Vertical magnetic structure of ocean crust determined from near‐bottom magnetic field measurements

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Abstract
Consensus on the source of marine magnetic anomaly stripes has remained one of the most elusive aspects of the Vine and Matthews seafloor spreading model which has been of such fundamental importance to global plate tectonics. The first attempt to measure the vertical magnetic structure of oceanic crust exposed at a scarp face using near‐bottom magnetic measurements is reported along with the theoretical basis for the reduction and analysis of these measurements. Magnetometers mounted to deep‐tow and submersible platforms were used to determine the horizontal and vertical variation of crustal magnetization of the upper 2 km of young oceanic crust exposed by a steep submarine escarpment at the Blanco fracture zone in the northeast Pacific Ocean. Results show a large‐amplitude magnetic anomaly is consistently found at the extrusive to intrusive transition indicating that the extrusives are strongly magnetized compared to the weakly magnetized intrusive crust. Profiles also show the systematic presence of more than one polarity with depth indicating the possibility of dipping polarity boundaries within the extrusives. Forward models find that a dipping polarity boundary model correctly predicts the amplitude and shape of the overlying sea surface anomaly, while a block model with vertical polarity boundaries overestimates the amplitude. These in situ results are used to directly assess the contribution made by the extrusive section to the overlying magnetic anomaly signal, in essence, to define the magnetic source layer. It is concluded that the extrusive lavas constitute the main magnetic source for magnetic anomalies in oceanic crust less than 2 m.y. in age.