Dynamics of detachment faulting at North Atlantic magma-poor rifted margins

At continent–ocean transition zones (COTs) of magma-poor rifted margins, the basement is typically shaped by highs and large domes with variable elevation and spacing. These features expose large portions of serpentinized mantle, locally intruded by variable volumes of gabbroic bodies. In these environments, the mantle is exhumed to the seafloor through detachment faulting, which promotes deep hydrothermal fluid circulation and pervasive alteration. However, how hydrothermal processes, magmatic accretion, and detachment faulting interact and evolve over geological timescales remains poorly understood. We address this problem using a 2-D geodynamic model coupled with thermodynamic calculations of water–rock interactions. The model accounts for sedimentation, magmatic accretion, and hydrothermal processes. We focus on the well-documented magma-poor Iberia margin, one of the best documented COTs, supported by extensive geophysical data and deep drilling results. Our simulations reproduce the observed basement morphology through successive episodes of detachment faulting. We find, however, that the development of multiple detachments does not necessarily take place following a flip-flop mode, in which, alternately, oppositely dipping detachments sequentially cut through their predecessors. Instead, deformation may evolve through sequential non-flipping detachment faulting, where polarity remains constant. While the flip-flop mode leads to a geologically symmetrical architecture between conjugate margins, the sequential non-flipping mode results in an asymmetric lithosphere structure, characterized by larger volumes of gabbros on one conjugate margin. The development of one mode or the other depends on the depth at which magma is partitioned across the lithosphere axis and on how faulting redistributes accreted magma and weaker serpentinized mantle. Model predictions for both symmetric (flip-flop) and asymmetric (sequential non-flipping) deformation modes closely match observations, reproducing basement morphology, P-wave velocity (Vp) structure, and the petrological architecture consistent with geological IODP samples from Iberia. This suggests that, in magma-poor settings, first-order Vp variations within the oceanic crust primarily reflect alteration paragenesis and fault geometries rather than mafic-ultramafic distinctions. Consequently, alteration may mask underlying geological differences, with a potentially non-flipping detachment mode that leads to widely spaced domes of exhumed serpentinized mantle at COTs. The choice between these modes hinges on the long-term interplay of axial magma-partitioning, detachment faulting, and hydration processes.