Magmatic sill formation during dike opening

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