Structure of the Southern Keweenawan Rift from COCORP Surveys across the midcontinent geophysical anomaly in northeastern Kansas

Abstract

Abstract.COCORP profiling across the midcontinent geophysical anomaly in north-

The Midcontinent Rift System

“…A recent COCORP seismic reflection profile over the MGA in northern Kansas has provided new insight into the structure of the axial trough of the midcontinent rift (Serpa et al 1984). Their interpretation of the reflection data (Figures 7a, 8) shows an asymmetric basin consisting of two stratigraphic units.…”

The Midcontinent Rift System

“…(Brown et al 1982), showing major reflectors. Note absence of any significant vertical offsets in Precambrian units, suggesting a relatively smooth basin (Serpa et al 1984). www.annualreviews.org/aronline Annual Reviews…”

“…However, gravity and magnetic studies have poor horizontal and vertical resolution, and seismic data have until recently been confined to regional crustal seismic measurements and shallow seismic investigations in Lake Superior. Two recent COCORP reflection profiles have helped in interpretation of the buried rift structure in Kansas (Serpa et al 1984) and central Michigan (Brown et al 1982), but in general structural detail poorly constrained. Figure 7 summarizes interpretations of the rift structure at four places along the rift system.…”

“…Contrary to classical views of surface deformation associated with rifts, a central graben with marginal, steeply deepening normal faults has not been observed for the midcontinent rift system. Instead, faulting is limited to tilted fault blocks, with extension presumably taken up by rotation of fault blocks (Serpa et al 1984); Weiblen & Morey (1980) have called on similar processes for the extension that accompanied emplacement of the Duluth Complex. The interpretations from the COCORP surveys also indicate that later high-angle reverse faulting, with associated horsts, is not a universal feature of the midcontinent rift system and may be limited to the western Lake Superior region and the northern part of the MGA (Figures 7b,c).…”

“…His results show a crust in central Lake Superior that reaches thicknesses in excess of 50 km, with arms of thickened crust extending southward from the ends of the lake along the MGA and mid-Michigan anomaly. COCORP seismic profiling in northern Kansas, across the MGA (Serpa et al 1984), indicates disturbed crust. Serpa et al infer the presence of intrusions beneath the rift trough from seismic velocities, reflection character, and modeling of the gravity anomaly data.…”