Showing papers on "Mid-ocean ridge published in 1973"

Stresses in the lithosphere caused by crustal thickness inhomogeneities

Abstract: A physical analysis is given of the state of stresses in the earth's lithosphere floating on the asthenosphere. Crustal thickness strongly varies in a horizontal direction and thus creates large variations in the potential energy stored in the crust. As a result, the crustal material tends to spread and to reduce crustal thickness inhomogeneities. A similar situation occurs where the masses of low-density mantle are located under the crust. Large nonhydrostatic stresses arise in the lithosphere from that tendency, even if the rate of spreading is very low. The main stresses are caused by regional and global crustal thickness inhomogeneities. Their magnitudes are of the order of the additional loads associated with the regional and global relief of the crustal surface. The stresses inherent in the variations in the thickness of crustal and low-density mantle layers form a global system. Especially high stresses of the order of thousands of bars exist in the regions of high uplifts on the continents. They may be both tensile and compressive, depending on the position of uplifts with respect to the boundaries between the lithospheric plates. They are responsible for rock deformations and earthquakes. The state of stress in the oceanic lithosphere is mainly defined by the low-velocity mantle distribution beneath the mid-ocean ridges. Compressive stresses of the order of a few hundred bars exist in the lithosphere in the oceanic basins. They may be responsible for sea floor spreading and continental drift. The stresses in the lithosphere produced by other mechanisms such as thermal convection and the lithosphere's sinking into the mantle are also considered. Crustal and low-density mantle thickness inhomogeneities represent the main factor determining the stresses in the lithosphere, and the plate motions are mainly caused by the spreading of low-density mantle material rising below the oceanic ridges. It is shown that the pressure of the oceanic lithosphere on the continents caused by sea floor spreading cannot produce folded mountains on the continents.

A simple mechanical model for oceanic spreading centers

Abstract: In a simple model for a spreading oceanic ridge, it is assumed that viscous material rises as a result of gravitational instability in a vertical-walled cleft between steadily diverging lithosphere plates of uniform thickness. The requirement that the walls must accrete at the same rate as they spread permits a simple discussion of the energetics of the conduit. A numerical example suggests that the rate of mechanical energy dissipation in the conduit may be comparable on a global scale to the total rate of seismic energy release. This much energy is available to drive or resist plate separation at oceanic ridges, depending on the vertical distribution of pressure in the conduit. If the mean pressure at the conduit wall were greater than the mean lithostatic pressure in the lithosphere, tectonic compression would develop in the spreading direction, dissipation in the conduit would decrease, and the energy saved would go into driving plate separation. If the mean pressure on the conduit wall were less than mean lithostatic, tectonic tension would develop. It would resist plate separation and would increase energy dissipation in the conduit accordingly. If the effective viscosity decreased upward in the conduit from values characteristic of the asthenosphere to values characteristic of crustal intrusions, the mean conduit pressure would be low, and ridges would offer substantial resistance to plate separation (∼300 bars in the numerical example). The model provides a relation between mean viscosity, conduit dimensions, and spreading velocity. For lower viscosities, much narrower conduits are required. An extension of the model to describe viscous drag on the conduit wall accounts for uplift adjacent to spreading centers and the occurrence of axial valleys for slow spreading and axial horsts or volcanic piles for rapid spreading. The model can evidently account for axial relief and its change with velocity in general agreement with observations. Reasonable conduit widths result if the relaxation time of the nearby lithosphere is assumed to be on the order of a million years. The stair-step offsets of oceanic ridges by transform faults suggest that the ridges offer much greater resistance to plate separation than the transforms. This is consistent with the present model, if transforms are loosely coupled with a confined fluid phase at depth and if their resistance is primarily in a superficial seismogenic zone. It is speculated that a similar decoupling mechanism might operate along the base of the lithosphere, with important implications for plate dynamics.

Magnetic Telechemistry of Oceanic Crust

Abstract: IT is now established that basalts from Iceland1–3, the Galapagos Islands4, and other volcanic island and seamount5,6 chains in general, are richer in iron and titanium than normal abyssal tholeiites produced by seafloor spreading on the Mid-Oceanic Ridge. According to present thought, these chains are the volcanic derivatives of mantle plumes over which the lithospheric plates make their way7. Schilling2 recently showed progressive decreases in minor and trace element concentrations along the Reykjanes Ridge south west away from Iceland; he interprets this as the result of mixing of a plume and a normal, depleted asthenosphere below the spreading axis, as the Icelandic asthenosphere travels southwards in a pipe or conduit below the spreading axis8. The model has been challenged9, but the geochemical variations seem to be characteristic of plumes the world over. From an economic point of view, mid-oceanic hydro-thermal muds and brines formed near plume centres would seem best suited as mineral reserves because of the inherently higher concentration of various metals in the upper crust. Is there any way to make a rapid assay of the ocean crust to identify such regions without tedious dredging and chemical analyses?

Origin of the Ninety East Ridge from studies near the equator

Abstract: A part of the Ninety East ridge near the equator was examined in 1971 by seismic profiling and gravity and magnetic observations. In the area examined, the topography of the ridge consists of blocklike or en echelon mountainous masses. A fracture zone trending north-south parallel to the overall trend was found along the eastern margin of the ridge topography. This fracture zone probably marks the principal boundary between the central Indian Ocean plate and the Wharton basin plate. The free air gravity anomalies associated with the Ninety East ridge are small, and thus the mass of the ridge must in some way be compensated at depth. The Ninety East ridge may have originated as a result of emplacement of gabbro and serpentinized peridotite beneath normal oceanic crustal layers. The lower density of the gabbro and serpentinized peridotite with respect to normal mantle at equivalent depths provides for both the uplift of the ridge and its compensation at depth.

Upper mantle structure beneath the north pacific and the marginal seas

Abstract: "Residual gravity anomalies", RGA, in the north Pacific are calculated from the explosion seismic results and examined as a function of the sea-floor age deduced from the geomagnetic anomalies. The RGA, which implies the mass anomaly in the upper mantle, increases systematically as the age increases; this suggests gradual cooling of the upper mantle away from the midocean ridges. The RGA in the Hawaii region, which is smaller than predicted by the standard RGA vs. age curve by about 100 mgal, implies hot material beneath this region. On the other hand, the RGA in the Parece Vela Basin is larger than predicted by the standard curve by about 100 mgal. This deviation can be expected if the upper mantle material beneath the marginal seas is of higher density than that beneath the normal ocean. The thickness of the "plate" inferred from the RGA is approximately proportional to the square root of the age.

Shear wave attenuation and melting beneath the Mid-Atlantic Ridge

Abstract: Because the attenuation of seismic waves is sensitive to variations in temperature and to partial melting, the mapping of seismic Q beneath the mid-ocean ridge systems is a useful tool to outline boundaries between lithosphere and asthenosphere and to constrain the mechanics of the intrusion process. Our approach is to measure the differential attenuation of long-period shear waves, using a spectral ratio technique, from earthquakes on the ridge and to look for variations in attenuation with propagation direction. We correct for propagation distance and, where known, the upper mantle attenuation beneath the receiving stations. The azimuthal dependence of attenuation of S waves from an earthquake offset from the ridge axis on a transform fault indicates the existence of a low-Q zone, no wider than 100 km and confined to depths shallower than about 50 to 150 km, beneath the crest of the mid-Atlantic ridge. The absence of appreciable azimuthal variation in shear wave attenuation for an earthquake on the ridge crest suggests that the low-Q zone is at least 50 km wide. Q within such a zone must be 10 or less for long-period S waves. The most likely explanation of such a low-Q zone of limited spatial extent and with sharply defined boundaries is that the zone is a region of extensive partial melting, probably at temperatures in excess of the anhydrous solidus of mantle material. Such a region of large melt concentration is consistent with the chemistry of rocks from the mid-ocean ridges and with models of the temperature field derived from numerical calculations of flow beneath spreading centers.

A Model for the Horst and Graben Structure of Midocean Ridge Crests based upon Spreading Velocity and Basalt Delivery to the Oceanic Crust

Abstract: Summary The East Pacific Rise crest from 17° N to 40° S is characterized by a central horst. This feature is suggested to be the steady-state product of constant spreading about a ridge with the standard deviation of dike injection rate smaller than the standard deviation of lateral strain rate. A model is developed, based only upon these two parameters, which adequately explains not only the central horst topography, but also that of central grabens. The latter feature is shown to occur within the model when the spreading velocity is so small that the standard deviation of the dike injection rate is larger than the standard deviation of the lateral strain rate. Such is believed to occur only when constant freezing and breaking of the central intrusion zone produces intermittent spreading. Thus, a ‘normal’ spreading centre has a central horst. Put simply, when material is intruded at the ridge crest over a distance greater than that required to accelerate the crestal material from zero horizontal velocity to the spreading velocity, a central graben forms. When material is intruded over a narrower distance than that required to accelerate to spreading velocity, a central horst forms.

Focal depth and mechanism of mid‐ocean ridge earthquakes

Abstract: Rayleigh wave phase and amplitude spectra are used to determine the focal depths and mechanisms of mid-ocean ridge earthquakes. The effect of propagation on the spectra is removed by analyzing the differential phase and the amplitude ratio of the Rayleigh waves from a pair of close events with different focal mechanisms. This analysis preserves the difference between the source spectra. By matching the observations with theoretical models, a best description of the source is derived. The two mid-Atlantic ridge dip slip events that we have studied have a focal depth of only 3 ± 2 km beneath the ocean floor. Two dip slip events in the northeast Pacific have depths consistent with these results but with an uncertainty of 20 km. The depths of two mid-Atlantic ridge strike slip events are 6 ± 3 km. Two strike slip events in the northeast Pacific are shallower than 25 km. The phase analysis greatly aids the amplitude analysis and, for these events, is indispensable.

Lead and Strontium Isotopic Studies in the Cascade Mountains: Bearing on Andesite Genesis

Abstract: Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement. New isotopic analyses of lead are reported for 40 volcanic rocks from the Cascade Mountains of Washington, Oregon, and California. Strontium isotopic compositions were also determined in 33 volcanic rocks from the same area. In addition, lead and strontium isotopic data are given for feldspar and whole-rock samples of prominent varieties of crystalline basement rocks from northern Washington. The Sr87/Sr86 values of the volcanic rocks average 0.7037, with no significant difference between andesite and high-alumina basalt. The ratios exhibit no measurable correlation with strontium concentrations over a range of 200 to 1,500 ppm. Strontium in the Cascade rocks is slightly more radiogenic than that in oceanic ridge basalts, but less radiogenic than in most continental basalt, including the Columbia River basalt. Comparisons with published data for strontium in Pacific Ocean sediments and coastal graywackes in Oregon indicate much higher Sr87/Sr86 values in the sedimentary rocks when compared to the Cascade volcanic rocks, thus ruling out anatectic models involving large sediment contributions. Lead isotopic compositions from the volcanic rocks are variable but tend to be rather constant at a single volcanic center. A noteworthy feature of the lead data are the higher Pb207/Pb204 values (or larger µ values in a model age diagram) for the Cascade volcanic rocks compared to published Pb207/Pb204 values and µ values for Pacific Ocean tholeiitic basalt. The Pb206/Pb204 values from the Cascade volcanic rocks are higher than in oceanic sediments from the northeast Pacific Ocean but are lower than the average ratio for three samples of Oregon coastal sedimentary rocks. There is no demonstrable difference between lead isotopes in high-alumina basalt and andesite at particular volcanos, although the lead data are more variable than the strontium isotope data. Strontium in crystalline basement rocks from northern Washington is substantially more radiogenic than that in the calcic and calc-alkalic volcanic rocks of the Cascade Mountains. The same is true of lead in some, though not all, of the basement rocks. The isotopic data are utilized, along with other trace-element data to test models of andesite genesis. The isotopic compositions of lead and strontium in andesite and high-alumina basalt from the Cascade Mountains are consistent with derivation of the two lava types from a common source, or with derivation of andesite by differentiation of high-alumina basalt magma. The data do not support models in which the Cascade calcic and calc-alkalic magmas were formed by melting of Oregon coastal eugeosynclinal sedimentary rocks, or by anatexis of basement rocks similar to those now exposed near volcanic centers in northern Washington. Various subduction zone melting models are also considered. Mixing models, whereby radiogenic strontium and lead are added to magmas containing lead and strontium with isotopic compositions similar to those in oceanic ridge tholeiite, do not fit the observed data satisfactorily because values of Pb206/Pb204 in Pacific Ocean sediments are too low to account for the Pb206/Pb204 values in the Cascade lavas. Mixtures of eugeosynclinal sedimentary rock and oceanic ridge tholeiite can probably account for the Cascade volcanic rock isotopic data in a general way, but the uniform isotopic composition of strontium in the lavas over a wide range of concentrations is hard to understand in terms of such a model. The same is true for strontium in the oceanic sediment mixing model. The uniform isotopic composition of strontium in the Cascade lavas, irrespective of strontium concentration and geologic setting, argues against contamination of magma with radiogenic strontium during ascent. A similar argument can be made for lead. The best explanation of the isotopic and trace-element data appears to be a multistage model in which orogenic andesite is derived from three or more stages of partial melting of mantle material in which crustal materials play an insignificant role. This model best explains the uniform Sr87/Sr86 values in the lavas over a wide range of strontium concentrations. It can also explain why strontium in lavas extruded at continental margins, such as in the Cascade Mountains and in Japan, has the same isotopic composition as strontium in lavas from the Mariana arc system in the Pacific Ocean. Pb206/Pb204 and Pb208/Pb204 values in Cascade lavas may show a correlation with crustal thickness, but this does not prove that crustal contamination is responsible for the trend. The lead and strontium data exhibit no correlation with the Quartz Diorite Line of Moore.

Mid-Atlantic Ridge from 47° to 51° North

Abstract: An extensive survey of the Mid-Atlantic Ridge from 47° to 51° N. has recovered a large quantity of detailed bathymetric data from the North Atlantic sea floor. Supplemented by seismic reflection and magnetic profiles, these data reveal some surprising characteristics of sea-floor spreading on the Mid-Atlantic Ridge. Closely spaced transform faults, generated about 60 m.y. B.P., extend to crust of about 20 m.y. B.P. age. Significantly, however, demonstrable transform faults are nearly absent on ocean crust younger than about 10 to 20 m.y. B.P. Possibly in response to recent increased spreading rates or increased asthenosphere flow below the spreading axis, the ridge axis has replaced the transform pattern within about the last 20 m.y. B.P. with alternating normal and oblique spreading axes. The normal spreading segments are generating a basement level 0.5 to 1 km higher than that produced by the oblique segments, probably because of greater viscous head losses for magma rising within oblique rifts. The late Tertiary spreading pattern of normal and oblique rift segments has not been stationary with respect to the transform direction but has migrated southward at 1 or 2 mm per yr, possibly a measure of slow southward subaxial motion in the asthenosphere. A problem with this hypothesis is that the rift-valley floor and the regional bathymetry seem to be shoaling very gradually (1:1,000) toward the Azores, which suggests a northward-directed pressure gradient in the asthenosphere. Although the top of the oceanic basement in the northeastern Atlantic deepens with increasing age, there are large depth anomalies with respect to crustal subsidence in the eastern Pacific. Crust of ages 60 to 50 and 10 to 0 m.y. B.P. is elevated about 1 km above coeval Pacific crust, with lesser anomalies occurring elsewhere. Because the highest basement levels seem to correlate with probable times of most intense plume convection, it is inferred that a lithosphere of lower density or thicker crust is formed at such times.

Viscous Remanent Magnetization in Oceanic Basalts

Abstract: THE magnetic properties of the basalts which form layer 2 of the oceanic lithosphere are important because of their relevance to the hypothesis1 of seafloor spreading. Most studies of these magnetic properties have been carried out on basalts obtained from dredge hauls taken predominantly from ocean ridge systems and fracture zones. These constitute special areas of the oceanic crust where the sediment cover is negligible. It is of interest to compare the magnetic properties of the dredged basalts with samples recovered from holes drilled through the overlying sediments into the basaltic layer at places distant from ridge axes. Samples obtained from the abandoned Mohole project and, more recently, from the Deep Sea Drilling Project (DSDP) possessed magnetic properties similar to those of dredged basalts2,3. Here I describe highly unstable magnetic characteristics found in basalts from DSDP hole 57.

A Topographic Interpretation of the Mathematician Ridge, Clipperton Ridge, East Pacific Rise System

Abstract: WITHIN the framework of plate tectonics, dating of the ocean floor has been almost exclusively achieved by studies of magnetic anomalies. But a north–south striking mid-ocean ridge system generating new seafloor in the equatorial region produces magnetic anomalies which are of small amplitude and are frequently distorted and disguised by magnetic “noise”. The correspondence between the absolute depth of oceanic crust and its age1 has provided an alternative method of age dating the oceanic crust anywhere in the world's oceans, provided, of course, that the crust was generated by a seafloor spreading system. Such a correlation tool has been used on the East Pacific Rise (Fig. 1) between 20° N and the equator1 and between 0° and 20° S (ref. 2).

Compressive Stress between Two Mid-Ocean Ridges

Abstract: ONE of the possible driving mechanisms of plate motions1–3 is a push from mid-ocean ridges. This could arise from the injection of magma at the ridge crest4, or to hydrostatic overpressure resulting from the elevation of the ridge above the ocean basins5. If the plates are pushed apart at ridges, an area between two mid-ocean ridges should be under compression. On May 9, 1971, an earthquake of magnitude 6.2 (mb reported by NOAA) occurred within the Antarctic plate between the Chile Ridge and the Pacific–Antarctic Ridge, followed within 24 h by several aftershocks of magnitude 5.0 or greater (Fig. 1). An analysis of the focal mechanism of the principal shock shows that the plate is at least locally under horizontal compressive stress, with the maximum stress axis approximately perpendicular to the Pacific–Antarctic Ridge.

Temperatures and compositions of magmas ascending along mid‐ocean ridges

Abstract: The application of plagioclase geothermometry to plagioclase-bearing volcanic ash layers and to the glassy margins of pillow basalts from the fast-spreading East Pacific rise, the moderately spreading Gorda and Juan de Fuca ridges, and the slow-spreading mid-Atlantic ridge has shown that magma temperatures, as well as average An contents of plagioclases, are negatively correlated with spreading rates. A detailed investigation of the major element chemistry of volcanic glasses from each of these areas suggests that the observed consistent element-element covariances among individual populations of samples have been caused by fractional crystallization of the magmas. The regularity of chemical variation and the similarity of magma temperatures within each population of samples suggest that magmas ascending from beneath each ridge have had similar evolutionary histories. Vector analysis of the chemical data of all samples of volcanic glasses indicate that each population of samples from each of the spreading centers is chemically distinct, even though all samples have been subjected to similar amounts of fractional crystallization. The compositional distinctiveness of each population of oceanic tholeiites probably reflects differences in the depths at which the magmas were generated. Calculated magma temperatures and geothermal gradients calculated from published heat flow measurements can be used to estimate depths of magma generation of about 16 km beneath the East Pacific rise and about 23 km beneath the mid-Atlantic ridge.

Limits of sediment involvement in the genesis of orogenic volcanic rocks

Abstract: The isotopic composition of Pb and Sr and the abundance of trace elements in oceanic sediments provide several independent mechanisms for determining the limits of oceanic sediment involvement in andesite genesis. These data, particularly the Th/U values and the Sr87/Sr86 values, indicate that oceanic sediment involvement is probably limited to 2% or less for andesitic magmas in the Cascade Mtns. The isotopic and chemical data on continental sedimentary rocks limit their involvement to 10% or less. The isotopic composition of Pb in all calcic and calc-alkaline or orogenic volcanic rocks however does not depend on crustal materials, and cannot be accounted for by mixing oceanic sediment and oceanic ridge basalt leads in all cases. One is not justified in using the lead growth curve for oceanic sediment as a convenient and arbitrary end member in these mixing model calculations, and therefore the model of Armstrong (1971) does not appear to offer a universal solution. It appears that “alkali basalt-like leads” may play an important role in the genesis of andesitic rocks.

Middle Cretaceous Nappe Structures in Puerto Rican Ophiolites and Their Relation to the Tectonic History of the Greater Antilles: Reply

Abstract: Radiolarian chert, serpentinized peridotite, pillow lava, and metamorphosed tholeiitic rocks form an ophiolite association in southwestern Puerto Rico These are the oldest rocks in Puerto Rico and apparently represent exposed oceanic crust Relations between the lithologies are complex; there is an older complex of serpentinized peridotite and metatholeiite 110 my old or older, an intermediate pillow lava and felsite association, and a younger radiolarian chert formation of middle Cretaceous (Albian) or older age A gravitational nappe structure, showing northward movement probably of Albian or earlier age, is composed of a disharmonically deformed sheet of radiolarian chert Serpentinite occurs in small lenses along the sole of the nappe and probably served as a lubricant for movement of the nappe A second nappe is suggested, on less extensive data, to have moved Campanian-Maestrichtian pelagic limestones northward over the older chert nappe This movement occurred in Maestrichtian or later time and is perhaps synchronous with smaller northward slides of the same formation during the Maestrichtian about 20 km to the north Other northward slides occurred in early Tertiary time in several places in southern Puerto Rico Northward gravity transport of the nappes and other slides is opposite to the sense of tectonic transport indicated by northward-dipping axial planes in the volcanic and volcano-sedimentary rocks in central Puerto Rico Similar relations exist in Cuba where thrust and gravity slides show northward motion toward and into the volcanic pile Gravity transport may have been from an oceanic ridge northward toward a trench, with volcanism developing between the two structures Early buckling of lithosphere plates into an oceanic ridge and complementary trench, before rupture and subduction, may provide the necessary relief for gravitational trench-ward slides A speculative geologic history of the Greater Antilles contains three stages: (1) pre-Albian buckling of the lithosphere into ridge and trench, with gravitational sliding of crustal rocks toward the trench; (2) rupture of the lithosphere and Albian-Campanian (or Maestrichtian) subduction, forming volcanism between ridge and trench; (3) choking of the Benioff zone by continental lithosphere of the northern plate, and further Maestrichtian to Eocene movement resolved along lateral faults with fault-controlled volcanism along an ancestral Cayman trough–Puerto Rico trench trend

Volcanic production rates: comparison of oceanic ridges, islands, and the columbia plateau basalts.

Abstract: New potassium-argon age data from the Columbia Plateau suggest a basalt production rate of 108 cubic meters per year during a middle Miocene volcanic episode. This is two to three times the production rate in some oceanic islands, and about four to six times the production rate in spreading mid-oceanic ridge systems.

North Atlantic Oceanic Topography and Lateral Variations in the Upper Mantle

Abstract: Summary A thermal model of sea-floor spreading applied to the northern Mid-Atlantic Ridge at several latitudes suggests that lateral variations of upper mantle structure in both the asthenosphere and the lithosphere may exist beneath the ocean, extending from Iceland to the Azores. The variation of ocean ridge cross-sectional area and the general shallowing of the sea towards the north which occur in the Atlantic may be explained by sea-floor spreading taking place above an asthenosphere whose temperature at a given depth increases northwards, with Iceland perhaps the focus of the thermal field. The relationships between asthenospheric temperatures, lithospheric thicknesses and ocean ridge dimensions derived here should be applicable to studies of other ridges.