A 3-dimensional analysis of magnetic anomalies over fracture zones in the Cretaceous Magnetic Quiet Zone (Madeira Abyssal Plain)

Abstract

Magnetic anomalies over fracture zones in a Magnetic Quiet Zone are not complicated by reversals in the earth's magnetic field and thus are easier to interpret, showing only the effect of fracture zones as such. Observations on the Kane Fracture Zone ( Twigt et al. 1983 ), Tyro Fracture Zone ( Slootweg & Collette 1985 ) and other fracture zones in the Cretaceous Magnetic Quiet Zone all point to an enhanced magnetization over fracture zones, mainly situated on the younger side of the fracture zone axis. A model which accounts for this, relates the asymmetric magnetization to the asymmetric development of topography at the intersection of fracture zone and transform fault. The three dimensional analysis of Hayes, Charis and Cruiser Fracture Zones, presented in this study, leads to a distribution of the magnetization of the oceanic crust which may be interpreted in terms of a varying magnetization intensity or a varying thickness of the magnetic layer. On the younger side of the fracture zone axis the magnetization is systematically higher than average (up to 6 A/m) in the intensity model. Also, there are indications that there exists a reduced magnetization on the older side in several places. In the study area a few large changes in seafloor spreading direction occur. These are accompanied by anomalies which do not conform to the general model. Evidently, the proposed mechanism of emplacement of magnetic bodies may not be effective under these extreme circumstances. For the remainder, the general validity of the magnetic fracture zone model is confirmed.