Segmented Mantle Melting, Lithospheric Rheology and Transform Fault Formation

Abstract

Mantle melting along mid-ocean ridges occurs in a segmented manner.  Melting and melt extraction are greatest within ridge segment interiors but near segment ends mantle upwelling decreases, cooling increases and melt extraction becomes inefficient.  Owing to the strong influence of water on mantle rheology, these effects have important consequences for the strength of oceanic lithosphere.  Residual mantle formed in ridge segment interiors is melt-depleted and dehydrated forming strong rheological bands.  Near segment ends, however, the formation of low-degree hydrous melts predominates, and these are inefficiently extracted from the mantle.  On solidification, these hydrous melts can re-fertilize surrounding mantle with water due to the high diffusivity of hydrogen in mantle material. This results in weak hydrous bands of mantle material near segment ends.  Thus, segmented mantle melting creates a corresponding segmented oceanic mantle rheological structure that favors the localization of shear deformation in the weak bands near segment ends.  Further strain localization within these weak zones may then facilitate additional weakening processes along discrete narrow transform fault zones. We Illustrate our model with geophysical observations from the Reykjanes Ridge and northern Mid-Atlantic Ridge south of Iceland.  The Reykjanes Ridge is a ~1000 km long linear axis without transform faults.  Rapid propagation of melting anomalies along its linear axis precludes a stable magmatic segmentation as shown by its linear mantle Bouguer anomaly.  Immediately south of the Reykjanes Ridge, the northernmost segments of the Mid-Atlantic Ridge have prominent mantle Bouguer anomaly lows indicating stable cells of segmented mantle melting. Transform and non-transform discontinuities immediately form at the ends of the mantle Bouguer anomaly lows.  This model can be extended to explain the occurrence (or absence) of transform faults over the full range of spreading rates from ultra-slow to ultra-fast ridges.Reference: Martinez, F., and R. Hey (2022), Mantle melting, lithospheric strength and transform fault stability: Insights from the North Atlantic, Earth and Planetary Science Letters, 579, doi:10.1016/j.epsl.2021.117351.