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.     

Apparent velocity measurements on an oceanic lithosphere

A direct measurement of apparent velocities for oceanic paths was made with an array of sensitive ocean bottom seismographs. The measurement was performed by recording waves from shallow earthquakes which occurred in the area close to trench axes and which were accurately located by the land seismological network in Japan. The range of epicentral distances is from 500 to 1,800 km.The observed P travel times are less than those in the Jeffreys-Bullen tables by 6–10 s for the range of distances.Since the dimension of the OBS array is about 400 km, the apparent velocities are determined quite precisely and show little dependence on the epicentral distances. The average value of the apparent velocities for the range 500–1,700 km is 8.64 ± 0.13 km/s.An offset of travel times, which is thought to be associated with a low-velocity layer underneath the oceanic lithosphere, has been observed.These results indicate that a high-velocity layer with a velocity of 8.6 km/s exists in the lower part of the oceanic lithosphere. Beneath the 8.6-km/s layer there is a thin low-velocity layer under which the velocity of the P wave is again 8.6 km/s.     

海洋岩石圈板块有效弹性厚度研究

本文在前人研究大陆岩石圈板块有效弹性厚度的基础上,建立研究海洋岩石圈板块有效弹性厚度的理论模型,推导出与大陆岩石圈不同的海洋岩石圈板块响应函数 Z(k,Te) 理论计算公式.并分析海洋岩石圈板块响应函数 Z(k,Te) 的特点.文中对实际的海洋测量数据的响应函数 Z(k,Te) 进行计算和分析,估算我国南海南沙海域和南海中央海盆岩石圈板块有效弹性厚度分别约为10 km和6~7 km.     

One thermal model of the young oceanic lithosphere: Direct problem

Summary The two-dimensional solution of the direct plate model of the oceanic lithosphere is computed for t ^1/2 dependences of its thickness using the multi-grid technique for the bilinear finite elements. If we assume the dependence of the lithospheric thickness on its age in the form h(t)=6·6t ^1/2 +7·5, then this model can roughly approximate the measured heat flow. In this model the horizontal conductive term is not negligible, the heat flow depends both on time and on the velocity of the sea-floor spreading which should be taken into account for the handling of heat flow measurements.

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Petrological models of the oceanic lithosphere: Geophysical and geochemical tests

Petrological models of the oceanic lithosphere are tested to satisfy geophysical and geochemical constraints within the framework of plate tectonics. Quartz eclogite, olivine eclogite, peridotite and dunite are considered as the material of the lithosphere. The temperature at the base of the lithosphere is assumed to be the solidus temperature. This temperature, the thermal conductivity, and the heat flow and topography changes with age are used as the geophysical constraints. The compressional wave velocity-depth profile is used to select preferred models. Among geophysically successful models, high-temperature models are preferred to wet low-temperature models, because the low-temperature models have difficulties in explaining the mechanism of generation of oceanic basalt magmas. A preferred model is a two-layer model 70 km thick consisting of peridotite at the upper lithosphere and olivine eclogite at the lower lithosphere bounded at the base by the dry solidus.     

Age-dependent subduction of oceanic lithosphere beneath western South America

A recently established relation between the penetration depth of oceanic lithosphere and the lithospheric age appears to be of special interest to the understanding of the South American subduction zone. The main characteristics of this complicated zone, such as the absence of deep-focus earthquakes south of 30°S, the variations in the dip angle of the descending Nazca plate and the gap in seismic activity between depths of approximately 300 and 525 km, can be understood if the spatial and temporal variations in the age of the descending oceanic lithosphere are taken into account. In view of the significance of local aspects of the subduction process the South American-Nazca plate interaction cannot simply be considered as a type-example of the interaction between a continental and an oceanic plate.     

On deep electric conductivity of the lithosphere in the Tobol-Ishim interfluve (West Siberia)

On the basis of exploration magnetotelluric soundings (MTS) carried out by the Tyumen Geological Survey in 1980–1981 within the Tobol-Ishim interfluve, the top of the conductive horizon with the electric resistivity of dozens of Ω m is identified in the geoelectrical section at a depth of 75–80 km. Two segments of the locally elevated top of this horizon are revealed. One segment, where the depth of the top of the conductor is 60–65 km, corresponds to the region of the Kiselevskii fault-the main fault in this territory; another segment is associated with the edge zone of the Ishim branch of the Triassic rift system within the West Siberian Plate. Within this segment, the top of the conductive layer rises to a depth of 55-60 km. It is assumed that the Ishim geoelectrical anomaly, which projects into the geothermal anomaly and is coherent with the features of the deep geological structure of its display region, is conditioned by the element of the mantle-crustal magmatogene fluid paleosystem.     

Stability of the oceanic lithosphere with variable viscosity: an initial-value approach

We have studied the problem concerning the onset of convective instabilities below the oceanic lithosphere. A system of linear partial differential equations, in which the background temperature field is time-dependent, is integrated in time to monitor the evolution of incipient disturbances. Two types of rheologies have been examined. One depends strongly on temperature. The other involves a viscosity which is both temperature- and pressure-dependent. The results from this initial-value approach, in which the viscosity profiles migrate downward with time, reveal the importance of considering temperature- and pressure-dependent rheology in issues regarding the development of local instabilities in upper mantle convection. For temperature-dependent viscosity, viscosities of 0(10²⁰P) are required to produce instabilities with growth-rates of 0(.1/Ma). In contrast, these same growth rates can be attained for a temperature- and pressure-dependent viscosity profile with a mean value close to 0(10²⁰P) in the upper mantle, owing to the presence of a low viscosity zone, 0(10²⁰P), existing right below the lithosphere. Unlike the results of temperature-dependent viscosity, whose growth-rates increase with time, the amplification of disturbances in a fluid medium with temperature- and pressure-dependent rheology reaches a maximum at an early age, < 50 Ma, and decreases thereafter with time. This suggests the potential importance played by initial disturbances in the evolution of the oceanic lithosphere.     

Rheological properties of deep subducted oceanic lithosphere and their geodynamic implications

According to the experimental studies on the rheology of two important mantle rocks (eclogite and harzburgite), the rheological properties of the deep subducted oceanic lithosphere are investigated by assuming a simplified harzburgite type slab model with moderate thickness of basaltic layer. When the mantle convergence rate is small or the subducting slab has been trapped in the mantle for an enough long time, the strength profile of the slab is characterized by a strong subducting crustal component lying on a weak subducting upper mantle. However, if the convergence rate is large enough, the subducting slab will be featured only by a rigid cold center. Our study suggests that the detachment of the subducting crust component from the underlying upper mantle is only likely to happen in hot slow subducting slabs, but not the cold fast subducting lithosphere. Rheological properties of the harzburgitic and the eclogitic upper mantle vary with depths. The eclogitic upper mantle is stronger than the peridotitic upper mantle across the upper mantle. Transition zone is the high strength and high viscosity layer in the upper mantle except the lithosphere.     

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.     

Thermally induced brittle deformation in oceanic lithosphere and the spacing of fracture zones

Brittle deformation of oceanic lithosphere due to thermal stress is explored with a numerical model, with an emphasis on the spacing of fracture zones. Brittle deformation is represented by localized plastic strain within a material having an elasto-visco-plastic rheology with strain softening. We show that crustal thickness, creep strength, and the rule governing plastic flow control the formation of cracks. The spacing of primary crack decreases with crustal thickness as long as it is smaller than a threshold value. Creep strength shifts the threshold such that crust with strong creep strength develops primary cracks regardless of crustal thicknesses, while only a thin crust can have primary cracks if its creep strength is low. For a thin crust, the spacing of primary cracks is inversely proportional to the creep strength, suggesting that creep strength might independently contribute to the degree of brittle deformation. Through finite versus zero dilatation in plastic strain, associated and non-associated flow rule results in nearly vertical and V-shaped cracks, respectively. Changes in the tectonic environment of a ridge system can be reflected in variation in crustal thickness, and thus related to brittle deformation. The fracture zone-free Reykjanes ridge is known to have a uniformly thick crust. The Australian-Antarctic Discordance has multiple fracture zones and thin crust. These syntheses are consistent with enhanced brittle deformation of oceanic lithosphere when the crust is thin and vice versa.     

Longshot experiments to study velocity anisotropy in the oceanic lithosphere of the northwestern Pacific

Several long-range explosion seismology experiments have been conducted in the northwestern Pacific basin, where one of the oldest oceanic lithospheres is postulated to exist. The experiments were conducted from 1974 to 1980. Highly sensitive ocean-bottom seismographs which had been developed for longshot experiments were used. The lengths of the profiles ranged from 1000 to 1800 km, and the directions were chosen to provide wide azimuthal coverage. One of the aims of this series of experiments was to test the existence of velocity anisotropy on a large, regional scale.The results show that the oceanic lithosphere has anisotropy wherein the velocity changes by 4–7%. The anisotropy extends from a depth of at least 40 to 140 km beneath the sea bottom; however, the magnitude of the anisotropy may vary with depth. The azimuth of the maximum velocity is 150–160° clockwise from north, and coincides with the “fossil” direction of spreading of the Pacific plate, whereas it differs from the present direction of plate motion by ～ 30°. The azimuth does not seem to depend on depth. In the direction of maximum velocity, the lithosphere is basically two-layered: 8.0–8.2 and 8.6 km s^?1. The depth of the interface is 50–60 km beneath the sea floor.     

Global electrical conductivity of the earth

The Banks (1969, 1972) and Parker (1970) models of the electrical conductivity distribution are critically reviewed along with classical models by Chapman (1919), Lahiri and Price (1939), Rikitake (1950a, b, c) and others. The modern models do not seem to account for the geomagnetic variations having a continuum spectrum and S_q at the same time. A large difference in response between the 1-day and 0.5-day period components of S_q is suspected to be caused by a resonance-like induction in the superficial layer of the earth. Dufficulties in determining the conductivity of the earth's top layer are also emphasized.An overall distribution of conductivity within the earth which seems to be the most reliable at present, is drawn mostly on the basis of Banks' model.     

The electrical conductivity of the Moon

Investigations of the lunar electrical conductivity were described in excellent reviews by Sonett (1974) and Dyalet al. (1976). In this paper we will try to consider some new aspects of this problem in comparison with the Earth's data.     

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.     

The electrical conductivity of clouds

Summary A modified Gerdien cell was designed, evaluated, and built for measurement of the polar conductivities in clouds. This conductivity dropsonde was attached to a U.S. Weather Bureau, 1680 mHz, radiosonde for telemetry and to measure pressure, temperature, and relative humidity profiles. The combined instruments were ejected from aircraft, and others were released from balloons into the region of interest.Eight flights were made during the 1967 thunderstorm season. Three of these drops were successful in measuring conductivity inside of electrically active clouds. Two fair-weather profiles were measured for comparison purposes, and three of the drops were faulty.These very preliminary results tend to indicate considerable electrical conductivity in thunderclouds. The data are too few to support a strong statement in favor of increased conductivity, but the instruments were sufficiently reliable to prove that the conductivity was not reduced, as is normally assumed, in the clouds investigated.This research was supported by the Atmospheric Sciences Section, National Science Foundation under Grant GA-701.     

A review on developments in the electrical structure of craton lithosphere

Cratons have a long history of evolution. In this paper, applications of the magnetotelluric method used in the study of craton lithosphere over the past 30 years were reviewed, examining case studies of cratons in North America, South America, Asia, Australia, and Africa. The nuclei of the Archean cratons, for example the Kalahari Craton and Rae Craton, are usually characterized by thick and highly resistive lithospheric roots. During or after the formation of the cratons, tectonothermal events, such as collision, mantle plume, and asthenosphere upwelling led to the formation of high-conductivity zones in the craton lithosphere, which could be attributed to the increased hydrogen content (of nominally anhydrous minerals), higher iron content, and formation of graphite films or sulfides along the grain boundary of minerals. These conductive zones are characterized by resistivity discontinuities in craton lithosphere. In particular, the conductive zones include (1) large-scale lithospheric mantle conductors beneath the Slave Craton, Gawler Craton, and central part of North China Craton(Trans-North China Orogen); (2) near-vertical high-conductivity zone associated with the fossil subduction zone beneath the Dharwar Craton and Slave Craton; and (3) regional lateral electrical discontinuities, such as a conductive anomaly under the Bushveld Complex of the Kaapvaal Craton. The eMoho refers to the electrical discontinuity in the crust-mantle boundary. In existing research, this has been detected under the condition of extremely high lithospheric resistivity with only a slight decrease in the lower crust, and in the case of a very thin conductive lower crust or the lack thereof. In the resistivity model, the unique “mushroom-like” lower crust-lithosphere mantle conductor and very thin lower crust layer of the North China Craton may represent lithosphere destruction and/or thinning. We also find that some of the cratons are still not well understood. Therefore, extensive three-dimensional inversion and joint interpretation of geochemical, geophysical, and geologic data are necessary to understand the tectonic evolutionary history of craton lithosphere.

     

Interarc spreading and Cordilleran tectonics as alternates related to the age of subducted oceanic lithosphere

Old, cold oceanic lithosphere is denser and therefore gravitationally more unstable than younger, hotter oceanic lithosphere. Hence, whereas old lithosphere will sink under its own weight, subduction of young lithosphere may require an additional force. Interarc spreading occurs or occurred recently in the western Pacific, in the southern Atlantic, and possibly in the Mediterranean, where the subducted sea floor appears to be more than 50 m.y. old, and in many cases, is more than 100 m.y. old. In most of these regions, the ease with which the old dense lithosphere sinks may have contributed to a seaward migration of the trenches, which led to interarc spreading. Cordilleran tectonics, including high mountains and broad zones of deformation, are present on the margins of the eastern Pacific where the subducted oceanic lithosphere is younger than about 50 m.y. An extra force, which we presume to be necessary to cause subduction of the young lithosphere, may be responsible for the deformation and mountains just as an extra force seems necessary to drive continental collision in Asia. The extensive early Tertiary deformation across a broad zone of western North America may be related to the long-term, continuous subduction of young lithosphere of the Farallon and Kula plates.     

Temperature-time dependence of the electrical conductivity of garnets

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On the question of the interrelation between variations in crustal electrical conductivity and geodynamical processes

The behavior of the variations in the crustal electrical conductivity in a wide range of periods is studied from the data of magnetotelluric soundings (MTS) during the Kambarata experiment (a strong industrial explosion to construct the blast-fill dam on the Naryn river), as well as at Aksu, a stationary geophysical monitoring point. The concept of the interrelation between the stress-strain state of the medium and the change in the apparent electrical resistivity, which is based on the idea of the redistribution of mineralized solutions between the crack networks, is confirmed experimentally. A procedure of azimuthal monitoring is developed, which allowed us not only to identify the anomalous changes in the module and phase of apparent resistivity but also to establish the directions of their maximum increases and decreases (the axes of compression and tension). For 34 points of deep MTS in the territory of Central Tien Shan, the depth intervals in the upper crust that are most sensitive to the changes in the stress-strain state of the medium are established. The variations in the electrical conductivity are compared with the solar-lunar tidal impacts. It is shown that by analyzing the recorded time series, it is possible to recognize the characteristic signs of the changes in the stress-strain state of the medium that are caused by seismic events.