Upper crust beneath the central Illinois basin, United States

Newly available industry seismic reflection data provide critical informati on for understanding the structure and origin of the upper crust (0-12 km d epth) beneath the central Illinois basin and the seismic-tectonic framework north of the New Madrid seismic zone in the central Mississippi Valley. Ma pping of reflector sequences furnishes the first broad three-dimensional pe rspective of the structure of Precambrian basement beneath the central Unit ed States Midcontinent, The highly coherent basement reflectivity is expres sed as a synformal wedge of dipping and subhorizontal reflections situated beneath the center of the Illinois basin that thickens and deepens to the n ortheast (e.g., 0 to similar to 5.3 km thickness along a 123 km south to no rth line). The thickening trend of the wedge qualitatively mimics the north ward thickening of the Late Cambrian Mt. Simon Sandstone; however, other Pa leozoic units in the Illinois basin generally thicken southward into the ba sin center. The seismic data also reveal an anomalous subsequence defined b y a spoon-shaped distribution of disrupted reflections located along the so uthern margin of the wedge, The boundaries of this subsequence are marked b y distinct steeply dipping reflections (possible thrust faults?) that conti nue or project up to antiformal disruptions of lower Paleozoic marker refle ctors, suggesting Paleozoic or possibly later tectonic reactivation of Prec ambrian structure. The areal extent of the subsequence appears to roughly c orrespond to an anomalous concentration of larger magnitude upper to middle crustal earthquakes. There are multiple hypotheses for the origin of the P recambrian reflectivity, including basaltic flows or sills interlayered wit h elastic sediments and/or emplaced within felsic igneous rocks. Such expla nations are analogous to nearby Keweenawan rift-related volcanism and sedim entation, which initiated during Proterozoic rifting, and were followed eve ntually by reverse faulting along the rift margins caused by Grenville comp ression.