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1.
Deformation within the Earth's lithosphere is largely controlled by the rheology of the rock. Fracture and faulting are characterized by elastic rheologies with brittle mechanisms, while folding and flow are characterized by plastic and/or viscous rheologies due to ductile mechanisms. However, it has been recognized that deformation that resembles ductile behavior can be produced within the confines of the brittle lithosphere. Specific examples are folds that form in the shallow crust, steep hinges at subduction zones that are accompanied by seismicity, and large-scale deformation at plate boundaries. In these cases, the brittle lithosphere behaves elastically with fracture and faulting yet produces ductile behavior. In this paper, we attempt to simulate such ductile behavior in elastic materials using continuum damage mechanics. Engineers utilize damage mechanics to model the continuum deformation of brittle materials. We utilize a modified form of damage mechanics that represents a reduction in frictional strength of preexisting fractures and faults. We use this empirical approach to simulate the bending of the lithosphere under the application of a constant moment.We use numerical simulations to obtain elastostatic solutions for plate bending and where the longitudinal stress at a particular node exceeds a yield stress, we apply damage to reduce Young's modulus at the node. Damage is calculated at each time step by a power-law relationship of the ratio of the yield stress to the longitudinal stress and the yield strain to the longitudinal strain. This results in the relaxation of the material due to increasing damage. To test our method, we apply our damage rheology to an infinite plate deforming under a constant bending moment. We simulate a wide range of behaviors from slow relaxation to instantaneous failure, over timescales that span six orders of magnitude. Using this method, stress relaxation produces elastic-perfectly plastic behavior in cases where failure does not occur. For cases of failure, we observe a rapid increase in damage leading to failure, analogous to the acceleration of microcrack formation and acoustic emissions prior to failure. The changes in the rate of damage accumulation in failure cases are similar to the changes in b-values of acoustic emissions observed in triaxial compression tests of fractured rock and b-value changes prior to some large earthquakes. Thus continuum damage mechanics can simulate the phenomenon of ductile behavior due to brittle mechanisms as well as observations of laboratory experiments and seismicity. 相似文献
2.
Seabeam, seismic and submersible surveys took place during the Kaiko Project and revealed significant compressive deformation at the northeastern end of the Philippine Sea plate, related to the recent collision of the Izu-Ogasawara Arc against Central Japan. Intraoceanic thrusting at the base of the Zenisu Ridge, a linear topographic high running a few tens of kilometers south of the Nankai Trough, is supported by tectonic, magnetic and gravimetric data. We investigate the formation of the Zenisu Ridge in terms of compressive mechanical failure of a thin elastic-perfectly plastic plate, subducting at a trench and subject to a regional compressive axial force. The rheological envelope concept is used throughout the numerical calculations. Based on a detailed study of flexure of the present-day bending far from the deformation zone, we evaluate the bending forces involved: the bulge is 120 to 150 m high and the compressive stress all along the Nankai Trough is about −100 MPa. In the Zenisu Ridge area, an additional compressive stress is superimposed due to the nearby collision at Izu-Peninsula. We compute the vertical distribution of the deviatoric stress before failure and find that the deviatoric stress is maximum at a depth of 20–25 km in the trench area, and again at the surface 60 to 100 km seaward, in the vicinity of the bulge. The development of a thrust joining these two maxima through the entire thickness of the lithosphere is discussed. The model predicts that the formation of the Zenisu Ridge did not occur before 4 Ma and is caused by progressive tectonic uplift due to the redistribution of bending stresses as the ridge approaches the Nankai Trough. 相似文献
3.
G. Ranalli 《Tectonophysics》1994,240(1-4):107-114
The elastic flexural thickness of the lithosphere depends on plate curvature. As curvature increases, the elastic core of the plate is thinned from above (frictional yielding) and from below (plastic creep). Sometimes decoupling between crust and mantle elastic cores can also occur. The mechanical thickness of the lithosphere, on the other hand, is the thickness of the rheologically layered plate having a strength above a critical threshold. In order to estimate the mechanical thickness from a simple uniform-rheology plate model, we adopt a plastic work-hardening constitutive equation, which results in lower stresses in the regions of high strain, relative to the elastic model. It is, therefore, a better approximation to the actual rheology of the flexed lithosphere where there is no lower-crustal decoupling (e.g., in oceanic and in cold continental lithosphere). The equivalent mechanical thickness of the nonlinear plate can be directly obtained, if the curvature is known, from the estimated elastic flexural rigidity or thickness. Comparison with numerical integration of bending moment in rheologically layered lithosphere shows that equivalent mechanical thickness is a good estimation of mechanical thickness. Examination of both oceanic and continental data suggests that mechanical thickness is controlled by the 900 ± 100°C isotherm. This corresponds to a creep strength of the order of 10 MPa in upper mantle material. 相似文献
4.
Using the finite-element method to study the deflection of the oceanic lithosphere at subduction zones shows that deflection of an elastic-plastic model agrees with observed bathymetry data, except for 2 examples, Kermadec and Mariana, where the lithospheric behaviour remains elastic seaward of the trench. Seven of the cases we studied could be easily fitted and enabled us to compute an effective yield strength, the average value of which is about 3.8 to 4.8 kbar and seems to be the same in every case. 相似文献
5.
6.
Detachment of the deeper part of subducted lithosphere causes changes in a subduction zone system which may be observed on the Earth's surface. Constraints on the expected magnitudes of these surface effects can aid in the interpretation of geological observations near convergent plate margins where detachment is expected. In this study, we quantify surface deformation caused by detachment of subducted lithosphere. We determine the range of displacement magnitudes which can be associated with slab detachment using numerical models. The lithospheric plates in our models have an effective elastic thickness, which provides an upper bound for rapid processes, like slab detachment, to the surface deformation of lithosphere with a more realistic rheology. The surface topography which develops during subduction is compared with the topography shortly after detachment is imposed. Subduction with a non-migrating trench system followed by detachment leads to a maximum surface uplift of 2–6 km, while this may be higher for the case of roll-back preceding detachment. In the latter situation, the back-arc basin may experience a phase of compression after detachment. Within the context of our elastic model, the surface uplift resulting from slab detachment is sensitive to the depth of detachment, a change in friction on the subduction fault during detachment and viscous stresses generated by sinking of the detached part of the slab. Overall, surface uplift of these magnitudes is not diagnostic of slab detachment since variations during ongoing subduction may result in similar vertical surface displacements. 相似文献
7.
We estimate the lateral variations of the elastic thickness of the Maracaibo block with a 3D numerical approach by using centered finite differences. The calculation is based on solving the fourth-order partial differential equation that governs the bending of a thin plate fixed on its boundaries (zero displacement) with variable thickness (or elastic thickness for this particular case). An initial plate-load model is built and is iteratively modified to fit the general basement configuration and gravity data. The final result is an elastic thickness map that covers the Maracaibo block and the surrounding sections of the South American plate. It shows that the elastic thickness ranges from 30 km to 18 km with a mean value of 23.6 km and a mode of 26 km. The largest elastic thickness values are associated with the location of the Santa Marta Mountains and the Barinas Apure Basin, while the smallest ones with the Mérida Andes-Maracaibo Basin flexural system. The current basement configuration within the Maracaibo basin, formed as a result of its geodynamic evolution, has affected the mechanical properties of the Maracaibo block near the current Mérida Andes position. The load of the Perijá Range is compensated by a complex stress tensor, and that of the Santa Marta Mountains does not have an isostatic root as it is held by a relatively strong lithosphere. 相似文献
8.
Thomas J. Owens 《Tectonophysics》1983,93(1-2)
An elastic-perfectly plastic plate model has been developed to analyze the flexure associated with normal faulting. The model consists of a thin layer, which is completely cut by a normal fault, overlying a fluid substratum. For a given applied bending moment at the fault, the relationship between the amount of displacement on the fault and the extent of the failure zone can be calculated. The model is applied to the Wasatch Front region in the eastern Basin and Range Province, USA to determine the correlation of its parameters with geological and geophysical data in the vicinity of a major normal fault, the Wasatch fault, along which there has been 3–4 km of Late Cenozoic uplift. In this region, most seismic activity occurs away from the Wasatch fault in a zone 30 km wide, roughly centered 30 km east of the fault. This activity occurs at depths of 15 km or less. In order to match the observations, the lithospheric layer must have a flexural rigidity of 0.5 to 1.1 · 1022 n-m and a yield stress of 1–2 kb and must have zero applied bending moment at the fault. The effective mechanical thickness of the lithosphere in this region is 20–25 km. These results indicate that the lithosphere in long-term mechanical studies in the eastern Basin and Range is thin and weak. Evaluating these results as compared to the seismic lithospheric thickness and temperature regime of the region produces some interesting correlations with studies in oceanic regions. 相似文献
9.
The lithosphere is the cold conductive boundary layer formed by cooling of the oceanic crust and upper mantle as it is convected away from oceanic ridges. Although its rheological properties vary continuously with depth, the lithosphere is conveniently divided into an upper elastic layer and a lower plastic layer, the latter overlying a zone of viscous flow. Chemically the lithosphere is vertically zoned with its uppermost part formed by variously hydrated oceanic crust; at M this overlies highly depleted dunite or harzburgite passing downwards over 50 km or so into garnet lherzolite. The vertical variation in density, and thus the gravitational stability of the lithosphere, is controlled by interplay of compositional variation and temperature distribution.As it enters an oceanic trench the lithosphere flexures elastically and plunges downwards at an average inclination close to 45°. During its descent it undergoes dissipative heating at its upper surface. Initially this heating drives a series of prograde metamorphic reactions in the oceanic crust ; because these are largely endothermic, the descending lithosphere heats less rapidly than previously expected, an effect which may be enhanced by percolation of the water of dehydration.Although it is commonly assumed that dehydration water is released upwards, it is not clear that this is true in the presence of the strong negative temperature gradients at the top of the slab, and water may initially be driven downwards into the slab to be released later at much greater depth. The magmatic activity which is associated with the partial melting of the uppermost part of the slab and with partial fusion of diapiric masses in the mantle above it, is critically dependent on the behaviour of the water carried down by the subduction process.The slab itself undergoes a series of phase changes during its descent some of which make a major contribution to the body force during subduction. By the time it reaches 700 km the slab has undergone significant thermal erosion, but the major compositional inhomogeneities within it are retained by the mantle into which it merges. 相似文献
10.
It is acknowledged that for extending the experimental results to real scale design, it is necessary to use an appropriate numerical analysis. The good analysis in geotechnical problems needs to adopt a suitable constitutive model for the materials. This paper presents a modeling approach to investigate the complex behavior of granular trench and reinforcement system. For this purpose, an experimental and numerical investigation has been carried out on the behavior of pullout resistance of an embedded anchor (circular plate) with and without geogrid reinforcement layers in stabilized loose and dense sand using a granular trench. Different parameters have been considered, such as number of geogrid layers, embedment ratios, relative density of soil, and height ratios of granular trench. Finite element analysis with Hardening Soil Model was utilized for sand and CANAsand constitutive model was used for granular trench to investigate failure mechanism and the associated rupture surfaces. Results showed that, when soil was improved with the granular-geogrid trench, the uplift force significantly increased, but in geogrid-reinforced granular trench condition, the ultimate pullout resistance at failure increased as the number of geogrid layers increased up to the third layer, the fifth layer had a negligible effect in comparison with the third layer of reinforcement. The ultimate uplift capacity of anchor plate and the variation of surface deformation for all the tests indicated a close agreement between the experimental and numerical models. 相似文献
11.
Temporary local seismic networks were installed in western Crete, in central Crete, and on the island Gavdos south of western Crete, respectively, in order to image shallow seismically active zones of the Hellenic subduction zone.More than 4000 events in the magnitude range between −0.5 and 4.8 were detected and localized. The resulting three-dimensional hypocenter distribution allows the localization of seismically active zones in the area of western and central Crete from the Mediterranean Ridge to the Cretan Sea. Furthermore, a three-dimensional structural model of the studied region was compiled based on results of wide-angle seismics, surface wave analysis and receiver function studies. The comparison of the hypocenter distribution and the structure has allowed intraplate and interplate seismicity to be distinguished.High interplate seismicity along the interface between the subducting African lithosphere and the Aegean lithosphere was found south of western Crete where the interface is located at about 20 to 40 km depth. An offset between the southern border of the Aegean lithosphere and the southern border of active interplate seismicity is observed. In the area of Crete, the offset varies laterally along the Hellenic arc between about 50 and 70 km.A southwards dipping zone of high seismicity within the Aegean lithosphere is found south of central Crete in the region of the Ptolemy trench. It reaches from the interface between the plates at about 30 km depth towards the surface. In comparison, the Aegean lithosphere south of western Crete is seismically much less active including the region of the Ionian trench. Intraplate seismicity within the Aegean plate beneath Crete and north of Crete is confined to the upper about 20 km. Between 20 and 40 km depth beneath Crete, the Aegean lithosphere appears to be seismically inactive. In western Crete, the southern and western borders of this aseismic zone correlate strongly with the coastline of Crete. 相似文献
12.
A mechanical model for plate deformation associated with aseismic ridge subduction in the new hebrides arc 总被引：1，自引：0，他引：1
Tectonic features associated with a subducting fracture zone-aseismic ridge system in the New Hebrides island arc are investigated. Several notable features including a discontinuity of the trench, peculiar locations of two major islands (Santo and Malekula), regional uplift, and the formation of a basin are interpreted as a result of the subduction of a buoyant ridge system. The islands of Santo and Malekula are probably formed from an uplifted mid-slope basement high while the interarc basin of this particular arc is probably a subsiding basin instead of a basin formed by backarc opening. The situation can be modeled by using a thin elastic half plate overlying a quarter fluid space with a vertical upward loading applied at the plate edge. This model is consistent with topographic and geophysical data. This study suggests that subduction of aseismic ridges can have significant effects on tectonic features at consuming plate boundaries. 相似文献
13.
Holger Sommer 《Mineralogy and Petrology》2009,96(1-2):113-120
The hypothesis that much of the lithosphere of the Archaean Tanzania Craton was hydrated, by the dehydration of a buoyant subduction 2 Ga ago is presented in this study. Buoyant subduction is a potential mechanism for thermal erosion and metasomatism of extensive regions of the cold overlying continental lithosphere. This hypothesis could explain why the Tanzania Craton forms an undeformed island within the intensely deformed mobile belts. Furthermore, it would explain the formation of the eclogite and lherzolite bearing kimberlites within the Tanzania Craton far away from the trench. A buoyant, slow subduction is required because this would provide sufficient cooling from the overlying cratonic lithosphere and therefore the dipping slab could retain hydrous minerals such as antigorite in hydrated aureoles in peridotites. To test this hypothesis, the release of water during prograde metamorphism of a flat-subducting plate was modeled. It is shown that water can be transported ～800 km laterally, inboard of the trench, which is close to the north-south extension of the Archaean Tanzania Craton. 相似文献
14.
An intrinsic feature of Cordillera-style orogenic systems is a spatial trend in the radiogenic isotopic composition of subduction-related magmatism. Magmatism is most isotopically juvenile near the trench and becomes increasingly evolved landward. A compilation of radiogenic isotopic data from the central Andes, U.S. Cordillera, and Tibet (the most well-studied examples of modern and ancient Cordilleran systems) demonstrate such spatial trends are long-lived and persist throughout the life of these continental subduction margins. The consistency of the isotopic trend through time in magmatic products is surprising considering the plethora of orogenic processes that might be expected to alter them. In addition to longevity, spatial isotopic trends encompass a broad spectrum of geochemical compositions that represent diverse petrogenetic and geodynamic processes. The two end-members of the spatial isotopic trends are represented by melts sourced within isotopically juvenile asthenospheric mantle and melts sourced from isotopically evolved continental lithospheric mantle and/or lower crust. Mantle lithosphere generally thins toward the magmatic arc and trench in Cordilleran orogens because sub-lithospheric processes such as delamination, subduction erosion, and subduction ablation, operate to thin or remove the continental mantle lithosphere. With time, magmatic additions may impart the isotopic composition of the mantle source on the lower crust, giving rise to an isotopically homogenous deep lithosphere. The results of this analysis have significant implications for interpreting temporal and spatial shifts in isotopic composition within Cordilleran orogens and suggest that the continental mantle lithosphere may be a significant source of magmatism in orogenic interiors. 相似文献
15.
中国大陆岩石圈厚度分布研究 总被引：11，自引：1，他引：10
利用不同物理性质所估计的岩石圈厚度可能具有不同的地球动力学意义。大陆岩石圈等效弹性厚度往往只与岩石圈内部的某些岩层相关,因此它可能不代表一般意义上的岩石圈厚度。地震学岩石圈厚度虽然有较高的精度,但依赖于人为地对岩石圈的定义;并且其具有的短时间尺度效应决定了它与长时间尺度的岩石圈概念不一致。热学岩石圈厚度体现了长时间尺度上的岩石圈热学作用,因此其厚度定义的标准是较合理的。地震-热学岩石圈厚度研究利用地震波速反演得到的温度数据按照热学岩石圈标准来对岩石圈厚度进行研究,具有地震学和热学岩石圈厚度两者的优点,是较合理的对岩石圈厚度的估计。中国大陆地震-热学岩石圈厚度分布有如下特点:(1)中国东部岩石圈较薄,厚度约100 km,其中包括中国东北、中朝克拉通、扬子克拉通东部和华南造山带;(2)青藏高原和塔里木克拉通以南地区的厚度变化较大,厚度约在160~220 km;(3)三大克拉通的岩石圈厚度有较大区别,扬子克拉通的核心最厚达约170 km,塔里木克拉通的核心厚度约140 km,中朝克拉通的厚度约100 km;(4)昆仑秦岭造山带的岩石圈上地幔内部较复杂,可能有大面积的部分熔融;(5)整个大陆岩石圈厚度分布并没有显示出与地壳年龄的线性相关关系,却表现出了与大地构造格局的直接关系。受板块碰撞强烈影响的地区,岩石圈较厚;受大洋俯冲带影响较强的地区,岩石圈较薄。 相似文献
16.
N.I. Pavlenkova 《Russian Geology and Geophysics》2011,52(9):1016-1027
Deep seismic investigation carried out in Russia in long-range profiles with peaceful nuclear explosions allowed clarifying in details the structure of the upper mantle and the transition zone down to the depth of 700 km within the huge territory of old and young platforms of Northern Eurasia. Variability of horizontal heterogeneity of the upper mantle depending on the depth serves to qualitative estimation of its rheological properties. The upper part of the mantle to the depth of 80–100 km is characterized by the block structure with significant velocity steps of seismic waves at the blocks often divided by deep faults. This is the most rigid part of lithosphere. Below 100 km horizontal heterogeneity is insignificant, i.e., at these depths the substance is more plastic and not capable to retain block structure. On the lithosphere bottom at the depth of 200–250 km plasticity increase is observed as well but the zone of the lower velocities that might have been bound with the area of partial melting (asthenosphere) has not been found. These three layers with different rheological properties are divided by seismic boundaries presented by thin layering zones with alternating higher and lower velocities. At the specified depths any phase boundaries have been distinguished. These thin layering zones are assumed to form due to higher concentration of deep fluids at some levels of depths where mechanical properties and permeability of substance change. Insignificant number of fluids may result in appearance of streaks with partial or film melting at relatively low temperature—to the rise of the weakened zones where subhorizontal shifts are possible. According to seismic data in many world regions seismic boundaries are also observed at the depth of about 100 and 200 km; they may be globally spread. There are signs that areas of xenoliths formation and earthquake concentration, i.e., zones of high deformations, are confined to these depths. 相似文献
17.
中国西部大陆岩石圈的有效弹性厚度研究 总被引：8，自引：0，他引：8
中国西部是地球上陆地隆升最显著的地区, 有世界上新构造运动最强烈的青藏高原、规模巨大的左行走滑位移的阿尔金断裂系和中亚地区最大的板块内部造山带———天山褶皱造山带.作为印度板块与欧亚板块相互碰撞的会聚带, 本区是研究岩石圈动力学的有利场所.主要运用重力资料和地形资料来研究中国西部地区显著上地壳结构和其上地幔变形之间的关系.依据岩石圈流变学的理论, 在空间域采用垂直和水平受力的多个变刚度的三维有限差分方法来计算弹性板的有效弹性厚度.模拟结果显示中国西部地区的岩石圈有效弹性厚度存在明显的横向不均匀性, 从6~10 km的造山带区域的有效弹性厚度变到大于60 km的古陆区域的有效弹性厚度.青藏高原地区的岩石圈有效弹性厚度平均为30 km, 塔里木盆地的有效弹性厚度为40~50 km, 南、北天山的岩石圈有效弹性厚度分别为10~15 km和30 km左右, 阿尔金断裂在东经90°以西部分的岩石圈有效弹性厚度要小于90°以东部分. 相似文献
18.
The Luzon Island is a volcanic arc sandwiched by the eastward subducting South China Sea and the northwestward subducting Philippine Sea plate.Through experiments of plane-stress,elastic,and 2-dimensional finite-element modeling,we evaluated the relationship between plate kinematics and present-day deformation of Luzon Island and adjacent sea areas.The concept of coupling rate was applied to define the boundary velocities along the subduction zones.The distribution of velocity fields calculated in our models was compared with the velocity field revealed by recent geodetic (GPS) observations.The best model was obtained that accounts for the observed velocity field within the limits of acceptable mechanical parameters and reasonable boundary conditions.Sensitivity of the selection of parameters and boundary conditions were evaluated.The model is sensitive to the direction of convergence between the South China Sea and the Philippine Sea plates,and to different coupling rates in the Manila trench,Philippine trench and eastern Luzon trough.We suggest that a change of±15° of the di rection of motion of the Philippine Sea plate can induce important changes in the distribution of the computed displacement trajectories,and the movement of the Philippine Sea plate toward azimuth330° best explains the velocity pattern observed in Luzon Island.In addition,through sensitivity analysis we conclude that the coupling rate in the Manila trench is much smaller compared with the rates in the eastern Luzon trough and the Philippine trench.This indicates that a significant part of momentum of the Philippine Sea plate motion has been absorbed by the Manila trench;whereas,a part of the momentum has been transmitted into Luzon Island through the eastern Luzon trough and the Philippine trench. 相似文献
19.
Mapping elastic lithospheric thickness variations in Canada 总被引：1，自引：0，他引：1
Mark Pilkington 《Tectonophysics》1991,190(2-4):283-297
The variation of elastic lithospheric thickness within the Canadian landmass is determined using the coherence between surface topography and Bouguer gravity anomalies. Estimates of elastic thickness (or, equivalently, rigidity) are derived by minimizing the difference between the observed coherence and that predicted by an isostatic model consisting of a thin elastic plate which is subject to surface and subsurface loading and overlies a fluid substratum (the asthenosphere).
Estimates of the elastic thickness vary from 17–18 km in coastal Labrador and Baffin Island to > 150 km near Lake Winnipeg, Manitoba. A broad correlation exists between lithospheric thickness and age. Exceptions to this correlation are areas of reduced plate thickness in North Atlantic coastal regions where magmatic activity associated with rifting and the formation of a passive margin may have been sufficient to reduce the plate strength but not enough to reset the thermal age of the basement rocks. Areas of greater plate thickness than that predicted by thermal cooling occur in the continental interior and may reflect lithospheric thickening due to the chemical and mechanical effects accompanying continental agglomeration. 相似文献
20.
To assess the possibility that the North Atlantic Ocean may subduct at Scotian basin east of Canada, we investigate the present compensation state of this deep basin. A Fourier domain analysis of the bathymetry, depth to basement and observed gravity anomalies over the oceanic area east of Nova Scotia indicates that the basin is not isostatically compensated. Moreover, the analysis emphasizes that in addition to the sediments, density perturbations exist beneath the basin. The load produced by the sediments and these density perturbations must have been supported by the lithosphere. We simulate the flexure of the lithosphere under this load by that of a thin elastic plate overlying an inviscid interior. It is shown that a plate with a uniform rigidity does not adequately represent the lithosphere beneath the basin as well as the oceanic lithosphere far from the basin, rather the rigidity of the lithosphere directly beneath the basin is about one to two orders of magnitude smaller than elsewhere. We relate this weakening to the thermal blanketing effects of the thick sediments and the fact that the lithosphere has a temperature-dependent rheology. We suggest that this weak zone would have a controlling effect on the reactivation of normal faults at the hinge zone of the basin, that were formed during the break-up of Africa and North America and were locked in the early stages after the break-up. The weak zone would facilitate reactivation of the faults if tensional stresses were produced by possible reorientation of the spreading direction of the North Atlantic Ocean in the future. The reactivation of the faults would create a free boundary condition at the hinge zone, allowing further bending of the lithosphere beneath the basin and juxtaposition of this lithosphere to the mantle beneath the continent. This may provide a favorable situation for initiation of slow subduction due to subsequent compressional forces. 相似文献