Seismic techniques provide the highest-resolution measurements of the structure of the crust and have been conducted on a worldwide basis. We summarize the structure of the continental crust based on the results of seismic refraction profiles and infer crustal composition as a function of depth by comparing these results with high-pressure laboratory measurements of seismic velocity for a wide range of rocks that are commonly found in the crust. The thickness and velocity structure of the crust are well correlated with tectonic province, with extended crust showing an average thickness of 30.5 km and orogens an average of 46.3 km. Shields and platforms have an average crustal thickness nearly equal to the global average. We have corrected for the nonuniform geographical distribution of seismic refraction profiles by estimating the global area of each major crustal type. The weighted average crustal thickness based on these values is 41.1 km. This value is 10% to 20% greater than previous estimates which underrepresented shields, platforms, and orogens. The average compressional wave velocity of the crust is 6.45 km/s, and the average velocity of the uppermost mantle (Pn velocity) is 8.09 km/s. We summarize the velocity structure of the crust at 5-km depth intervals, both in the form of histograms and as an average velocity-depth curve, and compare these determinations with new measurements of compressional wave velocities and densities of over 3000 igneous and metamorphic rock cores made to confining pressures of 1 GPa. On the basis of petrographic studies and chemical analyses, the rocks have been classified into 29 groups. Average velocities, densities, and standard deviations are presented for each group at 5-km depth intervals to crustal depths of 50 km along three different geotherms. This allows us to develop a model for the composition of the continental crust. Velocities in the upper continental crust are matched by velocities of a large number of lithologies, including many low-grade metamorphic rocks and relatively silicic gneisses of amphibolite facies grade. In midcrustal regions, velocity gradients appear to originate from an increase in metamorphic grade, as well as a decrease in silica content. Tonalitic gneiss, granitic gneiss, and amphibolite are abundant midcrustal lithologies. Anisotropy due to preferred mineral orientation is likely to be significant in upper and midcrustal regions. The bulk of the lower continental crust is chemically equivalent to gabbro, with velocities in agreement with laboratory measurements of mafic granulite. Garnet becomes increasingly abundant with depth, and mafic garnet granulite is the dominant rock type immediately above the Mohorovicic discontinuity. Average compressional wave velocities of common crustal rock types show excellent correlations with density. The mean crustal density calculated from our model is 2830 kg/m3, and the average SiO2 content is 61.8%.

Seismic velocity structure and composition of the continental crust: A global view

Expressions are derived for the vertical and horizontal components of the gravitational attraction due to a two-dimensional body of arbitrary shape by approximating it to an n-sided polygon. These expressions are put in forms suitable for solution by a high-speed digital computer. As an example of the application of this method, the crustal section across the Mendocino fracture zone is deduced from the gravity anomalies. Assuming the crust to consist of a single homogeneous layer, overlain by water and sediment, it is found to be about three km thicker to the north of the fracture zone than to the south of it.

Rapid gravity computations for two‐dimensional bodies with application to the Mendocino submarine fracture zone

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.

Mountain belts and the new global tectonics

Cosmology at low frequencies: The 21 cm transition and the high-redshift Universe

Seismic refraction results show that the igneous section of oceanic crust averages 7.1±0.8 km thick away from anomalous regions such as fracture zones and hot-spots, with extremal bounds of 5.0–8.5 km. Rare earth element inversions of the melt distribution in the mantle source region suggest that sufficient melt is generated under normal oceanic spreading centers to produce an 8.3±1.5 km thick igneous crust. The difference between the thickness estimates from seismics and from rare earth element inversions is not significant given the uncertainties in the mantle source composition, though it is of the magnitude that would be expected if partial melt fractions of about 1% remain in the mantle and are not extracted to the overlying crust. The inferred igneous thickness increases to 10.3±1.7 km (seismic measurements) and 10.7±1.6 km (rare earth element inversions) where spreading centers intersect the regions of hotter than normal mantle surrounding mantle plumes. This is consistent with melt generation by decompression of the hotter mantle as it rises beneath spreading centers. Maximum inferred melt volumes are found on aseismic ridges directly above the central rising cores of mantle plumes, and average 20±1 and 18±1 km for seismic profiles and rare earth element inversions respectively. Both seismic measurements and rare earth element inversions show evidence for variable local crustal thinning beneath fracture zones, though some basalts recovered from fracture zones are indistinguishable geochemically from those generated on normal ridge segments away from fracture zones. This is consistent with a model where the melt generated beneath spreading ridges is redistributed to intrusive centers along the ridge axis, from where it may flow laterally along the axis at crustal or surface levels. The melt may sometimes flow into the bathymetric lows associated with fracture zones. Oceanic crust created at very slow-spreading ridges, and in regions adjacent to some continental margins where rifting was initially very slow, exhibits anomalously thin crust from seismic measurements and unusually small amounts of melt generation from rare earth element inversions. We attribute the decreased mantle melting on very slow-spreading ridges to the conductive heat loss that enables the mantle to cool as it rises beneath the rift.

Oceanic crustal thickness from seismic measurements and rare earth element inversions

In a study of the dependence of the velocity of compressional waves in marine sediments upon the thickness of overburden, the velocity‐depth relationship in shelf sediments is shown to be distinctly different from that in deep basin sediments. The difference between the two cases may be illustrated by comparing the straight lines that best represent the data. These are V=1.70Z+1.70, shallow water, V=0.43Z+1.83, deep water where V is in km/sec and Z is in kilometers. Shallow and deep water are defined arbitrarily to be under 100 fathoms and over 1,500 fathoms respectively. The observed variation of average compressional velocity in the shallow and deep water sediments, taken together with the known limited range of variation of velocity for a given porosity, yields limits in turn upon the porosity‐depth dependence in the two environments. It is shown that at the same depth of overburden porosity is much greater in deep water sediments than in shallow. A physical argument is presented to show that there is ...

Variation with depth in shallow and deep water marine sediments of porosity, density and the velocities of compressional and shear waves

: The unconsolidated sediments that blanket the ocean floor are of widely varying thickness but seismic observations indicate that 200 to 400 meters in the Pacific and one kilometer in the Atlantic are fairly typical values for deep water. At present direct observation of these sediments is limited to such samples as may be recovered by dredging or coring operations, for drilling has been carried out only in the shallow waters of the coastal shelves. Knowledge of the physical properties of the great bulk of the sediments deeper than the few tens of feet reached by coring equipment is thus necessarily derived from geophysical observations.

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Physical properties of marine sediments

The relationship between the sound velocity and density in various oxide compounds at atmospheric pressure is relevant to problems of the earth's interior. Here, data on elastic constants of various compounds are collected and analyzed. It is shown that the bulk modulus-volume per ion pair relationship for oxide compounds differs in a remarkable degree from that found for alkali halides, fluorides, selenides, sulfides, and covalent compounds. It is shown that a change of volume has the same effect on the bulk modulus of oxide compounds, whether the volume change is produced by pressure, compositional variation, phase changes, temperature, or porosity. It thus appears that volume is the primary variable affecting the elastic moduli of oxide compounds, and all other variables affect the moduli only insofar as they affect the volume itself.

The bulk modulus-volume relationship for oxide compounds and related geophysical problems

Results of twenty-two seismic refraction stations over the northern mid-Atlantic ridge are combined with earlier data to obtain a better definition of the gross features of the ridge. The seismic crustal structure is found to be the same at equal water depths. The crust on the flanks is an uplifted ocean basin crust which does not thicken when water depth decreases. The relation between water depth, w km, and crustal thickness, C km, derived from the data is C = 0.80w + 1.20. It indicates that in the absence of a compensating mass within the mantle one would expect a positive gravity anomaly of 120 mgal per km of height of the ridge above the basin floor. In the axial zone, a 5.8-km/sec layer overlies a 7.3-km/sec ‘upper mantle.’ Three different areas in the North Atlantic Ocean are compared with the east Pacific rise section and a genetic relationship is inferred from these comparisons. The northern mid-Atlantic ridge is assumed to be in a later stage of evolution, the thickness and velocity of the basement layers being considered the best indicators of the degree of maturation of the mid-ocean ridges.

Crustal structure of the mid‐ocean ridges: 1. Seismic refraction measurements

A disturbance at one point of a dispersive medium resulting from an impulse applied at another point may be represented as a superposition of traveling plane waves. The phase and period of the disturbance at any instant are related by the principle of stationary phase to the phase and period of a traveling wave component. For the instantaneous phase of that traveling wave component the following equation may be written.

Ct-x=(N-ϕ0/2π)CT

where C is the phase velocity, x the distance, T the period, t the travel time, N an integer, and ϕ0 the initial phase of the traveling wave component. Since t and T may be measured from a record of the disturbance and x may be determined, the equation may be used to compute the phase velocity as a function of period, if the initial phases are known. If distance and the dispersion are known, initial phases may be determined. From distance, initial phases, and phase velocities the disturbance at any point may be constructed. The practical use of the method is demonstrated by application to antisymmetric waves in a cylindrical rod, Rayleigh waves from United States and Russian nuclear explosions, Rayleigh waves from the Hudson Bay earthquake of January 30, 1959, and Love waves from the Fairview Peak and Fallen, Nevada, earthquakes of 1954.

John E. Nafe

Papers

Variation with depth in shallow and deep water marine sediments of porosity, density and the velocities of compressional and shear waves

Physical properties of marine sediments

The bulk modulus-volume relationship for oxide compounds and related geophysical problems

Crustal structure of the mid‐ocean ridges: 1. Seismic refraction measurements

A simplified method for the analysis and synthesis of dispersed wave trains