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Tectonic Model for the Proterozoic Growth of North America

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Tectonic Model for the Proterozoic Growth of North America Tectonic model for the Proterozoic growth of North America Steven J. Whitmeyer* Department of Geology and Environmental Science, James Madison University, Harrisonburg, Virginia 22807, USA Karl E. Karlstrom* Department of Earth and Planetary Science, University of New Mexico, Albuquerque, New Mexico 87131, USA ABSTRACT ary provinces are composed of numerous 10 attempts to reconstruct the cycle of superconti- to 100 km scale terranes or blocks, separated nent forma tion and fragmentation in the Pre- This paper presents a plate-scale model by shear zones, some of which had compound cambrian. North America offers among the most for the Precambrian growth and evolution histories as terrane sutures and later crustal- complete geologic, geophysical, and iso topic of the North American continent. The core assembly structures. Major northeast-trend- data sets of any continent and so is an impor- of the North American continent (Canadian ing provinces are the Yavapai province (1.80– tant case study for continental evolution. It was shield) came together in the Paleoproterozoic 1.70 Ga), welded to North America during the also centrally located in the Precambrian super- (2.0–1.8 Ga) by plate collisions of Archean 1.71–1.68 Ga Yavapai orogeny; the Mazatzal continents of Nuna (1.8–1.6 Ga) and Rodinia continents (Slave with Rae-Hearne, then province (1.70–1.65 Ga), added during the (1.1–0.9 Ga); therefore, improved understanding Rae-Hearne with Superior) as well as smaller 1.65–1.60 Ga Mazatzal orogeny; the Granite- of the evolution of North America needs to be Archean continental fragments (Wyoming, Rhyolite province (1.50–1.30 Ga), added dur- directly linked with studies of past superconti- Medicine Hat, Sask, Marshfi eld, Nain cra- ing the 1.45–1.30 Ga tectonic event associated nent reconstructions (e.g., Li et al., 2007). tons). The resulting Trans-Hudson orogen was with A-type intracratonic magmatism; and The purpose of this paper is to present a visual a collisional belt similar in scale to the mod- the Llano-Grenville province (1.30–1.00 Ga), model for the development of the Precambrian ern Himalayas. It contains mainly reworked added during the 1.30–0.95 Ga broader core of North America (i.e., Laurentia) via a Archean crust, but remnants of juvenile vol- Grenville orogeny. During each episode of series of time-slice maps (Figs. 1–20) and ani- canic belts are preserved between Archean addition of juvenile lithosphere, the transfor- mations (Animations 1 and 21). These visualiza- masses. The thick, buoyant, and composi- mation of juvenile crust into stable continen- tions may be useful both for nongeology audi- tionally depleted mantle lithosphere that now tal lithosphere was facilitated by voluminous ences and for introductory geology teaching as a underlies North America, although domi- granitoid plutonism that stitched new and graphic display of plate tectonic models for con- nantly of Archean age, took its present shape existing orogenic boundaries. Slab roll back tinental growth and the time-integrated record by processes of collisional orogenesis and likely created transient extensional basins (1.70 and preserved in continents. For more advanced has a scale of mantle heterogeneity similar to 1.65 Ga) in which Paleoproterozoic quartzite- audiences, we hope this model will stimulate that exhibited in the overlying crust. rhyolite successions were deposited, then critical debate about the tectonic evolution of In marked contrast, lithosphere of south- thrust imbricated as basins were inverted. North America, resulting plate-scale hetero- ern North America (much of the conti nental The lithospheric collage that formed from geneity, processes that shape and modify con- United States) was built by progressive dominantly juvenile terrane accretion and tinents, and piercing points that can be used to addition of a series of dominantly juvenile stabilization (1.8–1.0 Ga) makes up about match the margins of ancestral North America vol canic arcs and oceanic terranes accreted half of the present-day North American con- (Laurentia) to neighboring continental margins along a long-lived southern (present coordi- tinent. Throughout (and as a result of) this of past supercontinents. We recognize that any nates) plate margin. Early juvenile additions long-lived convergent cycle, mantle litho- model at the scale of a whole continent and over ( Pembine-Wausau, Elves Chasm arcs) formed sphere below the accretionary provinces was 1 b.y. of history is necessarily simplifi ed, based at the same time (1.84–1.82 Ga) the core was more hydrous, fertile, and relatively weak on variable quality data, and limited by the biases assembling. Following fi nal assembly of the compared to mantle lithosphere under the and incomplete knowledge of the authors. Archean and Paleoproterozoic core of North Archean core. America by 1.8 Ga, major accretionary prov- METHODS inces (defi ned mainly by isotopic model ages) Keywords: Proterozoic, Rodinia, Laurentia, were added by arc-continent accretion, analo- continent assembly, North America This compilation of pre-Neoproterozoic tec- gous to present-day convergence between tonic belts in southern Laurentia is based pri- Australia and Indonesia. Also similar to Indo- INTRODUCTION AND SCOPE marily on geologic and geochronologic data nesia, some accreted terranes contain older from exposed Proterozoic outcrops (~10% of continental crustal material [Archean(?) The formation and long-term behavior of Mojavia], but the extent and geometry of continental lithosphere requires a plate-scale, 1If you are viewing the PDF, or if you are read- ing this paper offl ine, please visit http://dx.doi.org/ older crust are not well known. Accretion- time-integrated understanding of crust and 10.1130/GES00055.S1 and http://dx.doi.org/10.1130/ mantle formation and modifi cation events within GES00055.S2 or the full-text article on www. *[email protected]; [email protected] single continents, in association with continued gsajournals.org to view the animations. Geosphere; August 2007; v. 3; no. 4; p. 220–259; doi: 10.1130/GES00055.1; 20 fi gures; 2 animations. 220 For permission to copy, contact [email protected] © 2007 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/3/4/220/857342/i1553-040X-3-4-220.pdf by guest on 23 September 2021 Proterozoic growth of North America 2002). Aeromagnetic maps have been used in the matism (Finn and Sims, 2005). Thus, we believe interpretation of basement lithotypes and struc- that the maps show realistic portrayals of the ture, including the western United States (e.g., tectonic grain and nature of heterogeneity, and Lidiak, 1974; Ramberg and Smithson, 1975; perhaps even a crude estimate of the percentage Cordell and Grauch, 1985; Finn et al., 2001; of granitoid plutons versus country rock of the Sims and Stein, 2003; Grauch et al., 2003; Finn subsedimentary basement across the continental and Sims, 2005), Canada (e.g., Pilkington et al., United States. However, nearly all contacts on 2000; Ross et al., 1991), Australia (e.g., Gunn these map reconstructions have some degree of et al., 1997), and elsewhere (e.g., Henkel, 1991). uncertainty, and we have not tried to portray the Aeromagnetic anomalies primarily refl ect variable levels of uncertainty. variations in magnetization properties of Crust of rifted margins tends to be thinned Protero zoic crystalline basement (Grauch et al., and modifi ed during extension, after which 2003). Magnetic intensities refl ect distribution thinned crustal blocks can become incorporated of magnetite and other iron-bearing minerals and overprinted as they are reaccreted to conver- in the crust. Sedimentary strata generally have gent plate margins. Therefore we rely heavily on little magnetic character and are transparent in Nd and Pb isotopic data to indicate an average regional aeromagnetic mapping (Finn and Sims, crustal model age for a given province. When 2005). Large Cenozoic volcanic fi elds can model ages are within 10–100 m.y. of U-Pb Animation 1. Powerpoint sequence of the obscure magnetic patterns in the basement, but crystallization ages, we refer to the terranes as sequential assembly and growth of Lauren- for much of the western United States, Ceno- juvenile. However, when model ages are signifi - tia from ca. 2.0 Ga through ca. 0.535 Ga. zoic volcanics tend to have short-wavelength cantly older than crystallization ages (e.g., for Powerpoint slides follow the order of Fig- anomalies related to known vent areas and portions of the Trans-Hudson orogen, Mojave ures 2–20 in the text. fl ows such that the generally more persistent province, and the pre-Appalachian rifted mar- basement magnetic fabric is still decipherable. gin), the model ages commonly represent a In areas where there are outcrops (e.g., Rocky mixture of sources with different mantle sepa- Mountains), granite and granodiorite plutons ration ages that can be interpreted in different often are characterized by relatively high ampli- ways. (1) They may represent the presence of tude magnetic highs (20–200 nT), and this char- older crustal blocks in the subsurface that were acter has been used to extrapolate the contacts sampled by magmas during later partial melting of plutons into covered areas (Karlstrom et al., in the crust and/or tectonically imbricated with 2004; Finn and Sims, 2005). In the southern younger crust. (2) They may represent detritus Rocky Mountains, long-wavelength (>50 km), from older terranes that become mixed in with high-amplitude (>500 nT above base values) a younger terrane in a proportion to produce a magnetic anomalies commonly correspond to mixed model age. While we attempt to distin- 1.4 Ga plutons (Finn and Sims, 2005). In this guish areas of juvenile crust of a given age from paper, this has been invoked for interpretations areas of reworked older crust (reworked in the of the distribution and large areal extent of gran- sense of 1 and/or 2 above), the mixed model itoid plutons in the mid-continent (Fig.
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