Atlantic oceanic crust: seismic structure of a slow-spreading ridge

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Abstract
Summary: The seismic structure of the slow-spreading Atlantic ridge is reviewed using results constrained by synthetic seismogram modelling and all available seismic determinations of the crustal structure of fracture zones. The variation of velocity with depth in normal oceanic crust is characterized by a series of velocity gradients: large (1–2 s −1 ) gradients in the upper crust and more gentle ones (∼0.1 s −1 ) in the lower crust, with a variable crust-mantle transition. Petrologic changes, modified by hydrothermal circulation, metamorphic alteration and by cracks and fractures control the seismic structure. Superimposed on the ‘typical’ crustal structure are marked lateral variations caused by: (i) crustal formation in areas away from a mature spreading centre (e.g. aseismic ridges, the oceanic-continental transition); (ii) accretion within transform faults, generating low-velocity material of greatly attenuated or variable thickness; (iii) ageing of the young crust as it moves away from the spreading centre; (iv) localized and generally small variations in regions away from fracture zones caused by spatial and temporal changes in the igneous and tectonic activity. The observed variations in seismic structure can be explained by a model of the spreading centre consisting of a string of independently accreting segments separated by transform faults every 50–80 km. Restricted magma supply at the ends of the segments may result in accretion of thinned crust in transforms whilst faulting lowers the seismic velocity and allows the deep penetration of water and possibly serpentinization at depth. Growth faulting within the median valley causes the isovelocity contours in the lower crust to be rather flatter than the surface expression shown by the basement relief. Episodic accretion in the spreading centre results in small-scale lateral variability in the extrusive carapace.