Thermal Segmentation of Mid-Ocean Ridge Transform Faults

Physical processes along mid-ocean ridge transform faults (RTFs), including seismic behavior, melt migration and pooling, and frictional processes, are strongly controlled by the underlying thermal structure. Along some RTFs, pull apart basins, intra-transform spreading centers (ITSCs), and fault steps divide the fault system up into a series of two or more fault strands, reflecting a change in the underlying thermal structure of the fault system compared with that of a single, unbroken fault. In this study, three-dimensional finite element simulations are used to model the thermal structure underlying a simplified transform fault system, in which two adjacent fault segments are separated by an orthogonal offset. Based on fault segment length and slip rate, we develop a scaling relation for the critical offset length (OC) and use it to define three orders of RTF segmentation. (1) Offsets ≥ 3OC produce first-order RTF segments, where the thermal structure under the offset approaches that of a mature spreading ridge and the seismogenic area (AT, the area above the 600°C isotherm) under the adjacent fault segments approaches that of two isolated RTFs. (2) Offsets between OC and 3OC produce second-order RTF segments, where vertical mantle flow under the offset allows melt to reach the surface and the seismogenic areas of the adjacent fault segments are strongly influenced by one another, increasing AT above that of two isolated RTFs. (3) Lastly, offsets less than OC produce third-order RTF segments, where mantle flow under the offset is predominantly horizontal, focusing upwelling towards the adjacent fault segments, and AT approaches that of an unbroken RTF. The critical offset length marks a transition in offset type, between pull apart basins for offsets < OC and ITSCs for offsets ≥ OC, and may be a limiting factor for rupture propagation along RTFs.