Influence of magma injection on faulting and topography at mid-ocean ridges

Fault development at mid-ocean ridges is controlled by a combination of axial thermal structure, the rheology of the crust and mantle, and the rate and distribution of magma injection in the crust. Specifically, dike intrusion influences faulting through its combined effects on crustal temperature and the local stress field. However, while many studies have investigated the sensitivity of faulting to thermal structure, few have examined the mechanical implications of dike injection on geologic time-scales. In this study, we develop a kinematic model for dike intrusion in an extending 2-D elastic-viscoplastic layer. Dike injection is simulated by pressurizing a vertical column of model elements located within the brittle layer. We test the sensitivity of fault development to the geometry of the magma injection zone and find that continual dike intrusion can lead to the formation of a steady-state rift valley, even in the absence of an across-axis variation in lithospheric thickness. In addition, we show that the steady-state graben width increases linearly with the depth to the top of the injection zone, while graben depth reaches a maximum when dike intrusion is isolated in the lower half of the brittle layer. Furthermore, we find that contrary to previous models, periods of magmatic accretion can result in long-lived extension on graben bounding normal faults. These results imply that both the geometry of the intrusion zone, as well as the partitioning of strain between periods of diking and magmatic quiescence, influence the development of topography at mid-ocean ridges. Applying our results to observations of axial morphology and crustal thermal structure inferred from seismic imaging along the Galápagos Spreading Center, we hypothesize that the variations in axial relief are likely linked to changes in accretion zone geometry.