What is the difference between shields and stable platforms?
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A shield is a large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas.[1] These rocks are older than 570 million years and sometimes date back to around 2 to 3.5 billion years.[citation needed] They have been little affected by tectonic events following the end of the Precambrian, and are relatively flat regions where mountain building, faulting, and other tectonic processes are minor, compared with the activity at their margins and between tectonic plates. Shields occur on all continents. Show
Terminology[edit]The term shield cannot be used interchangeably with the term craton. However, shield can be used interchangeably with the term basement. The difference is that a craton describes a basement overlayed by a sedimentary platform while shield only describes the basement. The term shield, used to describe this type of geographic region, appears in the 1901 English translation of Eduard Suess's Face of Earth by H. B. C. Sollas, and comes from the shape "not unlike a flat shield"[2] of the Canadian Shield which has an outline that "suggests the shape of the shields carried by soldiers in the days of hand-to-hand combat."[3] Lithology[edit]A shield is that part of the continental crust in which these usually Precambrian basement rocks crop out extensively at the surface. Shields themselves can be very complex: they consist of vast areas of granitic or granodioritic gneisses, usually of tonalitic composition, and they also contain belts of sedimentary rocks, often surrounded by low-grade volcano-sedimentary sequences, or greenstone belts. These rocks are frequently metamorphosed greenschist, amphibolite, and granulite facies.[citation needed] It is estimated that over 50% of Earth's shields surface is made up of gneiss.[4] Erosion and landforms[edit]Being relatively stable regions the relief of shields is rather old with elements such as peneplains being shaped in Precambrian times. The oldest peneplain identifiable in a shield is called a "primary peneplain",[5] in the case of the Fennoscandian Shield this is the Sub-Cambrian peneplain.[6] The landforms and shallow deposits of northern shields that have been subject to Quaternary glaciation and periglaciation are distinct from those found in closer to the equator.[5] Shield relief, including peneplains, can be protected from erosion by various means.[5][7] Shield surfaces exposed to sub-tropical and tropical climate for long enough time can end up being silicified, becoming hard and extremely difficult to erode.[7] Erosion of peneplains by glaciers in shield regions is limited.[7][8] In the Fennoscandian Shield average glacier erosion during the Quaternary amounts to tens of meters, albeit this was not evenly distributed.[8] For glacier erosion to be effective in shields a long "preparation period" of weathering under non-glacial conditions may be a requirement.[7] An attempt is made in this paper to outline the general tectonic development of platforms. We hope that such a review will help to determine the directions which geological-geophysical research should take within the Upper Mantle Project. We apply the term platform, a synonym of craton, to any large portion of the continental crust bounded by folded belts or sometimes by oceans, and characterized by the following specific features: (a) tectonic stability over a very long period (several hundreds of million years or more); (b) general tectonic homogeneity (despite a complex inner structure and different times of formation of its separate parts); (c) an absence of high gradients of movement. Shields are elements of platforms which have been characterized by a tendency towards slow uplift over periods of hundreds of million years. They are not sharply delimited. We have accepted the following ages for the upper boundaries of Precambrian units: Archaean 2,600–2,800 m.y. (million years); Lower Proterozoic (Palaeoproterozoic, according to Salop) 2,100 m.y.; Middle Proterozoic (Mesoproterozoic) 1,850−1,650 m.y.; Upper Proterozoic (Neoproterozoic) 1,100 m.y.; Epi-Proterozoic (Epiproterozoic) 620 m.y.; Eocambrian 570 m.y. The authors believe, although the material here presented is based mainly on studies of Eurasian structures, that these results can be also applied to all other continents. Reference to data on other areas is always stipulated. References (33)
Stratigraphy of the Steeprock groupGeneral Geotectonics. NedraThe Evolution of North AmericaSome regularities of the structure and evolution of the earth's crust in intermontane troughs and foredeeps. Byull. Mosk. Obshchestva Ispytatelei PrirodyOtd. Geol.(1965) On the Question of the Earth Heating. Geol. Resultaty Prikladnoy Geochimii i GeofizikiGosgeolizdat(1960) The neomobilism and convection in the mantle. Article 1. Byull. Mosk. Obshchestva Ispytatelei PrirodyOtd. Geol.(1965) 2021, Precambrian Research 2012, Lithos Show abstractNavigate Down TTGs (tonalite–trondhjemite–granodiorite) are one of the archetypical lithologies of Archaean cratons. Since their original description in the 1970s, they have been the subject of many studies and discussions relating to Archaean geology. In this paper, we review the ideas, concepts and arguments brought forward in these 40 years, and try to address some open questions — both old and new. The late 1960s and the 1970s mark the appearance of “grey gneisses” (TTG) in the scientific literature. During this period, most work was focused on the identification and description of this suite, and the recognition that it is a typical Archaean lithology. TTGs were already recognised as generated by melting of mafic rocks. This was corroborated during the next decade, when detailed geochemical TTG studies allowed us to constrain their petrogenesis (melting of garnet-bearing metamafic rocks), and to conclude that they must have been generated by Archaean geodynamic processes distinct from their modern counterparts. However, the geodynamic debate raged for the following 30 years, as many distinct tectonic scenarios can be imagined, all resulting in the melting of mafic rocks in the garnet stability field. The 1990s were dominated by experimental petrology work. A wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to TTG liquids; whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate. From 1990s onwards, one of the key questions became the comparison with modern adakites. As originally defined these arc lavas are reasonably close equivalents to Archaean TTGs. Pending issues largely revolve around definitions, as the name TTG has now been applied to most Archaean plutonic rocks, whether sodic or potassic, irrespective of their HREE contents. This leads to a large range of petrogenetic and tectonic scenarios; a fair number of which may well have operated concurrently, but are applicable only to some of the rocks lumped together in the ever-broadening TTG “bin”. 2002, Journal of African Earth Sciences Show abstractNavigate Down This article introduces the name “Saharan Metacraton” to refer to the pre-Neoproterozoic––but sometimes highly remobilized during Neoproterozoic time––continental crust which occupies the north-central part of Africa and extends in the Saharan Desert in Egypt, Libya, Sudan, Chad and Niger and the Savannah belt in Sudan, Kenya, Uganda, Congo, Central African Republic and Cameroon. This poorly known tract of continental crust occupies ∼5,000,000 km2 and extends from the Arabian-Nubian Shield in the east to the Tuareg Shield to the west and from the Congo craton in the south to the Phanerozoic cover of the northern margin of the African continent in southern Egypt and Libya. The term “metacraton” refers to a craton that has been remobilized during an orogenic event but is still recognizable dominantly through its rheological, geochronological and isotopic characteristics. Neoproterozoic remobilization of the Saharan Metacraton was in the forms of deformation, metamorphism, emplacement of igneous bodies, and probably local episodes of crust formation related to rifting and oceanic basin development. Relics of unaffected or only weakly remobilized old lithosphere are present as exemplified by the Archean to Paleoproterozoic charnockites and anorthosites of the Uweinat massif at the Sudanese/Egyptian/Libyan boarder. The article explains why the name “Saharan Metacraton” should be used, defines the boundaries of the metacraton, reviews geochronological and isotopic data as evidence for the presence of pre-Neoproterozoic continental crust, and discusses what happened to the Saharan Metacraton during the Neoproterozoic. A model combining collisional processes, lithospheric delamination, regional extension, and post-collisional dismembering by horizontal shearing is proposed. 1993, Precambrian Research Show abstractNavigate Down Before the deposition of a Proterozoic cover and the repeated Proterozoic reworking of the older rocks, the presently exposed Archaean areas in northern Sweden formed part of a coherent craton. In the present study, we have used SmNd isotopic analyses of Proterozoic granitoids and metavolcanics to delineate the Archaean palaeoboundary. In a regional context, the transition from strongly negative ϵNd(t) values in the northeast to positive values in the southwest is distinct, and approximately defines the border of the old craton. The Archaean palaeoboundary extends in a WNW direction, and is subparallel to the longitudinal axis of the Skellefte sulphide ore district but it is situated ∼ 100 km farther to the north. The ∼ 1.9 Ga old granitoids on the two sides of the palaeoboundary were all formed in compressional environments, but those situated to the north have higher contents of LILE and LREE at similar contents of Si. This indicates that they were generated in an area with thicker crust and supports the location of the Archaean-Proterozoic palaeoboundary. There is no simple correlation between the Archaean palaeoboundary, as defined by the isotopic results, and any of the major fracture systems as interpreted from regional geophysical measurements. Reflection seismic work indicates that juvenile volcanic-arc terrains to the south have been thrust onto the Archaean craton. Possible thrust faults have been identified from aeromagnetic measurements. Rifting of the Archaean craton created a passive margin ∼ 2.0 Ga ago. Spreading shifted to convergence with subduction beneath the Archaean continent ∼ 1.9 Ga ago. Subsequently, the resulting juvenile volcanic arc collided with the old continent, and the Archaean palaeoboundary as existing today was formed by a collision characterized by overthrusting. The boundary then was disturbed by later deformation predominantly along NNE-trending fracture systems. 1990, Developments in Precambrian Geology Show abstractNavigate Down The Precambrian crust of the Aravalli Mountain Range comprises a number of Proterozoic fold belts underlain and delimited by schist-gneiss-granitic rocks of Archaean age. The basement, which was partly remobilized during the Proterozoic orogenesis, includes a heterogeneous assemblage of biotite gneiss, amphibolite, aluminous paragneiss, quartzite and marble formed during the time span 3.5-2.5 Ga. The three major Proterozoic cover units: the Aravalli Supergroup (−2.5-1.90 Ga), the Delhi Supergroup (−1.9-1.45 Ga), and the Champaner Group (closing at −0.9 Ga), evolved as ensialic fold belts through development of a series of rift-basins, grabens with intervening horsts. The basin-fills included sediments and volcanics laid down in several successive stages punctuated by great hiatuses. Sedimentation and volcanicity in these basins recorded several secular changes in terms of basin formation, lithologic characters, biological evolution, magmatism and metallogeny. The Proterozoic, which started with the formation of linear fold belts, culminated in abortive rifting and associated anorogenic multimodal magmatism in the axial zone of the mountain range and west of it. 1989, Journal of Southeast Asian Earth Sciences Show abstractNavigate Down The Precambrian crustal evolutionary events and tectonic pattern during successive megastages in the five Precambrian provinces of eastern Asia are reviewed. A tectonic province may consist of one or more stable platforms, including the surrounding contental margin tracts. Evidence available indicates that the most important period of crustal growth was during the early Precambrian, particularly in the late Archaean and early Proterozoic. Research article Russian Geology and Geophysics, Volume 57, Issue 11, 2016, pp. 1591-1605 Show abstractNavigate Down We performed geological, geochronological, and isotope-geochemical studies of two tectonized-granite bodies intruding the rocks of the lower plate of the Bezymyannyi metamorphic-core complex (MCC) (western Transbaikalia). The U-Pb zircon age of sheared granites sampled on the periphery of the Bezymyannyi MCC near the detachment zone is 202 ± 2 Ma (LA-ICP-MS). The U-Pb dating (SHRIMP-II) of zircon grains from gneissic granite intruding the rocks of the lower plate in the central zone of the Bezymyannyi MCC has yielded an age of 165 ± 2 Ma. The sheared granites dated at 202 Ma (Late Triassic) have low contents of Nb and Ta and high contents of Sr and Ba, probably inherited from magmatic subduction-related sources. These granites are characterized by £Nd(T) = -3.7, which, along with the high contents of K2O and medium contents of Th, testifies to the presence of continental-crust material in their source. Thus, they formed, most likely, from a mixed mantle-crust source. The studied granites, like other Triassic igneous rocks in Transbaikalia, might have originated in the suprasubduction setting during the evolution of the active continental margin of the Siberian continent. The gneissic granites dated at 165 Ma (Middle Jurassic) have high contents of K2O, Rb, and Th, are depleted in Nb, and are characterized by £Nd(T) = 0 and a negative Eu anomaly on the REE patterns. They formed, most likely, from an intermediate-felsic crustal source. These Middle Jurassic granites, like other small Jurassic granitoid massifs spatially associated with MCCs in western Transbaikalia, intruded in the western part of the Mongol-Okhotsk Ocean after its closure, during the change of the subduction regime for the collisional one. The studied Late Triassic granites of the Bezymyannyi MCC cannot be associated with MCCs in Transbaikalia because their intrusion was related to the subduction of the oceanic plate of the Mongol-Okhotsk Ocean beneath the Siberian continent. The Middle Jurassic granites of the Bezymyannyi MCC, together with other small Jurassic granitoid massifs, can be classified as prekinematic intrusions formed earlier than MCCs in western Transbaikalia. However, it is unlikely that their intrusion caused a large-scale crustal extension in Transbaikalia. Research article Russian Geology and Geophysics, Volume 57, Issue 1, 2016, pp. 47-68 Show abstractNavigate Down There are continuing issues concerning the formation and reconstruction of the geographic position of the Neoproterozoic Yenisei Ridge—a key element of the western framing of the Siberian craton and the Central Asian orogenic belt. This study focuses on the inner structure, composition, and boundaries of the Central Angara terrane, which is the largest in the Transangarian segment of the Yenisei Ridge. We propose a scheme of fault deformation of the region and demonstrate that the fault tectonics of the Central Angara terrane is distinct from that of adjacent terranes. We study in detail the Yeruda pluton granitoids of the Teya complex, which indicate accretionary-collisional magmatic events in this terrane prior to its collision with Siberia. New geochemistry and SHRIMP U–Th–Pb zircon geochronology of the granites indicate that they formed at 880–860 Ma in a collisional setting. Integrated petromagnetic and paleomagnetic investigations yield a paleomagnetic pole that is significantly different from the corresponding Neoproterozoic interval of the apparent polar wander path (APWP) for Siberia. The difference in paleolatitudes between the Central Angara terrane and the Siberian craton at the time of the Teya granites formation was at least 8.6 degrees, which equals a latitudinal separation of at least 1000 km. We consider various possible positions for the terrane relative to the Siberian craton. These results demonstrate that the 880–860 Ma magmatic events in the Central Angara terrane are not related to events in the western margin of the Siberian craton. Therefore, they do not indicate the existence of a Grenville-age orogenic belt in this location, as proposed by some authors. Research article Russian Geology and Geophysics, Volume 57, Issue 3, 2016, pp. 451-463 Show abstractNavigate Down High-pressure mafic granulites and garnet pyroxenites occur within the South Muya block as boudins or lenses among metamorphic rocks of the Kindikan Group. Their primary minerals crystallized at 670-750 °C and 9.5-12.0 kbar. Granulite metamorphism peaked at 630 Ma, according to LA-ICP-MS U-Pb zircon ages. Judging by their major- and trace-element compositions and Hf isotope ratios in zircons, the South Muya granulites were derived from differentiated within-plate basalts, which, in turn, resulted from melting of juvenile mantle source and Meso- or Paleoproterozoic crust. The events of granulite and eclogite metamorphism in the South and North Muya blocks, respectively, were coeval and the two blocks were spatially close to each other at the onset of Late Baikalian subduction and collision events. Research article Review of Palaeobotany and Palynology, Volume 246, 2017, pp. 109-119 Show abstractNavigate Down The end-Ordovician has received wide attention because it hosts major global events including mass extinctions, glaciations, significant sea-level fluctuations, and large-scale perturbations of the Earth's carbon cycle. Knowing the order and timing of these events and their components is crucial for understanding these environmental changes. Here, we use stable carbon isotope stratigraphy in combination with chitinozoan biostratigraphy to correlate the Upper Ordovician Belonechitina gamachiana chitinozoan Biozone. Its position has long been a matter of debate; some argue that it is of late Katian age and others that it is of early Hirnantian age. The Skogerholmen Formation from the Oslo-Asker District in Norway has been correlated with the lower-middle Pirgu Baltic Stage, hitherto believed to correspond to the international upper Katian Stage. Our study, however, reveals the presence of B. gamachiana, diagnostic of the eponymous Biozone, in the descending trend of a modest carbon isotope excursion in the lower part of this formation. This is strikingly similar to data from coeval end-Ordovician sections in North America, where the prevailing evidence suggests an early Hirnantian age for the B. gamachiana chitinozoan Biozone. This new correlation suggests that the lower Hirnantian boundary may be positioned within the Pirgu Baltic Stage. Research article Russian Geology and Geophysics, Volume 56, Issue 3, 2015, pp. 411-434 Show abstractNavigate Down The Dovyren intrusive complex includes the ore-bearing (Cu–Ni–PGE) Yoko–Dovyren layered pluton (728 Ma, up to 3.4 km in thickness), underlying ultramafic sills, and comagmatic leuconorite and gabbro-diabase dikes. Studies of Sr–Nd–Pb isotope systems were carried out for 24 intrusive rocks and five associated low- and high-Ti basalts. The high-Ti basalts show 0.7028 ≤ (87Sr/86Sr)T ≤ 0.7048 and 4.6 ≤ εNd(T) ≤ 5.8, similar to the values in MORB. The intrusive basic and ultrabasic rocks are geochemically similar to the low-Ti formation, making a compact cluster of compositions with extremely high ratios of radiogenic Sr and Pb isotopes and low εNd values. The maximum enrichment in radiogenic Sr is shown by the rocks near the pluton bottom ((87Sr/86Sr)T = 0.71387 ± 0.00010 (2σ); εNd(T) = –16.09 ± 0.06), which are the products of crystallization of the most primitive high-Mg magmas. The above-located dunites, troctolites, and gabbro show lower enrichment, probably because of the contamination of the host rocks during the filling of the magma chamber and/or because of the slight heterogeneity of the source. Calculations of the proportions of mixing of the parental melt with carbonate terrigenous material have shown that the variations in the Sr and Nd isotope ratios are due to the incredibly high contamination of the sediments, up to 40–50%. This contradicts the succession of the main rock types in the Yoko–Dovyren pluton in accordance with the crystallization of picrite-basaltic magma. The contribution of 5–10% high-Ti component seems more likely and suggests interaction between two isotopically contrasting magmas in this province in the Late Riphean. In general, the minor variations in εNd(T) of the intrusive rocks and metavolcanics (–14.3 ± 1.1) testify to the isotopically anomalous source of the low-Ti magmas. The time variation trend of εNd and geochemical features of the Dovyren rocks indicate that the products of melting of 2.7–2.8 Ga suprasubduction mantle might have been the massif protolith. Thus, the Dovyren parental magmas formed from a much older (sub)lithospheric source in the Late Riphean. The source was initially enriched in a mafic component with a low Sm/Nd ratio and was isolated from the convecting mantle and mantle melting processes for ~ 2 Gyr. The existence of such a long-living and at least twice reactivated lithospheric substratum is confirmed by the fact that the Nd isotope evolution trend of the initially nonanomalous mantle protolith includes not only the Dovyren rocks but also the Paleoproterozoic gabbro of the Chinei pluton and the Archean enderbites of the Baikal region. © 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Research article |
