The evolution of propagating rifts

Abstract

The magnetic survey carried out by Mason and Raff^1,2 off the north-west coast of the US has been central to the development of many of the concepts of seafloor spreading and plate tectonics. Recently Hey and Wilson³ have again used this survey to illustrate Hey's suggestion⁴ that ridge segments sometimes increase their length by propagation. This process leads to a characteristic pattern of magnetic anomalies offset by discontinuities which Hey called pseudofaults because, unlike fracture zones, they (Fig. 1) were never plate boundaries. We demonstrate here how the geometry of the pseudofaults, magnetic anomalies and the failed rift can be worked out using velocity triangles and the concept of stability. This method was originally developed to understand the evolution of triple junctions between three plates⁵.