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