Abstract

"In the beginning was the Word, and the Word was — Strike-slip"

The namesake "Nemaha Mountain Structure" lies across the common border of southeastern Nebraska and northeastern Kansas. The structure itself, is a buried, high-relief basement block, bounded on its east by the vertical, 2500 ft (810 m) Humboldt (also called the Nemaha) fault. There is a 25 mi (40 km) wide surface manifestation of the feature. It is basically a pop-up structure at the junction of two wrench-fault zones, the northwest-trending Central Plains megashear, and the southward-trending Nemaha fault. The Nemaha zone complex is about 400 mi (650 km) in length and extends south-southwesterly from the uplift, across Kansas and then south into south-central Oklahoma where it splays out and terminates against the Oklahoma megashear. The zone varies in width from about 4 to 15 mi (6-20 km) with many splits and returns, often a single plane in central Oklahoma. Its quasi-vertical hade reverses several times along its trace, with vertical displacements of up to several hundred feet, and in three places over 2500 feet (810 m).

The Nemaha zone has been described as a part of the late, mid-Proterozoic-aged (Keweenawan-1,000 Ma) Midcontinent Rift System. Neither geophysical analyses, geological dating nor tectonic framework support this. Detailed mapping of subsurface data from hundreds of wells in Oklahoma and Kansas and an abundance of related data show that the Nemaha zone has a structural style and history entirely different from that of the Midcontinent Rift. The Rift is a 25+ mi wide, trans-tensional graben with an inverted horst-block core consisting of basalt and rhyolite, dated at 1,200-1,000 Ma. It is embedded in an older 2,800-2,000 Ma terrane. The Nemaha is a rather narrow transpressional fault zone with no igneous components, rooted in a 1,650-1,350 Ma terrane. There is evidence to demonstrate that the Nemaha could be either a right-lateral, conjugate, Reidel R' second order right-lateral wrench fault, or a second order, right-lateral (Moody and Hill) wrench-fault, but it probably is simply a random break. Well control data in Oklahoma indicate that the zones initial movement was at least as old as mid-Ordovician (Taconic), but it probably originated much earlier, i.e., at the same time as its terminating, primal wrench-faults, i.e. in early mid-Proterozoic time (~1,600+ Ma) or older.

Evidence presented here will show that the Nemaha zone is a true wrench-fault, rooted in the deep crust, formed as the result of a slight counter-clockwise rotation of an intra-continental block lying between two bounding, left-lateral megashears. Its inherent structural features are the result of transpressional forces. Basically, it is a wrench-fault zone of limited horizontal displacement, where fault-dips along the trace flip from high-angle normal to high-angle reverse, and with pull-apart grabens and pop-up structures.

Beginning in mid-Proterozoic time and continuing, intense collisional surges produced during the westward-directed Taconic, Acadian, and Alleghenian orogenies on the east flank of the craton, and the opposing and contemporaneous eastward-directed Ruby and Antler orogenies on its west flank (Johnson, 1971), intermittently rejuvenated the cratonic wrench-faults during Paleozoic time.

The northward-directed Ouachita orogeny, while being contemporary with the westward-directed Allegheney orogeny, was entirely separate from it. It is believed that the Ouachita had no effect on the structural framework north of the Arkoma basin. The highly elevated and heavily eroded allochthon did provide many thousands of cubic mi of sediments into the adjacent sag and foreland basins from late Mississippian into Permian time.

East of the Nemaha zone in Oklahoma, there are a number of less prominent faults and associated structures parallel to it. They too, provide evidence of strike-slip displacements accompanied by syntectonic deposition that correlate temporally with events that occurred in the Nemaha zone and elsewhere on the craton. These faulted structures, like those along the Nemaha zone also provide repositories for many oil and gas fields, some of the giant variety.