Date of Award
Master of Science
Long-wavelength magnetic anomalies (LWMA) are broad scale magnetic fields that are usually observed at satellite altitudes. The origin of these fields have been assumed to be solely from the crust, disregarding possible contributions from the upper mantle. Using data from magnetic mantle xenoliths, the possible mantle contribution to LWMA was investigated for different regions including Siberian craton, Kamchatka subduction zone fore-arc, Hawaii hotspot, and French Massif Central plume. To do this, a MATLAB-based forward-modeling of magnetic anomalies from tectonic regions with different upper mantle geotherms and magnetized mantle geometries was developed. This model incorporated the increase in Curie temperature of magnetite with pressure, the current geotherms of the specific regions, and the statistical distributions of magnetic data from xenoliths in the specific regions. A Monte-Carlo method of random selection of values and repeated calculations was utilized in constraining the range of potential mantle contributions to satellite-observable LWMA. The Siberian craton shows the highest possible contribution to satellite magnetic anomalies with amplitudes ranging from 2 nT to 9 nT, with a wavelength equivalent to the long-axis of the craton. The Hawaii hotspot region displays a significant contribution of its upper mantle to satellite measured magnetic anomalies with an amplitude of the order of 2 nT, while the Massif Central plume regions shows an insignificant contribution of its upper mantle to satellite magnetic anomalies with potential total magnetic anomaly amplitude of 0.07 nT. Finally, the mantle portion of Kamchatka subduction zone shows a sizeable contribution to magnetic anomalies measurable at satellite altitude with an amplitude up to 1.3 nT. These results when compared with lithospheric total field intensity model derived from SWARM satellite data, show that the upper mantle can contribute significantly to LWMA depending on (a) the average remanent magnetization in xenoliths from such regions, (b) the thickness of magnetized mantle, and (c) the size of the region under consideration.
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