Magnetic modeling of the J anomaly and M-series at 31°N, NW Central Atlantic, constrained by crustal structure

The high amplitude J magnetic anomaly is usually described as corresponding to the M3-M0 anomalies of the M-series and marks the boundary with the Cretaceous Normal Superchron in the Central Atlantic. A seafloor spreading nature of J is undisputed, but remains debated a potential extension into the southern North Atlantic west of Iberia, with its implications for the kinematic reconstructions of the rift-to-drift transition of Iberia-Newfoundland margins. To image the structure of the J anomaly, coincident wide angle seismic, multichannel seismic and magnetic data were collected across the Mesozoic oceanic crust at ~31°N during the ATLANTIS cruise in 2022. A seismic tomography model reveals a complex velocity structure with significant lateral crustal thickness variation that is at odds with the classical view of a uniformly thick J anomaly crust. Instead of the invoked excess magma production, the structure supports complex variation of the seafloor spreading processes.

We present new magnetic modeling for the ATLANTIS profile that constrains the geometry of the magnetic layers with the seismic velocity model, the basement and Moho topography, and the crustal thickness. We first show the canonical approach of modeling the oceanic crust as a constant thickness layer with alternating polarity blocks, which is not able to match neither the amplitude nor the wavelength of anomalies, either considering constant depth or integrating top of basement topography for the magnetic layer. This may be related to the slow spreading processes that tend to cancel short wavelength anomalies and decrease the anomaly amplitude, which strongly suggests that the crustal structure should be integrated in magnetic modeling, especially of non-fast spreading crust.

We used the seismic velocity structure to constrain the thickness of the magnetic layer, either from 6.0 or 6.5km/s isovelocity contour, or varying proportionally to total crustal thickness. Our results show that the M-series domain (up to M3n) can be modeled by defining the magnetic layer thickness as ~20% of total crustal thickness and with a simplified alternating polarity sequence. For the J-anomaly domain, however, a simple relation doesn’t apply, and adjustments in the layer thickness and magnetization are required. A progressive increase in magnetization is needed from the time of M3n onwards, reaching maximum values between M1r and M0r anomalies and decreasing towards the CNS. The magnetic layer thickness follows the same tendency. However, the crustal thickness varies in a much distinct way: maximum magnetization values are modeled at the thinner crust, and intermediate magnetization is kept for the region where the crust is thicker, already located at the CNS and offset of the highest anomaly amplitude.

These results challenge the use of classical methods to model oceanic magnetic anomalies generated at slow spreading centers, and more particularly, the classical view of the J-anomaly structure. A temporal lag is suggested between the source mantle processes that originated the alteration of magma composition to higher magnetization (mantle fertility / chemical composition) and the increase in crustal thickness (likely more related to mantle temperature).

Work supported by the Portuguese FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025, UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020) and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020).