SHALLOW SEISMIC REFLECTION IMAGING OF THE KINGTOWN LINEAMENT WITHIN THE BIG CREEK FAULT ZONE, NORTHEAST ARKANSAS

Aerial photographs of the Kingtown lineament, located approximately 30 km west of Helena, Arkansas, show a set of tonal lineations (5-11 km in length) striking N52˚E to N64˚E. This trend is roughly parallel to the Big Creek escarpment within the Big Creek fault zone (BCFZ) and to faults associated with the southeast Reelfoot rift boundary. Although geomorphic expression is subtle, individual traces of the Kingtown lineament are recognized as broad (≤ 50-m-wide) linear escarpments typically less than 1 m in height. The linearity of the individual strands is indicative of steeply dipping faults in the shallow subsurface. Understanding the neotectonic history of the southern margin of the Reelfoot rift and outlying structures, such as the BCFZ, is of critical importance in evaluating seismic hazard in the central United States.

A 450-m-long shear-wave seismic reflection profile was collected across the central Kingtown lineament to provide an assessment of near surface structural deformation. The reflection data were collected in SH mode (sensitive to horizontally polarized shear waves) on a 24-channel seismograph. The active spread, consisting of 24 receivers spaced at 1.5-m intervals, was attached to a portable landstreamer and towed along a hard-packed dirt/gravel road. Seismic energy was generated by five horizontal impacts of a 1.8-kg sledgehammer on a 10-kg steel I-beam oriented perpendicular to the spread.

The processed seismic profile exhibits coherent seismic reflection energy to depths of greater than 50 m. Numerous strong diffractions indicting reflector terminations, offsets in reflections, and abrupt changes in reflection amplitude and coherency suggest the presence of a zone of high-angle faults in the shallow subsurface. Disruption and warping of very shallow reflections shows that some of the faults extend upward to within 5-10 m of the surface.