GEOELECTRIC IMAGES OF THE EASTERN US MIDCONTINENT: IMPLICATIONS FOR TECTONIC HISTORY AND MINERAL RESOURCE POTENTIAL

The Proterozoic history of the Eastern US Midcontinent has long been enigmatic due to the thick Phanerozoic sedimentary sequence that obscures basement geology. In order to provide new insights into the lithospheric architecture and history of this poorly understood region, we present new regional electrical conductivity images derived from long-period magnetotelluric data. Inverse solutions reveal a series of sub-vertical, northwest-southeast-trending (NW-SE) mid-lower crustal (~20-50 km depth) conductors beneath Missouri, Illinois, and Indiana. Expanding upon previous work (DeLucia et al., Tectonophysics, 2019), we interpret these NW-SE conductors as representing lithosphere-scale transtensional shear zones. These structures may have formed in association with the ~1.4 Ga Eastern Granite-Rhyolite magmatic event and could have served as conduits along which mantle-derived mafic magmas were channeled into the lower crust to generate large volumes of silicic magmas. Alternatively, these structures could have developed in association with younger Mesoproterozoic rifting. Significantly, these NW-SE conductors align with mapped deformation zones within Paleozoic strata; our geoelectric images provide support for the inference that Midcontinent fold-and-fault belts are related to Precambrian basement structures. These NW-SE conductors are truncated to the southeast, in southern Illinois and central Indiana, by a northeast-southwest (NE-SW) striking mid-lower crustal conductor that extends northeastward into northwestern Ohio. This conductor is subparallel to, but not coincident with, the Nd line; however, it is at least partially coincident with the Commerce Geophysical Lineament and with fault traces in overlying Phanerozoic sedimentary units. We tentatively interpret this NE-SW conductor as representing an early-mid Mesoproterozoic (pre-Grenville) suture zone. In addition to providing new, valuable insights into the lithospheric architecture of this enigmatic region, our electrical conductivity images highlight regions of the Midcontinent that may host Proterozoic-aged hydrothermal or magmatic-hydrothermal mineralization concealed under a relatively thin veneer (several hundred meters, potentially permissive for resource exploration) of Phanerozoic cover.