THE INFLUENCE OF THE MESOZOIC AND PALEOGENE HISTORY OF THE SALINE RIVER FAULT SYSTEM ON ITS NEOTECTONIC ACTIVITY

Although marked by only moderate historic seismicity, active tectonism along the NW-striking Saline River fault system (SRFS) of southeastern Arkansas is demonstrated by large fields of sand blows, regional stream migration against structural dip, and Quaternary surface faulting and folding. The seismic potential of the SRFS has been difficult to assess because its geometry has been poorly understood. To address this deficit, we mapped the SRFS in the subsurface using deep petroleum exploration seismic reflection profiles and shallow coal exploration borehole logs.

The seismic profiles show the SRFS is a series of NW-striking horsts and grabens that cut Ouachita thrust sheets to >8 km depth. Approximately 1 km of Triassic sediments preserved in the grabens is absent on the horsts. Following burial by 1 to 1.5 km of Jurassic to Eocene sediments, these grabens were compressed, and many normal faults were reactivated with a reverse component. These late-stage faults branch up-dip, consistent with flower geometry of a strike-slip system.

Faults within the SRFS mapped using coal logs are NW-striking, left-stepping, en echelon horsts and grabens that show 5 to 10 m of syn-sedimentation Eocene movement. These extensional faults were inverted with up to 30 m of net throw during a post-Eocene phase of compression. Restraining-bend uplift in step-over zones suggests right-lateral slip on the SRFS during this post-Eocene compression, inconsistent with the modern ENE-WSW compressive stress regime and field evidence of a left-lateral component of Quaternary slip. Thus, this phase of Neogene faulting was pre-Quaternary.

The SRFS is a series of parallel and en echelon faults that follow (and may be linked to) the Late Proterozoic North American plate margin transform fault below Ouachita thrust sheets, and its history of multiple reactivations shows the SRFS to be a fundamental weak zone.