DO LAYER-3 ROCKS MAKE A SIGNIFICANT CONTRIBUTION TO MARINE MAGNETIC-ANOMALIES - IN-SITU MAGNETIZATION OF GABBROS AT OCEAN DRILLING PROGRAM HOLE-735B

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 marin e magnetic anomalies. Results from Ocean Drilling Program Hole 735B sh ow that the drilled crustal section has a mean effective remanent magn etization of 2.5 A/m. Olivine gabbros, which make up 60% of the sectio n, have an average effective magnetization between 1 and 2 A/m. We use d two-dimensional forward modeling and the average effective magnetiza tion of olivine gabbros to predict the magnitude of a typical layer 3 anomaly observable at sea surface. These results indicate that if pola rity boundaries within oceanic layer 3 are near-vertical, crustal sect ions similar to 735B would contribute from 25% to 75% of the overlying magnetic anomalies at the sea surface. The shape of the polarity boun daries in lower crust will be controlled by its thermal evolution and potentially could be constrained by systematic analyses of the amplitu de and skewness of marine magnetic anomalies at individual spreading c enters.