Variability of Sea-surface Magnetic Anomalies at Ultraslow Spreading Centers: Consequence of Detachment Faulting and Contrasted Magmatism?

The capacity of oceanic crust to record geomagnetic polarity reversals makes sea-surface magnetic anomalies an essential tool to study plate tectonics. The anomalies are usually well-defined at magmatic spreading centers, but are distorted and eventually disappear on magma-poor mid-ocean ridges such as the ultraslow Southwest Indian Ridge (SWIR), making their interpretation difficult. We attribute the variability of the SWIR sea-surface magnetic anomalies to the alternance of magmatic spreading and detachment faulting. A three-layer magnetic model is used to simulate the influence of such an alternance on the sea-surface magnetic anomalies. Conversely, observed magnetic profiles at the SWIR are modelled to unravel their off-axis crustal structure and past mode of spreading. The intruding gabbro bodies on the footwall of detachment faults play a major role in explaining the variability of sea-surface magnetic anomalies at slow and ultraslow spreading ridges.