The Mid-Atlantic Ridge near 45° N. XI. Seismic velocity, density, and layering of the crust
TL;DR: In this paper, a study of the velocities of these layers at in situ pressures shows that they correlate well with the seismic layering of the oceanic crust of vesicular and massive basalts, metabasalts, meta-gabbroses, and serpentinites.
Abstract: Compressional wave velocities at pressures to 1000 kg/cm2 and densities are given for a representative suite of rocks selected from 42 dredge hauls on the Mid-Atlantic Ridge near 45° N. The spectrum of rocks studied includes vesicular and massive basalts, metabasalts, meta-gabbros, and serpentinites. Evidence is presented for block faulting of an originally continuously layered crust of vesicular basalt and massive basalt underlain by a metamorphosed basalt and gabbro sequence. A study of the velocities of these layers at in situ pressures shows that they correlate well with the seismic layering of the oceanic crust. The velocity of the massive basalt layer is in agreement with that reported for layer 2. The underlying layer, consisting of low-to-medium grade metamorphosed basalts and gabbros (greenstones and greenschists) exhibits higher velocities. None of these exceed 6 km/s but it is suspected that these rocks at greater burial depths will exhibit velocities comparable to those of layer 3. The occurre...
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TL;DR: In this paper, the authors proposed a model consistent with geophysical observations of heat flow, seismicity, gravity, and seismic structure at the ridge, and suggested that layer 3 is composed of hornblende metagabbro underlain by normal gabbroids.
Abstract: Vp from 6.g to 6.5 km/s within gO m.y.; both the mantle and layer 3 are statistically anisotropic. Dredge lithologies consist predominantly of serpentinized ultramafics and mafic igneous rocks ranging from basalt to gabbro, the gabbro often showing evidence of fractionation. Metamorphism of mafic rocks from zeolite to amphibolite facies grade is common. Velocities in oceanic serpentinites and basalts are generally lower than layer 3 refraction velocities. Unaltered gabbros have compressional wave velocities of approximately 7.0 km/s, which is high for layer 3, together with shear wave velocities V, of 3.g km/s and values of Poisson's ratio Uof 0.30. Metabasites containing hornblende and plagioclase have values of Vp = 6.g km/s, V, = 3.g km/s, and u = 0.2g, in good agreement with those of layer 3. On the basis of petrology and velocity it is suggested that layer 3 is composed of hornblende metagabbro underlain by normal gabbro. In a model consistent with geophysical observations of heat flow, seismicity, gravity, and seismic structure at the ridge it is proposed that layer 2 and the upper levels of layer 3 form near the median valley but that deeper levels of layer 3 thicken for 40 m.y. by intermittent offridge intrusion fed from the underlying anomalous mantle. Ophiolites in such a model represent segments of thin immature ridge crest obducted onto continental margins during subduction of a spreading ridge.
472 citations
TL;DR: In this article, the authors estimated the flow rate of sea water through sub-sea-floor hydrothermal systems at mid-ocean ridges by considering the rate at which circulating fluid must advect heat out of the spreading plates into the oceans.
Abstract: The flow rate of sea water through sub-sea-floor hydrothermal systems at mid-ocean ridges has been estimated at $$1.3-9 \times 10^{17} g/yr$$ by consideration of the rate at which circulating fluid must advect heat out of the spreading plates into the oceans. The rate of hydrothermal heat advection was obtained by computing the difference ($$40 \pm 4 \times 10^{18} cal/yr$$) between the theoretical heat production associated with sea-floor spreading and observed heat flow measurements. Effects of exothermic chemical reactions, direct heat loss from flows extruded on the ocean floor, and heat of crystallization of basalt were minor and yielded insignificant contributions to the estimate. The majority of dredged ocean-floor metamorphic rocks appear to represent the products of intensive hydrothermal reaction with hot sea water. The chemical trends (loss of Ca and K, gain of Mg, Na, and $$H_{2}O$$) and alteration mineral assemblages of these rocks closely resemble those observed in both laboratory alteration...
375 citations
TL;DR: In this paper, the transport rates for H2O and Cl between the mantle and surface reservoirs were investigated, taking into account alteration of oceanic crust, especially that of plutonic rocks, and possible subduction of sediments.
285 citations
TL;DR: The 18O/16O ratios of fresh unmetamorphosed basalts from the Reykjanes, Mid-Atlantic, and Gorda Ridges and the East Pacific Rise fall in a narrow range of 5.5-5.9
Abstract: The 18O/16O ratios of fresh unmetamorphosed basalts from the Reykjanes, Mid-Atlantic, and Gorda Ridges and the East Pacific Rise fall in a narrow range of 5.5–5.9‰ (SMOW), identical to those of bas...
284 citations
TL;DR: In this article, the authors show that the plastic upward flow of asthenosphere at mid-ocean ridge crests implies a linear basaltic magma chamber at the base of the ocean crust, and that it is asymmetric, with lavas and cumulates dipping towards the spreading center, and dykes predominantly chilled away from the spreading centre.
Abstract: Summary
The plastic upward flow of asthenosphere at mid-ocean ridge crests implies a linear basaltic magma chamber at the base of the ocean crust at ridge crests. This in turn implies a mature oceanic crust composed of a unit of lavas and dykes, overlying a unit of isotropic gabbro, formed by upward cooling of the magma chamber, overlying a unit of cumulate gabbros and ultramafics, overlying residual mantle from which the basaltic magma has been removed. Analysis of this structure shows that it is asymmetric, with lavas and cumulates dipping towards the spreading centre, and dykes predominantly chilled away from the spreading centre. Increase in spreading rate leads to an increase in lava dips, to a separation of lavas and dykes into distinct units, to a thinning of the dyke unit, and to a narrowing of individual dykes. Metamorphic facies and seismic layering may, but do not necessarily, correspond with this lithological layering. The degree of rifting of oceanic crust and the depth of the median valley are related to an interplay of spreading rate and viscosity of the rising asthenospheric material. Measurement of such parameters as direction and extent of one-way chilling of dykes, dyke thickness, lava dip, and degree of separation of lavas and dykes, in sequence-type ophiolite complexes should allow the determination of direction and rate of spreading at the mid-ocean ridges at which they were formed. The model implies cogenetic liquid-solid relationships between ocean-floor rocks, and suggests changes in the shape of the liquid-solid phase diagram over the pressure range 0.5–2 kb, as well as extensive development of ocean-floor metamorphic rocks.
236 citations
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TL;DR: The velocity of compressional waves has been determined by measurement of travel time of pulses in specimens of rock at pressures to 10 kilobars and room temperature as mentioned in this paper, mainly igneous and metamorphic rocks, furnished three specimens oriented at right angles to one another.
Abstract: The velocity of compressional waves has been determined by measurement of travel time of pulses in specimens of rock at pressures to 10 kilobars and room temperature. Most of the samples, mainly igneous and metamorphic rocks, furnished three specimens oriented at right angles to one another. The present paper gives experimental details, modal analyses, and numerical tables of velocity as function of direction of propagation, initial density, and pressure. Discussion of various aspects of the measurements is reserved for part 2.
2,185 citations
TL;DR: In this paper, a crustal section across the Puerto Rico Trench, from 450 km north to 250 km south of San Juan, was deduced from seismic refraction and gravity data.
Abstract: A crustal section across the Puerto Rico Trench, from 450 km north to 250 km south of San Juan, was deduced from seismic refraction and gravity data. The result, a refinement of previous work, was made possible through more complete seismic refraction coverage and a program for high-speed electronic computation of two-dimensional gravity problems. On the basis of refraction data, the crust was divided into five layers having compressional wave velocities of 1.54, 2.1, 3.8, 5.6, and 7.0 km/sec. Densities taken from a density-velocity curve compiled by Nafe and Drake are 1.03, 2.0, 2.4, 2.7, and 3.0 gm/cc, respectively. Depths to the Mohorovicic discontinuity were computed from the gravity data using sub-crustal density of 3.4 gm/cc, which corresponds to 8.2 km/sec on the density-velocity curve. (A similar calculation was made using a sub-crustal density of 3.3 gm/cc). Depth to M under the trench is about 20 km decreasing sharply on both sides. Northwards, it reaches a minimum of about 10 km under the Outer Ridge and then deepens gradually to about 13 km beneath the southern margin of the Nares Basin. South of the trench M rises under the Puerto Rico Shelf to about 17 km and then deepens sharply to about 30 km beneath Puerto Rico. South of Puerto Rico the depth decreases again to about 14 km under the Venezuelan Basin.
Depths to M were also obtained by using Airy isostatic anomalies and assuming constant crustal density of 2.67 gm/cc over a mantle of density 3.27 gm/cc. The crustal section thus deduced differs significantly from that obtained when the density structure within the crust was considered.
280 citations
TL;DR: In this article, it was shown that the mean crustal density of the continental crust is essentially constant (2.85 gm/cc to 2.88 gm /cc).
Abstract: Gravity data indicate that there is a regular relationship between crustal structure, crustal density (composition), and surface elevation. Earthquake and surface seismic refraction and reflection evidence as to the composition and structure of the earth's crust have not yielded a simple, unambiguous relationship to the surface elevation. The velocity dispersion of earthquake surface waves, on the other hand, indicates variations in the thickness and composition of the crust that are in general accord with the variations in surface elevation and the Bouguer gravity anomalies. Why seismic refraction measurements have not agreed everywhere with gravity and surface wave indications of crustal structure appears to be a result of masking of crustal layering. On the basis of the slope of the curve that describes the relationship between the seismic depth to the Mohorovicic discontinuity and Bouguer gravity anomalies, the density difference between the crust and the mantle appears to decrease as the thickness of the crust increases. On the assumption that the mantle has a constant mean density of 3.32 gm/cc, the mean crustal density would appear to increase from a minimum value of 2.86 gm/cc in the oceans to about 3.08 gm/cc beneath the high plateaus and mountains. If the mean crustal density is essentially constant, the effective density of the mantle must decrease by a comparable amount. The existence of a low-density zone in the upper part of the mantle, as suggested by the velocity dispersion of very long period Rayleigh waves, would explain the relationships observed. Isostatic relationships suggest that the mean density of the continental crust is essentially constant (2.85 gm/cc to 2.88 gm/cc). These values imply that a basaltic layer is present everywhere. That there is possibly an increase in mean crustal density as the crust thickens is suggested by U.S.S.R. seismic studies in Central Asia. These show that the intermediate (basaltic) layer is usually thicker beneath areas of uplift. Although the origin of the basaltic layer can only be surmised, its general inhomogeneity, as indicated by variations of seismic velocity from 6.4 to 7.3 km/sec, and its varying thickness suggest that it may be a zone of phase transformation within the underlying mantle rock. Despite the lack of homogencity in the crust, it appears possible that empirical relationships may be used to predict approximate crustal thickness from the regional Bouguer gravity anomalies or from surface elevations with a reliability approaching that for seismic measurements.
241 citations
TL;DR: A strip across the Crest Mountains and High-Fractured Plateau of the Mid-Atlantic Ridge has been surveyed systematically between latitudes 45 and 46° N.A..
Abstract: A strip across the Crest Mountains and High-Fractured Plateau of the Mid-Atlantic Ridge has been surveyed systematically between latitudes 45 and 46° N. Continuous bathymetric, magnetic and gravimetric data have been obtained. Seismic refraction experiments have revealed a complex structure lineated parallel to the axis of the Ridge. Seismic reflexion studies have revealed a picture of the sediment cover, and have shown the possible existence of block faulting of the underlying rocks, with faults alined both parallel and at right angles to the axis of the Ridge. The major rock types found in 46 dredge stations can be grouped as follows: (1) Ubiquitous basalts and tuffs (ranging from theoleiites to alkali basalts, with a few ferro-basalts and high-Al basalts). Basalts rich in resorbed high-calcic plagioclase xenocrysts are common; these occur both on the slopes of shield volcanoes and in the deepest hole of the Median Valley. A nearby fault scarp yielded coarse-grained gabbros. (2) Serpentinized mafic and ultramafic rocks are not restricted to elongated, presumably blockfaulted seamounts, but are also common on the slopes of what had been interpreted on morphological grounds as shield volcanoes; they are absent, however, on the Median alley floor and its immediate scarp slopes. The pre-serpentinization rock types include dunites, harzburgites, gabbros, troctolitic gabbros and amphibolitic peridotites showing crude cumulate textures. (3) The lower parts of the steep inner walls of the Median Valley have yielded metabasalts and metadiabases showing alteration within the greenschist facies of etamorphism, whilst still retaining original igneous characteristics. (4) Restricted to the fault scarps of elongated seamounts further removed from the Median Valley are higher grade metamorphic rocks of the almandine amphibolite facies of metamorphism. These rocks have lost all igneous textures and exhibit a strong gneissic fabric. (5) Three localities yielded dioritic rocks in association with serpentinized ultramafics. The diorites vary in character from hornblende-rich quartz diorites to more siliceous, almost hornblende-free trondhjemites. The latter show considerable albitization. The whole suite of rocks shows great affinities with similar suites found as late stage intrusives in alpine-type ultramafic complexes. About 23% of the specimens collected included gneissic, granitic and sedimentary rock types of erratic origin, ice rafted into the area in the Pleistocene. A study of their distribution indicates that there are no erratics in the Median Valley, that they are scarce on the mountain ranges immediately flanking the Valley, but beyond these areas they are abundant and are randomly distributed over the whole area. Such a distribution may be a result of ocean-floor spreading, indicating that the Median Valley is younger than the last ice age, or that extrusions subsequent to the last ice age have engulfed any erratics present in the Median Valley. The thickness of manganese coating on extrusive rocks and their K/Ar and fission track ages increase systematically with distance on either side of the axis of the M.A.R., strongly supporting the ocean-floor spreading hypothesis. The ages and coatings both show a marked change in their rate of increase beyond a distance of 50 to 60 km on either side of the axis. The position at which this occurs coincides with the thickening in these areas of sediments found in the inter-volcanic valleys, and the morphological changes between the Crest Mountains and the High-Fractured Plateau. The combined data strongly suggest that there was either a quiescent period sometime in the Pliocene during which ocean-floor spreading was inactive, or that the rate of spreading had accelerated during the Pliocene from less than 1 cm a -1 to a computed 2.5 cm a -1 in Pleistocene times.
102 citations