PRECAMBRIAN FRAMEWORK OF THE MIDCONTINENT USING GRAVITY AND MAGNETIC DATA: AN EXAMPLE FROM KANSAS

Regional gravity and magnetic maps, combined with well control down to the Precambrian, are a very useful tools for determining the geologic framework of Midcontinent. Much of the basement in the continental interior of the U.S. is buried beneath Phanerozoic platform sedimentary rocks. In order to understand the structure, origin, and evolution of the underlying Precambrian basement, we must rely on geophysical techniques constrained by borehole data and outcrops wherever possible. In addition, the structural framework of the Precambrian basement, including numerous faults and other shear zones, appears to largely control the Phanerozoic sedimentary framework above the basement due to reactivation during episodes of subtle tectonic movements within the continental interior.

One example of the utility of potential field maps to understanding the basement structure of the Midcontinent is the Kansas gravity and magnetic data set. This data set was collected over three major Precambrian terrains in Kansas: the southern Central Plains orogen, the southern granite rhyolite terrain, and the southern Midcontinent rift system. Numerous, isolated Precambrian plutons, largely epizonal granites, also occur within this area. The gravity and magnetic data show quite clearly the boundaries of these major terrains, including NW-SE and NE-SW structural and lithologic trends internal to them. Isolated intrusions with anomalous gravity or magnetic values are also clearly visible on the data. These boundaries, intrusions, and structural trends are particularly visible on the artificial sun-shaded maps, and appear to correspond to numerous younger structural features and stratigraphic trends mapped from the abundant Phanerozoic well control in Kansas.

As EarthScope and other large-scale seismic acquisition projects move eastward across the U.S., interpretation of regional gravity and magnetic maps such as those in this study will be critical to understanding the crustal component of the project. These data can also act as a bridge between geological information available for the crust and the deeper picture provided by the the Bigfoot array. In addition, the gravity and magnetic maps can provide a first look at the crustal structure of the Midcontinent in order to aid the sighting of projects relying on the Flexible array.