Plate Tectonic Paradigm Research Articles

INTRODUCTION: Initiation and Termination of Subduction: Rock Record, Geodynamic Models, and Modern Plate Boundaries

The plate tectonic paradigm revolutionized the way geologists and geophysicists look at the world, and produced an enormous increase in our understanding of how the Earth functions. The recognition of large, relatively rigid plates that float on the underlying asthenosphere, and interact with one

Global tectonics by PhilipKearey, Keith A.Klepeis and Frederick J.Vine. Third edition. Wiley‐Blackwell, Chichester, 2009. No. of pages: xiii+482. ISBN 978‐1‐4051‐0777‐8 (paperback).

Global tectonics by PhilipKearey, Keith A.Klepeis and Frederick J.Vine. Third edition. Wiley‐Blackwell, Chichester, 2009. No. of pages: xiii+482. ISBN 978‐1‐4051‐0777‐8 (paperback).

An overview of the Palaeoproterozoic geology of Peninsular India, and key stratigraphic and tectonic issues

Abstract The Archaean–Proterozoic rock successions in India have the potential to enrich the global database on Precambrian stratigraphic development, and to offer valuable clues to global tectonic reconstructions. Built over four distinct Archaean cratonic nuclei, the major Palaeoproterozoic supracrustal belts/cover sequences include the Dhanjori Group, the Singhbhum Group (including the Dalma volcanics and the Chandil Formation in eastern India), the Cuddapah Supergroup and the adjoining Nellore Schist Belt bordering the eastern Dharwar Craton, the Aravalli Supergroup in the Aravalli–Delhi Fold Belt in NW India, the lower Vindhyan (Semri Group) and the Mahakosal/Sausar/Betul belts close to the Central Indian Tectonic Zone (CITZ), and, possibly, the Dongargarh Supergroup in Bastar. A major erosional unconformity separates the Archaean tonalite–trondjhemite gneiss basement in these cratons from the overlying volcano-sedimentary successions. An overview of Palaeoproterozoic stratigraphic development in these cratonic blocks is presented to bring out the salient features for global comparison and to highlight issues requiring further attention. Multiple metamorphic, magmatic and deformation events are recorded in the fold belts at the join of the cratons or their margins, inviting application of the plate tectonic paradigm. However, a comprehensive tectonic model for the amalgamation of the Indian Archaean nuclei is yet to emerge, and is crucial for our understanding of Palaeoproterozoic supercontinent development.

Paleoproterozoic ultrahigh-temperature granulites in the North China Craton: Implications for tectonic models on extreme crustal metamorphism

Ultrahigh-temperature (UHT) metamorphic rocks associated with the ‘Khondalite Belt’ within the Inner Mongolia Suture Zone (IMSZ) provide robust evidence for extreme thermal metamorphism in the North China Craton (NCC). The IMSZ marks the collisional suture between the Yinshan Block to the north and the Ordos Block to the South as the NCC was incorporated within the Columbia supercontinent amalgam during Paleoproterozoic. Here we present a synthesis of the salient features of the UHT rocks from the NCC including petrologic indicators, fluid characteristics, and monazite and zircon chronometry on the extreme crustal metamorphism. The granulites carry diagnostic UHT mineral assemblages including sapphirine+quartz, low Zn/Fe3+ spinel+quartz, high alumina orthopyroxene+sillimanite+quartz and high temperature mesoperthite. The stability fields of the typical mineral assemblages, conventional geothermobarometry and phase equilibria modeling using pseudosections as reported in a number of recent studies converge to indicate that these UHT rocks experienced metamorphic temperatures up to or in excess of 1000°C at ca. 10kbar, followed by an isobaric cooling segment. The rocks were exhumed along a near-isothermal decompression path. Microstructures, mineral reactions and phase equilibria modeling suggest an anti-clockwise P–T path, similar to those displayed by metamorphic orogens developed in subduction-collision settings. The dominant category of fossil fluids preserved within the major UHT minerals is CO2, consistent with the stability of the broadly anhydrous mineral assemblage in these rocks. Both chemical and radiogenic isotopic ages from monazite and zircon chronometry suggest the timing of the UHT event as around 1.92Ga. The Paleoproterozoic high grade metamorphism younging from 1.95Ga in the western domain to 1.92Ga in the eastern domain of the Khondalite Belt might suggest a scissor-like closure of oblique collision between the Yinshan and the Ordos Blocks.The salient features of the UHT metamorphism in the NCC include: (1) extreme metamorphic temperatures at moderate pressures, (2) dominantly anhydrous nature of the mineral assemblages, typically the stability of orthopyroxene, (3) common presence of CO2-rich fluid inclusions as the trace of the ambient fluid, (4) regional extent of the UHT granulites, and (5) the association of the UHT orogen with an accretionary belt in a continental collisional suture. We evaluate the diverse models on the generation of UHT orogens including their formation in thickened and inverted back-arcs, orogen self-heating through heat producing elements, heat and CO2 input by plume impingement below a carbonated tectosphere, and asthenospheric upwelling through ridge subduction and slab-window process or during post-collisional slab break-off. The ultra-hot and dry UHT rocks in the NCC provide one of the well preserved examples from the Paleoproterozoic globe for investigating extreme metamorphism and related tectonic processes within the plate tectonic paradigm.

Structural Geology

Structural geology and continental tectonics were ushered in to the modern quantitative age of geosciences with the arrival of the global plate tectonics paradigm (circa 1968), derived using new data from the oceans' depths, and John Ramsay's 1967 seminal work, Folding and Fracturing of Rocks. Fossen is to be applauded for crafting a unique, high‐caliber, and accessible undergraduate textbook on structural geology that faithfully reflects this advance and the subsequent evolution of the discipline. This well‐written text draws on Fossen's wealth of professional experience, including his broad and diverse academic research and experience in the petroleum industry. This book is beautifully illustrated, with excellent original color diagrams and with impressive color field photographs that are all keyed to locations and placed into geologic context.

The young Earth and the story of the early Archaean rocks of West Greenland

Broadly the science of geology has passed through a number of distinct phases. In the early days attention was focussed on establishing a stratigraphic framework, concentrating on fossils and lithologies—the days of mapping and systematizing of sedimentary successions and the uncovering of the succession of life. Later, in the early twentieth century, geologists became much more interested in igneous rocks. By the 1960s attention turned to the ocean basins, culminating in the acceptance of the paradigm of plate tectonics. At the end of the 1960s, one area of geology that remained relatively little understood was the huge span of time represented by the Precambrian, about 80 per cent of Earth's history. By the 1960s this was changing. Radiometric dating was beginning to show the relative ages of such terranes and new methods of mapping were beginning to be used.

Europe from the bottom up: A statistical examination of the central and northern European lithosphere–asthenosphere boundary from comparing seismological and electromagnetic observations

The Lithosphere–Asthenosphere Boundary (LAB) is a fundamental boundary in the plate tectonic paradigm — it is the most pervasive on the planet, yet comparatively it is one we know little about. Defined initially on the basis of the mechanical response of the Earth to loading, its usage has become ubiquitous across the geosciences but the natural differences in its definition, due to differences in thermal, physical and chemical parameters, cause confusion. To advance this debate, comparisons are made both qualitatively and quantitatively, between the delineation of the LAB for Europe based on seismological and electromagnetic observations. We examine statistically, using robust methods, the LABs derived from independent datasets and methods. Essentially, all definitions of the LAB, as an impedance contrast from receiver functions (sLABrf), a seismic anisotropy change (sLABa) and an increase in conductivity from magnetotellurics (eLAB), are consistent with a deeper LAB beneath Precambrian Europe, and a shallower LAB beneath Phanerozoic Europe, with some exceptionally deep regions in Phanerozoic Europe, such as the Alps. All three LABs increase in depth significantly and rapidly at a location consistent with the surface expression of the Trans-European Suture Zone (TESZ) which separates Precambrian Europe to the north and east from Phanerozoic Europe in the centre. Two of the definitions, sLABrf and eLAB, are consistent for Phanerozoic Europe with mean values of 90–100 km, compared to an average sLABrf of 135 km. A different two, sLABa and sLABrf, are consistent for Precambrian Europe with mean values of 170–180 km compared to an eLAB mean of 250 km. The deeper eLAB depths for Precambrian Europe are more consistent with body wave and surface wave tomography models than are the sLABa and sLABrf ones, suggesting that the lower lithosphere beneath Precambrian Europe between 170 to 250 km is responding to shearing on the base of the lithosphere from plate driving forces. Taken together the definitions provide strong constraints on the nature of the LAB. For example, the electrical eLAB beneath the TESZ is anomalously thick, to possibly the transition zone, whereas sLABa and sLABrf seismological definitions would place it at far shallower depths. Given the sensitivity of electrical conductivity to partial melt and water content, the thick eLAB beneath the TESZ excludes interpretation of the seismic LAB in that location in terms of a thermal structure that would induce partial melting or of hydration.

Deformation of the Northwestern Okhotsk Plate: How is it happening?

Abstract. The Eurasia (EU) – North America (NA) plate boundary zone across Northeast Asia still presents many open questions within the plate tectonic paradigm. Constraining the geometry and number of plates or microplates present in the plate boundary zone is especially difficult because of the location of the EU-NA euler pole close to or even upon the EU-NA boundary. One of the major challenges remains the geometry of the Okhotsk plate (OK). whose northwestern portion terminates on the EU-OK-NA triple junction and is thus caught and compressed between converging EU and NA. We suggest that this leads to a coherent and understandable large scale deformation pattern of mostly northwest-southeast trending strike-slip faults which split Northwest OK into several extruding slivers. When the fault geometry is analysed together with space geodetic and focal mechanism data it suggests a central block which is extruding faster bordered east and west by progressively slower extruding blocks until the OK plate boundary faults are encountered. Taking into account elastic loading from both the intra-OK faults and the OK-Pacific (PA) boundary reconciles geodetic motions with geologic slip rates on at least the OK-NA boundary which corresponds to the Ulakhan fault.

Global plate motion frames: Toward a unified model

Plate tectonics constitutes our primary framework for understanding how the Earth works over geological timescales. High‐resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals, mid‐ocean ridges, transform faults, and seafloor spreading, cementing the plate tectonic paradigm. However, so‐called “absolute plate motions,” describing how the fragments of the outer shell of the Earth have moved relative to a reference system such as the Earth's mantle, are still poorly understood. Accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent‐ocean distributions change with time. A fundamental problem with deciphering absolute plate motions is that the Earth's rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system. Absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 Ma and ideally would require knowledge about the motions within the convecting mantle. In contrast, models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of Earth's history but cannot provide paleolongitudes because of the axial symmetry of the Earth's magnetic dipole field. We analyze four different reference frames (paleomagnetic, African fixed hot spot, African moving hot spot, and global moving hot spot), discuss their uncertainties, and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a “hybrid” model for the time period from the assembly of Pangea (∼320 Ma) to the present. For the last 100 Ma we use a moving hot spot reference frame that takes mantle convection into account, and we connect this to a pre–100 Ma global paleomagnetic frame adjusted 5° in longitude to smooth the reference frame transition. Using plate driving force arguments and the mapping of reconstructed large igneous provinces to core–mantle boundary topography, we argue that continental paleolongitudes can be constrained with reasonable confidence.

When did plate tectonics begin?

The plate tectonic paradigm has been the dominant model for understanding the solid Earth for over 40 years. However, although the model is hugely successful, there is still great uncertainty as to when the plate tectonic process began. Two recent papers have highlighted this difficulty by proposing two very different start times for plate tectonics. One model argues that plate tectonic processes took over from an earlier (unspecified) tectonic regime in a hotter, younger Earth at 1.0 Ga, whereas the other proposes a much earlier start at 4.4–4.5 Ga, and within a 100 Ma of planetary accretion. This feature discusses the evidence for the early and late start hypotheses and argues for a middle position in which plate tectonic processes began during the Archaean (>2.5 Ga ago).

Parallel computing of multi-scale continental deformation in the Western United States: Preliminary results

Lithospheric deformation in the western United States is one of the best examples of diffuse continental tectonics that deviate from the plate tectonics paradigm. Conceptually, diffuse continental deformation is known to result from (1) weak and heterogeneous rheology of continents and (2) driving forces that arise from plate boundaries as well as within the continental lithosphere. However, the dynamic interplay of continental rheology and driving forces, hence the geodynamics of continental tectonics, remains poorly understood. The heterogeneous rheology and multiple driving forces cause continents to deform over different spatiotemporal scales with different physical processes, yet most geodynamic models for continental tectonic avoid dealing with such multiphysics partly because of (1) the limited observational constraints of lithospheric structure and deformation, and (2) high demands on computing algorithms and resources. These constraints, however, have relaxed significantly in recent years to permit exploration of some of the multi-scale physics governing continental tectonics. Here we present preliminary results of modeling multi-scale tectonics in the western United States using parallel finite element computation. In a 3D subcontinental-scale model, we used fine numerical meshes to incorporate all major tectonic boundaries and rheological heterogeneities in the model to explore their interplay with tectonic driving forces in controlling active tectonics in the western US. In another model for the entire San Andreas Fault system, we explored strain localization and simulated fault behavior at multi-timescales ranging from rupture in seconds to secular fault creep in tens of thousands of years. These models can help to integrate data grids with distributed high-performance computing resources in the emerging geosciences cyberinfrastructure.

He, Sr, Nd, and Pb isotopic constraints on the origin of the Marquesas and other linear volcanic chains

The classic hotspot hypothesis [Morgan, W. J., 1971. Convection plumes in the lower mantle. Nature 230, 42–43], which posits that linear volcanic chains are traces of fixed plumes in the mantle on moving lithospheric plates, was instrumental in elevating the plate tectonics paradigm in the 1960s into a modern Earth Science theory. The hypothesis itself, however, remains conjectural because many of its predictions, particularly the simple age-progressive type of volcanism, are not observed in many linear volcanic chains. As an alternative explanation, it is proposed that linear volcanic chains are formed through magmatism along pre-existing lines of weakness such as transform zones and old sutures, or along cracks created by stresses on lithospheric plates. The Marquesas linear volcanic chain in south-central Pacific has geologic features that are consistent with some of the predictions of both hypotheses. To better constrain the origin of this volcanic chain, we collected major and trace element and Sr, Nd, Pb, and He isotopic data from several Marquesan lavas. Our new analyses combined with literature data classify the samples into the well established tholeiitic to mildly alkalic, low 87Sr/ 86Sr, high 143Nd/ 144Nd, shield-building volcanic phase lava group and highly alkalic, high 87Sr/ 86Sr, low 143Nd/ 144Nd, post-shield phase group. Lead isotopes show generally higher 206Pb/ 204Pb ratios and suggest evidence of crustal assimilation for the shield-building phase lavas, consistent with the argument that the shield-building phase volcanism has a lithospheric source component. On the other hand, post-shield phase lavas that are predicted to represent the true composition of the mantle source by the hotspot hypothesis have higher 3He/ 4He ratios and these are coupled to other geochemical tracers. Thus our results show that the Marquesas volcanic chain, similar to many other linear volcanic chains, has a high 3He/ 4He component in its mantle source. The presence of such a distinct source component cannot be easily explained by dispersed upper mantle heterogeneities, but provides a powerful constraint for the hotspot origin of many linear volcanic chains.

Analysis of airborne magnetic and gravity anomalies of peninsular shield, India integrated with seismic and magnetotelluric results and gravity anomalies of Madagascar, Sri Lanka and East Antarctica

Modelling of gravity and airborne magnetic data integrated with seismic studies suggest that the linear gravity and magnetic anomalies associated with Moyar Bhavani Shear Zone (MBSZ) and Palghat Cauvery Shear Zone (PCSZ) are caused by high density and high susceptibility rocks in upper crust which may represent mafic lower crustal rocks. This along with thick crust (44–45 km) under the Southern Granulite Terrain (SGT) indicates collision of Dharwar craton towards north and SGT towards south with N–S directed compression during 2.6–2.5 Ga. This collision may be related to contemporary collision northwards between Eastern Madagascar–Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC). Arcuate shaped N and S-verging thrusts, MBSZ-Mettur Shear and PCSZ-Gangavalli Shear, respectively across Cauvery Shear zone system (CSZ) in SGT also suggest that the WDC, EDC and SGT might have collided almost simultaneously during 2.6–2.5 Ga due to NW–SE directed compressional forces with CSZ as central core complex in plate tectonics paradigm preserving rocks of oceanic affinity. Gravity anomalies of schist belts of WDC suggest marginal and intra arc basin setting. The gravity highs of EGFB along east coast of India and regional gravity low over East Antarctica are attributed to thrusted high-density lower crustal/upper mantle rocks at a depth of 5–6 km along W-verging thrust, which is supported by high seismic velocity and crustal thickening, respectively. It may represent a collision zone at about 1.0 Ga between India and East Antarctica. Paired gravity anomalies in the central part of Sri Lanka related to high density intrusives under western margin of Highland Complex and crustal thickening (40 km) along eastern margin of Highland Complex with several arc type magmatic rocks of about 1.0 Ga in Vijayan Complex towards the east may represent collision between them with W-verging thrust as in case of EGFB. The gravity high of Sri Lanka in the central part falls in line with that of EGFB, in case it is fitted in Gulf of Mannar and may represent the extension of this orogeny in Sri Lanka.

YOSHIDA, M., WINDLEY, B. F. & DASGUPTA, S. (eds) 2003. Proterozoic East Gondwana: Supercontinent Assembly and Breakup. Geological Society Special Publication no. 206. x + 472 pp. London, Bath: Geological Society of London. Price £110.00, US $183.00; GSL member price £55.00, US $92.00; AAPG/SEPM/GSA member price £66.00, US $110.00; hard covers. ISBN 1 86239 125 4

YOSHIDA, M., WINDLEY, B. F. & DASGUPTA, S. (eds) 2003. Proterozoic East Gondwana: Supercontinent Assembly and Breakup. Geological Society Special Publication no. 206. x + 472 pp. London, Bath: Geological Society of London. Price £110.00, US $110.00; hard covers. ISBN 1 86239 125 4 - Volume 141 Issue 3

Discussion of "Analysis and reinterpretation of deformation features in the Rouge River valley, Scarborough, Ontario"

Structurally deformed Pleistocene sediments along the Rouge River are located < 7 km from Pickering Nuclear Generating Station (PNGS). They rest on structurally disturbed Paleozoic sedimentary strata and lie within an area of poorly understood geophysical and structural complexity in the underlying Proterozoic basement (Wallach et al. 1998). PNGS was constructed adjacent to a major population centre (now more than five million people) in the late 1960s largely in ignorance of local and regional geological conditions and well before the plate tectonic paradigm provided a model for basement evolution. The presence and significance of major bedrock lineaments, such as the Central Metasedimentary Belt Boundary Zone (CMBBZ) that passes directly under PNGS, together with several other structures that intersect below Pickering, was not then known. Today, such structures are recognised as being defined by persistent earthquake activity (Mohajer 1991, 1993, 1995; Wallach et al. 1998). A magnitude 3.1 earthquake occurred within 3 km of PNGS on May 24, 2000. Ten smaller magnitude earthquakes have been recorded in the last decade along the structure between Niagara and Pickering by the seismic networks of the Geological Survey of Canada and the United States Geological Survey. The more recently constructed Perry nuclear plant in the U.S.A. was temporarily closed in 1986 by a magnitude 5 temblor along the same CMBBZ structure. The local community has every right to be concerned about the presence of an aging nuclear reactor in their midst. The four Pickering “A” reactors were shutdown in 1997 as a result of “long standing management, process and equipment problems” (NPAG, Nuclear Performance Advisory Group 1997). As part of the recent Environmental Assessment regarding …

Magnetite reveals ambient field strength at low temperatures

Magnetite (Fe3O4) is the most important and oldest known magnetic mineral on Earth (Figure l). We have come a long way from the magnetite loadstone compasses of ancient China; magnetite and titanomagnetite have been established as the principal carriers of geologically important remanent magnetizations in rocks, the study of which led to the plate tectonic paradigm. We now recognize that magnetite plays an important role in the biosphere. Some organisms contain intra‐cellular particles of Fe3O4 that they use for spatial orientation and navigation. When preserved in rocks, these particles—called “magnetofossils”— can provide important insight into the origin and development of life here, and perhaps, on other planets [e.g., Thomas‐Keprta et al., 2000]. Magnetite is now used in the medical field and in nanotechnology research. Nanoparticles of magnetite are used as a contrasting agent in magnetic resonance imaging and are being developed to deliver site‐specific drugs for the treatment of cancer [Alexiou et al., 2000]. Such applications add to a long list of industrial uses of magnetite that range from magnetic ink to magnetic recording media.

Shifting paradigms in geomorphology: the fate of research ideas in an educational context

The acceptance of new ideas into the mainstream of geomorphological education is illustrated from the development of theories dealing with Earth history, glaciation, uniform flow, mass movement, continental mobility, cyclic erosion, and drainage networks. The lag between the conception of new ideas and their incorporation into mainstream texts has varied from negligible to more than 200 years. On one hand, despite its then untestable assumptions, the Davisian cycle of erosion gained rapid favor as the dominant paradigm of the early 20th century before it was found wanting. In contrast, concepts of uniform flow and slope stability, confirmed in the 18th century, waited almost 200 years for incorporation into geomorphology texts sensu stricto, although they had long been available in books on hydraulics and soil mechanics. Continental mobilism had a wild ride, culminating in the eventual acceptance of the plate-tectonics paradigm in the later 20th century. Explanations for the fate of these and other ideas are varied. New ideas are often opposed by establishment conservatism, language barriers, the perceived surrealism of new concepts, and simple ignorance. In contrast, new ideas may be accepted, sooner or later, by virtue of simplicity, forceful and well-connected leadership, or the death of opponents. Although mitigated by the information revolution of recent decades, these forces still persist and influence the extension of new ideas into a larger arena.

Late Cretaceous–Cenozoic deformation of northeast Asia

The plate tectonic paradigm implies rigid plates and narrow plate boundaries. In contrast, diffuse plate boundaries are common both in the oceans and continents [R.G. Gordon, Annu. Rev. Earth Planet. Sci. 26 (1998) 615–642], and their history is difficult to constrain, especially in remote, tectonically complex areas such as northeast Asia [M.E. Chapman, S.C. Solomon, J. Geophys. Res. 81 (1976) 921–930]. Here we show how extensive North Atlantic marine magnetic [R. Macnab et al., EOS 76 (1995) 449, 458] and gravity data [D.T. Sandwell, W.H.F. Smith, J. Geophys. Res. 102 (1997) 10039–10054] can be used to unravel, with tight confidence limits, successive periods of deformation over 80 million years, along the diffuse continental Eurasian–North American plate boundary. A period of compression in the Late Cretaceous (14 mm/yr in the Laptev Sea to 20 mm/yr in Kamchatka) led to thrusting in the Verkhoyansk Mountains, and was followed by extension from 68 to 40 Ma when ∼400 km of extension was accommodated by the formation of a series of grabens, including the Moma Rift system. Since 40 Ma, time-varying compression and transpression along the Moma Rift system created strike-slip faults, thrusts and folds at rates up to 6.3 mm/yr. In the Laptev Sea region, 600 km of extension from latest Late Cretaceous to present created the Laptev Sea and Lena Rift systems. The deformation predicted by our model fits most geological features formed in the Laptev Sea and central northeast Asia during Late Cretaceous–Cenozoic times. The most recent deformation (Late Miocene–Pliocene) is not very well constrained since our model lacks data younger than 11 Ma. The deformation that occurred in Kamchatka reflects a complex tectonic setting and our model’s predictions are only tentative.

Autochthonous development of Superior Province greenstone belts?

Models for Superior Province greenstone belt development have changed over the last 30 years from ensimatic, generally autochthonous models to suspect allochthonous terrane models based on the plate tectonic paradigm. Models intermediate between these end-members such as autochthonous terrane models have not been prominent. Within the Superior Province, there is a secular variation from early ca. 3 Ga platformal assemblages with overlying rift-related komatiite–tholeiite units representing breakup of ca. 3 Ga continental fragments to late (ca. 2.7 Ga) pull-apart basins. Autochthonous terranes should demonstrate some sort of logical order such as that described above, whereas allochthonous terranes will be largely oceanic in character and should be essentially random in their occurrence rather than exhibiting secular order. In the face of this situation, this paper reviews evidence from throughout the Superior Province that suggests many, but not all greenstone assemblages developed autochthonously or parautochthonously. The earliest greenstone assemblages in the Superior Province are generally quartzite and carbonate-bearing platform sequences in the North Caribou terrane and the Marmion terrane deposited unconformably upon granitoid or greenstone basement. The evidence for autochthonous/parautochthonous development includes the presence of unconformities between Keewatin type greenstone units such as: (1) basal unconformities between ca. 3 Ga granitoid or volcanic basement and slightly younger platformal assemblages; (2) between ca. 2.9 Ga platformal assemblages and younger platformal and Andean arc assemblages. Stratigraphic patterns such as adjacent greenstone belt assemblages representing sharply differing geodynamic settings with some indication of palinspastic order or progression in the northern, central and southern Superior Province are considered suggestive of orderly, autochthonous progression from platforms through rifting of continental fragments and late assembly during the Kenoran orogeny. Petrogenetic data indicating involvement of older granitoid crust in genesis of greenstone assemblages of a variety of ages throughout the Superior Province suggests the presence of older basement, implying autochthonous or parautochthonous development of many assemblages. Younger greenstone assemblages in the Abitibi–Wawa subprovince and the North Caribou terrane contain xenocrystic zircons similar in age to stratigraphically underlying greenstone assemblages. This pattern has been interpreted to represent autochthonous greenstone development. The current above enumerated evidence for autochthonous development of greenstone assemblages is generally indirect; few actual unconformities between Keewatin type greenstone assemblages have been recognized in the Superior Province. To aid in future recognition of unconformities in Keewatin type greenstone assemblages, examples of unconformities of varying degrees of subtlety in Archean sequences are reviewed. Archean unconformities ranging from sharp angular discordances in the northern Superior Province to subtle stratigraphic patterns, to weathering-related geochemical mobilization along inter-formational contacts are described. These styles of unconformity in Archean sequences suggest previously unrecognized major disconformities and unconformities may well exist between Keewatin type assemblages in the Superior Province generally and the Abitibi–Wawa subprovince specifically. The most important disconformity suggested in the Abitibi–Wawa subprovince is between the ca. 2726 Ma Deloro and the ca. 2708 Ma Tisdale assemblages. The presence of unconformities and disconformities coupled with the indirect lines of evidence cited above suggest an ‘autochthonous’ origin for many Superior Province greenstone belts. This ‘autochthonous’ origin involves either autochthonous or parautochthonous evolution of assemblages at least in part in place. That is, development of ca. 3 Ga platform sequences in selected locales followed by in place evolution of a succession of plume, oceanic and arc assemblages culminating in late pull-apart basin sequences. Implications of the autochthonous model for greenstone belts of the Superior Province, particularly the Abitibi–Wawa subprovince, include the future ability to recognize unconformity bounded sequences, potentially improving regional scale correlation.

From microscope to mountain belt: 150 years of petrology and its contribution to understanding geodynamics, particularly the tectonics of orogens

Thirty-five years ago the introduction of the plate tectonics paradigm led to a new understanding of orogeny. Subsequently, the development of advanced instruments for remote collection of information and for analysis of elemental and isotopic composition of materials, and the increases in computing power have enabled an unprecedented number of high-precision data about the Earth to be collected, analyzed, modelled and displayed. Within this revolution in global tectonics, the metamorphic petrologist has developed methods to unravel the depth, thermal, temporal and deformational history of orogens using detailed observations at map, hand sample and thin-section scales in combination with elemental and isotope data, and using inverse and forward modelling. Two exciting new directions in metamorphic petrology in relation to geodynamics concern the kinship between earthquakes and metamorphic reactions in subduction zones, and the petrology of the Earth’s mantle. Evidence of the changes in pressure ( P) and temperature ( T) in the Earth’s crust and upper mantle during the break up, movement, and collision of pieces of the continental lithosphere is sporadically recorded by the mineralogy and microstructures preserved in rocks exhumed to the surface. Better calibration of phase equilibria, the use of internally-consistent thermodynamic data sets and the development of techniques to retrieve close-to-peak P–T conditions from metamorphic rocks have yielded more precise P–T data that enhance our ability to characterize the path followed by individual rocks in P–T space. An improved ability to date segments of the P–T path, and to separate the length of time associated with the prograde (increasing T) evolution from the age of close-to-peak P–T conditions has enabled better understanding of the rates and processes involved in lithosphere thickening. At the same time, better constraints on the retrograde thermal history have contributed to our knowledge of the several tectonic processes that may operate during exhumation, although these are less well understood. The expanding database of key information, combined with predictions from modelling, has allowed the identification of characteristic P–T–t evolutions expected for rocks that have undergone distinct tectono-metamorphic histories. However, relating structural events recorded by rocks to specific points along the P–T evolution remains problematic, particularly regarding complex overprinting patterns of inclusion trails in porphyroblasts. These advances have improved our understanding of the tectonic evolution of orogens. At the extreme of conditions for crustal metamorphism are the recently discovered ultra-high pressure (UHP) and ultra-high temperature (UHT) facies of metamorphism. Both are problematic given our limited knowledge of processes at these conditions, particularly the return of UHP rocks from peak- P conditions and the mechanism for extreme heat in the crust in UHT metamorphism. The extreme depth inferred for metamorphism in some UHP terranes raises the issue of whether theoretically plausible tectonic overpressures can be dynamically maintained to affect metamorphic reactions. If the pressure gradient recorded by UHP rocks is greater than lithostatic, the UHP metamorphism may have occurred at depths shallower than currently believed. These studies have provided a reliable first-order framework for the comparison of rocks of ancient suture zones where the plate tectonics situation is less certain. However, orogens are spatially and temporally extended nonlinear systems with feedback relations. Such complex systems generate apparently simple behavior by self-organization, and the influence of unique histories must be respected.