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MRS MAGNETIC ANOMALY

Red ball iconTHE MIDCONTINENT RIFT SYSTEM IN IOWA

 

MRS MAGNETIC ANOMALY


Over most of its trend in Iowa the magnetic signature of the MRS is characterized by a series of linear anomalies following the trend of the gravity anomaly. Magnetic intensities are generally within a few hundred gammas (nanoteslas) of regional values, as compared to anomalies for typical intrusions which can exceed 1000 gammas (Carmichael and Black, 1986). The lack of a strong magnetic signature associated with the generally highly magnetic mafic igneous rocks of the Iowa Horst can probably be attributed to a combination of the strong remanent components in these rocks and the vertical sequence of normal and overlying reverse polarity rocks. Keweenawan igneous rocks from the Lake Superior region display Koenigsberger ratios (ratios of the remanent to induced magnetism) as high as 10 (Hinze et al., 1982) and display normal paleolongitude around 180 degrees E and paleolatitudes between 0 and 30 degrees N (Halls and Pesonen, 1982). Since most volcanic rock sequences display only a single normal / reverse remanent polarity cycle (the basal units reverse and the upper normal), the net magnetic signature observed at the land surface may be low. In much of Iowa it appears that the lower, reversely polarized rocks are more abundant on the central horst than are the upper, normally polarized rocks, producing a net magnetic signature that is very near regional intensities. In northern Iowa and Minnesota, the signature of the central horst becomes strongly positive. This suggests a northward increase in the thickness of the upper, normally polarized rocks compared to the underlying, reversely polarized rocks.

The clastic rocks that fill the flanking basins are magnetically transparent. The increase in the depth of the underlying magnetic basement results in a lowering of the magnetic surface and observed aeromagnetic field intensity. This creates, in effect, a filtering out of the shorter wavelength components of the field. On the aeromagnetic anomaly magnetic map of Iowa (Zietz et al., 1976), this produces a series of smooth, low intensity magnetic minima with a general trend parallel to the axis of the rift and flanking the Iowa Horst. On the more detailed, 20 miligal c.i. map of the Midcontainent Rift in Iowa by Henderson and Vargo (1965), this effect allows easy identification of the limits of the magnetic (igneous) rocks of the Iowa Horst and their lateral contact with the nonmagnetic (clastic) rocks of the flanking basins. The shallow depth of the magnetic rocks of the Iowa Horst produces a high relief anomaly surface characterized by closely spaced, tightly curving contours. Off the horst, the greater depth of the magnetic basement beneath the clastics produces much more subdued relief with more widely spaced, smoothly curving contours.

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