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Grenville Front Revised Stein et al. Grenville Front revised 1 Is the "Grenville Front" in the central United States 2 really the Midcontinent Rift? 3 4 5 Carol Stein1, Seth Stein2, Reece Elling2, Randy Keller3, Jonas Kley4 6 1University of Illinois at Chicago, 2Northwestern University, 7 3University of Oklahoma, 4Georg-August-Universität Göttingen 8 9 GSA Today 10 submitted August 8, 2017; revised October 13, 2017, accepted October 26, 2017 11 12 ABSTRACT 13 Two prominent Precambrian geologic features of central North America are the 14 Midcontinent Rift (MCR) and Grenville Front (GF). The MCR, an extensive band of 15 buried igneous and sedimentary rocks outcropping near Lake Superior, records a major 16 rifting event at ~1.1 Ga that failed to split North America. In SE Canada, the Grenville 17 Front is the continentward extent of deformation of the fold and thrust belt from the 18 Grenville orogeny, the sequence of events from ~1.3–0.98 Ga culminating in the 19 assembly of the supercontinent of Rodinia. In the central U.S. lineated gravity anomalies 20 extending southward along the trend of the front in Canada have been interpreted as a 21 buried Grenville Front. However, we use recent ideas and data analyses to argue that 22 these anomalies delineate the eastern arm of the MCR extending from Michigan to 23 Alabama, for multiple reasons: 1) These anomalies are similar to those along the 24 remainder of the MCR and quite different from those across the front in Canada. 2) The 25 Precambrian deformation observed on seismic reflection profiles across the presumed 26 "front" appears quite different from that across the front in Canada, cannot confidently be 27 assigned to the Grenville orogeny, and is recorded at least 100 km west of the "front". 3) 28 During the Grenville orogeny deformational events from Texas to Canada were not 29 caused by the same plate interactions and not necessarily synchronous. Hence the 1 30 commonly inferred position of the "Grenville Front" in the central U.S. is part of the 31 Midcontinent Rift, and should not be mapped as a separate entity. 32 33 INTRODUCTION 34 35 36 Two prominent Precambrian geologic features of central North America (Figures 1,2) 37 record different aspects of the Wilson cycle. One, the Midcontinent Rift (MCR), is a U- 38 shaped band of buried igneous and sedimentary rocks that outcrops near Lake Superior. 39 To the south it is buried by younger sediments, but easily traced because the igneous 40 rocks are dense and highly magnetized [Hinze et al., 1992; Merino et al., 2013]. The 41 western arm extends at least to Oklahoma, and perhaps Texas and New Mexico, as 42 evidenced by similar-age diffuse volcanism [Adams and Keller, 1994, 1996; Bright et al., 43 2014]. The eastern arm extends southward through lower Michigan to Alabama [Lyons, 44 1970; Keller et al., 1982; Dickas et al., 1992; Stein et al., 2014]. Although the MCR was 45 often viewed as two arms of a three-arm rifting event in a plate interior, it now appears 46 more likely that it formed as part of the rifting of the Amazonia craton (now in 47 northeastern South America) from Laurentia, the Precambrian core of North America 48 [Stein et al., 2014; 2016]. Hence the east and west arms were analogous to the east 49 and west branches of the East African rift, the broad zone forming one arm of the Nubia 50 (west Africa) - Somalia (east Africa) - Arabia 3-plate system. 51 52 A second major feature, east of the MCR, is the Grenville Front (GF), also known as the 53 Grenville Front Tectonic Zone. The front is the continentward boundary of deformation of 54 the fold and thrust belt from the Grenville orogeny, the sequence of orogenic events from 2 Stein et al. Grenville Front revised 55 ~1.3 – 0.98 Ga culminating in the assembly of the supercontinent of Rodinia [Li et al., 56 2008] (Figure 3). Studies in SE Canada, where Grenville rocks are exposed, find that 57 the orogeny involved discrete contractional phases, notably the Shawinigan from ~1200- 58 1140 Ma, Ottawan from ~1090-1030 Ma, and Rigolet from ~1010-980 Ma [Rivers, 2012; 59 McLelland et al., 2013] (Figure 4A). In SE Canada erosion has exposed deformed rocks 60 from these orogenic events, from ~54°N to Lake Ontario. 61 62 The orogeny's phases presumably reflect a series of continental blocks and arcs 63 colliding with and accreting to Laurentia at various locations along its eastern margin. 64 However, the specifics of the plate interactions remain unresolved because the limited 65 paleomagnetic data allow a range of scenarios. In one (Figure 4B), Amazonia collided 66 with Texas and then moved northward by strike-slip motion relative to Laurentia from 67 ~1.18 -1.12 Ga [Tohver et al., 2002; 2006]. It then rifted from Laurentia, leaving the MCR 68 as a failed third arm, with extension ending ~1.096 Ga [Stein et al., 2014; 2015]. 69 Amazonia is thought to have recollided with Laurentia somewhat later, causing the 70 Ottawan phase in Canada [McLelland et al., 2013]. The southern extent of this collision 71 varies between reconstructions [Li et al., 2008, 2013; Cawood and Pisarevsky, 2017; 72 Meredith et al., 2017]. 73 74 Even greater uncertainties arise in inferring what occurred during the Grenville orogeny 75 in the U.S. Although the front does not outcrop in the U.S., it has been assumed to 76 extend southward into the U.S. on geological and geophysical grounds. McLaughlin 77 [1954] proposed that it continued through Michigan and Indiana to the New Madrid 3 78 seismic zone because he considered features such as the Cincinnati Dome to be 79 Grenvillian, that are now considered to be much younger. Bass [1960] suggested that 80 the GF was located to the east, in western Ohio, because data from deep drill holes 81 indicated high-grade metamorphic rocks to the east and unmetamorphosed igneous and 82 sedimentary rocks to the west. 83 84 Geophysical data provide the other argument for southward extension of the Grenville 85 Front. In Canada the front is associated with weak gravity and magnetic anomalies 86 (Figures 1, 2). Zietz et al. [1966] noted that the proposed front in Ohio coincided with the 87 eastern edge of magnetic and gravity anomalies. Subsequent studies inferred that the 88 GF extended along the East Continent Gravity High (ECGH) through Kentucky and 89 Tennessee to southwest Alabama (Figure 1). As a result, the GF is often drawn 90 accordingly, although its position varies [e.g. Whitmeyer and Karlstrom, 2007; Baranoski 91 et al., 2009; Bartholomew and Hatcher, 2010; Stein et al., 2014]. The absence of the GF 92 between Alabama and the Grenville-age Llano uplift zone in Texas has been attributed 93 to the front's being rifted away from Laurentia during the latest Precambrian/Cambrian 94 rifting event [Thomas et al., 2012]. 95 96 REEVALUATING THE "GRENVILLE FRONT" IN THE U.S. 97 98 In this paper we argue that the inferred "Grenville Front" in the central U.S. is part of the 99 Midcontinent Rift, rather than the western edge of deformation from the Grenville 100 Orogeny. This interpretation is based on several aspects: 4 Stein et al. Grenville Front revised 101 102 1) Gravity anomalies in the "front" are similar to those along the remainder of the MCR, 103 and quite different from those across the front in Canada. 104 105 2) Although seismic reflection data near the presumed "front" show faults and possibly 106 suture zones, this deformation appears quite different from the SE-dipping layered 107 structures at the front in Canada and need not be Grenville-age. 108 109 3) The Grenville-age events in the Llano Uplift area of Texas and much of the eastern 110 U.S. differ, involved different continental fragments, and may have occurred at different 111 times from those in Canada. 112 113 THE "FRONT", THE EAST ARM OF THE MIDCONTINENT RIFT, AND WELL DATA 114 115 The often-assumed southward continuation of the "front" in the U.S. is based on gravity 116 and - to a lesser extent - magnetic lineaments. Hence a key question is whether the 117 gravity anomalies along the Fort Wayne Rift and ECGH are associated with the Grenville 118 Front or the east arm of the Midcontinent Rift. If they reflect the front, then its assumed 119 location near southeast Michigan implies that the east arm of the MCR ends there 120 [Cannon et al., 1989]. However, the gravity highs along the Fort Wayne rift and ECGH 121 seem to be the continuation of the east arm [Lyons, 1970; Keller et al., 1982; Dickas et 122 al., 1992; Stein et al., 2014], because they are similar in dimensions and magnitude to 123 those elsewhere along the MCR (Figures 1, 5) in showing a distinct central high. No 124 similar high occurs across the Grenville Front in Canada. 5 125 126 The Grenville Front exposed in Canada is severely eroded and represents a deep level 127 of the basal shear zone, not the deformation front observed in modern orogens. The 128 actual deformation front of the Grenville orogen must have been at least several tens of 129 km northwest of the front. As discussed above, the gravity highs in the U.S. reflect the 130 MCR's east arm, so it is unlikely that the Grenville Front lay immediately to their east. 131 Shallow-level thrusting of Grenville age would have directly impinged on the recently- 132 formed MCR.
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