Taking the pulse of planet earth: a proposal for a new multi-disciplinary flagship project in Canadian solid earth sciences.SUMMARY
Herein I propose a vision for a new multidisciplinary "big science" project for Canada's solid earth sciences. I call this proposed project: "Taking the Pulse of Planet Earth". At a modest cost, and over a 5 to 10 year life-span, it would aim at providing the most comprehensive and multidisciplinary knowledge base of the complete record of mafic magmatism in and around Canada, and through stimulating international cooperation, around the world.
A complete record of mafic magmatism (spatial distribution, ages, periodicities, rates, volume estimates, estimated geochemical fluxes to atmosphere and hydrosphere hydrosphere
Discontinuous layer of water at or near the Earth's surface. It includes all liquid and frozen surface waters, groundwater held in soil and rock, and atmospheric water vapour. Virtually all of these waters are in constant circulation through the hydrologic cycle. , tectonic settings, structural trends, sequence stratigraphic stra·tig·ra·phy
The study of rock strata, especially the distribution, deposition, and age of sedimentary rocks.
strat framework, evolving major and trace element compositions, evolving isotopic ratios, paleomagnetic information, paleo-intensities, associated ore deposits, etc.) constitutes critical input data for numerous first-order questions about the past and present evolution of our planet. Many of such questions relate to issues that are currently a focus of attention: global change, past climate extremes, complex Earth systems, planetary evolution, extinction events, flood volcanism volcanism
Any of various processes and phenomena associated with the surface discharge of molten rock or hot water and steam, including volcanoes, geysers, and fumaroles. , potential relationships with large impact events, and the discovery of new ore resources.
The proposed project is a focused, "smart", and highly efficient approach to solve a large number of these seemingly unrelated but first-order questions in contemporary earth science. At its core, it would have a large dating program, aiming to provide approximately 200 new, high-precision ages of mafic magmatic events across Canada Across Canada was an afternoon program that formerly aired on The Weather Network. The segment ran from early 1999 until mid 2002. The show ran from 3:00PM ET until 7:00 PM ET. and adjacent regions. A Supporting Geoscience ge·o·sci·ence
Any one of the sciences, such as geology or geochemistry, that deals with the earth.
ge grant system would ensure that other aspects of the magmatic record receive equal attention. Finally, I illustrate the impact this project would have on paleo-continental reconstructions. As part of this illustration, I synthesize existing data on two dyke swarms, the ca. 2.45 Ga Matachewan and Kaminak swarms, respectively, and propose a novel Superior-Hearne reconstruction within supercraton Superia.
Je propose ci-contre l'idee d'un nouveau projet multidisciplinaire de grande envergure dans le domaine des sciences des roches solides au Canada. J'ai nomme ce projet << Prendre le pouls de la planete Terre>>. D'un cout modeste et s'echelonnant sur cinq a dix ans, ce projet constituerait la base de connaissances multidisciplinaire la plus complete de tout le repertoire des evenements magmatiques mafiques au Canada et a son pourtour, cela, en profitant des effets stimulant de la cooperation internationale.
Un repertoire complet des evenements magmatiques mafiques (distribution spatiale, ages, periodicites, taux, estimations des volumes, estimations geochimiques des flux dans l'atmosphere et l'hydrosphere, cadres tectoniques, styles structuraux, cadres stratigraphiques des sequences, compositions des suites evolutives des elements majeurs et en trace, evolution des ratios isotopiques, donnees paleomagnetiques, paleo-intensites, gisements associes, etc.) constitue un registre de donees de base cruciales pour nombres de grandes questions sur revolution passee et actuelle de notre planete. Plusieurs de ces grandes questions sont liees a des problemes actuels qui mobilisent l'attention, tels: les changements a l'echelle planetaire, les evenements climatiques extremes du passe pas·sé
1. No longer current or in fashion; out-of-date.
2. Past the prime; faded or aged.
[French, past participle of passer, to pass, from Old French; see , les systemes planetaires complexes, l'evolution de la planete, les grandes extinctions biotiques, les grands epanchements volcaniques, les liens eventuels avec de grands impacts meteoritiques, ainsi que la decouverte de nouvelles sources de minerai.
Le projet propose constitue une approche ciblee, << habile hab·ile
Generally able or adroit; handy.
[Middle English habil, from Old French habile, from Latin habilis, from hab >>, et tres efficace permettant de solutionner un grand nombre de ces grands problemes, sans liens apparents, des geosciences contemporaines. Au coeur du projet on retrouve un grand programme de datation visant a etablir quelques 200 nouvelles datations de grande precision d'evenements magmatiques au Canada et dans les regions peripheriques. Un systeme de subvention d'appoint permettrait d'assurer que d'autres aspects de la problematique magmatique recoivent autant d'attention. Finalement, a titre titre
titer. d'illustration, je decris les repercussions repercussions npl → répercussions fpl
repercussions npl → Auswirkungen pl escomptees d'un tel projet sur les reconstitutions paleo-continentales. Dans le cadre de cette illustration, je presente une synthese des donnees disponibles concernant deux reseaux de dykes, soit les reseaux de Matachewan et de Kaminak de 2,45 Ga, respectivement, et propose une nouvelle reconstitution Superieur-Hearne dans le super-craton Superia.
LITHOPROBE, for more than two decades the premier flagship project of Canada's solid earth sciences, is winding down. Lithosphere-scale cross-sections have been gathered across the Canadian landmass land·mass
A large unbroken area of land.
a large continuous area of land
landmass and final syntheses are now being prepared (e.g., Percival et al., 2004). The inevitable ending of this project, on the heels of its success, will leave a growing vacuum across the Canadian earth science community at a time when competition for significant science funding is stronger than ever. Much of this competition revolves around highly visible "big science" projects in such fields as physics, astronomy, biochemistry and material science. Hence, we need to position ourselves to compete in this arena--we need a new flagship project. Here I will outline my vision for such a project, which I call "Taking the Pulse of Planet Earth" and which has at its core a large multidisciplinary study of all mafic magmatism through time. I use "mafic magmatism" as a shorthand here, but mean to include all mantle-derived magmatic activity other than steady-state arc magmatism; that is, all basalt basalt (bəsôlt`, băs`ôlt), fine-grained rock of volcanic origin, dark gray, dark green, brown, reddish, or black in color. Basalt is an igneous rock, i.e., one that has congealed from a molten state. ([+ or -] komatiite Komatiites are ultramafic mantle-derived volcanic rocks. They have low SiO2, low K2O, low Al2O3, and high to extremely high MgO.
Komatiites were named for their type locality along the Komati River in South Africa. )-dominated and (or) bimodal bi·mod·al
1. Having or exhibiting two contrasting modes or forms: "American supermarket shopping shows bimodal behavior magmatism associated with divergent margins, or more generally extensional regimes, intraplate magmatic provinces, mantle plumes, anorogenic provinces, kimberlite kimberlite: see diamond.
or blue ground
Dark, heavy, often fragmented igneous rock that may contain diamonds in the rock matrix. and alkaline provinces.
Before describing this vision in some detail, it is important to understand the key reasons why LITHOPROBE was successful as a "big science" project, so that we may emulate some of the same proven formulae in any future project. First of all, proponents of LITHOPROBE had a grand vision (e.g., Keen, 1981; Clowes et al., 1984)--to understand the fundamental architecture of the Canadian crust and lithosphere lithosphere (lĭth`əsfēr '), brittle uppermost shell of the earth, broken into a number of tectonic plates. The lithosphere consists of the heavy oceanic and lighter continental crusts, and the uppermost portion of the mantle. from coast to coast, in the third dimension, and through time. Second, rapid advances in seismic reflection and deep probing electromagnetic techniques, largely made possible by rapid growth in modern electronics and computing power, opened an entirely new observational window into the crystalline crust and, increasingly over the last decade, into the lithospheric mantle (e.g., Calvert et al., 1995; Cook et al., 1999). Third, the transect tran·sect
tr.v. tran·sect·ed, tran·sect·ing, tran·sects
To divide by cutting transversely.
[trans- + -sect. approach divided the overall project up into manageable chunks, linking regional expertise pertinent to particular transects with thematic expertise from across the country. Fourth, the Supporting Geoscience grant system assured "buy-in" from a multi-disciplinary science community, bringing together the best minds from different disciplines and organizations to work together on cross-sections through the Canadian lithosphere. And finally, competent leadership and management made it all happen. Of course, favourable timing of these and other factors (e.g., funding sources and cycles, willingness of the Geological Survey The term geological survey can be used to describe both the conduct of a survey for geological purposes and an institution holding geological information.
A geological survey of Canada to be a significant partner, industry interest), all coming together at the right time, was equally critical.
Although all of the reasons above are necessary for a successful "big science" project, the first two--a grand vision, and a powerful new observational window--are undoubtedly the most important at a conceptual stage of any new project, and hence I will focus on those. The "vision" I propose is to understand the first-order pulse of the Earth, as reflected in its record of mafic magmatic events, at a global scale and over 4.5 billion years of Earth history. The "new observational window" is provided by ever increasing precision, accuracy, and resolving power resolving power: see telescope.
Resolving power (optics)
A quantitative measure of the ability of an optical instrument to produce separable images. in a broad range of disciplines and techniques: geochronology geochronology
Dating and interpretation of geologic events in the history of the Earth. The classical technique of geochronology was stratigraphy, including faunal succession. , isotope and trace element geochemistry, paleomagnetism paleomagnetism, study of the intensity and orientation of the earth's magnetic field as preserved in the magnetic orientation of certain minerals found in rocks formed throughout geologic time. , sequence stratigraphy Sequence stratigraphy is a relatively new branch of geology that attempts to link prehistoric relative sea-level changes to sedimentary deposits. The essence of the method is mapping of strata based on identification of time lines (e.g. , and geophysical approaches such as tomography, receiver function analysis, and geodynamic modeling.
THE PULSE OF PLANET EARTH
Silicate silicate, chemical compound containing silicon, oxygen, and one or more metals, e.g., aluminum, barium, beryllium, calcium, iron, magnesium, manganese, potassium, sodium, or zirconium. Silicates may be considered chemically as salts of the various silicic acids. planets regulate their internal thermal state through melting events in their mantles; through transport of these melts and entrained heat to their surfaces; through creation of new lithosphere; and through recycling of cold crust and lithosphere back into their hot convective interiors (e.g., Davies, 1999). Today, on Earth, plate tectonics plate tectonics, theory that unifies many of the features and characteristics of continental drift and seafloor spreading into a coherent model and has revolutionized geologists' understanding of continents, ocean basins, mountains, and earth history. is the dominant process and is driven to a large extent by the negative buoyancy of aged, and therefore cool, rigid, and relatively dense slabs of oceanic lithosphere (i.e., the upper thermal boundary layer boundary layer
In fluid mechanics, a thin layer of flowing gas or liquid in contact with a surface (e.g., of an airplane wing or the inside of a pipe). The fluid in the boundary layer is subjected to shear forces. of the convective mantle). Mantle plumes (e.g., Morgan, 1971; Ritsema and Allen, 2003; Montelli et al., 2004), originating from instabilities at deeper thermal boundary layers (i.e., the core-mantle boundary in whole-mantle convection, e.g., Campbell, 2001), are thought to contribute about 10% of the global heat loss (Davies, 1999). Plate tectonics and mantle plumes are two independent manifestations of mantle convection Mantle convection is the slow creeping motion of Earth's rocky mantle in response to perpetual gravitationally unstable variations in its density. Material near the surface of Earth, particularly oceanic lithosphere, cools down by conduction of heat into the oceans and atmosphere, , one strongly controlled by the mechanical behaviour of the cold upper boundary layer (e.g., Anderson, 2002a), the other by the fluid dynamics fluid dynamics
n. (used with a sing. verb)
The branch of applied science that is concerned with the movement of gases and liquids. of a hot deep boundary layer. Each process produces voluminous mafic magmas, but on different and independent time scales and with different characteristic plan forms.
At longer time-scales, plate tectonic activity appears quasi-periodic and self-organizes into the supercontinent cycle The supercontinent cycle describes the quasi-periodic aggregration and dispersal of Earth's continental crust. There are varying opinions as to whether Earth's budget of continental crust is increasing, decreasing, or remaining about constant, but it is agreed that this inventory (e.g., Schuiling, 1973; Anderson, 1982; Gurnis, 1988). There are subtle indications that the periodicity periodicity /pe·ri·o·dic·i·ty/ (per?e-ah-dis´i-te) recurrence at regular intervals of time.
1. between successive supercontinent cycles is getting shorter (Fig. 1), perhaps primarily because the average size of continental plates has increased, allowing less time lag between break-up and subsequent aggregation events (Bleeker, 2003; see also Hoffman, 1997).
[FIGURE 1 OMITTED]
Running time forward into the future, average plate size may grow further while the overall vigour of mantle convection will decrease as a result of secular heat loss and exponential decay Noun 1. exponential decay - a decrease that follows an exponential function
decay, decline - a gradual decrease; as of stored charge or current of heat-producing elements. At some point in the future, plates will seize up (e.g., Anderson, 2002b) and ultimately planet Earth will die a "thermal death", a state long reached by our much smaller neighbouring planet, Mars. There will be a transient era, in which heat production will still exceed conductive heat conductive heat
Heat transmitted to the body by direct contact, as by an electric pad. loss so that the mantle underneath the seized-up lithosphere will heat up, and transient plume activity or mantle overturn events may temporarily bring back mobility to the lithosphere, resulting in significant resurfacing. This is the probable state of our closest sister planet, Venus. Smaller neighbouring planets, although each having unique attributes, provide to some extent future snapshots of planet Earth as the rate of thermal evolution scales inversely with planetary radius (but, see also Stevenson, 2003). In this sense, Venus, with a radius 0.95 that of Earth, is marginally ahead of us, whereas our much smaller Moon, after initial cooling of its magma ocean, may never have had a mobile lithosphere, although the 3.9-3.1 Ga mare basalts suggest significant transient heating and possibly one or more mantle overturn events (e.g., Hess and Parmentier, 1995; Stegman et al., 2003).
Going back in time, it is possible that mantle plumes may have played a more significant role, although this remains uncertain. Perhaps plume activity was highly episodic, in response to strong spikes in heat flow across the core-mantle boundary due to tidal resonances in the fluid outer core (Greff-Lefftz and Legros, 1999; see Fig. 1). Increased core heat flow would have temporarily destabilized the boundary layer (D") at the base of the mantle and sharply increased plume activity. Because past rates of deceleration deceleration /de·cel·er·a·tion/ (de-sel?er-a´shun) decrease in rate or speed.
early deceleration of the Earth's spin (due to tidal drag) are variable and not very well known (e.g., Denis Denis, king of Portugal: see Diniz. et al., 2002, and references therein), the absolute timing of these resonances are poorly constrained but are estimated at ca. 3.0[+ or -]0.2 Ga and 1.8[+ or -]0.2 Ga (Greff-Lefftz and Legros, 1999). Could this explain the post-Archean spike in komatiite magmatism at ca. 1.9 Ga, as observed in circum-Superior belts?
In the past, plate tectonic recycling must have operated faster (e.g., Burke et al., 1976; Hargraves, 1986) to keep pace with significantly higher heat production and a still significantly higher primordial heat budget retained from the era of accretion and internal differentiation (e.g., Richter, 1988; Pollack, 1997; Davies, 1999). Thermal arguments suggest that at some point in the distant past, plate tectonics may not have been able to keep pace with the required rate of heat loss (e.g., Davies, 1992, 1999) for the simple reason that oceanic lithosphere, on reaching a nearby trench, was on average too young (hence too buoyant to subduct subduct /sub·duct/ (-dukt´) to draw down.
To pull or draw downward.
to draw down. ), and too thin (and hence less capable of cooling the mantle). Therefore, other and less familiar geodynamic processes must have mediated the required heat loss, perhaps involving 1) catastrophic mantle overturn and melting events (Davies, 1995; Condie, 1998; e.g., the 2.72-2.70 Ga basalt-komatiite-rhyolite magmatism documented in virtually every fragment of Archean crust around the world?); and 2) a less organized return flow of lithospheric material into the deep mantle in the form of delamination delamination /de·lam·i·na·tion/ (de-lam?i-na´shun) separation into layers, as of the blastoderm.
1. A splitting or separation into layers.
2. or "drip" (e.g., Davies, 1992) of subcreted mafic and ultramafic ul·tra·maf·ic
Containing mainly mafic minerals. Used of igneous rocks and often used interchangeably with ultrabasic. Dunite is an ultramafic rock. material after its partial transformation into dense, refractory eclogite eclogite
Any member of metamorphic rocks whose original composition is similar to that of basalt. Eclogites consist primarily of green pyroxene (omphacite) and red garnet (pyrope), with small amounts of various other minerals such as kyanite and rutile. . As part of the transformation into dense eclogite, aqueous fluids and fusible fusible /fu·si·ble/ (fu´zi-b'l) capable of being melted. tonalite-trondjhemite-granodiorite (TTG tTG Tissue Transglutaminase
TTG Telltale Games (website)
TTG TiVo To Go
TTG Tea Tree Gully (South Australia)
TTG Tom Tom Go ) components were extracted from the subcreted mafic-ultramafic material and ascended into the overlying overlying
suffocation of piglets by the sow. The piglets may be weak from illness or malnutrition, the sow may be clumsy or ill, the pen may be inadequate in size or poorly designed so that piglets cannot escape. crust to contribute to the formation of granite-greenstone terrains. Numerous questions remain, but these less familiar, non-uniformitarian processes explain in part why Archean cratons do not look like modern continental plates, even though many of the second-order processes were similar (e.g., Vlaar, 1986; Zegers and van Keken, 2001; Bleeker, 2002; Hamilton, 2003). Certainly there was great mobility, involving both horizontal and vertical tectonics, but not necessarily in the form of plate tectonics sensu stricta, driven by strongly asymmetrical subduction sub·duc·tion
A geologic process in which one edge of one crustal plate is forced below the edge of another.
[French, from Latin subductus, past participle of of rigid, negatively buoyant slabs.
Most importantly Adv. 1. most importantly - above and beyond all other consideration; "above all, you must be independent"
above all, most especially for the present purpose, all these processes of first-order geodynamic interaction between core, mantle, and lithosphere, and of planetary evolution in general, leave a record in the form of mafic magmatic events. The products of these mafic magmatic events are preserved in the continental geological record through time, and for the last 180 million years also in the oceanic record. With increasing precision and accuracy these mafic magmatic events can be dated (e.g., Krogh et al., 1987; Harlan et al., 2003), while at the same time the mafic magmas provide chemical probes into the evolution of the lithosphere and convective mantle (e.g., Sylvester et al., 1997; Campbell, 2003). Some of them entrain entrain /en·train/ (en-tran´) to modulate the cardiac rhythm by gaining control of the rate of the pacemaker with an external stimulus. deep crustal crust·al
Of or relating to a crust, especially that of the earth or the moon.
Adj. 1. crustal - of or relating to or characteristic of the crust of the earth or moon and mantle xenoliths to the surface, providing the only direct rock samples of the lithosphere available to us. Remanent magnetism (Physics) magnetism which remains in a body that has little coercive force after the magnetizing force is withdrawn, as soft iron; - called also residual magnetism ltname>.
See also: Remanent of mafic magmatic rocks, if primary, can provide paleo-intensities of the geomagnetic field geomagnetic field
Magnetic field associated with the Earth. It is essentially dipolar (i.e., it has two poles, the northern and southern magnetic poles) on the Earth's surface. Away from the surface, the field becomes distorted. at the time of crystallization Crystallization
The formation of a solid from a solution, melt, vapor, or a different solid phase. Crystallization from solution is an important industrial operation because of the large number of materials marketed as crystalline particles. (e.g., Macouin et al., 2003), thus providing constraints on processes in the liquid outer core. When did the inner core start to form (e.g., Labrosse et al., 2001)? And when did the geodynamo ignite?
The very presence of these mafic magmatic events signifies extension, rifting and break-up, and arrival of mantle plumes or astheno-spheric upwellings. Some of the largest mafic magmatic events may be related to catastrophic mantle overturn events or slab avalanches. Basalt flows, dykes, and sills signify critical events on rifted margins and in the evolution of many sedimentary basins and, where they can be dated, allow time calibration of basin evolution. Some of the largest events are capable of venting enough C[O.sub.2], other volcanic gases, and (or) ash into the atmosphere to result in major environmental deterioration and upsets of the greenhouse balance. Although ultimate causes are still being debated (e.g., Courtillot and Renne, 2003), the two largest Phanerozoic extinction events, the Permo-Triassic and K-T K-T Cretaceous-Tertiary boundaries, overlap in time or immediately follow the extrusion of some of the largest continental flood basalt A flood basalt or trapp basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Flood basalts have occurred on continental scales (large igneous provinces) in prehistory, creating great sequences, the Siberian (e.g., Kamo et al., 2003) and Deccan (Hofmann et al., 2000) traps, respectively. Collectively, mafic magmatic events provide perhaps the most important record of "the pulse of the Earth" (e.g., Larson, 1991) in its overall thermal evolution from a hot, young planet to a cold, dead planet.
DATING ALL MAFIC MAGMATIC EVENTS THROUGH TIME
Many mafic magmatic events have been precisely dated (e.g., see Ernst and Buchan, 2001 for a recent compilation; see also Ernst and Buchan, 2004); however, many others remain undated un·dat·ed
1. Not marked with or showing a date: an undated letter; an undated portrait.
2. or have yielded imprecise, less robust, or questionable ages (e.g., many old K-Ar ages). A few mafic magmatic events may not be datable directly, although with continuing advances in U-Pb and [sup.40][Ar-.sup.39]Ar dating, their number is shrinking. Hence, given the significance of mafic magmatic events to nearly all first-order geodynamic processes, either as direct manifestations of these processes, or as time calibrators, it is surprising that no concerted national or international effort has yet been made to date all such events and obtain a complete time series of mafic magmatism and its critical attributes. Perhaps this deficiency is more conspicuous now then ever, at a time when highly capable robotic rovers are investigating the surface of a neighbouring planet, yet key elements of the much more readily available terrestrial record remain poorly known.
Significant interest in a complete record of all terrestrial mafic magmatism exists in Canada and, indeed, Canadian geochronologists have been leading efforts to date such rocks. There is equal interest in this around the world, and a Canadian initiative may be the critical impetus for a global initiative (e.g., see plans for an international dating campaign on pre-Mesozoic large igneous provinces being developed by the Large Igneous Provinces Commission, at www.largeigneousprovinces.org). A comprehensive study of all mafic magmatism on Earth would have major impacts across numerous earth science disciplines, ranging from the study of ancient tectonic regimes, and the chemical and dynamic evolution of the core and mantle, to testing putative links between major flood volcanism, impacts, and extinctions, and the study of what triggers major climatic excursions. It is beyond the scope of this paper to do equal justice to all these disciplines and questions; below, I will merely illustrate potential impacts on two related fields: Precambrian evolution and paleo-continental reconstructions.
Let us examine plate tectonic reconstructions of various times in the past and the impact a global dating program would have on such reconstructions.
The extant record of Archean crust is highly fragmented and scattered around the globe in about 35 significant pieces, the "Archean cratons" (Bleeker, 2003). Each of these cratons is crosscut by numerous mafic dykes swarms, the precise ages of which provide a "bar code" (Fig. 2) that identifies that piece of crust and which can be expected to match, at least in part, the "bar code" from a distant craton craton (krā`tŏn): see continent. that has since rifted off and been dispersed around the globe. For example, from the time of its formation in an ancestral landmass (Kenorland or Superia? see Bleeker, 2003), up to its incorporation into the Laurentian collage (Hoffman, 1988, 1989), the Superior craton The Superior craton (or Superior Province) forms the core of both the North American continent and the Canadian Shield. It extends from Quebec in the east to eastern Manitoba in the west. was cut by as many as 20 mafic dyke swarms and intrusive events (Buchan and Ernst, 2004). To date, only about half of these events have been dated precisely. In contrast, the Slave craton The Slave craton is a relatively small Archean craton (about 300,000 km²) located in the Northwest Territories of Canada. It is dominated by ca. 2.73-2.63 Ga greenstones and turbidite sequences and ca. 2.72-2. in northwestern Canada was intruded, over a similar time interval, by about 10 dyke swarms, few if any of which match those in the Superior. Hence, these two distinct pieces of Archean crust were never close neighbours and likely originated from different ancestral supercratons (Bleeker, 2003). Only by 1.9-1.8 Ga were these crustal fragments brought into relative proximity during their amalgamation into Laurentia (Hoffman, 1988).
[FIGURE 2 OMITTED]
Subsequently, both cratons were cut by radiating dykes of the giant Mackenzie swarm at 1267 Ma (LeCheminant and Heaman, 1989), which fan out from a focal point focal point
See focus. along the northern edge of the Slave craton to the southeast, as far as the northwestern Superior craton (e.g., Ernst and Baragar, 1992; Baragar et al., 1996). Were Laurentia to break up, it would be the shared record of the Mackenzie swarm dykes, and their inherent paleomagnetic record, that would allow reconstruction of Laurentia and reveal, most conclusively, a shared, transient, residence of the Slave and Superior cratons in this continent from some time prior to 1267 Ma to the time of breakup.
Although correlation of platformal sequences on dispersed cratons is one of the more definitive tools to track initially adjacent pieces of crust (e.g., the sequence stratigraphic correlation of the Transvaal Supergroup, overlying the Kaapvaal craton The Kaapvaal craton (Limpopo province of South Africa), along with the Pilbara craton of Western Australia, are the only remaining areas of pristine 3.6-2.5 Ga crust on Earth. , with that of the Mount Bruce Supergroup, overlying the Pilbara craton The Pilbara craton (the Pilbara province in northwest Western Australia), along with the Kaapvaal craton (the Kaapvaal province of South Africa) are the only remaining areas of pristine Archaean 3.6-2.7 Ga crust on Earth. ; e.g., Cheney, 1996), it is not the most generally applicable tool, because it relies on good preservation. During collision and orogenesis o·rog·e·ny also or·o·gen·e·sis
The process of mountain formation, especially by a folding and faulting of the earth's crust.
or , platformal sequences may be entirely removed by uplift and erosion. Even where platformal sequences are preserved and potential sequence stratigraphic correlations can be made, they may not provide the detailed spatial and directional information to tightly constrain past configurations.
Mafic dyke swarms, on the other hand, have a significant aerial and depth extent and commonly survive significant uplift. Equally important, they provide a trend and thus directional information, as well as potentially precise piercing points (Park et al., 1995; Buchan and Ernst, 1997). In theory, an analysis of flow direction (e.g., from magnetic fabrics or imbrication imbrication
surgical pleating and folding of tissue to realign organs and provide extra support, e.g. chronically stretched joint capsule.
Flo imbrication of tabular plagioclase plagioclase
Any member of the series of abundant feldspar minerals that usually occur as light- to medium-grey-coloured, transparent to translucent grains or crystals. Plagioclase ranges in composition from albite to anorthite. phenocrysts in the laminar flow laminar flow
Fluid flow in which the fluid travels smoothly or in regular paths. The velocity, pressure, and other flow properties at each point in the fluid remain constant. regime near dyke margins) can provide unique azimuthal az·i·muth
1. The horizontal angular distance from a reference direction, usually the northern point of the horizon, to the point where a vertical circle through a celestial body intersects the horizon, usually measured clockwise. directions. Integrated mapping, high-precision age dating, fabric studies, and paleomagnetism of mafic magmatic events and their dyke swarms thus allow continental fragments to be placed:
1) at a specific latitude;
2) at a specific time;
3) with a known orientation;
4) in an position that optimizes geological continuity prior to break up and dispersal; and
5) that satisfies the specific requirements of precise piercing points.
In an ideal case, information from multiple dyke swarms (e.g., all dyke swarms of the Superior, precisely dated) would allow definition of improved apparent polar wander paths, providing additional information on how a certain continental fragment arrived at a specific location and which other blocks were fellow travelers. When a supercraton breaks up, apparent polar wander paths bifurcate To divide into two. ; when two cratons are joined by collision, two previously independent paths merge into a common path. Better-defined and more densely populated polar wander paths can greatly reduce or even eliminate the ambiguities due to polarity. They may also contribute to the understanding of true polar wander events (e.g., Evans, 2003).
None of these various approaches is strictly new and various pioneers have applied them successfully, particularly over the last two decades. However, their scale of application and integration, under the proposed project, would be new. As recent examples, Wingate et al. (2002) have shown the power of every new precisely dated mafic event and associated paleopole in testing the putative Laurentia-Australia connection in the mid-Proteozoic, whereas Buchan et al. (2000) have applied similar tests to Laurentia-Baltica connections. However, no comprehensive effort has been made to address these questions for all mafic magmatic events across Canada. For instance, about half of the mafic magmatic events in the Slave and Superior, two of the best-studied cratons worldwide, remain to be dated precisely. Worse, few if any mafic dyke swarms have been dated from the Rae craton The Rae craton is an Archean craton located in northern Canada north of the Superior craton. Ungava Peninsula
The Ungava Peninsula, situated on the northeast portion of the Canadian Shield, is where the Rae Province connects with the Superior Province. and, hence, the ancestry of this second-largest building block of Laurentia remains entirely unknown.
Given the improved precision in many of the underlying techniques, and the growing awareness that mafic magmatic events and their inherent information (precise age, composition, causative tectonic event, orientation, paleolatitude and polar wander path) provide the most efficient, robust, and generally applicable tools to unravel past plate configurations, the time seems right for a national and international effort.
AN EXAMPLE: A SUGGESTED SUPERIOR-HEARNE CORRELATION AT 2.45 GA
As one further illustration of the power of this approach, I will present a possible reconstruction of the Hearne and Superior cratons prior to their breakup and dispersal sometime before 2.0 Ga. The Superior craton is cut by the large Matachewan dyke swarm (e.g., Halls and Zhang 1998, and references therein), which radiates from an apparent focal point along its Huronian margin--a probable break-up margin that was later obscured by younger Grenvillian overthrusts. Although complicated by the presence of two age populations, the younger Matachewan dykes have been dated precisely at 2446 Ma (e.g., Heaman, 1997). Many of these dykes contain abundant, large plagioclase megacrysts (Fig. 3a, b, c), which set these dykes apart petrographically from many other dyke swarms. I estimate that such an abundance of large plagioclase megacrysts occurs in fewer than 10% of mafic dyke swarms worldwide. Following up on earlier suggestions of similar dykes in the Kaminak area of the Hearne craton (Christie et al., 1975), Heaman (1994) showed that these dykes, characterized by a similar abundance of large plagioclase megacrysts (Fig. 3d, e), are essentially identical in age (ca. 2.45 Ga, Heaman, 1994). Remarkably, both dyke swarms trend in a northerly direction, at high angles to dominant late Archean tectonic trends in each craton.
[FIGURE 3 OMITTED]
Existing paleomagnetic data (Christie et al., 1975) allow the Hearne craton to be placed, at ca. 2.45 Ga, just to the south of the Huronian margin of the Superior craton (Fig. 4a, b, c)--a position that could equate the Kaminak dykes with a southern continuation of the younger age population of the Matachewan dykes, perhaps as another branch of a giant radiating dyke swarm associated with a triple junction A triple junction is the point where the boundaries between three tectonic plates, and three plate margins, meet. At the triple junction a boundary will be one of 3 types - a ridge, trench or transform fault and triple junctions can be described according to the types of plate along the Huronian margin (Fig. 4d). This configuration (Fig. 4d) suggests that dyke intrusion, rifting, and eventual break-up of supercraton Superia may have occurred in response to the arrival of a mantle plume head--the Matachewan plume of Heaman (1994). The proposed reconstruction would suggest a more or less direct correlation Noun 1. direct correlation - a correlation in which large values of one variable are associated with large values of the other and small with small; the correlation coefficient is between 0 and +1
positive correlation between the partly glaciogenic Huronian Supergroup (e.g., Young et al., 2001) and Hurwitz Group (e.g., Young, 1973, 1975), perhaps on either side of a large continental rift continental rift
A long, narrow fissure in the Earth marking a zone of the lithosphere that has become thinner due to extensional forces associated with plate tectonics. (Fig. 5). By 2218 Ma, the age of the Nipissing diabase diabase: see basalt.
Fine- to medium-grained, dark gray to black intrusive igneous rock. Diabase is one of the dark rocks known commercially as “black granite. sills in the Huronian Supergroup (Corfu and Andrews, 1986; Krogh et al., 1987), this rift was evidently wide enough to impede southward propagation of Nipissing magmas across the rift into the Hurwitz basin. Nipissing sill magmas are thought to be derived, via a radiating swarm of feeder dykes, from a focal point northeast of the Superior craton (Buchan et al., 1998).
[FIGURES 4-5 OMITTED]
This Superior-Hearne reconstruction (Fig. 4d), prior to Paleoproterozoic break-up and dispersal of these cratonic fragments, is arguably one of the most specific, and testable, pre-2.0 Ga plate reconstructions proposed to date, although the Kaminak swarm paleomagnetic pole (Christie et al., 1975) and age need to be improved. An immediate and straightforward test would be to investigate the flow direction in Kaminak dykes, as the reconstruction of Figure 4 would predict a north-to-south magma flow in these dykes, away from a northerly plume source.
If correct, the new reconstruction would further suggest that Hearne crust and its large, juvenile Kaminak greenstone belt Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies. represent a continuum of the belt-like crustal growth observed in the southern Superior craton. Alternatively, if refined 2.45 Ga paleopoles for both the Kaminak and Matachewan swarms would prove to be statistically identical (i.e., the original interpretation of Christie et al., 1975), it would place both cratons in approximately their current relative position at ca. 2.45 Ca. This alternate result may require a radical re-interpretation of Trans-Hudson Orogen in terms of being the product of opening and closure of a narrow "intra-cratonic" ocean rather than the prevailing view of a wide Manikewan ocean (Stauffer, 1984; see also Symons, 1998; Halls and Hanes, 1999). These first-order questions about the fundamental architecture and evolution of Laurentia have presented themselves loud and clear since the 1975 paper of Christie et al., yet they remain unresolved. A related question is the identity and significance of the two or more age populations among the dykes of the composite Matachewan swarm.
Geography of selected portions of the Earth's surface at specific times in the geologic past. The simplest kind of paleogeography is a map showing the locations of ancient lands and seas, but paleogeographic maps may also AT CA. 1885 MA: WHERE ARE THE CONJUGATE conjugate /con·ju·gate/ (kon´jdbobr-gat)
1. paired, or equally coupled; working in unison.
2. a conjugate diameter of the pelvic inlet; used alone usually to denote the true conjugate diameter; see MARGINS OF THE SUPERIOR?
Although possibly distracting the discussion from the specific late Archean Superior-Hearne reconstruction proposed above, it is worth pointing out that very similar paleogeographic questions present themselves again when we consider the distribution of ca. 1883 Ma mafic-ultramafic magmatism around the margins of the Superior craton (e.g., Molson dykes, Fox River Sill). If this widespread magmatism is related to rifting and break-up of the Superior craton, perhaps related to arrival of a mantle plume underneath the western part of this craton, why do we find mafic sills of identical age (1883 [+ or -] 5 Ma; R. Parrish, unpublished date; mentioned in Van Kranendonk et al., 1993) in the Piling Group on the southern flank of the Rae craton? Are these identical ages mere coincidence, or a product of insufficient age accuracy? Or do they suggest proximity of the Piling Group basin, and by inference the Rae margin, to the Superior craton and the Molson plume centered on this latter craton?
In general, which margins were conjugate to the circum-Superior margin at ca. 1.88 Ga and where are they now? Given the presence of world-class mineral deposits along the western and northern Superior margin (e.g, Thompson, at 80 Mtonnes the largest komatiite-associated Ni-Cu-PGE deposit in the world), this question is of more than scientific interest. Again, all these questions are most clearly and critically addressed by a comprehensive survey of the record of ca. 1.88 Ga mafic magmatism around the Superior and other cratons and the information inherent therein: spatial distribution, sequence stratigraphic framework, high-resolution age data, and paleomagnetic information.
The time is right for a comprehensive, multidisciplinary, Canada-wide study of all mafic and intraplate magmatic events through time, their precise ages, their tectonic settings, and other inherent information, which not only addresses critical paleogeographic questions but has the potential to illuminate secular evolution of the mantle and the core.
Results from such a project would produce a quantum leap quantum leap
An abrupt change or step, especially in method, information, or knowledge: "War was going to take a quantum leap; it would never be the same" Garry Wills. in the understanding of Precambrian terrains, younger break-up margins, and past plate configurations such as the Superior-Hearne reconstruction proposed herein, particularly if a Canadian project would help stimulate similar initiatives on other continents (e.g., Australia, southern Africa
The overall price tag of the proposed project would be substantial but modest compared to that of LITHOPROBE. Funding on the order of 10 MS over 10 years would allow many of the goals to be achieved, including approximately 200 new high-precision ages. Of course, increased funding would allow more supporting geoscience and hence assure broader buy-in from a wider spectrum of disciplines.
Naturally, high-precision geochronology would be a lead discipline, as precise ages are critical to the ordering of events in their correct sequence, the unraveling of cause and effect, and the determination of rates. However, the following disciplines would all be natural and equal partners in this potential "big science" project: major and trace element geochemistry, isotope geochemistry Isotope geochemistry is an aspect of geology based upon study of the relative and absolute concentrations of the elements and their isotopes in the Earth. Broadly, the field is divided into two branches: stable and radiogenic isotope geochemistry. , paleomagnetism, paleo-intensity studies and insights into core dynamics, structural geology structural geology
Scientific discipline concerned with rock deformation on both small and large scales. Its scope ranges from submicroscopic lattice defects in crystals to fault structures and fold systems of the Earth's crust. and tectonics, regional mapping, sequence stratigraphy and basin analysis, petrology petrology, branch of geology specifically concerned with the origin, composition, structure, and properties of rocks, primarily igneous and metamorphic, and secondarily sedimentary. and xenolith studies, geophysics, geodynamic modeling, economic geology economic geology
Scientific discipline concerned with the distribution of mineral deposits, the economic considerations involved in their recovery, and assessment of the reserves available. (Ni-Cu-PGEs, Cr, diamonds), Earth systems science and the study of extinction events.
A strategic link with the Integrated Oceanic Drilling Program (IODP IODP Integrated Ocean Drilling Program
IODP Incremental Operations Design Panel ; http://www.iodp.org) would extend the scope into the marine and oceanic realm. Finally, this project would aim to understand the evolution of planet Earth in its true natural context--the dynamic evolution of the Solar System solar system, the sun and the surrounding planets, natural satellites, dwarf planets, asteroids, meteoroids, and comets that are bound by its gravity. The sun is by far the most massive part of the solar system, containing almost 99.9% of the system's total mass. . Hence it would seek close links with the planetary science planetary science or planetology, study of planets and planetary systems as a whole. Planetary science applies the theories and methods of traditional disciplines such as astronomy, geology, physics, chemistry, and mathematics to the study of community. Perhaps more so than for LITHOPROBE, international participation should be encouraged, to benefit from expertise not currently resident within Canada.
Major industry players around the world are spending considerable time and money on the understanding and exploration of mafic magmatic provinces, with the potential rewards of world-class mineral deposits such as those of Norilsk, Thompson, or a Merensky Reef The Merensky Reef, is a layer of igneous rock in the Bushveld Igneous Complex (BIC) in the Transvaal which together with an underlying layer, the Upper Group 2 Reef (UG2), contains most of the the world's known reserves of platinum group metals (PGMs) or platinum group elements . Even other deposit types, not traditionally related to mafic magmatic events (VMS (1) (Virtual Memory System) A multiuser, multitasking, virtual memory operating system for the VAX series from Digital. VMS applications run on any VAX from the MicroVAX to the largest unit. See OpenVMS. , SEDEX SEDEX Sedimentary Exhalative ), may owe their ultimate origin to mafic or ultramafic intrusions, i.e. as the heat engines that drove hydrothermal circulation (e.g., Pirajno, 2000). Key examples would be the 2716-2710 Ma, giant Kidd Creek Cu-Zn-Ag deposit, closely associated with ultramafic flows and intrusions (Bleeker, 1999; Hannington and Barrie, 1999), or the Sullivan Pb-Zn deposit in the Belt-Purcell basin, closely associated with the 1468 Ma Moyie sills (e.g., Anderson and Davis, 1995). Hence, there is no doubt that industry would be a supportive partner, allowing significant opportunities for matching funding.
Finally, the same tectonic questions that control mafic magmatic events are also relevant to the triggering and emplacement of kimberlite provinces. Indeed, as emphasized in the introduction, this project should include all intra-plate magmatic events as they reflect equally on the tectonic pulse of the Earth. This project will clearly look into the mantle, and in some ways even into the core. In this sense it is a logical follow-up on LITHOPROBE, which had a definite crustal and lithospheric perspective. Through its mantle perspective there is potential for synergies with the POLARIS project already underway in the geophysical community.
"Taking the Pulse of Planet Earth", by means of a comprehensive multidisciplinary study of the complete record of mafic magmatism, including a large dating program, is a "smart" and highly efficient approach to address a number of first-order questions in contemporary Earth science over the next 5 to 10 years.
A complete record of mafic magmatism (ages, periodicities, rates, volume estimates, geochemical fluxes, spatial distribution, structural trends, evolving isotope and trace element ratios, paleomagnetic information, associated ore deposits, etc.) will provide critical constraints on issues as diverse as paleogeographic reconstructions and the supercontinent cycle; the triggers of past climate extremes; complex Earth systems through time; crustal growth, core and mantle evolution; evolution of the core dynamo; causes of flood volcanism, and the potential relationships with extinction events; the hypothesized relationship between flood volcanism and large impact events; and, last but not least, the discovery of new ore deposits of strategic minerals.
The proposed project is certain to stimulate a quantum leap in the understanding of Precambrian terrains and their paleogeographic distribution through time. More importantly, a more complete understanding of mafic magmatism through time will provide new insights into the evolution of planet Earth. Finally, this project would promote, at a modest cost, new links between numerous earth science disciplines and facilitate further integration between earth and planetary sciences.
Ideas discussed herein were first presented at the 2002 GAC GAC Great American Country
GAC Global Assembly Cache (Microsoft .NET)
GAC Global Assembly Cache
GAC Granular Activated Carbon
GAC Gustavus Adolphus College (St. conference in Saskatoon Saskatoon (săskətn`), city (1991 pop. 186,058), S central Sask., Canada, on the South Saskatchewan River. and have since developed through discussions with many, in particular Ken Buchan, Richard Ernst, Don Davis, Sandra Kamo, Bill Davis, Cees van Staal, Simon Hanmer, Bob Barager, Tom Skulski, John Waldron, Paul Sylvester, Larry Heaman, Henry Halls, Keith Sircombe, John Ketchum, Kevin Cassidy, Dave Evans, Ian Dalziel, and many others. Many of these scientists share my vision, although individual approaches or emphases would naturally differ somewhat--this is to be expected. At a minimum, I hope this paper will stimulate discussion on the future of the Solid Earth Sciences in Canada. More so, I hope to convince fellow scientists that mafic magmatism is a first-order process and diagnostic of both long-term and short-term planetary evolution. As a tool, its enormous potential remains grossly underutilized. I thank Ken Buchan for kindly assisting with the plotting of Figure 4. The two field photos of Kaminak dykes were provided by Simon Hanmer. Bob Baragar suggested the flow direction test of the Superior-Hearne reconstruction. Geological Survey of Canada contribution number 2003305.
[FIGURE 4 OMITTED]
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Scientists have several theories as to the origins of the early human population of the North America. , in Bally, A.W. and Palmer, A.R., eds., The Geology of North America: An Overview: Geological Society of America, Boulder, Colorado, p. 447-512.
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atomic number 40, zirconium, Zr - a lustrous grey strong metallic element resembling titanium; it is used in nuclear reactors as a neutron absorber; it occurs in baddeleyite but is obtained chiefly from and zircon zircon
Silicate mineral, zirconium silicate, ZrSiO4, the principal source of zirconium. Zircon is widespread as an accessory mineral in acid igneous rocks; it also occurs in metamorphic rocks and, fairly often, in detrital deposits. , in Halls, H.C. and Fahrig, W.F., eds., Mafic Dyke Swarms: Geological Association of Canada, Special Paper 34, p. 147-152.
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LeCheminant, A.N. and Heaman, L.M., 1989, Mackenzie igneous ig·ne·ous
1. Of, relating to, or characteristic of fire.
a. Formed by solidification from a molten state. Used of rocks.
b. Of or relating to rock so formed; pyrogenic. events, Canada: middle Proterozoic hotspot magmatism associated with ocean opening: Earth and Planetary Science Letters, v. 96 (1-2), p. 38-48.
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[Middle English, multitude, from Old French, from Latin frequentia; see frequency.]
Noun 1. tomography reveals a variety of plumes in the mantle: Science, v. 303, p. 338-342.
Morgan, W.J., 1971, Convection plumes in the lower mantle: Nature, v. 230 (5288), p. 42-43.
Ojakangas, R.W., 1988, Glaciation: an uncommon "mega-event" as a key to intracontinental and intercontinental correlation of early Proterozoic basin fill, North American North American
named after North America.
North American blastomycosis
see North American blastomycosis.
North American cattle tick
see boophilusannulatus. and Baltic cratons, in Kleinspehn, K.L. and Paola, C., eds., New Perspectives in Basin Analysis: Springer-Verlag, New York, USA, p. 431-444.
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Prokoph, A., Ernst, R.E. and Buchan, K.L., 2004, Time-series analysis Time-series analysis
Assessment of relationships between two or among more variables over periods of time. of large igneous provinces: 3500 Ma to present: The Journal of Geology, v. 112, p. 1-22.
Richter, F.M., 1988, A major change in the thermal state of the Earth at the Archean-Proterozoic boundary: consequences for the nature and preservation of continental lithosphere, in Menzies, M.A. and Cox, K.G., eds., Oceanic and Continental Lithospheres: Similarities and Differences: Journal of Petrology, Special Lithosphere Issue, p. 39-52.
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n. pl. detritus
1. Loose fragments or grains that have been worn away from rock.
a. Disintegrated or eroded matter: the detritus of past civilizations. zircon geochronology and grain-size analysis of ~2800 Ma Mesoarchean proto-cratonic cover succession, Slave Province, Canada: Earth and Planetary Science Letters, v. 189 (3-4), p. 207-220.
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Sylvester, P.J., Campbell, I.H., and Bowyer bow·yer
1. One who makes or sells bows for archery.
2. Archaic An archer. , D.A., 1997, Niobium/uranium evidence for early formation of the continental crust: Science, v. 275, p. 521-523.
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Of, relating to, or containing aluminum or alum.
Adj. 1. aluminous - pertaining to or containing aluminum or alum quartzites as possible time marker for middle Precambrian (Aphebian) rocks of North America, in Young, G.M., ed., Huronian Stratigraphy and Sedimentation: Geological Association of Canada, Special Paper 12, p. 97-127.
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Accepted as revised 7 August 2004