Curie-temperature depth estimation using a self-similar magnetization model

The Earth's crust is magnetized down to the Curie-temperature depth at about 10 to 50 km. This limited depth extent of the crustal magnetization is discernible in the power spectra of magnetic maps of South Africa and Central Asia. At short wavelengths, the power increases as rapidly towards longer wavelengths as expected for a self-similar magnetized crust with unlimited depth extent. Above wavelengths of about 100 km the power starts increasing less rapidly, indicating the absence of deep-seated sources. To quantify this effect we derive the theoretical power spectrum due to a slab carved out of a self-similar magnetization distribution. This model power spectrum matches the power spectra of South Africa and Central Asia for a self-similarity parameter of β = 4 and Curie temperature depths of 15 to 20 km.     

Seismic evidence for a sharp lithospheric base persisting to the lowermost mantle beneath the Caribbean

Broad-band data from South American earthquakes recorded by Californian seismic networks are analysed using a newly developed seismic wave migration method—the slowness backazimuth weighted migration (SBWM). Using the SBWM, out-of-plane seismic P -wave reflections have been observed. The reflection locations extend throughout the Earth's lower mantle, down to the core–mantle boundary (CMB) and coincide with the edges of tomographically mapped high seismic velocities. Modelling using synthetic seismograms suggests that a narrow (10–15 km) low- or high-velocity lamella with about 2 per cent velocity contrast can reproduce the observed reflected waveforms, but other explanations may exist. Considering the reflection locations and synthetic modelling, the observed out-of-plane energy is well explained by underside reflections off a sharp reflector at the base of the subducted lithosphere. We also detect weaker reflections corresponding to the tomographically mapped top of the slab, which may arise from the boundary between the Nazca plate and the overlying former basaltic oceanic crust. The joint interpretation of the waveform modelling and geodynamic considerations indicate mass flux of the former oceanic lithosphere and basaltic crust across the 660 km discontinuity, linking processes and structure at the top and bottom of the Earth's mantle, supporting the idea of whole mantle convection.     

Global heat budget, plate tectonics and climatic change

For the past 2000 Ma, the temperature of the Earth's surface has fluctuated around a mean similar to that of today, although individual locations have undergone long-term changes of ∼30°C at different times in different places. Water bodies absorb at least five times as much solar radiation as land surfaces, and ocean currents transport the excess heat absorbed in the tropics towards the poles. Changes in the distribution of land and sea due to plate tectonics explain the major temperature fluctuations (>25°C) around the globe in the last 350 Ma, and are first-order controls. Large-scale changes in ocean currents and thermohaline circulations are probably second-order controls (15–25°C). The Milankovitch orbital cycles are third-order controls producing variations in air temperature of the order of 10°C, while massive volcanic eruptions and changes in carbon dioxide are amongst the fourth-order controls producing minor perturbations (<5°C). The major climatic fluctuations are continuous but regional in effect and not global. Extraterrestrial factors may not cause major changes in climate when viewed from a geological perspective.     

Properties of iron at the Earth's core conditions

Summary. The phase diagram of iron up to 330 GPa is solved using the experimental data of static high pressure (up to 11 GPa) and the experimental data of shock wave data (up to 250 GPa). A solution for the highest triple point is found ( P = 280 GPa and T = 5760 K) by imposing the thermodynamic constraints of triple points. This pressure of the triple point is less than the pressure of the inner core–outer core boundary of the Earth. These results indicate that the density of iron at the inner core–outer core boundary pressure is close to 13 g cm⁻³, which lies close to the seismic solutions of the Earth at that pressure. It is thus concluded that the Earth's inner core is very likely to be virtually pure iron in its hexagonal close packed (hcp) phase.
It is shown that four properties of the Earth's inner core determined from seismology are close in value to the corresponding properties of hcp iron at inner core conditions: density, bulk modulus, longitudinal velocity, and Poisson's ratio. The density–pressure profile of hcp iron at inner core conditions matches the density–pressure profile of the inner core as determined by seismic methods, within the spread of values given by recent seismic models.
This indicates that the Earth is slowly cooling, the Earth's inner core is growing by crystallization, and the impurities of the core are concentrated in the outer core. The calculated temperature at the Earth's centre is 6450 K.     

COCORP deep seismic reflection traverses of the U.S. Cordillera

Summary. COCORP seismic reflection traverses of the U.S. Cordillera at 40°N and 48.5°N latitude reveal some fundamental similarities as well as significant differences in reflection patterns. On both traverses, autochthonous crust beneath thin-skinned thrust belts of the eastern part of the Cordillera is unreflective; immediately to the west the Cordilleran interior is very reflective above a flat, prominent reflection Moho. Mesozoic accreted terranes in the western part of the orogen are underlain on both traverses by very complex reflection patterns, in constrast to more easily deciphered patterns beneath areas of Cenozoic accretion. The prominent reflection Moho beneath the orogenic interior on both transects probably evolved through a combination of magmatic and deformational processes during Cenozoic extension. The main differences between the two traverses lie in the reflection patterns of the middle and lower crust in the Cordilleran interior; these differences are probably related to the way Cenozoic extension was accommodated at depth. Laminated middle and lower crust above the reflection Moho in the western Basin and Range (40°N) may be related to magmatism, ductile pure shear and large-scale transposition during Cenozoic extension. By contrast, beneath the eastern Basin and Range (40°N), and the orogenic interior in the NW United States (48.5°N), Cenozoic extension was probably accommodated along dipping deformation zones throughout the crust.     

Palaeomagnetism of nine dated Phanerozoic dykes in south-east Greenland

Summary Nine basic dykes were sampled near Angmagssalik, east Greenland. Specimens have been treated by alternating field demagnetization in 11 steps up to 3000 (peak) oersted (300 ml). The 'cleaned' direction at all sites is recognized after treatment at 150 oersted. All specimens are reversely magnetized. The mean of the site mean directions has declination = 182°.0, inclination =−66°.9, it = 45, α₉₅= 7°.7. This direction yields a palaeomagnetic pole (reversed) at 73°.4N, 139°.5E ( dp = 10°.7, dm = 12°.9) which is near, but significantly different from, that derived from lower Tertiary rocks in Greenland, namely 63°.2N, 184°.6E ( A ₉₅= 4°.5). K-Ar ages of the nine dykes, based upon whole-rock and mineral separates, range from mid-Tertiary to Cambrian. It is impossible to reconcile these ages with the palaeomagnetic results. The palaeomagnetic evidence, supported by geological inference, suggests that all nine dykes are members of the east Greenland lower Tertiary dyke swarm, designated THOL1, of probable age c. 52 Ma.
The difference between the poles given above can be explained by supposing that the sampling area has tipped about a horizontal axis directed along 013°/193°, the angle of rotation being 13° (± 11°) anti-clockwise, when the axis is viewed along 013°. This local effect could have been due to block faulting when the north-east Atlantic started to open, or may be attributed to upwarping of the coast due to the weight of the ice-cap inland.     

Crustal Structure and Origin of Peake and Freen Deeps, NE Atlantic

Summary Peake and Freen Deeps are elongate structures some 30 nautical miles long by 7 miles wide situated near 43° N 20° W on the lower flanks of the Mid-Atlantic Ridge. Seismic reflection records show that underneath about 400 fm of layered sediment the bedrock lies at a depth greater than 3600 fm in Peake Deep and 3300 fm in Freen Deep; the surrounding seafloor is at about 2100 fm. Freen Deep is the eastern end of King's Trough, a flat floored feature some 400 fms deeper than the adjacent seafloor. The Trough extends 220 miles west-north-westwards towards the crest of the Mid-Atlantic Ridge. The area is aseismic and heat flow is normal; there is no displacement of the crest of the mid-ocean ridge on the projected line of King's Trough. Gravity and magnetic surveys have been made. With minor exceptions, magnetic anomalies are not due to bodies elongated parallel with the structure, which, therefore, cannot be a volcanic collapse caldera. Seismic refraction results in the Peake-Freen area show that the crust is not thinned under the deeps although the Moho may be depressed by 2 km. Bouguer anomalies also suggest that the Moho is flat and does not rise to compensate the deeps. Models consistent with gravity and seismic information suggest there is a dense block in the upper mantle under the area. Since no reason to ascribe the origin of the structure to tear faulting has yet been acquired, it is interpreted in terms of over thrusting perpendicular to the deeps, followed by inversion of the lower part of the thickened basaltic crust to eclogite, and its subsequent sinking into the mantle.     

A geomagnetic scattering theory for evaluation of earth structure

Summary. Structural features of the Earth's lower crust and upper mantle can be mapped by the analysis of temporal geomagnetic fluctuations using the electromagnetic scattering theory developed in this paper. Decomposing geomagnetic field fluctuations at the Earth's surface into an excitation part and a scattered part forms the basis of a power series development. The vertical field component is interpreted as a scattering of the excitation field. The horizontal gradient and geomagnetic depth sounding methods are special cases of the theory developed. The horizontal gradient sounding method has a tensorial aspect which has not been recognized before; it should be included to obtain correct penetration depth parameter evaluations from field data.     

Crustal and uppermost mantle structure in southern Africa revealed from ambient noise and teleseismic tomography

Rayleigh wave phase velocity maps in southern Africa are obtained at periods from 6 to 40 s using seismic ambient noise tomography applied to data from the Southern Africa Seismic Experiment (SASE) deployed between 1997 and 1999. These phase velocity maps are combined with those from 45 to 143 s period which were determined previously using a two-plane-wave method by Li & Burke. In the period range of overlap (25–40 s), the ambient noise and two-plane-wave methods yield similar phase velocity maps. Dispersion curves from 6 to 143 s period were used to estimate the 3-D shear wave structure of the crust and uppermost mantle on an 1°× 1° grid beneath southern Africa to a depth of about 100 km. Average shear wave velocity in the crust is found to vary from 3.6 km s^–1 at 0–10 km depths to 3.86 km s^–1 from 20 to 40 km, and velocity anomalies in these layers correlate with known tectonic features. Shear wave velocity in the lower crust is on average low in the Kaapvaal and Zimbabwe cratons and higher in the surrounding Proterozoic terranes, such as the Limpopo and the Namaqua-Natal belts, which suggests that the lower crust underlying the Archean cratons is probably less mafic than beneath the Proterozoic terranes. Crustal thickness estimates agree well with a previous receiver function study of Nair et al. . Archean crust is relatively thin and light and underlain by a fast uppermost mantle, whereas the Proterozoic crust is thick and dense with a slower underlying mantle. These observations are consistent with the southern African Archean cratons having been formed by the accretion of island arcs with the convective removal of the dense lower crust, if the foundering process became less vigorous in arc environments during the Proterozoic.     

Two-dimensional finite-difference model of seafloor compliance

Seafloor compliance is the measure of seafloor deformation under a pressure signal. Our new 2-D finite-difference compliance modelling algorithm presents several advantages over the existing compliance models, including the ability to handle any gridded subsurface structure with no limitations on the gradients of the material properties, as well as significantly improved performance. Applying this method to some of the problems inaccessible to previously existing methods, demonstrates that lateral variations in subsurface structure must be accounted for to adequately interpret compliance data. In areas with significant lateral variations, the utilization of 1-D modelling and inversion is likely to result in high interpretation errors, even when additional subsurface structure information is available. We find that flattened pure melt bodies have a significantly higher compliance than cylindrical melt bodies with the same cross-sectional area. The compliance created by such bodies often has side peaks over their edges, which are as strong as or stronger than the central peak, requiring a series of measurements to best constrain their size and shear velocity. Finally, we find that the compliance data are far and away most sensitive to the broad, thick, lower-crustal partial melt zone. Our simple data fitting model for the compliance measurements on the East Pacific Rise at 9°48'N required shear velocities as low as 700 m s⁻¹ in the centre of this zone, far below the values previously estimated using 1-D model based inversions, suggesting higher melt percentages than those previously estimated, while small melt bodies in the upper part of the crust were found to have little or no effect on the measured compliance.     

Structural effects of the crust on the geoid modelled using deep seismic sounding interpretations

According to the theory of isostasy, the Earth has a tendency to deform its surface in order to reach an equilibrium state. The land-uplift phenomenon in the area of the Fennoscandian Shield is thought to be a process of this kind. The geoid, as an equipotential surface of the Earth's gravity field, contains information on how much the Earth's surface departs from the equilibrium state. In order to study the isostatic process through geoidal undulations, the structural effects of the crust on the geoid have to be investigated.
  The structure of the crust of the Fennoscandian Shield has been extensively explored by means of deep seismic sounding (DSS). The data obtained from DSS are used to construct a 3-D seismic-velocity structure model of the area's crust. The velocity model is converted to a 3-D density model using the empirical relationship that holds between seismic velocities and crustal mass densities. Structural effects are then estimated from the 3-D density model.
  The structural effects computed from the crustal model show that the mass deficiency of the crust in Fennoscandia has caused a geoidal depression twice as deep as that observed from the gravimetric geoid. It proves again that the crust has been isostatically compensated by the upper mantle. In other words, an anomalously high-density upper mantle must exist beneath Fennoscandia.     

Numerical models of mantle convection with secular cooling

A 2-D time-dependent finite-difference numerical model is used to investigate the thermal character and evolution of a convecting layer which is cooling as it convects. Two basic cooling modes are considered: in the first, both upper and lower boundaries are cooled at the same rate, while maintaining the same temperature difference across the layer; in the second, the lower boundary temperature decreases with time while the upper boundary temperature is fixed at 0°C. The first cooling mode simulates the effects of internal heating while the second simulates planetary cooling as mantle convection extracts heat from, and thereby cools, the Earth's core. The mathematical analogue between the effects of cooling and internal heating is verified for finite-amplitude convection. It is found that after an initial transient period the central core of a steady but vigorous convection cell cools at a constant rate which is governed by the rate of cooling of the boundaries and the viscosity structure of the layer. For upper-mantle models the transient stage lasts for about 30 per cent of the age of the Earth, while for the whole mantle it lasts for longer than the age of the Earth. Consequently, in our models the bulk cooling of the mantle lags behind the cooling of the core-mantle boundary. Models with temperature-dependent viscosity are found to cool in the same manner as models with depth-dependent viscosity; the rate of cooling is controlled primarily by the horizontally averaged variation of viscosity with depth. If the Earth's mantle cools in a similar fashion, secular cooling of the planet may be insensitive to lateral variations of viscosity.     

EARTH MODELS WITH CONTINUOUS DENSITY DISTRIBUTION

Summary. Eight different Earth models have been set up, all with the property that the density p varies continuously from just below the crust to the centre. The distributions of the pressure p , gravity g , incompressibility k and rigidity are also given; and values of a parameter equal to (k/p)dp/dp , indicate the deviations from (chemical) homogeneity in the lower mantle and outer core. The models are designed to provide a numerical background towards testing the view that there is no density jump between the Earth's mantle and core.
A discussion shows that this view is difficult to reconcile with a homogeneous core unless an implausibly low value is assumed for the density just below the crust.     

Deep seismic reflection studies in Israel - an update

Summary. The results of three deep crustal reflection lines are presently available from Israel. A 90 km line from near the Dead Sea rift to the Mediterranean coast was carried out for deep study. Two other lines in the Mediterranean coastal area were derived by recorrelation of oil exploration lines. The data shows a division between continental inner Israel and the coastal plain. In the first area a reflective lower crust is apparent with transparent upper crust and almost transparent upper mantle. Near the coast, in an area which was previously suggested as underlain by an ancient fossil oceanic crust, strong reflections characterize the uppermost mantle. Comparison between the reflection pattern and previous deep refraction and MT data indicates some agreement away from the coast and lack of correlation in the area of possible fossil oceanic crust near the coast.     

Crustal heterogeneity and velocity anisotropy from seismic studies in the USSR

Summary. The analysis of data of seismic crustal studies in the USSR, obtained from waves propagating at different azimuths, reveals considerable horizontal and vertical inhomogeneity of the crust. Against this background it is difficult to predict what kind of velocity anisotropy can be expected in the continental crust. The rare cases of disagreement in velocities on intersecting profiles can be attributed both to anisotropy and to horizontal crustal inhomogeneity. There is a definite disagreement in layer velocities measured by reflected waves: fine layers in the crust and upper mantle have been found to have anomalously high velocities. The role of anisotropy in these events is not clear. The frequently observed splitting of S -wave with different polarization, however, positively implies anisotropy in the Earth's crust.     

Dislocation-mediated melting of iron and the temperature of the Earth's core

Summary . Dislocation theories of melting provide a possibility of calculating the melting temperature, from first principles, as the temperature at which the free energy of a crystal saturated with dislocations becomes equal to that of the dislocation-free crystal. After a brief review of the physical bases of the dislocation melting theories, Ninomiya's theory is used to determine the melting temperature as well as the volume and entropy of melting and the slope of the melting curve for iron at atmospheric pressure and under conditions prevailing at the Earth's inner core boundary. The necessary parameters (elastic moduli, Grüneisen parameter) are drawn from seismological earth models. We find a melting temperature of the material of the inner core of about 6150 K, independent of shock-wave experiments but in good agreement with them and with extrapolations using Lindemann's law. With usually accepted values of the melting point depression due to light elements in solution, the temperature at the inner core boundary is found to be T _ICB≅ 5000 K. This temperature is compatible with a temperature of the outer core at the core-mantle boundary T _CMB≅ 3800 K. Dislocation melting theories can thus help constrain the temperature profile in the Earth's core.     

Coseismic rotation changes from the 2004 Sumatra earthquake: the effects of Earth's compressibility versus earthquake induced topography

Polar motion is modelled for the large 2004 Sumatra earthquake via dislocation theory for an incompressible elastic earth model, where inertia perturbations are due to earthquake-triggered topography of density–contrast interfaces, and for a compressible model, where inertia perturbation due to compression-dilatation of Earth's material is included; density and elastic parameters are based on a multilayered reference Earth. Both models are based on analytical Green's functions, propagated from the centre to the Earth's surface. Preliminary and updated seismological solutions are considered in elucidating the effects of improving earthquake parameters on polar motion. The large Sumatra thrust earthquake was particularly efficient in driving polar motion since it was responsible for large material displacements occurring orthogonally to the strike of the earthquake and to the Earth's surface, as imaged by GRACE gravity anomalies over the earthquake area. The effects of earthquake-induced topography are four times larger than the effects of Earth's compressibility, for l = 2 geopotential components. For varying compressional Earth properties and seismic solution, modelled polar motion ranges from 8.6 to 9.4 cm in amplitude and between 117° and 130° east longitude in direction. The close relationship between polar motion direction, earthquake longitude and thrust nature of the event, are established in terms of basic physical concepts.     

Seismic refraction profiles between Cyprus and Israel and their interpretation

Summary. A long seismic refraction profile was carried out between southern Israel and Cyprus. The seismic energy was generated by 33 sea shots each of 0.8 t explosives and was recorded by land stations in Israel and Cyprus and by ocean bottom seismographs deployed along the profile.
The results showed that the continental crust of southern Israel thins towards the Mediterranean underneath a northward thickening sedimentary cover. Cyprus is underlain by a 35 km thick continental crust thinning south-wards and extending to Mt Eratosthenes. Between Mt Eratosthenes and the Israel continental shelf the crystalline crust is composed of high velocity (6.5 km s^-1)material and is about 8 km thick. It is covered by 12–14 km of sediments and may represent a fossil oceanic crust.     

A seismological study of normal faulting in the Demirci, Alaşehir and Gediz earthquakes of 1969–70 in western Turkey: implications for the nature and geometry of deformation in the continental crust

Summary. In this study, seismological techniques are combined with surface observations to investigate the faulting associated with three large earthquakes in western Turkey. All involved normal faulting that nucleated at 6–10 km depth with dips in the range 30–50°. The two largest earthquakes, at Alaşehir (1969.3.28) and Gediz (1970.3.28), were clearly multiple events and their seismograms indicate that at least two discrete subevents were involved in producing the observed surface faulting. In addition, their seismograms contain later, longer-period signals that are likely to represent source, not structure or propagation, complexities. These later signals can be modelled by subevents with long time functions on almost flat detachment-type faults.
As a result of these observations, we propose a model for the deformation of the lower crust, in which brittle failure of the top part occurs when high strain rates are imposed during an earthquake that ruptures right through the upper, brittle crust. Under these special circumstances, seismic motion occurs on discrete faults in the lower crust, which otherwise normally deforms by distributed creep. In the case of the normal faults studied here, motion in the uppermost lower crust takes place on shallow dipping faults that are downward continuations of the steeper faults that break to the surface. The faults thus have an overall listric geometry, flattening into a weak zone below the brittle layer at a depth that is probably dependent on the termperature gradient. This interpretation explains why detachment-type mechanisms are not seen in first motion fault plane solutions of normal faulting earthquakes, and suggests an origin for the Metamorphic Core Complexes seen in the Basin and Range Province, which probably represent flat lower crustal faults, analogous to those postulated at Alaşehir and Gediz, that have been uplifted to the surface.