the advection of the grid in the Lagrangian grid. The equation for sediment erosion is: \(\frac{\partial h } {\partial t} = k\frac{\partial^2 h}{\partial x^2} + S_x\) where \(k\) is the diffusivity of the sediments in m2*s-1 and \(S_x\) (\(10^{-11}m^2 \cdot s^{-1}\)) is the source for the deposition of sediments, \(x\) is the horizontal direction and \(h\) the topography. Transport occurs as the numerical grids deform. k varies whether the sediments are under water (\(k = 10^{-7}m^2 \cdot s^{-1}\)) or above sea-level (\(k = 5 \times 10^{-7} m^2\cdot s^{-1}\)). If the sediments are below sea-level, they are water loaded and compacted following Atty's law for standard siliciclastic sediments (Allen & Allen, 2013). We did not vary the erosional parameters to study their effect on the thermomechanical evolution of the margin.
4 Numerical modeling results
We present nineteen models that possess homologous features with respect to observed magma-poor margins from both field geology and seismic experiments by varying mantle potential temperature (MPT); which controls LAB depth, surface heat flux (SHF), and extension rate. (Homologies are here defined as similarities between models and observations with respect to structure, composition, and spatio-temporal relations). All are relatively cold rifts with MPTs from 1300 to 1400ºC and SHFs from 45 to 75 mW*m-2 (Table 2). For each model, we record the percentage of partial melting, oceanic lithosphere thickness (depth to the melt region), margin widths, difference between conjugate flanks (asymmetry), number of lithospheric boudins, crustal boudins, and mantle boudins, the minimum and maximum length of boudins, the number of mantle core complexes, and the modal width of each model’s anastomosing, extensional duplexes. The relationship between these observables and MPT, SHF, and extension rate are shown in Figs. S1-9, and the table of observed values is Table S1.