ABSTRACT The rifted Precambrian margin of western Laurentia is hypothesized to have consisted of a series of ~330°-oriented rift segments and ~060°-oriented transform segments. One difficulty with this idea is that the 87 Sr/ 86 Sr i = 0.706 isopleth, which is inferred to coincide with the trace of this rifted margin, is oriented approximately N-S along the western edge of the Idaho batholith and E-W in northern Idaho; the transition between the N-S– and E-W–oriented segments occurs near Orofino, Idaho. We present new paleomagnetic and geochronologic evidence that indicates that the area around Orofino, Idaho, has rotated ~30° clockwise since ca. 85 Ma. Consequently, we interpret the current N-S–oriented margin as originally oriented ~330°, consistent with a Precambrian rift segment, and the E-W margin as originally oriented ~060°, consistent with a transform segment. Independent geochemical and seismic evidence corroborates this interpretation of rotation of Blue Mountains terranes and adjacent Laurentian block. Left-lateral motion along the Lewis and Clark zone during Late Cretaceous–Paleogene time likely accommodated this rotation. The clockwise rotation partially explains the presence of the Columbia embayment, as Laurentian lithosphere was located further west. Restoration of the rotation results in a reconstructed Neoproterozoic margin with a distinct promontory and embayment, and it constrains the rifting direction as SW oriented. The rigid Precambrian rift-transform corner created a transpressional syntaxis during middle Cretaceous deformation associated with the western Idaho and Ahsahka shear zones. During the late Miocene to present, the Precambrian rift-transform corner has acted as a fulcrum, with the Blue Mountains terranes as the lever arm. This motion also explains the paired fan-shaped contractional deformation of the Yakima fold-and-thrust belt and fan-shaped extensional deformation in the Hells Canyon extensional province.

ABSTRACT Ferroan granite is a characteristic rock type of the Laurentian margin. It is commonly associated with anorthosite and related rocks. Ferroan granites are strongly enriched in iron, are alkalic to alkali-calcic, and are generally metaluminous. These geochemical characteristics reflect their tholeiitic parental magma source and relatively reducing and anhydrous conditions of crystallization. Their compositions distinguish them from arc magmas, which are magnesian and calcic to calc-alkalic. Ferroan granite magmas are hot, which promotes partial melting of their crustal wall rocks. Assimilation of these silica-rich and peraluminous melts drives the resulting magmas to higher silica and aluminum saturation values. Where Proterozoic ferroan granites intrude Archean crust, their mantle component is readily identified isotopically, but this is more difficult where they intrude relatively juvenile crust. Ferroan granite forms in tectonic environments that allow partial melts of tholeiitic mantle to pond and differentiate at or near the base of the crust. Phanerozoic examples occur in plume settings, such as the Snake River Plain and Yellowstone, or under certain conditions involving slab rollback, such as those that formed the Cenozoic topaz rhyolites of the western United States or ferroan rhyolites of the Sierra Madre Occidental. It is possible that the long-lived supercontinent Nuna-Rodinia, of which Laurentia was a part, formed an insulating lid that raised underlying mantle temperatures and created a unique environment that enabled emplacement of large volumes of mafic melt at the base of the crust. Ascent of felsic differentiates accompanied by variable crustal assimilation may have created large volumes of Proterozoic ferroan granite and related rocks.

ABSTRACT The North American continent has a rich geologic record that preserves evidence for tectonic processes throughout much of Earth’s history. Within this long history, however, particular times—e.g., “turning points”—have had specific and lasting impact on the evolution of Laurentia (ancestral North America). This volume is focused on seven of these “turning points”: (1) The Neoarchean (2.7–2.5 Ga), characterized by cratonization and the Kenoran orogen(s); (2) the Paleoproterozoic (1.9–1.7 Ga) and the initial assembly of Laurentia; (3) the Mesoproterozoic (1.5–1.4 Ga) Andean-style margin on the southern edge of Laurentia with the Pinware-Baraboo-Picuris orogeny; (4) the 1.2–1.0 Ga Midcontinent rift, and the Grenville orogeny and assembly of Rodinia; (5) the 700–500 Ma Neoproterozoic breakup of Rodinia; (6) the mid-Paleozoic (420–340 Ma) closure of the Iapetus and Rheic oceans and the development of the Appalachian-Caledonian orogen; and (7) the Jurassic–Paleogene (200–50 Ma) assembly of the North American Cordilleran margin by terrane accretion and subduction. The assembled chapters provide syntheses of current understanding of the geologic evolution of Laurentia and North America, as well as new hypotheses for testing. The inclusion of work from different geological time periods within a single volume provides continent-wide perspectives on the evolution of tectonic events and processes that acted on and within Laurentia.

ABSTRACT It is proposed that the Pinware orogen of eastern Canada, the Baraboo orogen of the midcontinent, and the Picuris orogen of the southwestern United States delineate a previously unrecognized, ~5000-km-long, ca. 1520–1340 Ma trans-Laurentian orogenic belt. All three orogenic provinces are characterized by Mesoproterozoic sedimentation, magmatism, metamorphism, and deformation—the hallmarks of a tectonically active plate margin. Tectonism was diachronous, with the earliest stages beginning ca. 1520 Ma in eastern Canada and ca. 1500 Ma in the southwest United States. Magmatic zircon age distributions are dominated by Mesoproterozoic, unimodal to multimodal age peaks between ca. 1500 and 1340 Ma. The onset of magmatism in the Pinware and Baraboo orogens was ca. 1520 Ma, and onset for the Picuris orogen was ca. 1485 Ma. Detrital zircon age distributions within each orogenic province yield maximum depositional ages between ca. 1570 and 1450 Ma. Minimum depositional ages generally fall between ca. 1500 and 1435 Ma, as constrained by crosscutting intrusions, metatuff layers, or the age of subsequent metamorphism. Metamorphic mineral growth ages from zircon, garnet, and monazite yield peak ages between ca. 1500 and 1350 Ma and tend to be older in the Pinware and Baraboo orogens than in the Picuris orogen. The 40 Ar/ 39 Ar cooling ages for hornblende, muscovite, and biotite yield significant peak ages between ca. 1500 and 1350 Ma in the Baraboo and Picuris orogens. We propose that the Pinware-Baraboo-Picuris orogen formed in a complex, diachronous, convergent margin setting along the southern edge of Laurentia from ca. 1520 to 1340 Ma.

ABSTRACT Nearly all models of Earth’s oxygenation converge on the premise that the first notable rise of atmospheric oxygen occurred slightly above the Archean-Proterozoic boundary, with the second notable rise occurring just below the Proterozoic-Phanerozoic boundary. Plate tectonic–driven secular changes found above the Archean-Proterozoic boundary are thought to have been partly or wholly responsible for the initial rise in atmospheric O 2 in the Great Oxidation Event; however, the role of plate tectonics in oxygen levels thereafter is not well defined. Modern plate tectonics undoubtedly play a role in regulating atmospheric O 2 levels. Mountain building, for example, promotes high erosion rates, nutrient delivery to oceans, and efficient biogeochemical cycling of carbon, resulting in the net burial of organic carbon—thought to be the primary regulator of atmospheric O 2 levels on geological time scales. The trajectory of atmospheric O 2 and oceanic redox conditions in the Proterozoic Eon, representing almost 2 b.y. of geological history, shows a dynamic history with global trends that indicate overall high-low-high O 2 levels throughout the Proterozoic Eon, with low-oxygen conditions established by ca. 2.0–1.8 Ga. This contravenes the tenet that major orogenic events (e.g., the Himalaya-scale Trans-Hudson orogen and other coeval orogens that formed the supercontinent Nuna) should yield higher O 2 levels, not lower. The contrast of higher O 2 early in the Paleoproterozoic with lower O 2 later in the Paleoproterozoic is particularly striking, and mechanisms that might have caused this secular change remain unclear. This contribution explores feedbacks related to the tectonic evolution associated with the building of proto-Laurentia and Earth’s first supercontinent, Nuna, and how this impacted the trajectory of atmospheric O 2 in the latest Paleoproterozoic Era.

ABSTRACT The Neoarchean is generally considered to have been the final era of major crust formation and may have been characterized by the onset of modern plate tectonics. The Neoarchean may also have been the time interval during which subduction processes prevailed and became global. Evidence from individual cratons around the world suggests that this transition in geodynamic processes may have included diachronous and episodic major changes (i.e., turning points) and a more gradual evolution at the global scale, possibly largely driven by the secular cooling of the mantle and increasing stability of the lithosphere. The Superior craton, Canada, is the largest and best-preserved Archean craton in the world, making it an ideal location in which to investigate the occurrence (or absence) of turning points in the Neoarchean. This contribution examines the changes in geodynamic and magmatic processes that occurred during the Neoarchean, using geochemical data and new insights garnered from isotopic surveys from the southern part of the Superior craton. We summarize current understanding of the evolution of the youngest (southern) part of the Superior craton that led to the stabilization (cratonization) of this continental lithosphere and how this evolution aligns with local and global geodynamic processes.

ABSTRACT Using images from an updated and expanded three-dimensional electrical conductivity synthesis model for the contiguous United States (CONUS), we highlight the key continent-scale geoelectric structures that are associated with the Precambrian assembly of southern Laurentia. Conductivity anomalies are associated with the Trans-Hudson orogen, the Penokean suture, the ca. 1.8–1.7 Ga Cheyenne belt and Spirit Lake tectonic zone, and the Grenville suture zone; the geophysical characteristics of these structures indicate that the associated accretionary events involved the closure of ancient ocean basins along discrete, large-scale structures. In contrast, we observe no large-scale conductivity anomalies through the portion of southern Laurentia that is generally viewed as composed of late Paleoproterozoic–early Mesoproterozoic accretionary crust. The lack of through-going conductors places constraints on the structure, petrology, and geodynamic history of crustal growth in southern Laurentia during that time period. Overall, our model highlights the enigmatic nature of the concealed Precambrian basement of much of southern Laurentia, as it in some places supports and in other places challenges prevailing models of Laurentian assembly. The revised CONUS electrical conductivity model thus provides important constraints for testing new models of Precambrian tectonism in this region.

ABSTRACT Many of the hallmarks of modern plate-tectonic processes first occurred in the Paleoproterozoic Era, indicating that the mechanical, thermal, and compositional parameters of Earth’s lithosphere had evolved to approximately modern ranges of values by that time. The core of Laurentia preserves widespread examples of both convergent and divergent tectonic processes in the time span from 2.2 to 1.7 Ga, particularly within the Trans-Hudson composite orogen. Large continental masses or supercontinents previously accreted during the Neoarchean Era began to break up between 2.4 and 2.0 Ga, leading to the deposition of widespread passive-margin sedimentary prisms and locally voluminous emplacement of mafic magma in radiating dike swarms. Further rifting and drifting led to the formation of incipient (e.g., Bravo Formation) to fully developed oceanic crust (e.g., Manikewan Ocean). Plate convergence beginning ca. 1.92 Ga heralded the demise of the Manikewan Ocean ~150 m.y. after its postulated opening. Protracted subduction of oceanic lithosphere over a period of ~90 m.y. produced a series of island arcs, some of which (Lynn Lake, Flin Flon, Snow Lake) host world-class volcanogenic massive sulfide (VMS) ± Au deposits. Plate convergence also led to progressive southeastward (present-day coordinates) accretion of microplates on a pre-amalgamated core consisting of the Slave craton and the Rae and Hearne “Provinces,” forming the Churchill plate. Following the formation of the Churchill plate collage ca. 1.86 Ga, subduction of oceanic lithosphere organized along an ~4000-km-long, north-dipping subduction zone along the southeastern edge of the Churchill plate, producing voluminous continental arc magmas in an Andean-type setting. The final phase of tectonic evolution involved collision of the Superior and North Atlantic cratons with the Churchill plate and intervening juvenile oceanic arc terranes. That phase was strongly influenced by the irregular shape of the indenting Superior craton, favoring the development of oroclines and leading to escape tectonics and lateral extrusion of continental microplates. For the most part, the Trans-Hudson was a hot but not necessarily thick orogen, perhaps reflecting a higher geothermal gradient during the Paleoproterozoic Era.

ABSTRACT The North American Cordillera experienced major contractional deformation during the Cretaceous–Paleogene, which is commonly attributed to normal subduction transitioning to shallow-slab subduction. We provide details of an alternative hit-and-run model, wherein the Insular superterrane obliquely collided with the North American margin from 100 to 85 Ma (the “hit”), followed by northward translation during continued oblique convergence with North America from 85 to 55 Ma (the “run”). This model assumes that the paleomagnetic evidence from the accreted terranes of the northern North American Cordillera, indicating up to thousands of kilometers of northward movement primarily between ca. 85 and 55 Ma, is correct. The hit-and-run model also incorporates new advances: (1) A worldwide plate reorganization occurred ca. 105–100 Ma; and (2) multiple subducted slabs have characterized subduction systems of the North American Cordillera since ca. 120 Ma. Finally, we explicitly address along-strike variations, such as the role of the preexisting rifted Precambrian margin and Permian–Triassic truncation of North America, in margin-parallel movement along western North America. The 100–85 Ma “hit” phase of the orogeny was characterized by dextral transpressional deformation that occurred simultaneously in the magmatic arcs of Idaho, northern Nevada, eastern California, and the Peninsular Ranges of southern California and northern Mexico. The hit phase also recorded incipient plateau formation, foreland block uplifts in the northern Rocky Mountains, and significant foreland sedimentation in adjacent North America. The transition from “hit” to “run” is hypothesized to have occurred because of the clockwise rotation of a Precambrian promontory in Washington State that was blocking northward translation: This rotation was accommodated by sinistral motion along the Lewis and Clark deformation zone. The 85–55 Ma “run” phase resulted in dextral strike-slip faulting of coastal blocks and significant contractional deformation in adjacent continental North America. The hit-and-run model is consistent with first-order geological and geophysical constraints from the North American Cordillera, and the proposed type of oblique orogeny requires a three-dimensional, time-dependent view of the deformation along an irregular and evolving continental margin.

ABSTRACT The North American Cordillera experienced significant and varied tectonism during the Triassic to Paleogene time interval. Herein, we highlight selected questions and controversies that remain at this time. First, we describe two tectonic processes that have hindered interpretations of the evolution of the orogen: (1) strike-slip systems with poorly resolved displacement; and (2) the closing of ocean basins of uncertain size, origin, and mechanism of closure. Next, we divide the orogen into southern, central, and northern segments to discuss selected controversies relevant to each area. Controversies/questions from the southern segment include: What is the origin of cryptic transform faults (Mojave-Sonora megashear vs. California Coahuila transform fault)? Is the Nazas an arc or a continental rift province? What is the Arperos basin (Guerrero terrane), and did its closure produce the Mexican fold-and-thrust belt? How may inherited basement control patterns of deformation during subduction? Controversies/questions from the central segment include: Can steeply dipping mantle anomalies be reconciled with geology? What caused high-flux events in the Sierra Nevada batholith? What is the origin of the North American Cordilleran anatectic belt? How does the Idaho segment of the orogen connect to the north and south? Controversies/questions from the northern segment include: How do we solve the Baja–British Columbia problem? How big and what kind of basin was the Early Cretaceous lost ocean basin? What connections can be found between Arctic geology and Cordilleran geology in Alaska? How do the Cretaceous tectonic events in the Arctic and northern Alaska connect with the Cordilleran Cretaceous events? What caused the Eocene tectonic transitions seen throughout the northern Cordillera? By addressing these questions along the length of the Cordillera, we hope to highlight common problems and facilitate productive discussion on the development of these features.

ABSTRACT The Neoproterozoic to Cambrian rifting history of Laurentia resulted in hyperextension along large segments of its Paleozoic margins, which created a complex paleogeography that included isolated continental fragments and exhumed continental lithospheric mantle. This peri-Laurentian paleogeography had a profound effect on the duration and nature of the Paleozoic collisional history and associated magmatism of Laurentia. During the initial collisions, peri-Laurentia was situated in a lower-plate setting, and there was commonly a significant time lag between the entrance of the leading edge of peri-Laurentia crust in the trench and the arrival of the trailing, coherent Laurentian landmass. The final Cambrian assembly of Gondwana was followed by a global plate reorganization that resulted in Cambrian (515–505 Ma) subduction initiation outboard of Laurentia, West Gondwana, and Baltica. Accretion of infant and mature intra-oceanic arc terranes along the Appalachian-Caledonian margin of the Iapetus Ocean started at the end of the Cambrian during the Taconic-Grampian orogenic cycle and continued until the ca. 430–426 Ma onset of the Scandian-Salinic collision between Laurentia and Baltica, Ganderia, and East Avalonia, which created the Laurussian continent and closed nearly all vestiges of the Iapetus Ocean. Closure of the Iapetus Ocean in the Appalachians was followed by the Devonian Acadian and Neoacadian orogenic cycles, which were due to dextral oblique accretion of West Avalonia, Meguma, and the Suwannee terranes following the Pridolian to Lochkovian closure of the Acadian seaway and subsequent outboard subduction of the Rheic Ocean beneath Laurentia. Continued underthrusting of Baltica and Avalonia beneath Laurentia during the Devonian indicates that convergence continued between Laurentia and Baltica and Avalonia, which, at least in part, may have been related to the motions of Laurentia relative to its converging elements. Cambrian to Ordovician subduction zones formed earlier in the oceanic realm between Laurentia and Baltica and started to enter the Arctic realm of Laurentia by the Late Ordovician, which resulted in sinistral oblique interaction of the Franklinian margin with encroaching terranes of peri-Laurentian, intra-oceanic, and Baltican provenance. Any intervening seaways were closed during the Middle to Late Devonian Ellesmerian orogeny. Exotic terranes such as Pearya and Arctic Alaska became stranded in the Arctic realm of Laurentia, while other terranes such as Alexander and Eastern Klamath were translated further into the Panthalassa Ocean. The Middle/Late Devonian to Mississippian Antler orogeny along the Cordilleran margin of Laurentia records the first interaction with an outboard arc terrane built upon a composite block preserved in the Northern Sierra and Eastern Klamath terranes. The Carboniferous–Permian Alleghanian-Ouachita orogenic cycle was due to closure of the vestiges of the Rheic Ocean and assembly of Pangea. The narrow, continental transform margin of the Ouachita embayment of southern Laurentia had escaped accretion by outboard terranes until the Mississippian, when it collided with an outboard arc terrane.

ABSTRACT The amalgamation of Laurentia’s Archean provinces ca. 1830 Ma was followed by ~700 m.y. of accretionary orogenesis along its active southeastern margin, marked by subduction of oceanic lithosphere, formation of arcs and back-arcs, and episodic accretion. This prolonged period of active-margin tectonic processes, spanning the late Paleoproterozoic and Mesoproterozoic eras, resulted in major accretionary crustal growth and was terminated by closure of the Unimos Ocean (new name). Ocean closure was associated with rapid motion of Laurentia toward the equator and resulted in continental collision that led to profound reworking of much of the accreted Proterozoic crust during the ca. 1090–980 Ma Grenvillian orogeny. The Grenvillian orogeny resulted in formation of a large, hot, long-duration orogen with a substantial orogenic plateau that underwent extensional orogenic collapse before rejuvenation and formation of the Grenville Front tectonic zone. The Grenvillian orogeny also caused the termination and inversion of the Midcontinent Rift, which, had it continued, would likely have split Laurentia into distinct continental blocks. Voluminous mafic magmatic activity in the Midcontinent Rift ca. 1108–1090 Ma was contemporaneous with magmatism in the Southwestern Laurentia large igneous province. We discuss a potential link between prolonged subduction of oceanic lithosphere beneath southeast Laurentia in the Mesoproterozoic and the initiation of this voluminous mafic magmatism. In this hypothesis, subducted water in dense, hydrous Mg-silicates transported to the bottom of the upper mantle led to hydration and increased buoyancy, resulting in upwelling, decompression melting, and intraplate magmatism. Coeval collisional orogenesis in several continents, including Amazonia and Kalahari, ties the Grenvillian orogeny to the amalgamation of multiple Proterozoic continents in the supercontinent Rodinia. These orogenic events collectively constituted a major turning point in both Laurentian and global tectonics. The ensuing paleogeographic configuration, and that which followed during Rodinia’s extended breakup, set the stage for Earth system evolution through the Neoproterozoic Era.

ABSTRACT The discovery of multiple deformed and metamorphosed sedimentary successions in southwestern Laurentia that have depositional ages between ca. 1.50 and 1.45 Ga marked a turning point in our understanding of the Mesoproterozoic tectonic evolution of the continent and its interactions with formerly adjacent cratons. Detrital zircon U-Pb ages from metasedimentary strata and igneous U-Pb zircon ages from interbedded metavolcanic rocks in Arizona and New Mexico provide unequivocal evidence for ca. 1.50–1.45 Ga deposition and burial, followed by ca. 1.45 and younger deformation, metamorphism, and plutonism. These events reflect regional shortening and crustal thickening that are most consistent with convergent to collisional orogenesis—the Mesoproterozoic Picuris orogeny—in southwestern Laurentia. Similar metasedimentary successions documented in the midcontinent of the United States and in eastern Canada help to establish ca. 1.45 Ga orogenesis as a continent-scale phenomenon associated with a complex and evolving convergent margin along southern Laurentia. Metasedimentary successions of similar age are also exposed across ~5000 km of the western Laurentian margin and contain distinctive 1.6–1.5 Ga detrital zircon populations that are globally rare except in select cratonic provinces in Australia and Antarctica. The recognition of these distinctive detrital zircon ages provides a transient record of plate interactions prior to breakup of Nuna or Columbia ca. 1.45 Ga and provides key constraints on global plate reconstructions.

ABSTRACT The Appalachian Mountains were formed through multiple phases of Paleozoic orogenesis associated with terrane accretion. The timing, tempo, and significance of each event in New England are obscured by overprinting, the limits of geochronologic tools, and differences between lithotectonic domains. We present new monazite and xenotime geochronology, 40 Ar/ 39 Ar thermochronology, and major- and trace-element thermobarometry from major tectonic domains in southern New England and across multiple structural levels. These data show contrasting pressure-temperature-time ( P-T-t ) paths across tectonic domains and highlight eastward metamorphic overprinting associated with younger tectonic events. Our data and geochemical proxies suggest two major periods of crustal thickening, ca. 455–440 Ma and 400–380 Ma, and a heterogeneous record of thinning/exhumation. Ordovician (Taconic) crustal thickening postdates the interpreted accretion of the Moretown terrane by ~20 m.y. and may have been related to shallow subduction after subduction polarity reversal. Subsequent cooling and exhumation (440–430 Ma) may have been related to the end of the Taconic orogeny and opening of the Connecticut Valley basin. (Neo)Acadian tectono-metamorphism is recognized in accreted terranes of New England and is absent in the Taconic block. Amphibolite- to (high-pressure) granulite-facies metamorphism, slow cooling, and protracted anatexis ca. 400–340 Ma support the existence of a long-lived orogenic plateau in southern New England. Exhumation, which began at 340–330 Ma, may have involved ductile (channel) flow. The boundary between continental Laurentia and accreted terranes has been reactivated at multiple times and is presently manifested as a 12–15 km Moho step. At the latitude of our samples, Alleghanian-age tectonism (ca. 310–285 Ma) was limited to retrograde metamorphism, and relatively minor loading and exhumation in the vicinity of the Pelham dome. Our results highlight the sensitivity of the integrative petrochronologic approach and the transition of the eastern margin of Laurentia from terrane accretion to the formation of a high-elevation plateau.

ABSTRACT Rapid midcrustal cooling (>10 °C/m.y.) is typical of Phanerozoic orogens, but it is less commonly reported from Precambrian orogenic belts. Abundant new 40 Ar/ 39 Ar (predominantly plateau) dates reveal a period of late, rapid cooling following slow postpeak metamorphic cooling during the evolution of the Paleoproterozoic Cape Smith belt, a greenschist- to amphibolite-facies foreland thrust belt in the ca. 1.83–1.76 Ga Trans-Hudson orogen. We conducted 40 Ar/ 39 Ar step-heating analyses on biotite, hornblende, and/or muscovite from 38 samples sourced from the thrust belt and its footwall basement, the Archean Superior craton. The 40 Ar/ 39 Ar dates from the Cape Smith belt and re-equilibrated Superior craton ranged ca. 1948–1708 Ma in biotite, ca. 1801–1697 Ma in muscovite, and ca. 1764–1694 Ma in hornblende. Of these, ~70% were ca. 1740–1700 Ma plateau dates, which we interpret as cooling ages following Cape Smith belt metamorphism; gas-release spectra of older outlying dates exhibit characteristics of excess Ar. Following the metamorphic thermal peak, the belt cooled at slow rates of up to ~1 °C/m.y. until ca. 1740 Ma. Concordant biotite, muscovite, and hornblende cooling dates of ca. 1740–1700 Ma require fast, late cooling of the belt (≥4 °C/m.y.) through upper midcrustal levels (~500–300 °C), and they allow for very rapid cooling rates (≤200 °C/m.y.). Accelerated cooling rates may have been triggered by uplift in response to detachment of lower crust or subcontinental lithosphere, facilitated by the postcollisional relaxation of isotherms and structural uplift in basement-involved folds. In Superior craton basement, ca. 2704–2667 Ma 40 Ar/ 39 Ar hornblende plateau dates reflect undisturbed cooling ages following Neoarchean metamorphism, whereas younger and wide-ranging 40 Ar/ 39 Ar biotite dates (ca. 2532–1743 Ma) with variable gas-release spectra suggest spatially heterogeneous degrees of Ar resetting in biotite during Cape Smith belt tectonism. Partially reset 40 Ar/ 39 Ar biotite dates in the Superior craton up to ~100 km south of the belt suggest that the pre-erosional thrust wedge extended at least that far south, and that it imposed a widespread low-temperature (<300 °C) and/or short-lived thermal overprint on the footwall basement. Integration of multimineral 40 Ar/ 39 Ar data with structural and metamorphic constraints for the Cape Smith belt indicates that modern-style postcollisional exhumation and rapid cooling were viable processes during the middle Paleoproterozoic.

ABSTRACT An accretionary tectonic model for the Mesoproterozoic ca. 1500–1340 Ma tectonic evolution of the southern Laurentian margin is presented. The tectonic model incorporates key observations about the nature and timing of Mesoproterozoic deposition, magmatism, regional metamorphism, and deformation across the 5000-km-long southern Laurentian margin. This time period was one of transition in the supercontinent cycle and occurred between the breakup of Columbia and the formation of Rodinia, and the southern Laurentian margin was a significant component of a much greater accretionary margin extending into Baltica and Amazonia and possibly parts of Antarctica and Australia. However, fundamental questions and contradictions remain in our understanding of the tectonic evolution of Laurentia and paleogeography during this time interval.

ABSTRACT The Archean Wyoming Province formed and subsequently grew through a combination of magmatic and tectonic processes from ca. 4.0 to 2.5 Ga. Turning points in crustal evolution are recorded in four distinct phases of magmatism: (1) Early mafic magmatism formed a primordial crust between 4.0 and 3.6 Ga and began the formation of a lithospheric keel below the Wyoming Province in response to active plume-like mantle upwelling in a “stagnant lid”–type tectonic environment; (2) earliest sialic crust formed in the Paleoarchean by melting of hydrated mafic crust to produce rocks of the tonalite-trondhjemite-granodiorite (TTG) suite from ca. 3.6 to 2.9 Ga, with a major crust-forming event at 3.3–3.2 Ga that was probably associated with a transition to plate tectonics by ca. 3.5 Ga; (3) extensive calc-alkalic magmatism occurred during the Mesoarchean and Neoarchean (ca. 2.85–2.6 Ga), forming plutons that are compositionally equivalent to modern-day continental arc plutons; and (4) a late stage of crustal differentiation occurred through intracrustal melting processes ca. 2.6–2.4 Ga. Periods of tectonic quiescence are recognized in the development of stable platform supracrustal sequences (e.g., orthoquartzites, pelitic schists, banded iron formation, metabasites, and marbles) between ca. 3.0 and 2.80 Ga. Evidence for late Archean tectonic thickening of the Wyoming Province through horizontal tectonics and lateral accretion was likely associated with processes similar to modern-style convergent-margin plate tectonics. Although the province is surrounded by Paleoproterozoic orogenic zones, no post-Archean penetrative deformation or calc-alkalic magmatism affected the Wyoming Province prior to the Laramide orogeny. Its Archean crustal evolution produced a strong cratonic continental nucleus prior to incorporation within Laurentia. Distinct lithologic suites, isotopic compositions, and ages provide essential reference markers for models of assembly and breakup of the long-lived Laurentian supercontinent.

ABSTRACT Supermature siliciclastic sequences were deposited between 1.64 Ga and 1.59 Ga over a broad swath of southern Laurentia in the Archean, Penokean, Yavapai, and Mazatzal Provinces. These siliciclastic sequences are notable for their extreme mineralogical and chemical maturity, being devoid of detrital feldspar and ferromagnesian minerals, containing the clay mineral kaolinite (or its metamorphic equivalent, pyrophyllite), and having a chemical index of alteration >95. Such maturity is the result of a perfect confluence of tectonic and climatic conditions, including a stable continental crust with low topographic relief (the Archean, Penokean, and Yavapai Provinces ca. 1.70 Ga), a warm humid climate, an elevated level of atmospheric CO 2 , and relatively acidic pore fluids in the critical zone. The weathered detritus was transported and deposited by southward-flowing streams across the Archean, Penokean, and Yavapai Provinces, ultimately to be deposited on 1.66 Ga volcanic and volcaniclastic rocks in the Mazatzal continental arc along the southern margin of Laurentia.

ABSTRACT The Neoarchean marked an important turning point in the evolution of Earth when cratonization processes resulted in progressive amalgamation of relatively small crustal blocks into larger and thicker continental masses, which now comprise the ancient core of our continents. Although evidence of cratonization is preserved in the ancient continental cores, the conditions under which this geodynamic process operated and the nature of the involved crustal blocks are far from resolved. In the Superior craton, deep-crustal fault systems developed during the terminal stage of Neoarchean cratonization, as indicated by the cratonwide growth of relatively small, narrow, syn-to-late tectonic (ca. 2680–2670 Ma) sedimentary basins. The terrigenous debris eroded from the uplifted tectono-magmatic source regions was deposited as polymictic conglomerate and sand successions in fluvial-dominated basins. The composition of the sedimentary rocks in these unique basins, therefore, offers a unique record of crustal sources and depositional settings, with implications for the geodynamic processes that formed the world’s largest preserved craton. Here, we compare the geochemical compositions of sandstone samples from six sedimentary basins across the Abitibi greenstone belt and relate them to their mode of deposition, prevailing provenance, and geodynamic setting during crustal growth and craton stabilization. The sandstones represent first-cycle sediment that is poorly sorted and compositionally very immature, with variable Al 2 O 3 /TiO 2 ratios and index of chemical variability values >1 (average of 1.36), reflecting a large proportion of framework silicate grains. The sandstones display chemical index of alteration values between 45 and 64 (average of 53), indicating that the detritus was eroded from source regions that experienced a very low degree of chemical weathering. This likely reflects a high-relief and active tectonic setting that could facilitate rapid erosion and uplift with a short transit time of the detritus from source to deposition. Multi-element variation diagrams and rare earth element patterns reveal that the lithological control on sandstone composition was dominated by older (>2695 Ma) pretectonic tonalite-trondhjemite-granodiorite and greenstone belt rocks. The sandstone units display large variations in the proportions of felsic, mafic, and ultramafic end-member contributions as a consequence of provenance variability. However, an average sandstone composition of ~65% felsic, ~30% mafic, and ~5% komatiite was observed across the basins. This observation is in agreement with recent models that predict the composition of the Neoarchean emerged continental crust for North America and supports the presence of a felsic-dominated Archean crust. The high proportion of felsic rocks in the upper crust requires continuous influx of H 2 O into the mantle and is best explained by subduction-related processes. In such a scenario, the detritus of the fluvial sandstones is best described as being controlled by uplifted and accreted continental arcs mainly composed of tonalite-trondhjemite-granodiorite and greenstone belt rocks.