Thrusting of young lithosphere in subduction zones with special reference to structures in ophiolitic peridotites

Two contrasted types of structures have been recognized in peridotites from ophiolites and from the oceanic environment. The first one, typical of high-temperature/moderate-stress conditions, is observed in the upper part of ophiolitic peridotites and has been ascribed to plastic flow in an oceanic ridge environment. The second one, typical of moderate-temperature/high-stress conditions, is more specially dealt with here. It is printed in the peridotites above the basal metamorphic aureole found in many ophiolites. The strain increases downward over 1–2 km to produce peridotite mylonites at the contact with the metamorphic aureole. Similarities with rocks from trench and island arc environments suggest ascribing this deformation in ophiolites to a trench environment. We propose that shear fracturing in a young oceanic lithosphere is initiated by the compressive elastic stress in its lower part which is produced by bending of the subducted plate. An externally applied compressive stress is responsible for subsequent overthrusting of the fractured lithosphere. This interpretation is in good agreement with the available geophysical data on young subducted plates and with the physical data on ophiolitic peridotites.     

Thermal cooling of the oceanic lithosphere: new constraints from geoid height data

Up to now, tests of thermal models of the oceanic lithosphere as it cools and moves away from the ridge crest have been based mainly on topography and heat flow data. However, large areas of the ocean floor deviate from the normal subsidence due to thermal contraction and heat flow data are not very sensitive to the form of the model.

Cooling of the lithosphere causes a short-wavelength step in the geoid across fracture zones that can also be used to constrain thermal models. We have analyzed geoid data at fracture zones from the SEASAT altimeter measurements in the entire Pacific Ocean and redetermined parameters of the cooling models. We find that the data reveal two distinct regimes of cooling; one for seafloor ages in the range 0–30 Ma, the other beyond 30 Ma; this does not appear to be correlated with particular fracture zones but rather it is representative of the whole area studied, i.e., the entire south Pacific and northeast Pacific Ocean. These two trends may be interpreted in terms of two different (asymptotic) thermal thicknesses of the plate model. The smaller thermal thickness ( 65 km) found for ages <30 Ma—compared to 90 km in the age range 30–50 Ma—calls for some kind of thermal perturbation in the vicinity of the ridge crest.

From the results obtained in this study, we conclude that the half-space cooling model is unable to explain the data, that beyond 30 Ma, a simple plate model gives a satisfactory fit to the data but in the younger plate portion (ages < 30 Ma) the cooling history of the oceanic lithosphere is much more complex than predicted by the usual cooling models. Furthermore, the depth-age relationship obtained from the geoid-derived thermal parameters departs significantly beyond 30 Ma from the widely used Parsons and Sclater's depth-age curve, predicting a lesser subsidence.     

The concentrations of the heavy metals in the four new Antarctic meteorites Yamato (a), (b), (c) and (d) and in Orgueil, Murray, Allende, Abee, Allegan, Mocs and Johnstown

All active midocean ridges show a uniform relationship between depth and age of the oceanic crust. Recently, it has been shown by numerical methods that convective flow in a Newtonian fluid will have a positive gravity anomaly and an upward surface deformation associated with an ascending limb. If there is thermal convection in the upper mantle, these calculations predict that there may be a correlation between free air gravity anomalies and differences from the uniform relationship between oceanic depth and age. To investigate such a correlation, we considered the crestal elevation and free air gravity anomaly over the crest of the midocean ridges. It has been suggested that the differences from the depth versus age relationship are related to spreading rate. Thus, we also considered a correlation between crestal elevation and changes in rate along the ridge axis.We found a positive correlation between free air gravity and differences in crestal depth of the midocean ridge system. We found no correlation between spreading rate and gravity and no uniform relationship which holds in all the oceans between spreading rate and observed crestal depths.The long wavelength gravity anomalies which are correlated with the differences in crestal depth cannot be supported by an 80 km thick lithosphere. Thus, they are considered evidence of flow within the aesthenosphere. Further, the correlation between gravity anomaly and differences in crestal depth has the same sign and gradient as predicted by the investigations of convection in a Newtonian fluid.     

Mantle flow patterns at the Neyriz Paleo-spreading center, Iran

The mantle peridotites of Neyriz record two successive episodes of plastic deformations; the first one related to the igneous accretion of the lithosphere and the second one developed during the first stage of the emplacement of the peridotites. These two events have been distinguished on the basis of microstructural criteria. The diapiric pattern, particularly relevant to the mantle process beneath spreading ridges, features vertical flow lines and elliptic flow plane trajectories in a pipe and extends along the ridge axis about 5 km. These structures rotate to horizontal and diverge in every direction in a narrow transition zone, a few hundred meters thick, below the Moho discontinuity. Such a diapiric pattern has been recognized in a few places along the Neyriz paleo-ridge. A large amount of magma passed through these mantle diapirs that were probably the main zones feeding the overlying magma chamber. The most common pattern features very regular structures over several kilometers along the strike of the paleo-ridge: the flow plane dips away from the ridge axis, and the flow line is parallel to the spreading direction. This flow pattern is frozen during the gradual accretion of the lithospheric mantle away from the ridge in a steady-state spreading regime. A shear-sense inversion at just below the Moho is commonly observed, pointing to forced asthenospheric flow. The reconstructed orientation of the Neyriz paleo-spreading center is 105°, compatible with the geometry and orientation of harzburgite foliations and lineations and sheeted dikes.     

Heat loss from the earth: A constraint on Archaean tectonics from the relation between geothermal gradients and the rate of plate production

The models suggested for the oceanic lithosphere which best predict oceanic heat flow and depth profiles are the constant thickness model and a model in which the lithosphere thickens away from the ridge with a heat source at its base. The latter is considered to be more physically realistic. Such a model, constrained by the observed oceanic heat flow and depth profiles and a temperature at the ridge crest of between 1100°C and 1300°C, requires a heat source at the base of the lithosphere of between 0.5 and 0.9 h.f.u., thermal conductivities for the mantle between 0.005 and 0.0095 cal cm⁻¹ °C⁻¹ s⁻¹ and a coefficient of thermal expansion at 840°C between 4.1 × 10⁻⁵ and 5.1 × 10⁻⁵ °C⁻¹. Plate creation and subduction are calculated to dissipate about 45% of the total earth heat loss for this model. The efficiency of this mechanism of heat loss is shown to be strongly dependent on the magnitude of the basal heat source. A relation is derived for total earth heat loss as a function of the rate of plate creation and the amount of heat transported to the base of plates. The estimated heat transport to the base of the oceanic lithosphere is similar to estimates of mantle heat flow into the base of the continental lithosphere. If this relation existed in the past and if metamorphic conditions in late Archaean high-grade terrains can be used to provide a maximum constraint on equilibrium Archaean continental thermal gradients, heat flow into the base of the lithosphere in the late Archaean must have been less than about 1.2–1.5 h.f.u. The relation between earth heat loss, the rate of plate creation and the rate of heat transport to the base of the lithosphere suggests that a significant proportion of the heat loss in the Archaean must have taken place by the processes of plate creation and subduction. The Archaean plate processes may have involved much more rapid production of plates only slightly thinner than at present.     

The existence of a thin low-viscosity layer beneath the lithosphere

The horizontal temperature gradient at the base of the lithosphere at an oceanic fracture zone, where plate of different ages is juxtaposed, is expected to drive a local circulation, the characteristics of which can be constrained by the amplitude, wavelength and age-dependence of the geoid. Two-dimensional numerical models of convection in a fluid layer overlain by a solid conducting lid have been used to generate theoretical geoid profiles at right angles to the fracture zone. Only a thin, low-viscosity layer provides a reasonable fit to the data. The best model so far obtained has a fluid layer 150 km thick with viscosity 1.5 × 10¹⁹ Pa s under a 75 km lid. Such a layer, which is incapable of transmitting strong horizontal shear stresses, could provide the decoupling mechanism between plate and deep mantle flow required to balance the forces on the plates.     

On the applicability of a universal elastic trench profile

Using thin elastic plate theory and neglecting horizontal applied forces, a universal deflection profile applicable to many oceanic trenches is derived. This theoretical profile is compared with bathymetric profiles from the central Aleutian, Kuril, Bonin, and Mariana trench-outer rise regions. The profiles were corrected for sediment thickness and age variation of the lithosphere. Good agreement between theory and observation is found. The distance from the first point of zero deflection seaward of the trench to the point of maximum height of the outer rise is directly related to the flexural rigidity of the lithosphere. The thickness of the elastic lithosphere is found to vary between 20 and 29 km for the trench profiles considered. The good agreement obtained shows that horizontal forces may be neglected and that the bending lithosphere behaves elastically in the cases considered. The analysis shows that only unreasonably large horizontal forces would affect the universal deflection curve. It is concluded that although the near-surface lithosphere may be subject to brittle fracture, the deeper lithosphere is capable of transmitting elastic stresses as high as 9 kbar.     

Geotectonic bipolarity, evidence from the pattern of active oceanic ridges bordering the Pacific and African plates

Active oceanic ridges are part of the global system of diverging plate boundaries encircling the Earth. They represent weak zones of the lithosphere. They are isostatically equilibrated. The system as a whole is considered to be well adapted to the present field of plate driving forces. The search for regularities in the pattern of active oceanic ridges may, therefore, provide valuable information as to the large-scale characteristics of structures and processes in the Earth’s mantle. Two large belts of active oceanic ridges are envisaged: (1) The semi-circular belt bordering the Pacific plate which extends from South of Tasmania to Northwest of Vancouver Island over a length of 20,000 km. It appears to encircle a center P1 in the central Pacific region. (2) The circum-African belt bordering the African plate which extends from the Azores to the Gulf of Aden over a length of 24,000 km. It appears to encircle a center A1 in central Africa. The attempt is made to determine the position of these centers. Extent and position of the ridge systems are described by 34 fixed points. Points R01–R20 mark the circum-African ridge system, points R21-R34 the Pacific ridge system. A least-squares adjustment is used to determine the optimum position of the centers P1 and A1. Center P1 of the Pacific ridge system is located at 169.8°W/2.6°S. Center A1 of the circum-African ridge system is located at 11.6°E/2.4°N. The location error of the centers is less than 2.8°. In view of the great extent of the ridge systems, and considering the fact that the location of P1 and A1 is based on independent data sets, the nearly antipodal and equatorial position of the centers is remarkable. The newly defined centers P1 and A1 are located close to the Pacific pole P, at 170°W/0°N, and the African pole A, at 10°E/0°N. Within the limits of error the center P1 coincides with pole P, the center A1 with pole A. Originally, these poles were introduced in order to describe a fundamental hemispherical symmetry which is apparent in the evolution of the Earth’s lithosphere during the last 180 Ma. The new results confirm the unique position of poles P and A in the global tectonic framework.     

Approximate solutions for the formation of the lithosphere

The lithosphere is interpreted as a thermal boundary layer. Approximate solutions of the boundary layer cooling problem are developed which include mantle radioactivity, partial melt in the asthenosphere, a temperature gradient in the asthenosphere, and a non-zero lithospheric thickness at the ridge crests. The cooling history of oceanic lithosphere is found to be remarkably insensitive to assumptions about the amount of radioactivity in the upper mantle and the extent of melting in the asthenosphere. Determinations of the thickness of oceanic lithosphere and the depths of oceans as a function of age are in excellent agreement with boundary layer predictions which include a heat flux from the asthenosphere. However, the determinations do not resolve how much of the total asthenospheric heat flux might be caused by a temperature gradient in the asthenosphere. Simple thermal arguments indicate that the initial lithospheric thickness, L₀, at ridge crests should depend on the local half-spreading rate, V, as L₀ = 3 km/V(cm/year).     

Subsidence and flexure along the Pratt-Welker seamount chain

Chemical and age data led Turner, Jarrard and Forbes to conclude that the origin of the Pratt-Welker seamount chain in the Gulf of Alaska cannot be attributed to a single cause. They argued instead that some seamounts in the chain formed over a hotspot, away from a ridge, while others formed near a ridge. They also noted that the latter group of guyots were generally deeper than the former and they used this observation to predict the origin of the unsampled seamounts in the chain. A second geophysical test of the origin hypothesis is to examine the degree of isostatic compensation of the guyots; seamounts formed near a ridge should be in a state of local isostatic compensation, while seamounts formed away from a ridge should be regionally compensated. This test has been carried out using GEOS 3 and SEASAT altimeter data. The effective flexural rigidity of the lithosphere below all seamounts is found to be less than about 10²⁰ Nm, such that the isostatic state is nearly local, rather than regional. This may be a consequence of all the seamounts having formed on an initially weak lithosphere, of stress relaxation subsequant to their formation away from the ridge, or both. If the seamounts from Giacomini to Durgin formed away from the ridge then these results point to an effective flexural rigidity at the time of loading of about 3 × 10²¹ Nm and to a stress-relaxation time of about 10⁶ years. These values are for an ocean lithosphere that was about 20–22 my old when loaded. Corresponding values for 60 my old lithosphere in the southern Pacific were previously found to be about 3 × 10²² Nm and 5 × 10⁶ years. This comparison suggests that both the initial elastic response and the rate of stress relaxation are functions of the age of the lithosphere. The subsidence of guyots is due to numerous factors including thermal contraction of the seafloor, sediment loading, the flexure of the lithosphere prior to its subduction along the Aleutian Trench and, in view of the above short stress relaxation time, stress relaxation. A principal uncertainty in evaluating the subsidence that has occurred subsequent to the seamount having been eroded to sealevel is the erosion time interval. The comparison of the predicted subsidence with observed depths points to an erosion time constant of the order of 5 my and which is a function of seamount size. The conclusions from the flexure and subsidence analysis as to where the various seamounts formed are in agreement with those of Turner et al. Giacomini, Quinn, Surveyor, Pratt and Durgin formed away from a ridge and are consistent with a fixed hotspot and uniform spreading rate model. The geophysical information for Denson, Davidson and Hodgkins is consistent with the hypothesis that these guyots formed near or on a ridge. The case for Welker seamount is ambiguous, and this guyot may have formed over a second hotspot, located at an intermediate distance between the first and the ridge. The geophysical evidence for Bowie seamount is also ambiguous. Possibly it has a similar source to Welker, suggesting that there may actually be three different origin mechanisms that led to the chain.     

Modelling mixing and circulation in subglacial Lake Vostok,Antarctica

Lake Vostok, isolated from direct exchange with the atmosphere by about 4 km of ice for millions of years, provides a unique environment. This inaccessibility raises the importance of numerical models to investigate the physical conditions within the lake. Using a three-dimensional numerical model and the best available geometry, we test different parameter settings to define a standard model configuration suitable for studying flow in this subglacial lake. From our model runs we find a baroclinic circulation within the lake that splits into three different parts: Along a topographic ridge in the northern part of Lake Vostok, bottom water masses are transported eastward, diverging away from the ridge. In the lake’s surface layer, the flow in these two vertical overturning cells has opposite directions. In the southern part of the lake, where freezing occurs across about 3,500 km², two opposing gyres split the water column vertically. The general flow is stronger in the southern basin with horizontal velocities in the order of 1 mm/s. The strongest upwelling, found in the eastern part of this basin, is about 25 μm/s. We estimate the lower limit of the overturning timescale to be about 2.5 years vertically and 8.6 years horizontally. The basal mass loss of ice from the ice sheet floating on the lake is 5.6 mm/year (equivalent to a fresh water flux of 2.78 m³/s, or a basal ice loss of 0.09 km³/year). This imbalance indicates either a constant growth of the lake or its continuous (or periodical) discharge into a subglacial drainage system.     

Heat flow and thickness of the oceanic lithosphere

Existing thermal models of the oceanic lithosphere predict too sharp an increase of heat flow towards the ridge axis. A new mathematical model of a thickening lithosphere is presented. The temperature distribution is computed by the use of observed surface heat flow as a boundary condition. If observed heat flow values represent flow of heat from the mantle, the model predicts a rather rapid growth of the lithosphere within the first 30 m.y. and a nearly steady state after 100 m.y. Heat flow from the asthenosphere to the lithosphere shows a minimum near the ridge axis, suggesting a down-going convective flow in the asthenosphere at both sides of a spreading center.     

Diapirism of depleted peridotite — a model for the origin of hot spots

A model is proposed for the origin of hot spots that depends on the existence of major-element heterogeneities in the mantle. Generation of basaltic crust at spreading centers produces a layer of residual peridotite ～20–25 km thick directly beneath the crust which is depleted in Fe/Mg, TiO₂, CaO, Al₂O₃, Na₂O and K₂O, and which has a slightly lower density than undepleted peridotite beneath it. Upon recycling of this depleted peridotite back into the deep mantle at subduction zones, it becomes gravitationally unstable, and tends to rise as diapirs through undepleted peridotite. For a density contrast of 0.05 g cm^?3, a diapir 60 km in diameter would rise at roughly 8 cm y^?1, and could transport enough heat to the base of the lithosphere to cause melting and volcanism at the surface. Hot spots are thus viewed as a passive consequence of mantle convection and fractionation at spreading centers rather than a plate-driving force.It is suggested that depleted diapirs exist with varying amounts of depletion, diameters, upward velocities and source volumes. Such variations could explain the occurrence of hot spots with widely varying lifetimes and rates of lava production. For highly depleted diapirs with very low Fe/Mg, the diapir would act as a heat source and the asthenosphere and lower lithosphere drifting across the diapir would serve as the source region of magmas erupted at the surface. For mildly depleted diapirs with Fe/Mg only slightly less than in normal undepleted mantle, the diapir could provide not only the source of heat but also most or all of the source material for the erupted magmas. The model is consistent with isotopic data that require two separate and ancient source regions for mid-ocean ridge and oceanic island basalts. The source for mid-ocean ridge basalts is considered to be material upwelling at spreading centers from the deep mantle. This material forms the oceanic lithosphere. Oceanic island basalts are considered to be derived from varying mixtures of sublithospheric and lower lithospheric material and the rising diapir itself.     

Dynamics of the mid-ocean ridge plate boundary: Recent observations and theory

The global mid-ocean ridge system is one of the most active plate boundaries on the earth and understanding the dynamic processes at this plate boundary is one of the most important problems in geodynamics. In this paper I present recent results of several aspects of mid-ocean ridge studies concerning the dynamics of oceanic lithosphere at these diverging plate boundaries. I show that the observed rift valley to no-rift valley transition (globally due to the increase of spreading rate or locally due to the crustal thickness variations and/or thermal anomalies) can be explained by the strong temperature dependence of the power law rheology of the oceanic lithosphere, and most importantly, by the difference in the rheological behavior of the oceanic crust from the underlying mantle. The effect of this weaker lower crust on ridge dynamics is mainly influenced by spreading rate and crustal thickness variations. The accumulated strain pattern from a recently developed lens model, based on recent seismic observations, was proposed as an appealing mechanism for the observed gabbro layering sequence in the Oman Ophiolite. It is now known that the mid-ocean ridges at all spreading rates are offset into individual spreading segments by both transform and nontransform discontinuities. The tectonics of ridge segmentation are also spreading-rate dependent: the slow-spreading Mid-Atlantic Ridge is characterized by distinct bulls-eye shaped gravity lows, suggesting large along-axis variations in melt production and crustal thickness, whereas the fast-spreading East-Pacific Rise is associated with much smaller along-axis variations. These spreading-rate dependent changes have been attributed to a fundamental differences in ridge segmentation mechanisms and mantle upwelling at mid-ocean ridges: the mantle upwelling may be intrinsically plume-like (3-D) beneath a slow-spreading ridge but more sheet-like (2-D) beneath a fast-spreading ridge.     

Modelling the composition of melts formed during continental breakup of the Southeast Greenland margin

We have developed a generic dynamic model of extension of the lithosphere, which predicts major element composition and volume of melt generated from initial extension to steady state seafloor spreading. Stokes equations for non-Newtonian flow are solved and the mantle melts by decompression. Strengthening of the mantle due to dehydration as melting progresses is included. The composition is then empirically related to depletion. Using a crystallisation algorithm, the predicted primary melt composition was compared with mean North Atlantic mid-ocean ridge basalt (MORB). At steady state, using half spreading rates from 10 to 20 mm yr^− 1 and mantle potential temperatures of 1300 to 1325 °C we predict a major element composition that is within the variation in the mean of North Atlantic MORB.

This model is applied to the Southeast Greenland margin, which has extensive coverage of seismic and ODP core data. These data have been interpreted to indicate an initial pulse of magmatism on rifting that rapidly decayed to leave oceanic crustal thickness of 8 to 11 km. This pattern of melt production can be recreated by introducing an initial hot layer of asthenosphere beneath the continental lithosphere and by having a period of fast spreading during early opening. The hot layer was convected through the melt region giving a pulse of high magnesian and low silica melt during the early rifting process. The predicted major element composition of primary melts generated are in close agreement with primary melts from the Southeast Greenland margin. The observed variations in major element composition are reproduced without a mantle source composition anomaly.     

岩浆与构造作用对洋壳厚度的影响——以西北印度洋为例

洋中脊及邻区洋盆的洋壳厚度能很好地反映区域岩浆补给特征,对于研究洋中脊内部及周缘岩浆活动和构造演化过程具有很好的指示意义.西北印度洋中脊作为典型的慢速扩张洋中脊,其扩张过程与周缘构造活动具有很强的时空关系.本文利用剩余地幔布格重力异常反演了西北印度洋洋壳厚度,由此分析区域内洋壳厚度分布和岩浆补给特征.研究发现,西北印度洋洋壳平均厚度为7.8 km,受区域构造背景影响厚度变化较大.根据洋壳厚度的统计学分布特征,将区域内洋壳分为三种类型:薄洋壳(小于4.5 km)、正常洋壳(4.5~6.5 km)和厚洋壳(大于6.5 km),根据西北印度洋中脊周缘(~40 Ma内)洋壳厚度变化特征可将洋中脊划分为5段,发现洋中脊洋壳厚度受区域构造活动和地幔温度所控制,其中薄洋壳主要受转换断层影响造成区域洋壳厚度减薄,而厚洋壳主要受地幔温度和地幔柱作用影响,并在S4洋中脊段显示出较强的热点与洋中脊相互作用,同时微陆块的裂解和漂移也可能是导致洋壳厚度差异的原因之一.     

大陆岩石圈、地幔底部异常体与地幔对流相互作用的数值模拟

在地球表层存在着占地表面积约30%的具有低固有密度、高黏度的大陆岩石圈.由于其特殊的物理化学性质,大陆岩石圈通常不直接参与下方的地幔对流,但其与地幔对流格局有着重要的相互影响.大量研究显示,在中太平洋和非洲的下地幔底部,存在着两块占核幔边界（CMB）面积约20%的高密度热化学异常体（由于其剪切波速度较低,常称作低剪切波速度省（LSVPs））.LSVPs的演化既受地幔对流的影响,同时也影响地幔物质运动的格局和动力学过程.本文系统研究了存在大陆岩石圈,下地幔LSVPs的地幔对流模型.模拟结果显示：（1）当大陆体积较小时,其边缘常伴随着俯冲,大陆区域地幔常处于下涌状态,其上地幔温度较低,大陆岩石圈在水平方向处于压应力状态.随着大陆体积的增大,大陆边缘的俯冲逐渐减弱,大陆区域地幔由下涌转为上涌,其上地幔温度较高,大陆岩石圈水平方向处于拉应力状态.（2） 岩石圈与软流圈边界（LAB）在大陆下方较深,温度较低;在海洋区域较浅,温度较高.随着大陆体积的增大,陆洋之间LAB深度、温度的差异逐渐减小.（3）大陆区域地幔底部LSVPs物质的丰度与大陆的体积呈正相关.当大陆体积较小时,大陆下方的LSVPs丰度比海洋区域少.随着大陆体积的增大,大陆下方LSVPs的丰度逐渐增大.（4）海洋地区地表热流高,且随时间波动大,大陆地区地表热流低,随时间波动较小;LSVPs区域的核幔边界热流低.     

Lithospheric model with thick oceanic crust at the continental boundary: A mechanism for shallow spreading ridges in young oceans

In a general lithospheric model of a simple divergent ocean and continental margin that satisfies the constraints of isostasy and gravity anomalies, the free-air gravity anomaly at the margin is modelled by an oceanic crust that thickens exponentially toward the margin from its common value of 6.4 km about 600 km from the margin to 17.7 km at the margin; this postulated thickening is supported empirically by seismic refraction measurements made near continental margins. The thickness of the oceanic crust matches that of the continental lithosphere at breakup, as observed today in Afar and East Africa, and is interpreted as the initial oceanic surface layer chilled against the continental lithosphere. With continued plate accretion, the chilled oceanic crust thins exponentially to a steadystate thickness, which is achieved about 40 m.y. after breakup. These findings contrast with the generally held view that the oceanic crust has a uniform thickness.During the first 40 m.y. of spreading, the thicker oceanic crust, of density 2.86 g/cm³, displaces the denser (3.32 g/cm³) subjacent material; by isostasy, the spreading ridge and the rest of the seafloor thus stand higher in younger( <40m.y.) oceans than they do in older(>40m.y.) oceans. This is postulated to be the cause of the empirical relationship between the crestal depth of spreading ridges and the age (or half-width) of ocean basins.     

Effect of the oceanic lithosphere velocity on free convection in the asthenosphere beneath mid-ocean ridges

The paper presents results obtained in experiments on a horizontal layer heated from below in its central part and cooled from above; the layer models the oceanic asthenosphere. Flow velocity and temperature profiles are measured and the flow structure under boundary layer conditions is determined (at Rayleigh numbers Ra > 5 × 10⁵). The flow in the core of a plane horizontal layer heated laterally and cooled from above develops under conditions of a constant temperature gradient averaged over the layer thickness. The flow core is modeled by a horizontal layer with a moving upper boundary and with adiabatic bounding surfaces under conditions of a constant horizontal gradient of temperature. Exact solutions of free convection equations are found for this model in the Boussinesq approximation. Model results are compared with experimental data. Temperature and flow velocity ranges are determined for the boundary layer regime. Based on the experimental flow velocity profiles, an expression is found for the flow velocity profile in a horizontal layer with a mobile upper boundary heated laterally and cooled from above. Free convection velocity profiles are obtained for the asthenosphere beneath a mid-ocean ridge (MOR) with a mobile lithosphere. An expression is obtained for the tangential stress at the top of the asthenosphere beneath an MOR and the total friction force produced by the asthenospheric flow at the asthenosphere-lithosphere boundary is determined.