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Volume 51 • Number 3 • March 2014

John Tuzo Wilson, a man who moved mountains, and his contributions to earth sciences

Title page

Introduction

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Vol. 51No. 3pp. v–viii
John Tuzo Wilson (1908–1993) was one of the greatest Canadian scientists of the 20th century. His contributions to Earth Sciences, leading the formulation of the theory of plate tectonics, have revolutionized our understanding of how the planet Earth works and evolved over the past 4 billion years. This 50th anniversary special issue of the Canadian Journal of Earth Sciences is dedicated in honour of John Tuzo Wilson, who inspired tens of thousands of students all around the world to study the Earth. This special issue contains 12 papers dealing with various aspects of the “Wilson Cycle” in the geologic record, plate tectonics, mantle plumes, and how John Tuzo Wilson accepted “continental drift” and formulated the theory of plate tectonics. The contributions have mostly been made by geoscientists who directly or indirectly associated with John Tuzo Wilson and have contributed significantly to the plate tectonics paradigm.
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Vol. 51No. 3pp. ix–xvi
It is noted that different physicists and geologists have in recent years espoused not less than four groups of theories of the physical behavior of the Earth’s interior. Recent observations of submarine geology, heat, and rock magnetism have tended to support some form of continental drift rather than the older concept of a rigid earth.The Hawaiian Islands are one of seven, parallel, linear chains of islands and seamounts in the Pacific Ocean of Tertiary to Recent age. Their nature had previously been explained in terms of a series of volcanoes along parallel faults. Horizontal shear motion along these faults was supposed to be extending them southeasterly.The inadequacies of this explanation are pointed out. If there are convection currents in the Pacific region and if the upper parts of these cells move faster than the central parts, sources of lava within the slower moving cores could give rise to linear chains of progressively older volcanic piles such as the Hawaiian Islands. This view is shown to be compatible with seismic observations and age determinations.

Tribute

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Vol. 51No. 3pp. xvii–xxxi
Fifty years ago, the world’s Earth Scientists experienced the so-called “Revolution in the Earth Sciences”. In the decade from 1960 to 1970, a massive convergence took place from many diverse and contradictory theories about the tectonic processes operating on Earth (then loosely called “mountain building”) to a single widely accepted paradigm now called Plate Tectonics. A major player in leading the international “Revolution” was Canadian geophysicist J. Tuzo Wilson. This tribute reviews how he helped define and promote the Plate Tectonic paradigm, and also, from 1946 to 1967, how he led a rapid expansion of the role of geophysics in Canadian and international earth science. Wilson was a controversial figure before and during the “Revolution”, but his influence was large. It was not coincidental that earth science research in Canada grew by 1964 to the point where the National Research Council of Canada could add the Canadian Journal of Earth Sciences to its group of Canadian research journals.

Review

Vol. 51No. 3pp. 187–196
John Tuzo Wilson coined the term “plate” in plate tectonics. He is famous for inventing transform boundaries, hot spot tracks, and the Wilson cycle of ocean birth, growth, and decline. Less well remembered is his work in the 1950s on tectonic and radiometric age provinces of the Canadian Shield, as part of which he fathered U/Pb geochronology in Canada. This work gave strong support to the notion of continental growth through accretion of successively younger terranes onto an ancient cratonic core. The present paper reviews how paleomagnetism can trace the motions of continents to test Wilson’s ideas. Continental accretion often involves deep burial of one of the colliding elements through subduction or crustal underplating; such was the case with the Grenville orogen and its subprovinces in their Proterozoic accretion onto the Laurentian craton. The resulting heating and metamorphism erases most pre-collisional magnetic information but adds something new: the possibility of following the post-metamorphic uplift and cooling history, in time and space. The time element is provided by a new form of isotopic geochronology, thermochronometry, which provides dates for specific minerals together with the temperatures at which they became closed to isotopic migration. U/Pb dating of sphene is one method used; another is the 40Ar/39Ar variant of K/Ar dating applied to hornblende, micas, and feldspars, which have a wide range of Ar closure temperatures. The two specific Grenville studies described deal with parallel uplift histories determined by 40Ar/39Ar dating and by magnetics for the accreted terranes of the Central Metasedimentary Belt in Ontario and with the paleomagnetic detection of the post-1240 Ma closing of a small ocean between the Elsevir terrane and Laurentia during the Grenvillian orogeny.

Articles

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Vol. 51No. 3pp. 197–207
Tuzo Wilson’s well-known pre-1961 opposition to continental drift stemmed from his early experience as a geologist in the Appalachians and the Canadian Shield, which convinced him that orogenesis did not change drastically over geologic time. Conversely, Taylor (in 1910) and Wegener (in 1912) hypothesized that continental drift began in Cenozoic or Mesozoic time. Between 1949 and 1960, Tuzo Wilson with Adrian Scheidegger developed a quasi-uniformitarian model of progressive continental accretion around fixed Archean nuclei. Tuzo abruptly jettisoned this model in 1961 when, under pressure from paleomagnetic evidence for continental drift and a nascent concept of sea-floor spreading, he finally entertained the possibility of pre-Mesozoic as well as younger continental drift. He immediately found it a superior fit to Appalachian and Shield geology, while his uniformitarian conviction remained intact. Tuzo had blinded himself to the evidence for continental drift so long as he confined it to Taylor or Wegener’s conception. In continental drift operating continuously over geologic time, he found a theory he could eagerly accept.
Vol. 51No. 3pp. 208–221
Having discerned competition among vigorous plumes on the 108 year timescale, Tuzo Wilson suggested that plumes control plate behavior and are “the mainsprings of geological history”. Here we revisit that idea by discussing selected examples of plume–plate interaction and find that modern observational, instrumental, computational, and modeling capabilities are revealing a wonderful variety and complexity in plume–plate interaction. The degree to which plumes control plate behavior is poorly constrained. However, the examples we consider suggest complex interactions between plumes and plates that, during the past 70 million years, have led to separate episodes of extreme plate acceleration and near complete cessation of plate motion in the deep mantle reference frame. The recognition of contrasting convective behavior within two newly distinguished “Active Plume Heads”, both reaching to depths of ∼1200 km, one beneath Hawaii and the other between Iceland and Norway, represent new opportunities in studying plume–plate interaction. Wilson’s suggestion continues to inspire stimulating questions for future research.
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Vol. 51No. 3pp. 222–242
The North Anatolian Fault is a 1200 km long strike-slip fault system connecting the East Anatolian convergent area with the Hellenic subduction zone and, as such, represents an intracontinental transform fault. It began forming some 13–11 Ma ago within a keirogen, called the North Anatolian Shear Zone, which becomes wider from east to west. Its width is maximum at the latitude of the Sea of Marmara, where it is 100 km. The Marmara Basin is unique in containing part of an active strike-slip fault system in a submarine environment in which there has been active sedimentation in a Paratethyan context where stratigraphic resolution is higher than elsewhere in the Mediterranean. It is also surrounded by a long-civilised rim where historical records reach well into the second half of the first millennium BCE (before common era). In this study, we have used 210 multichannel seismic reflexion profiles, adding up to 6210 km profile length and high-resolution bathymetry and chirp profiles reported in the literature to map all the faults that are younger than the Oligocene. Within these faults, we have distinguished those that cut the surface and those that do not. Among the ones that do not cut the surface, we have further created a timetable of fault generation based on seismic sequence recognition. The results are surprising in that faults of all orientations contain subsets that are active and others that are inactive. This suggests that as the shear zone evolves, faults of all orientations become activated and deactivated in a manner that now seems almost haphazard, but a tendency is noticed to confine the overall movement to a zone that becomes narrower with time since the inception of the shear zone, i.e., the whole keirogen, at its full width. In basins, basin margins move outward with time, whereas highs maintain their faults free of sediment cover, making their dating difficult, but small perched basins on top of them in places make relative dating possible. In addition, these basins permit comparison of geological history of the highs with those of the neighbouring basins. The two westerly deeps within the Sea of Marmara seem inherited structures from the earlier Rhodope–Pontide fragment/Sakarya continent collision, but were much accentuated by the rise of the intervening highs during the shear evolution. When it is assumed that below 10 km depth the faults that now constitute the Marmara fault family might have widths approaching 4 km, the resulting picture resembles a large version of an amphibolite-grade shear zone fabric, an inference in agreement with the scale-independent structure of shear zones. We think that the North Anatolian Fault at depth has such a fabric not only on a meso, but also on a macro scale. Detection of such broad, vertical shear zones in Precambrian terrains may be one way to get a handle on relative plate motion directions during those remote times.
Vol. 51No. 3pp. 243–265
Remnants of the early-Ottawan thrust-sheet stack are exposed in the Central Gneiss Belt (CGB, lower portion of stack) and the Composite Arc Belt (upper portion of stack). Post-collisional vertical thinning and associated horizontal extension of the stack produced structures ranging over eight orders of magnitude in horizontal length, and both orogen-parallel and orogen-perpendicular in orientation. At the 100 km scale, the fold-induced constriction in the northern Parry Sound domain appears to have been enhanced, and lineation trend lines in its footwall locally deflected, by a component of NW–SE (i.e., orogen-perpendicular) flattening and a component of NE–SW (i.e., orogen-parallel) ductile extension. At the 10 km scale, four non-cylindrical lenticular bodies of gabbro–anorthosite gneiss within the domain, inferred to be triaxial mega-boudins or heterogeneously strained plutons, are separated by large extensional bending folds, the complementary structures attesting to a component of NW–SE flattening and a component of NE–SW extension. Non-cylindrical lenticular structures in other domains of the CGB, interpreted as triaxial foliation mega-boudins, exceed 30 km in length. Their moderately strained granulite-facies interiors give way to highly strained amphibolite-facies margins, thus documenting subvertical ductile flattening and multi-lateral extension during retrogression. Well-layered, highly strained gneiss is commonly deformed by steep NE–SW-trending extensional faults and associated monoclinal fault-propagation folds (FPFs). The short limbs of the FPFs bend the regional elongation lineation and host a set of fault-parallel, unstrained to slightly deformed, granite–pegmatite dikes. Dilation vectors of most dikes are oblique to the granite–pegmatite contacts, and the sense of their tangential components attests to orogen-perpendicular extension. The fault-parallel dikes and associated FPFs are cut by a set of unstrained dikes. Collectively these observations document a prolonged history of post-collisional extension of the mid crust, from ductile structures indicative of a significant component of orogen-parallel extension shortly after the metamorphic peak at mid-crustal depths, to brittle–ductile structures indicative of a component of orogen-perpendicular extension and associated magmatic dilation following its exhumation and cooling in the upper crust.
Vol. 51No. 3pp. 266–271
The plate tectonic revolution, culminating with the formulation of the Wilson Cycle, took place over a period of less than a decade in the 1960s and early 1970s. The model provided a framework for understanding the formation of almost every type of mineral deposit then known on Earth, ranging from base and precious metal deposits associated with rifting, to porphyry Cu–Mo and epithermal Au deposits associated with subduction, and collision-related mesothermal Au deposits. By the end of the 1970s, satisfactory tectonic models for most of these deposit types had been established. Modern geological and economic geology research is largely built on these models, which have been expanded in detail but remain largely unchanged in concept and function.
Vol. 51No. 3pp. 272–285
The Sanandaj–Sirjan Zone is a basement culmination northeast of the Neo-Tethys suture in Iran. In this zone near Azna, granite has a magmatic zircon U–Pb age of 568 ± 11 Ma, with 900–800, ca. 2400, and ca. 3600 Ma inherited cores. The ca. 3600 Ma inherited zircon is the oldest crustal component yet detected in Iran. Near Chadegan, orthogneiss has a magmatic zircon U–Pb age of 637 ± 15 Ma, and carries ca. 1000 and 2000 Ma inherited zircons. Inherited 900–1000 Ma zircons have juvenile initial εHf values of ca. +8 to +9, whereas the younger 630 and 568 Ma magmatic zircons show lower initial εHf values; however, the 3600 Ma core has initial εHf = 0.0. A Neoproterozoic rim on the inherited 3600 Ma core has the most extreme initial εHf value of −18. The Hf isotopic data indicates generation of the magmatic protoliths from a mixture of juvenile Neoproterozoic and Archean sources. Previous studies showed that in Turkey the Central Anatolian Crystalline Complex is underlain by Neo-Eoarchean rocks, the Menderes Massif contains Neoproterozoic granitoids, and that central Iran’s basement and the northern Sanandaj–Sirjan Zone contain Neoproterozoic granitic rocks. This basement terrane is from Gondwana, and was transferred across Paleo-Tethys to dock against Eurasia’s southern margin. Occurrence in Iran and Turkey of Eoarchean crust raises the possibility of sinistral migration of this terrane in the closure of Tethys because the nearest known early Archean crust occurs in northeast India.
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Vol. 51No. 3pp. 286–296
As the asthenospheric mantle rises at oceanic spreading centres, it undergoes partial melting, producing oceanic crust and depleted mantle, both of which have lower intrinsic density than the asthenospheric mantle from which they were derived. With a warmer asthenosphere in the Archean, these effects are enhanced, leading to the possibility that subduction was no longer feasible. I investigate the density of the oceanic crust and underlying mantle for a mantle with temperatures 200 °C higher than today, using models of the chemistry of melting and the mineralogy of the ensuing rocks. For the melting model used, crustal thicknesses are 21 km and the depth to which the mantle is partially melted is 114 km, compared with 7 and 54 km for a comparable model of modern Earth. Two thermal-evolution models for Archean oceanic lithosphere are examined. One assumes twice the heat flow into the base of the plates, which severely restricts the depths to which the plates can cool with age. A second assumes the plates can cool to the depth to which the asthenosphere undergoes partial melting, resulting in heat flow into the base of the plates only 1.3 times as large as today. With the first model, oceanic plates do not become denser than an equivalent column of asthenosphere. With the second, they do after ∼50 Ma of cooling. In both cases, however, the cooling is sufficient to provide a significant driving force for the initiation of subduction because the sole requirement for a subduction-initiation drive is that the cooled lithosphere be denser than the column of differentiated asthenosphere that would replace it. This, combined with the low flexural rigidity of Archean plates, makes the initiation of subduction probably slightly easier than it is today. The relatively low density of oceanic plates results in lower slab pull, but this effect is counterbalanced both by the likelihood that some of the low-density crust may have been delaminated, and by the effect of passage of the thicker crust through the eclogite transition. Given our present knowledge of Archean thermal conditions, there does not appear to be a compelling theoretical argument against efficient subduction processes at that time.
Vol. 51No. 3pp. 297–311
A review and comparison of the tectonic history of the North China and Slave cratons reveal that the two cratons have many similarities and some significant differences. The similarities rest in the conclusion that both cratons have a history of a Wilson Cycle, having experienced rifting of an old continent in the late Archean, development of a rift to passive margin sequence, collision of this passive margin with arcs within 100–200 Ma of the formation of the passive margin, reversal of subduction polarity, then eventual climactic collision with another arc terrane, microcontinental fragment, or continent. This cycle demonstrates the operation of Paleozoic-style plate tectonics in the late Archean. The main differences lie in the later tectonic evolution. The Slave’s post-cratonization history is dominated by subduction dipping away from the interior of the craton, and later incorporation into the interior of a larger continent, whereas the North China Craton has had a long history of subduction beneath the craton, including presently being located above the flat-lying Pacific slab resting in the mantle transition zone, placing it in a broad back-arc setting, with multiple mantle hydration events and collisions along its borders. The hydration enhances melting in the overlying mantle, and leads to melts migrating upwards to thermochemically erode the lithospheric root. This major difference may explain why the relatively small Slave craton preserves its thick Archean lithospheric root, whereas the eastern North China Craton has lost it.
Vol. 51No. 3pp. 312–325
The geological history and evolution of the Dharwar craton from ca. 3.5–2.5 Ga is reviewed and briefly compared with a second craton, Kaapvaal, to allow some speculation on the nature of global tectonic regimes in this period. The Dharwar craton is divided into western (WDC) and eastern (EDC) parts (separated possibly by the Closepet Granite Batholith), based on lithological differences and inferred metamorphic and magmatic genetic events. A tentative evolution of the WDC encompasses an early, ca. 3.5 Ga protocrust possibly forming the basement to the ca. 3.35–3.2 Ga Sargur Group greenstone belts. The latter are interpreted as having formed through accretion of plume-related ocean plateaux. The approximately coeval Peninsular Gneiss Complex (PGC) was possibly sourced from beneath plateau remnants, and resulted in high-grade metamorphism of Sargur Group belts at ca. 3.13–2.96 Ga. At about 2.9–2.6 Ga, the Dharwar Supergroup formed, comprising lower Bababudan (largely braided fluvial and subaerial volcanic deposits) and upper Chitradurga (marine mixed clastic and chemical sedimentary rocks and subaqueous volcanics) groups. This supergroup is preserved in younger greenstone belts with two distinct magmatic events, at 2.7–2.6 and 2.58–2.54 Ga, the latter approximately coincident with ca. 2.6–2.5 Ga granitic magmatism which essentially completed cratonization in the WDC. The EDC comprises 2.7–2.55 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses and migmatites, approximately coeval greenstone belts (dominated by volcanic lithologies), with minor inferred remnants of ca. 3.38–3.0 Ga crust, and voluminous 2.56–2.5 Ga granitoid intrusions (including the Closepet Batholith). An east-to-west accretion of EDC island arcs (or of an assembled arc – granitic terrane) onto the WDC is debated, with a postulate that the Closepet Granite accreted earlier onto the WDC as part of a “central Dharwar” terrane. A final voluminous granitic cratonization event is envisaged to have affected the entire, assembled Dharwar craton at ca. 2.5 Ga. When Dharwar evolution is compared with that of Kaapvaal, while possibly global magmatic events and freeboard–eustatic changes at ca. 2.7–2.5 Ga may be identified on both, the much earlier cratonization (by ca. 3.1 Ga) of Kaapvaal contrasts strongly with the ca. 2.5 Ga stabilization of Dharwar. From comparing only two cratons, it appears that genetic and chronologic relationships between mantle thermal and plate tectonic processes were complex on the Archaean Earth. The sizes of the Kaapvaal and Dharwar cratons might have been too limited yet to support effective thermal blanketing and thus accommodate Wilson Cycle onset. However, tectonically driven accretion and amalgamation appear to have predominated on both evolving cratons.
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Volume 61
Issue 2
February 2024
Volume 61
Issue 1
January 2024
Volume 60
Issue 12
December 2023
Volume 60
Issue 11
November 2023
Volume 60
Issue 10
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Volume 60
Issue 9
September 2023