Segmented mantle melting, lithospheric strength, and the origin of transform faults: Insights from the North Atlantic

Mantle melting at mid-ocean ridges is thought to strengthen residual mantle by extracting water (hydroxyl defects) thereby increasing its viscosity by over two orders of magnitude to create a “compositional” lithosphere. Although water is strongly partitioned into basaltic melt relative to olivine, mantle dehydration also requires that melt extraction be efficient. Otherwise, retained low-degree hydrous melts will rapidly reinfuse surrounding mantle with hydrogen on solidifying owing to its high diffusivity in mantle materials. The pattern of mantle melting at ridges varies strongly both within segments and with spreading rate. We examine these patterns along the northern Mid-Atlantic Ridge and the adjoining Reykjanes Ridge using mantle Bouguer anomalies (MBAs). The Reykjanes Ridge has a linear axis, no transform faults, and a continuous MBA low, indicating continuous axial mantle melting. In contrast the adjoining Mid-Atlantic Ridge is segmented with transform and non-transform discontinuities and has pronounced “bulls-eye” MBA lows indicating focused mantle melting beneath each segment. We hypothesize that the pattern of mantle melting explains the absence or occurrence of transform faults on these systems. Segmented mantle melting results in dry, depleted, and strong mantle beneath ridge segment interiors but at segment ends, low extents of melting and inefficient melt extraction preserve damp and weak mantle.  Since the rheological changes created by segmented melting develop rapidly near the ridge axis and extend from the Moho to the dry solidus depth, a pronounced rheological banding is formed in the mantle. The weak segment ends localize shear zones oriented in the spreading direction where transform faults may form whereas the ridges, flanked by strong compositional lithosphere, will be oriented orthogonally.  Our hypothesis also explains the variation of transform fault spacing with spreading rate or their absence. At ultra-slow ridges, overall melting is limited and irregular and melt extraction is inefficient so that no systematic rheological bands form and transform faults are not favored. At slow spreading rates, mantle melting forms three-dimensional diapiric instabilities at typical spacings of ~40-80 km so that transform faults also have this spacing. As spreading increases to fast rates mantle melting becomes two-dimensional and typical magmatic segment length and corresponding transform spacing increases to >100 km.  At ultra-fast ridges (>145 km/my) mantle melting is ubiquitous and melt extraction is everywhere efficient so that a systematic rheological banding does not form and transform faults are again not favored. Our model implies that beyond cooling and strengthening with age, the pattern of mantle melting shapes the rheological structure of oceanic lithosphere and the geometry of plate tectonics. Reference:  Martinez and Hey, 2022, https://doi.org/10.1016/j.epsl.2021.117351