Architecture of the Hidden Penokean Terrane Suture and Midcontinent Rift System Overprint in Eastern Minnesota and Western Wisconsin from Magnetotelluric Profiling

RESEARCH

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

R.L. Wunderman1,*, P.E. Wannamaker2,†, and C.T. Young1,§ 1DEPARTMENT OF GEOLOGICAL AND MINING ENGINEERING AND SCIENCES, MICHIGAN TECHNOLOGICAL UNIVERSITY, 1400 TOWNSEND DRIVE, HOUGHTON, MICHIGAN 49931, USA 2ENERGY & GEOSCIENCE INSTITUTE, UNIVERSITY OF UTAH, 423 WAKARA WAY, SUITE 300, SALT LAKE CITY, UTAH 84108, USA

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 Flam- beau 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 electri- cally 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.

LITHOSPHERE; v. 10; no. 2; p. 291–300; GSA Data Repository Item 2018112 | Published online 1 March 2018 https://doi.org/10.1130/L716.1

INTRODUCTION end of our magnetotelluric data set coverage overlies these gneisses and the Isle and Warman granitic panel of the East-Central Minnesota batholith. The northern midwestern region of the United States, and in particu- The gneisses appear to be remobilized from the Archean Minnesota River lar the Lake Superior region, contains unique elements in the construc- Valley terrane, more preserved to the southwest, which in turn is separated tion and evolution of North America. Here lie remains of the Penokean from the Superior granitoid Wawa terrane by the Great Lakes tectonic compressional orogeny, the oldest Proterozoic accretionary orogen along zone (Fig. 1; Chandler et al., 2007; Bauer et al., 2011; Southwick, 2014). southern Laurentia (Van Schmus et al., 2007; Schulz and Cannon, 2007; The Keweenawan Midcontinent rift system was imposed upon the ter- Whitmeyer and Karlstrom, 2007). It was the forerunner of Laurentia’s rane suture in our study area at ca. 1100 Ma. This event nearly separated southward growth from the Archean Superior core via island-arc and small the ancient continent and imparted enormous volumes of mafic magma terrane assembly (Fig. 1). In Michigan and Wisconsin to the northeast to various depths in the crust (Hinze et al., 1997; Nicholson et al., 1997; of our study, the Superior-Penokean suture is marked by the well-known Vervoort et al., 2007; Stein et al., 2015), engendering numerous high-grade Niagara fault zone. In Minnesota, where our study lies, the suture has been base and precious metal deposits (Miller and Nicholson, 2013). Northeast- associated with the Malmo structural discontinuity, but it is considered to ward from our transect area in Michigan, a strong clockwise rotation of have been extensively disrupted and obscured by post-Penokean gneiss Midcontinent rift system strike regionally appears to be primary and not upwelling and plutonism, for example, by the McGrath gneiss dome and subsequent to emplacement, suggesting influence by preexisting large- the East-Central Minnesota batholith (Schulz and Cannon, 2007; Holm et scale structures in the evolution of the Midcontinent rift system (Hnat et al., 2007; Chandler et al., 2007, 2008; Boerboom et al., 2011). The west al., 2006). Similarly, major Penokean structures continuing eastward into Ontario, Canada, may have influenced rift orientation there (Manson and Phil Wannamaker http://orcid.org/0000-0001-5744-6474 Halls, 1997; Riller et al., 1999). Recently, the Midcontinent rift system has *Current affiliation: Global Volcanism Program, Smithsonian Institution, National been interpreted as the failed arm of a triple junction as seafloor spread- Museum of Natural History, 1000 Constitution Avenue NW, Washington, DC 22560, ing between Laurentia and Amazonia was established (Stein et al., 2015, USA (Retired) †Corresponding Author: [email protected] 2016). The volume of magmatism appears to require mantle hotspot input §Emeritus beyond passive rifting (Stein et al., 2015, 2016).

-95.5-94 -92.5-91 -89.5 47.5 47.5

46 46

44.5 44.5

-95.5-94 -92.5-91 -89.5 Units of Penokean External Domain Units of Penokean Internal Domain Animikie Group & Marquette Range Groups Xam Xmg Early Proterozoic gabbro plutons - Ea Prot. foreland deposit Mille Lacs Group and North Range Groups Xmn Xgd Late tectonic granite plutons emplaced ~1835 Ma - Ea Prot. continental margin rocks Little Falls Formation Pembine-Wausau terrane Xlf Xgg - Ea Prot. argillite, slate, pelitic schist - Ea Prot. syntectonic granites Archean gneiss & Ea Prot. volc/sed rocks Wgd Xtg Pembine-Wausau terrane metamorphic arc rocks - metamorphosed in Penokean and Yavapai Marshfield terrane - Mainly Late Archean gneisses Wwa Archean rocks of Superior (Wawa) Province Wgn - metamorphosed in Penokean orogeny Ea Archean gneiss of Minnesota River Valley Amv - weakly metamorphosed in Penokean Keweenawan and Paleozoic Units Pz Paleozoic sedimentary rocks Post-Penokean Units Middle Proterozoic Keweenawan fine grained Zks Xwg Wolf River Batholith - 1450 Ma sedimentary rocks Quartzite of Ea Prot. "Baraboo interval" Middle Proterozoic Keweenawan volcanic rocks - Xbq Zkv - <1750 Ma basalt and rhyolite

Xgr Granitic rocks emplaced 1850-1760 Ma Modern Lake

MT CSAMT SPREE SN Normal Fault Thrust Fault Figure 1. Generalized basement geologic map of the Penokean orogen and Midcontinent rift system (MRS). Magnetotelluric (MT) sites are red squares, and controlled-source audio (CSA) MT sites are red dots. The Wgn unit in our study area and to the west-southwest comprises the McGrath–Little Falls gneiss panel, including the particularly uplifted McGrath Dome (MG). Primary normal faults of the Midcontinent rift system regime are the Douglas fault (DF) and Lake Owens fault (LOF), which may have been reactivated in later compression to form the St. Croix horst (SCH). Four MT sites that were examined in closer detail are labeled (e.g., N03). Red profile line approximates location of the model views of Figures 4 and 5. Urban or geographic features noted include Duluth (DL), Aitkin (AT), Saint Paul (SP), Ironwood (IW), and Flambeau Ridge (FR). Geologic units are from digital base map of Cannon et al. (1999), modified from work of Ojakangas et al. (2001), Schulz and Cannon (2007), Van Schmus et al. (2007), Chandler et al. (2007, 2008), Holm et al. (2007), and Boerboom et al. (2011). Inset shows simplified regional basement geology mainly after Southwick (2014). Its elements include Superior Wawa Province (SWW), Minnesota River Valley subprovince of the Superior Province (MRV), Penokean foreland terrane (PFT), Midcontinent rift system (MRS), Penokean Pembine-Wausau arc terrane (PPW), Archean Marshland terrane (MAT), and Yavapai terrane (YVP). Major structures include the Great Lakes tectonic zone (GLTZ), Niagara fault zone (NFZ), and Spirit Lake tectonic zone (SLTZ), where the last represents the regional Yavapai terrane suture zone. Abbreviations: ECMB—East-Central Minnesota batholith; EPSZ—Eau Pleine shear zone; MD—Malmo discontinuity; ML—Mille Lacs Lake; LS—Lake Superior; LM—Lake Michigan. Ea Prot—early Proterozoic; SPREE—Superior Province Rifting EarthScope Experiments data.

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Geophysical methods can illuminate deep crustal architecture in the The 14 MT soundings, for which positions are shown in Figure 1, were face of event overprinting and later sedimentary cover. The magnetotellu- obtained in early 1986 by Michigan Technological University (Wunder- ric (MT) method is well known for imaging subsurface electrical resistiv- man, 1988; see Appendix). Sites were located according to land access, ity variations related to temperature and fluid/melt content (Wannamaker avoidance of substantial overburden, and lack of urban development. Also, et al., 2009; Chave and Jones, 2012). However, MT also has a role in the general trend toward the southeast kept the MT profile within the delineating fossil terrane sutures based upon low resistivity created by Penokean Pembine-Wausau arc terrane in that direction rather than enter- underthrust, metamorphosed, and remobilized graphite- or sulfide-bearing ing the Marshfield terrane. The x axis for all wave periods and all sites metasediments (e.g., Boerner et al., 1996; Wannamaker, 2000, 2005; Jones is N060E, which is approximately parallel to overall strike of both the et al., 2005; Yang et al., 2015; Bedrosian, 2016). The Paleoproterozoic Penokean suture and the Midcontinent rift system in this area. The wave Era was prodigious in creation and preservation of organic matter and period range of the data set is ~0.01 to 300 s (frequencies of 100–0.0033 sulfide in sedimentary sections along continental margins (Des Marais Hz), which we show is sensitive to resistivity structure from around 300 et al., 1992; Des Marais, 1994; Canfield, 2005). The Penokean domain m to 100 km depth. The eight CSAMT soundings, also located in Figure has been known since the early MT data of Dowling (1970) to possess 1, extend over a distance of ~17.5 km and were obtained under private deep crustal resistivity over broad scales that is lower than that expected contract (EMS Inc., 1985; see Appendix). Only ρxy and ρyx values were for purely crystalline silicate rocks of cratonic interiors. recorded (no φ) at seven discrete frequencies over a range of 1000–10 Hz. During the middle 1980s, 14 tensor MT soundings plus a dense profile This yields a useful depth of exploration of ~5 km. Given the CSAMT of controlled-source audio magnetotelluric (CSAMT) data were acquired profile orientation, the x axis for these sites differs modestly from the in eastern Minnesota and northwestern Wisconsin over the presumed loca- MT data set at N080E. tion of the Superior-Penokean suture from its near foreland area at the The MT and CSAMT data are assembled into pseudosections in northwest end, across the Midcontinent rift system, and into the internal Figure 2, where the location serves as the abscissa and the wave period domains of the accreted Penokean arc (Fig. 1). The MT data acquisition serves as the ordinate for contour plots of ρa and φ. As indicated in Figure was motivated in part by nearly contemporaneous Consortium for Conti- 1, the CSAMT sites lie just north of the Malmo discontinuity upon meta- nental Reflection Profiling (COCORP) and Great Lakes International Mul- morphosed, early Proterozoic deposits of the Mille Lacs Group. These tidisciplinary Program on Crustal Evolution (GLIMPCE) regional seismic rocks originally were continental margin sediments and volcanics, with investigations of the Midcontinent rift system (Wunderman, 1988; Hinze et possible early Penokean foreland sediments (Schulz and Cannon, 2007; al., 1992), while the CSAMT data were taken for mineral resource assess- Boerboom et al., 2011). Two small discrete conductors are evident: the ment by the State of Minnesota Department of Natural Resources (EMS ρa minimum centered on T = 0.01 s under the third CSAMT site from the Inc., 1985). These older data have limitations, as discussed herein, but their profile’s northern end, and a minimum toward T = 0.1 s at the fifth and sixth quality is sufficient to reveal first-order features of the Penokean suture site from that end (Fig. 3). However, the sites otherwise are predominantly zone and the Midcontinent rift system overprint, and to locally focus the resistive (few thousand Ω·m), showing no further evidence of low resistiv- larger-scale resistivity structure imaged by the EarthScope Magnetotelluric ity, at least in the upper crust under this area of the Mille Lacs lithology. Transportable Array (MT TA; Yang et al., 2015; Bedrosian, 2016). In this Close to the southeast portion, over the Snake River area, the west- study, we recovered these results and applied modern methods of inver- ernmost four MT sites show both ρxy and ρyx values plunging toward long 1 sion imaging (for analysed MT data file, see GSA Data Repository Item ). periods with high values of φxy and φyx (Fig. 2). These lie mainly upon high-grade Archean-cored gneisses of the McGrath–Little Falls domain RECORDED MT/CSAMT DATA and post-Penokean plutons such as the Isle and Warman granitic panel of the East-Central Minnesota batholith (Chandler et al., 2008; see Fig. 1). The MT method makes use of naturally occurring, low-frequency elec- This behavior signifies a large-scale conductor at depth in the crust, which tromagnetic (EM) wave fields as sources for imaging subsurface electrical we will argue represents remnants of the Penokean suture in our study resistivity (or its inverse, conductivity) structure (Chave and Jones, 2012). area. We note that the westernmost MT site (N03), in particular, shows

Propagation of MT wave fields in the conducting media of Earth is diffusive; rising ρyx and low φyx values for T > 10 s (Fig. 3A, feature A) relative to for example, in a uniform half-space, penetration depths are determined by sites immediately to the southeast (Fig. 3B, feature C in site S01). This an e-folding distance (skin depth), which is dependent upon wave period suggests that the large conductor is pinching out toward the surface to (inverse of frequency) and resistivity. The observed horizontal electric (E) the northwest and is underlain by resistive material. At site S01, a maxi- and magnetic (H) fields in the frequency domain are related in a tensor mum in ρyx and a low in φyx develop at high frequencies (feature B in Fig. fashion to provide apparent resistivities (ρa) and impedance phases (φ; see 3B), which expand in magnitude for sites southeastward, as seen in the Appendix), which are primary MT response functions determined solely by pseudosections. This reflects an increase in depth to top of the crustal Earth properties. The x and y coordinate orientations of the responses com- conductor in that direction. monly have x assigned to geological strike, if strike is apparent, so that ρxy Further southeast, below St. Croix State Park and Clam Falls area, and φxy in a two-dimensional (2D) situation correspond to electric current over the core of the Midcontinent rift system, ρxy and ρyx rise to moder- flow parallel to strike (transverse electric or TE mode), whileρ yx and φyx ate values (Fig. 2). In particular, there are anomalies of low φxy and φyx correspond to current across strike (transverse magnetic or TM mode) (e.g., in the 0.1 to 2 s period range, signifying a laterally confined resistor in Wannamaker et al., 2009, 2014, 2017). The MT response functions become the middle crust, which we will equate to a cooled rift igneous volume the inputs to nonlinear inversion algorithms, a form of tomography, to derive (Fig. 3C, feature D). Only a relatively weak high in φyx around T = 10 s quantitative and stable resistivity structural models (Chave and Jones, 2012). persists southeastward along the profile, representing a fading amplitude for the large dipping conductor. Reaching the Rice Lake and Flambeau Ridge area, near the southeast end of the MT profile, very high values 1 ρa GSA Data Repository Item 2018112, Text file of MT/CSAMT response functions appear, corresponding to the Penokean internal (arc) domain. However, analyzed in this paper, with terms defined in the header information, is available at http://www.geosociety.org/datarepository/2018, or on request from editing@ somewhat elevated values of φxy and φyx, spanning the 1–100 s range, geosociety.org. reveal that resistivity in the lowermost crust may diminish slightly to

LITHOSPHERE | Volume 10 | Number 2 | www.gsapubs.org 293 WUNDERMAN ET AL.

)/ *' 31.6 5,)7 31., )/ *' 31.6 5,)7 31.,

0'0/ ;JU 65) =NY 6&3 &) 3] 5/ )5 0'0/ ;JU 65) =NY 6&3 &) 3] 5/ )5 ȡ\[ ȍP 6 1 6 6 6 1 6 6 ORJ 7 V A E ੮ \[ GHJ ORJ 7 V B F ȡ ȍP [\ ORJ 7 V C G ੮ GHJ [\ ORJ 7 V D H NP NP 1: 2EV 6( 1: &DO 6( Figure 2. Pseudosections of magnetotelluric (MT) responses for the eight high-frequency controlled-source audio (CSA) MT soundings plus the 14 wide- band MT soundings across the Midcontinent rift system and western Penokean orogen, Minnesota and Wisconsin. Observed (Obs) quantities in left column are: (A) nominal TM (yx) mode apparent resistivity; (B) transverse magnetic (TM) mode impedance phase; (C) Transverse Electric (TE) (xy) mode apparent resistivity; and (D) TE mode impedance phase. Corresponding calculated (Cal) responses from the nonlinear two-dimensional (2-D) inversion model are in the right column (E–H). Only the TM mode data were used explicitly in the inversion, so the TE computations simply accompany the TM mode data. Tectonic domains labeled at the top: foreland thrust belt (FL), gneiss doming (GD), Penokean suture (PNK-S), igneous rift domain (RIFT), and Penokean interior (PNK-I). Physiographic features labeled across the top include: Lake Mille Lacs (ML), Snake River State Forest (SRF), St. Croix State Park (SCP), Clam Falls (CF), Rice Lake (RL), and Flambeau Ridge (FR), taken from U.S. Geological Survey 7.5′ and 1:100,000 scale maps. Geologic features include the Malmo discontinuity (MD), post-Penokean East-Central Minnesota batholith pluton (Xgr), Keweenawan surface volcanics (Zkv), and Paleozoic cover (Pz). Locations of four specific MT stations discussed in main text are plotted across the top of both observed and computed results. moderate values (Fig. 3D, feature E). Features such as these will be cor- tectonic context, they provide new insights into the Penokean-Superior related further in the discussion of the inversion model. deep crustal transition. A two-dimensional (2-D) resistivity model was produced from the TWO-DIMENSIONAL (2-D) RESISTIVITY INVERSION MODEL observations in Figure 2 using a stabilized fitting (inversion) algorithm developed in-house that produces smoothed images of the subsurface Although the older MT/CSAMT data presented here have some similar to seismic tomography (Fig. 4). In particular, the inversion empha- limitations, their reinterpretation using modern methods produces an sizes fitting of the TM mode data subsetρ yx and φyx values, because these insightful crustal model for the survey area. The MT data lack the on- data are understood to be more robust (usually) to departures in the 2-D diagonal elements Zxx and Zyy (see Appendix) and so cannot be rotated to assumption (Wannamaker, 1999). Close attention was paid to replicating explore two-dimensionality. They also lack vertical magnetic field (tipper) the responses from the western MT sites into the CSAMT profile, because responses, which provide complementary information about resistivity the Penokean suture is presumed to lie in this area. Further details on strike and lateral variation (Chave and Jones, 2012). Nevertheless, the model construction are described in the Appendix. pronounced drop in ρa and high φ values that the westernmost MT sites The most striking feature of the inversion cross section of Figure 4 is exhibit are coherent and unique to that region. These patterns must cor- the pronounced low-resistivity zone under the MT stations on the East- respond to a first-order conductive structure, and, when interpreted in Central Minnesota batholith in the Snake River area, which exhibits a

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A B 1 [\7( 6 [\7( \[70 \[70 % RKPP RKPP D D ʌ ʌ Figure 3. Four example magneto- ORJ ORJ & telluric (MT) soundings of apparent $ resistivity, ρa, and impedance phase, φ, from the total profile showing data quality and character. Loca- tions are shown in Figure 1. The orientation of assumed average $ GHJ GHJ geoelectric strike is x = N060E. ੮ ੮ Several features pertinent to major inversion structures are labeled A–E and discussed in text. Solid lines are ORJ 7 V ORJ 7 V the computed nominal transverse C D magnetic (TM) mode responses of the inversion model in Figure 4. For site N03, given the agreement between observed and computed

φyx, the modest drop of ρyx below ( the computed value in the 0.3–5 s RKPP ' RKPP period range is believed to repre- D D ʌ ʌ sent magnetic field auto-power bias and not data-model misfit. 6 [\7( 6 [\7( ORJ ORJ \[70 \[70

' GHJ GHJ ੮ ੮

ORJ 7 V ORJ 7 V

)/ *' 31.6 5,)7 31., 0/ 65) 6&3 &) 5/ )5 Figure 4. Two-dimensional (2-D) 0' ;JU =NY 3] nonlinear resistivity inversion ȡ ȍP model of the 22 magnetotelluric (MT) and controlled-source audio (CSA) MT soundings across the Penokean suture–Midcontinent rift system tectonic domain, Min- nesota and Wisconsin. Individual tectonic, physiographic, and geologic features are as in Figure 3. Tectonic, physiographic, and geologic domains are labeled at the 'HSWK NP top as in Figure 2. Strong conductor (warm colors) within the suture is interpreted as graphitized metasediments subducted during Penokean orogen and multiply NP metamorphosed. 1: 6(

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moderate dip to the southeast from ~5 km depth near profile km 30 to good, even though these data were not formally inverted out of con- ~40 km depth near km 100 under the Midcontinent rift system domain. cern for more severe three-dimensional (3-D) effects (see Appendix).

The conductor appears to pinch out at its west end and not extend fully An exception is the compact high ρxy anomaly near profile km 130, just across the McGrath gneiss dome terrane to reach the Mille Lacs Group east of the bounds of the Midcontinent rift system, which persists to lithologies. It is conceivable that a large volume of low resistivities could the longest periods. This behavior is characteristic of the finite strike be hidden somewhere at sufficient depth below the CSAMT profile and length of a structure and cannot be reproduced in a purely 2-D response

not influence the response. Such a geometry was entertained in the early in ρxy and φxy (Wannamaker, 1999). Similarly, the low computed φxy at

forward modeling of Wunderman (1988). However, given the elevated ρa the longest periods (T > 100 s) for the four western MT soundings in values of most CSAMT stations, which imply via skin depth that resis- the Snake River area, unlike the observed values, can be explained by

tivities are high to ~5 km depth, the low values of φyx, the rising ρyx for the finite strike of the large conductive body. Third, the anomalous high

T > 10 s at the northwesternmost MT site N03, and the tight data fit of in φxy is at least as great as that of φyx at the southeast end of the profile the inversion model, some pinchout of the conductor under the McGrath under Rice Lake and Flambeau Ridge, suggesting low resistivity exists gneisses seems likely. somewhere beneath the profile. However, 3-D structural variation can The resistivity section implies that much of the Penokean conductive locally amplify such anomalies to imply higher than actual contrasts zone underlies the Yavapai-aged (post-Penokean) Isle and Warman gran- from a 2-D analysis (e.g., Wannamaker et al., 2017), and so we do not ites of the East-Central Minnesota batholith, which rose buoyantly and wish to overemphasize those observations. mushroomed over this apparent suture. This is where the higher frequencies and denser lateral sampling of the recovered MT/CSAMT data offer LITHOTECTONIC INTERPRETATION OF RESISTIVITY STRUCTURE improvement in resolution over that from the EarthScope MT TA data, having 70 km average spacing and ~10 s as the shortest wave period. In Despite data limitations, the MT resistivity model section provides new particular, this large conductor at depth tends to show up centered under insights with regard to Paleoproterozoic terrane assembly and subsequent the McGrath gneiss dome or even the Malmo discontinuity in the regional late Mesoproterozoic rift-related modification (Fig. 5). We maintain that TA models (Yang et al., 2015; Bedrosian, 2016). The apparent suturing the Penokean suture is marked by a massive, southeast-dipping conductor conductor is distinct from the large, quasi-horizontal ABA conductor of that extends below the post-Penokean East-Central Minnesota batholith Bedrosian (2016) to the north of our study, which lies in the upper crust, and westernmost Keweenawan volcanic cover in east-central Minnesota probably in the younger, more distant foreland zone of the Animikie Basin. and is most prominent in the depth range 5–20 km. As interpreted region- Moving southeast to the main axis of the Midcontinent rift system ally, and in other settings globally, the highly conducting material of the in the St. Croix State Park to Clam Falls area, from profile km 85 to km suture volume is taken to be mainly metamorphic graphite with possible 125, the model conductor visually steps downward and is overlain by a sulfide contributions (Boerner et al., 1996; Wannamaker, 2005; Yang et compact resistor reaching 5000 Ω·m in the 5–20 km depth range. This al., 2015; Bedrosian, 2016). The carbonaceous material generally is of

resistor results from the low values of φyx in the 0.1–2 s range, shown biogenic origin, originally laid down in reduced rift basins or in sediment- in Figures 2 and 3C. Moreover, the deep crustal conductor appears to starved compressional foreland basins (cf. Giles and Dickinson, 1995), terminate below the southeast side of the Midcontinent rift system near subsequently overridden and possibly thickened through imbrication in

profile km 125 as the high inφ yx also diminishes (Fig. 4). Toward the far convergence. Specifically, the suture would be represented by the inter- southeast end of the profile, the limited drop in model resistivity to the face between the highly conducting metasediments associated primarily 500–700 Ω·m range in the deepest crust and uppermost mantle follows with the Superior Province margin and the overlying Penokean arc rocks

from the moderate high in φyx from 1 to 100 s. This structure is considered (Pembine-Wausau terrane) accreted from the southeast. tenuous and, among other things, could be influenced by the tectonically The most likely representative of the carbon-bearing lithology in our related, crustal-scale Flambeau low-resistivity anomaly to the northeast study appears to be early Proterozoic Mille Lacs and North Range schists, of our profile (Sternberg and Clay, 1977; Yang et al., 2015; Bedrosian, and possibly Little Falls schist, containing rifted Superior margin reduced 2016). However, early MT soundings over Wisconsin imply that such sediments plus perhaps initial Animikie foreland basin material at the mild decreases in resistivity toward the deepest crust may be widespread outset of Penokean convergence (Southwick et al., 1988; Ojakangas et under the Pembine-Wausau terrane and at odds with whole-crustal resis- al., 2001; Schulz and Cannon, 2007; Boerboom et al., 2011). The exposed tivities of 10,000 Ω·m or more thought to be characteristic of cold, solely schists now lie in a series of north-verging, low-angle thrust sheets (Fig. crystalline shield lithologies (Dowling, 1970). 1), which were likely created mainly during the middle Penokean orogeny,

The computed response in ρyx and φyx from the inversion model appears when the Marshfield terrane collided along the Eau Pleine shear zone, also in Figures 2 and 3. These response variations are smoother than the but which were thoroughly remetamorphosed by post-Penokean events observed data because statistical noise (scatter) in the calculations is (Holm et al., 2007; Schulz and Cannon, 2007; see also Fig. 1). They are absent, and the inversion balances closeness of fit against model smooth- interpreted to predate and unconformably underlie the main Penokean ness. However, the primary features of the observed data are reproduced Animikie foreland basin (Boerboom et al., 2011). Graphite is abundant

well, for example, the high φyx and drop in ρyx defining the strong, east- in these schists, and its increasing crystallinity southward with metamor- dipping crustal conductor under the Snake River area of Figure 4. The phic grade (McSwiggen and Morey, 1989) should promote conductivity

low φyx near T = 10 s at the westernmost MT site and the high ρyx of the (Wannamaker and Doerner, 2002; Bedrosian, 2016). Most occurrences southern CSAMT sites again imply that the rocks under the McGrath originally were carbonaceous mudstones, commonly holding abundant gneiss dome should largely be resistive. The compact midcrustal resistor pyrite, which could contribute (with graphite) to the low resistivity. The 13 12 under the central Midcontinent rift system reproduces the low φyx response C/ C isotope ratios of all graphitic samples studied strongly imply a

there well. Very high ρyx values toward the far southeast end are reproduced biogenic origin (McSwiggen and Morey, 1989). Bedrosian (2016) also by the high model resistivities there in the upper 20 km of the model. proposed that graphite of the Mille Lacs and North Range schists could Coincidentally, the agreement between the observed and computed be responsible for the conductor in this general area resolved in the Earth-

responses in the TE mode ρxy and φxy in Figure 2 is also reasonably Scope MT TA data.

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80 Bouguer Gravity 0 mgal -80 FL GD PNK-S RIFT PNK-I Figure 5. Two-part interpretive cross section assigning Proterozoic tectonic/geologic ele- MDML Xgr SRF Zkv SCP CF Pz RL FR ments to major resistivity variations. In the 0 upper view, element boundaries are overlain DF Xmn Xtg ? F upon reduced hue and saturation color ver- Xgg Xmn Zki LO sion of Figure 4. In the lower view, elements Wwa +Xlf Wgd +eXam? are colored in accord with the geological map Xtg of Figure 1. Age of deformative tectonism is Wgd? Xtg denoted by arrow type labeled at bottom. Under- Tectonic, physiographic, and geologic labels 50 Moho plate? along top are as in Figures 2 and 4, and section is not balanced due to probable large- scale ductile deformation. Bouguer gravity,

Depth (km) Moho, and upper bound of crustal transitional Flm or zone are after Zhang et al. (2016). Litholo- EPSZ? gies are as in Figure 1, plus Keweenawan NW Mdz? SE Midcontinent rift system subsurface intru- 100 sives (Zki), zone of seismically interpreted magmatic underplating, and upper mantle. MD Xgr Zkv Pz Unit Xgr in the Snake River area is the Isle 0 and Warman granitic panel of East-Central DF Xmn Xtg ? F Minnesota batholith (Chandler et al., 2008). Xgg Xmn Zki LO Opposing arrow directions on the Douglas Wwa +Xlf Wgd +eXam? and Lake Owen faults (DF, LOF) recognize the Xtg possibility of later tectonic inversion produc- Wgd? Xtg ing the St. Croix horst. Also speculative are a Under- minor influence of Flambeau- or Eau Pleine 50 Moho plate? shear zone–related (Flm or EPSZ), mildly low- resistivity material, plus possible resistive Keweenawan melt depletion zone in upper-

Depth (km) most mantle approximately beneath the rift Flm or (Mdz). The latter resistor alternatively could EPSZ? represent underthrust Archean lithosphere. NW Mdz? SE 100 0 50 100 150 200 km late Penokean Penokean / Yavapai Keweenawan

Some samples classify as siliceous carbonites, reaching 44 wt% car- subducted paleomargin, or fluidized redistribution during Penokean and bon, and appear to be chemical precipitates lacking a significant clastic post-Penokean events. component (McSwiggen and Morey, 1989). Fluid mobilization and repre- The Penokean suture was not destroyed per se in our study area (cf. cipitation of ordered graphite during metamorphism are widely recognized, Schulz and Cannon, 2007), but post-Penokean emplacement from below producing vein-like textures amenable to long-range electrical connec- the Archean-cored McGrath Dome gneisses and Yavapai-aged (1775– tion (Luque et al., 1998; Wannamaker, 2000, 2005). Circulation of fluids 1750 Ma) East-Central Minnesota batholith (Figs. 1 and 5) disrupted and from subduction closure or orogenic collapse is suggested to be abundant obscured it at the level of the present-day bedrock surface. Intrusive-cored within the suture (Vallini et al., 2007). However, the bulk of the original gneiss doming replaces prior lithology with highly resistive crystalline carbonaceous material today lies deep in the suture, according to the rocks, as seen elsewhere along the orogen (Young and Rogers, 1985). The resistivity model, so that correlation with surface units is tentative. The foreland rocks overlying the gneiss belt that were not downfolded and limited extent of low-resistivity material resolved by the CSAMT data incorporated would have been uplifted and eroded away, separating the across the Mille Lacs Group rocks exemplifies the heterogeneity pos- surviving Mille Lacs and North Range Groups from the conductive litholo- sible in the rock properties, despite abundant graphite occurrences. Local gies detected down the former trench. These events would complicate an airborne and ground EM surveys, plus outcrop studies, have revealed interpretation of the Malmo discontinuity as the original Penokean suture the sheet-like, anisotropic fabric of the graphite and sulfide conductors due to deformation and the degree of later Proterozoic injected material at finer scales here and near the Niagara fault zone (Sternberg and Clay, (gneiss plus pluton). Our interpretation compares reasonably with that of 1977; Young, 1977; McSwiggen, 1987). We suggest that the deep con- Chandler et al. (2007, 2008), where they viewed the Malmo discontinuity ductor may have such a high magnitude because of thrust imbrication, a as a structure diminished by subsequent events and suggested that one greater quantity of original graphite or sulfides along this portion of the should perhaps seek the Penokean suture south of the McGrath gneiss

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dome in our study area. Boerboom et al. (2011) viewed the discontinuity Farther southeast toward the Penokean interior, the upper half of the today as a Yavapai-aged (post-Penokean) boundary along which doming crust is generally resistive and made up of high-grade meta-igneous rocks and exhumation of gneiss (i.e., the McGrath) took place. of the Pembine-Wausau arc and subsequent intrusives. At lower-crustal We consider the large crustal conductor resolved here to be correlative levels and into the upper mantle nominally, resistivities faintly fall below with the Flambeau anomaly and related conductors to the northeast of our 1000 Ω·m, particularly below the Rice Lake area. This might represent study along the Niagara fault zone (Sternberg and Clay, 1977; Wunder- trace amounts of solid phase conductors such as graphite introduced during man, 1988; Yang et al., 2015; Bedrosian, 2016). The 70 km spacing of the precollisional subduction or redistributed during protracted metamorphism EarthScope MT TA sites again is an issue for precise resolution, but the (e.g., Wannamaker, 2005), and possibly locally hydrated upper mantle at conductors also appear to approximately correspond with organic carbon– 100 km and beyond (Bedrosian, 2016). However, it also may include 3-D or sulfide-bearing schists metamorphosed to moderately high grades in a effects (sideswipe) from an offline structure such as the Flambeau anomaly similar setting to ours. The coarse TA site sampling leaves questionable or metasediments around the Eau Pleine shear zone (Marshfield suture) to the degree to which conductors extend southward down the Niagara fault the south, such that the true depth extent may be less than that in the image. zone, and regional erosion since early Proterozoic time may have caused Note that the color contours in Figures 4 and 5 are logarithmic, so the neces- southward shifts in the surface trace of the suture. Comparable MT stud- sary volume of grain boundary conductors below the southeast model area ies in the Slave craton of northwest Canada have resolved conductors is very small compared to the main suture zone. Nevertheless, the early MT dipping to ~150 km depth, interpreted to be ancient subduction material, soundings of Dowling (1970) spanning much of Wisconsin imply that such through the full range of the graphite stability field (Jones et al., 2003). mild decreases in resistivity toward the deepest crust may be widespread. The prominent conductor in Figure 4 dips southeastward, as expected Clearly, additional 3-D data coverage and modeling are warranted. for such a setting, but it becomes obscure below the superimposed Mid- The MT transect data improve resolution of the Penokean and Keween- continent rift system. The conductor appears to step downward at ~85 km awan structure in east-central Minnesota and western Wisconsin from that along the profile and is overlain by a compact resistor reaching 5000 Ω·m provided by the 70-km-spaced, long-period (T ≥ 10 s) EarthScope MT TA in the 5–20 km depth range from 90 to 120 km along profile. The resistor soundings. In both the Yang et al. (2015) and Bedrosian (2016) models, properties are compatible with intruded (synrift and near postrift) mafic in the ~10–30 km depth range, the main western Penokean “suture” con- rocks of the Midcontinent rift system event as imaged in detail in the seis- ductor lies between four regional MT stations and roughly centered upon mic reflection and gravity lines across Lake Superior to the northeast and the Malmo discontinuity. Our results suggest that the main conductivity elsewhere (Hinze et al., 1997; Stein et al., 2015). Adams (1990) showed lies farther southeast under gneiss and plutonic rocks and, presumably, that possibly related rocks of the outcropping Duluth complex had resis- Penokean arc rocks covered by Keweenawan rift strata (Figs. 1 and 5). tivities often exceeding 10,000 Ω·m. Lateral limits are compatible with This is in accord with tectonic history at this locale involving doming, principal control by the Douglas and Lake Owens regional normal fault intrusion, and suture disruption. Our model also offers tighter geometric systems (Fig. 1). Asymmetry of the crustal rift structure is not resolved in bounds for the crustal intrusive units of the Midcontinent rift system, our MT data, but we presume the controlling geometry is predominantly which affect the MT data almost entirely at periods shorter than the 10 s down on the east side, as our transect lies across the Brules second-order limit (frequency >0.1 Hz) of the MT TA coverage. This volume likely was rift segment of the Superior rift zone (Dickas and Mudrey, 1997; see emplaced in the main magmatic stage at 1101–1094 Ma during crustal- also Fig. 5). Upper-crustal rift strata contribute in only a modest way to scale graben formation (Miller and Nicholson, 2013). the MT response, and there is no observable manifestation in the MT of structural inversion from later (e.g., Grenville) compression that formed CONCLUSIONS the St. Croix horst (Hinze et al., 1997). However, an unknown proportion of the Pembine-Wausau arc rocks (Xtg) labeled as overlying the main Older geophysical data, even if not necessarily of modern-day stan- conductor west of the rift could alternatively be rift volcanics. dards, can yield fresh insights into Earth structure and processes. The Seismic P-wave receiver function (RF) imaging close to the northeast MT/CSAMT results we interpreted here are somewhat reconnaissance of our profile in the Superior Province Rifting EarthScope Experiments in their coverage, but first-order features are strongly suggestive of con- (SPREE) project (Zhang et al., 2016; their line SN) revealed the Moho vergent and rift structures. The recovered MT data in east-central Min- deepened to >55 km under the Midcontinent rift system from an average nesota and western Wisconsin confirm an extension of Penokean orogen regional value of ~40 km (Keller, 2013; see also Fig. 5). In addition, a deep structure into eastern Minnesota from the comparable terranes in second imaged interface overlies this thickened crustal zone with a depth Wisconsin and upper Michigan. They improve upon the resolution of the to top as shallow as 35 km, centered beneath the Midcontinent rift system. regional EarthScope MT TA coverage by showing that the bulk of the RF polarity signifies increased velocity from crustal background, and the deeply underthrust, former margin and early foreland organic-rich, sulfide- zone is interpreted as mainly dense magmatic underplate, explaining much bearing sediments probably underlie the eastern portion of the uplifted of the positive Bouguer gravity anomaly characterizing the majority of gneiss belt and post-Penokean intrusives and the westernmost Keween- the Midcontinent rift system (Merino et al., 2013; Zhang et al., 2016). awan volcanic cover. To an extent, these carbonaceous volumes have This structure, projected along strike of the Bouguer anomaly, does not been disconnected from their possible correlatives in the Mille Lacs and show in the MT structure, as it should be highly resistive like the upper North Range schists by subsequent uplift. Additional MT data of similar mantle immediately below. It also would be obscured by overlying, mod- frequency range to the current study would be needed to further resolve estly lower resistivities of the suture, which are smoothed spatially in the details of the transition from the Mille Lacs Group across the McGrath inversion (Fig. 5). However, some of the gravity anomaly could be due to gneisses and East-Central Minnesota batholith, and of the suture geometry the middle-crustal resistive body noted earlier herein, as considered by in other parts of the orogen such as across the Niagara fault zone. The Hinze et al. (1997). Speculatively, higher-resistivity upper mantle below suture zone conductive structure appears to be displaced below the later the rift (Mdz of Fig. 5) could represent a possible Keweenawan melt deple- igneous Midcontinent rift system by formation of a compact resistive tion zone, perhaps corresponding to the high seismic Vs body of Shen et body in the 5–20 km depth range approximately bounded laterally by the al. (2013) near there or, alternatively, underthrust Archean lithosphere. regional Douglas and Lake Owens fault zones. The bounds of this resistor

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agree well with similar interpretations from reflection seismic imaging grew in thickness geometrically to a total model depth of ~400 km. Horizontally, there were two columns of element parameters per MT/CSAMT station, except where there were larger and gravity modeling elsewhere along the Superior zone of the Midcon- gaps between clusters of sites, in which case there were more parameter columns. The start- tinent rift system. No distinct Moho-level, fossil magmatic underplating ing model was a uniform half-space of 1000 Ω·m. The inversion converged monotonically in zone beneath the rift is resolved in the MT data, but it would be difficult eight iterations from an initial normalized root-mean-square misfit (nRMS) of 14.1 to nRMS = 1.94, which is considered a good fit. The model roughness matrix norm was 0.3 times the to distinguish such a deep resistor from generally high overall resistivi- norm of the error-weighted Jacobian matrix, and decreasing this factor did not improve fit ties of the uppermost mantle today. The influence of prior (Penokean) significantly but did generate needlessly rougher models (see, e.g., deGroot-Hedlin and Con- lithospheric structure on the geometry of the Midcontinent rift system stable, 1990). Close attention was paid to fitting the responses from the western MT sites into the CSAMT profile, as the Penokean suture is presumed to lie in this area. emplacement locally is obscure because the two systems converge at an acute angle through our study area. ACKNOWLEDGMENTS Institutional support for Wunderman included Michigan Technological University, Phoenix APPENDIX: METHODS FOR RECOVERY AND INVERSION OF LEGACY Geophysics (Denver), and a Minnesota Geological Survey Grant-in-Aid to students. Magneto- MT/CSAMT DATA telluric (MT) data collection field assistance was provided by Mark Kitchen, Tim Seiss, Alan In the MT method, the observed horizontal electric (E) and magnetic (H) field compo- Koivunen, Eric Hepp, and Josie O’Brien. Individuals aiding in the early data collection and nents in the frequency domain are related through the complex tensor impedance Z (Chave analysis together with published abstracts are listed in Wunderman (1988). Recovery and and Jones, 2012), expanded as: inversion of the MT and controlled-source audio (CSA) MT data set were supported under U.S. National Science Foundation grant EAR11-48081 to Wannamaker, as a component of the EZxx=+xxHZxyHy, (1) Superior Province Rifting EarthScope Experiments (SPREE) project (Stein et al., 2011, 2016). EZyy=+xxHZyyHy. (2) A similar tensor relation can be defined for the observed vertical magnetic field as: Anders Jeppson, formerly of Electromagnetic Surveys, Inc., kindly loaned us an archival copy of Minnesota Department of Natural Resources Report 8714 in order to allow high-resolution HKyz=+xxHKzyHy. (3) The xy and yx MT impedances typically are transformed through simple arithmetic to scans of the plates for CSAMT data recovery. Suzan van der Lee is thanked for providing locations of the SPREE SN line passive seismic sites. Wannamaker wishes to acknowledge apparent resistivities (ρa) and impedance phases (φ) for display and analysis (Chave and numerous insightful discussions with Professor Charles R. Bentley of the University of Wis- Jones, 2012). For example, converting MT impedance to ρa over a uniform half-space returns consin, who in particular pointed out the early MT work of Dowling (1970). Greg Nash and the resistivity of that half-space, so ρa is intended to depict a smoothed version of resistivity structure toward depth as period increases (frequency decreases). Virginie Maris produced several of the graphics. The 14 MT soundings were measured using instrumentation constructed by Phoenix Geophysics, Ltd., and owned by Michigan Technical University. As is standard for MT sound- REFERENCES CITED ings in our wave period range (~0.01 to 300 s), magnetic fields were recorded using induction Adams, D.C., 1990, Footwall Structure of the Duluth Complex in Northeastern Minnesota, a coils buried several inches below surface for mechanical and thermal stability. Electric fields Geophysical Investigation [M.S. thesis]: Houghton, Michigan, Michigan Technological were obtained as voltage differences across bipoles spanning from 25 to 125 m, grounded at University, 189 p. the end points with quiet Pb-PbCl2 chemical electrodes. Field time series were digitized to 16 Bauer, R.L., Bickford, M.E., Satkoski, A.M., Southwick, D.L., and Samson, S.D., 2011, Geology bits accuracy and then processed to the Fourier (frequency) domain assuming an exp(+iωt) and geochronology of Paleoarchean gneisses in the Minnesota River Valley, in Miller, J.D., time dependence on a Hewlett Packard 9845-B mini-computer. Fourier components were Hudak, G.J., Wittkop, C., and McLaughlin, P.I., eds., Archean to Anthropocene: Field Guides computed via cascade decimation with quality sorting according to field multiple coherence to the Geology of the Midcontinent of North America: Geological Society of America Field (Stodt, 1983). Instrument calibrations were carried out before and after field deployment. Guide 24, p. 47–62, https://doi.org/10.1130/2011.0024(03). The original MT data exist as paper plotting records only for the off-diagonal impedance Bedrosian, P., 2016, Making it and breaking it in the Midwest: Continental assembly and rift- quantities (xy, yx). Neither the on-diagonal (xx, yy) impedance values nor tipper elements ing from modeling of EarthScope magnetotelluric data: Precambrian Research, v. 278, are available. Hence, good resolution plots of the xy and yx components of ρ and φ (ρ , ρ , a xy yx p. 337–361, https://doi.org/10.1016/j.precamres.2016.03.009. φ , φ ), including error intervals from Wunderman (1988, his Appendix A), were scanned at xy yx Boerboom, T., Holm, D., and van Schmus, R., 2011, Late Paleoproterozoic deformational, 300 dots per inch (dpi), and a graphical grid was overlain for manual digitization. We esti- metamorphic, and magmatic history of east-central Minnesota, in Miller, J.D., Hudak, mate the digitized ρ values and error intervals to be represented to ~5%, with phases to a G.J., Wittkop, C., and McLaughlin, P.I., eds., Archean to Anthropocene: Field Guides to 1°–2°. These are less than the data error floors we applied for inversion. Example soundings the Geology of the Midcontinent of North America: Geological Society of America Field of the off-diagonal apparent resistivities and impedance phases are presented in Figure 3. Guide 24, p. 1–23, https://doi.org/10.1130/2011.0024(01). The CSAMT method substitutes EM fields at discrete frequencies from a man-made trans- Boerner, D.E., Kurtz, R.D., and Craven, J.A., 1996, Electrical conductivity and Paleoprotero- mitter located outside a survey area for the natural fields of MT (Goldstein and Strangway, zoic foredeeps: Journal of Geophysical Research, v. 101, p. 13,775–13,791, https://doi 1975; Wannamaker, 1997). This can be a logistical advantage for shallower, focused surveying. .org/10.1029/96JB00171. The original CSAMT recordings comprise a dense profile of 45 sites, ~17.5 km long, running Canfield, D.E., 2005, The early history of atmospheric oxygen: Homage to Robert M. Garrels: slightly west of north from the northeast margin of Mille Lacs Lake (profile A of EMS, Inc., Annual Review of Earth and Planetary Sciences, v. 33, p. 1–36, https://doi .org/10.1146 1985; see also Fig. 1). A crossed-bipole transmitter with legs of ~2 km length was located /annurev.earth.33.092203.122711. ~12 km to the west of center of the profile. The crossed bipoles broadcast source fields with Cannon, W.F., Kress, T.H., Sutphin, D.M., Morey, G.B., Meints, J., and Barber-Delach, R., 1999, E dominantly normal to the profile (Z ), and separately with E dominantly along the profile xy Digital Geologic Map and Mineral Deposits of the Lake Superior Region, Minnesota, Wis- (Zyx), which is a scalar recording setup. Only ρa values were recorded (no φ) at seven discrete consin, Michigan: U.S. Geological Survey Open-File Report 97-455 (v3). frequencies over a useful range of 1000 to 10 Hz. Values of ρxy and ρxy for the original 45 sites Chandler, V.W., Boerboom, T.J., and Jirsa, M.A., 2007, Penokean tectonics along a promontory- at seven discrete frequencies from 1000 to 10 Hz were entered from paper records (EMS, Inc., embayment margin in east-central Minnesota: Precambrian Research, v. 157, p. 26–49,

1985), converted to |Zxy| and |Zyx|, and averaged laterally using a triangle function in accord https://doi.org/10.1016/j.precamres.2007.02.009. with Electromagnetic Array Profiling (EMAP) principles (Torres-Verdin and Bostick, 1992) to Chandler, V.W., Jirsa, M.A., and Boerboom, T.J., 2008, A Gravity and Magnetic Investigation yield eight equivalent binned sites with ~2 km separation (Fig. 1). These binned soundings of East-Central Minnesota: Insights into the Structure and Evolution of the Paleoprotero- well represent the major trends in the original 45 sites. Given that the CSAMT recording sen- zoic Penokean Orogen: Minnesota Geological Survey Report of Investigations 66, 33 p.

sors were oriented along and across the profile, the CSAMTρ xy and ρyx values have a uniform Chave, A.D., and Jones, A.G., eds., 2012, The Magnetotelluric Method: Theory and Prac- x orientation of ~N080E for all sites and frequencies. tice: Cambridge, UK, Cambridge University Press, 552 p., https://doi .org/10.1017 The digitized CSAMT and MT data were combined as represented in Figure 2 for input to /CBO9781139020138. inversion imaging. When carrying out 2-D inversion, generic 3-D model studies suggest that deGroot-Hedlin, C.M., and Constable, S.C., 1990, Occam’s inversion to generate smooth, two- useful resistivity cross sections can be recovered in the face of along-strike (3-D) variations dimensional models from magnetotelluric data: Geophysics, v. 55, p. 1613–1624, https://

in structure if the ρyx and φyx data defined with fixed axes havingy approximately parallel to doi.org/10.1190/1.1442813. the profile trend are inverted as the equivalent 2-D transverse magnetic (TM) mode (electric Des Marais, D.J., 1994, Tectonic control of the crustal organic carbon reservoir during the current flow in the profile direction; Wannamaker, 1999; Ledo, 2006). Fortunately, the paper Precambrian: Chemical Geology, v. 114, p. 303–314, https://doi.org/10.1016/0009-2541 records of the MT soundings were so defined, withx = N060E throughout. This is somewhat, (94)90060-4. though not seriously, oblique to the profile trend and is nearly along strike of the rift and Des Marais, D.J., Strauss, H., Summons, R.E., and Hayes, J.M., 1992, Carbon isotope evidence suspected suturing structures. The CSAMT data were defined withx = N080E, approximately, for the stepwise oxidation of the Proterozoic environment: Nature, v. 359, p. 605–609, which deviates somewhat from the MT, but we do not consider this a large angle, and the https://doi.org/10.1038/359605a0. CSAMT soundings are not strongly anisotropic (Fig. 2). Thus, it is feasible to merge these Dickas, A.B., and Mudrey, M.G., Jr., 1997, Segmented structure of the middle Proterozoic sites with the MT in a single profile. Midcontinent rift system, North America, in Ojakangas, R.W., Dickas, A.B., and Green,

Inversion of ρyx and φyx in Figure 2 as TM mode data was performed using the University J.C., eds., Middle Proterozoic to Cambrian Rifting, Central North America: Geological of Utah finite-element algorithm described in Wannamaker et al. (2009, 2014). This stabilized Society of America Special Paper 312, p. 37–46, https://doi.org/10.1130/0-8137-2312-4.37. (regularized) inversion code minimizes an objective function, which is a weighted sum of data Dowling, F.L., 1970, Magnetotelluric measurements across the Wisconsin Arch: Journal of misfit and model roughness defined according to lateral model slope iny and z (cf. Chave Geophysical Research, v. 75, p. 2683–2698, https://doi.org/10.1029/JB075i014p02683.

and Jones, 2012). An error floor of 7% forρ yx and 2 degrees for φyx was applied for the MT EMS (ElectroMagnetic Surveys), Inc., 1985, CSAMT Survey, Aiken County, Minnesota (Ac-

data, with a 20% floor applied to the CSAMTρ yx. The mesh was 178 elements columns wide quired by ElectroMagnetic Surveys, Inc.): Minnesota Department of Natural Resources, by 60 element rows deep. The thinnest row at the surface was 25 m thick, and deeper rows Mineral Division Report 8417, 29 p., plus appendices and plates.

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