anomaly basics - topex.ucsd.edu · anomaly basics •earth filter: shorter wavelengths more...
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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]
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• 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]
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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]
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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
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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]
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Magnetic anomalies from modern geomagnetic polarity timescale GPTS
From Gee and Kent, 2007, Treatise on Geophysics, v. 5.12
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Vector anomalies can allow identification of anomalies in geometries with very low amplitude total field anomalies
Engels et al. (2008)
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Vector data allow orientation of magnetic contrasts to be estimated from a single profile
Engels et al. (2008)