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Oceanic detachment faulting, a major mode of seafloor accretion at slow and ultraslow spreading ridges, is thought to occur during magma‐poor phases and be abandoned when magmatism increases. In this framework, detachment faulting is the result of temporal variations in magma flux, which is inconsistent with recent geophysical observations at the Longqi segment on the Southwest Indian Ridge (49°42′E). In this paper, we focus on this sequentially active detachment faulting system that includes an old, inactive detachment fault and a younger, active detachment fault. We investigate the mechanisms controlling the temporal evolution of this tectonomagmatic system by using 2D mid‐ocean ridge spreading models that simulate faulting and magma intrusion into a visco‐elasto‐plastic continuum. Our models show that temporal variations in magma flux alone are insufficient to match the inferred temporal evolution of the sequentially active detachment system. Rather we find that sequentially active detachment faulting spontaneously occurs at the Longqi segment as a function of lithospheric thickness. This finding is in agreement with an analytical model, which shows that a thicker axial lithosphere results in a smaller fault heave and that a flatter angle in lithosphere thickening away from the accretion axis stabilizes the active fault. A thicker axial lithosphere and its flatter off‐axis angle combined have the potential to modulate sequentially active detachment faulting at the Longqi segment. Our results thus suggest that temporal changes of magmatism are not necessary for the development and abandonment of detachment faults at ultraslow spreading ridges. Plain Language Summary: The oceanic lithosphere is frequently generated through slip on long‐lived faults called oceanic detachment faults, which exhume lower crustal and upper mantle materials on the seabed and form dome‐shaped features. Yet, oceanic detachment faulting is a sequentially active process and previous studies have focused on temporal variations in magma flux as the primary cause. Magma flux refers to the rate at which the continuous flow of magma moves from melted mantle at depth toward the ridge axis through volcanic vents or fissures. In the presented manuscript we show that this standard model does not work for the ultraslow spreading Southwest Indian Ridge (49°42′E). Here, we present geological and geophysical evidence that the magma flux had remained stable during the two most recent phases of detachment faulting. We investigate the mechanisms controlling the life cycle of the detachment fault by using a 2D geodynamics model. We compare model results with observations and propose that sequentially active detachment faulting at the Longqi segment is a function of lithospheric structure rather than variations in magma flux. These findings show that temporal changes of magmatism are not required for sequentially active detachment faulting to occur. Key Points: Our tectonomagmatic models provide a mechanical explanation for the sequentially active detachment fault system at the Longqi segment on the ultraslow spreading ridgesLithospheric structure at spreading centers plays a key role in the lifecycle of oceanic detachment faults [ABSTRACT FROM AUTHOR]

Hydrothermal processes in detachment settings at slow and ultraslow spreading ridges differ greatly from those at melt-rich faster spreading ridges. Active detachment faulting provides the possibility for off-axis high-temperature hydrothermal vents located far away from the heat source beneath the axial volcanic ridge. Seismic data from the slow spreading Mid-Atlantic ridge revealed that hydrothermal fluids may exploit detachment faults to extract heat from a melt zone near the crust-mantle interface. However, knowledge of the subsurface structure and the kinematic processes of detachment faults, and their interaction with hydrothermal fields at the slowest spreading ridges is still insufficient. Here, we report new insights on the seismicity beneath the Longqi hydrothermal field at the ultraslow spreading southwest Indian ridge from three ocean bottom seismometer monitoring experiments. The seismicity outlines the subsurface geometry of a detachment faulting system (DF1 and DF2). The strongly flexed DF1 (45°) is a mature detachment fault with a domed-shaped OCC where ultramafic rocks are exposed on the seafloor. An active young DF2 with intense earthquake activity along the steep subsurface (67°) suggests the initiation phase of rotation. The diversity of hydrothermal activities in the Longqi detachment-hydrothermal systems is closely related to the evolution of detachment faults. Additionally, both the non-transform discontinuity on the western margin of the Longqi-1 field and local faults facilitate hydrothermal circulation. Our study provides baseline observations for a Longqi-type of hydrothermal circulation at the inside corner associated with detachment faulting and non-transform offsets. • Micro-earthquakes outline the subsurface geometry of a detachment system beneath the Longqi hydrothermal field at the SWIR. • The diversity of hydrothermalism at Longqi depends on the evolution of the detachment faults. • Seismicity and hydrothermal activity fade out with distance to the non-transform discontinuity. [ABSTRACT FROM AUTHOR]