Showing papers in "Geophysical Journal International in 1978"

Active tectonics of the Alpine—Himalayan belt: the Aegean Sea and surrounding regions

Abstract: Summary. New fault plane solutions, Landsat photographs, and seismic refraction records show that rapid extension is now taking place in the northern and eastern parts of the Aegean sea region. The southern part of the Aegean has also been deformed by normal faulting but is now relatively inactive. In northwestern Greece and Albania there is a band of thrusting near the western coasts adjacent to a band of normal faulting further east. The pre-Miocene geology of the islands in the Aegean closely resembles that of Greece and Turkey, yet seismic refraction shows that the crust is now only about 30 km thick beneath the southern part of the sea, compared with nearly 50 km beneath Greece and western Turkey. These observations suggest that the Aegean has been stretched by a factor of two since the Miocene. This stretching can account for the high heat flow. The sinking slab produced by subduction along the Hellenic Arc may maintain the motions, though the geometry and widespread nature of the normal faulting is not easily explained. The motions in northwestern Greece and Albania cannot be driven in the same way because no slab exists in the area. They may be maintained by blobs of cold mantle detaching from the lower half of the lithosphere, produced by a thermal instability when the lithosphere is thickened by thrusting. Hence generation and destruction of the lower part of the lithosphere may occur beneath deforming continental crust without the production of any oceanic crust.

A study of geomagnetic storms

Abstract: The interplanetary energy flux is estimated on the basis of the Poynting flux and its variations with the rate of energy dissipation in terms of: (1) the ring-current particle injection, (2) Joule dissipation in the ionosphere, and (3) auroral particle injection for 15 major geomagnetic storms. A relationship, in terms of the angle between the interplanetary magnetic field vector and the magnetospheric field vector, is defined by which the growth of geomagnetic storms is closely associated with the Poynting flux. It is found that the energy flux that enters the magnetosphere is dissipated through intramagnetospheric substorm processes. Geomagnetic storm phenomena represent the combined influence of such effects.

A new method for computing synthetic seismograms

Abstract: Summary. The computation of theoretical seismograms for models in which the elastic parameters and density vary only with depth (in a plane, cylindrical or spherical geometry) reduces to the solution of an ordinary differential equation plus the evaluation of inverse transformations. In principle, the problem is straightforward. In practice, many techniques and approximations can be used at each stage and many combinations and variants are possible. In this paper, we discuss a new method of evaluating the inverse transforms. Any method can be used to solve the differential equation and we only discuss a few analytic approximations to illustrate the new method. The inverse transformations are a frequency and wavenumber integral. Essentially four techniques can be used to evaluate these depending on the order of integration and whether the wavenumber integral is distorted from the real axis. Three of these have been widely used, but the technique of evaluating the frequency integral first and keeping the wavenumber real is new. In this paper, we discuss some of the advantages of this combination.

Seismic-wave propagation through a cracked solid: polarization as a possible dilatancy diagnostic

Abstract: Summary. A new technique is presented for modelling the elastic constants of cracked structures with application to systems with weak concentrations of parallel cracks, and of simple biplanar and triplanar cracks. The velocities and V,lVs ratios of these anisotropic structures are used to provide quantitative models for some earthquake precursors. These results indicate the great importance of crack geometry to the behaviour of precursors. The velocities of saturated cracks appear to favour the dilatancy-diffusion model of precursory phenomena. Synthetic seismograms are calculated for propagation through possible dilatancy zones. The seismograms show some characteristic features which may be useful for the investigation of earthquake dilatancy .

Thermal origin of mid‐plate hot‐spot swells

Abstract: Summary. Additional evidence supports the idea that thc shallow rises surrounding mid-plate, hot-spot volcanoes are caused by a broad-scale reheating of the lithosphere above hot-spots. Firstly, as required by the reheating concept, the rises appear to be supported by a density deficiency within the normal thickness of the lithosphere. The gravity anomalies over the Bermuda, Cape Verde, Hawaii and Cook-Austral swells indicate that the compensation of these swclls is only 40 to 100 km deep. The geoid anomaly over the Hawaiian swell is consistent with these depths. Secondly, as also required by the reheating concept; swells and the volcanoes formed on swells subside at the same rate as younger, hotter lithosphere which is at the same ocean depth. Almost all mid-plate swells rise to an ocean depth of 4250 ni, the depth of normal 25 Myr-old lithosphere. The llawaiian Swell, Emperor Guyots, Cook-Austral Swell and Bikini and Enewetok Atolls all subside as 25 Myr-old lithosphere subsides.

Present tectonics and Late Cenozoic evolution of New Zealand

Abstract: Summary. The boundary between the Pacific and Indian Plates in New Zealand is a 200-km-wide zone of pervasive deformation manifest in seismic activity, distortion of old triangulation nets and differential vertical movements of the land. The nature of the deformation within the zone is investigated using geodetic observations and earthquake slip vectors. Two components of shear strain have been measured at 22 locations using changes in the observed angles of retriangulation nets. Earthquake slip vectors for 20 published fault-plane solutions show the direction of slip at the time and location of the earthquake. Together these data indicate an axial belt of rapid dextral shear some 70-100 km in width crossing diagonally from the Hikurangi active margin on the northeast to the Fiordland margin on the south-west. Within this belt maximum shear rates reach values of 7 x IO-'yr-'. The rates and the directions of the principal axis of horizontal compression are the same, within the limits of error, as those deduced for the relative movement between the Pacific and Indian Plates from the Minster et al. instantaneous pole of rotation. Two zones of almost pure compression occur to the west of the axial tectonic belt; they are here named the Nelson and Milford tectonic zones. Behind the two active margins are zones of extension named the Taupo rift in the North Island and the Waiau rift in the South Island. The present rate of compression across the boundary zone, taken up by crustal thickening, can account for the Quaternary uplift of the mountains of the tectonic belt and zones. Two types of deformation in crustal rocks are inferred: a seismic, elastic deformation resulting in fault displacements, and an aseismic, ductile flow. The relative proportions of these varies along the length of the boundary zone. In central South Island flow is dominant, fault displacements are small and seismicity low. In the south, fault displacements account for most, and flow a minor part, of the movement between the plates. Extrapolation of present-day deformation into the past indicates that much of the observed deformation of New Zealand is of Late Cenozoic age and includes the displacement of the Alpine Fault and the bending of an original straight Mesozoic geosyncline into its present arc. Crustal thickening,

Evolution of the Labrador Sea and its bearing on the early evolution of the North Atlantic

Abstract: Summary. Geophysical data collected from 1972 to 1975 during a systematic mapping program of the Labrador Sea have been analysed to investigate its geological history and evolution. The data have been used to establish the location of the ridge axis, the age of the ocean floor, and the direction of movement of Greenland relative to North America. Different poles of rotation for the Eurasian and Greenland plate relative to the North American plate in the Late Cretaceous have been derived in order to fit together satisfactorily the plate boundaries defined by the magnetic anomalies in the Labrador Sea and the North Atlantic. The analysis shows that active seafloor spreading commenced in the southern Labrador Sea during the Campanian (anomaly 32) and in the northern Labrador Sea during the Maastrichtian (anomaly 28), with little or no spreading in the Baffin Bay region during this period. With the commencement of active seafloor spreading in the Norwegian Sea during the lower Paleocene (anomaly 24), the direction of seafloor spreading changed in the Labrador Sea and spreading commenced in Baffin Bay. The spreading ceased in the Labrador Sea and Baffin Bay during the lower Oligocene (pre-anomaly 13) when Greenland started to move with the North American plate. Paleogeographic reconstruction of the three plates shows that Greenland moved north relative to North America during the first phase of opening of the Labrador Sea (75–60 Myr), giving rise to compressive forces between northern Greenland and the Canadian Arctic Islands. During the second phase of opening of the Labrador Sea (60–40 Myr) Greenland moved past Ellesmere Island in the left lateral sense along Nares Strait. Some compression is also inferred from these constructions between the margins of northeast Greenland and Svalbard. The poles of rotation obtained for the three plates show a different set of events which may have been responsible for the separation of Rockall Plateau from the British Isles during the early evolution of the North Atlantic.

The Effect of A General Aspherical Perturbation on the Free Oscillations of the Earth

Abstract: summary. The Lagrangian governing the infinitesimal elastic-gravitational oscillations of a completely general earth model with interior fluid—solid boundaries is given. Rayleigh's principle is then used to derive a formula for calculating the first-order perturbations in the eigenfrequencies due to an arbitrary slight perturbation of a spherically symmetric, non-rotating, isotropic starting model. The perturbations considered include rotation, asphericity, elastic anisotropy, and a deviatoric initial stress, as well as a change in the positions of both welded and fluid—solid boundaries.

LISPB — IV. Crustal structure of Northern Britain

Abstract: Summary. This paper presents those results from the 1974 Lithospheric Seismic Profile in Britain (LISPB) which relate to the compressional velocity structure of the crust and uppermost mantle beneath Northern Britain. A combination of interpretation techniques suitable for modelling laterally inhomogeneous media, including two-dimensional ray-tracing and time-term analysis, has resulted in a detailed seismic cross-section across the Caledonian orogenic belt. The main features of this section are a possible horizontal discontinuity in the Pre-Caledonian’basement, a change in the relationship between the lower crust and the uppermost mantle from north to south and a considerable thickening of the crust beneath the Caledonian fold belt. These results place considerable constraints upon tectonic models for the evolution of the Caledonides in particular in their implication of differing crustal structures north and south of the Southern Uplands and their indication of the primary significance of the Southern Uplands Fault.

Double-planed deep seismic zone and upper-mantle structure in the northeastern Japan arc

Abstract: Summary. The ScSp wave converted from the ScS wave at the boundary between the descending lithospheric slab and the mantle above it was clearly observed from a nearby deep earthquake with magnitude 7.7 at some stations of the seismic network of Tohoku University which covers the Tohoku District, the northeastern part of Honshu, Japan. By applying the threedimensional seismic-ray tracing method, the location of this boundary was determined from the difference in arrival time between the ScS and ScSp waves. The result shows that the upper boundary of the descending slab lies exactly on the upper plane of the double-planed deep seismic zone found in the Northeastern Japan Arc. There is an additional evidence that the boundary is located on the upper plane of the double-planed deep seismic zone. The hypocentre distribution of intermediate-depth earthquakes located by the small-scale seismic-array observation is extremely different from that obtained by the relatively largescale seismic network. The discrepancy in the distribution of hypocentres of the same earthquake independently located is well explained by the inclined lithospheric slab model derived from the difference in arrival time between the ScS and ScSp waves. The earthquakes with reverse faulting or with down-dip compressional stresses occur at the upper boundary of the descending slab. Within the descending slab, the earthquakes with down-dip extensional stresses also occur in a very narrow zone from 30 to 40 km below the dipping boundary in the depth range from 50 to about 200 km, and these shocks form the lower plane of the double-planed deep seismic zone.

Shear deformation zones along major transform faults and subducting slabs

Abstract: Summary. Narrow zones of intense shear deformation, i.e. viscous slip zones, are studied analytically with a one-dimensional time-dependent model of two half-spaces of identical or contrasting rheologies and ambient temperatures in relative motion. The rheologies of the half-spaces are strongly temperature-dependent and viscous heating maintains a thin zone of high temperature, low viscosity and large strain rate. The mathematical model is used to describe the structures of slip zones at ridge and plate-boundary transform faults, major continental strike—slip faults and at the top of subducting oceanic crust. No a priori assumption about slip-zone width or shear-stress magnitude is necessary; the thermal-mechanical structure of the slip zone evolves in time and all its characteristics are self-consistently determined. Slip-zone widths and shear stresses depend on the ambient temperatures, the relative velocity, the rheology and the length of time following the onset of relative motion; for reasonable geologic times, 0.1–10 M yr for example, slip zones are generally several kilometres wide and shear stresses are several hundred bars (tens of MPa). The region of intense shear in a viscous slip zone is an order of magnitude narrower than the width of the accompanying thermal anomaly. The maximum temperature generated by viscous dissipation in a slip zone depends only on the relative motion and the creep properties of the rocks; it is independent of slip-zone age and ambient temperature. Maximum temperatures associated with frictional heating are always less than those required for partial melting. The slip zone on a descending slab is influenced most strongly by the contrast in creep behaviour between the relatively soft oceanic crustal rocks and the hard, overlying mantle rocks; as a result, the slip zone is confined entirely within the oceanic crustal layer. Oceanic crustal rocks deform so readily that frictional heating in a slip zone on a descending slab cannot by itself lead to partial melting and thermal conduction from the hotter overriding mantle must play an essential role in heating the descending crust if melting is to occur therein. Because of the increase in the mantle temperature with depth, narrow slip zones in oceanic regions probably do not exist below depths of about 100 km; they may extend to greater depths beneath continents.

The evolution of sedimentary basins on a viscoelastic lithosphere: theory and examples

Abstract: Summary. A systematic approach is suggested for modelling the development of sedimentary basins. The theory, which partitions basin formation into initiating and isostatic adjustment processes, is applicable to all modes of basin formation if these processes are linear, or can he represented with sufficient accuracy in an incrementally linear form. The dynamics of regional isostatic adjustment are characterized by the Heaviside space-time Green functions for the response of elastic and viscoelastic (Maxwell) thin plate models of the lithosphere. It is shown, by convolving the Heaviside—Green functions with cylindrical surface loads, that the rate of isostatic adjustment on a viscoelastic lithosphere is a function of the wavelength of the surface load, long wavelengths being compensated most rapidly. Six archetypal initiating processes for sedimentary basin development are presented. These processes are those responsible for the subsidence of the Earth's surface which creates a depression in which water and sediments collect. Isostatic amplification of subsidence by sediment and water loads is cast in the form of an integral equation with isostatic Heaviside—Green functions as kernel. Specific examples, the basins that result from a graben initiating process, are compared with the largest scale structure of the North Sea Basin, a basin that is known to be underlain by a graben system. A model, in which a 50-km wide graben subsides exponentially with a time constant of 5 × 107yr during the interval 180–100 Myr bp, is shown to be consistent with the largest scale structure of the North Sea Basin if the underlying lithosphere is viscoelastic with a flexural rigidity of ∼5 × 1025 Nm and relaxation time constant ∼ 106 yr.

The gravitationally powered dynamo

Abstract: Summary. The energetics of the gravitationally powered dynamo have been studied with the aid of a compressible-earth model which allows for the growth of the solid inner core. The basic premise of this study is that as the Earth gradually cooled over geological time the solid inner core continually accreted dense material which crystallized from an outer core composed of a molten binary alloy. This process requires a continual rearrangement of matter which generates the fluid motions needed to sustain the dynamo. These motions maintain the outer core in a well-mixed state, in apparent contradiction to Higgins & Kennedy’s hypothesis that the outer core is stably stratified. The vigour of these motions is dependent primarily upon the composition of the solid inner core, but is surprisingly independent of the density of the light constituent in the core. If the solid core is composed entirely of heavy metal, then as much as 3.7 x 1OI2 W may be transferred from the core to the mantle as a result of cooling and gravitational settling. This is roughly equal to estimates of the amount of heat conducted down the adiabat in the core, but it is argued that there is no direct relation between the amount of heat conducted down the adiabat and the amount transferred to the mantle if the convection is driven non-thermally. The gravitational energy released per unit mass of the solid inner core is remarkably constant and may be as much as 2 x 106J/kg, roughly five times the value of the latent heat of iron. These values are reduced if the solid inner core contains some light constituents. It was found that the efficiency of the gravitationally powered dynamo may exceed 50 per cent, a much higher figure than is possible for either the thermally or precessionally driven dynamo. Also, the amount of gravitational energy available to drive the dynamo in the future is many times that expended so far. The size of the magnetic field sustained by gravitational settling was related to the density jump at the inner-outer core boundary and the field strength was estimated to lie between 390 and 685 G, strongly suggesting that the dynamo is of the nearly-axisymmetric type developed by Braginsky.

The interaction of surge and tide in the North Sea and River Thames

Abstract: Summary. Although the tendency for surge peaks in the Thames to occur on rising tide has been recognized for some time, no satisfactory physical explanation has been presented. The phenomenon almost certainly results from non-linear interaction between tide and surge and it is the mechanism of this interaction which is examined in the present study. A statistical analysis of surges recorded at 10 ports located along the east coast of Britain demonstrated the development of interaction as surges propagate southwards. This analysis showed that surges tend to develop a peak on the rising tide in the Thames irrespective of the phase relationship between tide and surge in the northern North Sea. A one-dimensional model of the River Thames was used to examine how surge-tide interaction varied for surges of differing types. In order to identify the mechanics of interaction, a new modelling technique was developed involving two models, one of tidal propagation and one of surge propagation, operated simultaneously with cross-linkages in the form of perturbation terms providing the effects of interaction. By this means it was shown that quadratic friction is the dominant interaction mechanism in the Thames.

Dynamical rupture process on a three‐dimensional fault with non‐uniform frictions and near‐field seismic waves

Abstract: Summary Dynamical rupture process on the fault is investigated in a quasithreedimensional faulting model with non-uniform distributions of static frictions or the fracture strength under a finite shearing pre-stress The displacement and stress time functions on the fault are obtained by solving numerically the equations of motion with a finite stress-fracture criterion, using the finite difference method If static frictions are homogeneous or weakly non-uniform, the rupture propagates nearly elliptically with a velocity close to that of P waves along the direction of pre-stress and with a nearly S wave velocity in the direction perpendicular to it The rise time of the source function and the final displacements are larger around the centre of the fault In the case when the static frictions are heavily non-uniform and depend on the location, the rupture propagation becomes quite irregular with appreciably decreased velocities, indicating remarkable stick-slip phenomena In some cases, there remain unruptured regions where fault slip does not take place, and high stresses remain concentrated up to the final stage These regions could be the source of aftershocks at a next stage The stick-slip faulting and irregular rupture propagation radiate highfrequency seismic waves, and the near-field spectral amplitudes tend to show an inversely linear frequency dependence over high frequencies for heavily non-uniform frictional faults

A procedure for estimating lateral variations from low-frequency eigenspectra data

Abstract: Summary. Expressions are derived for the zeroth and first moments of an isolated normal mode multiplet in the presence of rotational anisotropy, lateral heterogeneity and attenuation. It is demonstrated that, to first order, the zeroth moment is independent of the heterogeneity and the first moment is a linear functional of the heterogeneity. The location of the multiplet, defined in terms of the ratio of the first moment to the zeroth moment, is used to set up a linear inverse problem for lateral variations. The approximations employed in deriving the differential kernels are uniformly valid for all angular orders. In addition, formula for the differential kernels asymptotically valid in the limit of large angular order are developed, and the correspondence between these formulae and the geometrical optics approximation is established. It is shown that, in the limit of weak heterogeneity and large angular order, the location of the multiplet is the average of the local perturbation to the eigenfrequency over the great circular path containing the source and receiver.

Statistical study of the occurrence of shallow earthquakes

Abstract: Summary. The time-space-magnitude interaction of shallow earthquakes has been investigated for three catalogues: worldwide (M > 7.0), Southern and Northern California (A4 2 4.0) and Central California (A4 2 1 S). The earthquake sequences are considered as a multi-dimensional stochastic point process; the estimates of the parameters for a branching model of the seismic process are obtained by a maximum-likelihood procedure. After applying magnitude-time and magnitude-distance scaling, the pattern of relationship among earthquakes of different magnitude ranges is almost identical. The number of foreshocks diminishes as the magnitude difference between the main shock and the foreshocks increases, while the magnitude distribution of aftershocks has the opposite property. The strongest aftershocks are likely to occur at the beginning of the sequence; later they migrate away with velocities of the order of km/day. The sequences which are composed of smaller aftershocks last longer and there are indications that they remain essentially in the focal region. Foreshocks also appear to migrate, but in this case, toward the main shock. The rate of occurrence of dependent shocks increases as t -' as the origin time of the main shock is approached, effectively making every earthquake a multi-shock event. This interaction of earthquakes was modelled by a Monte-Carlo simulation technique. The statistical inversion of simulated catalogues was undertaken to derive the information we would be able to retrieve from actual data, as well as possible errors of estimates. The possibility of using these results as a tool for seismic risk prediction is discussed and evaluated.

Measurements and interpretation of normal mode attenuation

Abstract: summary. Measurements of Q for modes of free oscillation provide the most accurate information about the anelastic properties of the whole Earth in the period range from 100 to 3000 s. We have obtained more than 230 Q measurements, by using two different techniques. Individual LaCoste—Romberg gravimeter recordings of three large earthquakes were used to observe the time rate of decay of spectral peaks corresponding to different modes. This method provided measurements of Q for 37 different modes. By stacking 211 WWSSN recordings of two deep earthquakes, we were able to measure Q for 197 modes, including many overtones which cannot be analysed using the spectra of individual recordings.

A detailed rock magnetic and opaque mineralogy study of the basalts from the Nazca Plate

Abstract: Summary. The penetration of igneous basement in the Nazca Plate during DSDP Legs 16 and 34 provided samples of both fine-grained pillow-basalt and coarse-grained massive flow units. The magnetic mineral in these basalt samples is initially a titanomagnetite (Fe,-,Ti,O4) with a narrow range of composition of x = 0.62 f 0.05. Subsequent to formation, the titanomagnetite grains are generally subjected to low temperature oxidation to titanomaghemite with a corresponding rise in Curie temperature from the initial values of 120-150°C up to a maximum of 400°C. Both grain size and low-temperature oxidation state play important, and interrelated, roles in controlling the intensity and stability of magnetic remanence and other magnetic properties. Overall grain size can, in some cases, be related to oxidation state since some sections of the relatively impermeable massive flows can remain unoxidized for as long as 40 Myr while pillow basalts are extensively oxidized within 34 Myr. Low-temperature alteration in turn effects magnetic grain size since oxidation and subsequent Fe cation migration results in grain subdivision by the formation of shrinkage cracks. A five-stage sequence of the microscopic changes that are associated with progressive low-temperature oxidation is proposed and illustrated with photomicrographs from these basalt samples. A hierarchy in the intensity of magnetic remanence may exist with unoxidized pillow basalts having a much higher intensity and oxidized pillow basalts having a much lower intensity than the massive flow units. While pillow basalts are relatively immune to the addition of secondary components of magnetization, the coarse-grained massive flows readily acquire components of viscous remanence. Although they oxidize much more slowly than pillows, when oxidation does take place, components of chemical remanence can be acquired by the multi-domained grains in the massive flow units.

On the elastic‐perfectly plastic bending of the lithosphere under generalized loading with application to the Kurd Trench

Abstract: Summary. The bending of the lithosphere as it approaches an oceanic trench is modelled using an elastic-perfectly plastic rheology. The rock within the elastic lithosphere is assumed to behave elastically until a yield stress is reached; for larger strains the stress is assumed to remain at the yield value (no strain hardening). The deflection of the lithosphere is obtained numerically for a range of applied moments, vertical forces and horizontal forces at the trench axis. The results of the analysis are compared with a series of topographic and gravity profiles across the Kuril Trench. In order to model the large observed curvatures of the lithosphere on the outer trench slope plasticity is required. The location of the region of high curvatures gives a value for the compressive axial load. One conclusion is that the lithosphere is under compression seaward of the Kuril Trench.

The Mid-Atlantic Ridge at 37 and 45° N: some geophysical and petrological constraints

Abstract: Summary. At present there is a strong conflict between, on the one hand, seismological and thermal models of the Mid-Atlantic Ridge, which indicate that no large crustal magma chamber can exist, and on the other hand petrological models many of which stress the importance of such a chamber. We review the available geophysical and petrological information from the FAMOUS area and 45° N in an attempt to resolve this conflict and demonstrate that a model (the infinite leek) can be constructed which satisfies all the available seismological, thermal, petrographic, major element and trace-element information from these two areas. This mode is as follows: mantle rising from depth begins to melt at about 60 km, and rises in equilibrium with its melt to about 15–25 km below the sea surface. At this level melt segregates and rises rapidly to the base of the crust. Magma injection above this takes place by a process of crack propagation, or by the development of a narrow vertical magma chamber, but no large crustal chamber is present. This model successfully explains the marked petrographic zonation of the floor of the median valley (Hekinian, Moore & Bryan).

Frictional heating on a fault zone with finite thickness

Abstract: Summary. The problem of frictional heating on a fault of finite thickness is considered. Temperature distributions during and after faulting are obtained for faults of various thicknesses. A thick fault approximation is shown to be valid if a fault has a thickness greater than about 1 cm. The thickness of the melted zone is predicted for various frictional stress levels and displacements. The predicted thicknesses are shown to be in reasonable agreement with field observations.

Stress and temperature in subduction shear zones: Tonga and Mariana

Abstract: Summary. Because there is secondary sea-floor spreading in the Tonga and Mariana subduction systems, the island arcs are separate plates. Horizontal forces on the two sides of the arc must balance, and the maximum force on the back-arc side can be calculated from a lithostatic ridge model. This, in combination with gravity data, allows calculation of the average shear stress in the top 100 km of the subduction shear zone. Stress in Tonga is 220±100 bar, and in the Mariana it is 165±75 bar. These low stresses are probably made possible by a fluid pore pressure almost equal to the least compressive stress. Knowledge of stress allows approximate calculation of temperature in the shear zone by integration of a single differential equation. These temperatures are too low to activate most dehydration reactions in the subducted crust. As it approaches the volcanic line, this crust is at 150–350°C in Tonga and 150–300°C in the Mariana. Shear melting of the crust is ruled out, and conductive melting of the slab by contact with the asthenosphere meets with geochemical objections. Magmas in these systems are probably produced by partial melting of asthenosphere, triggered by a sudden release of water from the slab.

The northward motion of India since the Late Cretaceous

Abstract: Summary. Palaeomagnetic data are presented from the sediments and basalts at DSDP sites 213, 214, 215, 216, and 217 on or near the Ninetyeast Ridge in the eastern Indian Ocean. The palaeolatitudes confirm that the ridge is attached to the Indian plate and that both have moved rapidly northwards since the Late Cretaceous. The Indian plate moved northwards with respect to the South Pole at an average rate of 14.9±4.5 cm/yr from 70 Myr ago until about 40 Myr ago when it slowed to its present rate of 5.2±0.8 cm/yr. Basement palaeolatitudes on the Ninetyeast Ridge indicate that its volcanic source was approximately fixed in latitude near 50° S, supporting the concept that the ridge is the trace of the Kerguelen hotspot on the northward moving Indian plate. The existence of a‘mirror ridge’on the Antarctic plate and the very shallow depths of basement formation on the ridge suggest that the Indian/Antarctic spreading centre must have remained near the hotspot from 80 to 40 Myr in spite of one-limb spreading rates of up to 12 cm/yr. This is unexpected in view of the apparently small amount of motion of the Antarctic plate during this time. It is suggested that Antarctica was held nearly fixed by the geometry of other plate motions, and therefore the Kerguelen hotspot caused asymmetric accretion of new plate material at the southwestern end of the Ninetyeast Ridge. Evidence of such asymmetry has been reported in the form of an 11° southerly migration or jump of that spreading centre. The Ninetyeast Ridge palaeolatitudes are consistent with the Deccan Traps palaeomagnetic poles. However, a comparison of the Australian palaeomagnetic poles and these data shows a major inconsistency between 50 and 40 Myr. Although the reason for this inconsistency is not known, the error may be in the age of the Harrington volcano pole for Australia. The two data sets compare favourably in both rate and palaeolatitude back to 40 Myr. Prior to 50 Myr they compare favourably in rate but the DSDP data imply that India was 13° farther south than the Australian poles indicate.

Magnetic properties of dredged oceanic gabbros and the source of marine magnetic anomalies

Abstract: Summary. Magnetic property studies (natural remanent magnetization, initial susceptibility, progressive alternating field demagnetization and magnetic mineralogy of selected samples) were completed on 45 samples of gabbro and metagabbro recovered from 14 North Atlantic ocean-floor localities. The samples are medium to coarse-grained gabbro and metagabbro which exhibit subophitic intergranular to hypidiomorphic granular igneous textures. The igneous mineralogy is characterized by abundant plagioclase, varying amounts of clinopyroxene and hornblende, and lesser amounts of magnetite, ilmenite and sphene. Metamorphic minerals (actinolite, chlorite, epidote and fine-grained alteration products) occur in varying amounts as replacement products or vein material. The opaque mineralogy is dominated by magnetite and ilmenite. The magnetite typically exhibits a trellis of exsolution-oxidation ilmenite lamellae that appears to have formed during deuteric alteration. The NRM intensities of the gabbros range over three orders of magnitude and give a geometric mean of 2.8 x 10-4gau~~ and an arithmetic mean of 8.8 x 10-4gauss. The Konigsberger ratio, a measure of the relative importance of remanent to induce magnetization, is greater than unity for the majority of the samples and indicates that remanent magnetization on average dominates the total magnetization of oceanic gabbros in the Earth's magnetic field. The magnetic properties of fresh and metamorphosed gabbros appeared to be similar. The majority of gabbros studied were characterized by median destructive fields greater than 200 Oe. The high stability is attributed largely to the effective subdivision of the magnetite grains by the

Effect of compaction on the acquisition of a detrital remanent magnetization in fine-grained sediments

Abstract: Summary. Measurements of the detrital remanent magnetization (DRM) of redeposited deep-sea sediment of the silty clay grade are described. Variations in the magnitude of an observed remanence inclination error are related to conditions of sediment accumulation, contrasted here as grain-by-grain settling from a dilute dispersion or by settling from a concentrated slurry. For these artificial redepositions post-depositional compaction is shown to be a major factor in shallowing the observed inclination from the ambient field inclination. The term compactive DRM is tentatively assigned to describe such behaviour.

Self-consistent equilibrium ocean tides

Abstract: Summary. We compute the static response of the world ocean to an external zonal gravitational potential. The computation includes the effects of the self-attraction of the ocean, and the yielding of the Earth caused both by the external potential and the change in ocean load. We compare the computed tide with measurements of the fortnightly and monthly ocean tides. The short-wavelength departures from equilibrium found by Wunsch are still present. An average of observations at Pacific islands shows that the fortnightly tide departs significantly from equilibrium but the monthly may not. We have also calculated the effects of our computed tide on measurements of tidal gravity and tidal fluctuations in the length of day. Existing tidal gravity data are too imprecise to enable us to determine whether or not the spatial average of the ocean tides departs from equilibrium. The length of day data suggest that the monthly tide is farther from equilibrium than the fortnightly. We have not been able to resolve the apparent discrepancy between the length of day and ocean tide data.

Reliability of palaeointensity methods using alternating field demagnetization and anhysteretic remanence

Abstract: Summary. Using natural volcanic rocks which acquired thermoremanence (TRM) in known fields, reliability of various palaeointensity methods using alternating field (AF) demagnetization were evaluated. Natural remanence (NRM), TRM and anhysteretic remanences (ARM's) before and after heating were stepwisely AF demagnetized following Shaw's method. It was found that the coercivity spectra of TRM and ARM in these samples are very similar, and that even when changes occurred during heating, the changes for two remanences are quite similar in many samples. Therefore, Shaw's method of palaeointensity determination, which incorporates ARM checks to the conventional comparison of NRM and TRM coercivity spectra, gives results as reliable as those obtained by the Thellier method. Many examples were demonstrated in which TRM and ARM intensities changed substantially by heating, but without changes in the shape of their coercivity spectra. Such changes cannot normally be detected and erroneous palaeointensities with apparent internal consistency would be obtained by usual AF demagnetization methods. Although ARM is quite similar to TRM, the rate of acquisition of ARM and TRM in weak fields varies by a factor of five among the present samples. To determine palaeointensities from the linear relation between ARM and TRM, it is necessary to determine experimentally the relative acquisition rate of these remanences. Therefore, methods based only on NRM-ARM relations would not give palaeointensities with acceptable errors.

LISPB — V. Studies of crustal shear waves

Abstract: Summary. Many shots in the LISPB profiles produced shear waves with large amplitudes which were recorded by three-component stations. However, S waves seem to be strongly attenuated when they propagate through complex velocity structures. Upper crustal refractions (mainly land shots) and wideangle reflections (mainly sea shots) were picked with the help of particle motion plots. S to P travel-time ratios (ts/tp) were used to calculate the distribution of Poisson’s ratios in a crustal model. The results were generally close to u = 0.25 except in the upper crust south of the Southern Uplands Fault (u = 0.231) and in the middle crust under the Midland Valley (a = 0.224).

On seismic-wave scattering by a rough core-mantle boundary

Abstract: Summary. A formulation is given for the seismic-wave scattering by a rough solid-liquid interface, in analogy to results derived for a solid-solid interface and a heterogeneous volume. Using Kennett’s approach and the reciprocity theorem, the scattering is formulated as the excitation by an equivalent dislocation. Using interface parameters relevant to the core-mantle boundary (CMB), computational results for several types of body-wave scattering are given and compared to scattering by a heterogeneous volume. In an application to the generation of PKP precursors it is concluded that, whereas some data groups point to heterogeneity (which may not be small) in the lower mantle above CMB, in other cases a rough CMB may be considered equally feasible. Scattering at the source or receiver side of the core by both a slightly rough CMB (radial variations up to a few hundred metres) and a slightly heterogeneous lower mantle (relative variations in physical parameters up to a few per cent) produces the energy level that is observed in most of the PKP precursors; also the relevant scale lengths of variation are about the same ie both mechanisms (10-20 km with possibly somewhat higher values at relatively long epicentral distances).