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A stochastic model for the emplacement of dikes and lava flows at a fast spreading ridge can generate an upper oceanic crustal structure similar to that observed in seismic data from the East Pacific Rise (EPR), in ocean drill holes, and in ophiolites. In this model the location of successive dike intrusion events relative to the ridge axis is determined by a Gaussian probability function and the cumulative flow lengths of the erupted lavas are chosen to build a Gaussian-shaped lava pile. We interpret wide-angle seismic reflections from the steep velocity gradient at the base of seismic layer 2A to be the extrusive/sheeted dike contact. Seismic data from the northern and southern EPR place constraints on the on-axis extrusive layer thickness (230 ± 50 m), the distance over which the thickening of the extrusive layer occurs (width of the accretion zone = 1–3 km) and its off-axis thickness (300–800 m). Ophiolites and ocean …