Flip-Flop Detachments Require Moderate Strength Contrasts Between Fault Zone and Fault Blocks

Abstract

Large-offset detachment faults frequently occur at slow- and ultraslow-spreading mid-ocean ridges, especially where magma supply is limited. Observations show that detachments at ultraslow ridges often alternate in polarity (“flip-flop” regime), but the mechanical controls governing their formation remain poorly understood. Here we use two-dimensional thermo-mechanical models to systematically investigate how frictional and cohesive strength contrasts between fault zones and surrounding lithosphere influence faulting styles in nearly amagmatic ultraslow ridge environments. Our models incorporate a self-evolving thermal structure and Mohr-Coulomb elasto-plasticity with friction and cohesion weakening during deformation, along with parameterizations of fluid circulation. We identify three primary deformation regimes: distributed minor faults, sequential flip-flop detachments, and persistent runaway detachments. Flip-flop detachments emerge under moderate frictional and cohesive weakening, where faults alternate polarity as antithetic faults periodically replace dominant detachments. Persistent runaway detachments, as produced by models, show excessive cross-axis migration that is not typically observed in nature. A strong, viscous lower lithosphere promotes distributed deformation and hinders the onset of persistent detachments. Analytical force balance modeling further shows that flip-flop faulting occurs when fault strength loss balances the defocusing effect of lithospheric flexure, which necessitates moderate frictional weakening (∼0.2–0.3) achievable via serpentinization of the fault zone. In contrast, runaway detachments require further weakening. These findings clarify the mechanical conditions driving flip-flop detachment formation and highlight how relatively modest rheological contrasts between intact and deformed lithosphere enable large-offset faulting at ultraslow mid-ocean ridges.