Possible mechanism of horizontal overpressure generation of the Khibiny,Lovozero, and Kovdor ore clusters on the Kola Peninsula

The paper discusses questions related to the generation of increasing crustal horizontal compressive stresses compared to the idea of the standard gravitational state at the elastic stage or even from the prevalence of horizontal compression over vertical stress equal to the lithostatic pressure. We consider a variant of superfluous horizontal compression related to internal lithospheric processes occurrin in the crust of orogens, shields, and plates. The vertical ascending movements caused by these motions at the sole of the crust or the lithosphere pertain to these and the concomitant exogenic processes giving rise to denudation and, in particular, to erosion of the surfaces of forming rises. The residual stresses of the gravitational stressed state at the upper crust of the Kola Peninsula have been estimated for the first time. These calculations are based on the volume of sediments that have been deposited in Arctic seas beginning from the Mesozoic. The data speak to the possible level of residual horizontal compressive stresses up to 90 MPa in near-surface crustal units. This estimate is consistent with the results of in situ measurements that have been carried out at the Mining Institute of the Kola Science Center, Russian Academy of Sciences (RAS), for over 40 years. It is possible to forecast the horizontal stress gradient based on depth using our concept on the genesis of horizontal overpressure, and this forecasting is important for studying the formation of endogenic deposits.     

Computer modeling of folding in rocks

The results of numerical mathematical modeling of rock deformations under compression are given. The numerical solutions are obtained using discretization of the equations of the solid mechanics with the finite element method. The model of an ideal elastic-plastic material with a Huber-Mises yield surface was used in the calculations. The layered medium structure is taken into account in modeling of the compression of layers of the lower/middle crust on a local scale. The natural folds in strongly deformed metamorphic sequences were reproduced by the mathematical deformation models. It is shown that folding in the lower part of the Earth’s crust is possible, when the yield stresses of the host rocks are approximately two orders of magnitude less than those of the hard layers. The effect of the boundary conditions and thickness of the compressed rocks on the folding is shown.     

Numerical modelling of the three-dimensional stress field in southwestern Japan

It has been inferred from the focal mechanism of earthquakes and their hypocenter distribution (Shiono, 1977) that the stress field in southwestern Japan indicates complicated features; a NW-SE compression at shallow depths along the Nankai trough, an E-W or ESE-WNW compression in the inland crust, an extension parallel to the leading edge of the Philippine Sea plate at subcrustal depths in the region from the southern Chubu to northwestern Shikoku, and a down-dip tension beneath the Kyushu island.In order to investigate possible sources of these complex features of the stress state, a three-dimensional finite element method is employed to model the configuration of the subducting Philippine Sea plate, taking into consideration the following three possible types of forces:

1. (1) A negative buoyancy due to the density contrast between the subducting plate and the surrounding mantle.

2. (2) A northwestward compressive force generated by the movement of the Philippine Sea plate.

3. (3) A westward compressive force due to the movement of the Pacific plate.

For various combinations of different magnitudes of these forces, and of different elastic moduli, the stresses at a number of selected sites are calculated, and their directions are compared with those inferred from the focal mechanism and other geophysical information.It is found that the observed extensional stresses parallel to the leading edge of the subducting Philippine Sea plate may be caused mainly by the negative buoyancy. The northwestward compressive force may not play an important role in generating the complex stress field in southwestern Japan. The observed E-W compressional stress field prevailing in the inland region appears to result mainly from the westward-moving Pacific plate. The present results suggest that if a thin low-velocity transitional layer exists just above the subducting Philippine Sea plate, it could give appreciable effects on the mechanism of low-angled thrust faulting off the Kii peninsula and the Shikoku island.The magnitude of the shear stress in the continental crust and in the subducting plate is estimated to be of the order of several hundred bars, although the calculated shear stresses are considerably affected by the configuration of the subducting plate and also by the applied forces.It is interesting that the stress distribution appears to have some relations to seismicity of subcrustal earthquakes, and to the rupture process of large thrust earthquakes along the plate boundary.     

Collisional plateaus

In the fifteen years since the importance of collisional plateaus with thickened continental crust began to be recognized as one of the inevitable consequences of the processes of plate tectonics, rapid progress in their understanding has come from studies of the world's only active terminal collision zones in the Himalayan-Tibetan and Turkish-Iranian plateaus.Ancient collisional plateaus are being recognized throughout the geological record (back to 3.8 Ga) from the occurrence of extensive areas (typically > 500,000 km²) of 8 kbar metamorphism in granulite facies or from the occurrence of extensive areas of higher level minimum-melt composition granite rocks whose isotopic signatures indicate reactivation of existing continental crust rather than addition of new crust from the mantle at the time of collision. Recognition of strike-slip faulting in the ancient collisional plateau areas indicates that “tectonic escape” may have been as important in the past as it is today.Earth may not be the only planet on which collisional plateaus are important. The highlands of Venus (approximately 7% of the surface with elevations over 1.5 km above mean planetary radius) can only exist as a result of crustal thickening, and not as a product of lithospheric thinning. Most of these highlands can be explained by models involving volcanic construction. However, the highest peaks, including Maxwell Montes, the highest mapped area of Venus rising over 10 km above mean planetary radius, require much greater crustal thickening to support them than can reasonably be explained by a volcanic mechanism. Geological features of Maxwell Montes inferred from radar images suggest some analogy between Maxwell Montes and the Tibetan plateau.It is somewhat paradoxical that extensional tectonics are commonly associated with continental collision, and that collision-related rifts may be the only sites where the uppermost layers of a collision-thickened crust are preserved from erosion. Extensional stress fields are generated during continental collision, primarily in areas associated with strike-slip faulting and “tectonic escape”. Additional extensional stresses are gravitationally generated associated with the topography and thickened crust in a collision zone. Tectonically thickened crust is particularly susceptible to rifting as its lithosphere is weak as a result of heating associated with magmatism. This lithosphere is also compositionally weak because of the relatively thick crust, dominated by a weak quartz rheology, and thin mantle lithosphere, dominated by a strong olivine rheology, in comparison with a lithosphere with a more normal crustal thickness. Thus, the common association of rifts and collision zones may be a consequence of both stresses generated during collision and modification of the lithosphere by collision.     

The Stress Field in the Upper Crust as Determined from In Situ Measurements

This paper provides a synopsis of the state of stress in the upper parts of the earth's crust based uponin situ rock stress determinations. Despite the large scatter of the data, two dominant trends can be detected in the variations of average horizontal stress with depth in various geological environments. Basement rocks in ancient shields and deformed rocks in fold belts usually show horizontal stresses larger than the theoretical overburden pressure. Sedimentary cover rocks and fissured massive rocks show horizontal stresses smaller than the overburden pressure. The ratio of the maximum to the minimum horizontal stress exhibits a clear stress anisotropy in most cases. Directions of maximum horizontal compression are fairly consistent in areas where sufficient measurements are available (North America and Fennoscandia), although in many instances they do not conform to any simple predicted stress pattern. Many factors complicate the interpretation ofin situ stress determinations. There is no simple relationship between the stress trajectories and the free surface. Topographic features and erosional processes may cause horizontal stress concentrations. Remanent stresses of great age can be superimposed on current tectonic stresses, while sometimes current stresses no longer coincide with the stress systems that caused observable faulting and folding. Observation of remanent stresses in ancient rocks shows that rocks in the upper crust have finite strength even under geological time intervals. From the viewpoint of global tectonics,in situ stress determinations ought to be used with great caution, and in conjunction with focal mechanism solutions of earthquakes. Although the state of stress is everywhere compressive, the fact that undeformed sedimentary cover rocks often show no excess horizontal stress would seem to indicate that no active global horizontal compression is required. Horizontal stresses larger than the overburden pressure in regions of intense palaeodeformation may be due to remanent stress effects and to the influence of the local structure. It is premature to advance any general statement on tectogenesis on the basis ofin situ stress determinations. More measurements, and a quantitative evaluation of the factors affecting them, are required before further progress can be made.     

Numerical simulation of fault reactivation phenomenon

Two-dimensional finite element method was used for evaluating the effect of orthogonal compression on precursor faults. The tendency of reactivation of precursor faults as thrust or normal was analyzed involving the positions and angles of precursor faults with the stresses, strains and displacements. Twelve cases were taken up with different combinations of precursor fault angles (high, >45° and low, <45°) and fault positions for analysis. Different positions and angles of precursor faults are correlated with stresses, strains, and displacements and are discussed in detail. It is hoped that this would help in understanding the past and the present geodynamics of the earth’s crust.     

Topography of the Moho undulations in France from gravity data: their age and origin

The complete gravity data set from France and part of the neighboring countries has been analyzed to compute the topography of the Moho undulations. This work is based on an improved filtering technique and an appropriate assumed density contrast between the crust and the upper mantle. Comparison with deep seismic refraction data reveals that this relief map expresses the continuity and geometry of the Moho undulations better than the sparsely distributed seismic refraction data in France. This gravity Moho map, though may not give absolute depths at places, provides a far better correlation with surface geology than the result from other geophysical techniques. Four domains have been recognized: (a) the Alpine domain where all the Moho undulations are concentric with the Alps; (b) the Armorican domain in which all the undulations are north-west/south-east oriented; (c) the Pyrenean domain, in which the undulations are parallel with the Mountain chain; and (d) the Massif Central Domain which does not show clear structural orientation because of the influence of the strong heat flow located at the lower crust/upper mantle interface. Study of the topography and of the superficial structures associated with these undulations reveals that the undulations delineated in the Alpine Domain result from the Tertiary compression which shaped the Alps. The Armorican Domain was first created during the Lower to Middle Cretaceous opening of the Bay of Biscay. It is now slightly affected by the Tertiary to Quaternary closure of this Bay. The Pyrenean Domain was successively shaped by the Lower Cretaceous oblique opening of the Bay of Biscay and by the Upper Cretaceous to Eocene northward displacement of Spain. Comparison between the Moho undulations map and the stress map of France reveals that most of the undulations are perpendicular to the actual shortening directions. This observation suggests that the Mesozoic, Cenozoic and Quaternary stress directions were roughly the same. Massif Central is characterized by the convergence of these three sets of undulations. Its Post-Oligocene uplift was probably the result of the converging stresses recognized in the three surrounding domains. When the Moho undulations and the topography are compared, two types of periodic crustal instabilities can be recognized. One corresponds to the buckling of the crust developed under compression, the other to boudinage which was associated with extension. Both phenomena show a typical wavelength of 200–250 km which is in agreement with the results of the actual physical and numerical modeling currently available.     

Two layer model of lithospheric compression and uplift/exhumation in an intracratonic setting: an example from the Cooper–Eromanga Basins,Australia

Seismic reflection profiles indicate the compressive nature of the structural style associated with the major uplift events in the Cooper–Eromanga Basins. Inversion geometries and reactivated features attest to a period of compression during Late Triassic–Early Jurassic times. In the Eromanga Basin, compressional structural styles associated with Late Cretaceous–Tertiary are apparent. Many of the Late Cretaceous–Tertiary structures coincide with exhumation highs in Late Cretaceous–Tertiary times. The two-layer lithospheric compression model is considered as the most complete explanation of both the uplift of areas subject to compression and crustal thickening, and of the regional uplift of areas not subject to any apparent Late Cretaceous–Tertiary compression. In the model, compression and thickening in the lower lithosphere is decoupled and laterally displaced from that in the upper crust. Thickening of the mantle lithosphere without thickening of the overlying crust can account for the initial subsidence then uplift of not inverted platform areas. The opening of the Tasman Sea and the Coral Seas can lead to stress transmission in the interior of the continent. These stresses are likely to generate uplift but cannot explain the distribution of uplift in areas not subject to compression.     

Formation of diapiric structure in the deformation zone,central Indian Ocean: A model from gravity and seismic reflection data

Analyses of bathymetry, gravity and seismic reflection data of the diffusive plate boundary in the central Indian Ocean reveal a new kind of deformed structure besides the well-reported structures of long-wavelength anticlinal basement rises and high-angle reverse faults. The structure (basement trough) has a length of about 150 km and deepens by up to 1 km from its regional trend (northward dipping). The basement trough includes a rise at its center with a height of about 1.5km. The rise is about 10 km wide with rounded upper surface and bounded by vertical faults. A broad freeair gravity low of about 20 mGal and a local high of 8 mGal in its center are associated with the identified basement trough and rise structure respectively. Seismic results reveal that the horizontal crustal compression prevailing in the diffusive plate boundary might have formed the basement trough possibly in early Pliocene time. Differential loading stresses have been generated from unequal crust/sediment thickness on lower crustal and upper mantle rocks. A thin semi-ductile serpentinite layer existing near the base of the crust that is interpreted to have been formed at mid-ocean ridge and become part of the lithosphere, may have responded to the downward loading stresses generated by the sediments and crustal rocks to inject the serpentinites into the overlying strata to form a classic diapiric structure.     

Vertical tectonic movements of the crust in transform fracture zones of the Central Atlantic

The most significant vertical movements of the oceanic crust in the Central Atlantic are characteristic of transverse ridges confined to transform fracture zones. These movements are also recorded in some local depressions of the Mid-Atlantic Ridge (MAR) and in older structures of deep-sea basins. The amplitude of such movements substantially exceeds that related to the cooling of lithospheric plates. Vertical movements can be driven by various factors: the thermal effect of a heated young MAR segment upon a cold plate, thermal stress, thermal energy released by friction in the course of displacement of fault walls relative to each other, serpentinization of the upper mantle rocks in the transform fault zone, and lateral compression and extension. The alternation of compression and extension that arises because of the nonparallel boundaries of the transform fracture zone and the unstable configuration of the rift/fracture zone junction was the main factor responsible for the formation of the transverse ridge in the Romanche Fracture Zone. The most probable cause of the vertical rise of the southern transverse ridge in the Vema Fracture Zone is the change in the spreading direction. In general, the fracture zones with active segments more than 100 km long are characterized by extension and compression oriented perpendicularly to the main displacement and related to slight changes in the spreading configuration. It is impossible to single out ambiguously the causes of vertical movements in particular structural features. In most cases, the vertical movements are controlled by several factors, while the main role belongs to the lateral compressive and tensile stresses that appear owing to changes in the movement of lithospheric blocks in the course of MAR spreading.     

New data on relationship between earth's crust and upper mantle

New data on velocities of elastic waves in rocks and minerals under pressures, anisotropism of elastic properties of monocrysts, oceanic volcanism, deep seismic profiles, isotopic composition of strontium and distribution-abundance of Sr⁸⁷ in ancient and in young rocks, and others, tend to show that the "M" discontinuity is but an expression of the state of compaction of the rocks, devoid of any petrographic or geochemical connotations, that the sialitic shell of the Earth, with its heterogeneities, extends to depths exceeding 100 km (i.e. deeper than the "M"), and that the two types of the crust, "oceanic" and "continental," created by geophysicists, are actually one and the same type. Tentatively drawn analogies between the terrestrial and the lunar crust, on the assumption of a lunar origin of tektites, and the.crust of Mars, with regard to densities and planetary size relationships, are used as illustrations of the re-evaluated ratios between the crust and the upper mantle of the earth. — IGR Staff.     

Compression behaviour of minerogenic low energy intertidal sediments

Existing geotechnical approaches that describe volumetric changes in intertidal sediments in response to applied vertical effective stresses are limited by a lack of empirical research into their one-dimensional compression behaviour. In this paper we address this deficiency by presenting the results of an investigation into the compression behaviour of minerogenic low marsh and tidal flat sediments. We have tested samples of these sediment types obtained from Greatham Creek (Cowpen Marsh, Tees Estuary, UK). Analysis of physical properties and oedometer compression tests demonstrates that, contrary to the implicit assumptions of existing models, the surface sediments studied are overconsolidated. Structural variability between samples arises due to sedimentological factors, notably variations in organic content. We attribute overconsolidation to tidal exposure and falls in groundwater level that permit desiccation and cause capillary suction stresses. Greater rates of compression with respect to effective stress occur in sediments with higher initial voids ratios and more open, unstable initial structures. Variability in structure decreases with application of higher effective stresses due to the destructuration of the sediments, which also creates increased homogeneity of compression behaviour under higher effective stresses. We subsequently develop a new conceptual framework to describe compression behaviour in minerogenic intertidal sediments that incorporates overconsolidation. We advocate a statistical approach that accounts for structural variability and variations in compression behaviour at effective stresses less than and greater than the yield stress. We argue that our conceptual framework is broadly applicable to minerogenic intertidal sediments at different locations and burial depths within Holocene stratigraphic sequences providing site-specific compression data are collected. Inter-site transfer and application of measured material properties should not be undertaken due to local variations in compression behaviour resulting from varying ecological, sedimentological, geochemical, climatic, geomorphic and hydrographic conditions. The individual characteristics of different field locations should be carefully considered before the suggested framework is routinely applied.     

The role of impact and radiogenic heating in the early thermal evolution of Mars

Journal of Earth System Science - The planetary differentiation models of Mars are proposed that take into account core–mantle and core–mantle–crust differentiation. The numerical...     

Thicknesses and types of the crust beneath the Sea of Japan inferred from marine and satellite gravimetric investigations

The combined gravimetric, altimetric, and seismic data used for calculating the crust thickness beneath the Sea of Japan revealed its different types in this region: oceanic and continental with different degrees of destruction. It is shown that the crust thickness derived from satellite altimetry is well consistent with that calculated from the data obtained by on-board gravimetric observations.     

Effects of Tectonic Force on Hydrostatic Pressure in Crust

The research into the hydrostatic pressure in the crust has been previously conducted from the viewpoint that the hydrostatic pressure is equal to the gravity, based on the fact that the hydrostatic pressure is derived mainly from the gravity of its overlying rocks. In this paper, the stress state of any point in the crust is suggested to have been caused by both the gravity and the tectonic force. The author proposes that the hydrostatic pressure is a combination or superposition of two isotropic stresses in the tectonic force and gravity stress fields. The results obtained with a finite-element simulation indicate that the additional hydrostatic pressure borne by rocks decreases gradually from the compression zone (pc^s),the shear zone (psh^s) to the tensile zone (pt^s), and that the difference in the additional tectonic hydrostatic pressure between these deformed zones tends to increase, following the increase in the absolute value and/or the difference in external forces between different directions. This paper presents the foundation for the research into the tectonic physicochemistry.     

On the role of heat flow, lithosphere thickness and lithosphere density on gravitational potential stresses

Gravitational potential stresses (GPSt) are known to play a first-order role in the state of stress of the Earth's lithosphere. Previous studies focussed mainly on crust elevation and structure and little attention has been paid to modelling GPSt using realistic lithospheric structures. The aim of the present contribution is to quantify gravitational potential energies and stresses associated with stable lithospheric domains. In order to model realistic lithosphere structures, a wide variety of data are considered: surface heat flow, chemical depletion of mantle lithosphere, crustal thickness and elevation. A numerical method is presented which involves classical steady-state heat equations to derive lithosphere thickness, geotherm and density distribution, but additionally requires the studied lithosphere to be isostatically compensated at its base. The impact of varying surface and crustal heat flow, topography, Moho depth and crust density on the signs and magnitudes of predicted GPSt is systematically explored. In clear contrast with what is assumed in most previous studies, modelling results show that the density structure of the mantle lithosphere has a significant impact on the value of the predicted GPSt, in particular in the case of thick lithospheres. Using independent information from the literature, the method was applied to get insights in the state of stress of continental domains with contrasting tectono-thermal ages. The modelling results suggest that in the absence of tectonic stresses Phanerozoic and Proterozoic lithospheres are spontaneously submitted to compression whereas Archean lithospheres are in a neutral to slightly tensile stress state. These findings are in general in good agreement with global stress measurements and observed geoid undulations.     

The role of viscous heating in Barrovian metamorphism of collisional orogens: thermomechanical models and application to the Lepontine Dome in the Central Alps

Thermal models for Barrovian metamorphism driven by doubling the thickness of the radiogenic crust typically meet difficulty in accounting for the observed peak metamorphic temperature conditions. This difficulty suggests that there is an additional component in the thermal budget of many collisional orogens. Theoretical and geological considerations suggest that viscous heating is a cumulative process that may explain the heat deficit in collision orogens. The results of 2D numerical modelling of continental collision involving subduction of the lithospheric mantle demonstrate that geologically plausible stresses and strain rates may result in orogen‐scale viscous heat production of 0.1 to >1 μW m^?3, which is comparable to or even exceeds bulk radiogenic heat production within the crust. Thermally induced buoyancy is responsible for crustal upwelling in large domes with metamorphic temperatures up to 200 °C higher than regional background temperatures. Heat is mostly generated within the uppermost mantle, because of large stresses in the highly viscous rocks deforming there. This thermal energy may be transferred to the overlying crust either in the form of enhanced heat flow, or through magmatism that brings heat into the crust advectively. The amplitude of orogenic heating varies with time, with both the amplitude and time‐span depending strongly on the coupling between heat production, viscosity and collision strain rate. It is argued that geologically relevant figures are applicable to metamorphic domes such as the Lepontine Dome in the Central Alps. We conclude that deformation‐generated viscous dissipation is an important heat source during collisional orogeny and that high metamorphic temperatures as in Barrovian type metamorphism are inherent to deforming crustal regions.     

Types of crust beneath the Ligurian Sea

The structural setting beneath the Ligurian Sea resuJts from several tectonic events reflected in the nature of the crust. The central-western sector, called the Ligurian basin, is part of the northwestern Mediterranean. It is a marginal basin that was generated in Oligocene-Miocene time by subduction of the Adriatic plate beneath the European plate and by the eastward drift of the Corsica-Sardinia block. The eastern sector belongs to the Tyrrhenian basin system and is characterized by extensional activity which since Tortonian time superimposed an earlier compressional regime. Our effort has been addressed in particular towards simplifying the complex nature of the crust of the Ligurian basin by modelling its genesis using uniform extension and sea-floor depth variation with age. In the rift stage of the basin's evolution, the initial subsidence reaches the isostatic equilibrium level of the asthenosphere by a thinning factor of 3.15. The additional passive process, corresponding to the cooling of the lithosphere since 21 Ma, leads to a total tectonic subsidence of 3.4 km, representing the boundary of the extended continental crust. For values up to 4.1 km a transitional-type crust is expected, whereas for higher tectonic subsidence values a typical oceanic crust should exist. After setting these constraints, the boundaries of the different crust types have been drawn based on total tectonic subsidence observations deduced from bathymetry and post-rift sediment thickness. Although there is a general agreement with the previous reconstructions deduced from other experimental data, the oceanic realm has wider extent and more complex shape. The northernmost part of this realm shows crust of sub-oceanic type altemating basement highs with lower subsidence values. The observed surface heat flux is consistent with the predicted geothermal held in the Alpine-Provençal continental margin and in the oceanic domain. However, a characteristic thermal asymmetry is clearly visible astride the basin, due to the enhanced heat flux of the Corsica margin. Even if the uniform extension model accounts well at a regional level for the present basement depth, a remarkable tectonic subsidence excess has been found in the Alpine-Provençal continental margin. This evidence agrees with the reprise in compression of the margin; the direction of the greatest principal stress is N120°E on average.     

从岩浆岩看青藏高原地壳的生长演化

青藏巨厚陆壳的性质和形成机制,是重要的科学问题.通过对青藏岩浆岩,特别是其Nd、Hf同位素的研究,证明青藏高原并存着新生地壳和再循环地壳两类地壳.地壳的结构和属性对成矿矿种、矿床类型和分布有重要的控制作用.青藏巨厚地壳是由两种机制（构造加厚;地幔物质通过岩浆作用注入加厚）共同造成的,二者贡献比例约6:4.地壳增厚主要发生在50~25 Ma期间.大陆碰撞带是造成地壳净生长的重要构造部位.      

青藏高原地球物理特征分析

根据区域性航空磁测、重力区测及现代地震活动观察分析表明,青藏高原是一个十分独特和年青的巨大的构造单元。它具有薄的磁性壳层,透镜状的地壳结构,断块镶嵌的构造特征。高的热流和地震活动,以及地磁场的剧烈衰减和大面积分布的均衡重力异常,揭示出青藏高原目前仍然在升温上拱和推覆调整中。青藏高原地壳内部普遍发育低速低阻层,壳下发育壳幔混合层。地壳显示为两类不同特征的结构层:上部地壳以断错叠推增厚为特征,下部地壳因混入大量上地幔及软流圈物质而增生为透镜层。地壳挤压推叠破坏了岩石圆的均衡状态,而地幔混合层的发育则是对壳幔不均衡状态的补偿和调整。由于地壳深部的高温高压环境,古构造岩相痕迹被改造熔蚀,现今观察到的青藏高原地球物理特征主要反映的是新近发育和目前存在的构造特征。青藏高原是由两个不同历史演化的大陆板块碰撞缝合而成。航磁及其它地球物理测量表明,两大板块的结合带发育在金沙江—红河断裂带及南侧的唐古拉—三江构造带。在上述地带两边,区域磁场背景差别十分明显。