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STRUCTURE

Red ball iconTHE MIDCONTINENT RIFT SYSTEM IN IOWA

 

STRUCTURE


The combined seismic and gravity modelling of a series of profiles across the Iowa Horst (click to view Iowa MRS profiles) has yielded a wealth of information about the poorly understood structure. The upper 8 km (5 miles) or so of the gravity models, the region controlled by the seismic profiles, generally yielded model interpretations with a high degree of confidence. Below the region of seismic control, the models are based on continuation of shallower trends, and an increasing density with depth. The entire crust, to a depth of 48 km (30 mi), was modelled by Anderson (1992), with several variations attempted on a general Iowa Horst model. These models included polygons representing mafic volcanics-dominated rocks in the upper crust, to 13 km (8 miles), gabbro dominated rocks in the middle crust, to 23 km (14.5 miles), and a zone of mafic dikes in the lower crust.

Anderson (1992) noted that the geometry of the MRS in the middle and lower crust was especially difficult to interpret. A goal of the interpretation of the deep crustal geometry of the MRS along each profile was to be consistent with the deep crustal structures interpreted from other rifts, and deep MRS structure as interpreted in the Lake Superior region, while yielding model gravity anomalies consistent with observed gravity data. Profile 11 was chosen to test various deep crustal models of Iowa data because of its relatively simple geometry and the challenge of modeling its steep, narrow observed gravity anomaly maxima. In addition to the model finally chosen as representative of the geology along Profile ll, five other deep crustal models were tested and ultimately rejected. These rejected models tested various configuration of basal feeder dikes and pluton configurations.

The development of the Iowa Horst can be documented by examination of the signature of the feature at various crustal levels. The accepted horst model assumes a uniform width for the zone of dikes through the lower crust, so a map of the feature at the base of the lower crust would be identical to the base of the middle crust. Consequently, no basal crust map was prepared by Anderson (1992). At the base of the middle crust, and through the lower crust, the zone of dikes beneath the Iowa Horst marks the axis of the MRS through the state. The rift trends N 20 degrees E from Minnesota south to central Webster County where it abruptly assumes a more easterly N 45 degrees E trend, similar to the general grain of the Penokean Volcanic Belt. Of note is the fact that the axis of the N20 degreesE segment lies several miles east of the axis of the Bouguer gravity anomaly of the feature. The zone of dikes ranges in width from about 3 km (2 miles) near Minnesota to about 35 km (22 miles), just north of area where the northern segment of the rift changes its axial trend.

At the Greene/Guthrie County line, the axis of the rift is offset to the south about 16 km (10 miles) along a transfer fault named the Sheeder Prairie Structural Zone. The Sheeder Prairie Structural Zone (named by Anderson, 1992) is coincident with the western edge of an apparent north-trending felsic pluton first identified by Anderson (1988) and named the Central Iowa Arch Granite. The feature is best observed on aeromagnetic anomaly maps. On the Aeromagnetic Anomaly Map of North America, the Central Iowa Arch Granite is characterized by an orange colored region both north and south of the MRS. On the Aeromagnetic Anomaly Map of Iowa, the granite is characterized by yellow and orange colored areas both north and south of the MRS.

Although the axis of the southern segment of the Iowa Horst is slightly concave to the north between the Sheeder Prairie Structural Zone and the Nebraska border, it follows a general N35 degree E trend. The zone of dikes beneath the southern segment of the Iowa Horst displays a relatively uniform width of about 5 km (3 miles).

At the base of the upper crust the model suggests that the middle crust component of the Iowa Horst, modelled as dominated by gabbroic rocks, is significantly wider than the zone of dikes in the lower crust. The northern segment ranges in width from about 5 km (3 miles) at the Minnesota border to about 60 km (37 miles) near the center of the segment. The easterly bend of the rift axis in the northern segment of the Iowa Horst that was evident in the lower crust is mostly obscured at the base of the upper crust, and the segment displays a more linear trend. The Sheeder Prairie Structural Zone is still present as a rift offset, and the southern segment of the Iowa Horst is a relatively uniform 19 km (12 miles) in width.

click for references cited

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