INSIGHTS ON THE COMMERCE GEOPHYSICAL LINEAMENT (CGL), THE SOUTH CENTRAL MAGNETIC LINEAMENT (SCML), AND PROTEROZOIC IGNEOUS COMPLEXES IN THE EASTERN GRANITE RHYOLITE PROVINCE

Transformations and inversions of existing magnetic and gravity data provide insights on the structural underpinnings of the Illinois Basin and surrounding region. The results of 2D and 3D inversion techniques suggest that the northeast-trending Commerce geophysical lineament (CGL) (a 10-15 km wide deformation zone) follows the southeast boundary of a dense and magnetic igneous center named the Vincennes igneous center (VIC). The VIC may have contributed to the extrusion/intrusion of igneous material of the overlying Centralia volcanoclastic sequence, expressed as highly coherent seismic reflectors. A comparison of gravity and magnetic fields over the VIC and the St. Francois mountains suggests that the basement of the VIC may be analogous to the ~1.45-Ga-old St. Francois granite-rhyolite complex with high-silica rocks in the upper crust that are genetically related to more abundant mafic components at depth. We suggest that numerous such volcanic centers, although not exposed, are expressed in the potential-field data as coincident and roughly circular gravity and magnetic anomalies. These centers may have been the source of the massive volumes of granites and rhyolites characterizing this province. Another prominent geophysical element with bearing on the tectonic evolution of this region is the South-central magnetic lineament (SCML), a conspicuously linear, northwest-trending zone of strong magnetic anomalies (positive to the northeast and negative to the southwest) crossing the CGL. The analytic signal of the magnetic data highlights the associated magnetic igneous sources, perhaps related to Proterozoic incipient rifting. The overall patterns of the analytic signal field of the Illinois Basin, the Rough Creek graben, and the Reelfoot rift suggest that the source of the SCML may be younger than the ~1.45-Ga-old eastern granite-rhyolite terrain. The SCML terminates in the southeast in Tennessee against gravity and magnetic anomaly patterns similar to those associated with the Grenville front in Ohio, suggesting that the associated magmatic event occurred during or prior to the Grenville orogeny.