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Journal ArticleDOI

Rises, trenches, great faults, and crustal blocks

15 Mar 1968-Journal of Geophysical Research (American Geophysical Union (AGU))-Vol. 73, Iss: 6, pp 1959-1982
TL;DR: In this article, the transform fault concept is extended to a spherical surface, where the motion of one block relative to another block may then be described by a rotation of a rigid crustal blocks relative to the other block.
Abstract: The transform fault concept is extended to a spherical surface. The earth's surface is considered to be made of a number of rigid crustal blocks. It is assumed that each block is bounded by rises (where new surface is formed), trenches or young fold mountains (where surface is being destroyed), and great faults, and that there is no stretching, folding, or distortion of any kind within a given block. On a spherical surface, the motion of one block (over the mantle) relative to another block may then be described by a rotation of one block relative to the other block. This rotation requires three parameters, two to locate the pole of relative rotation and one to specify the magnitude of the angular velocity. If two adjacent blocks have as common boundaries a number of great faults, all of these faults must lie on ‘circles of latitude’ about the pole of relative rotation. The velocity of one block relative to the other must vary along their common boundary; this velocity would have a maximum at the ‘equator’ and would vanish at a pole of relative rotation. The motion of Africa relative to South America is a case for which enough data are available to critically test this hypothesis. The many offsets on the mid-Atlantic ridge appear to be compatible with a pole of relative rotation at 62°N (±5°), 36°W (±2°). The velocity pattern predicted by this choice of pole roughly agrees with the spreading velocities determined from magnetic anomalies. The motion of the Pacific block relative to North America is also examined. The strike of faults from the Gulf of California to Alaska and the angles inferred from earthquake mechanism solutions both imply a pole of relative rotation at 53°N (±3°), 53°W (±5°). The spreading of the Pacific-Antarctic ridge shows the best agreement with this hypothesis. The Antarctic block is found to be moving relative to the Pacific block about a pole at 71°S (±2°), 118°E (±5°) with a maximum spreading rate of 5.7 (±0.2) cm/yr. An estimate of the motion of the Antarctic block relative to Africa is made by assuming closure of the Africa-America-Pacific-Antarctica-Africa circuit and summing the three angular velocity vectors for the cases above.

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Citations
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Journal ArticleDOI
TL;DR: In this paper, a global set of present plate boundaries on the Earth is presented in digital form, taking into account relative plate velocities from magnetic anomalies, moment tensor solutions, and geodesy.
Abstract: [1] A global set of present plate boundaries on the Earth is presented in digital form. Most come from sources in the literature. A few boundaries are newly interpreted from topography, volcanism, and/or seismicity, taking into account relative plate velocities from magnetic anomalies, moment tensor solutions, and/or geodesy. In addition to the 14 large plates whose motion was described by the NUVEL-1A poles (Africa, Antarctica, Arabia, Australia, Caribbean, Cocos, Eurasia, India, Juan de Fuca, Nazca, North America, Pacific, Philippine Sea, South America), model PB2002 includes 38 small plates (Okhotsk, Amur, Yangtze, Okinawa, Sunda, Burma, Molucca Sea, Banda Sea, Timor, Birds Head, Maoke, Caroline, Mariana, North Bismarck, Manus, South Bismarck, Solomon Sea, Woodlark, New Hebrides, Conway Reef, Balmoral Reef, Futuna, Niuafo'ou, Tonga, Kermadec, Rivera, Galapagos, Easter, Juan Fernandez, Panama, North Andes, Altiplano, Shetland, Scotia, Sandwich, Aegean Sea, Anatolia, Somalia), for a total of 52 plates. No attempt is made to divide the Alps-Persia-Tibet mountain belt, the Philippine Islands, the Peruvian Andes, the Sierras Pampeanas, or the California-Nevada zone of dextral transtension into plates; instead, they are designated as “orogens” in which this plate model is not expected to be accurate. The cumulative-number/area distribution for this model follows a power law for plates with areas between 0.002 and 1 steradian. Departure from this scaling at the small-plate end suggests that future work is very likely to define more very small plates within the orogens. The model is presented in four digital files: a set of plate boundary segments; a set of plate outlines; a set of outlines of the orogens; and a table of characteristics of each digitization step along plate boundaries, including estimated relative velocity vector and classification into one of 7 types (continental convergence zone, continental transform fault, continental rift, oceanic spreading ridge, oceanic transform fault, oceanic convergent boundary, subduction zone). Total length, mean velocity, and total rate of area production/destruction are computed for each class; the global rate of area production and destruction is 0.108 m2/s, which is higher than in previous models because of the incorporation of back-arc spreading.

1,853 citations


Cites background from "Rises, trenches, great faults, and ..."

  • ...[2] An idealized plate of lithosphere is a region which rotates (with respect to some other specified plate) without internal deformation about an imaginary axis through the center of the planet [Morgan, 1968]....

    [...]

Journal ArticleDOI
TL;DR: In this article, a preliminary model for the Cenozoic kinematic evolution of the western Mediterranean oceanic basins and their peripheral orogens is presented, which integrates the motion of Africa relative to Europe based upon a new study of Atlantic fracture zones using SEASAT data and the Lamont-Doherty magnetic anomaly database.
Abstract: Summary The kinematic understanding of the relationship between relative plate motion and the structure of orogenic belts depends upon a knowledge of relative plate motion across the plate boundary system, the relative motion of small blocks and flakes within the system, an evaluation of orogenic body forces, and an understanding of the thermomechanical evolution of the upper part of the orogenic lithosphere in determining strength and detachment levels. We have built a preliminary model for the Cenozoic kinematic evolution of the western Mediterranean oceanic basins and their peripheral orogens that integrates (1) the motion of Africa relative to Europe based upon a new study of Atlantic fracture zones using SEASAT data and the Lamont-Doherty magnetic anomaly database, (2) a new interpretation of the rotation of Corsica/Sardinia and the opening of the Balearic and Tyrrhenian oceanic basins, (3) sedimentary facies sequences in the Apennines, Calabria, and Sicily, and (4) Apennine/Calabrian structure and structural sequence.

1,545 citations

Journal ArticleDOI
TL;DR: A detailed assembly of the outlines of the continents around the North and central Atlantic, before the initial dispersion of Gondwanaland in Early Jurassic times, is presented in this paper.
Abstract: It is contended that the Late Triassic to present-day gross evolution of the Alpine system in the Mediterranean region has been the result of activity along an evolving network of accreting, transform, and subducting plate boundaries between the large stable cratons of Europe and Africa. A refined assembly of the outlines of the continents around the North and central Atlantic, before the initial dispersion of Gondwanaland in Early Jurassic times, is presented. By considering geologic facies, structural fabric, and paleomagnetic criteria, the smaller continental fragments now found within the Alpine system are restored to their proposed initial positions relative to each other in the reconstruction offered. The motion of the major plate of Africa relative to Europe, commencing with the initial continental fragmentation, is documented by analysis of the sea-floor spreading history of the Atlantic Ocean, with the assumption that plate accretion there has occurred between torsionally rigid lithospheric plates. By the computerized fitting of well-defined and well-dated key pairs of symmetric magnetic anomaly lineations back together by a series of finite rotations, the relative position of North America to both Europe and Africa has been determined for the following times: 180 m.y. (Toarcian Stage, Early Jurassic); 148 m.y. (Kimmeridgian Stage, Late Jurassic); 80 m.y. (Santonian Stage, Late Cretaceous); 63 m.y. (Danian Stage, Paleocene); 53 m.y. (Ypresian Stage, Eocene); and 9 m.y. (Tortonian Stage, Miocene). From these positions, a series of rotation poles presumed to describe the stepwise motion of Africa relative to Europe were computed. The motions of the smaller intervening microplates have been inferred from the style of tectonic deformation on their borders, and these motions have been constrained to satisfy both changes in paleo-latitude with time and progressive rotations relative to the large macroplates that can be deduced from paleomagnetic measurements. The evolution of Tethys does not involve a single simple plate boundary between Europe and Africa, as has been envisioned previously, but, instead, a constantly evolving mosaic of subsiding continental margins, migrating mid-oceanic ridges, transform faults, trenches, island arcs, and marginal seas (back-arc basins). The periods of passive-continental margin development are recognized by a transgressive facies of platform carbonate rocks and thick prisms of continental-rise type sedimentation; accreting ridges by ultramafic rocks, gabbro, pillow basalt, deep-sea pelagic ooze, and abyssal red clay of the ophiolite suite; trenches by a migrating series of progressively younger linear flysch troughs whose immature mineral composition reflects nearby andesitic and metamorphic source terrains; the arcs themselves by calc-alkaline volcanism and the intrusion of silicic to intermediate plutons; the polarities of these arcs by the direction of overthrust nappe sheets and gradients in the ratio of potash to silica in the extrusives; their orientation by paired belts of high T and P and high P-T metamorphics; and finally the spreading back-arc basins by outpourings of basaltic magmas and evidence of flipping Benioff planes. A compilation of eight phases or chapters in Atlantic spreading history are outlined, which are based on the recognition of discrete differences and (or) relative motion between the continents bordering the Atlantic. All of these changes are reflected in the Tethys by reorganizations of the intervening plate boundaries and, we believe, are most explicitly recorded in the deformational history of the subducting zones. A montage of geometrically assembled plate-boundary interpretations are pictorially displayed as time-lapse frames of the evolving Alpine system. The montage begins with the Late Triassic (pre-Atlantic) setting of the Tethys 1 Ocean and extends to the present through nine phases of Tethyan history. Each phase is recognized on the basis of the age of intrusion and extrusion of basic lavas in ophiolite complexes, which mark the creation of new oceanic areas by both axial accretion in rift valleys of mid-oceanic ridges between rigid plates or by a more uncertain type of spreading in basins behind active island arcs. All the schemes presented are best estimates of the gross geometrical arrangements at discrete time intervals and should be treated as merely educated guesses. Despite the fact that we only have rigorous constraints for the relative positions of the nondeformed forelands of Europe and Africa, our models nevertheless imply that the motions of the larger plates will, by and large, dictate the general behavior of the smaller microplates through the particular styles of deformation set up along the adjoining plate boundaries. The Tethys 1 Ocean, located between Africa and Europe in Triassic times, has been almost entirely swallowed up in subduction zones of the Major Caucasus Mountains along its former northern margin and in similar zones of the Pontides and Minor Caucasus along its southern margin. The only remnants of Tethys 1 are the areas of oceanic crust in the Black and South Caspian Seas. There is considerable evidence to suggest that the Tethys 1 Ocean had an actively spreading ridge. Some tens of millions of years prior to the opening of the central North Atlantic, a branch of this ridge system entered into the Vardar Zone of eastern Greece and broke off fragments of northeast North Africa to initiate the development of the present-day Ionian and Levantine Basins of the eastern Mediterranean. Additional fragments (the Moroccan and Oranaise Meseta) were ruptured from northwest Africa following its separation from North America. The intervening Jurassic Atlas, seaway developed along an accreting plate boundary extending from the eastern Tethys to the crest of the embryonic Mid-Atlantic Ridge where it formed a migrating triple junction whose trace, we believe, follows the trend of the New England seamount chain. The western Mediterranean basins of the Alboran, Balearic, and Tyrrhenian Seas are very much younger, being initially opened in the early Miocene as a string of back-arc marginal seas behind the developing Apennine, Tel Atlas, and Rif suture zone that today marks the sites of subduction of Jurassic and Lower Cretaceous oceanic crust. The contemporary Alpine system displays a spectrum of stages in the building of mountain belts. Embryonic nappes within the Mediterranean Ridge in proximity to melange zones of the inner wall of the Hellenic Trench are, perhaps, signs of the initial deformation of sedimentary passengers on oceanic crust arriving at a subduction zone. Total closure of an ocean followed by the partial consumption of a passive continental margin leads to events such as the tectonic emplacement of crystalline basement nappes of the European “chaine calcaire” onto northwest Africa. Arc-continent collisions of this type which have then been succeeded by total destruction of marginal back-arc basins are recognizable in the Hellenides and Pontides. There are, as well, collisions that have not involved the disappearance of large oceanic areas; these are most apparent in the particular tectonic style of the Pyrenees and High Atlas Mountains.

1,538 citations

Journal ArticleDOI
TL;DR: In this paper, the relative strength of the plausible driving forces, given the observed motions and geometries of the lithospheric plates, was analyzed. But the results indicate that the forces acting on the downgoing slab control the velocity of the oceanic plates and are an order of magnitude stronger than any other force.
Abstract: Summary A number of possible mechanisms have recently been proposed for driving the motions of the lithospheric plates, such as pushing from mid-ocean ridges, pulling by downgoing slabs, suction toward trenches, and coupling of the plates to flow in the mantle. We advance a new observational method of testing these theories of the driving mechanism. Our basic approach is to solve the inverse problem of determining the relative strength of the plausible driving forces, given the observed motions and geometries of the lithospheric plates. Since the inertia of the plates is negligible, each plate must be in dynamic equilibrium, so that the sum of the torques acting on a plate must be zero. Thus, our problem is to determine the relative sizes of the forces that minimize the components of net torque on each plate. The results indicate that the forces acting on the downgoing slab control the velocity of the oceanic plates and are an order of magnitude stronger than any other force. Namely, all the oceanic plates attached to substantial amounts of downgoing slabs move with a ' terminal velocity ' at which the gravitational body force pulling the slabs downward is nearly balanced with the resistance acting on the slab; regardless of the other features of the trailing horizontal part of the plates. The drag on the bottom of the plates which resist motion is stronger under the continents than under the oceans.

1,462 citations

Journal ArticleDOI
TL;DR: In this paper, it is proposed that mountain belts develop by deformation and metamorphism of the sedimentary and volcanic assemblages of Atlantic-type continental margins, resulting from the events associated with the rupture of continents and the expansion of oceans by plate generation at oceanic ridges.
Abstract: Analysis of the sedimentary, volcanic, structural, and metamorphic chronology in mountain belts, and consideration of the implications of the new global tectonics (plate tectonics), strongly indicate that mountain belts are a consequence of plate evolution. It is proposed that mountain belts develop by the deformation and metamorphism of the sedimentary and volcanic assemblages of Atlantic-type continental margins. These assemblages result from the events associated with the rupture of continents and the expansion of oceans by lithosphere plate generation at oceanic ridges. The earliest assemblages thus developed are volcanic rocks and coarse clastic sediments deposited in fault-bounded troughs on a distending and segmenting continental crust, subsequently split apart and carried away from the ridge on essentially aseismic continental margins. As the continental margins move away from the ridge, nonvolcanic continental shelf and rise assemblages of orthoquartzite-carbonate, and lutite (shelf), and lutite, slump deposits, and turbidites (rise) accumulate. This kind of continental margin is transformed into an orogenic belt in one of two ways. If a trench develops near, or at, the continenal margin to consume lithosphere from the oceanic side, a mountain belt (cordilleran type) grows by dominantly thermal mechanisms related to the rise of calc-alkaline and basaltic magmas. Cordilleran-type mountain belts are characterized by paired metamorphic belts (blueschist on the oceanic side and high temperature on the continental side) and divergent thrusting and synorogenic sediment transport from the high-temperature volcanic axis. If the continental margin collides with an island arc, or with another continent, a collision-type mountain belt develops by dominantly mechanical processes. Where a continent/island arc collision occurs, the resulting mountains will be small (e.g., the Tertiary fold belt of northern New Guinea), and a new trench will develop on the oceanic side of the arc. Where a continent/continent collision occurs, the mountains will be large (e.g., the Himalayas), and the single trench zone of plate consumption is replaced by a wide zone of deformation. Collision-type mountain belts do not have paired metamorphic belts; they are characterized by a single dominant direction of thrusting and synorogenic sediment transport, away from the site of the trench over the underthrust plate. Stratigraphic sequences of mountain belts (geosynclinal sequences) match those asciated with present-day oceans, island arcs, and continental margins.

1,462 citations

References
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Journal ArticleDOI
03 Oct 1969-Science
TL;DR: The two sets of variables primary production and the associated food chain dynamics may act additively to produce differences in fish production which are far more pronounced and dramatic than the observed variability of the individual causative factors.
Abstract: Numerous attempts have been made to estimate the production in the sea of fish and other organisms of existing or potential food value to man (1-4). These exercises, for the most part, are based on estimates of primary (photosynthetic) organic production rates in the ocean (5) and various assumed trophic-dynamic relationships between the photosynthetic producers and the organisms of interest to man. Included in the latter are the number of steps or links in the food chains and the efficiency of conversion of organic matter from each trophic level or link in the food chain to the next. Different estimates result from different choices in the number of trophic levels and in the efficiencies, as illustrated in Table 1 (2). Implicit in the above approach is the concept of the ocean as a single ecosystem in which the same food chains involving the same number of links and efficiencies apply throughout. However, the rate of primary production is known to be highly variable, differing by at least two full orders of magnitude from the richest to the most impoverished regions. This in itself would be expected to result in a highly irregular pattern of food production. In addition, the ecological conditions which determine the trophic dynamics of marine food chains also vary widely and in direct relationship to the absolute level of primary organic production. As is shown below, the two sets of variables primary production and the associated food chain dynamics may act additively to produce differences in fish production which are far more pronounced and dramatic than the observed variability of the individual causative factors.

1,423 citations

Journal ArticleDOI
TL;DR: In this article, a geometrical model of the surface of the earth is obtained in terms of rigid blocks in relative motion with respect to each other, and 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.
Abstract: 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.

1,293 citations

Journal ArticleDOI
TL;DR: Fits made by numerical methods, with a ‘least squares’ criterion of fit, for the continents around the Atlantic ocean are described, finding the best fit to be at the 500 fm contour which lies on the steep part of the continental edge.
Abstract: The geometrical fit of the continents now separated by oceans has long been discussed in relation to continental drift. This paper describes fits made by numerical methods, with a ‘least squares’ criterion of fit, for the continents around the Atlantic ocean. The best fit is found to be at the 500 fm. contour which lies on the steep part of the continental edge. The root-mean-square errors for fitting Africa to South America, Greenland to Europe and North America to Greenland and Europe are 30 to 90 km. These fits are thought not to be due to chance, though no reliable statistical criteria are available. The fit of the block assembled from South America and Africa to that formed from Europe, North America and Greenland is much poorer. The root-mean-square misfit is about 130 km. These geometrical fits are regarded as a preliminary to a comparison of the stratigraphy, structures, ages and palaeomagnetic results across the joins.

1,141 citations

Journal ArticleDOI
24 Jul 1965-Nature
TL;DR: In this article, the authors proposed the notion of dextral transform faults, which can be seen as a pair of half-shears joined end-to-end, which is the case of the San Andreas Transform Fault.
Abstract: T and half-shears. Many geologists1 have maintained that movements of the Earth's crust are concentrated in mobile belts, which may take the form of mountains, mid-ocean ridges or major faults with large horizontal movements. These features and the seismic activity along them often appear to end abruptly, which is puzzling. The problem has been difficult to investigate because most terminations lie in ocean basins. This article suggests that these features are not isolated, that few come to dead ends, but that they are connected into a continuous network of mobile belts about the Earth which divide the surface into several large rigid plates (Fig. I). Any feature at its apparent termination may be transformed into another feature of one of the other two types. For example, a fault may be transformed into a mid-ocean ridge as illustrated in Fig. 2a. At the point of transformation the horizontal shear motion along the fault ends abruptly by being changed into an expanding tensional motion across the ridge or rift with a change in seismicity. A junction where one feature changes into another is here called a transform. This type and two others illustrated in Figs. 2b and c may also be termed half-shears (a name suggested in conversation by Prof. J. D. Bernal). Twice as many types of half-shears involve mountains as ridges, because mountains are asymmetrical whereas ridgos have bilateral symmetry. This way of abruptly ending large horizontal shear motions is offered as an explanation of what has long been recognized as a puzzling feature of large faults like the San Andreas. Another type of transform whereby a mountain is transformed into a mid-ocean ridge was suggested by S. W. Carey when he proposed that the Pyrenees Mountains were compressed because of the rifting open of the Bay of Biscay (presumably by the formation of a midocean ridge a.long its axis). The types illustrated are all dextra.l, but equivalent sinistral types exist. In this article the term 'ridge' will be used to mean midocean ridge and also rise (where that term has been used meaning mid-ocean ridge, as by Menard\" in the Pacific basin). The terms mountains and mountain system may include island arcs. An arc is described as being convex or concave depending on which face is first reached when proceeding in the direction indicated by an arrow depicting relative motion (Figs. 2 and 3). The word fault may mean a system of several closely related faults. Transform faults. Faults in which the displacement suddenly stops or changes form and direction are not true transcurrent faults. It is proposed that a separate class of horizontal shear faults exists which terminate abruptly at both ends, but which nevertheless may show great displacements. Each may be thought of as a pair of half. shears joined end to end. Any combination of pairs of the three dextral half-shears may be joined giving rise to the six types illustrated in Fig. 3. Another six sinistral forms can also exist. The name transform fault is proposed for the class, and members may be described in terms of the features which they connect (for example, dextral transform fault, ridge--convex arc type). The distinctions between types might appear trivial until the variation in the habits of growth of the different types is considered as is shown in Fig. 4. These distinctions are that ridges expand to produce new crust, thus leaving residual inactive traces in the topography of their former positions. On the other hand oceanic crust moves down under island arcs absorbing old crust so that they leave no traces of past positions. The convex sides of arcs thus advance. For these reasons transform faults of types a, b and d in Fig. 4 grow in total width, type f diminishes and the behaviour of types c and e is indeterminate. It is significant that the direction of motion on trarn,form faults of the type shown in Fig. 3a is the reverse of that required to offset the ridge. This is a fundamental difference between transform and transcurrent faulting.

1,137 citations


"Rises, trenches, great faults, and ..." refers background in this paper

  • ...This presentation is an extension of the transform fault concept [Wilson, 1965] to a spherical surface....

    [...]

Journal ArticleDOI
F. J. Vine1
16 Dec 1966-Science
TL;DR: The hypothesis that magnetic anomalies may indicate the nature of oceanic fracture zones and distinguish the parts of the ridge system that are actively spreading is suggested and data derived during the past year lend remarkable support to the hypothesis.
Abstract: It is suggested that the entire history of the ocean basins, in terms of oceanfloor spreading,is contained frozen in the oceanic crust. Variations in the intensity and polarity of Earth's magnetic field are considered to be recorded in the remanent magnetism of the igneous rocks as they solidified and cooled through the Curie temperature at the crest of an oceanic ridge, and subsequently spread away from it at a steady rate. The hypothesis is supported by the extreme linearity and continuity of oceanic magnetic anomalies and their symmetry about the axes of ridges. If the proposed reversal time scale for the last 4 million years is combined with the model, computed anomaly profiles show remarkably good agreement with those observed, and one can deduce rates of spreading for all active parts of the midoceanic ridge system for which magnetic profilesor surveys are available. The rates obtained are in exact agreement with those needed to account for continental drift. An exceptionally high rate of spreading (approximately 4.5 cm/year) in the South Pacific enables one to deduce by extrapolation considerable details of the reversal time scale back to 11.5 million years ago. Again, this scale can be applied to other parts of the ridge system. Thus one isled to the suggestion that the crest of the East Pacific Rise in the northeast Pacific has been overridden and modified by the westward drift of North America, with the production of the anomalous width and unique features of the American cordillera in the western United States. The oceanicmagnetic anomalies also indicate that there was a change in derection of crustal spreading in this region during Pliocene time from eastwest to southeast-northwest. A profile from the crest to the boundary of the East Pacific Rise, and the difference between axial-zone and flank anomalies over ridges, suggest increase in the frequency of reversal of Earth's magnetic field, together, possibly, with decrease in its intensity, approximately 25 million years ago. Within the framework of ocean-floor spreading, it is suggested that magnetic anomaliesmay indicate the nature of oceanic fracture zones and distinguish the parts of the ridge system that are actively spreading. Thus data derived during the past year lend remarkable support to thehypothesis that magnetic anomalies may reveal the history of the ocean basins.

693 citations