Concept Of Plate Tectonics Research Articles

The evolution of propagating rifts

The magnetic survey carried out by Mason and Raff1,2 off the north-west coast of the US has been central to the development of many of the concepts of seafloor spreading and plate tectonics. Recently Hey and Wilson3 have again used this survey to illustrate Hey's suggestion4 that ridge segments sometimes increase their length by propagation. This process leads to a characteristic pattern of magnetic anomalies offset by discontinuities which Hey called pseudofaults because, unlike fracture zones, they (Fig. 1) were never plate boundaries. We demonstrate here how the geometry of the pseudofaults, magnetic anomalies and the failed rift can be worked out using velocity triangles and the concept of stability. This method was originally developed to understand the evolution of triple junctions between three plates5.

Seismicity and structural investigations of the Romanian Vrancea Region: Evidence for azimuthal variations of p-wave velocity and poisson's ratio

A study of the shallow and intermediate depth seismicity of the Romanian Vrancea region in the period 1964–1981 has been performed. The seismic events have been relocated by a standard location procedure using a regional velocity model. From the temporal and spatial distribution of the seismic activity, aspects of the seismicity before the large March 4, 1977 earthquake are treated, in particular the seismic gap in space and time prior to this event, found by Mârze (1979), which is critically discussed and revised. The concept of the precursor time/magnitude relationships of different authors is applied and its validity to the Vrancea region assessed. The hypocentral distribution shows that the intermediate depth seismic activity is confined to a small volume with dimensions of only some tens of kilometers. The results are interpreted in terms of the tectonics of the region. From an analysis of the travel-time residuals at different local stations, evidence for lateral velocity heterogeneities beneath the region is obtained e.g. a high velocity zone southeastwards of the Carpathian chain. Finally mean ν p ν s ratios, (i.e. Poisson's ratios), for various stations are calculated from P- and S-wave travel times. They show azimuthal variations of up to 6% for stations within the area where the intermediate seismic activity occurs in comparison with the station Focsani, situated eastwards in the Carpathian foredeeps. All these results are compatible with the plate tectonic concept for the Vrancea region, that is the subduction of an oceanic lithospheric slab under the Carpathian mountain arc, giving rise to such a highly active seismic zone.

Problems of the „Pannonian Median Massif“ and the plate tectonic concept. Contributions based on the distribution of Late Paleozoic — Early Mesozoic isopic zones

After a short historical review of conceptions about the “Pannonian Median Massif” or “Tisia”, the distribution of Middle-Upper Carboniferous and Lower Triassic-Liassic facies is briefly examined on the two sides of the Zagreb-Zemplin line dividing into two main segments the basement of the Carpathian basin. It shows on the one hand, that the Igal-Bukk zone (“Igal-Bukk eugeosyncline” ofWein, 1969), thought to establish a connection between the NW-Dinarides and the Dinaric-type Bukkium, is only a tectonic zone and could never have been a paleogeographical unit. On the other hand, the northern, marginal part of “Tisia” (“Mecsek-Bihor crystalline belt” and its sedimentary cover;Dank B it was rather part of the northern (northeastern) marginal complex of Tethys. Therefore allochthonous models suggesting largerscale horizontal movements give a much more plausible explanation. The style of Late Hercynian development and the distribution of Lower Triassic-Liassic facies zones suggest, that “Tisia” was part of the northern (northeastern) shelf of the Tethys until the end of the Liassic (in accordance with the opinion ofBleahu), then split off and was wedged in by horizontal movements between the NW-Dinarides and the Dinaric-type Bukkium by the end of the Jurassic — beginning of the Cretaceous.

The origin and peculiarities of the nigerian benue trough: Another look from recent gravity data obtained from the middle benue

The origin of the Benue trough has been a controversial subject. Previous workers in the area have evolved several theories ranging from a rift, with or without plate tectonic concepts (King, 1950; Wright, 1968, 1970; Burke et al., 1970; Grant, 1971; Olade, 1975) to a geosyncline (Lees, 1952) and a combination of both (Cratchley and Jones, 1965; Offodile, 1976) to explain the origin of the trough. One of the aims of the gravity survey, carried out in the middle Benue and discussed in this paper, was to attempt to resolve some of the problems associated with the origin of the trough as proposed by previous workers and in so doing to explain the geological peculiarities associated with the trough. Interpretations of the gravity data along two north—south profiles in the survey area indicated the existence of a rift, of normal width (about 40 km) buried under the Cretaceous cover and located between the central axis (ridge) of the trough and the northern boundary of the trough. The gravity minimum which occurs south of the central axis of the trough, was also ascribed to another rift. Thus the Benue trough is considered to consist of two parallel rifts separated by the axial ridge which partly explains the abnormal width of the trough. The interpreted models also indicate a low-angle thrust fault, with a small displacement, which may be related to the folding of the sediments in the trough and also to the asymmetry of the trough. Furthermore, the gravity data suggest that oceanic crust might not have reached the ground surface in the Benue trough, as in the Red Sea, before rifting ceased.

Wegener's theory: The role of geophysics in its eclipse and triumph

Wegener's hypothesis of continental drift was based on a synthesis of world geological observations. Although there was much debate about the correctness of the geological interpretations, the chief objection to the theory came from the absence of a viable geophysical mechanism and in particular from the argument in favour of the mantle having finite strength. By about 1950 the hypothesis was widely rejected by geoscientists but soon after the development of palaeomagnetism provided what was needed to test it: a quantitative method of determining the position of the continents in the geological past. Comparisons of palaeomagnetic polar wandering curves from the different continents provided strong support for the essential ideas which Wegener had put forward. By the early 1960s the hypothesis of convection in the Earth's mantle made possible by solid state creep removed the basic geophysical objections to Wegener's theory. Sea floor spreading neatly complemented continental drift, and integrated into the concept of plate tectonics, has provided a framework in which much other geological and geophysical data fit.

Geomagnetic variation sounding of the asthenosphere beneath the Juan de Fuca Ridge

Two ocean bottom magnetometers have been deployed at sites on either side of the Juan de Fuca Ridge, 20 and 40 km from the crest, near latitude 47°N. Twelve days of data were recorded simultaneously at the two sites, and these data have been used to determine the electrical conductivity structure beneath the region. The vertical field variations at both stations are of very low amplitude in the period range 0.25 to 8 hours, with no detectable phase reversal across the ridge. Electric fields in the ocean layer have been estimated from the attenuation of sea floor horizontal fields with respect to Victoria, and the resulting impedance spectra inverted to find resistivity as a function of depth using the technique of Oldenburg (1979). Resistivity models which fit the data within the specified bounds are characterized by a comparatively resistive crust and upper lithosphere overlying a low resistivity zone between depths of 55 and 75 km which in turn appears to be underlain by a higher resistivity section. The average resistivities are used to estimate temperature and melt fraction on the basis of the effective medium theory of Shankland and Waff (1976) for a basalt melt fraction within an olivine matrix. For the low resistivity zone, and depending on the water content of the melt (0 to 10%), the estimated temperature range is 1400°C to 1250°C and the corresponding melt fraction 0.05 to 0.09. The data suggest that the melt fraction decreases below 80 km. The conductivity profile from magnetotelluric data on 72 Ma ocean floor in the North Central Pacific (Filloux, 1977) has similar characteristics to the 1 Ma old conductivity structure near the Juan de Fuca Ridge, but with the low resistivity layer at depths of 140 to 220 km. In both cases, the resistivity minimum should be equated with the region of maximum partial melt within the asthenosphere, rather than with the base of the lithosphere. The increase in depth to this zone with increasing lithosphere age is in general agreement with plate tectonic concepts.

Washita Valley Fault System--A New Look at an Old Fault: ABSTRACT

With the application of plate tectonic concepts to southern Oklahoma, the structural style has more recently been characterized as a wrench fault system. In particular, the Washita Valley fault (WVF) is generally considered by many geologists to be a major left-lateral strike-slip fault with an offset of approximately 40 mi (64 km). The map most commonly used to demonstrate this magnitude of lateral offset is the basal Oil Creek sand distribution map. The zero edge of the basal Oil Creek sand provides the necessary piercing point to judge lateral offset along the WVF. However, the published basal Oil Creek map depicts the present-day distribution of the sand which is the result of cumulative movements since the deposition of the basal Oil Creek sand. Individual orogenic episodes must be sorted to determine what contribution possible strike-slip movements have made toward the present-day basal Oil Creek sand distribution. To unravel these various orogenic episodes a palinspastic restoration must be made. In such a sequential restoration, the last movement should be restored first or the known movements restored first to determine the unknown movements. In this presentation, the observable folds along the WVF have been unfolded and the known reverse faults have been restored to a pre-fault position. When this has been accomplished the partly restored basal Oil Creek sand map may be used to determine the amount of lateral offset on the WVF. The resulting restoration indicates that the commonly quoted figure of 40 mi (64 km) of lateral offset is too large. Surface and subsurface data demonstrate that the crustal shortening represented by folds and reverse faults alone can account for the present-day basal Oil Creek sand distribution and thus very little strike-slip movement is needed along the Washita Valley fault system. End_of_Article - Last_Page 1496------------

Plate Tectonics, Organic Matter, and Basin Evaluation for Petroleum Potential: ABSTRACT

Application of plate-tectonic concepts to sedimentary basin development has been important in petroleum geology. Physical aspects have been stressed, but a complete classification must include the amount, type, and distribution of organic matter because this is the material that generates petroleum. Organic matter can be classified into two broad categories: (1) land-derived (terrestrial) organic matter that may include major amounts of lignin and surface coatings which give gas or waxy crudes respectively, and (2) aquatic organic matter which commonly is dominantly of algal origin and generates normal crudes. The relative amounts of these two types of organic matter, and hence the relative amounts of oil and gas, depend on the depositional environment and this can be r lated to plate-tectonic setting. Rifts that form in the early stages of continental breakup receive high percentages of terrestrially derived organic matter initially, but aquatic organic matter becomes quantitatively more significant as the rift widens and marine conditions develop. Thus pull-apart continental margins develop a vertical profile from terrestrial organic matter deep to aquatic organic matter shallow. The deeper continentally derived sedimentary sections of these margins produce waxy crudes, gas, and some condensate reflecting the character of the organic matter. Pull-apart margins and rifts with waxy crudes were within 20° of the equator at the time of rifting. They include the Sirte, Cambay, Reconcavo, Gabon, and Cuanza basins. When rifting occurred farther from the equator, gas-dominated provin es were developed, such as northwest Australia, offshore Newfoundland, Baltimore Canyon, and the central Viking graben. Rivers transport continental sediments and organic matter to continental margins, and the association of transported terrestrial organic matter with clastic sediments makes deltas one of the most gas-prone depositional environments. Organic materials are not distributed uniformly in deltas because terrestrial organic matter has its highest concentration nearshore whereas aquatic material is also produced in large amounts offshore. This separation and distribution lead to gas fields near paleoshorelines and oil farther out. As the delta progrades, terrestrial organic matter is deposited over the previously deposited aquatic organic matter. This profile of organic matter type is exactly the opposite of that developed on passive continental margins and leads to the opposite trend for the distribution of oil and gas with depth. Although organic-matter type exercises initial control over the nature of the hydrocarbons generated, the composition may be changed later by maturation and migration. Maturation is the thermally induced trend from oil to gas and the depth for this conversion will depend on geothermal gradient. Migration distance is also controlled by plate-tectonic setting with distances in excess of 100 km being well documented for structurally simple interior basins. In contrast, migration distances are much shorter when migration pathways are interrupted, for example, by block faulting in rifts and growth faults in deltas. End_of_Article - Last_Page 1356------------

Geological events during the early formation of a passive margin

The concept of plate tectonics implies that the normal sea floor spreading stage is preceded by a sequence of events associated with the break-up of continental crust. Thus, evidence of the early development of “non-failed” rifts is to be found at passive continental margins. Of special interest is the question of the extent of the continental crust and the structural and compositional changes associated with the change in crustal type. In addressing these topics, we have focused attention on the Norwegian margin between the Jan Mayen and Senja fracture zones (66°–70°N) in an attempt to understand its history of rifting and early sea floor spreading. p ]The southern part of this rifted margin is characterized by a wide shelf and the marginal Vøring Plateau interrupts a gentle slope at a level of about 1500 m. However, the margin becomes progressively narrower towards the north and a typical narrow shelf and steep slope emerge off the Lofo—tenVesterålen Islands (Fig. 1). In a reconstructed pre-opening configuration (Talwani and Eldholm, 1977) the narrowest part of the juxtaposed EastGreenland margin is found in the south and a wide shelf and slope corresponds to the Lofoten-Vesterålen margin. The most prominent structural element is a buried basement high underneath the Vøring Plateau. The high is bounded landward by the Vøring Plateau Escarpment, a major structural boundary which defines typical changes in the geophysical parameters. These are: (1) a sudden increase of depth to acoustic basement; (2) changes in the velocity-depth function; (3) a gravity gradient; and (4) a magnetic edge anomaly separating sea-floor spreading type anomalies from a quiet zone on the landward side (Talwani and Eldholm, 1972). These observations were interpreted in terms of a sharp ocea—ncontinent crustal transition along the escarpment with sea-floor spreading commencing between anomaly 24 and 25 time (56–58 m.y. B.P.). Alternatively, the concept of ancient oceanic crust landward of this escarpment and the possible existence of continental crust under the outer basement high have been argued and we refer to Eldholm et al. (1979) for a detailed discussion.

Geophysical review of the continental margins of eastern and western Canada

The evolution and geophysical features of the continental margins of eastern and western Canada are reviewed in light of recent plate-tectonic concepts. The two margins are very different in age, structure, and origin. The eastern margins were formed either by rifting or by transform motion during the latest separation of the continents around the Atlantic that occurred from Jurassic to Tertiary times. Studies of these margins centre around a reconstruction of plate motions, the inception of which occurred over 70 Ma ago, and on subsequent processes such as subsidence and sedimentation. The subsidence of the margin is explicable in terms of cooling of the lithosphere and sediment loading. Deep crustal features are inferred from seismic, gravity, and magnetic data. The recognition of the ocean–continent boundary at these margins involves consideration of edge effects, magnetic quiet zones and rifting mechanisms. The western Canadian margins are present active plate boundaries. Recent geophysical studies of these margins centre around the detailed definition of the present plate boundaries and relative plate motions, and those of the recent past (about the past 10 Ma), and involve spreading ridges, transform faults, and subduction zones. The plate convergence predicted by offshore geophysical data has a pronounced effect on the continental crust and upper mantle extending several hundred kilometres inland from the coast. In southwestern Canada patterns characteristic of subduction zones are seen in seismic structure, the gravitational and magnetic fields, heat flow, and deep electrical structure.

Possible evidence of a Precambrian continental collision in the Rio Pardo basin of eastern Brazil

Research Article| September 01, 1979 Possible evidence of a Precambrian continental collision in the Rio Pardo basin of eastern Brazil Augusto J. Pedreira Augusto J. Pedreira 1Companhia de Pesquisa de Recursos Minerais-CPRM Rua Barros Falcão, 21-40000 Salvador, Brazil Search for other works by this author on: GSW Google Scholar Author and Article Information Augusto J. Pedreira 1Companhia de Pesquisa de Recursos Minerais-CPRM Rua Barros Falcão, 21-40000 Salvador, Brazil Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1979) 7 (9): 445–448. https://doi.org/10.1130/0091-7613(1979)7<445:PEOAPC>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Augusto J. Pedreira; Possible evidence of a Precambrian continental collision in the Rio Pardo basin of eastern Brazil. Geology 1979;; 7 (9): 445–448. doi: https://doi.org/10.1130/0091-7613(1979)7<445:PEOAPC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Some features in the basement rocks near the eastern coast of Brazil probably can be explained by plate-tectonics concepts. The area studied contains east-west-oriented diabase dikes, intrusive alkalic rocks, and miogeosynclinal sediments in a fault-bounded sedimentary basin now metamorphosed to greenschist facies, all affected by a northwest-southeast-trending shear zone. These features evolved chronologically in the sequence indicated, about 1,160 to 600 m.y. ago. The model devised by Sengör indicates that these features are possibly the product of a Precambrian collision between two continents with irregular margins. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

An analysis of the palaeomagnetic field

The paper is devoted to a critical evaluation of the hypothesis of the prevalent character of the palaeomagnetic field in the course of the whole Phanerozoic. The study ties in with the synthetic processing of all so far published palaeomagnetic data for Eurasia and Africa respecting some of the models ensuing from the concept of plate tectonics. Initial material for testing the dipolar hypothesis was obtained by statistical processing of the palaeomagnetic pole positions and by analyzing the values of the so-called palaeorotation, the palaeoinclination difference and of palaeodivergence. It was shown that, by defining tectonically stable blocks (plates, palaeoplates) within the scope of the Eurasian continent and within the scope of the African continent as a whole, data were obtained bringing evidence of the predominating dipolar character of the mean geomagnetic field for the period from the Quaternary to the Cambrian, with the exception of the Silurian which has not been studied sufficiently. The deviations of the palaeomagnetic directions from the direction of the dipolar field within the scope of the stable blocks have the character of a Gaussian distribution and the standard deviations only amount to a few degrees.

Book Vignettes

Book reviewed in this article:Schaff, William, L., Mathematics and Science: An Adventure in Postage Stamps.Moore, Patrick, Comets: An Illustrated Introduction.Berston, Hyman Maxwell and Fisher, Paul, Collegiate Business Mathematics.Fodor, R.V., Earth in Motion: Concept of Plate Tectonics.

AGU Midwest Meeting Report

Approximately 150 Midwestern geophysicists attended the third annual AGU Midwest Meeting, which was held at Purdue University, September 26–28, 1977. The majority of the 75 papers presented were related to topical special sessions arranged by seven sections of the Union to be of particular, current importance to Midwestern geophysics. The interest shown in the meeting indicates not only the vigor of current geophysical research in the Midwest but also the desirability of continuing these meetings.The majority of the papers presented were in the area of solid‐earth geophysics, and these primarily were concerned with the paleogeodynamics and contemporary geodynamics of the Midwest. They reflect the growing interest in deciphering the complex evolutionary history of the continental interior by utilizing a wide range of methodologies and the concepts of plate tectonics which have generally emphasized oceanic and continental margin features. The papers in the session on Geology and Geophysics of the Precambrian Basement Rocks of the Midcontinent Region demonstrated that geophysical data can be used to clarify and amplify our understanding of the basement geology in poorly exposed regions. By extension of the data base to include buried portions of the basement, it soon may be possible to determine the major structural, lithologic, and chronologic provinces of the basement in the Midwest fairly accurately. The session dealing with Geodynamics of Plate Interiors demonstrated that the structures present in the midcontinent reflect a long and complex midplate tectonism which is not completely understood. The 15 papers of the sessions on the New Madrid Seismic Zone, the 38th Parallel Lineament, and the Seismicity of the Midwest illustrate the rapidly broadening interest in and knowledge of the tectonic framework of the seismically active New Madrid area and environs. It was emphasized that this area is an excellent example of reactivation of ancient structures during the Phanerozoic and locally into the present. Interpretation of surface, air, and satellite geophysical and geological data suggests the presence of an ancient easterly striking rift south of the 38th Parallel Lineament and a northeast‐trending graben lying along the axis of the New Madrid Seismic Zone. Microearthquakes together with historical seismicity are being used to delineate active faults of this area and estimate earthquake recurrence rates and the local stress state.

Tectonic Terminology: Some Proposed Changes

Plate tectonics concepts require a definition of fault, a new term to compliment epeirogeny, and a clarification of transform fault characteristics.

Precambrian mobile belts of southern and eastern africa — ancient sutures or sites of ensialic mobility? a case for crustal evolution towards plate tectonics

There is still considerable controversy among earth scientists whether plate-tectonic processes as envisaged for the Phanerozoic have been operative since the generation of first sialic crust in the Archaean or whether global tectonics underwent an evolutionary process during the Precambrian history of the earth thereby gradually leading, through various stages, to the present plate-tectonic phase. Southern and eastern Africa represent excellent examples to study the development of a variety of mobile belts ranging in age from more than 3.0 b.y. to the end of the Precambrian and it is attempted here to test whether field geology, tectonics, geochronology and palaeomagnetic data in such belts as the Limpopo, Ubendian, Irumide and Pan African are compatible with plate-tectonic processes, notably the continental-collision model. Most of the above belts have in common that orogenic reworking of pre-existing sialic crust has taken place on a grand scale and that mafic volcanic rocks of ophiolite affinity are completely lacking, even in those belts which are not eroded to their roots. It is seen as significant that some linear belts cut older ones without completely destroying the ancient geologic record and that some belts end within the continent and were apparently not associated with a plate margin. It is also demonstrated that considerable continental thickening, following collision in a plate-tectonic model, has obviously not taken place behind some belts and that potash granites are often not generated in the lower crust through fractional crystallization of calc-alkaline magma but through melting and mobilization of the geosynclinical fill and its underlying sialic basement. Features in accordance with modern plate-tectonic concepts such as well developed suites of mafic volcanics, including ophiolites, and clearly defineable plate margins are found for the first time in Africa during the development of some Pan-African belts. It is proposed that ensialic orogeny is caused by the same subcrustal forces which are operative during plate separation, ocean opening and continental collision. If these forces are not strong enough to cause complete continental rupture, linear zones of weakness and intraplate graben systems, aulacogens or geosynclinal basins develop in the crust. The orogenic cycle is set in motion through compressive stress in the lithosphere, gravitational instability, and increased heat flow through the fractured crust in the zone of weakness. All transitional stages from non-orogenic intraplate graben systems through ensialic mobile belts to collision belts following plate separation and ocean closure should therefore exist. It is further proposed that horizontal mobility of large crustal plates increased through geologic time. An early phase of predominantly ensialic orogenic evolution up to the mid-Proterozoic was followed by a phase where plate separation and modern plate-tectonic processes became increasingly common (late Precambrian/early Paleozoic), thus leading over to the Phanerozoic phase of widespread ocean opening and plate collision.

Plate tectonics, paleomagnetism and the tectonic history of the N.E. Pacific

The geology of Alaska has long been recognized as very complex. Recent paleomagnetic data, combined with paleogeographic reconstructions based on the concepts of plate tectonics, indicate that at least the southern parts of Alaska may be made up of a number of slivers of continental material rafted up along the western edge of North America.

Reversals of the earth's magnetic field

One of the most intriguing problems in geophysics to-day is why the Earth's magnetic field reverses. This review summarizes the progress in our knowledge of the subject since Bullard's [1] Bakerian Lecture to the Royal Society in 1967. Reversals have played a major role in the changed outlook in geological thinking through the development of the concept of plate tectonics. Polarity transitions also provide the geologist with horizons which may be applied to stratigraphic correlations and dating problems. The mean frequency of reversals has also shown marked changes in the past and these may well be related to major tectonic changes. In spite of all these increased benefits to the geologist, the physics of the reversal mechanism is still not well understood. The detailed behaviour of the magnetic field during a polarity transition is described as well as the more recently observed phenomenon of excursions of the field or aborted reversals. Physical and mechanical models of the geodynamo are examined in some detail in so far as they may affect reversals. The role of convection in the Earth's core is also discussed.

Evolution of Nigeria's Benue Trough (Aulacogen): a tectonic model

SummaryThe tectonic evolution of Nigeria's Benue Trough has recently been the focus of investigations, especially in relation to continental drift and opening of the South Atlantic Ocean during Cretaceous times. Most of the tectonic models presented for the Benue depression have placed undue emphasis on certain plate tectonic concepts which are inconsistent with the salient geologic features of the trough. A possible model, involving the rise and cessation of mantle upwelling beneath a Cretaceous hot spot, and attendant patterns of doming, rifting, sedimentation, magmatism and tectonism, is proposed for the Benue aulacogen.

On crustal plates

Abstract During the decade just past, developments in Seismology have played an active and central role in the development of the concept of Plate Tectonics. Observational Seismology has provided support for and verification of a number of the dynamic aspects of the hypotheses of continental drift, sea-floor spreading, transform faults and the underthrusting of the lithosphere at island arcs and some continental margins. Those types of seismological evidence which bear on the question of the thickness of the lithosphere are either indirect or circumstantial, or both. As early as 1926, Gutenberg postulated the existence of a layer at a depth of 80 to 150 or 200 km, probably worldwide in extent, in which the velocities of seismic waves are slightly lower than in the immediately overlying layers. Some plate tectonics workers equate this low-velocity layer to the relatively-weak asthenosphere required by Plate Tectonics to underlie the stronger, more brittle lithosphere. In this review, several lines of evidence are marshalled in support of a plate model of the continental crust in seismically active regions in which a layer of decoupling of an upper, lithospheric layer from the weaker substrate may lie in the crust itself at a depth of perhaps 10 to 15 km.