Gravity and Magnetic Modeling of Central Segment of Mid-Continent Rift in Iowa: New Insights into Its Stratigraphy, Structure, and Geologic History

Raymond R. Anderson

Middle Proterozoic (approximately 1,000 Ma) rocks of the central segment of the Mid-Continent Rift (MCR) occupy an area of about 50,000 km² of the Precambrian surface but are completely buried by Phanerozoic rocks. Gravity and magnetic modeling of the MCR, constrained by seismic reflection and well data, provides new insight into the stratigraphy, structure, and geologic history of this portion of the MCR. The MCR is dominated by the Iowa horst, a mafic volcanics-dominated axial horst reaching a maximum model thickness of at least 30 km and a maximum width of 55 km. It is segmented near its midpoint by an accommodation zone that displays about 20 km of left-stepping offset and is flanked by deep basins filled with up to 10 km of clastic rocks. The horst was app rently uplifted along high-angle reverse faults, over the clastic rock sequences, with as much as 5 km of horizontal overlap. Modeling indicates that the central horst is comprised of two structurally distinct packages. The uppermost package is distinguished by remarkably continuous multiple parallel reflectors, interpreted as individual volcanic flows or groups of flows, that can frequently be traced across much of the horst. The flanking clastic-filled basins also appear to be composed of two distinct lithologic packages, with the lower package displaying higher densities and more continuous reflectors than the upper. In transverse cross section, the Iowa horst and flanking basins have approximate bilateral symmetry, unlike the half-graben structure recently suggested for the western L ke Superior segment of the MCR. This symmetry suggests that at least the upper volcanic package was deposited in a uniformly subsiding graben.

AAPG Search and Discovery Article #91025©1989 AAPG Midcontinent, Sept. 24-26, 1989, Oklahoma City, Oklahoma.