GEOPHYSICAL INVESTIGATIONS IN THE MIDCONTINENT RIFT OF SOUTHEAST NEBRASKA AND EASTERN KANSAS

The 1.1 Ga Mid-continent Rift System (MRS) of North America has been subject to intense geologic investigations for well over a century. Established as a failed rift system nearly 2000 km in length, it consists of a southwest extension from the Lake Superior region into Oklahoma with a short southeast extension into Michigan. It has also been suggested that the MRS may even continue into Canada. The MRS is commonly compared to the East African Rift System (EARS) and the Baikal rift zone (BRZ), both of which are rift systems considered to have different modes of origin, although still are subject to debate themselves. The two models of formation include (1) an “active” lithospheric response to the presence of a rising mantle plume head and (2) the “passive” lithospheric response to far-field tectonic processes, which in this case would be the Grenville Orogeny.

In order to reconstruct the origins of the MRS it is necessary to collect data in regards to its geophysical, geochemical and structural make-up. In particular, geophysical data such as gravity, magnetic and magnetotellurics (MT) are fundamental in obtaining subsurface information, and it is here we can begin modeling the structural details of the MRS. Since the discovery of the mid-continent geophysical anomaly (MGA) it is known that a deflection 60 km in length occurs near the Kansas-Nebraska border in the southwest extensional arm. By modeling the existing data in this region and combining it with previous seismic reflection and broadband seismic results it may lead to a better understanding in rift geometry and structure, and ultimately constrain theories regarding its origin.

In this study, we analyzed the existing gravity and magnetic data in conjunction with newly acquired gravity data in SE Nebraska to determine the general lithospheric structure of the MRS. The analysis consists of constructing a series of residual and derivative anomaly maps and three models perpendicular to the MRS. In addition, recently acquired MT data from the EarthScope project will be analyzed along the same three profiles to aid in determining the lithospheric structure and aid in determining if the upper mantle was depleted during the formation of the large amounts of mafic material related to the MRS.