Gravity and magnetic data have been simultaneously inverted to derive a crustal model of the northern Mississippi Embayment which is compatible with geologic and seismic refraction data. The results indicate that this region is a site of a late Precambrian--early Paleozoic rift system. An anomalous dense layer is present at the base of the crust and thickens beneath a broad northeast trending graben that formed during the intial phases of rifting. The thickest part of anomalous crust occurs beneath the region of greatest seismic activity, which lies within the geographic limits of the graben. It is suggested from this correlation that strain accumulates within the anomalous crust layer, which acts as an inhomogeneity in a relatively homogeneous lithosphere. Based on the results of this study and previous geophysical studies, the complex tectonic evolution of the upper Mississippi Embayment since late Precambrian time is rift formation possibly along a pre-existing shear zone, graben development in Cambrian time, reactivations in Permian and Cretaceous time, and subsequent formation of the present Mississippi Embayment and fault zones with large influences from rift structures.