Ervin, C. Patrick
M.S. (Master of Science)
Department of Geology
Gravity--Wisconsin; Geology--Wisconsin; Geomagnetism--Wisconsin
Geophysical investigations of the Precambrian rocks of Wisconsin are made imperative by the ubiquitous cover of glacial and postglacial deposits. Detailed gravity and aeromagnetic data of north-central Wisconsin serve to distinguish an Early Precambrian gneissic terrane, Middle Precambrian metamorphosed mafic and felsic volcanic rocks, and the products of Late Precambrian volcanism and plutonism. The Middle Precambrian metavolcanic zone in the northern part of the study area is characterized by alternating positive and negative Bouguer gravity anomalies and low intensity magnetic anomalies of considerable relief that tend to be elongated northeasterly reflecting the structural grain of the rocks. Observed anomalies can be correlated with the different average densities and magnetic susceptibilities of mafic and felsic volcanic rocks. Theoretical two-dimensional gravity models reveal faulting, folding and intrusive bodies. The differing Precambrian terranes are identified by their dissimilar gravity and magnetic fields. Granite and other felsic intrusions are defined by broad, very negative (-70 mgal) gravity anomalies and areas of low magnetic intensity and many magnetic irregularities; higher density gneisses are associated with correspondingly more positive (-35 to -65 mgal) gravity values. Mafic volcanics (gabbro, metagabbro) are associated with positive gravity anomalies (-55 to -65 mgal) and both higher than average magnetic level and greater magnetic relief. Greenstones should appear as areas of low magnetic intensity (-400 to -500 gammas), low magnetic relief, and relatively positive gravity anomalies. Basic dikes appear as 'noses' or gradient breaks against the magnetic background.
Tuftee, K. K., "Interpretation of the gravity and magnetic fields of the southwest part of the Iron Mountain Quadrangle, Wisconsin" (1981). Graduate Research Theses & Dissertations. 3622.
Northern Illinois University
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