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.
THE MIDCONTINENT RIFT SYSTEM IN
IOWA
