Understanding basement tectonics of an interior cratonic basin: southern Illinois Basin, USA

Abstract

Although the Illinois Basin is one of the world's most studied interior cratonic basins, little is known of its deep structure or seismotectonic framework. In this study, seismic reflection profiles over the major structures of the basin reveal reverse faults that penetrate the upper Precambrian crust, disrupt the surface of Precambrian basement, and accommodate folding of Paleozoic sediments. Observations of deformation indicate concordant folding of the basement surface and overlying strata and/or possible `Laramide-style' basement-controlled folding. Most profiles show evidence of dual fold vergence and some profiles suggest transpression involving lower Paleozoic strata and the basement surface. The moderate seismicity in the southern Illinois Basin, just north of the New Madrid seismic zone (NMSZ), defines an active deformation regime of interpreted NNE-striking dextral strike-slip and reverse faulting. Upper crustal fault structure, such as shown in this study, could provide a fabric capable of reactivation by stress manifested as contemporary seismicity. A tectonic framework for the possible kinematic linkage of deformation in the NMSZ and the southern Illinois Basin is proposed on the basis of a hypothetical distribution of branched fault (and/or fold) patterns in southern Illinois that broaden northward beyond the NMSZ. Such a dispersive deformation pattern is thought to be associated with the gradual cessation in displacement north of the NMSZ (i.e., displacement is dissipated over many faults). The abrupt change in seismotectonic regime from a relatively discrete, highly seismic NMSZ with infrequent great earthquakes to a multi-stranded fault pattern with more frequent moderate earthquakes in southern Illinois is typical of active strike-slip systems involving a bend or shift in direction.