Mechanisms of lithospheric extension at mid-ocean ridges

SUMMARY We examine the extensional deformation of oceanic plates at mid-ocean ridges, especially within an axial yield zone where pervasive faulting occurs. Thermal models of ridges are developed which include the effects of lithospheric thickening on the mantle flow, the heat of magmatic crustal accretion at the ridge axis, and the hydrothermal cooling due to seawater circulation in the crust. When hydrothermal circulation occurs, the brittle lithospheric plate at slow-spreading ridges could be as thick as 8-9 km, thicker than the crust; while the plate at fast-spreading ridges is only 1-2 km. For a typical slow-spreading ridge, several kilometres of plate thickening are expected within a distance of 15 km from the ridge axis. When subjected to the extensional force due to horizontal stretching, shear failure by normal faulting will occur pervasively in an axial zone, where the lithospheric plate is the thinnest. Adopting perfectly plastic rheology as a continuum description of deformation on the distributed faults, we obtain approximate solutions for the stress distribution in the yield zone. Within this yield zone, sea-floor topography increases significantly away from the ridge axis so that the resuiting gravity sliding force balances the differential horizontal extensional force due to the thickening of the lithospheric plate. Basal stresses induced by the viscously deforming asthenosphere could significantly influence the stresses inside the lithospheric plate only if the mantle viscosity beneath the ridge is on the order of 10" Pa s, significantly higher than generally accepted values of 10" Pa s. Model calculations reveal that although the seafloor topography at the Mid-Atlantic Ridge at 13-15 ON is regionally compensated, it is locally supported by stresses in the lithospheric plate. The deviation of the lithospheric plate from its thermal isostatic equilibrium position can be explained by necking due to plastic stretching in the axial yield zone and elastic deflection of strong plates outside the yield zone. The best fit models require the yield zones to have a half width of 10-15 km. We find systematic variations in the gravity and topography of the East Pacific Rise, which indicate strong influence of plate spreading rate on the ridge thermal and mechanical structure. At the 16-17 ON area, where the half-spreading rate is 4.3 cm yr-', a prominent axial topographic high and an axial mantle Bouguer gravity low exist, implying a crustal or sub-crustal low density body. The gravity low disappears at the 20-21"N area, where the half-spreading rate is 3.6 cm yr-'. As the plate spreading rate decreases from 4.3 cm yr-' at the 16-17 ON area to 2.7 cm yr-' at the 22-23 ON area, axial ridge topography changes from higher than the thermal isostatic equilibrium position to lower than the isostatic position. The low axial topography at the 22-23"N area can be explained by the existence of a low amplitude median valley.

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