THE MIDCONTINENT RIFT SYSTEM IN
IOWA
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MRS GRAVITY ANOMALY |
The Bouguer Gravity Anomaly Map of Iowa (Anderson, 1981) clearly displays anomalies produced by the major structures that characterize the MRS in Iowa. The strongly positive axial anomaly is the result of the thick sequence of dense mafic-dominated volcanic and plutonic rocks that comprise the Iowa Horst, and its shallow location at the Precambrian surface. The flanking gravity minima are produced by the thick sequences of less dense, rift- related, clastic sedimentary rocks that are preserved in the basins marginal to the horst. Five clastic-filled basins are defined by closed contours of gravity minima along the flanks of the Iowa Horst. The very steep gravity gradient between the axial high and the flanking lows attests to the abrupt contact between the two features, their density contrast, and the very large volumes of the disparate lithologies.
2-D Gravity Modelling along Seismic Profiles
To better understand the structure of the MRS in Iowa, Anderson (1992) modeled a series of cross-sectional profiles across the feature. The locations of these profiles were based on the locations of five of the reflection seismic lines. Initial visual interpretation of the migrated seismic data was used as the basis for the development of a series of coincident 2-D gravity profiles.
Gravity modelling was conducted using GRAVMAG, a 2-D modelling program created in part from GRAV2D (a 2-D gravity modelling program obtained from Purdue University where it had been adapted by Shanabrook, Ciolek, and Locher from equations developed by Talwani and Worzel, 1959).
The combination of the seven interpreted reflection seismic profiles of the Midcontinent Rift System in Iowa, the nine 2-D gravity profile models, visual interpretation of the Bouguer gravity and aeromagnetic anomaly maps, examination of samples from wells that penetrate the MRS, and a review of previous work on the MRS, especially in its Lake Superior exposure belt, has allowed the first detailed examination of the MRS in Iowa. These data reveal the structure and history of a truly extraordinary geologic feature.
click to view Bouguer gravity anomaly map of Iowa
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