Magmatic sill formation during dike opening at plate spreading centers

The identification of thin, quasi-horizontal magmatic sills along the axis of some spreading centers profoundly changed the accepted view of magmatic process for both oceanic and continental magmatic systems. However, 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. Here, we simulate long-term extension followed by short-term dike opening in a 2D viscoelastic medium representing a plate-spreading center. Our results show that dike opening at a spreading center can rotate stresses and reduce vertical stresses so that nearly horizontal sills should open. A key to the rotation of stresses is that the reduction of horizontal stresses is limited by magma pressure as the dike propagates below the lithosphere. This allows the vertical stress to become lower than the horizontal stress and so to be lower than the magma pressure. This mechanism only works given three conditions: the Maxwell time of the asthenosphere is ~>5 times smaller than the time interval between dike episodes; the average density of the lithosphere is not much greater than the magma density, and the depth of an axial valley is smaller than a few hundred meters. Our model 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.