Effects of Hydrothermal Cooling and Magma Injection on Mid-Ocean Ridge Temperature Structure, Deformation, and Axial Morphology

Fault development at mid-ocean ridge spreading centers is strongly dependent on the thermal state of the axial lithosphere. Thermal conditions at a ridge axis are a combined function of spreading rate, mantle temperature, magma injection, and hydrothermal circulation. In this study, we test the sensitivity of fault development in slow-spreading environments to the efficiency of hydrothermal cooling and the depth extent of magma injection near the ridge axis. A 3-D finite difference scheme is first used to calculate axial temperature structure, and deformation is then modeled in 2-D vertical sections of lithosphere using a visco-plastic finite element model. Strain-rate softening in the brittle regime is used to simulate the rate-dependence of frictional strength observed in laboratory studies. This formulation results in the formation of localized zones of high strain rate (analogous to faults) that develop in response to the rheology and boundary conditions and are not imposed a priori. Comparing our numerical experiments with observed faulting at the center and ends of several segments along the slow-spreading Mid-Atlantic Ridge, we find that temperatures near the segment end must be warmer than predicted by previous models. These predicted high temperatures can be explained by either inefficient hydrothermal cooling in the shallow crust or heating of the upper mantle through magmatic accretion below the Moho. Because geophysical and geochemical evidence support efficient hydrothermal cooling in young oceanic lithosphere, we favor a model in which heat is supplied to the upper mantle beneath the ends of slow-spreading segments by either crystallization of rising asthenospheric melts or episodic lateral dike propagation from the segment center.