The origin of horizontal magma-filled sills is disputed, particularly for extensional settings where the opening of vertical dikes is the predicted mode of magma intrusion. We simulate long-term extension followed by short-term dike opening in a two-dimensional viscoelastic medium representing a plate spreading center. We show that dike opening in extensionally stressed lithosphere can reduce sublithospheric vertical stresses enough for sill opening given three conditions: (1) the Maxwell time of the asthenosphere is <5× the time interval between dike episodes; (2) the average density of the lithosphere is not much greater than the magma density; and (3) the depth of an axial valley is smaller than a few hundred meters. This mechanism explains the presence of sills along much of the axis of faster-spreading ridges and their absence along slower-spreading centers where thick dense lithosphere and/or sizeable axial valleys exist.

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