BASEMENT DEFORMATION NEAR THE TERMINATION OF A FAILED MAGMATIC CONTINENTAL RIFT: INSIGHTS FROM NORTH-CENTRAL OKLAHOMA

Structural deformation associated with failed continental rifts is often characterized in the context of rift-axial/along-rift basin inversion and their associated structures; however, little is known about the deformational styles at the off-rift and near rift-tip regions. In this study, we explore basement deformation around the Nemaha Uplift and Fault Zone (NFZ), considered to be the southern continuation of the Precambrian Mid-Continent (Keweenawan) Rift. We analyze top-basement and intra-basement structures in seven post stack time-migrated 3-D seismic reflection volumes (cumulative 639 km² area) around Kay and Osage counties, utilizing structure-oriented seismic attributes to better resolve structures. Overall, our results reveal 90 fault lineaments at the top-basement surface with minimal throw and dominant trends along N-S, NE-SW and NW-SE. We observe that at the NFZ, the mapped faults dominantly strike N-S; whereas, farther away from the NFZ, the faults exhibit N-S, NE-SW and NW-SE trends. Further, analyses of the of the basement interval reveal pervasive sub-horizontal (10-20˚) intra-basement reflectors that show cross-cutting geometries and terminate at the top-basement surface. Some mapped faults at top-basement extend downwards and offset the intra-basement reflectors. Based on the interpreted reflector geometries and samples from near-by basement wells, we interpret the intra-basement reflectors to represent diabase sills and inclined sheets possibly associated with off-rift/rift flank magmatism along the Mid-Continent Rift. Therefore, we infer that the mapped faults are possibly rift-related faults that experienced multiphase reactivation after rift failure and the later Phanerozoic tectonic events that affected the mid-continent of the United States. On a regional-scale, we consider that the extents/geometry of the Mid-Continent Rift as revealed in potential field data, with respect to the NFZ describe a structural pattern similar to deformation at a small-scale fracture tip observed in laboratory experiments and in the field. In summary, our findings provide interesting insights into the styles of off-rift/rift flank basement deformations around a failed magmatic continental rift and have implications for other failed rifts around the world.