Global trends of axial relief and faulting at plate spreading centers imply discrete magmatic events

Spreading centers exhibit systematic variations in across-axis topographic relief and faulting with spreading rate and crustal thickness. Standard models for axial relief do not produce the observed range of faulting modes. Models that fit faulting modes do not produce observed gradations in axial relief. Both previous classes of models treat magmatic dike opening as a continuous process. A recently developed mechanical model (Liu and Buck., 2018) incorporating the effect of discrete diking events suggests a way to produce both faulting modes and gradual changes of axial relief. A key concept is that during a sudden dyke event the asthenosphere responds as a brittle elastic solid that can be split by the opening of an overlying dyke. Dikes should open not only in the brittle axial lithosphere but also some distance into the underlying ductile crust/mantle, where they would cool to form gabbro. The amount of gabbro so intruded depends on magma pressure that is related to axial relief. To allow comparison of model predictions with data we linked that mechanical model with a thermal model and ran scores of cases with different spreading rate and crustal thickness. In contrast to the accepted view that the axial relief is mainly a function of axial lithospheric thickness, we find a direct dependence on crustal thickness. We then complied data on axial relief, and faulting mode for all places where seismically determined crustal thickness has been measured. Only for dike widths on the scale of meters does the model fit the observed axial relief and faulting mode as functions of spreading rate and crustal thickness. When the axial lithospheric thickness is between ~1/2 and ~3/4 of the crustal thickness, our new model predicts that magma input to dikes controls the depth of axial valleys. In this mode, the magma supply and so the steady-state crustal thickness directly translates into changes in axial depth. For this spreading mode, oscillations in magma supply in the period ranges of climate driven sea level variations produces a potentially detectable in the amplitude of seafloor axial relief.