Intrusive Magmatism, Plate Stresses, and the Cause of Orthogonal Versus Oblique Spreading Mid‐Ocean Ridges

The origin of orthogonally spreading ridge segments separated by oceanic transform faults, versus obliquely spreading ridge segments without transform faults (TFs), is a long‐standing enigma of plate tectonics. We address this problem using three‐dimensional (3D) geodynamic models that simulate axial magmatic intrusions along two ridge segments, initially oriented orthogonally to seafloor spreading and the intervening TF. The intrusion zones dynamically relocate to local maxima in the depth‐averaged horizontal “effective stress.” Both 3D numerical models, which simulate a viscoelastic–plastic rheology as well as 2D thin elastic plate models predict shear stress on the TF to induce an asymmetry in lithospheric tension, causing the ridge segment ends to migrate toward each other and eventually form an obliquely spreading ridge. Orthogonal spreading with a TF is promoted by weak TFs, rapid lithospheric thickening beneath the plate boundary zone, higher rates of magmatically accommodated spreading, and intrusions that are confined to a narrow zone beneath the ridge axis. These conditions are observed or expected at slow and faster spreading rates, where orthogonal spreading occurs. In contrast, oblique spreading is promoted by strong TFs, slow lithospheric thickening beneath the plate boundary, lower rates of magmatically accommodated extension, and a broader width about the ridge axis where intrusions occur. These conditions are predicted to promote the observed oblique spreading in typical slow and ultra‐slow spreading environments. At the atypical, hotspot‐influenced Reykjanes Ridge, oblique spreading is inferred to be promoted by a broad subaxial zone of magma intrusion caused by anomalously high magma supply.