Mechanism Of Plate Tectonics Research Articles

Genesis of the earliest (3.20-2.83 Ga) terranes of the Fennoscandian shield

Our study of magmatic rocks provides grounds for discussing the successive phases and geodynamic conditions of continental crust generation between 3.2-2.83 Ga within the Fennoscandian shield. Three tectono-magmatic phases, 3.2-3.1 Ga, 3.0-2.92 Ga, and 2.92-2.83 Ga, are established. During the 3.2-3.1 Ga phase, voluminous intrusives were emplaced, creating the large Vodlozero Positive Nd(t) values for mafites and, partly, granitoids point to a weighty contribution of juvenile material derived from a depleted mantle. At the same time, Nd isotope composition for a number of granitoid massifs and zircon ages suggest the presence of an earlier (as old as 3.5 Ga) crustal component. Endogenic processes that occurred during the second, 3.0-2.92 Ga phase, have been recorded in southeastern and western Karelia and are inferred to have occurred in the Kola Peninsula as well. During this phase, oceanic plateaus and island arcs were formed near, to be accreted onto, the western and eastern margins of the ancient Vodlozero core. Simultaneously, the central part of the Vodlozero core was the locus of emplacement of gabbronorite-diorite intrusions and purely dioritic bodies, as well as vigorous tonalite- granodiorite magmatism, to form the Vodlozero domain, the oldest on the shield. In the western Karelian domain, rocks younger than 2.92 Ga are exposed at the current erosional surface. The presence of ancient material in western Karelian crust is pinpointed by Nd model ages for granitoids and volcanites and by a detrital zircon age from granite. The third, 2.92-2.83 Ga phase entailed further reworking of ancient terranes and initiation of new sialic cores. At the northern margin of the Vodlozero domain and within the western Karelian one, a system of rift-related features came into being, eventually to evolve into bimodal greenstone belts largely dominated by mafic and ultramafic volcanites. The Kola province provided the stage for inception of rift-related greenstone belts with their associated komatiite-tholeiite (2.92-2.87 Ga) series followed by the basalt-andesite-dacite (2.88-2.79 Ga) series. Apparently, these belts in their present-day form, just like those of the western and eastern margins of the Vodlozero domain, result from tectonic juxtaposition of rock assemblages that originated from a variety of geodynamic settings. Archean continental crust of the Fennoscandian shield is shown to have formed through both progressive addition of sialic crust over time, mainly at convergent boundaries of ancient plates, and via reworking of ancient fragments, which involved input of juvenile material resulting from rising mantle plumes. In all likelihood, ascending mantle plumes are responsible for the formation of accretionary and collisional orogens, whereas the coeval magmatism at active plate margins was due to subduction (in the context of the plate tectonic mechanism). Generation of rift-related structures and associated magmatism may have been driven by rising mantle plumes. Brittle deformations leading to rifting and associated with the ascent of mantle plumes did not result in break-up of the young continental crust. The main outcome of this mechanism was massive inflow of high-temperature magma into the lithosphere forming within-crust layers of asthenosphere, in which granite melts originated, eventually to migrate to shallower levels.

Which forces drive North America?

Understanding the mechanism of plate tectonics is one of the most important problems in the geosciences. Are the tectonic plates pulled and pushed from the side, as the Orowan-Elsasser model suggests, or does mantle convection play an active role in driving the plates? This question can be addressed by studying the deformation of deep continental roots. The application to North America shown here indicates that the deeper mantle moves at a higher velocity than the North American plate and that the mantle plays an important role in driving the plates. I suggest that this finding (1) provides a natural explanation for why the motion of North America slowed down dramatically during the past 100 m.y. and (2) predicts that North American motion will eventually come to a halt.

Crustal evolution in the early Archaean of South America: example of the Sete Voltas Massif, Bahia State, Brazil

The Sete Voltas massif (São Francisco craton, Brazil) appears as a composite crustal segment built in at least three successive accretional events dated by RbSr, 207Pb 206 Pb monozircon and U-Pb SHRIMP methods. (a) At ca 3.4 Ga, generation and emplacement of magmatic precursors to the old grey gneisses. These are the oldest rocks so far recognized in South America. Their composition is typical of Archaean TTG (Tonalite, Trondhjemite and Granodiorite) and geochemical modelling indicates that they were produced by partial melting of an Archaean tholeiite, leaving a hornblende garnet residue. (b) Between 3.17 and 3.15 Ga, younger grey gneisses and porphyritic granodiorites intruded the old grey gneisses. Their geochemical composition is clearly different from that of typical TTG and geochemical modelling shows that they were produced by partial melting of an older continental crust. (c) Grey granite dykes emplaced at ca 2.6 Ga, during a late-stage magmatic event in the Sete Voltas massif. All the units belonging to the Sete Voltas massif yield homogeneous T DM ages at 3.66 Ga which are interpreted as reflecting derivation from older protoliths, or rather their contamination by older pre-existing continental crust. It is tentatively proposed that a 3.66-Ga-old continental crust existed prior to the emplacement of the older grey gneisses. Although such crustal material is unknown at the present day in South America, Nutman and Cordani (1993) have nevertheless reported a single zircon core from Sete Voltas giving a 207Pb 206 Pb age of 3.473 ± 0.008 Ga. The present study, together with some other recent data (Mougeot et al., 1995; Santos Pinto et al., 1995a) indicates that the 3.4-Ga-old South American continental crust, until now considered as of very small geographical extent, in fact made up a true continental block corresponding in size to at least the whole present-day Gavião block. Major and trace element petrogenetic modelling for the young grey gneisses and the porphyritic granodiorites indicates that the precursor magmas are derived from the melting of pre-existing crustal rocks similar to the old grey gneisses. This conclusion is supported by the intense migmatization of the old grey gneisses which took place before the intrusion of the younger intrusives. The calculated residual mineral assemblage in equilibrium with the magmatic liquid is quartz + hornblende + plagioclase, a condition that is achieved between P = 10 kbar, T ≈ 800°C, <5% weight H 2O, or P = 15 kbar, T ≈ 700°C, ∼15% weight H 2O (Johnson and Wyllie, 1988). This suggests that the hydrous melting of the old grey gneisses occurred at ca 3.17 Ga at depths of ca 30–45 km, thus giving a minimum estimate of the thickness of this part of the Archaean crust. This estimate, as well as the development of an intense migmatization and the strong foliation in the old grey gneisses, can be interpreted as classical features of collisional thickening. Taken with the existence of horizontal tectonics in Archaean cratons older than 3.0 Ga, such a conclusion indicates that, more or less modern types of plate tectonic mechanisms operated in the early Archaean in the studied terrain.

Ridge subduction: kinematics and implications for the nature of mantle upwelling

Ridge subduction follows the approach of an oceanic spreading centre towards a trench and subduction of the leading oceanic plate beneath the overriding plate. There are four possible kinematic scenarios: (1) welding of the trailing and overriding plates (e.g., Aluk–Antarctic Ridge beneath Antarctica); (2) slower subduction of the trailing plate (e.g., Nazca–Antarctic Ridge beneath Chile and Pacific–Izanagi Ridge beneath Japan); (3) transform motion between the trailing and overriding plates (e.g., San Andreas Transform); or (4) divergence between the overriding and trailing plates (e.g., Pacific – North America). In case 4, the divergence may be accommodated in two ways: the overriding plate may be stretched (e.g., Basin and Range Province extension, which has brought the continental margin into collinearity (and, therefore, transform motion) with the Pacific – North America relative motion); or divergence may occur at the continental margin and be manifest as a change in rate and direction of sea-floor spreading because the pair of spreading plates changes (e.g., from Pacific–Farallon to Pacific – North America), spawning a secondary spreading centre (i.e., Gorda – Juan de Fuca – Explorer ridge system) that migrates away from the overriding plate.Mantle upwelling associated with sea-floor spreading ridges is widely regarded as a passive consequence, rather than an active cause, of plate divergence. Geological and geophysical phenomena attendant to ridge–trench interaction suggest that regardless of the kinematic relations among the three plates, a thermal anomaly formerly associated with the ridge migrates beneath the overriding plate. The persistence of this thermal anomaly demonstrates that active mantle upwelling may continue for tens of millions of years after ridge subduction. Thus, regardless of whether the mantle upwelling was active or passive at its origin, it becomes active if the spreading continues for sufficient time and, thus, must contribute to the driving mechanism of plate tectonics.

The driving mechanism of plate tectonics

Plate velocities reflect the global mantle circulation which carries the heat from the deep part of the Earth to the surface. Three other measurable parameters put strong constraints on the physical processes involved in mantle dynamics: the surface topography, the geoid and the lateral density heterogeneities mapped throughout the mantle by seismic tomography. Using these internal loads and the Navier-Stokes equations, a first generation of dynamic Earth models have been developed in the last five years. They have produced satisfactory predictions for the divergence of the surface velocities and for the geoid. These predictions are quite sensitive to the assumed radial viscosity variations. The best match with observations has therefore helped to discriminate between various possible viscosity structures. A new set of models taking the existence of plates into account has been particularly successful in predicting the plate velocities, including their vorticity. The driving mechanism of plate tectonics is thus put on a firm physical basis and the computed solutions are quantitatively reasonable.

Blueschists and eclogites: a possible plate tectonic mechanism for their emplacement from the upper mantle

The most significant problem of blueschists and eclogite origin is explaining the mechanism by which they were transported from great depths to the surface. Three aspects of this problem are discussed: 1. (1) prograde and retrograde trajectories in blueschist- and eclogite-bearing belts and the preservation conditions for such minerals as aragonite, lawsonite and coesite; 2. (2) the mechanism by which low density crustal rocks (with coesite and diamond) descend to a depth of 100–150 km, and the high rate of return from these depths to the surface; 3. (3)the origin of a tectonic mixture of rocks from different depths in melange zones. Using estimations of P- T conditions and constructed P- T- t trajectories four types of blueschist belt are proposed. These include two of the Ernst types (the Alpine and Franciscan) and two additional types (the Dora-Maira and Sanbagawa types). Key examples of geologically and mineralogically well-studied complexes in the Soviet Union are discussed (the Kokchetav massif, Maksutov complex, Syumkeu belt, Borus belt). The various models for the origin of different types of high-pressure metamorphic belts are considered: 1. (1) slow uplift, or uplifting of the thickened crust after cessation of subduction; 2. (2) more rapid tectonic “pushing out”; 3. (3) the same as in case 2 but with an additional “champagne cork” effect; 4. (4) the most complicated process, with repeated deformation and metamorphism.

Eustatic sea level fluctuations induced by polar wander

THE observation of apparently simultaneous episodes of deposition or non-deposition of marine sediments in different parts of the world has led to the proposal that at least some sea level rises and falls must be global, or eustatic, in character1–3. Here we show that polar wander of a viscoelastic stratified Earth can induce global sea level fluctuations comparable to the short-term component in the eustatic sea-level curves. The sign of these fluctuations, which are very sensitive to the rheological stratification, depends on the geographical location of the observation point; rises and falls in sea level can thus be coeval in different parts of the world. This finding is in distinct contrast to the main assumption underlying the reconstruction of eustatic curves, namely that global sea-level events produce the same depositional sequence everywhere. This apparent contradiction is due to the poor time resolution of the stratigraphie records in the distant past, which is comparable to the timescale of polar motion4–6, and to non-uniform data coverage. We propose that polar wander should be added to the list of geophysical mechanisms (the others are glacial instabilities, plate tectonic mechanisms, sea-floor spreading, and thermal and compaction-induced subsidence) that can control the third-order cycles in sea level.

A Simple Class Exercise on Plate Tectonic Motion

It is possible to construct a useful model of plate divergence with two sheets of paper and to show the separation of two continental plates in a system of spreading ridges and faults. The model can help students gain an appreciation of the nature of constructive and conservative plate margins, and can lead to the discussion of the underlying mechanism of plate tectonics.

Relations entre modifications biologiques et phénomènes géologiquessur les plates-formes carbonatées du domaine périméditerranéen au passage Bédoulien-Gargasien

On the perimediterranean carbonate platforms the Bedoulian-Gargasian transition corresponds to a major biologic crisis, well expressed for Dasyclads (calcareous algae), benthic foraminifera and rudists. Extinctions dominate speciation processes. These biologic modifications are controlled by: taxonomic diversity, size of the organisms, thallus or test mineralogy and paleobiogeographic dispersion. They are correlated with important paleogeographic (bathymetric and sedimentologic), oceanographic and climatic changes linked to plate tectonic mechanisms. The most important changes concern chemical properties of the ocean (CO 2 content, Mg/Ca value, deep water anoxic conditions resulting from hydrologic stratification), biologic production, hydrologic structure and trophic patterns. Consequently biologic modifications of carbonate platform biota is more dependent from oceanic events than from those on platforms themselves.

Seismic detection of the untransformed 'basaltic' oceanic crust subducting into the mantle

Summary. Subcrustal earthquakes occur in a slab-like zone beneath SW Japan in association with the subduction of the Philippine Sea plate. They generate a distinct pair of P and S later phases with anomalously low apparent velocities of about 7 and 4 km s−1, which cannot be explained by a laterally homogeneous velocity structure. These later phases are observed for particular source–receiver geometries: the sources at depths shallower than 50–60 km and the receivers beneath which the subcrustal seismic zone comes in contact with the bottom of the continental crust. We examined the nature of these later phases using two methods. One is to determine two types of apparent velocities, an ‘event-common’ apparent velocity and a ‘station-common’ apparent velocity, which may be different from one another for a laterally inhomogeneous velocity structure. The other is to trace seismic rays numerically for a laterally heterogeneous velocity model. An extensive analysis using these two methods leads to a conclusion that the slab-like seismic zone constitutes a low-velocity channel in the uppermost mantle with P and S velocities comparable to those of the lower continental crust beneath SW Japan. The waves trapped within the low-velocity channel escape from it through its contact with the continental Moho and are observed as a distinct pair of later phases. The head waves guided by the bottom boundary of the low-velocity channel are observed as weak arrivals of initial phases. The thickness (<10 km) and the velocities (VP=7 km s−1, VS=4 km s−1) of the subcrustal seismic zone indicates that it is made up of the ‘untransformed basaltic oceanic crust’ subducted with the underlying lithospheric mantle and that the transformation of basalt to eclogite does not take place in the subducting slab at least down to depths of 50–60 km. We present laterally heterogeneous structural models of the crust and the uppermost mantle along two profiles in SW Japan. The models are characterized by an apparent thickening of the lower continental crust in particular regions due to a direct contact of the subducting oceanic crust to the continental Moho. Such features are qualitatively consistent with the results of explosion seismology and with the Bouguer anomaly pattern in the relevant regions. The oceanic lithosphere with the untransformed ‘light’ oceanic crust at its top might still be buoyant after subduction, a factor which must be taken into account in considering the driving mechanisms of plate tectonics, the slab geometry and the earthquake-related tectonic process.

India–Eurasia collision chronology has implications for crustal shortening and driving mechanism of plates

The motion of the Indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of Eurasia deduced from palaeomagnetic data in Tibet. The 2,600±900 km of continental crust shortening observed is shown to have occurred in three different episodes: subduction of continental crust, intracontinental thrusting and internal deformation, and lateral extrusion. The detailed chronology of the collision and plate reorganizations in the Indian and Pacific oceans supports the hypothesis that slab-pull is a dominant driving mechanism of plate tectonics.

Thermodynamic constraints on phase changes as earthquake source mechanisms in subduction zones

Application of thermodynamic equations for phase transitions with a shock wave output from metastable to stable phases that are believed to occur in subduction zones indicates that while many show a volume reduction, and thus may contribute to slab pull mechanisms of plate tectonics, they do not satisfy criteria for the generation of shock waves. Of the reactions considered only one (antigorite + clinoenstatite → forsterite + talc) satisfies the criteria, and for this case the shock wave pressure is, in fact, less than the pressure calculated for the leading edge of the subduction zone. This would imply that if the transition does occur under shock conditions in the subduction zone, dilatational rather than compressional first motions for the associated earthquake are expected.

What happens in the mantle?

While it is generally agreed that continental drift occurs by the mechanisms of plate tectonics, the forces that drive these motions remain largely undescribed. But change may be on the way.

Correction [to “The driving mechanism of plate tectonics: Relation to age of the lithosphere at trenches”

Geophysical Research LettersVolume 10, Issue 10 p. 987-987 CorrectionsFree Access Correction [to “The driving mechanism of plate tectonics: Relation to age of the lithosphere at trenches”] AGU, AGUSearch for more papers by this author AGU, AGUSearch for more papers by this author First published: October 1983 https://doi.org/10.1029/GL010i010p00987AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume10, Issue10October 1983Pages 987-987 RelatedInformation

The driving mechanism of plate tectonics: Relation to age of the lithosphere at trenches

Sea‐floor age‐depth and age‐trench‐depth relations suggest that the oceanic lithosphere continues to thicken and subside with age beyond 80 Ma. If so, slab‐pull (FS) and ridge‐push (FR) forces can be calculated from the age of the sea floor at trenches (t), and if the shear stress acting on the base of the plate increases with absolute velocity (νa), the approximate force balance equation is math image where D, S and R are constants related to drag, slab pull and ridge push, respectively. The relation between velocity and age is urn:x-wiley:00948276:media:grl2256:grl2256-math-0002 We have tested this model by linear regression using rate and age data for 15 trenches: urn:x-wiley:00948276:media:grl2256:grl2256-math-0003 The strength of this correlation (r² = 0.98) is strong evidence in favor of the validity of this simple model. These results suggest that ridge push is just sufficient to overcome drag at the base of the plate, and does not contribute significantly to the motions of oceanic plates, though the value of R′ is consistent with the motions of plates not attached to subducted slabs.

A review of: “Review of mechanisms of continental drift and plate tectonics”

Abstract Edited by P. A. Davies and S. K. Runcorn. Academic Press, 1980. 362 pp. (£32.00, $72.00) (ISBN 0 122061608)

Geological evidence for the geographical pattern of mantle return flow and the driving mechanism of plate tectonics

Tectonic features at the earth's surface can be used to test models for mantle return flow and to determine the geographic pattern of this flow. A model with shallow return flow and deep continental roots places the strongest constraints on the geographical pattern of return flow and predicts recognizable surface manifestations. Because of the progressive shrinkage of the Pacific (averaging 0.5 km2 /yr over the last 180 m.y.) this model predicts upper mantle outflow through the three gaps in the chain of continents rimming the Pacific (Caribbean, Drake Passage, Australian‐Antarctic gap). In this model, upper mantle return flow streams originating at the western Pacific trenches and at the Java Trench meet south of Australia, filling in behind this rapidly northward‐moving continent and providing an explanation for the negative bathymetric and gravity anomalies of the ‘Australian‐Antarctic Discordance’. The long‐continued tectonic movements toward the east that characterize the Caribbean and the easternmost Scotia Sea may be produced by viscous coupling to the predicted Pacific outflow through the gaps, and the Caribbean floor slopes in the predicted direction. If mantle outflow does pass through the gaps in the Pacific perimeter, it must pass beneath three seismic zones (Central America, Lesser Antilles, Scotia Sea); none of these seismic zones shows foci below 200 km. Mantle material flowing through the Caribbean and Drake Passage gaps would supply the Mid‐Atlantic Ridge, while the Java Trench supplies the Indian Ocean ridges, so that deep‐mantle upwellings need not be centered under spreading ridges and therefore are not required to move laterally to follow ridge migrations. The analysis up to this point suggests that upper mantle return flow is a response to the motion of the continents. The second part of the paper suggests a possible driving mechanism for the plate tectonic process which may explain why the continents move. This hypothetical driving mechanism has four aspects: (1) the lower mantle convects, (2) this convection drives the continents by drag on their deep roots, (3) return flow in the upper mantle provides a volumetric balance for the motion of the continental masses without passing under them, and (4) oceanic plates are effectively decoupled from the asthenosphere and are driven largely by slab pull. This mechanism accounts for the opening of the Atlantic, the ability of spreading ridges to stay on the midlines of oceans, and the penetration of India into Asia following their collision. Continental motions strongly imply lower mantle upwelling and divergence beneath the Atlantic and southeast Indian Oceans, and convergence and subsidence along the Tethyan belt. This picture disagrees with the concept, derived from hot spot studies, of an undeforming, absolute reference framework at depth, but weaknesses in current hot spot theory would make a rejection of the present model on this ground premature. If the present model is generally correct, a fairly simple pattern of lower mantle convection, with as few as four cells, may explain much of the tectonic complexity of the earth.

Proterozoic tectonic evolution and late Svecokarelian plate deformation of the Central Baltic Shield

The continental crust of the Central Baltic Shield evolved by accretion towards the west during the Svecokarelian orogeny 1700–2200 Ma ago. The following features are consistent with a plate tectonic mechanism involving subduction of oceanic crust below an Archean craton in the east: flysch-sediments with serpentinite masses and pillow lavas, linear high-grade metamorphic zones, island-arc type volcanic belts and late tectonic batholiths with porphyry type Cu-Mo deposits. Semi-consolidated new crust was affected by late Svecokarelian deformation (Dn) after 1850 Ma; NNE-trending folds with crenulation cleavage were overprinted on older structures together with associated NW trending ductile transcurrent shear zones that curve the Fn folds into gentle S and Z shapes. The late tectonic batholiths intruded partly at the same time as and partly after the Dn deformation.

The conference on magnitude of deviatoric stresses in the Earth's crust and uppermost mantle

The magnitude of deviatoric stresses in the earth's crust and uppermost mantle, and especially the magnitude of shear stresses that resist plate motions across the major plate boundaries, is a matter of central importance to solid earth geophysics. Since currently available estimates vary by at least a factor of 10, even when the same features or set of observations are involved, such fundamental issues as the driving mechanism(s) of plate tectonics and the energetics of crustal faulting are entirely unresolved. More generally, there is very little in the study of active geologic processes of geophysical interest that does not involve material deformation of some kind or another; in understated terms the analysis of material deformation to reveal causative processes is poorly constrained when the magnitude of the deviatoric stresses involved is so uncertain.Apart from the obvious problem of our inability to measure directly deviatoric stresses in the earth's crust and uppermost mantle through all but a small fraction of its depth—or sample directly this region in any other way at the present time—a confounding difficulty exists in the great range of time scales involved in geophysical processes of interest, from fractions of seconds for the spontaneous failure of small crustal earthquakes to billions of years for the relaxation of certain topographic and gravimetric features of the continental crust. It is probably naive then to expect that the same magnitude of deviatoric stresses should exist for all these geologic processes, given the great range of time scales over which they operate. Moreover, the earth's crust and uppermost mantle are heterogeneous enough, especially in continental regions, that significant spatial variations probably exist as well. Certainly, an important problem is whether the actual variations in space and time of deviatoric stresses in the earth's crust and uppermost mantle can be large enough to accommodate the order of magnitude differences as presently perceived or whether our present perceptions are only imagined, in one sense or another. Even so, we can be assured that at least some of our perceptions must be imagined, when deviatoric stress estimates differing by an order of magnitude or more are offered to explain the same basic processes or the same observations of them.

Ocean crustal dynamics program

The Joint Oceanographic Institutions, Inc. (JOI), commissioned a panel of active researchers in marine geology and geophysics to consider the goals of their field in the 1980's. In 1979 the panel, called the Ocean Crustal Dynamics Planning Committee (OCDPC), convened a series of eight workshops in which experts from academia, industry, and government research units met and debated portions of the research effort in detail. Their reports have been distilled into an Ocean Crustal Dynamics (OCD) program.The committee was concerned with several specific questions: (1) What is the composition and mineralogical make‐up of the ocean crust? How does this composition vary in space and time? (2) What processes are involved in the formation and evolution of oceanic crust? (3) What is the nature and origin of the boundary between the lithosphere and asthenosphere? (4) What is the driving mechanism of plate tectonics? (5) What type of plate interactions occur at active boundaries? (6) What are the nature and evolutionary development of rifted (passive) ocean‐continent boundaries? (7) What is the relationship between plate tectonics and the geochemical and thermal balance of the lithosphere?