NEAR-SURFACE GEOPHYSICAL IMAGING OF THE BIG CREEK FAULT ZONE SOUTH OF THE EASTERN REELFOOT RIFT MARGIN

The Big Creek fault zone (BCFZ), a potential seismic source zone located south of the eastern Reelfoot rift margin (ERRM) was investigated using near-surface geophysical data [shear-wave seismic reflection (SH-wave) and ground penetrating radar (GPR)]. The northeast-trending BCFZ is subparallel to the ERRM (~50 km to the north) and has been mapped in detail in east-central Arkansas. The most significant surficial features of the BCFZ in the study area are the 50-km-long, southeast-facing Big Creek escarpment (BCE), and the parallel-aligned Kingtown lineament (KL), comprised of two strands characterized by small escarpments and deflected drainages, located 5 km north of the BCE.

Four SH-wave profiles were collected across the BCE to characterize shallow deformation. Two profiles were acquired across the BCE near Pigtail Point (PP), where previous borehole analysis indicated the presence of near-surface faulting. The other two profiles were collected at the northeast end of the BCE, near West Helena (WH), where the escarpment bends anomalously to the east. The interpreted SH-wave profiles show that the topographic scarp is underlain by high-angle faults that extend upward into Quaternary sediments (shallower than ~40 m). Interestingly, the faults imaged on the profiles near PP dip northwest and indicate reverse faulting while the principal fault zone imaged on the WH profiles shows normal separation (south dipping). This structural style, assuming the BCFZ has dextral strike-slip characteristics like other fault zones associated with the ERRM (i.e., Crittenden County and Meeman-Shelby), suggests the change in scarp orientation near WH is a releasing bend.

Four additional SH-wave profiles, along with coincident GPR data, were acquired across the KL. Three profiles targeted the northern strand of the KL and one targeted the southern strand. Diffractions and abrupt changes in reflection amplitude and coherency suggest the presence of a high-angle fault zone underlying the northern strand. The profile targeting the southern strand displays a strong reflection from a depth of approximately 50-60 m. This reflection is continuous across the profile and exhibits a monoclinal flexure. The GPR data shows correlative zones of deformation in the upper 3 m indicating upward continuation of the deeper structure.