TRANSFORM-PARALLEL AND RIFT-PARALLEL INTRACRATONIC FAULT SYSTEMS AROUND THE OUACHITA EMBAYMENT OF THE IAPETAN RIFTED MARGIN OF SOUTHERN LAURENTIA

The Southern Oklahoma fault system and Mississippi Valley graben each extend ~500 km into Laurentian crust from the Ouachita embayment of the Iapetan rifted margin, conforming to the definition of an aulacogen, a system of basement faults extending into the continent from a mountain belt along the margin. A gabbro-dominated bimodal suite of igneous rocks (539–530 Ma) extends along the Southern Oklahoma fault system. High-amplitude gravity and magnetic anomalies with steep gradients on both sides show that the dense mafic rocks have steep boundaries; composition indicates deep magma sources (upper mantle). Crust-penetrating near-vertical fractures as magma conduits are consistent with a leaky transform fault. The Mississippi Valley graben is a fault-bounded basement graben with a thick clastic sedimentary fill of probable Early to biostratigraphically documented early Late Cambrian age, indicating the time of movement on the graben-boundary faults. Middle Late Cambrian carbonates overlap the graben-boundary faults and graben fill, documenting the end of fault movement.

The Southern Oklahoma fault system (aulacogen) previously was interpreted to be a failed rift, specifically the failed arm of a three-armed radial-rift triple junction. That interpretation relied in part on a buried thick metasedimentary succession, inferred to be rift-fill sediment. More recent work has shown the age of the metasedimentary succession to be >1.0 Ga (age of metamorphism) and unrelated to the Cambrian fault system. Previous analogy with the Benue trough of West Africa as the failed arm of a rift triple junction has been superceded by new documentation of the Benue trough as a strike-slip fracture system extending into the African continent from transform faults that intersect the African rift margin, providing an analog for the Southern Oklahoma fault system as a transform-parallel intracratonic fault system.

In the context of an Iapetan rift system of northeast-striking rift segments offset by northwest-striking transform faults, the Southern Oklahoma fault system is parallel but not aligned with the Alabama-Oklahoma transform fault, which offsets the Iapetan margin from the Blue Ridge rift to the Ouachita rift. The Mississippi Valley graben is parallel with and has the same sense of extension as the Blue Ridge rift.