A global plate motion model, named NUVEL-1, which describes current plate motions between 12 rigid plates is described, with special attention given to the method, data, and assumptions used Tectonic implications of the patterns that emerged from the results are discussed It is shown that wide plate boundary zones can form not only within the continental lithosphere but also within the oceanic lithosphere; eg, between the Indian and Australian plates and between the North American and South American plates Results of the model also suggest small but significant diffuse deformation of the oceanic lithosphere, which may be confined to small awkwardly shaped salients of major plates

Current plate motions

SUMMARY 
We determine best-fitting Euler vectors, closure-fitting Euler vectors, and a new global model (NUVEL-1) describing the geologically current motion between 12 assumed-rigid plates by inverting plate motion data we have compiled, critically analysed, and tested for self-consistency. We treat Arabia, India and Australia, and North America and South America as distinct plates, but combine Nubia and Somalia into a single African plate because motion between them could not be reliably resolved. The 1122 data from 22 plate boundaries inverted to obtain NUVEL-1 consist of 277 spreading rates, 121 transform fault azimuths, and 724 earthquake slip vectors. We determined all rates over a uniform time interval of 3.0m.y., corresponding to the centre of the anomaly 2A sequence, by comparing synthetic magnetic anomalies with observed profiles. The model fits the data well. Unlike prior global plate motion models, which systematically misfit some spreading rates in the Indian Ocean by 8–12mm yr−1, the systematic misfits by NUVEL-1 nowhere exceed ∼3 mm yr−1. The model differs significantly from prior global plate motion models. For the 30 pairs of plates sharing a common boundary, 29 of 30 P071, and 25 of 30 RM2 Euler vectors lie outside the 99 per cent confidence limits of NUVEL-1. Differences are large in the Indian Ocean where NUVEL-1 plate motion data and plate geometry differ from those used in prior studies and in the Pacific Ocean where NUVEL-1 rates are systematically 5–20 mm yr−1 slower than those of prior models. The strikes of transform faults mapped with GLORIA and Seabeam along the Mid-Atlantic Ridge greatly improve the accuracy of estimates of the direction of plate motion. These data give Euler vectors differing significantly from those of prior studies, show that motion about the Azores triple junction is consistent with plate circuit closure, and better resolve motion between North America and South America. Motion of the Caribbean plate relative to North or South America is about 7 mm yr−1 slower than in prior global models. Trench slip vectors tend to be systematically misfit wherever convergence is oblique, and best-fitting poles determined only from trench slip vectors differ significantly from their corresponding closure-fitting Euler vectors. The direction of slip in trench earthquakes tends to be between the direction of plate motion and the normal to the trench strike. Part of this bias may be due to the neglect of lateral heterogeneities of seismic velocities caused by cold subducting slabs, but the larger part is likely caused by independent motion of fore-arc crust and lithosphere relative to the overriding plate.

We have constructed a magnetic polarity time scale for the Late Cretaceous and Cenozoic based on an analysis of marine magnetic profiles from the world's ocean basins. This is the first time, since Heirtzler et al. (1968) published their time scale, that the relative widths of the magnetic polarity intervals for the entire Late Cretaceous and Cenozoic have been systematically determined from magnetic profiles. A composite geomagnetic polarity sequence was derived based primarily on data from the South Atlantic. Anomaly spacings in the South Atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles. Fine-scale information was derived from magnetic profiles on faster spreading ridges in the Pacific and Indian Oceans and inserted into the South Ariantic sequence. Based on the assumption that spreading rates in the South Atlantic were smoothly varying but not necessarily constant, a time scale was generated by using a spline function to fit a set of nine age calibration points

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A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic

A comprehensive study of the observations of seismology provides widely based strong support for the new global tectonics which is founded on the hypotheses of continental drift, sea-floor spreading, transform faults, and underthrusting of the lithosphere at island arcs. Although further developments will be required to explain certain part of the seismological data, at present within the entire field of seismology there appear to be no serious obstacles to the new tectonics. Seismic phenomena are generally explained as the result of interactions and other processes at or near the edges of a few large mobile plates of lithosphere that spread apart at the ocean ridges where new surficial materials arise, slide past one another along the large strike-slip faults, and converge at the island arcs and arc-like structures where surficial materials descend. Study of world seismicity shows that most earthquakes are confined to narrow continuous belts that bound large stable areas. In the zones of divergence and strike-slip motion, the activity is moderate and shallow and consistent with the transform fault hypothesis; in the zones of convergence, activity is normally at shallow depths and includes intermediate and deep shocks that grossly define the present configuration of the down-going slabs of lithosphere. Seismic data on focal mechanisms give the relative direction of motion of adjoining plates of lithosphere throughout the active belts. The focal mechanisms of about a hundred widely distributed shocks give relative motions that agree remarkably well with Le Pichon's simplified model in which relative motions of six large, rigid blocks of lithosphere covering the entire earth were determined from magnetic and topographic data associated with the zones of divergence. In the zones of convergence the seismic data provide the only geophysical information on such movements.

Two principal types of mechanisms are found for shallow earthquakes in island arcs: The extremely active zone of seismicity under the inner margin of the ocean trench is characterized by a predominance of thrust faulting, which is interpreted as the relative motion of two converging plates of lithosphere; a less active zone in the trench and on the outer wall of the trench is characterized by normal faulting and is thought to be a surficial manifestation of the abrupt bending of the down-going slab of lithosphere. Graben-like structures along the outer walls of trenches may provide a mechanism for including and transporting sediments to depth in quantities that may be very significant petrologically. Large volumes of sediments beneath the inner slopes of many trenches may correspond, at least in part, to sediments scraped from the crust and deformed in the thrusting.

Simple underthrusting typical of the main zone of shallow earthquakes in island arcs does not, in general, persist at great depth. The most striking regularity in the mechanisms of intermediate and deep earthquakes in several arcs is the tendency of the compressional axis to parallel the local dip of the seismic zone. These events appear to reflect stresses in the relatively strong slab of down-going lithosphere, whereas shearing deformations parallel to the motion of the slab are presumably accommodated by flow or creep in the adjoining ductile parts of the mantle. Several different methods yield average rates of underthrusting as high as 5 to 15 cm/yr for some of the more active arcs. These rates suggest that temperatures low enough to permit dehydration of hydrous minerals and hence shear fracture may persist even to depths of 700 km. The thickness of the seismic zone in a part of the Tonga arc where very precise hypocentral locations are available is less than about 20 km for a wide range of depths. Lateral variations in thickness of the lithosphere seem to occur, and in some areas the lithosphere may not include a significant thickness of the uppermost mantle.

The lengths of the deep seismic zones appear to be a measure of the amount of under thrusting during about the last 10 m.y. Hence, these lengths constitute another ‘yardstick’ for investigations of global tectonics. The presence of volcanism, the generation of many tsunamis (seismic sea waves), and the frequency of occurrence of large earthquakes also seem to be related to underthrusting or rates of underthrusting in island arcs. Many island arcs exhibit a secondary maximum in activity which varies considerably in depth among the various arcs. These depths appear, however, to correlate with the rate of underthrusting, and the deep maxima appear to be located near the leading (bottom) part of the down-going slab. In some cases the down-going plates appear to be contorted, possibly because they are encountering a more resistant layer in the mantle. The interaction of plates of lithosphere appears to be more complex when all the plates involved are continents or pieces of continents than when at least one plate is an oceanic plate. The new global tectonics suggests new approaches to a variety of topics in seismology including earthquake prediction, the detection and accurate location of seismic events, and the general problem of earth structure.

Seismology and the new global tectonics

A geometrical model of the surface of the earth is obtained in terms of rigid blocks in relative motion with respect to each other. With this model a simplified but complete and consistent picture of the global pattern of surface motion is given on the basis of data on sea-floor spreading. In particular, the vectors of differential movement in the ‘compressive’ belts are computed. An attempt is made to use this model to obtain a reconstruction of the history of spreading during the Cenozoic era. This history of spreading follows closely one previously advocated to explain the distribution of sediments in the oceans.

Sea-floor spreading and continental drift

Magnetic Anomalies Over Oceanic Ridges

The Western Isles–North Channel (WINCH) traverse (Fig. 1), an extension of the successful MOIST profile1, was recorded in 1982 at sea along the west coast of Britain for BIRPS (British Institutions Reflection Profiling Syndicate) by the Geophysical Company of Norway (GECO). The purpose of the WINCH traverse was to study crustal structure of the Caledonian foreland, to cross the Caledonian orogen, and to establish the three-dimensional geometry of mantle reflectors originally seen on MOIST. We describe here the data, first emphasizing British Caledonian structures, and then discussing features of the deep crust of wider significance. The data are of very good quality and contain clear Moho and upper mantle reflections. The lower crust is surprisingly reflective, deeply penetrative thrusts are observed and there is firm evidence that several of the Mesozoic basins round the shores of the United Kingdom were formed by reactivation of older features.

BIRPS deep seismic reflection studies of the British Caledonides

The South West Approaches Traverse comprises 1600 km of profiling to 15 seconds two-way time. The profiles were planned to investigate Variscan structures in the crust and upper mantle. In Ireland, SW England and NW France, Variscan deformation and granite intrusion extend from Middle Devonian to Early Permian time, about 370 to 270 Ma ago. We have imaged a normal fault dipping gently south under the Celtic Sea that corresponds at surface, in position and trend, with the Variscan 'front' and can be traced down to at least 20 km depth. As under Caledonian Britain to the north, the lower crust is everywhere strongly reflective from about 6 seconds two-way time down to the Moho at about 10 seconds. The Haig Fras granite and Cornubian batholith, crossed west of the Isles of Scilly, appear only as unusually shallow portions of the lower crustal reflectors.

Deep seismic reflection profiling between England, France and Ireland

The Moine and Outer Isles Seismic Traverse (MOIST) is a deep crustal reflection profile shot at sea off the north coast of Scotland in 1981. Spectacular reflections are observed from the Moho and from thrust zones within the Caledonian fold belt and foreland. Deep reflection profiling of the continental crust of the United States under the direction of the COCORP group, has amply demonstrated the value of this technique when applied to suitable geological problems (see, for example, refs 1,2). A similar group, the British Institutes Reflection Profiling Syndicate (BIRPS), has now been set up in the UK to organize such projects. Funds are provided by the Natural Environment Research Council (NERC) through the core group based at Cambridge University. However, a preliminary experiment, of which MOIST is the result, was organized through the Institute of Geological Sciences (IGS).

Deep structure of the Scottish Caledonides revealed by the MOIST reflection profile

Summary
Surveys of the crest of the Mid-Atlantic Ridge and similar portions of the other mid-ocean ridges characteristically reveal a pattern of magnetic anomalies which have an elongation of about 4:1 parallel with the median line of the ridge. The linear pattern can be roughly simulated by a model in which steep-sided blocks within the oceanic crust between the sea floor and the depth of the Curie point isotherm have alternately normal and reversed directions of remanent magnetization. Vine & Matthews (1963) suggested that this structure might be due to the continued injection of basaltic feeder dykes along the median line of the ridge, each dyke shouldering aside its predecessors and being magnetized in the direction of the Earth's magnetic field. As the direction of the Earth's field reverses periodically this mechanism can account in a crude way for the formation of the model. A survey of the crest of the Mid-Atlantic Ridge at 45° N (Loncarevic, Mason & Matthews 1966) showed a less regular pattern of blocks suggesting that the mechanism of dyke injection is less regular than that utilized in the original theory. It seems plausible that dyke injections do not always occur precisely along the median line but are scattered about it. An analogue model experiment involving marbles, and a computer program written for titan are used to investigate the consequence of such scatter. First results indicate that the great majority of dyke injections must occur within the width of the median valley; the standard deviation of the distribution of dykes (assumed normal) must be less than about 5 km (half the width of the median valley).

D. H. Matthews

Papers

Magnetic Anomalies Over Oceanic Ridges

BIRPS deep seismic reflection studies of the British Caledonides

Deep seismic reflection profiling between England, France and Ireland

Deep structure of the Scottish Caledonides revealed by the MOIST reflection profile

Formation of Magnetic Anomaly Pattern of Mid-Atlantic Ridge