Fracture mechanics controls on mid-ocean ridge segmentation: insights from laboratory experiments

Mid-ocean ridge (MOR) axes are segmented over scales of 10s to 100s of kilometers by several types of offsets including transform faults (TF), overlapping spreading centers (OSC) and non-transform, non-overlapping offsets (NTNOO). Variations in segmentation have been attributed to changes in magma supply, axial thermal structure (which depends on mantle temperature and spreading rate), and axial mechanical properties. To isolate the effect of each of these processes is difficult with field data alone. We therefore present a series of analogue experiments using colloidal silica dispersions as an Earth analogue. Diffusion of salt from saline solutions placed in contact with these fluids cause formation of a skin, whose rheology evolves from viscous to elastic and brittle with increasing salinity. Applying a fixed spreading rate to this pre-formed, brittle plate results in cracks, faults, and ridge segments. Lithospheric thickness is varied independently by changing the surface water layer salinity. Our previous work shows that experimental results depend on the axial failure parameter π_F, the ratio of a mechanical length scale (Z_m) and the axial elastic thickness (Z_axis), which depends on mantle temperature and spreading velocity (Sibrant et al., 2018). Indeed, we showed that the differences in shape of slow-spreading, fault-dominated and fast-spreading, fluid intrusion-dominated ridges on Earth and in the laboratory are separated by the same critical value π_FC~0.024, suggesting that axial failure mode governs ridge geometry. Here, we examine ridge axis segmentation. Measurements of >4000 experimental ridge segments and offsets yield an average segment length L_m that is quasi-constant at all spreading velocities. Scaled to the Earth, this L_m value is ~55 km, in agreement with natural data. Experiments with low π_F show offset size varying as d_l = ~√(L_m × Z_axis) regardless of offset type, a correlation well explained by fracture mechanics. Finally, as on Earth, experimental ridge segments are separated by transform and non-transform discontinuities, and their nature and occurrence vary with π_F. NTNOOs develop when π_F < π_FC, while OSCs develop when π_F > π_FC. In contrast, TF may form at any π_F, but the proportion of TFs relative to OSCs or NTNOOs decreases when π_F/π_FC>> 1 or << 1_, in agreement with natural MORs.