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Neotectonics of the northern Mojave Desert

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Mapping with magnetism: Maps of the earth's magnetic properties is another means of generating information about rocks materials in the subsurface. Unlike gravity measurements that are labor intensive (carrying equipment to locations on the ground), magnetism can be recorded by aircraft and on the ground. Rocks of different types have different magnetic properties based on the amount of iron minerals present in the rock. For instance, Intrusive igneous and metamorphic rocks of mafic composition (iron-rich rocks such as basalt, gabbro, serpentinite) have strong magnetic signatures, whereas materials that lack abundant iron, like quartz sand, shale, limestone, and some iron-poor granites, will have low magnetic properties. In many cases, massive iron-rich bodies and ore deposits may not be visible on the surface or on gravity maps. The combination of magnetism and gravity with surficial mapping adds an additional powerful view of the earth in 3 dimensions. Map below is an aeromagnetic map of the same region in the two previous maps (surficial geology and gravity). The additional map below shows how magnetic data can be used to determine the amount of slip that has taken place along a fault.

Aerial magnetism map of the central Mojave Desert region of California

When the magnetic properties of rocks in the subsurface are compared with maps resolved on the surface, the amount of movement along a fault can sometimes be estimated. Terms magnetic "anomalies" and "ambiguities" are used because the source of high or low magnetic properties of the rocks in the subsurface may not be known. In some cases, other useful data about rocks in the subsurface and be derived from evaluation of deep well data, electrical resistivity measurements, and seismic data from earthquakes or man-made shockwave sources.
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graphics provided by Kevin Schmidt

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