Transform versus non-transform offsets controlled by offset length and the variation in magmatic accretion within the offset zone

Transform faults and non-transform offsets define the bounds of mid-ocean ridge spreading segments, but tectonic and magmatic controls on the length of segments and the morphology of intervening offsets are poorly understood. A general observation at intermediate and slow-spreading oceanic environments is that localized strike-slip motion along transform faults tends to occur on larger offsets in space or crustal age, whereas more diffuse deformation at non-transform zones occurs at shorter offsets distances. In addition, variables such as lithospheric thickness, the size and spacing of faults, and the fraction (M) of extension accommodated by magmatic accretion (rather than faulting) are known to influence the overall morphology of the ridge segment and its vicinity. We hypothesize that the decrease in the amount of magmatic extension along the ridge segment towards the discontinuity along with the ridge segment offset play a role in defining the transition between transform and non-transform offsets.

In this study, we employ a 3D-numerical model to investigate how the relative amounts of fault- or magma-accommodated spreading and distance offset (D) between ridge segments control the development of transform versus non-transform offsets. Our model employs a ridge-like initial temperature structure, with magma intrusion simulated by adding a divergence to the right-hand-side of the continuity equation within a magmatic accretion zone at the ridge axis. M, the fraction of magmatically compensated spreading inside the magmatic accretion zone, can be varied along strike. By using a visco-elasto-plastic formulation the model can simulate the spontaneous formation and evolution of normal faults that accommodate part of the spreading. The temperature field is allowed to evolve and the model accounts for an increased, temperature-dependent conductivity around each ridge segment. We vary both the offset distance D separating two axes of magmatic accretion as well as the length L over which M decreases along the ridge axes towards the discontinuity. We find that increasing L leads to non-transform offsets, particularly for small offset distances D. As D increases, the occurrence of the offset zone is less prominently dominated by L. Depending on M, the style of faulting differs along the magmatic segments. While for M>0.5 we observe migrating faults creating topography similar to abyssal hills, values for M that are smaller or equal to 0.5 lead to stationary faults which are located closer to the ridge axis.