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Observed variations in across‐axis topographic relief and faulting style at spreading centers have been challenging to explain. Axial highs are seen at fast‐spreading centers, while valleys occur for slow‐spreading centers. Fault offsets range from tens of meters at fast‐spreading ridges to tens of kilometers at some slow‐spreading ridges. Models that fit the axial relief fail to produce observed fault patterns, while models that fit the fault patterns fail to produce observed variations in axial relief. A recent mechanical analysis (Liu & Buck, 2018,https://doi.org/10.1016/j.epsl.2018.03.045) suggests that including the effect of many discrete diking events can result in a gradual change in axial relief with crustal thicknesses. To compare this mechanical model directly with observations requires us to couple it with a two‐dimensional thermal model. This allows us to estimate the axial lithospheric thickness consistently as a function of the spreading rate and crustal thickness. For thinner axial lithosphere the model predicts an axial high with relief supported by low‐density material beneath the axial lithosphere. For axial lithospheric thickness between approximately one half and approximately three fourths of the crustal thickness, the axial depth decreases with magma supply increase. For thicker axial lithosphere the axial valley relief is controlled by axial brittle lithospheric thickness and near‐axis lithospheric geometry. We compared model predictions to data by compiling observations on axial relief and faulting mode for all spreading centers where seismic crustal thickness has been measured. Good fit to the data is obtained for model parameters giving dike widths in the axial lithosphere close to a meter.

 

Seaward dipping reflectors (SDRs) are large piles of seaward thickening volcanic wedges imaged seismically along most rifted continental margins. Despite their global ubiquity, it is still debated whether the primary cause of SDR formation is tectonic faulting or magmatic loading. To study how SDRs might form, we developed the first two‐dimensional thermomechanical model that can account for both tectonics and magmatism development of SDRs during rifting. We focus here on the magmatic loading mechanism and show that the shape of SDRs may provide unprecedented constraints on lithospheric strength at volcanic rifting margins. For mapping SDRs geometries to lithospheric strength, a sequence of model lithospheric rheologies are treated, ranging from analytic thin elastic plates to numerical thick elasto‐visco‐plastic crust and mantle layers with temperature and stress‐dependent viscosity. We then analyzed multichannel seismic depth‐converted images of SDRs from Vøring and Argentinian rifted margins in terms of geometric parameters that can be compared to our model results. This results in estimates for the lithospheric thickness during rifting at the two margins of 3.4 and 5.7 km. The plate thickness correlates inversely with mantle potential temperature at these margins during rifting, as estimated by independent studies. 

Seaward dipping reflectors (SDRs) are large piles of seaward thickening volcanic wedges imaged seismically along most rifted continental margins. Despite their global ubiquity, it is still debated whether the primary cause of SDR formation is tectonic faulting or magmatic loading. To study how SDRs might form, we developed the first two‐dimensional thermomechanical model that can account for both tectonics and magmatism development of SDRs during rifting. We focus here on the magmatic loading mechanism and show that the shape of SDRs may provide unprecedented constraints on lithospheric strength at volcanic rifting margins. For mapping SDRs geometries to lithospheric strength, a sequence of model lithospheric rheologies are treated, ranging from analytic thin elastic plates to numerical thick elasto‐visco‐plastic crust and mantle layers with temperature and stress‐dependent viscosity. We then analyzed multichannel seismic depth‐converted images of SDRs from Vøring and Argentinian rifted margins in terms of geometric parameters that can be compared to our model results. This results in estimates for the lithospheric thickness during rifting at the two margins of 3.4 and 5.7 km. The plate thickness correlates inversely with mantle potential temperature at these margins during rifting, as estimated by independent studies.