The Missouri High-Conductivity Belt, revealed by magnetotelluric imaging: Evidence of a trans-lithospheric shear zone beneath the Ozark Plateau, Midcontinent USA?

Abstract

Crust in the central Midcontinent of the USA, part of North America's intracratonic platform, consists of Precambrian crystalline basement overlain by a cover of Phanerozoic sedimentary strata. Analysis of EarthScope long-period magnetotelluric (MT) data provides an opportunity to characterize lithosphere-scale structures of this region. Inversion of these data reveals a distinct belt of high electrical conductivity (≤30 Ωm) traversing Missouri. This belt strikes northwest-southeast, dips steeply to the northeast, and extends at least from the mid-crust into the lithospheric mantle. Significantly, this “Missouri high-conductivity belt” (MHCB) spatially coincides with the Missouri Gravity Low (a distinctive feature on Bouguer anomaly maps), lies adjacent to the southwestern edge of Precambrian bedrock exposure in the Ozark Plateau, trends parallel to both magnetic-anomaly lineaments and mapped fault traces, and aligns with lateral steps (transfer or transform faults) in the Midcontinent rift and the Reelfoot rift. We suggest that the MHCB initiated as a lithosphere-scale transtensional shear zone, which linked rift segments that were active during Proterozoic extensional events. Conceivably, pull-aparts within this shear zone localized felsic igneous activity and produced depressions that filled with sediment in association with the development of the 1.5–1.3 Ga Eastern Granite-Rhyolite Province, yielding a region of less-dense crust that could account for the Missouri Gravity Low. The shear zones likely also provided pathways for fluid migration. High conductivity in the MHCB may reflect deep crustal and lithospheric mantle alteration in response to the passage of fluids during dilatational shearing. We postulate that fluid movement accompanied by shear resulted in graphite and perhaps sulfide concentration along the shear zone in the lower crust, and hydration and/or grain diminution and crystallographic preferred orientation in the lithospheric mantle. This alteration also may have weakened lithosphere along the MHCB, explaining the spatial association of the belt with Paleozoic reactivated faults and with contemporary seismicity.