THE COMMERCE GEOPHYSICAL LINEAMENT AND ITS POSSIBLE RELATION TO PROTEROZOIC IGNEOUS COMPLEXES AND LARGE EARTHQUAKES IN THE CENTRAL ILLINOIS BASIN

New 2D and 3D inversions of magnetic and gravity data lead to insights on upper crustal structures in the central Illinois Basin. These results suggest that the source of Commerce geophysical lineament follows the southeast boundary of a dense and magnetic, northeast-trending igneous center, which we call the Vincennes igneous center. Thus the Commerce geophysical lineament or "Commerce deformation zone," possibly related to a rifted margin or a Late Precambrian accretionary boundary, appears to have influenced the structural development of the proposed Vincennes igneous center. Overlying this igneous center is the Proterozoic Centralia seismic reflection sequence, expressed as highly coherent reflectors. We suggest that the Vincennes igneous center is the source of inferred volcanic units of the Centralia sequence. A comparison of gravity and magnetic fields over the Vincennes igneous center and the St. Francois Mountains suggests that the associated sources in these regions are similar in composition and perhaps origin. Such a conclusion combined with drillhole data leads us to speculate that ~ 1.4-Ga-year-old volcanic fields of high-silica rocks with more abundant mafic components at depth characterize basement in the regions of the Vincennes igneous center and the St. Francois Mountains. Further, we conclude that the Commerce deformation zone evolved in the Precambrian (1.1 to 1.5 Ga) as a major cratonic structural boundary, associated with the proposed volcanism, and was the focus of episodic reactivation related to varying stress regimes throughout its history.

Spatial relations of the Commerce deformation zone with large earthquakes associated with Quaternary and Holocene faulting suggest that the zone represents a major crustal feature intimately related to both surface and deep structures and to the seismic hazard of the Illinois Basin region. Assuming recent right-lateral slip along the Commerce deformation zone, we propose that a jog or left-step in the Vincennes area leads to a restraining bend, where associated stress accumulations may have caused nearby large prehistoric earthquakes and may also be the nucleation zone of future large earthquakes.