Showing papers on "Mid-ocean ridge published in 1970"
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
TL;DR: Oceanic ridge volcanic rocks alkali metal, alkaline earth, rare earth, nickel and major element content, observing partial melting as mentioned in this paper. But this is not the case for all volcanic rocks.
Abstract: Oceanic ridge volcanic rocks alkali metal, alkaline earth, rare earth, nickel and major element content, observing partial melting
581 citations
TL;DR: In this paper, the epicenters of about 900 earthquakes in the Indian Ocean, Africa, and adjacent areas that occurred from 1950 to 1966 were relocated by computer and the locations of the earthquakes delineated many transform faults, ridge crests, and triple junctions.
Abstract: The epicenters of about 900 earthquakes in the Indian Ocean, Africa, and adjacent areas that occurred from 1950 to 1966 were relocated by computer. These epicenters delineate many transform faults, ridge crests, and triple junctions much more precisely than was done previously. A series of great NNW-striking transform faults south of Tasmania successively offset the mid-ocean ridge a total of about 1500 km. The distribution of epicenters and of two focal mechanism solutions for the southwest brance of the mid-Indian ridge are indicative of sea-floor spreading with ridge crests oriented approximately WNW and transform faults NNE. Previous reports of sediment thicknesses near this branch of the ridge are in good accord with the inferred pattern of spreading ridge crests and transform faults. Likewise, failures to detect measurable sea-floor spreading along this branch can be ascribed to the unfortunate orientation of the few published profiles nearly parallel to ridge crests. An incipient tectonic feature, possibly a nascent island arc, may be associated with a seismic zone in the northeast Indian Ocean between Ceylon and Australia. The large width of the zone of shallow shocks, the relative abundance of earthquakes larger than magnitude 7, and the absence of observed topographic features associated with the seismic zone are indicative of a nascent tectonic feature that may be related to a decrease or change in the relative motion between the Indian-Australian plate and-the Eurasian plate, possibly as a result of continental collision. Other interpretations of the tectonics of this unusual zone are also discussed. Regardless of its tectonic significance, this zone deserves further study since it is the most seismically active region in the oceans that has not been identified as either a ridge, an arc, or a transform fault.
The results of the relocations and other pertinent data are given in a separate appendix, which is available on microfiche along with the entire article. Order from the American Geophysical Union, Suite 435, 2100 Pennsylvania Ave., N.W., Washington, D.C. 20037. Document J 70-003; $1.00. Payment must accompany order.
195 citations
TL;DR: In this article, the formation of olivine and plagioclase from abyssal tholeiite magma has been studied, showing remarkable differentiation, producing high-iron, high-titanium gabbros and aplite in later stages.
165 citations
TL;DR: In this paper, it is shown that the upper mantle is probably unstable and that the instability expresses itself in diapirism under the ridges and sinking of the lithosphre under the island arcs.
Abstract: It is shown that the upper mantle is probably unstable. The instability expresses itself in diapirism under the ridges and sinking of the lithosphre under the island arcs. Both processes exert a force on the plate that can move it against the viscous drag from the flow of the asthenosphere. Order of magnitude estimates of the forces and of the required energy demonstrate that the process is plausible and that the drift rate can be determined from the resistance the plate encounters at its edges, particularly below the island arcs where it plunges into the mesosphere. The proposed mechanism can be tested by studying, e.g., the deep structure of the ridges, the stress field in the plates, and the gravity field over ridges and trenches.
146 citations
TL;DR: Using the ocean floor spreading/plate tectonics theory as a starting point, examining the implications of this theory for processes taking place at mid-ocean ridge crests, and adding data collected from dredged rocks and geophysical measurements in the oceans, a well controlled model of the structure of the ocean crust is built.
Abstract: USING the ocean floor spreading/plate tectonics theory as a starting point, examining the implications of this theory for processes taking place at mid-ocean ridge crests, and adding data collected from dredged rocks and geophysical measurements in the oceans, it is possible to build up a well controlled model of the structure of the ocean crust and the way in which it is formed.
121 citations
TL;DR: In this article, the authors show that compressional wave velocities in rocks are consistent with an oceanic crust composed of an assemblage of hornblende and plagioclase (layer 3) overlain by tholeiitic basalt (layer 2).
52 citations
TL;DR: A geologic and geophysical reconnaissance of the continental margin of the north coast of Brazil led to the discovery of a narrow basement ridge, referred to as the North Brazilian Ridge as mentioned in this paper, which is essentially a continuous topographic and/or structural feature across a distance of 1,300 km.
Abstract: A geologic and geophysical reconnaissance of the continental margin of the north coast of Brazil led to the discovery of a narrow basement ridge, herein called the North Brazilian Ridge The ridge closely parallels the coast of Brazil between the Amazon cone and the easternmost tip of Brazil and is essentially a continuous topographic and/or structural feature across a distance of 1,300 km The ridge lies 150-200 km seaward from the base of the continental slope Its topographic relief ranges from about 300 m to about 4 km Although a conspicuous sediment ponding is observed on the continental side of the ridge, there is no marked contrast in type for the upper few meters of sediment in piston cores from opposite sides of the ridge The primary process of sediment transport and deposition thus appears to be acting parallel with the ridge rather than downslope A prominent acoustic reflector, which may be synchronous with Horizon "A" in the North Atlantic, abuts the north flank of the ridge This observation, together with ages extrapolated from sedimentation rates determined on the ridge at JOIDES drill sites 25-25A, suggests that the ridge is relatively old, of the order of 100 my The ridge is gravimetrically compensated, and gravity values are roughly the same in the basins on opposite sides of the ridge Magnetic anomalies over the ridge are typically of low amplitude and have an apparently random character Rock outcrops were photographed along the steep flanks of the ridge and are very similar to outcrops of mafic igneous rocks on the flanks of the Mid-Atlantic Ridge The North Brazilian Ridge is not continuous with the major fracture zones of the equatorial Atlantic A collective examination of all data leads us to the conclusion that the ridge probably formed by oceanic volcanism shortly after the separation of Africa and South America Its location and extent may be the direct consequence of the manner and geometry of the opening of the equatorial South tlantic Ocean
34 citations
TL;DR: In this paper, the authors analyzed sediment samples taken from traverses across active oceanic ridges for Pb, Ag, Sn, Tl, and Zn and found that the ridge effects are not more than 10%.
29 citations
TL;DR: In this article, a matrix-based approach is proposed to deduce the distribution of magnetization within a specified layer which causes a given magnetic anomaly, and applied to a profile across the Sheba Ridge, Gulf of Aden.
Abstract: Summary This paper presents a matrix method for deducing the distribution of magnetization within a specified layer which causes a given magnetic anomaly. It extends the earlier method given by Bott by allowing irregular upper and lower surfaces for the magnetic layer. The method is particularly applicable to interpretation of oceanic magnetic anomalies in terms of a magnetization distribution within Layer 2 of the oceanic crust. It provides a method for applying the Vine-Matthews hypothesis to regions of irregular topography. The method is applied to a profile across the Sheba Ridge, Gulf of Aden. Strip-like magnetic anomalies of several hundred gamma amplitude were first discovered off the California coast (Raff & Mason 1961; Mason & Raff 1961) and are typical of much of the oceanic regions. The strip-like anomalies tend to be parallel to the crest of mid-ocean ridges. The normal interpretation of the anomalies is in terms of the hypothesis of Vine & Matthews (1963) which is itself dependent on the idea of ocean-floor spreading. Following the Vine-Matthews hypothesis, the study of oceanic magnetic anomalies has made outstandingly important contributions to our knowledge of the history of formation of ocean basins in relation to continental drift, and to determining the time-scale of reversals of the Earth’s magnetic field over the last 70 My and more. Vine (1966) and Heirtzler er al. (1968) have used the indirect method of magnetic interpretation to show that typical magnetic profiles of several hundreds of kilometres length can be simulated by a series of two-dimensional rectangular blocks of alternating magnetic polarity which are symmetrical about the ocean ridge crests. These blocks nominally represent Layer 2 of the oceanic crust, which is 2km thick on average: indeed the main source of the anomalies must lie within Layer 2 because Layer 1 consists of effectively non-magnetic sediments and Layer 3 is too deep to explain the anomalies without unacceptably high and irregular variations in magnetization (Bott 1967). A particular sequence of blocks representing the past time-scale of reversals of the geomagnetic field over the last 70 My has been shown by Heirtzler et al. (1968) to give good agreement with observations across the ocean ridge system at widely separated places, provided different relative spreading rates are assumed.
23 citations
TL;DR: A series of dredgings were made by Miami's Institute of Marine Sciences on the walls of the Romanche trench, where alkali gabbros are of special interest, particularly because rocks with modal nepheline have not been reported in the past from oceanic ridges.
Abstract: IN the course of a long range programme aimed at the study of the petrology of the Equatorial Mid-Atlantic Ridge, a series of dredgings were made by Miami's Institute of Marine Sciences on the walls of the Romanche trench. The Romanche is a major tectonic fracture which intersects the Ridge close to the Equator, and offsets its axis, perpendicular to its elongation. At one locality on the north wall of the trench (station P6707-25; 00° 22′ S, 20° 09′ W) about 120 kg of rock fragments was recovered from 5,100–5,300 m below sea level. The dredged material consists of about 95 per cent (by weight) of basalt and glassy basaltic breccias; 2.5 per cent of gabbros; 2 per cent of cataclastic breccias; 0.5 per cent of serpentinized peridotite. Among the gabbros norites, troctolites and gabbros with alkali affinities were observed; among the latter, some contain modal nepheline and are similar to theralites. These alkali gabbros are of special interest, particularly because rocks with modal nepheline have not been reported in the past from oceanic ridges. We present here a description of them and a brief discussion of their origin.
TL;DR: Deep sea carbonate lithification and the interbedding of carbonate and ferromanganese are common processes on oceanic ridges and seamounts as discussed by the authors, and temperature and salinity changes caused by climatic fluctuation are also common processes.
Abstract: Deep sea carbonate lithification and the interbedding of carbonate and ferromanganese are common processes on oceanic ridges and seamounts. Temperature and salinity changes caused by climatic fluct...
TL;DR: The products of volcanism in the African rift valleys differ in composition and volume from those in the median valleys of oceanic ridges as discussed by the authors, which can be explained in terms of a petrogenetic model which requires a low heat flow under the rifts and a high geothermal gradient under oceanic slopes.
TL;DR: A survey of geophysical research in Iceland related to this problem is given in this paper, which includes measurements of geothermal gradient and electrical conductivity, seismic sounding, observation of earthquakes, crustal displacement, gravity, paleomagnetism and geomagnetic surveys.
Abstract: As the largest island on the worldwide ocean ridge system, Iceland may offer the possibility to study on land some of the characteristic features of ocean ridges. The paper gives a survey of geophysical research in Iceland related to this problem. It includes measurements of geothermal gradient and electrical conductivity, seismic sounding, observation of earthquakes, crustal displacement, gravity, paleomagnetism and geomagnetic surveys.
TL;DR: A seismic reflection profile across the fracture zones on the Mid-Atlantic Ridge between 43° N and 44° N shows that the thickness of sediment increases markedly south of the fracture zone at 43° 05 as mentioned in this paper.
Abstract: A seismic reflection profile across the fracture zones on the Mid-Atlantic Ridge between 43° N and 44° N shows that the thickness of sediment increases markedly south of the fracture zone at 43° 05...
01 Jan 1970
TL;DR: In the field of seafloor geology, the work of as discussed by the authors has shown that the ocean basins are geologically young (less than 200 million years), and the distribution of land and sea may be a passive and ephemeral consequence of interactions between adjacent plates of lithosphere.
Abstract: Drilling and coring by JOIDES consists of over 150 stations in the world oceans including coring beneath water depths in excess of 20,000 feet, sub-bottom penetration of over 3,000 feet, and re-entry of the drill hole beneath 13,000 feet of water. Oil was discovered beneath 11,700 feet of water in the Gulf of Mexico and other cores have supplied supporting evidence for concepts of sea floor spreading, and plate tectonics as derived from bathymetry, geomagnetism, earthquake seismology and tectonic events recorded by the rocks an land. Evidence is accumulating that the ocean basins are geologically young (less than 200 million years); that new oceanic lithosphere forms along crests of oceanic ridges; that older oceanic lithosphere is resorbed beneath linear trends of deep focus earthquakes associated with oceanic trenches; that extensive plates of the earth's lithosphere move laterally at rates up to several inches per year; that these plates interact to cause deformation, mountain building, and unconformities; and although the areal extent of continental material may be increasing on balance, the distribution of land and sea may be a passive and ephemeral consequence of interactions between adjacent plates of lithosphere. The practical consequences of these concepts may lie in the potential for reconstruction of land-sea relationships of the past and thereby the distribution of sediment along continental margins of the past and present; the potential for predicting structural style resulting from plate divergence, convergence and oblique interaction; and the effects of abnormal temperature in the generation of petroleum.
TL;DR: In this paper, the evolution of the continental crust can be distinguished into three stages: growth of continental cores by differentiation, the existence of peripheral growing zones around the first core, and the intergrowth of the existing continents.
Abstract: Although active volcanic territories are not characterised by extreme high heat flow, yet terrestrial heat is one of the main, if not hte first cause of volcanic activities. About 94 per cent of the known active volcanoes are in connection with orogenic zones including ocean ridges, which mean that processes responsible for the evolution of the continents cause the overwhelming majority of volcanism. There stages of the evolution of the continental crust can be distinguished: 1.) Growth of the continental cores by differentiation, 2.) Existence of peripheral growing zones around the first core, 3.) Intergrowth of the existing continents. Volcanism, as well as seismicity are manifestations of the evolution of the crust which is caused by the development of heat sources in the mantle. The heat partly melt the upper mantle and induce slow plastic flow in connection of differentiation. The space, left empty by continental uplift is filled by the slow plastic inflow of mantle from the ocean. This slow redistribution of mantle and curstal rocks disturbs the stress field, the rocks fail along the strained zones and zones of fractures appear sometimes with volcanic activity.
TL;DR: This paper showed that the crust of Dampier Ridge has a quasi-continental thickness, and therefore can not be an extinct mid-ocean ridge, and Three Kings Rise is also regarded as sialic in origin.