Thermal-rheological controls on deformation within oceanic transforms

Transform faults that offset mid-ocean ridge (MOR) segments accommodate plate motion through deformations that involve complex thermal and mechanical feed-backs involving both brittle and temperature-dependent ductile rheologies. Through the implementation of a 3D coupled thermal-mechanical modelling approach, we have developed a more detailed picture of the geometry of plate boundary deformation and its dependence on plate velocity and the age offset of MOR transforms. The modelling results show that cooling of near-ridge lithosphere (lateral heat transfer) has significant effects in the ductile mantle lithosphere for both the location and style of deformation. The region where strain is accommodated in the subjacent mantle lithosphere is systematically offset from the position of the overlying linear transform fault in the brittle crust. This offset causes the boundary to be oblique to plate motions along much of the transform's length, producing extension in regions of significant obliquity modifying the location of the surface fault segments. An implication of this complex plate-boundary geometry is that in the near-ridge region, the older (cooler) lithosphere will extend beneath the ridge tip, restricting the upwelling of mantle to the MOR. The melt to generate the oceanic crust adjacent to the transform must migrate laterally from its offset source, resulting in a reduced volume and thinner crust. This near-ridge plate boundary structure also matches the pattern of core-complex extension observed at inside corners of many slow-spreading ridges. The oblique extensional structure may also explain magmatism that is observed along 'leaky' transforms, which could ultimately result in the generation of new ridge segments that effectively 'split' large transforms.