Seismic reflection images of shallow faulting, northernmost Mississippi embayment, north of the New Madrid seismic zone

High-resolution seismic reflection surveys document tectonic faults that di splace Pleistocene and older strata just beyond the northeast termination o f the New Madrid seismic zone, at the northernmost extent of the Mississipp i embayment. These faults, which are part of the Fluorspar Area fault compl ex in southeastern Illinois, are directly in line with the northeast-trendi ng seismic zone. The reflection data were acquired using an elastic weight- drop source recorded to 500 msec by a 48-geophone array (24-fold) with a 10 -ft (similar to3.0 m) station interval. Recognizable reflections were recor ded to about 200 msec (100-150 m). The effects of multiple reflections, num erous diffractions, low apparent velocity (i.e., steeply dipping) noise, an d the relatively low-frequency content of the recorded signal provided chal lenges for data processing and interpreting subtle fault offsets. Data proc essing steps that were critical to the detection of faults included residua l statics, post-stack migration, deconvolution, and noise-reduction filteri ng. Seismic migration was crucial for detecting and mitigating complex faul t-related diffraction patterns, which produced an apparent 'folding' of ref lectors on unmigrated sections. Detected individual offsets of shallow refl ectors range from 5 to 10 m for the top of Paleozoic bedrock and younger st rata. The migrated sections generally indicate vertical to steeply dipping normal and reverse faults, which in places outline small horsts and/or grab ens. Tilting or folding of stratal reflectors associated with faulting is a lso locally observed. At one site, the observed faulting is superimposed ov er a prominent antiformal structure, which may itself be a product of the Q uaternary deformation that produced the steep normal and reverse faults. Ou r results suggest that faulting of the Paleozoic bedrock and younger sedime nts of the northern Mississippi embayment is more pervasive and less locali zed than previously thought.