Using fracture zones magnetic anomalies to unravel the long-term behavior of the geomagnetic field

Marine magnetic profiles oriented along the spreading direction have been used to study paleointensity variations of the geomagnetic field. These data-sets have confirmed the link between crustal magnetization and past geomagnetic field variations. An additional set of globally distributed anomalies, oriented perpendicular to the spreading flow-line direction, are located above oceanic fracture zones. These anomalies have never been used to study the geomagnetic field nor has their origin been systematically studied. Here we present the first attempt to use these anomalies to study the long-term behavior of the geomagnetic field in the Cretaceous normal polarity superchron. We use archival data and new magnetic and bathymetry data acquired on geophysical cruise MGLN44MV aboard the R/V Melville on the Pioneer and the Murray fracture zones, North Pacific. These Fracture zones straddle the fast-spreading Pacific Cretaceous Quiet Zone (KQZ) and provide a relatively simple setting to test the feasibility of our approach. We use data from 150 crossings that cover a time-span of 27 million years (110 to 83 Ma). The anomalies demonstrate a remarkable uniform shape and size for thousands of kilometers implying that they were generated by spatially and temporally uniform processes. Two dimensional inversion solutions together with 3D forward models suggest that remanent magnetization governs the magnetic signal. These models also imply that enhanced magnetization, primarily situated within the uplifted regions adjacent to fracture zones, is responsible for the observed anomalies. The actual mechanism that creates these magnetization highs is not known, yet the inference of remanent magnetization allows us to use the anomalies to explore the behavior of the geomagnetic field. The nearly uniform amplitude of the anomalies suggests that the strength of the geomagnetic field remained relatively constant over most of the superchron. If a change in the geodynamo behavior is linked with changes in frequency of reversals then this change should have taken place after (and probably before) the superchron. Initial analysis of crossings located on crust younger then the KQZ suggests that in-fact, the Cenozoic field had similar strength as the superchron.