Deformation of the “Anorogenic” Wolf River Batholith, Wisconsin, USA: Understanding the Baraboo Orogeny Hinterland

Abstract

The Mesoproterozoic (~1470 Ma) Wolf River batholith (WRB) is exposed over 6500 km², encompassing 11 plutons that crosscut the Archean Marshfield and Proterozoic Penokean terranes. As the WRB is the classically defined anorogenic batholith, to test this hypothesis, seven igneous phases were analyzed using anisotropy of magnetic susceptibility (AMS), as a proxy for magmatic flow during intrusion, and the samples recorded a sub-horizontal emplacement in six different orientations. Paleopoles from six of eight igneous samples preserve a wide variety of sub-vertical orientations with two reversed and four normal polarities. The synorogenic Baldwin Conglomerate is the youngest rock (<1460 Ga) associated with WRB. Magnetic fabrics are horizontal, but multidomain and paleopole signatures, where interpretable, are sub-vertical. The North American APWP places middle Laurentia at low-latitude during Geon 14, and all our paleopoles are sub-vertical, not sub-horizontal, again suggesting post-intrusion deformation. Moreover, the McCauley gneiss (1886 Ma; U-Pb zircon), Rib Mountain Quartzite (1750 Ma MDA; U-Pb zircon, n = 150), Dells of the Eau Claire rhyolite (1483 Ma; U-Pb zircon, 1469 Ma; monazites-in-garnet), and Baldwin conglomerate (1460 Ma MDA; U-Pb zircons, n = 150) are sub-vertical inliers (xenoliths) in the igneous suite; the Proterozoic Wausau turbidite (1850 Ma MDA; U-Pb zircon, n = 150) was intruded by the WRB and dips 25°W. Here, we present a reinterpretation of the WRB as a deformed synorogenic rather than an anorogenic intrusion.

1. Introduction

During the Proterozoic, Laurentia grew through lateral orogenic accretion that youngs to the southeast, where subduction was northwest-dipping (Penokean orogen, Geon 18; Yavapai orogen, Geon 17; Mazatzal orogen, Geon 16; Baraboo orogen Geon 14 [1,2,3] including the emplacement of the ~1470 Ma Wolf River Batholith (WRB) [4,5] (Figure 1).

The Penokean Orogeny (1800–1900 Ma) is one of four contemporaneous Proterozoic orogenic belts that sutured the various Archean terranes together to form Laurentia [4,6]. The Penokean Orogeny deformed the Huron and Animikie basins, which include a diverse suite of metasedimentary and metavolcanics rocks [7]. The Penokean Orogeny had two phases: (1) A juvenile island arc called the Pembine–Wausau terrane collided with the southern margin of the Superior province with volcanoes formed in its back-arc basin. And (2) an Archean-cored microcontinent, called the Marshfield terrane, which likely rifted off the Wyoming province, collided to the south of the Pembine–Wausau terrane. The Yavapai (~1760 Ma), Mazatzal (~1660 Ma), and Baraboo (~1460 Ma) orogenies contributed to local deformation, metamorphism, and magmatism [2]. The Eau Pleine shear zone is a south-dipping Penokean terrane boundary that is crosscut by the WRB. The extent of the WRB is defined by a uniform magnetic low (Figure 2 and Figure 3), where the six inliers are too small (non-magnetic) to appear as magnetic anomalies.

Figure 2. Aeromagnetic map of Wisconsin (scale in nanoTesla, nT) highlighting the WRB, a magnetic low (from USGS Open file data).

Figure 2. Aeromagnetic map of Wisconsin (scale in nanoTesla, nT) highlighting the WRB, a magnetic low (from USGS Open file data).

Figure 3. Geologic map of the phases of the WRB and its relationship to host rocks. Dashed boxes indicate detailed local maps for Rib Mountain, the Dells of the Eau Claire River, and the Mountain shear zone. Sample locations are indicated (Table 1, Appendix A).

Figure 3. Geologic map of the phases of the WRB and its relationship to host rocks. Dashed boxes indicate detailed local maps for Rib Mountain, the Dells of the Eau Claire River, and the Mountain shear zone. Sample locations are indicated (Table 1, Appendix A).

Table 1. WRB samples *.

Detrital Zircon
Igneous	Age (Ma)	Orientation	Age Spectra (Ma)	Strain	AMS	Paleopole	Comment
Wauson Syenite (Site 9)	1520				X	X
Bowler Anorthosite (Site 11)					X	X
Tigerton Rapalivi Granite (Site 13)					X	X
Mountain Granite (Site 3)	1470				X	X	Mountain Shear Zone
Big Falls Granite (Site 14)	1468				X	X
Keshena Falls Granite (Site 12)					X	X
Badgley Rapids Mangerite (Site 2)					X	X	Crosscutting Pseudotachylite; Mountain Shear Zone
Hager Granite (Site 1)	1470				X	X
Inliers
Rib Mountain Quartzite (Site 10)	MDA	75°, 90°	1780, 2650, 3400	LNS		X	Craddock et al., 2018 [7]; This study
Wausau Turbidite (Site 8)	MDA	0°, 30° W	1880			X
Dells of Eau Claire Rhyolite (Site 7)	1470	300°, 90°			X	X	Zircon; Monazites in garnet; This study
Waupee Greenstone (Site 4)	1812	65°, 90°			X	X	Hines Quartz Diorite (1812 Ma); Mountain Shear Zone
Baldwin Conglomerate (Site 5)	MDA	65°, 90°	1460	LNS		X	Medaris et al., 2021 [2]; Mountain Shear Zone
McCauley Gneiss (Site 6)	1876	65°, 90°					Mountain Shear Zone; This study

* Sample locations in Figure 3 and Appendix A.

Table 1. WRB samples *.

Detrital Zircon
Igneous	Age (Ma)	Orientation	Age Spectra (Ma)	Strain	AMS	Paleopole	Comment
Wauson Syenite (Site 9)	1520				X	X
Bowler Anorthosite (Site 11)					X	X
Tigerton Rapalivi Granite (Site 13)					X	X
Mountain Granite (Site 3)	1470				X	X	Mountain Shear Zone
Big Falls Granite (Site 14)	1468				X	X
Keshena Falls Granite (Site 12)					X	X
Badgley Rapids Mangerite (Site 2)					X	X	Crosscutting Pseudotachylite; Mountain Shear Zone
Hager Granite (Site 1)	1470				X	X
Inliers
Rib Mountain Quartzite (Site 10)	MDA	75°, 90°	1780, 2650, 3400	LNS		X	Craddock et al., 2018 [7]; This study
Wausau Turbidite (Site 8)	MDA	0°, 30° W	1880			X
Dells of Eau Claire Rhyolite (Site 7)	1470	300°, 90°			X	X	Zircon; Monazites in garnet; This study
Waupee Greenstone (Site 4)	1812	65°, 90°			X	X	Hines Quartz Diorite (1812 Ma); Mountain Shear Zone
Baldwin Conglomerate (Site 5)	MDA	65°, 90°	1460	LNS		X	Medaris et al., 2021 [2]; Mountain Shear Zone
McCauley Gneiss (Site 6)	1876	65°, 90°					Mountain Shear Zone; This study

* Sample locations in Figure 3 and Appendix A.

The ~1470 Ma WRB is a ferroan rapakivi granite massif that underlies an area of ~9200 km² in east–central Wisconsin, where it intrudes the Penokean terrane (both the Wausau–Pembine and Marshfield terranes), the Yavapai Province, and Baraboo Interval quartzites [8,9]. Historically, the WRB has been interpreted to be anorogenic, and its emplacement was only accompanied by contact metamorphism rather than regional metamorphism and deformation [9]. The WRB is now known to have been emplaced as part of a major plutonic, tectonic, sedimentary, and metamorphic event that occurred throughout southern Laurentia now known as the Baraboo Orogeny. Much of the Baraboo orogenic belt is composed of high-silica plutonic rocks of the south granite–rhyolite and east granite–rhyolite suites [10,11], known from sparse outcrop and many drill cores, whereas the WRB is ~1000 km inboard of the 1600 km wide Baraboo orogen margin and ~500 km inboard of the Nd anomaly boundary [10] that separates mid-crustal melts (north) from melts generated by underplating by mafic crust with a mantle source. The Baraboo orogen is the equivalent to the Picuris orogen to the southwest and the Pinware orogen to the northeast [3] and deforms the Baraboo Interval strata [2,12,13], the first sheet of mature sand to cover Laurentia [14].

To the west of the WRB occurs the older Wausau syenite (1520 Ma) and a younger, eastern group of five plutons [9,15,16,17]. Collectively, these plutons represent an A-type, alkali-rich granite massif. Petrologic and geochemical investigations suggest that the WRB derived from tonalitic crust at depths of 25 to 36 km and intruded at shallow (<4 km) depths [4,5].

WRB cooling ages of 1455–1385 Ma that young to the south indicate post-intrusion uplift [15,18]. Vertical microfractures (N40°W, 90°) occur throughout the WRB, the only hint of post-intrusion penetrative deformation [19]. Our contribution is to better understand the intrusion and deformation history by dating the six inliers (xenoliths, roof pendants), five of which are vertical within WRB, and by using magnetic techniques (paleopoles; AMS-anisotropy of magnetic susceptibility) to document their structural fabrics and histories. During Geon 14, central Laurentia was at a low latitude [20], which predicts sub-horizontal paleopoles and, because most plutons intrude vertically, sub-vertical AMS intrusion fabrics.

Field exposures are confined to road and stream cuts, so pluton–pluton contacts are rare. Similarly, most igneous outcrops are void of any foliation, primary layering, or fracture-joint fabric. There are, however, a variety of inliers around the complex which are well-exposed: Rib Mountain is a vertical quartzite, the Wausau turbidite is nicely exposed along the Wisconsin River and adjacent roads, the vertical metavolcanic sequence along the Dells of the Eau Claire River is a broad outcrop, and the vertical Baldwin Conglomerate and vertical Waupee greenstones are nicely exposed along the Mountain shear zone (Figure 3). Regional expression of the WRB and Baraboo orogeny is found throughout central Wisconsin in the form of Geon 14 metamorphic and hydrothermal alteration [17,21] and all the folded quartzites, mostly synclines [12].

Here, we present paleomagnetic and geochronologic data that reveal the “anorogenic” WRB is in fact internally deformed and has sheared margins. It is thus an integral part of the hinterland of the Baraboo orogenic belt [2,3]. Thus, fabrics in adjacent rocks of the Penokean orogenic belt, and the Proterozoic geology of the southern Superior Province need to be reconsidered in light of these observations.

2. Materials and Methods

U-Pb geochronologic analyses were conducted by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) at the Arizona LaserChron Center. Please refer to the Element2 methodology at www.laserchron.org (accessed on 10 January 2025) for the details of our analytical techniques. These U-Pb geochronology methods also have been described by [22,23]. The details of detrital zircon U-Pb age data are provided in Appendix B.

Samples for paleomagnetic analysis were collected from both the main igneous suite and five of the inliers (Table 1). Anisotropy of magnetic susceptibility (AMS) results are provided for seven WRB localities and two inliers. Alternating Field (AF) demagnetization results are provided for eight igneous rocks and five inliers (three sedimentary, two igneous). Anisotropy of magnetic susceptibility (AMS) is a technique used to determine the orientation of magnetic minerals within sediment or rock and depicts the primary structures and fabric within a rock body. This technique also detects weak deformation in rock bodies where foliation and lineation are not present in hand samples. Cores (or cubes) of these samples were oriented and prepared for AMS analysis. The Kappa Bridge is an AC susceptibility bridge with an automated sample handler for determining the anisotropy of low-field magnetic susceptibility at room temperature at the Institute for Rock Magnetism, University of Minnesota. The AMS technique has been used as a proxy for strains in foreland strata [24,25] and magmatic flow in dikes [26,27] and plutons [28,29,30]. An alternating current in the external “drive” coils produces an alternating magnetic field in the sample space with a frequency of 680 Hz and an amplitude of up to 1 mT. The induced magnetization of a sample is detected by a pair of “pickup” coils, with a sensitivity of 1.2E-6SI volume units. For anisotropy determination, a sample is rotated about three orthogonal axes, and susceptibility is measured at 1.8° intervals in each of the three measurement planes. In most samples, the K_max and K_min axes roughly perpendicular. Eight cubes were then demagnetized to determine a paleopole signature for each sample using stepwise alternating-field (AF) methods. The details of the paleomagnetic analysis is available by request from the authors.

We use earthquake data collected by the Earthscope Transportable Array to calculate receiver functions for stations across the region and, from these, generate Common Conversion Point (CCP) stacks. These new cross-sectional images of seismic impedance contrasts across east–central Wisconsin provide insight into crustal-scale structures.

3. Results

3.1. Local Geology and Geochronology

There is tremendous diversity in the field relations around the batholith (Figure 3), including Rapakivi granites and the older, intruded Wausau turbidite (Figure 4 and Figure 5). Other local observations include mangerite with cross-cutting pseudotachylite, deformed polymictic clasts in the Baldwin Conglomerate, flattened lapilli in the Dells of the Eau Claire River rhyolites, flattened, vertical pillows in the Waupee Greenstone inlier (Figure 6). Six inliers are known in and around the WRB. From west to east, these are Rib Mountain, the Wausau metasedimentary and metavolcanic rocks, the rhyolites and garnet schist section exposed along the Dells of Eau Claire River, and the Baldwin Conglomerate, Waupee Greenstone and McCauley gneiss in the Mountain shear zone (Figure 3). From oldest to youngest, we describe the local geological details.

3.1.1. Eau Pleine Shear Zone (Sites 15 and 16)

The Marshfield terrane (Figure 1) includes Archean crust as old as 3000 Ma and was accreted to Laurentia during the Penokean orogen and the northern bounding fault is the Eau Pleine shear zone [6]. The Spirit Lake shear zone is the southern boundary of the Marshfield terrane, and these two terrane boundaries merge into the younger WRB. The Eau Pleine shear zone includes mylonitic gneisses (N120° E, 65° S), with pseudotachylite and top-to-the-north kinematics (Figure 7). The back-scatter SEM image reveals a nice foliation and many monazites, but the monazites were low-U and could not be dated. Zircons in the gneiss preserve a Concordia upper intercept age of ~2476 Ma (Lake DuBay), and mylonite zircons record ~1868 Ma Penokean orogenesis (Meyer’s Landing; Figure 8). The Lake DuBay sample has a ~259 Ma lower intercept, consistent with far-field Appalachian fluid migration and deformation [7,31]. The Meyer’s Landing sample is consistent with Penokean intrusions in the Marshfield terrane throughout the region [32].

3.1.2. The McCauley Gneiss (Site 6)

The N60°E striking Mountain shear zone [33,34] occurs in a reentrant along the northeast margin of the WRB. Major rock units here are the undated Waupee volcanics, which consist of deformed and metamorphosed felsic–mafic volcanic rocks, interbedded sedimentary rocks, and the previously undated McCauley granite and its deformed gneissic equivalent. Deformed (4:1:1 aspect ratios) pillows are present in the Waupee volcanics (65°, 90°, where layering is vertical, and the pillows face north). The same author reported an age of ~1815 Ma for an undeformed quartz diorite at the SW end of the shear zone. These rocks are overlain (to the north) by the metamorphosed and deformed Baldwin Conglomerate that has a maximum depositional age of ~1460 Ma [21], and the entire succession is intruded by the ~1470 Hagar Porphyry, a phase of the WRB [9].

The McCauley gneiss (Figure 9) has an upper Concordia intercept age of ~1876 Ma. The Baldwin has the same fabric orientation as the McCauley and Waupee and is intruded by the Hagar Porphyry, the contact of which is parallel to the fabric in the shear zone. The age of the Baldwin and the orientation of the contact suggest that at least part of the deformation within the shear zone must have occurred during the Baraboo orogeny.

The high Pb²⁰⁴ concentrations in zircon suggests that the Mountain shear zone may have been a conduit for hydrothermal fluids during the Paleozoic. Circulation may occur between the Precambrian basement and Paleozoic units during mineralization based on Pb-isotopes on Paleozoic-hosted galena [33]. Regional gradients in the Pb²⁰⁶/Pb²⁰⁴ isotopic signature of the galena stretch from northeastern Wisconsin to Iowa, Illinois, and Minnesota. These values are similar to the late-stage Pb-isotopic signature of Pb and Au mineralization in a Paleoproterozoic deposit in central Wisconsin [31], suggesting that fluids exchanged in both directions between Precambrian and Paleozoic rocks.

3.1.3. Rib Mountain (Site 10)

At 1942′ (592 m) in elevation, Rib Mountain is the second highest point in Wisconsin and is a large exposure of vertical, south-facing quartzite oriented at 90°, 90°. Shortening axes are 20°, 2°, a layer-normal fabric [12]. The detrital zircon age peaks are at 1900 and 2600 Ma, a Baraboo sand, with an MDA of 1784 ± 12 Ma [14]. The Rib Mountain inlier is a roof pendant in the Wausau syenite, which is surrounded by Penokean-age volcanic and plutonic rocks.

3.1.4. Brokaw Metasedimentary and Metavolcanic Rocks (Site 8)

A post-Penokean, pre-Baraboo succession of deformed metasedimentary and metavolcanic rocks occur north of Wausau near town of Brokaw, WI (Figure 4) and are best exposed in the large 3 M quarry and Highway W north of Wausau and west of the river near the town of Brokaw, WI. LaBerge and Myers [35] completed the definitive mapping of Marathon County, Wisconsin. They interpreted Penokean metamorphosed and deformed Penokean plutonic, volcanic, and sedimentary rocks to be the oldest units in the Wausau, WI area. These rocks were subsequently eroded and blanketed by quartzites of the Baraboo interval and then intruded by syenite phases of the WRB during the Baraboo Orogeny. Reichoff [36] provided a detailed account of the petrology and sedimentology of what he interpreted to be Penokean rocks and presented an excellent account of the local geology in the vicinity of the 3 M quarry near Brokaw Wisconsin. Here, he defined a >1 km succession of dark colored rhyolitic volcanic rocks that are unconformably overlain by dark gray–green immature metasedimentary rocks that include argillite, metagraywacke and conglomerate that are lightly deformed (i.e., no strong cleavage or folding present in most Proterozoic supracrustal successions in the region), and tilted gently and moderately to the northeast. The succession is cut by an E-W trending diabase dike and steeply inclined-faults with small (>20 m) displacement. A prominent unimodal age peak of 1850 Ma is evident in one sample of a conglomeratic graywacke collected from the metasedimentary rock. The maximum age of deposition is 1812.1 ± 8.1 Ma. All zircons separated from unconformably underlying meta-rhyolite are interpreted to be inherited.

3.1.5. Baldwin Conglomerate (Site 5)

The ~100 m thick polymictic conglomerate is vertical (65°, 90°) and in unconformable contact with the Hager granite to the north and is truncated along the Mountain shear zone to the south. The conglomerate youngs to the south, and clasts have an aspect ratio of ~4:1:1, preserving a layer-normal shortening (LNS) fabric. The conglomerate was derived locally during the Baraboo orogen based on the detrital zircon spectra [2].

3.1.6. Dells of the Eau Claire River (Site 7)

The geology at the Dells of the Eau Claire River includes locally mylonitized rhyolitic volcanic rocks and associated garnet-bearing tuffs (Figure 10). The locality occurs less than 1 km west of the WRB. LaBerge and Myers [35] report ductile and brittle structures in a several km wide shear zone in volcanic rocks adjacent to the batholith and recognized that some of this deformation must have occurred during an unrecognized event after batholith emplacement and consequent regional garnet-grade metamorphism of Penokean-age volcanic rocks (Figure 11). In light of the newly defined Baraboo Orogeny [2], our aim was to determine the eruption age of felsic volcanic rocks at the Dells of the Eau Claire River. The Dells of Eau Claire county park, Highway Y in Marathon county contains a ~5 km area of rhyolites with flattened ashfall lapilli (Figure 6) and garnet-rich schist (sedimentary protolith) oriented 300°, 90°. A younging direction is not known.

Zircon fertility is low, but we managed to date eight grains. The oldest fraction (n = 3) ranges from 2713 to 2730 Ma. There is one grain with an age of 1746 Ma. They youngest suite (n = 4) ranges from 1463 to 1491 Ma, with a peak age of 1483 Ma (Figure 12). U-Pb monazite ages within garnets preserve an age of 1493 Ma but with huge (Pb²⁰⁷/Pb²⁰⁶; 280 Ma; Figure 13) errors and are included in Appendix B.

We interpret that the ~1483 Ma subset represents the emplacement (eruption) age of the meta-rhyolite, which means that these rocks are temporally and genetically related to the emplacement of the WRB and thus represent the first known occurrence of Wolf River age volcanic rocks. As the volcanic rocks here are deformed, this deformation must have occurred during the Baraboo Orogeny.

The Archean and Yavapai zircons are interpreted to be inherited. They may be xenocrysts in the rhyolitic magma, which would support the interpretation of Archean crust beneath the Penokean Pembine–Wausau terrane [6]. They also may be detrital and derived from the Superior Province to the north. The Yavapai zircon is most likely detrital and derived from the south.

3.1.7. Sibley Group

Atop the Sibley Peninsula, north of Lake Superior and near Nipigon, Ontario, is the flat-lying, 950 m thick Sibley Group [37]. The Group is sub-divided into the lower Pass Lake, middle Rossport, and upper Kama Hill sections. We have analyzed a sandstone bed from the basal Pass Lake Fm. (Loon Lake member), the maximum depositional age is ~1456 Ma, with age peaks at ~1860 (Penokean orogen) and ~2682 Ma (Superior province; Figure 14; Appendix B.1). Paleocurrents for the Sibley Group are mixed but mostly indicate sediment transport from the south [7]. We include this sample in this paper, as it is the first distal (to the north) Baraboo synorogenic deposit recognized, and is a large, undeformed outlier siting unconformably on Archean Wawa crystalline rocks.

3.2. Geophysical Investigation

3.2.1. Magnetic Fabrics

Lower hemisphere plots of AMS (K_max is the long axis of the magnetic ellipsoid, K_min is the short axis) indicate that intrusion of the various igneous phases was sub-horizontal. All the igneous samples are weakly anisotropic. The Bagley Rapids mangerite was intruded horizontally, ~E-W, the Big Falls granite was ~SW-NE, the Wausau syenite was NW-SE, the Mountain granite was NW-SE, and the Tigerton Rapakivi granite was ~N-S, as was the Hager granite; the Bowler anorthosite has no magnetic fabric. A summary AMS plot is part of the Discussion Section. We analyzed two batholith inliers, and both the Dells of the Eau Claire rhyolite (320°, 90°) and Waupee Greenstone (65°, 90°) are strongly anisotropic and preserve sub-vertical flow nearly within the vertical plane of the host (Figure 15).

3.2.2. Paleomagnetism

Eight WRB igneous phases were analyzed for their paleopole orientations. Alternating field (AF) demagnetization was effective, but there was considerable scatter in most samples, with reversed polarities in many, and only six samples yielded a meaningful result (Table 1 and

Table 2. WRB paleopoles *.

	Rock Type	N (#Samples)	Declination (°)	Inclination (°)	α95	k	Comment
Wauson Syenite	Igneous	8	318.81	52.95	94.62	14.52
Bowler Anorthosite	Igneous	8	254.97	44.75	92.47	5.29
Tigerton Rapalivi Granite	Igneous	8					Complex Demagnetization
Mountain Granite	Igneous	8	40.72	51.15	103.33	28.98
Big Falls Granite	Igneous	8					Complex Demagnetization
Keshena Falls Granite	Igneous	8	121.49	−56.93	92.57	32.35
Badgley Rapids Mangerite	Igneous	8	328.05	−30.04	91.11	76.25
Hager Granite	Igneous	8	76.36	45.26	93.19	27.02
Rib Mountain Quartzite	Inlier	8	348.63	67.07	92.4	25.84
Wausau Turbidite	Inlier	8	275.52	−20.3	91.11	158.9
Dells of Eau Claire Rhyolite	Inlier	8					Complex Demagnetization
Waupee Greenstone	Inlier	8					Complex Demagnetization
Baldwin Conglomerate	Inlier	8	13.99	60.17	91.11	146.85

* Sample locations in Appendix A.

; Figure 3, Figure 16 and Figure 17). The Big Falls and Tigerton Rapakivi granites were so altered that a paleopole vector could not be calculated, and a summary plot of acceptable paleopoles shows a wide variety of orientations (Figure 18).

We also analyzed five inliers and discarded the sheared Waupee Greenstone (Figure 6) and Dells of the Eau Claire rhyolite as uninterpretable. The remaining three inliers produced acceptable paleopoles, two in the lower hemisphere (normal polarity; Baldwin Conglomerate, Rib Mountain quartzite) and one in the upper hemisphere (reversed polarity; Wausau turbidite; Figure 17). The Wausau turbidite is older than the WRB and was tilted by this igneous intrusion. A summary plot is presented in Figure 18.

3.2.3. Seismic Profiles

We crafted a south-to-north section through eastern Wisconsin centered on the WRB (Figure 2) using receiver functions and Rayleigh wave tomography (Figure 19). The Moho discontinuity is located at 40 km depth. Wave velocity variations occur within the upper crust (<15 km), indicating either a layered intrusion that may thicken to the north or that the batholith extends to ~15 km depth. Regional analysis of velocity variations demonstrates a circular batholith profile at 8 km and in the mid-crust crust but seismic uniformity at depth (>25 km; Figure 20; yellow circle).

3.2.4. The Mountain Shear Zone

Sims [34] presents a map of the Mountain shear zone (65°, 90°) in the northeast portion of the WRB where a variety of rocks are enveloped by the Hager (north) and Mountain granites (south) in the vicinity of Mountain, WI, along Country Rd. W (Figure 21). From north to south, the Hager granite (1470 Ma) has a vertical contact with the Baldwin Conglomerate (<1450 Ma). The contact has no evidence of deformation and is interpreted as unconformable. The Baldwin Conglomerate is ~100 m thick (Figure 6B) and youngs to the south; its bedding is vertical, and its clasts have aspect ratios of ~4:1:1, with the shortening axis being horizontal (bedding-normal). Along county highway W, in a valley, are outcrops of vertical Waupee Greenstone. Pillows face north with aspect ratios of ~4:1:1 (Figure 6D); shortening is also horizontal (bedding-normal). The Waupee is co-eval with the Hines quartz diorite (1812 Ma) and in contact along strike (NE) with the McCauley gneiss (1877 Ma). South of the gneiss is the Mountain granite (1470 Ma). The Bagley Creek mangerite is also along the strike to the southwest and includes aplite dikes crosscut by pseudotachylite (Figure 6A) and may be part of the shear zone. While there is good outcrop, there are no contacts and no fault kinematic structures. Paleopoles for rocks in the shear zone are all very different (Figure 18C).

4. Discussion

Greenburg and Brown [38] reported ~300 outcroppings of purple–pink quartzite in Wisconsin and Minnesota, and these were all included as part of Baraboo interval strata. The analysis of U-Pb ages of detrital zircons for many of these quartzites reveals that there was once a continent-wide sheet of sand deposited across central North America [14,39]. The tectonic mystery is how this sheet of sand became a quartzite, deposited on crusts of many ages (Figure 1), and often folded into those crusts at ~1470 Ma [2] as upright synclines. Intrusion of the WRB, and its metasedimentary inliers, is part of this story.

4.1. Structural Overview

Malone et al. [14] have defined the extent of Baraboo-interval quartzites, and Medaris et al. [2] defined the ~1470 Ma Baraboo orogen based on the regional prevalence of Geon 14 mineral overgrowths, especially micas with a cleavage selvage in Waterloo quartzites. Deformation in the quartzites was first measured by [40]. Finite strains and compression axes derived from quartz deformation lamellae indicate SSE-NNW shortening across the region with a combination of layer-parallel and layer-normal fabrics (Figure 22) with a deformation gradient that weakens to the north. Vertical microfractures reported by [19] have the same orientation. Many of the Baraboo Quartzites are upright or south-verging synclines with top-to-the-south kinematics (Figure 23) and no observable local or regional detachment. Comparison of fold axes and axial planar cleavages, quartz fabrics, strain shortening axes, paleopoles, and inlier bedding (with southerly younging) all plot as SE-NW girdles on stereonets, a direction parallel with regional Baraboo orogen shortening (Figure 22) that conforms with the plane strain (shortening in the plane of transport) observation in most orogenic belts.

Metamorphic core complexes were first described in the Cordillera [41,42,43]. Because the WRB is enigmatic and anorogenic, ref. [37] suggested the WRB (Figure 1 and Figure 2) was a highland from which the surrounding deformed quartzites slid and these outcrops present a visually pleasing pattern of “allochthonous” quartzite outcrops with long run-out distances around the intrusion (Figure 23). The ~NE-SW alignment of the inliers in the batholith are roughly parallel to the lineation in Cordilleran core complexes that is parallel to the long axis of the periclinal dome. There are numerous shortcomings with a core complex model to explain the Baraboo orogen: the WRB is not a periclinal dome, there is no capping mylonite zone (i.e., no kinematic indicators either), there are large, vertical inliers atop (within) the intrusion, the upper plate Baraboo sands are now quartzites and up to 300 km from the alleged core complex source, and the folds have a consistent regional pattern, not a radial fold axis. Axial planar cleavages in all the folds strike SW-NE, consistent with a fold-and-thrust belt [44] (

Table 3. Wolf River core complex?

Structural Feature	Wolf River Complex	Core Complex	Comment
Antiformal dome	No	Yes
Detachment Mylonite	No	Yes
Syn-uplift Intrusions	No	Yes	WRB intruded Penokean crust
Pristine Upper Plate	No	Yes	WRB upper plate rocks are quartzite
Upper Plate Runout?	Yes?	No	If true, 1–100 km
Tectonic Setting (km from margin)	1000 km	500 km
Many MCC, margin-parallel?	No	Yes
Synorogenic Strata?	Yes	No	Baldwin Conglomerate and Sibley Group have 1460 Ma detrital zircons.
Roof Pendants, Xenoliths, and Inliers?	Yes	No	WRB inliers are vertical, oriented ~E-W

). Cordilleran core complexes have a distinctive, highly deformed mylonitic lower plate and a pristine, undeformed (no cleavage) upper plate with small (<1 km) transport distances.

4.2. Synorogenic Sedimentation and Inliers

Synorogenic sediment is transported by fluvial systems up to 3000 km across continental cratons, with the Grenville orogen being the first documented example with synorogenic detrital zircons prevalent in the Brock and Minto outliers in northwest Canada [44,45,46]. Remnants of the Baraboo orogen synorogenic sediments include the Baldwin Conglomerate (MDA: 1464 Ma) and the Sibley Group (MDA: 1454 Ma; Figure 14) north of Lake Superior. The Baldwin Conglomerate is 1500 km from the Baraboo subduction margin and is rotated to a vertical orientation as an inlier in the WRB, but most of the detrital zircons were derived locally. The Sibley Group section (1000 m) is 2000 km from the margin, is undeformed, and has a number of detrital zircon age peaks. The Sibley strata represent the Baraboo orogeny distal foreland and were subsequently buried by Grenville orogen synorogenic sediments from the east. That burial did not metamorphose the Sibley, whereas the Baraboo interval strata [14] were metamorphosed to quartzite, some flat-lying (Sioux, Barron, Necedah, etc.) and some folded and cleaved at 1470 Ma (Flambeau, McCaslin, Baraboo, Waterloo) [2].

The WRB includes two distinctive inliers, the vertical, south-facing Rib Mountain quartzite that has a Baraboo-interval detrital zircon spectra, and the vertical rhyolite-schist (sedimentary protolith) sequence exposed along the Dells of the Eau Claire River with zircon and monazite ages ~1455 Ma. Each of these successions was once horizontal and has since been rotated (folded and faulted) within the pluton to a vertical orientation. The Mountain shear zone includes the vertical Baldwin Conglomerate (100 m thick; MDA: 1464 Ma) in unconformable contact with the 1460 Ma Hager Granite on the north side of the valley (Figure 21). The south side includes the Mountain Granite (1470 Ma) and Bagley Mangerite (with pseudotachylite) in a complex relationship with the Penokean crustal rocks (Hines Quartz Diorite, McCauley Gneiss and Waupee Greenstone); all the contacts are sub-vertical, and the Mountain shear zone is poorly exposed, with no kinematic indicators, but presumed to be vertical. The Baldwin Conglomerate faces south on the north side of the fault, and the Waupee Greenstone pillows face north on the south side of the fault, presenting a challenge to palinspastic restoration (see Section 4.4).

4.3. Emplacement and Deformation Paleomagnetism

Our AMS results indicate a complex, sub-horizontal intrusion pattern, unlike AMS fabrics in layered intrusions [47] or in larger plutons [48,49]. Demagnetization of inliers within host plutonic intrusions resulted in random paleopole orientations, an expected result for rocks of different ages intruded at the same time (Figure 18). Paleopoles for the igneous suite were successful for six of eight samples; two have a reversed polarity, four have normal polarities, but none have a sub-horizontal orientation for rocks intruded at equatorial latitudes at ~1470 Ma [50,51,52]. Post-intrusion thrusting, either north or south-dipping, provides a mechanism to rotate sub-horizontal paleo-inclinations to being sub-vertical. Our seismic profiles (Figure 19 and Figure 20) suggest a shallow, nearly horizontal shape, perhaps a thrust sheet. Thick-skinned thrusting 1000 km in the foreland of the Baraboo orogen margin also redefines far-field orogenic shortening with deformation in Yavapai and Penokean crust and suggests shallow, N-dipping slab dip at 1460 Ma as a mechanism. There are no observed faults offsetting WRB, but the older Eau Pleine shear zone (south dip, top-to-the-north kinematics) and a thrust near Veedum, WI (E-W, 65°S dip) [12] are reactivation fault candidates.

Magnetic fabrics (AMS; n = 8) were measured as a proxy for magmatic intrusion; K_max values were mostly sub-horizontal (K_min was mostly sub-horizontal; Figure 18) and in many orientations. These horizontal K_max orientations are either primary and part of a shallow intrusion (Figure 19) or are vertical flow fabrics rotated to horizontal by younger fault motions.

4.4. Palinspastic Restoration

The Wausau turbidites are a Penokean-aged deposit and dip 25°W following intrusion of the younger Wausau syenite; this is evidence of deformation related to the Baraboo orogen on the edge of the pluton. Rib Mountain is a large xenolith encased in Penokean crust and WRB [35] and is rotated from horizontal to vertical (Figure 17 and Figure 19). The paleopole is 348°, 67° and, when rotated to the south to horizontal, had a pre-deformation paleopole of 170°, 23°, more in-line with the location of central Wisconsin at 1470 Ma. A similar reconstruction of the vertical inlier rocks at the Dells of the Eau Claire River is not possible because there is no facing direction, and the paleopole data were uninterpretable.

The Mountain shear zone places south-facing rocks (Baldwin Conglomerate) on the north side of the fault in contact with north-facing rocks (pillows in the Waupee Greenstone) on the south side of a vertical contact (65°, 90°). Lacking any kinematic observations [34], interpreted an oblique-slip fault motion. On the north side of the fault the Hager Granite (75°, 45°) and Baldwin Conglomerate (14°, 60°) share a vertical unconformity, so a horizontal rotation to the south of the south-facing conglomerate results in paleopoles of 327°, 8° and 144°, 19°, respectively. These are similar values, supporting a low-latitude crystallization and subsequent rotation into the hanging wall of a north-dipping thrust. The rotated AMS K_max contoured maxima become sub-vertical when the rocks are rotated to horizontal. On the south side of the fault, the north-facing, vertical Waupee Greenstones were rotated 90°to the north, rotating the intrusive Mountain Granite (40°, 51°) and Bagley Mangerite (328°, −30°), resulting in new paleopoles of 148°, 9° and 146°, −60°, respectively. All four samples, the Hager, Baldwin, Mountain, and Bagley, have rotated paleopole declinations and inclinations that fall on a common great circle; only the Bagley inclination (−60°, upper hemisphere) is confusing. Perhaps the Mountain shear zone is a boundary where south- and north-directed thrusts meet, a triangle zone, which is normal for the distal Baraboo orogen margin. Rotated AMS fabric-contoured K_max values are vertical (Mountain Granite) and horizontal (Bagley: NE, 0°, Waupee: NW, 0°).

Thin-skin fold-and-thrust belts are dominated by hanging-wall anticlines that often verge in the direction of thrust transport, and footwall synclines are reported in two locations, both in the Cordilleran Sevier belt [53,54]. The Baraboo orogen presents the reverse relationship, no hanging-wall anticlines and a variety of synclines (Baraboo, Flambeau and McCaslin) folded into crystalline crust across the distal foreland (Figure 22 and Figure 23). The Baraboo syncline is topographically high, has a vertical northern limb, and is periclinal, plunging west on the west end and east on the east end. The fold continues to the east in the subsurface (Figure 2 and Figure 22). The Baraboo syncline has an axial-planar cleavage and kinematic indicators that suggest material movement into the fold core on the outer arc of the fold and material movement out of the core on the inner arc. This is a two-dimensional observation that is consistent with plane strain deformation, especially in thrust belts [9]. The base of the fold is underlain by a paleosol [55], which is underlain by a layer of rhyolite, then crystalline rocks (Baxter Hollow granite, locally). There is no observed fault surface or detachment under either limb of the fold. Refs. [13,56] proposed (strike-slip bounded) pull-apart basins into which the Baraboo succession was deposited non-horizontally (pre-folded), then folded into the north-verging syncline by north-dipping thrust faults [57]. The total thrust shortening of the Yavapai and Mazatzal crust must be significant to provide the overburden to metamorphose all the Baraboo arenites into quartzite; a palinspastic reconstruction of the Baraboo syncline is impossible since both fold limbs are eroded, so there are no pinpoints.

4.5. Regional Tectonics

The tectonic configuration of Laurentia when the WRB intruded included a north-dipping subduction complex on the southern margin. The WRB is the northernmost intrusion, 1000 km from the margin, and is a shallow intrusion emplaced into the Penokean and Yavapai crust (Figure 1) [1]. Ref. [2] reports penetrative deformation and widespread hydrothermal alteration across the Baraboo orogen foreland. We explored the shortcomings of a metamorphic core complex model (Figure 23; Table 3) which leaves a Laramide-style (thick-skinned; [58,59]) process of numerous faults that offset crystalline rocks metamorphosing and folding the arenites. Laramide structures in the Wyoming province resulted from flat-slab subduction [60]. The proximal Baldwin conglomerate is metamorphosed and deformed. The distal Sibley quartzite is not deformed. We need a hybrid tectonic mechanism to deform southern Laurentia, forming folded and cleaved quartzites across the region at <1470 Ma, a model supported by cooling (uplift) ages preserved in biotite [15,18] (Ar-Ar on biotite; 1475–1404 Ma) that young to the south, supporting south-directed thrusting (north-dipping thrusts) of the WRB (Figure 24). Faulting of Proterozoic crust, with south- and north-dipping thrusts, would also explain the rotation of paleopoles from horizontal (low-latitude) to sub-vertical and the AMS K_max fabrics from sub-vertical to sub-horizontal (Figure 24 and Figure 25).

5. Conclusions

The WRB had a ~20 Ma intrusive history spanning 1470–1450 Ma in a foreland setting ~1000 km from the Baraboo orogen margin to the south. The high-silica melts were sourced in the shallow crust, and intrusion also captured a variety of shallow crustal remnants now present as inliers within the WRB. The WRB is deformed, based on the poor preservation of clean paleopoles, the variety of paleopole orientations (none are sub-horizontal), pseudotachylite, the rotation of inliers to vertical orientations, and microfractures. Deformation is younger than the youngest unit, the ~1460 Ma Baldwin Conglomerate, and deformation involved thrust faulting, perhaps making the WRB and much of the Yavapai and Mazatzal crust allochthonous.