MAGMATIC SEGMENTATION AND THE FORMATION OF OCEANIC TRANSFORM FAULTS

Although oceanic transform faults are one of the most prominent expressions of plate tectonics, their formation remains a major unresolved question. A requirement for transform fault formation is the development of relatively strong segments separated by weak zones where shear deformation can be localized. Examination of North Atlantic spreading systems provides insights into how this condition is achieved. The Reykjanes Ridge is a ~1000 km long and linear divergent boundary without transform faults. The Mid-Atlantic Ridge immediately to the south is offset by transform and non-transform discontinuities. A striking difference between these systems is the occurrence of discrete mantle Bouguer anomaly (MBA) “bull’s eye” lows at the Mid-Atlantic Ridge segments and their absence on the Reykjanes Ridge which has a continuous low. MBA lows indicate focused mantle melting and crustal accretion and reflect the magmatically segmented nature of seafloor spreading. Mantle melting and efficient melt extraction have been proposed to dehydrate the residual mantle, thereby increasing its viscosity by over two orders of magnitude [Hirth & Kohlstedt, 1996, EPSL]. However, mantle melting at oceanic spreading systems is generally a segmented process. At slow spreading ridges, mantle upwelling, melting, and melt extraction are focused to segment interiors but suppressed near ridge segment ends. At fast spreading ridges, melting is two-dimensional, but segment offsets suppress upwelling, melting and melt extraction. Suppression of mantle melting and melt extraction at ridge segment ends prevent dehydration of mantle maintaining weak zones there. This concept also explains why endmember spreading rate ridges generally do not have transform faults. At ultra-slow rates, melting and/or melt extraction is generally suppressed so that a systematic magmatic segmentation does not form. These systems tend to form continuous divergent plate boundaries, despite significant curvature and oblique spreading of the axis. At ultra-fast rates (>145 mm/yr) melting and melt extraction is ubiquitous so that weak hydrous zones fail to form and these ridges also lack transform faults. Our model implies that the pattern of mantle melting shapes oceanic lithospheric rheology and thereby the ridge-transform geometry of plate tectonics.