Magnetic modelling of the J-anomaly in the Central Atlantic constrains the structure of slow-spreading oceanic crust

We investigate the magnetic structure of the M-series and J-anomaly oceanic crust in the Central Atlantic with the integration of seismic data in magnetic anomaly modelling. We find that traditional magnetic models of oceanic crust, which assume uniform layers with constant magnetization and geomagnetic polarity reversals, fail to explain the observed anomalies, especially the lack of high-frequency reversals. This suggests that the complex 3D crustal structure created at slow to intermediate spreading rates plays a significant role in the anomaly patterns, because faulting and magmatic processes act as geological filters. Our new modelling approach includes defining the magnetic structure using the crustal seismic structure to adjust the magnetic layer thickness, and laterally varying the magnetization intensity. We explain the high amplitude of the J-anomaly due to a Fe-Ti enrichment in the magma and not to increases in layer thickness as previously proposed. We also discover a previously unrecognized K-anomaly, younger than the J-anomaly. We provide a new kinematic reconstruction of the central Atlantic showing asymmetry in the magnetic and crustal structure between the African and American plates. These findings emphasize the importance of combining seismic data to realistically model magnetic data and better understand crustal formation at slow-spreading ridges.

This work has been funded by the Spanish Ministry of Science and Innovation MCIN/AEI under project ATLANTIS (ref. PID2019-109559RB-I00), and by the Portuguese Fundação para a Ciência e Tecnologia FCT, I.P. / MCTES under project LISA (https://doi.org/10.54499/PTDC/CTA-GEF/1666/2020) and through national funds (PIDDAC): LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020), UID/50019/2025  (https://doi.org /10.54499/UID/PRR/50019/2025), UID/PRR2/50019/2025.