A steady-state model for plate accretion at oceanic ridges, developed using constraints mainly from western Newfoundland ophiolite complexes, involves four main rheological components: (1) a convectively cooling lid accelerating from the ridge axis across a zone of decreasing dike-injection rate and thickening by the addition of extrusive basalts above and gabbro underplating beneath; (2) a wedge-shaped magma chamber with a flat floor; (3) a differentially subsiding wedge of cumulates; and (4) a narrow axial partially melting Iherzolite-derived diapir from which basalts are liberated over a narrow axial welt into the magma chamber and from which residual harzburgites are plated near the axis below the base of the subsiding cumulates. The axial depth of the magma chamber determines the thickness of plated gabbro. The basal width of the magma chamber and the height of the partially melted welt control the thickness of cumulates and the final attitude of cumulate banding.

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