STRUCTURAL EVIDENCE OF THE COMPLEX TECTONIC HISTORY OF THE LEXINGTON FAULT SYSTEM, KENTUCKY

The north-northeast trending Lexington Fault System (LFS) in Central Kentucky has been described in the literature as a series of sub-parallel normal faults associated with post-Mississippian extension. We investigated this fault system to document the occurrence of structures that are spatially and temporally related to LFS and to constrain its structural and tectonic history within the framework of the regional geology of the Appalachian Foreland Basin (AFB). The AFB strata in the study area consist mainly of Ordovician to Mississippian limestones, locally interbedded with shales. LFS coincides with the eastern flank of the Cincinnati Arch, a probable forebulge to the Appalachian Orogenic Belt. High-angle reverse and thrust faults are predominant structures along LFS with northeast to easterly orientations and occur as discrete fault populations with a spacing of 1 to 4 m in outcrop and steeply to moderately dipping planes. Limited mineral and slickenside lineations on fault planes display subhorizontal (4° to 35°) plunges suggesting oblique-slip along many of the LFS components. Macroscopic-scale open folds with gently eastward plunging fold axes are common in the central section of the study area and appear to be temporally associated with contractional faulting that produced the LFS. Normal faults, although to a lesser extent, occur in the same rocks and change in strike from northeast in the northern and central sections of the LFS to west-northwest in the south. We interpret the origin of contractional reverse/thrust faults and folds as a result of regional shortening in the Appalachian Foreland Basin during the latest stages of the orogenic buildup in the Late Paleozoic. Extensional normal faults were probably a result of flexural loading/subsidence within this basin. This inferred origin of the LFS and its relation to the AFB are compatible with the evolution of foreland basins in other orogenic belts. We do not think that LFS represents a discrete, post-orogenic extensional deformation feature and a local rift structure within AFB, as previous studies have suggested.