Structure and Inferred Geological History of Mid-Continent Rift in Iowa

Raymond R. Anderson

Recent completion of a gravity and magnetic modeling project has provided new insight into the structure and geologic history of the central segment on the Mid-Continent rift (MCR) in Iowa. The rift zone is dominated by the Iowa horst, a mafic volcanics-dominated axial horst thrust horizontally, as much as 5 km or more, over postvolcanic rift clastics that fill basins on both its eastern and western flanks. The models indicate the volcanics-dominated sequence of the Iowa horst reaches thicknesses of at least 30 km and is composed of at least two structurally distinct packages. The lower package shows more structural complexity than the overlying package. Multiple parallel seismic reflectors in the upper volcanic package, interpreted as individual lava flows or sets of flo s, are subhorizontal and commonly are continuous across most of the horst. The flanking clastic-filled basins reach a maximum depth of about 10 km, also are composed of two distinct packages that frequently can be discriminated in seismic data, and apparently are separated by an erosional unconformity. Keweenawan clastic rocks are present in several small basins that have been preserved on the Iowa horst. In cross section, the central segment of the MCR has approximate bilateral symmetry. This symmetry suggests that at least the upper volcanic package was deposited in what best can be described as a uniformly subsiding trough, not the half graben suggested for the MCR in the Lake Superior region.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.