Do layer 3 rocks make a significant contribution to marine magnetic anomalies? In situ magnetization of gabbros at Ocean Drilling Program hole 735B

Abstract

We estimate the average magnetization of a vertical section of oceanic gabbros using paleomagnetic and downhole magnetic logging techniques and evaluate the ability of these crustal rocks to contribute to marine magnetic anomalies. Results from Ocean Drilling Program Hole 735B show that the drilled crustal section has a mean effective remanent magnetization of 2.5 A/m. Olivine gabbros, which make up 60% of the section, have an average effective magnetization between 1 and 2 A/m. We used two‐dimensional forward modeling and the average effective magnetization of olivine gabbros to predict the magnitude of a typical layer 3 anomaly observable at sea surface. These results indicate that if polarity boundaries within oceanic layer 3 are near‐vertical, crustal sections similar to 735B would contribute from 25% to 75% of the overlying marine magnetic anomalies at the sea surface. The shape of the polarity boundaries in lower crust will be controlled by its thermal evolution and potentially could be constrained by systematic analyses of the amplitude and skewness of marine magnetic anomalies at individual spreading centers.