A comparison of the isostatic response of bathymetric features in the north Pacific Ocean and Philippine Sea

Summary. This paper concerns the calculation and analysis of admittance functions from large and uniform data sets of gravity and topography in four regions of the northern and western Pacific Ocean. The purpose is to separate and describe possible differences in isostatic compensation between several 'type' regions of oceanic crust: a mid-ocean ridge (Juan de Fuca), a mid-plate seamount chain (Hawaiian Ridge), fracture zone topography on old crust (north of Hawaii) and a marginal basin (Philippine Sea). Results suggest that there are significant differences in the degree to which long wavelength topography has been compensated which can be distinguished between regions. These differences are set in the perspective of three simple compensation mechanisms. Two of these consider local Airy models in which raised topography is compensated at depth either by crustal roots or low density mantle. A third considers the effects of an elastic plate of variable thickness supporting crustal variations. Conclusions are that: (a) a thick plate possibly in excess of 30 km supports the Hawaiian Ridge; (b) a much thinner plate of 5 to 15 km existed when the fracture zone topography was formed; (c) the Juan de Fuca Ridge is compensated either regionally by a plate 5 to 10 km thick or locally by sub-crustal low densities at depths of 15 to 20 km; and (d) the Philippine Sea shows no evidence for regional support: ridges are compensated locally by differences in crustal thickness whereas the basins are underlain by density variations at depths comparable to those of the much younger Juan de Fuca Ridge. The major difference between admittance functions for the Philippine Sea and comparably aged regions of the north Pacific Ocean adds further new evidence of possible evolutionary differences between it and normal ocean basins.     

Timing of depth‐dependent lithosphere stretching on the S. Lofoten rifted margin offshore mid‐Norway: pre‐breakup or post‐breakup?

Depth‐dependent stretching, in which whole‐crustal and whole‐lithosphere extension is significantly greater than upper‐crustal extension, has been observed at both non‐volcanic and volcanic rifted continental margins. A key question is whether depth‐dependent stretching occurs during pre‐breakup rifting or during sea‐floor spreading initiation and early sea‐floor spreading. Analysis of post‐breakup thermal subsidence and upper‐crustal faulting show that depth‐dependent lithosphere stretching occurs on the outer part of the Norwegian volcanic rifted margin. For the southern Lofoten margin, large breakup lithosphere β stretching factors approaching infinity are required within 100 km of the continent–ocean boundary to restore Lower Eocene sediments and flood basalt surfaces (～54 Ma) to interpreted sub‐aerial depositional environments at sea level as indicated by well data. For the same region, the upper crust shows no significant Palaeocene and Late Cretaceous faulting preceding breakup with upper‐crustal β stretching factors <1.05. Further north on the Lofoten margin, reverse modelling of post‐breakup subsidence with a β stretching factor of infinity predicts palaeo‐bathymetries of ～1500 m to the west of the Utrøst Ridge and fails to restore Lower Eocene sediments and flood basalt tops to sea level at ～54 Ma. If these horizons were deposited in a sub‐aerial depositional environment, as indicated by well data to the south, an additional subsidence event younger than 54 Ma is required compatible with lower‐crustal thinning during sea‐floor spreading initiation. For the northern Vøring margin, breakup lithosphere β stretching factors of ～2.5 are required to restore Lower Eocene sediments and basalts to sea level at deposition, while Palaeocene and Late Cretaceous upper‐crustal β stretching factors for the same region are < 1.1. The absence of significant Palaeocene and late Cretaceous extension on the southern Lofoten and northern Vøring margins prior to continental breakup supports the hypothesis that depth‐dependent stretching of rifted margin lithosphere occurs during sea‐floor spreading initiation or early sea‐floor spreading rather than during pre‐breakup rifting.     

Three-dimensional gravity inversion for Moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction

This paper describes a method for determining Moho depth, lithosphere thinning factor (γ= 1 − 1/β) and the location of the ocean–continent transition at rifted continental margins using 3-D gravity inversion which includes a correction for the large negative lithosphere thermal gravity anomaly within continental margin lithosphere. The lateral density changes caused by the elevated geotherm in thinned continental margin and adjacent ocean basin lithosphere produce a significant lithosphere thermal gravity anomaly which may be in excess of −100 mGal, and for which a correction must be made in order to determine Moho depth accurately from gravity inversion. We describe a method of iteratively calculating the lithosphere thermal gravity anomaly using a lithosphere thermal model to give the present-day temperature field from which we calculate the lithosphere thermal density and gravity anomalies. For continental margin lithosphere, the lithosphere thermal perturbation is calculated from the lithosphere thinning factor (γ= 1 − 1/β) obtained from crustal thinning determined by gravity inversion and breakup age for thermal re-equilibration time. For oceanic lithosphere, the lithosphere thermal model used to predict the lithosphere thermal gravity anomaly may be conditioned using ocean isochrons from plate reconstruction models to provide the age and location of oceanic lithosphere. A correction is made for crustal melt addition due to decompression melting during continental breakup and seafloor spreading. We investigate the sensitivity of the lithosphere thermal gravity anomaly and the predicted Moho depth from gravity inversion at continental rifted margins to the methods used to calculate and condition the lithosphere thermal model using both synthetic models and examples from the North Atlantic.     

Explosive volcanic eruptions—IX. The transition between Hawaiian-style lava fountaining and Strombolian explosive activity

The commonest eruption styles of basaltic volcanoes involve Hawaiian lava fountaining or intermittent Strombolian explosions. We investigate the ways in which magma rise speed at depth, magma volatile content and magma viscosity control which of these eruption styles takes place. We develop a model of the degree of coalescence between gas bubbles in the magma which allows us to simulate the transition between the two extreme styles of activity. We find that magma rise speed is the most important factor causing the transition, with gas content and viscosity also influencing the rise speed at which the transition occurs. Counter to intuitive expectations, a decrease in gas content does not cause a transition from Hawaiian to Strombolian activity, but instead causes a transition to passive effusion of vesicular lava. Rather, a change from Hawaiian to Strombolian style requires a significant reduction in magma rise speed.     

State of stress in the oceanic lithosphere in response to loading

Summary. Flexure studies of the oceanic lithosphere constrained by bathymetry and gravity data suggest that the lithosphere behaves elastically over geological time-scales. For loads to be supported, however, large bending stresses (approaching 10 kb in some cases) are required at the top and bottom of the elastic plate.
These stress-differences can be significantly reduced by introducing more complex rheologies: we propose a model of layered lithosphere, consisting of a purely elastic upper layer, a transition zone with viscosity varying with depth and a perfectly plastic lower layer. The transition layer is grossly centred at the bottom of the elastic plate. Such a model results in a noticeable reduction of stress differences; reaching 60 per cent for flow laws representing creep mechanisms in olivine. When applied to a number of seamount loads, this model leads to maximum stress-differences which do not exceed 1–2kb.
The approach used in this study allows us to follow stress relaxation over time. Taking account of the thermal cooling of the lithosphere, we show that the elastic thickness of the lithosphere is stabilized after a given time, while the time required for stabilization is found to be of the order 5—6 per cent of the age of the lithosphere at the date of loading.     

Localization of gravity and topography: constraints on the tectonics and mantle dynamics of Venus

We develop a method for spatio-spectral localization of harmonic data on a sphere and use it to interpret recent high-resolution global estimates of the gravity and topography of Venus in the context of geodynamical models. Our approach applies equally to the simple spatial windowing of harmonic data and to variable-length-scale analyses, which are analogous to a wavelet transform in the Cartesian domain. Using the variable-length-scale approach, we calculate the localized RMS amplitudes of gravity and topography, as well as the spectral admittance between the two fields, as functions of position and wavelength. The observed admittances over 10 per cent of the surface of Venus (highland plateaus and tessera regions) are consistent with isostatic compensation of topography by variations in crustal thickness, while admittances over the remaining 90 per cent of the surface (rises, plains and lowlands) indicate that long-wavelength topography is dominantly the result of vertical convective tractions at the base of the lithosphere. The global average crustal thickness is less than 30 km, but can reach values as large as 40 km beneath tesserae and highland plateaus. We also note that an Earth-like radial viscosity structure cannot be rejected by the gravity and topography data and that, without a mechanical model of the lithosphere, admittance values cannot constrain the thickness of the thermal boundary layer of Venus. Modelling the lithosphere as a thin elastic plate indicates that at the time of formation of relief in highland plateaus and tesserae, the effective elastic plate thickness, T_e , was less than 20 km. Estimates of T_e at highland rises are consistently less than 30 km. Our inability to find regions with T_e > 30 km is inconsistent with predictions made by a class of catastrophic resurfacing models.     

Multimode surface waveform tomography of the Pacific Ocean: a closer look at the lithospheric cooling signature

We present a regional surface waveform tomography of the Pacific upper mantle, obtained using an automated multimode surface waveform inversion technique on fundamental and higher mode Rayleigh waves, to constrain the   V_SV   structure down to ∼400 km depth. We have improved on previous implementations of this technique by robustly accounting for the effects of uncertainties in earthquake source parameters in the tomographic inversion. We have furthermore improved path coverage in the South Pacific region by including Rayleigh wave observations from the French Polynesian Pacific Lithosphere and Upper Mantle Experiment deployment. This improvement has led to imaging of vertical low-velocity structures associated with hotspots within the South Pacific Super-Swell region. We have produced an age-dependent average cross-section for the Pacific Ocean lithosphere and found that the increase in   V_SV   with age is broadly compatible with a half-space cooling model of oceanic lithosphere formation. We cannot confirm evidence for a Pacific-wide reheating event. Our synthetic tests show that detailed interpretation of average   V_SV   trends across the Pacific Ocean may be misleading unless lateral resolution and amplitude recovery are uniform across the region, a condition that is difficult to achieve in such a large oceanic basin with current seismic stations.     

Estimation of ocean primary productivity and its spatio-temporal variation mechanism for East China Sea based on VGPM model

According to calculation results of ocean chlorophyll concentration based on SeaWiFS data by SeaBAM model and synchronous ship-measured data, this research set up an improved model for CaseⅠand CaseⅡwater bodies respectively. The monthly chlorophyll distribution in the East China Sea in 1998 was obtained from this improved model on calculation results of SeaBAM. The euphotic depth distribution in 1998 in the East China Sea is calculated by using remote sensing data of K490 from SeaWiFS according to the relation between the euphotic depth and the oceanic diffuse attenuation coefficient. With data of ocean chlorophyll concentration, euphotic depth, ocean surface photosynthetic available radiation (PAR), daily photoperiod and optimal rate of daily carbon fixation within a water column, the monthly and annual primary productivity spatio-temporal distributions in the East China Sea in 1998 were obtained based on VGPM model. Based on analysis of those distributions, the conclusion can be drawn that there is a clear bimodality character of primary productivity in the monthly distribution in the East China Sea. In detail, the monthly distribution of primary productivity stays the lowest level in winter and rises rapidly to the peak in spring. It gets down a little in summer, and gets up a little in autumn. The daily average of primary productivity in the whole East China Sea is 560.03 mg/m2/d, which is far higher than the average of subtropical ocean areas. The annual average of primary productivity is 236.95 g/m2/a. The research on the seasonal variety mechanism of primary productivity shows that several factors that affect the spatio-temporal distribution may include the chlorophyll concentration distribution, temperature condition, the Yangtze River diluted water variety, the euphotic depth, ocean current variety, etc. But the main influencing factors may be different in each local sea area.     

东海初级生产力遥感反演及其时空演化机制

According to calculation results of ocean chlorophyll concentration based on SeaWiFS data by SeaBAM model and synchronous ship-measured data, this research set up an improved model for Case I and Case Ⅱ water bodies respectively. The monthly chlorophyll distribution in the East China Sea in 1998 was obtained from this improved model on calculation results of SeaBAM. The euphotic depth distribution in 1998 in the East China Sea is calculated by using remote sensing data of K490 from SeaWiFS according to the relation between the euphotic depth and the oceanic diffuse attenuation coefficient. With data of ocean chlorophyll concentration, euphotic depth, ocean surface photosynthetic available radiation (PAR), daily photoperiod and optimal rate of daily carbon fixation within a water column, the monthly and annual primary productivity spatio-temporal distributions in the East China Sea in 1998 were obtained based on VGPM model. Based on analysis of those distributions, the conclusion can be drawn that there is a clear bimodality character of primary productivity in the monthly distribution in the East China Sea. In detail, the monthly distribution of primary productivity stays the lowest level in winter and rises rapidly to the peak in spring. It gets down a little in summer, and gets up a little in autumn. The daily average of primary productivity in the whole East China Sea is 560.03 mg/m^2/d, which is far higher than the average of subtropical ocean areas. The annual average of primary productivity is 236.95 g/m^2/a. The research on the seasonal variety mechanism of primary productivity shows that several factors that affect the spatio-temporal distribution may include the chlorophyll concentration distribution, temperature condition, the Yangtze River diluted water variety, the euphotic depth, ocean current variety, etc. But the main influencing factors may be different in each local sea area.     

Glacial rebound of the British Isles—II. A high-resolution, high-precision model

Observations of ice movements across the British Isles and of sea-level changes around the shorelines during Late Devensian time (after about 25 000 yr BP) have been used to establish a high spatial and temporal resolution model for the rebound of Great Britain and associated sea-level change. The sea-level observations include sites within the margins of the former ice sheet as well as observations outside the glaciated regions such that it has been possible to separate unknown earth model parameters from some ice-sheet model parameters in the inversion of the glacio-hydro-isostatic equations. The mantle viscosity profile is approximated by a number of radially symmetric layers representing the lithosphere, the upper mantle as two layers from the base of the lithosphere to the phase transition boundary at 400 km, the transition zone down to 670 km depth, and the lower mantle. No evidence is found to support a strong layering in viscosity above 670 km other than the high-viscosity lithospheric layer. Models with a low-viscosity zone in the upper mantle or models with a marked higher viscosity in the transition zone are less satisfactory than models in which the viscosity is constant from the base of the lithosphere to the 670 km boundary. In contrast, a marked increase in viscosity is required across this latter boundary. The optimum effective parameters for the mantle beneath Great Britain are: a lithospheric thickness of about 65 km, a mantle viscosity above 670 km of about (4-5) 10²⁰ Pa s, and a viscosity below 670 km greater than 4 × 10²¹ Pa s.     

Late Jurassic subsidence and passive margin evolution in the Vulcan Sub-basin, north-west Australia: constraints from basin modelling

The Vulcan Sub-basin, located in the Timor Sea, north-west Australia, developed during the Late Jurassic extension which ultimately led to Gondwanan plate breakup and the development of the present-day passive continental margin. This paper describes the evolution of upper crustal extension and the development of Late Jurassic depocentres in this subbasin, via the use of forward modelling techniques. The results suggest that a lateral variation in structural style exists. The south of the basin is characterized by relatively large, discrete normal faults which have generated deep sub-basins, whereas more distributed, small-scale faulting further north reflects a collapse of the early basin margin, with the development of a broader, 'sagged' basin geometry. By combining forward and reverse modelling techniques, the degree of associated lithosphere stretching can be quantified. Upper crustal faulting, which represents up to 10% extension, is not balanced by extension in the deeper, ductile lithosphere; the magnitude of this deeper extension is evidenced by the amount of post-Valanginian thermal subsidence. Reverse modelling shows that the lithosphere stretching
factor has a magnitude of up to β=1.55 in the southern Vulcan Sub-basin, decreasing to β=1.2 in the northern Vulcan Sub-basin. It is proposed that during plate breakup, deformation in the Vulcan Sub-basin consisted of depth-dependent lithosphere extension. This additional component of lower crustal and lithosphere stretching is considered to reflect long-wavelength partitioning of strain associated with continental breakup, which may have extended 300–500 km landward of the continent–ocean boundary.     

The relationship between gravity and bathymetry in the Pacific Ocean

Summary. Surface-ship and satellite derived data have been compiled in new free-air gravity anomaly, bathymetry and geoid anomaly maps of the Pacific Ocean basin and its margin. The maps are based on smoothed values of the gravity anomaly, bathymetry and geoid interpolated on to a 90 × 90 km grid. Each smoothed value was obtained by Gaussian filtering measurements along individual ship and subsatellite tracks. The resulting maps resolve features in the gravity, bathymetry and geoid with wavelengths that range from a few hundred to a few thousand kilometres. The smoothed values of bathymetry and geoid anomaly have been corrected for age. The resulting maps show the Pacific ocean basin is associated with a number of ENE–WSW-trending geoid anomaly highs with amplitudes of about ± 5 m and wavelengths of about 3000 km. The most prominent of these highs correlate with the Magellan seamounts–Marshall Gilbert Islands–Magellan rise and the Hess rise–Hawaiian ridge regions. The correlation between geoid anomaly and bathymetry cannot be explained by models of static compensation, but is consistent with a model in which the geoid anomaly and bathymetry are supported by some form of dynamic compensation. We suggest that the dynamic compensation, which characterizes oceanic lithosphere older than 80 Myr, is the result of mantle convection on scales that are smaller than the lithospheric plates themselves.     

The dynamical origin of subduction zone topography

Summary. Subduction zones are expressed topographically by long linear oceanic trenches flanked by a low outer rise on the seaward side and an island arc on the landward side. This topographic structure is reflected in free air gravity anomalies, suggesting that much of the topography originates from dynamical forces applied at the base of the crust. We have successfully reproduced the general topographic features of subduction zones by supposing that the stresses generated by the bending of the viscous lower lithosphere as it subducts are transmitted through the thin elastic upper portion of the lithosphere. The trench is due to a zone of extensional flow (associated with low pressure) in the upper part of the viscous lithosphere.
The stresses in the subducting slab are computed using a finite element technique, assuming a Maxwell viscoelastic constitutive relation. Various dips (10 to 90°) were investigated, as well as depth dependent and non-Newtonian (power law, n = 3) viscosities. Observed subduction zone dimensions are well reproduced by these models. The effective viscosity required at mid-depth in the lithosphere is about 6 × 10²² P. This low value is probably due to the stress dependence of the effective viscosity. However, these models also show that the topography of the subduction zone depends primarily upon the geometry of the subducting slab (dip, radius of curvature of the bend) rather than upon its rheology. Shear stresses beneath the trench reach maxima of approximately 50 MPa. An interesting feature of some solutions is a dynamically supported bench or platform between the trench and island arc.     

Anisotropy in the oceanic lithosphere of the North-western Pacific Basin

Summary. A series of long-range explosion seismological experiments has been conducted by the use of specially designed ocean bottom seismographs (OBSs) in the Western Pacific. OBS studies of apparent velocity measurements by the use of natural earthquakes have also been made. The experiments have made clear that large-scale P -wave anisotropy exists in the entire thickness of the oceanic lithosphere. The existence of the large-scale anisotropy in the oceanic lithosphere has been demonstrated for the first time by seismic body-wave studies. Previously, anisotropy had been found only in the uppermost oceanic mantle in the Eastern Pacific.
The azimuth of the maximum velocity, 8.6 km s^-1, is about 155° clock-wise from north. The direction is perpendicular to the magnetic lineation of the region, however, the direction differs from the direction of the present plate motion by about 30°. So it appears that the anisotropy has been 'frozen' at least since the change of the plate motion that occurred 40 Myr ago. The frozen anisotropy should set important constraints on the mechanical properties of the lithosphere such as the viscosity and temperature of the lower lithosphere.     

Anomalous seismic crustal structure of oceanic fracture zones

Summary. The seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the North Atlantic. The crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3. Anomalous crust is present in both large- and small-offset fracture zones. Since they are among the most common tectonic features in the ocean basins, and are particularly closely spaced on slow-spreading ridges, fracture zones represent a major source of seismic crustal heterogeneity. We interpret the anomalous crust as a thin, intensely fractured, faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that, in places, are extensively serpentinized. The unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes; firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section; and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point, so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness, culminating in the very thin crust of the fracture zones at either end of the ridge segment.     

东海初级生产力遥感反演及其时空演化机制

针对基于SeaWiFS的海洋叶绿素浓度SeaBAM模型反演结果,在中国东海海域分别建立了Ⅰ、Ⅱ类水体的修订模式,反演计算获得了我国东海海域1998年各月叶绿素浓度的分布,并根据真光层深度与海水漫射衰减系数之间的关系,利用SeaWiFS的K490遥感资料反演获得的1998年各月真光层深度的分布,在VGPM模型支持下,反演计算获得了中国东海海域1998年的逐月初级生产力时空分布以及全年累积初级生产力分布状况。对东海海域海洋初级生产力逐月时空变化特征及其影响机制的初步研究结果表明,整个东海海域初级生产力的逐月变化具有明显的双峰特征,表现为冬季最低,春季迅速上升达到最高,夏季略有下降,秋季又略有回升。海域初级生产力日平均值为560.03 mg/m²/d,远高于世界亚热带海域平均状况。年平均值为236.95 g/m²/a。控制东海海洋初级生产力时空变化的主要因素可能包括叶绿素浓度分布、温度条件、长江冲淡水变化,以及真光层深度、海流锋面过程等,不同海区初级生产力时空变化的主要控制因素有所不同。     

The Canary Islands swell: a coherence analysis of bathymetry and gravity

The Canary Archipelago is an intraplate volcanic chain, located near the West African continental margin, emplaced on old oceanic lithosphere of Jurassic age, with an extended volcanic activity since Middle Miocene. The adjacent seafloor does not show the broad oceanic swell usually observed in hotspot-generated oceanic islands. However, the observation of a noticeable depth anomaly in the basement west of the Canaries might indicate that the swell is masked by a thick sedimentary cover and the influence of the Canarian volcanism. We use a spectral approach, based on coherence analysis, to determine the swell and its compensation mechanism. The coherence between gravity and topography indicates that the swell is caused by a subsurface load correlated with the surface volcanic load. The residual gravity/geoid anomaly indicates that the subsurface load extends 600 km SSW and 800 km N and NNE of the islands. We used computed depth anomalies from available deep seismic profiles to constrain the extent and amplitude of the basement uplift caused by a relatively low-density anomaly within the lithospheric mantle, and coherence analysis to constrain the elastic thickness of the lithosphere ( T_e ) and the compensation depth of the swell. Depth anomalies and coherence are well simulated with T_e =28–36 km, compensation depth of 40–65 km, and a negative density contrast within the lithosphere of ∼33 kg m⁻³. The density contrast corresponds to a temperature increment of ∼325°C, which we interpret to be partially maintained by a low-viscosity convective layer in the lowermost lithosphere, and which probably involves the shallower parts of the asthenosphere. This interpretation does not require a significant rejuvenation of the mechanical properties of the lithosphere.     

Sediment blanketing and the flexural strength of extended continental lithosphere

The flexural rigidity of the oceanic lithosphere is strongly dependent on its temperature structure at the time of loading. It is commonly assumed that the depth to the 450°C isotherm defines the effective elastic thickness T_e of the lithosphere. However, recent gravity studies across the Baltimore Canyon and Nova Scotian margins suggest that temperature may play a more complicated role in controlling the mechanical strength of extended continental lithosphere. For example, the flexural strength of the Baltimore Canyon margin (with sediment thicknesses of ? 15 km) appears to be controlled by the depth to the 150°C isotherm whereas the strength of the Nova Scotian margin (with sediment thicknesses cf ? 10 km) is controlled by the depth to the 250°C isotherm. The apparent correlation between sediment thickness and controlling isotherm suggests that sediment blanketing may play a role in modifying the flexural strength of extended continental lithosphere. This hypothesis was investigated by simulating the sedimentation history of a margin as a Gaussian function in which sedimentation peak and rate are determined by the mean and standard deviation of the function. The temperature structure of the lithosphere is continually modified as sediments are deposited on, and incorporated into the temperature structure of, the underlying lithosphere. Given a ‘starting’ value of T_e defined by the degree of extension of the lithosphere, the modification of T_e appears to be directly proportional to the sedimentation rate and cumulative sediment thickness, and inversely proportional to the time at which the sedimentation rate is a maximum. The first-order consequence of sediment blanketing is to reduce the cooling rate of the lithosphere relative to cooling in the absence of sediments. At thermal equilibrium, the initial value of T_e is reduced by the cumulative sediment thickness. Local isostatic conditions (i. e. T_e? 0) can only be approached when the sedimentation rate is unrealistically high (> 1000 m/Myr) during the rift or early post-rift phase of basin development. However, while these early loads may be locally compensated, any subsequent loads will be regionally compensated. Thus, it is unlikely that the low present-day flexural strengths interpreted from the Baltimore Canyon and Nova Scotian passive continental margins are a consequence of sediment blanketing.     

A Maastrichtian palaeomagnetic pole for the Pacific plate from a skewness analysis of marine magnetic anomaly 32

The asymmetry (skewness) of marine magnetic anomaly 32 (72.1–73.3  Ma) on the Pacific plate has been analysed in order to estimate a new palaeomagnetic pole. Apparent effective remanent inclinations of the seafloor magnetization were calculated from skewness estimates of 108 crossings of anomaly 32 distributed over the entire Pacific plate and spanning a great-circle distance of ~12  000  km. The data were inverted to obtain a palaeomagnetic pole at 72.1°N, 26.8°E with a 95 per cent confidence ellipse having a 4.0° major semi-axis oriented 98° clockwise of north and a 1.8° minor semi-axis; the anomalous skewness is 14.2° ± 3.7°. The possible dependence of anomalous skewness on spreading rate was investigated with two empirical models and found to have a negligible effect on our palaeopole analysis over the range of relevant spreading half-rates, ~25 to ~90  mm  yr⁻¹ . The new pole is consistent with the northward motion for the Pacific plate indicated by coeval palaeocolatitude and palaeoequatorial data, but differs significantly from, and lies to the northeast of, coeval seamount poles. We attribute the difference to unmodelled errors in the seamount poles, mainly in the declinations. Comparison with the northward motion inferred from dated volcanoes along the Hawaiian–Emperor seamount chain indicates 13° of southward motion of the Hawaiian hotspot since 73  Ma. When the pole is reconstructed with the Pacific plate relative to the Pacific hotspots, it differs by 14°–18° from the position of the pole relative to the Indo–Atlantic hotspots. This has several possible explanations including bias in one or more of the palaeomagnetic poles, motion between the Pacific and Indo–Atlantic hotspots, and errors in plate reconstructions relative to the hotspots.     

On the application of asymptotic analysis to the dynamical theory of the pole tide

Summary. This article examines the effects of boundaries on the pole tide in an ocean of constant depth. The cyclically continuous global ocean solutions to Laplace's tidal equations with the pole tide forcing are used as the particular solutions to the problem. The approach here is to find approximate asymptotic solutions to the homogeneous tidal equations which can be added to the global particular solutions so that the normal component of velocity will vanish at the boundary. At the very long period of the pole tide, the unforced motions are assumed to be non-divergent, and so only the homogeneous vorticity equation must be solved.
The first case considered is a zonal ocean bounded by parallels of latitude equidistant from the equator. Asymptotic solutions are found in order to satisfy the zonal boundary condition, and this gives rise to a narrow zonal boundary current. The contribution of these solutions is exponentially small compared to the forced global pole tide except in the immediate vicinity of the northern and southern boundaries.
Next, the effect of meridional boundaries is considered. When a linear form of bottom friction is assumed, two approximate homogeneous solutions are found to construct a general solution that satisfies the meridional boundary conditions. One solution decays exponentially in longitude and gives rise to a western boundary current, while the other solution is independent of longitude. The meridional boundary conditions are used to match the homogeneous and particular solutions, and so the solution for the interior of the ocean satisfies the eastern boundary condition. The resulting solution for the pole tide has a western boundary current term, while over the majority of the ocean domain the solution has a term varying with the wavelength of the forcing (the global solution) and a zonal motion term used to satisfy the eastern boundary condition. Comparisons are made with the wind-driven ocean circulation problem.