Abstract
Three-dimensional inversion of regional long-period magnetotelluric (MT) data re- veals the presence of two distinct sets of high-conductivity belts in the Precambrian basement of the eastern U.S. Midcontinent. One set, beneath Missouri, Illinois, Indiana, and western Ohio, is defined by northwest– southeast-oriented conductivity structures; the other set, beneath Kentucky, West Vir- ginia, western Virginia, and eastern Ohio, in- cludes structures that are generally oriented northeast–southwest. The northwest-trend- ing belts occur mainly in Paleoproterozoic crust, and we suggest that their high con- ductivity values are due to graphite precipi- tated within trans-crustal shear zones from intrusion-related CO2-rich fluids. Our MT inversion results indicate that some of these structures dip steeply through the crust and intersect the Moho, which supports an inter- pretation that the shear zones originated as “leaky” transcurrent faults or transforms during the late Paleoproterozoic or the early Mesoproterozoic. The northeast-trending belts are associated with Grenvillian orogen- esis and also potentially with Iapetan rifting, although further work is needed to verify the latter possibility. We interpret the differ- ent geographic positions of these two sets of conductivity belts as reflecting differences in origin and/or crustal rheology, with the northwest-trending belts largely confined to older, stable, pre-Grenville cratonic Lauren- tia, and the northeast-trending belts largely having formed in younger, weaker mar- ginal crust. Notably, these high-conductivity zones spatially correlate with Midcontinent fault-and-fold zones that affect Phanerozoic strata. Stratigraphic evidence indicates that Midcontinent fault-and-fold zones were par- ticularly active during Phanerozoic orogenic events, and some remain seismically active today, so the associated high-conductivity belts likely represent long-lived weaknesses that transect the crust.
Document Type
Article
Publication Date
2023
Funding Information
The conductivity model presented herein is avail- able through the IRIS (EarthScope) Earth Model Collaboration (EMC) (http://ds.iris.edu/ds/products /emc/). The model has also been incorporated into the revised CONUS-MT national-scale conductiv- ity model of the contiguous United States (CONUS- MT-2021, https://doi.org/10.17611/dp/emc.2021 .conusmt.1; Murphy et al., 2023). We thank Jamey Jones, Brandon Chase, and an anonymous reviewer for helpful feedback on this work, and we thank Krissy Lewis, Brian Shiro, and Janet Carter for fa- cilitating U.S. Geological Survey internal review. B.S. Murphy acknowledges support from the National Science Foundation Graduate Research Fellowship Program under grant 1314109-DGE to Oregon State University for the beginning of this work and support through the U.S. Geological Survey Mendenhall Re- search Fellowship Program for the conclusion of this work. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorse- ment by the U.S. government.
Repository Citation
Murphy, Benjamin S.; DeLucia, Michael S.; Marshak, Stephen; Ravat, Dhananjay; and Bedrosian, Paul A., "Magnetotelluric insights into the formation and reactivation of trans-crustal shear zones in Precambrian basement of the eastern U.S. Midcontinent" (2023). Earth and Environmental Sciences Faculty Publications. 39.
https://uknowledge.uky.edu/ees_facpub/39
Notes/Citation Information
© 2023 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license.