A NW-TRENDING TRANS-LITHOSPHERIC SHEAR ZONE CUTTING THROUGH THE USA MIDCONTINENT, AS REVEALED BY MAGNETOTELLURIC (MT) IMAGING

Analysis of EarthScope long-period magnetotelluric (MT) data provides an opportunity to characterize lithosphere-scale structures of North America's cratonic platform in the Midcontinent. 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 middle crust into the lithospheric mantle. Significantly, this “Missouri high-conductivity belt” (MHCB) spatially coincides with the Missouri Gravity Low (a distinctive yet enigmatic feature on gravity maps recognized in the early 1980s), trends parallel to both magnetic anomaly lineaments and fault traces, underlies major ore deposits, and aligns with transfer or transform faults in the Midcontinent rift and the Reelfoot rift. Furthermore, it lies within a previously recognized series of surface structures and geophysical features called the “Dakota-Carolina corridor,” and is parallel to the Mojave-Sonora Megashear. Resolution tests of our MT data emphasizes that the MHCB appears to extend down to a depth of ~200 km. We suggest that the MHCB initiated either as a purely trans-tensional shear zone, or as an extensional, and later transtensional shear zone associated with Proterozoic rifting. 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 presumably weakened the lithosphere along the MHCB, explaining the spatial association of the belt with Paleozoic reactivated faults and with contemporary seismicity. The length, deep conductivity, continuous dip, and correlation with other geologic and geophysical features of this zone differ from those of other Midcontinental fault zones in the USA. The MHCB shear zone does, however, resemble examples of known trans-lithospheric fault zones (such as the Altyn Tagh and San Andreas), though with a longer history of reactivation. This similarity suggests that the MHCB shear zone accommodated significant displacement during the tectonic evolution of the craton.