Anomaly Basics

• Earth filter: shorter wavelengths more attenuated

• Geomagnetic signal includes both direction and intensity

• Crustal accretion processes as filter

Source magnetization and sea surface and near bottom anomalies generated at pole (i.e. with remanence and ambient field both vertical). 55 km/m.y. half spreading rate.

[Slides from Jeff Gee, 2017]

• Shape (skewness) of anomalies depends on geometry of spreading lineation, Mdir, Bdir

• Skewness = phase shift to make similar to pattern at pole

• Anomalous skewness = not predicted by geometry

[Slides from Jeff Gee, 2017]

Variations in magnetization (calculated from anomaly data) are best modeled by lava thickness variations

Williams et al. (2008)

Along-Axis Magnetization Variations

[Slides from Jeff Gee, 2017]

Temporal Changes in Magnetization

• Anomaly amplitudes and sample remanence higher at ridge

• Attributed to low T oxidation of titanomagnetite

sample NRM data from Irving (1970) oxidized TM (from Furuta, 1993)

Cracking in large titanomagnetitetypical of low T oxidation

Spreading Rate Dependence

Gee & Kent (2007)

Large decrease in amplitude anomaly off-axis confined to full spreading rates less than about 30 km/m.y.

Central Anomaly not such a pronounced feature at faster spreading rates.

[Slides from Jeff Gee, 2017]

Magnetic anomalies from modern geomagnetic polarity timescale GPTS

From Gee and Kent, 2007, Treatise on Geophysics, v. 5.12

Vector anomalies can allow identification of anomalies in geometries with very low amplitude total field anomalies

Engels et al. (2008)

Vector data allow orientation of magnetic contrasts to be estimated from a single profile

Engels et al. (2008)