Architecture of the hidden Penokean terrane suture and Midcontinent rift system overprint in eastern Minnesota and western Wisconsin from magnetotelluric profiling

Abstract

We resolved the architecture of the early Proterozoic Penokean orogen suture and late middle Proterozoic (Keweenawan) Midcontinent rift system magmatic overprint in east-central Minnesota and western Wisconsin through recovery and analysis of a legacy magnetotelluric (MT) data set. We digitized printed plots of off-diagonal MT and controlled-source audio (CSA) MT responses, including error intervals, to provide 22 soundings along a profile of ~225 km length extending from north of Lake Mille Lacs, Minnesota, southeastward to Flambeau Ridge, Wisconsin. The MT data were inverted to a smoothed electrical resistivity structure using a two-dimensional finite-element, regularized Gauss-Newton algorithm emphasizing the transverse magnetic (TM) mode data subset. Our model reveals a major electrically conductive zone dipping moderately to the southeast for >50 km in the 5-35 km depth range, which marks the probable Penokean suture in easternmost Minnesota. We interpret the conductor to reflect a package of graphitized metasediments of the former Archean continental margin and near foreland zone, underthrust as the Penokean terrane collided with the Superior Province. The large-scale conductor is now hidden beneath mainly Yavapai-aged plutonic rocks of the East-Central Minnesota batholith. Below the axis of the later Midcontinent rift (St. Croix horst, subsequently), a compact resistive body ranging from 5 to 20 km deep overlies the large conductor. We interpret this resistor to be mafic volcanic and intrusive rocks of the Midcontinent rift event, which correlate spatially with a high Bouguer gravity anomaly similarly modeled. The rift here coincides with the lower-crustal reaches of the suture, but the specific influence of the suture on rift emplacement is unclear.

Magnetotelluric Sampling and Geoelectric Hazard Estimation: Are National‐Scale Surveys Sufficient?

“…Bedrosian, 2016) crystalline intrusive rocks. In the southern and eastern portions of this focus region, relatively electrically conductive (~100 Ωm; Wunderman et al, 2018) Paleozoic and Mesozoic sedimentary rocks, which reach thicknesses of more than a kilometer (Marshak et al, 2017), overlie these electrically heterogeneous Precambrian domains.…”

“…We use geomagnetic time series records of the St. Patrick's Day Geomagnetic Storm of March 17-18, 2015 (Dst = −223 nT;Wu et al, 2016), from U.S. Geological Survey (USGS; Love & Finn, 2011) and Natural Resources Canada (NRCan; Newitt & Coles, 2007) geomagnetic observatories for the driving magnetic fields in our analyses here. We specifically use 1-s definitive data, which are provided in the cardinal geographic coordinate frame.…”

“…The major constituent geoelectric structural domains for the Northern Midcontinent, as well as the MT data and transmission lines used in this regional analysis, are shown in Figure 5a. The Archean Superior Province, part of the ancient cratonic core of the North American continent, is characterized by bulk electrically resistive lithosphere (>1,000 Ωm; Bedrosian, 2016; Wunderman et al., 2018; Yang et al., 2015). The Paleoproterozoic Basins are the result of continental margin and subsequent orogenic processes (e.g., Sims et al., 1993); they comprise variably deformed crystalline basement and supracrustal (i.e., sedimentary and volcanic) rocks that vary widely in electrical properties.…”

Hypogenic Versus Epigenic Origin of Deep Underwater Caves Illustrated by the Hranice Abyss (Czech Republic)—The World's Deepest Freshwater Cave

“…A combination of geophysical techniques including electrical resistivity tomography (ERT), gravity, and seismic methods (e.g., Válois et al, 2011) can be beneficial for characterization of the near‐surface structure of karst regions. Information about the deeper structure and morphology requires the use of deeper penetrating techniques such as magnetotellurics (e.g., Hill et al, 2015; Wunderman et al, 2018) and/or using deep‐reaching audiomagnetotellurics (AMT) in combination with expanded investigation scales of geophysical methods (resistivity, gravity, and seismic) typically used for near‐surface investigations (Blecha et al, 2018).…”

Three‐Dimensional Electrical Resistivity Characterization of Mountain Pass, California and Surrounding Region

“…Magnetotellurics (MT) is a common tool for imaging electrically conductive structures associated with mineral deposits (e.g., Cherevatova et al., 2015; Fernandes et al., 2018; Hayward et al., 2016; Hübert et al., 2009; Kalscheuer et al., 2017; Padilha et al., 2013; Thiel et al., 2016; Wannamaker and Doerner, 2002; Lindsay et al., 2017; Wunderman et al., 2018). A demonstrative example is from South Australia where MT has been used to image the subsurface below Olympic Dam, one of the world's largest iron‐oxide copper gold (IOGC) deposits (Heinson et al., 2006, 2018; Thiel & Heinson, 2013).…”