Structural Underpinnings and Neotectonics of the Southern Illinois Basin: An Overview

Abstract

The southern end of the Illinois Basin is one of the most structurally complex regions in the Midcontinent United States. Two major structural elements characterize this part of the basin: (1) A broad southwestward-plunging cratonic depression extends across central Illinois and southwestern Indiana. Investigations of historical and prehistorical earthquakes in this part of the basin indicate that moderate to high earthquake potential exists; the seismogenic sources, however, remain enigmatic. (2) The southernmost part of the basin is underlain by the Reelfoot Rift and Rough Creek Graben, a rift system that formed during late Precambrian to Middle Cambrian time. Geodynamic processes operating within the rift since the late Precambrian have had a major influence on the tectonic, diagenetic, and depositional history of the region. In addition, tectonic compressive stress appears to be reactivating ancient faults within the Reelfoot Rift, resulting in coherent linear segments of earthquake epicenters called the New Madrid Seismic Zone. Geological and geophysical information suggests that the cause of earthquakes in the New Madrid Seismic Zone is unrelated to that in the region north of the rift system.

The 6 June 2003 Bardwell, Kentucky, Earthquake Sequence: Evidence for a Locally Perturbed Stress Field in the Mississippi Embayment

“…The Reelfoot rift may be underlain by a rift pillow, a high-velocity and highdensity crustal layer 30-40 km deep (Ginzburg et al, 1983;Mooney et al 1983). The east-trending Rough Creek graben is approximately the same age as and contiguous with the Reelfoot rift (Kolata and Hildenbrand, 1997).…”

Introduction: Seismicity, Quaternary Faulting, and Seismic Hazard

“…Carefully focused efforts to identify the geological controls on Illinois basin seismicity and hazard may eventually lead to reduced uncertainty in hazard assessments throughout the cratonic part of the North American midcontinent. Note that the boundaries of the Illinois basin are drawn in different places by different authors, according to the geographic extent of the stratigraphic horizon or interval of interest (for example, Buschbach and Kolata, 1991;Oltz et al, 1991;Pryor et al, 1991;Nelson, 1995;Kolata and Hildenbrand, 1997). The two parts of the midcontinent of the United States and southern Canada that least resemble the Illinois basin are the generally nonseismic region bordering the Gulf of Mexico and the intensely active Reelfoot rift that includes the NMSZ.…”

“…The basin is part of the craton, being geologically and tectonically similar to most of the North American midcontinent between the Appalachian and Rocky Mountains. The chosen boundary is the one shown by Kolata and Hildenbrand (1997, Figure 1). Both of these areas underwent younger rifting than the rest of the midcontinent and therefore are no longer part of the craton (Johnston, 1989(Johnston, , 1994Kanter, 1994).…”

Seismotectonic map showing faults, igneous rocks, and geophysical and neotectonic features in the vicinity of the lower Wabash Valley, Illinois, Indiana, and Kentucky

“…The map area is in the southern part of the Illinois Basin (Kolata and Hildenbrand, 1997). Most of the map area is underlain by flat-lying or gently dipping upper and middle Paleozoic platform strata of the midcontinent (Willman and others, 1967;McDowell and others, 1981;Gray and others, 1987;Noger, 1988).…”

“…Thus, the sparsely seismic Rough Creek Graben separates the Reelfoot Rift and its seismicity from the Grayville Graben, the Wabash Valley Fault System, and the scattered northern seismicity. The nature of the structural and seismological connection, if any, between the New Madrid seismic zone of the Reelfoot Rift and the scattered seismicity in and around the northern half of the map area remains unclear (Hildenbrand and Ravat, 1997;Kolata and Hildenbrand, 1997;Wheeler, 1997; see also other papers in Seismological Research Letters,v. 68,no.…”