Gravity and Magnetic Evidence for the Eastern Termination of the Lake Superior Syncline

The Midcontinent Rift System: A major Proterozoic continental rift

“…Gravity and magnetic surveys had an important role in the early definition and study of the Midcontinent Rift (Woollard, 1943;Thiel, 1956;Lyons, 1959Lyons, , 1970Craddock et al, 1963;Hinze, 1963;Hinze et al, 1966Hinze et al, , 1975White, 1966;King and Zietz, 1971;Oray et al, 1973;Halls, 1978;McSwiggen et al, Figure 9. General stratigraphic sequence in the M. G. Eischeid #1 drill hole, Carroll County, Iowa (after Witzke, 1990).…”

“…A rift origin for the structure was strongly suggested, but caution in acceptance of the rift hypothesis was expressed due to the lack of clear surface evidence for grabens and associated normal faults (White, 1966). Furthermore, it was at this time that the continuity of the structure as a series of segments from Kansas to Lake Superior and across the Michigan basin was established (Hinze et al, 1966(Hinze et al, , 1975King and Zietz, 1971;Oray et al, 1973).…”

“…This interpretation of the Kansas segment of the rift and the iso- topic age of 1,097.5 Ma for the igneous rocks, which falls within the narrow age range of Keweenawan igneous rocks in the Lake Superior region, confirms the general consistency in structural style, tectonic evolution, and age along the length of the rift system. The extension of the Midcontinent Rift from southeastern Lake Superior across northern Michigan (Fox, 1988;Cannon et al, 1991) into southern Michigan is evident in the continuity of the gravity anomaly (Oray et al, 1973;Hinze et al, 1975). However, the available basement drillhole data and the seismic reflection profiling show no evidence of late-stage rift sedimentary rocks along the margins of the rift basin over much of its length in southern Michigan.…”

A structural analysis of the Midcontinent Rift in Michigan and Minnesota

“…Further geophysical work conducted over the Lake Superior region during the 1960s and 1970s was in the form of gravity studies (Canada Department of Energy, 1974;Craddock et al, 1970;Ervin and Hammer, 1974;Weber and Goodacre, 1966;Wold and Berkson, 1977), magnetic studies (Hinze et al, 1966;White, 1966;Wold and Ostenso, 1966), heat flow (Steinhart et al, 1968), and seismic reflection studies (Wold, 1979;Wold and Ostenso, 1966;Zumberge and Gast, 1961), all of which confirmed the synclinal structure of the Lake Superior Basin and approximated thicknesses of the volcanic and sedimentary rocks. Land gravity and magnetic studies of the Keweenawan rocks in the area were published by Bacon (1966), Meshref and Hinze (1970), Oray et al (1973), and Klasner et al (1979). Paleomagnetic studies during the past 20 years (Dubois, 1962;Palmer, 1970;Books, 1972;Green and Books, 1972;Halls, 1974) determined the polarity of the basalts, while geochronological studies were undertaken by Silver and , 1963), Goldich (1968), and Van Schmus (1976 to obtain some of the ages for the Keweenawan Super Group.…”

The Midcontinent Rift System

“…Oray et al (1973) demonstrated the direct connection of this anomaly to the anomalies at the eastern end of the Lake Superior basin, and Hinze et al (1975) interpreted the anomaly as a rift feature that is continuation of the structure associated with the MGA and Lake Superior basin. This interpretation was supported by results from a deep drill hole located in central Michigan on the maximum of the mid-Michigan geophysical anomaly (Figure 3 ; Sleep & Sloss 1978).…”