Showing papers on "Mid-ocean ridge published in 1969"

Tectonics of the Caribbean and Middle America Regions from Focal Mechanisms and Seismicity

Abstract: Seismic data strongly support recent theories of tectonics in which large plates of lithosphere move coherently with respect to one another as nearly rigid bodies, spreading apart at ocean ridges, sliding past one another at transform faults, and underthrusting at island arcs. Boundaries between adjacent plates of lithosphere are defined by belts of high seismic activity. Redetermination of more than 600 hypocenters in the Middle America region and previous studies in the Galapagos and Caribbean regions define the boundaries of two relatively small, nearly aseismic plates in the region of interest. The first, the Cocos plate, is bordered by the East Pacific rise, the Galapagos rift zone, the north-trending Panama fracture zone near 82° W., and the Middle America arc; the second, the Caribbean plate, underlies the Caribbean Sea and is bounded by the Middle America arc, the Cayman trough, the West Indies arc, and the seismic zone through northern South America. Focal mechanisms of 70 earthquakes in these regions were determined to ascertain the relative motion of these two plates with respect to the surrounding regions or plates. The results show underthrusting of the Cocos plate beneath Mexico and Guatemala in a northeasterly direction and beneath the rest of Central America in a more north-northeasterly direction. The Cocos plate is spreading away from the rest of the Pacific floor at the East Pacific rise and at the Galapagos rift zone. Motion is right-lateral strike-slip along the Panama fracture zone, a transform fault connecting the Galapagos rift zone and the Middle America arc. At the same time, the Caribbean plate is moving easterly with respect to the Americas plate, which is here taken to include both North and South America and the western Atlantic. Left-lateral strike-slip motion along steeply dipping fault planes is observed on the Cayman trough. The Americas plate is underthrusting the Caribbean in a westerly direction at the Lesser Antilles and near Puerto Rico. Unlike the Lesser Antilles, however, motion at present is not perpendicular to the Puerto Rico trench but instead is almost parallel to the trench along nearly horizontal fault planes. Computations of rates of motion indicate that underthrusting is at a higher rate in southeastern Mexico and Guatemala than in western Mexico and that the Caribbean is moving at a lower rate relative to North America than is the Cocos plate.

Lateral variations of attenuation in the upper mantle and discontinuities in the lithosphere

Abstract: This study demonstrates the existence and determines the pattern of lateral variations of attenuation in the uppermost mantle on a worldwide scale. The evidence comes mostly from a comparison of the gross characteristics of the seismic phase Sn for over 1500 paths, which taken together cross as many regions of the earth as possible with the current configuration of the World-Wide Standardized Seismograph Network. Sn is a seismic shear wave that propagates in the uppermost mantle and that does not penetrate the low-velocity channel. It propagates very efficiently across the stable regions of the earth, the continental shields, and deep-ocean basins, but propagation is very inefficient when paths cross the crests of the mid-ocean ridge system or the concave sides of most island arcs. These observations suggest that attenuation is more pronounced in the uppermost mantle near the ridge crests and the islands arcs than in the more stable regions. If low attenuation, or high Q, correlates with high strength, the data imply that the uppermost mantle is considerably weaker under the ridge crests and the concave sides of the island arcs than it is elsewhere. Thus, the part of the strong outer shell, or lithosphere, in the mantle is discontinuous with gaps in it at the ridges and island arcs. The low attenuation for Sn for paths crossing the transform faults that connect ridge crests suggests that any gap at the transform faults is very narrow. In addition, S waves recorded at stations in and near the Mariana, New Britain, Solomon, and South American arcs from local deep shocks were found to follow the pattern previously observed in the Tongan and Japanese arcs. As was suggested for the Tongan arc, the observations imply that one piece of lithosphere has underthrust another to great depth at these arcs. Hence, data presented in this paper are in accord with recent ideas of sea-floor spreading in which large plates of lithosphere move with respect to each other as rigid bodies, spreading apart at the ocean ridges, sliding past one another at the transform faults, and underthrusting at the island arcs.

Aluminum‐poor ferromanganoan sediments on active oceanic ridges

Abstract: The results of 399 new analyses and 144 analyses from the literature indicate that the carbonate-free fraction of sediments from active oceanic ridges is characterized by very low aluminum and titanium content and high iron and manganese content compared with sediments from other volcanic regions and from inactive ridges. These active ridge sediments appear to originate by volcanic processes associated with ocean-floor spreading and high heat flow. The total mass of these sediments is probably only a few per cent of all deep-sea sediments.

Underthrusting of the Eastern Margin of the Antilles by the floor of the western North Atlantic Ocean, and origin of the Barbados Ridge

Abstract: Two seismic reflection profiles of the southeastern end of the Puerto Rico trench show that the strata forming the ocean floor, including the oceanic basement, dip westward beneath the eastern flank of the Antilles arc. Farther south a profile across the eastern margin of the northern end of the Barbados ridge shows a similar westward dip of oceanic strata beneath the ridge and suggests that thrust faults dipping 20°–30° west cut the overlying sediments that form the east flank of the ridge. Similar thrust-faults appear in a profile southeast of Barbados. The evidence from the reflection profiles is compatible with seismological and magnetic evidence that suggests that the ocean floor is spreading west from the mid-Atlantic ridge and is being thrust westward beneath the Antilles. The Barbados ridge is composed of sedimentary material that formerly formed a continental rise overlying the oceanic crust along the northern margin of South America. Thrusting of oceanic crust beneath the Lesser Antilles arc and displacement of South America westward relative to the Antilles during the Cenozoic resulted in the crumpling, thickening, and accumulation of the material of the continental rise in front (east) of the Antilles arc to form the Barbados ridge.

Convection in a mantle with variable physical properties

Abstract: The aspects of mantle convection that do not require a knowledge of the viscosity are considered first. Using spreading rates obtained from magnetic anomaly data, the temperature distribution in the thermal boundary layers adjacent to the ocean floors is determined. The values of the surface heat flux are compared with measurements. A temperature distribution in the upper mantle is obtained by matching the boundary-layer profiles to adiabatic profiles consistent with the boundary-layer theory for finite amplitude convection. Mechanisms for a fluid-like mantle are considered. Assuming that diffusion creep is occurring, a semi-empirical expression for the temperature and pressure dependence of the viscosity is given. The viscosity is found to have a strong minimum near a 100-km depth. The effect of a strong temperature-dependent viscosity on the ascending, diverging flow near ocean ridges is studied. The topography of ocean ridges is attributed to the hydrostatic head required for the horizontal flow. The excess temperature associated with the ascending flow is found to be of the order of 50°K.

Heat flow in the eastern pacific and sea floor spreading

Abstract: A simple model based on the hypothesis of sea floor spreading can account for the main features of two major high heat flow anomalies in the eastern Pacific; the broad band of high values along the crest of the East Pacific Rise and the large concentration of high values centered on the Galapagos Rift Zone. Using the same model to interpret both the surface shape of the midocean ridges and the heat loss of the entire ridge system, the calculated elevation is found to be comparable, though smaller than, that observed for the ridges and the heat dissipated by crustal production along the axis of the entire ridge system is shown to be approximately 15% of the total heat loss through the oceans.

Peridotite-Gabbro Complexes as Keys to Petrology of Mid-Oceanic Ridges

Abstract: Two suites of olivine-rich ultramafic and feldspathic rocks appear to be present in the Mid-Atlantic Ridge: one which seems to have alkalic affinities, and one similar to the chromitite-bearing alpine peridotite-gabbro complexes. The similarities of rocks in the two environments— continental and oceanic—imply that much about the petrology of mid-oceanic ridges may be learned from studies of continental complexes, and that silicic rocks have been formed in the mantle. Although gabbros in St. Paul Rocks and similar rocks at Tinaquillo, Venezuela, and Lizard, England, have been interpreted as not comagmatic with intimately associated peridotite by some petrologists, evidence to the contrary at Lizard is discussed. Association of fresh gneissic gabbro, some containing quartz, with talcose serpentinite, amphibole schist, quartz diorite and epidotic but unsheared basalts along the Mid-Atlantic Ridge is believed to indicate presence of alpine-type rocks that occur normally in eugeosynclinal belts. Gabbro, described as partly interlayered with peridotite by gravitational differentiation, forms major parts of three widely separated ultramafic complexes which have been interpreted as slices of oceanic crust and upper mantle: the Troodos massif in Cyprus, the Bowutu Mountains in Papua, and the Camaguey complex in central Cuba. If, as Dietz has suggested, peridotite and related rocks in eugeosynclines represent fragments of ocean rind formed along mid-oceanic ridges and moved laterally by ocean-floor spreading, gabbro must be an essential constituent of the upper mantle. This could account for many geophysical anomalies, but would complicate some postulated mechanisms involved in ocean-floor spreading.

Growth of drifting volcanoes

Abstract: In some well-surveyed areas, volcanic seamounts increase in size with distance from a midocean ridge crest. This suggests that they remain active as they drift with the crust. Hardly any volcanic islands rise above the oceanic crust produced during the last 10 million years by sea-floor spreading. This indicates that active, drifting volcanoes rarely grow to the sea surface in less than that time. Guyots and atolls generally occur only on much older crust, which suggests that ocean basin volcanoes commonly remain active for tens of millions of years. Consequently the paleobathymetry of the Darwin rise is compatible with sea-floor spreading. The discharge of lava in the Hawaiian Islands in historical times has been many times greater than average for an equivalent length of midocean ridge crest. These volcanoes in the center of the Pacific tectonic plate grow rapidly with minimum drifting before they reach the surface. Smaller volcanoes in the basin thus may be built in a single magnetic polarity period, as suggested by paleomagnetism.

Circum-pacific late cenozoic structural rejuvenation: implications for sea floor spreading.

Abstract: The hypothesis of sea floor spreading and lithosphere plates seems to unify the origins of both oceanic ridges and volcanic arc-trench systems; therefore knowledge of well-known land areas should shed light upon sea floor tectonics. Impressive evidence of a major mid-Cenozoic discontinuity in the tectonic history of circum-Pacific land areas suggests a roughly synchronous change in sea floor development, more evidence for which may be anticipated in the future.

Mid-Atlantic Ridge near 43°N latitude

Abstract: The results of a reconnaissance bathymetric and magnetic survey of the mid-Atlantic ridge between 42° and 45°N latitude show a symmetrical north-south magnetic pattern parallel to the axis over the two southernmost of six profiles across the ridge. The southern profiles are also characterized by a well developed central rift valley. The four northern profiles do not contain obvious magnetic symmetry elements and are over a portion of the ridge characterized by rugged topography without a central rift. A detailed survey made over the ridge crest between 42°30′ and 43°30′N spanned the region of apparent change. Here a narrow zone transverse to the ridge axis shows abrupt termination of the symmetrical north-south magnetic trends and central rift valley of the southern area. Dredged rocks and heat-flow measurements in the two distinct areas also differ markedly. Basalts and relatively low heat flow (<0.9 μcal/cm2 sec, two observations) characterize the southern area, even in the median valley, whereas serpentinized peridotites and very high heat flow (3–7 μcal/cm2 sec, four observations) were observed in the northern area. The contrasting nature of the magnetic, topographic, petrologic, and heat-flow character in the northern and southern areas suggests that the ridge crest at 43°N latitude is interrupted by a complex fracture zone. Model studies suggest that the configuration of magnetic bodies that could account for the symmetrical magnetic pattern of the southern area may be interpreted in terms of the Vine-Matthews hypothesis of seafloor spreading. A change in spreading rate is suggested to have occurred 4 m.y. ago. However, it is difficult to identify specific magnetic features, and several models can be fitted to the observed profile. The determination of spreading history here may not be possible from the magnetic models alone.

Upper Mantle Structure Along the Axis of the Mid-Atlantic Ridge near Iceland

Abstract: Summary Apparent velocity measurements of P arrivals from Mid-Atlantic Ridge earthquakes across pairs of stations in Iceland and Greenland have been made using bulletin data. They define both the vertical and horizontal extent of the low velocity anomalous mantle zone between 50° and 70° N.

Long waves on oceanic ridges

Abstract: A ridge in an ocean of uniform depth acts as a waveguide for long waves. A uniform rectangular ridge is used as a simple model, and it is shown that in this case an infinite number of modes of propagation along the ridge are possible. Actual dispersion curves are given for numerical examples based on the Mid-Atlantic Ridge. In the case of a ridge of general shape, a simple variational technique is suggested as a means of achieving numerical solutions. Two numerical examples of these techniques are given. The effect of the Earth’s rotation is considered at the end of the paper, and it is shown that in the numerical examples in this paper, this effect is very small.

Determination of Focal Depths of Earthquakes in the Mid-Oceanic Ridges from Amplitude Spectra of Surface Waves

Abstract: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1969.

Increased Estimate for Heat Flow at Oceanic Ridges

Abstract: THE theory of ocean floor spreading requires that oceanic crust be generated at mid-ocean ridges. Evidence from a variety of sources suggests that the new crust is made largely of basalt and gabbro, and their low-grade metamorphic equivalents1,2, formed by igneous activity close to the ridge axis. If the rate of spreading and the thickness of the crust are known, the rate of heat transfer to the surface by magmas during crust formation in the axial zone of the ridge can be estimated. It is of interest to compare this rate with the heat flux actually measured on oceanic ridges.

Undeformed Sediments in Oceanic Trenches with Sea Floor Spreading

Abstract: THE study of magnetic anomalies associated with mid-ocean ridges indicates that sea floor spreading is active in the vicinity of these ridges1. Mid-ocean ridges are thought likely to be the sites of upwelling convection currents and oceanic trenches to be the sites of descending currents2. Most trenches are associated with intense seismic activity3–6. The hypocentres of earthquakes occur along almost planar zones (Benioff zones) which may be the result of movement of the sea floor under continents or island arcs. Seismic refraction studies of oceanic trenches, however, indicate that the sediments within the trenches are essentially undeformed and that there is no large accumulation of deformed sediments with a low seismic velocity associated with these trenches7–10. The absence of deformed low velocity sediments has been cited as evidence that the sea floor is not descending in the vicinity of these trenches7,8,10. Scholl et al.10 assume that deformed pelagic sediments would be expected within the trenches if the sea floor is actively underthrusting a continent or island arc. The centre of the Benioff zone, however, typically intersects the ocean floor landward of the axes of oceanic trenches4–6. This indicates that downward movement of the sediments occurs landward of the axes of the trenches rather than directly under the axes of the trenches.

Origin of the Gulf and Caribbean and Implications Regarding Ocean Ridge Extension, Migration, and Shear (1)

Abstract: The Gulf and Caribbean are a zone of north-south extension and left lateral shear opened between the Americas as these continents moved westward from Africa. The movements are related to ocean floor spreading away from the mid-Atlantic ridge. In order to accommodate spreading, the ridge itself migrates westward from Africa. Ridge migration is radial outward from Africa and results in opening triangular sheared grabens with apexes against Africa. A new ridge segment extends across these openings. Spreading rates vary and the migrating and extending ridge is sheared on fracture zones in response to these variations. The currently popular related concepts of plate tectonics and transform faults are inconsistent with ridge migration and shear because these theories deny shear on fracture zones beyond ridge offsets and in the sense indicated by the position of ridge segments. Ridge migration and shear are a necessary complication of the spreading hypothesis. T-intersections of ridges are explained as intersections between a spreading and migrating ridge and a shear. The shear is only active on the side of the ridge toward which the migration is taking place. The junction of the mid-Atlantic ridge with the Azores-Gibraltar ridge is an example of such a feature.

Seismology and the new global tectonics: Implications for earthquake prediction

Abstract: A comprehensive study of the observations of seismology provides widely based, strong support for the new global tectonics that 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 parts 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 explained, in general, 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.

Floor of the North Atlantic--Summary of Geophysical Data: Chapter 4: Introductory Papers

Abstract: The geophysical data for the Atlantic Ocean north of 30°N, and for the Labrador, Greenland, and Norwegian Seas, are sparse in comparison with those from the midlatitudes, but new information is being accumulated rapidly due to the stimulus of new ideas about ocean-floor spreading This summary is based on, published data and such unpublished information as investigators have released The North Atlantic and the northern marginal seas are dominated by the Mid-Atlantic Ridge and its extension toward the Arctic Ocean As elsewhere, the axial zone of the ridge is also an earthquake epicentral belt and is associated with high heat flow Where sufficient magnetic observations have been made, magnetic lineations have been found to parallel the ridge axis In deep water north of 50°N, almost irrespective of geographic location, seismic-refraction results more closely resemble those from the Mid-Atlantic Ridge than those from ocean basins In only one instance has a velocity as high as 8 km/sec been reported Velocity below the deepest interface is commonly 74-77 kim/sec In contrast to the zero or negative free-air gravity anomalies elsewhere in the Atlantic, a broad zone with free-air anomalies greater than +20 mgal spreads north of 25-30°N along the ridge, possibly extending over Iceland, the Denmark Strait, and the Iceland-Faeroe Ridge South of Iceland the sediment cover in the North Atlantic is greater west of the ridge than it is on the east, and is thin or absent over the axial zone The ocean-floor spreading rate inferred from magnetic data is lower than elsewhere, being of the order 1 cm/yr There is some indication of a magnetic pattern similar to that observed over midocean ridges paralleling the buried ridge structure in the Labrador Sea and Davis Strait Anomalies are most apparent away from the center of the Mid-Atlantic Ridge, and are small or absent near the axis This situation suggests that, although spreading may have occurred there, the region is now quiescent In summary, the region north of 50°N is characterized (1) by the almost complete absence of sub-moho velocities of 79-85 km/sec; (2) by a broad zone of positive free-air anomalies; (3) by widespread volcanism; (4) by magnetic anomalies in the ridge zone indicating a low rate of spreading that was rejuvenated recently; and 5) by topography strongly reflecting the presence of nearby landmasses and the ridge structure