Global geometry of mid-ocean and back-arc ridge systems: Ridge segments, transform faults, and non-transform offsets

The total length of the global mid-ocean ridge system length is currently estimated at 50000 to 100000 km. To define in detail the complex structure of divergent plate boundaries, and to accurately evaluate its length and the geometry of ridge offsets, we present the MAPRIDGES database. It provides the first comprehensive global compilation of mid-ocean ridge geometry interpreted from multibeam bathymetry, encompassing in detail individual ridge segments, transform faults, and non-transform offsets (NTOs). The database includes 262 transform faults and 10 transform zones, totalling ~28.000 km. We quantify for the first time both overlapping and underlapping NTOs at divergent plate boundaries and find a cumulative lateral offset of ~9.000 km, representing one-third of the cumulative length of transform offsets. The database incorporates 1471 ridge segments reaching a total length of ~71200 km, while the global ridge length corrected for NTOs overlaps and underlaps amounting to ~67900 km. Mid-ocean ridge spreading rate exerts a first-order control on the geometry and nature of mid-ocean ridge segmentation. We document that at rates below a spreading rate threshold of ~80-100 mm/yr, the proportion of overlapping NTOs increase with spreading rate, while the cumulative lateral offset decreases. At rates above this spreading rate threshold there is no apparent dependence of NTOs geometry on spreading rate. We also document that the lateral offset of NTOs and transforms show a significant overlap, with numerous transforms displaying offset lengths below 40 km, and NTOs exceeding 50 km. NTOs affect about 10% of the oceanic crust along-axis, reshaping the lithosphere with tectonic deformation, volcanism, hydrothermal activity and alteration. Consequently, these non-transform discontinuities, together with transform faults, should be systematically included in global assessments of lithospheric accretion, alteration, and associated fluxes of fluids and elements at mid-ocean ridges. We suggest that ridge segmentation and the nature of discontinuities are influenced by multiple factors beyond spreading rate, including mantle temperature, melt supply, and ridge obliquity, in addition to local intra- and inter-segment variability. Analyses of the patterns reported here might help understanding the nature and evolution of magmatic vs. tectonic-dominated ridge segments and their terminations.