ARE THE MISSISSIPPI VALLEY GRABEN AND THE ROME TROUGH CONNECTED? AN ANALYSIS OF CAMBRIAN FAULTING PATTERNS WITH RESPECT TO OBLIQUITY OF RIFTING

Data suggest that the Mississippi Valley Graben, Rough Creek Graben of western Kentucky, and Rome Trough of West Virginia and eastern Kentucky formed at the same time (Early - Middle Cambrian), and thus probably from within the same tectonic environment. This raises the possibility that these three structures are fundamentally connected and part of a continent-scale rift zone that underlies the eastern United States, including parts of the Illinois Basin and a significant portion of the southern Appalachian Basin. These structures have vastly different strikes, vertical offsets, and fault characteristics, however, which have led many researchers to dismiss them as separate deformation events.

Ironically, these seemingly incompatible characteristics are what imply that these structures are connected. The Mississippi Valley Graben has a typical “pure shear” rifting profile: a wide zone of crustal-thinning, normal faults with roughly 60° dip, and with vertical offsets that decrease away from the center of the rift. The Rough Creek Graben, however, is a relatively narrow and very deep (estimated over 9 km) rift graben with nearly vertical bounding faults. The Rome Trough is characterized by complex structural elements that include large changes in strike (one of 30° and another of 60° in the opposite direction), apparent asymmetric rift growth, and major differences in structural style along strike between Kentucky and West Virginia. If these structures are assumed to be connected, then the unrifted regions northwest and southeast of them may have moved as two separate, rigid crustal plates, with the Mississippi Valley, Rough Creek, and Rome structures accommodating the extensional strain between them. In this two-plate model, the changes in strike along the extensional zone dictate variations in obliquity with respect to the single extensional direction. When the differing faulting and rifting patterns are analyzed from this point of view, it becomes apparent that the geometric orientation of the fault systems relative to σ³ determined the rifting style. Changes in structural style and rifting patterns were not the result of different tectonic events, but a consequence of their relative angle to the regional extension direction.