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1.
Annie Souriau 《Earth and Planetary Science Letters》1984,68(1):101-114
Gorringe Ridge is a strong uplifted block of oceanic crust and upper mantle lying at the eastern end of the Azores-Gibraltar plate boundary. The geoid over this structure derived from Seasat altimeter data exhibits a 9-m height anomaly with a north-south lateral extension smaller than 200 km. An attempt is made to interpret this geoid together with the gravity anomalies and with the seismicity, which has been compiled as a function of depth.It is first shown that the flexure of the oceanic lithosphere due to the ridge loading does not provide a good fit of the geoid anomalies and probably should be discarded, as it assumes a continuous unfractured elastic plate.Models involving local heterogeneities are then tested. The comparison of the observed geoid anomalies with the anomalies due to the uncompensated relief indicates that the topographic high has no shallow compensation.Uncompensated models, previously proposed to explain the gravity anomalies, are tested using the geoid. One model (Purdy and Bonnin, in Bonnin [11]), which involves an uplift of upper mantle material at depth, generates too strong geoid anomalies and must be discarded. Another model, which represents a nascent subduction zone (Le Pichon et al. [25]), fits both the gravity and geoid anomalies, but leads to difficulties in explaining the deep seismicity north of Gorringe Ridge.A model in isostatic equilibrium is also able to fit both gravity and geoid anomalies. This model involves a deep root of density 3.0 g cm?3, as has been previously proposed for many oceanic ridges and plateaus. This model is compatible with the deep seismicity, but the origin of this low-density material at great depth is up to now an unresolved question.More likely, dynamical models taking into account the forces induced by the convection related to the slow plate convergence in this area will have to be considered. 相似文献
2.
Seasat altimetry profiles across the Falkland-Agulhas fracture zone (FZ) and the Ascension FZ in the South Atlantic were examined for evidence of step-like geoid offsets predicted from thermal modeling of the lithosphere. The geoid profiles exhibit much short-wavelength power and the step-like offsets are often small, making reliable estimation of the heights of the observed geoid offsets difficult. The offsets were estimated by the least-squares fitting of quadratic curves incorporating a step function to the altimetry profiles. A preferred offset value was determined for each profile by taking the average of step heights computed with various distances around the fracture zone excluded from the fit. The age of the crust surrounding the fracture zones, necessary for computing a theoretical geoid offset, was determined from surface ship magnetic anomaly data and from existing ocean floor age maps.Observed variations in geoid step height with age of the lithosphere are not consistent with those predicted from standard thermal plate models. For ages less than 30 Ma, the step offsets across both fracture zones decrease in a manner appropriate for an unusually thin plate with a thickness of 50–75 km. At greater ages, the offsets show complex behavior that may be due to bathymetric features adjacent to the fracture zones. Similar geoid patterns on opposite branches of the Falkland-Agulhas FZ are indicative of processes that act symmetrically on both sides of the Mid-Atlantic Ridge. This behavior of the geoid is consistent both with small-scale convection occurring beneath the lithosphere and with bathymetric features originally produced along the ridge crest and now located symmetrically on opposite sides of the ridge. The west flank of the Ascension FZ displays a regrowth in step height at about 40 Ma consistent with small-scale convection and in agreement with other studies of Pacific and South Atlantic fracture zones. 相似文献
3.
Thermal-rheological structure of lithosphere beneath the northern flank of Tarim Basin, western China: Implications for geodynamics 总被引：2，自引：3，他引：2
LIU Shaowen WANG Liangshu LI Cheng LI Hua HAN Yongbing JIA Chengzao & WEI Guoqi .Department of Earth Sciences Nanjing University Nanjing China .Chinese Petroleum Exploration Development Institution Beijing China 《中国科学D辑(英文版)》2004,47(7):659-672
Based on the data of geo-temperature and thermophysical parameters of rocks in the Kuqa Depression and the Tabei Uplift, northern flank of the Tarim Basin, in terms of the analytical solution of 1-D heat transfer equation, the thermal structure of the lithosphere under this region is determined. Our results show that the average surface heat flow of the northern flank of the Tarim Basin is 45 mW/m2, and the mantle heat flow is between 20 and 23 mW/m2; the temperature at crust-mantle boundary (Moho) ranges from 514℃ to 603℃ and the thermal lithosphere where the heat conduction dominates is 138-182 km thick. Furthermore, in combination with the P wave velocity structure resulting from the deep seismic sounding profile across this region and rheological modeling, we have studied the local composition of the lithosphere and its rheological profile, as well as the strength distribution. We find that the rheological stratification of the lithosphere in this region is apparent. The lowermost of the lower crust is ductile; however,the uppermost of the mantle and the upper and middle parts of the crust are both brittle layers,which is typically the so-called sandwich-like structure. Lithospheric strength is also characterized by the lateral variation, and the uplift region is stronger than the depression region. The lithospheric strength of the northem flank of the Tarim Basin decreases gradually from south to north; the Kuqa Depression has the lowest strength and the south of the Tabei Uplift is strongest.The total lithospheric strength of this region is 4.77× 1012-5.03 × 1013 N/m under extension, and 6.5 × 1012-9.4× 1013 N/m under compression. The lithospheric brittle-ductile transition depth is between 20 km and 33 km. In conclusion, the lithosphere of the northern flank of the Tarim Basin is relatively cold with higher strength, so it behaves rigidly and deforms as a whole, which is also supported by the seismic activity in this region. This rigidity of the Tarim lithosphere makes it little deform interior, but only into flexure under the sedimentation and tectonic loading associated with the rapid uplift of the Tianshan at its northern margin during the Indian-Eurasian continental collision following the Late Eocene. Finally, the influences of factors, such as heat flow, temperature,crustal thickness, and especially basin sediment thickness, on the lithospheric strength are discussed here. 相似文献
4.
Using Pioneer Venus line-of-sight gravity data and orbit simulation procedures, we have estimated apparent depths of isostatic compensation (ADCs) for twelve Venusian highland features: Asteria, Atla, Bell, Beta, Ovda, Phoebe, Tellus, Thetis and Ulfrun Regiones, and Nokomis, Gula and Sappho Montes. ADCs range from 50 km to 270 km; half of the values are less than 100 km. Using these ADCs, we estimate geoid to topography ratios (GTRs) for each area to allow comparison with convection calculations and with terrestrial data for oceanic hot spots, swells and plateaus. The geoid is estimated in the wavenumber domain from the isostatic formula, using the topography and ADC for each region. In the space domain, the GTR is equal to the least squares slope of the linear fit of the geoid to the topography. The resulting GTR range is 7–31 m/km, which is much higher than terrestrial oceanic values (−1 to 5 m/km). The features fall into two distinct groups, one with a GTR range of 7–13 m/km, and one with a range of 19–25 m/km. The exception is Beta Regio, which has a GTR of 31 m/km. A model for thermal thinning of a 100 km thick lithosphere fits all values in the lower GTR group to within one standard deviation. Airy compensation could also be present, but cannot fully compensate these features. Partial dynamic compensation of the lower GTR group in combination with lithospheric mechanisms is also possible, but not required to fit the data. The upper GTR range, 19–25 m/km, can be fit with an upper mantle, constant viscosity convection model. The large GTR values are inconsistent with the presence of a low viscosity zone. If more than one compensation mechanism is present in the regions in the higher GTR group, the GTRs will be underestimated in terms of a dynamic interpretation. We thus fit the convection models to the upper end of the GTR range, 25 m/km. Rayleigh numbers in the range 104–106 will produce a GTR of 25 m/km when combined with conductive lid thicknesses of 85–150 km. The 6 m/km range in both of the GTR groups is probably due to varying degrees of crustal and thermal compensation, combined with dynamic compensation in the upper GTR group. The difference between terrestrial and Venusian GTR ranges can be explained largely by the lack of a low viscosity zone on Venus. 相似文献
5.
Kurt Lambeck 《Earth and Planetary Science Letters》1981,55(3):482-496
The response of the ocean lithosphere to volcanic island loading has been investigated for the Southern Cook Islands of Rarotonga, Mangaia and the Aitutaki-Mauke group using (1) estimates of the change—with distance from the load—of uplift of islands located on the flexure-created arch, (2) geoid height perturbations as observed with the GEOS 3 and SEASAT radar altimeters, and (3) bathymetric evidence for the moat and arch about the island loads. The effective flexural rigidity for Rarotonga is found to be higher than for the older and nearby islands of Mangaia and Aitutaki. This, together with an earlier result for Tahiti [1], suggests that stress relaxation of the lithosphere has occurred and that this layer behaves as a viscoelastic material with an effective viscosity of about 5–10×1025 poise. The observations for the flexural rigidity do not indicate any dependence on the age of the lithosphere at the time the loading occurred. This suggests that there are no significant differences in lithospheric properties for plates in the age range of 6–7×107 years. Altimeter passes to the east of Mauke indicate that there may be an uncharted submarine extension of the Aitutaki-Mauke chain. 相似文献
6.
We have determined the elastic thicknessTe of the oceanic lithosphere along two volcanic chains of the South Central Pacific: Cook-Austral and Society islands. We used a three-dimensional spatial method to model the lithospheric flexure assuming a continuous elastic plate. The model was constrained by geoid height data from the SEASAT satellite.Along the Cook-Austral chain the elastic thickness increases westward, from 2–4 km at McDonald hot spot to 14 km at Rarotonga. At McDonald seamount, however, the data are better explained by a local compensation model. The observed trend shows an increase ofTe with age of plate at loading time. However, the elastic layer under the Cook-Austral appears systematically thinner by several kilometers than expected for “normal” seafloor, suggesting that substantial thermal thinning has taken place in this region. Considering the apparent thermal age of the plate instead of crustal age improves noticeably the results. Along the Society chainTe varies from 20 km under Tahiti to 13 km under Maupiti which is located 500 km westward. When plotting together the Society and Cook-AustralTe results versus age of load, we notice that within the first five million years after loading,Te decreases significantly while tending rapidly to an equilibrium value. This may be interpreted as the effect of initial stress relaxation which occurs just after loading inside the lower lithosphere and suggests that the presently measured elastic thickness under the very young Tahiti load ( 0.8 Ma) is not yet the equilibrium thickness. 相似文献
7.
本文基于Vening Meinesz区域均衡模型,通过试验不同参数计算Vening Meinesz均衡补偿深度,将其与CRUST1.0模型给出的莫霍面深度进行拟合,得到适应于天山及邻区的平均补偿深度、"地区性指标"以及区域补偿半径.结合地球重力场模型EIGEN-6C4与地形数据,利用球冠体积分方法进行地形效应、沉积层效应计算和均衡校正,得到了研究区的Vening Meinesz均衡重力异常.结果显示天山及邻区的均衡重力异常幅值在-110~120 mGal之间,表明了天山及周边盆地岩石圈所处于的均衡状态,同时揭示了研究区的壳幔密度分布特征.天山、塔里木盆地、准噶尔盆地等块体的地壳垂向形变可能部分地由均衡调整引起,且均衡调整趋势与地面形变测量结果相契合.通过对均衡重力异常成因的解释,从地壳均衡角度分析了该地区复杂的构造背景及其新生代以来的演化历程. 相似文献
8.
将地形高、地球内部质量异常以及重力、大地水准面展开成球谐级数,依据岩石圈弹性挠曲均衡补偿理论建立地形—均衡补偿重力、大地水准面异常的球谐级数表达式.由此我们可以研究地形—均衡补偿重力、大地水准面异常与球谐级数阶次的关系,以及不同波长地形荷载与岩石圈挠曲补偿的关系,探讨地形—均衡补偿重力、大地水准面的频谱特性.从观测大地水准面异常和自由空气重力异常扣除地形—均衡补偿大地水准面、重力异常,可以得到均衡大地水准面异常和均衡重力异常.均衡大地水准面异常已经消除了浅层物质不均匀的影响,反映的是地球深部物质密度不均匀分布.均衡重力异常显示出中短波长特性,反映的是地壳上地幔物质分布的失衡和物质调整的动力学特征. 相似文献
9.
The origin of the high topography of the Norwegian Mountains is currently much debated. Several geophysical studies show that the uppermost mantle below southern Norway has anomalously low velocities as compared to other parts of the Baltic Shield. This study aims to shed lights on the structure of the lithospheric mantle below southern Norway by adapting and further refining a method based on isostatic and thermal equilibrium to compute temperature, temperature-related density and synthetic S-wave velocity in stable continental domains. The one-dimensional steady-state heat equation is used with topographic, Moho depth, crustal density and surface heat flow data. A condition of local isostasy is assumed and geoid undulations are used to constrain the range of possible lithosphere models.Results derived from this method suggest a thickening of the thermal lithosphere below southern Norway from west to east. The western part is found to have higher temperatures, lower densities and lower synthetic S-wave velocities than the eastern part, compatible with results from a recent P-wave travel time residual study. Comparison of the synthetic shear-velocity profiles beneath southwestern Norway with velocity profiles inverted from Rayleigh wave dispersion data suggests that the higher temperatures associated with a thinner lithosphere can explain parts of the seismic low-velocity anomaly.The inferred lithospheric structure is sensitive to uncertainties in the crustal input model, but the main features remain undisturbed by changes in the input data. The results show that the lithosphere of southwestern Norway can be in local isostatic equilibrium, if it is thinner and warmer than the lithosphere of eastern Norway. The present-day high topography may therefore be partially sustained by lower densities in the mantle lithosphere. 相似文献
10.
Heat flow in the Sohm abyssal plain is measured to be 53 mW/m2 at an age of 163 Ma. This is 25% higher than predicted by conductive cooling models, even though the sediment-corrected basement depth of 6.5 km at this location is normal for its age. An analysis of existing heat flow, depth and geoid anomalies in the northwest Atlantic shows that there is little correlation between heat flow and depth throughout the entire region. Depth and geoid are clearly related to the Bermuda swell while the associated heat flow anomaly, once adjusted for variations with age, is limited to 5 mW/m2 and only decays to the south. This means that the Bermuda swell is probably not caused by extensive thermal reheating within the lithosphere, but instead by dynamic uplift at its lower boundary due to the convective upwelling of a mantle plume. The regionally high heat flow in the northwest Atlantic may be a thermal remanent of previous plumes which passed beneath this region early in its history. Therefore, depth and heat flow anomalies from this region cannot be used to provide constraints on steady-state parameters of the lithosphere, such as the presence or absence of a long-term boundary layer at its base. 相似文献
11.
基于SIO(Scripps Institute of Oceanography)最新全球重力和高程模型,计算了巴颜喀拉地块东部及邻区的布格重力异常、均衡重力异常、岩石圈有效弹性厚度及荷载比.结合大地热流、地震速度结构、地震活动和断裂构造分布等,分析了地壳均衡状态和岩石圈有效弹性厚度、地质构造单元间的差异及与地震活动的相关性特征.研究结果表明,该区域布格重力变化范围约为-500～0mGal(1mGal=10~(-5)m·s~(-2),下同),在巴颜喀拉块体东部区域形成弧形重力梯度带,近年来的中强地震活动频发于该梯度带不同部位,应与其应力依次释放有关;均衡重力异常结果表明,其变化范围约为-80～+100mGal,且大部分区域处于±20mGal以内的被认为处于重力均衡的状态,重力非均衡(正或负)多出现于块体边界带附近,地震多发生在靠近块体边界的均衡重力异常(正或负,主要为正)区域内;巴颜喀拉地块东部及邻区岩石圈有效弹性厚度(T_e)为10～65km,不同构造单元之间T_e空间分布差异明显,较低的T_e值出现在龙门山构造带附近,T_e值为20km左右,岩石圈荷载加载比为0.5～0.8,表明现今的岩石圈挠曲状态主要由莫霍面加载形成.进一步分析表明,巴颜喀拉地块东部挤压增生与横向流动同时发生,是造成该区域地震发生与重力均衡异常高值重合、岩石圈有效弹性厚度和大地热流值较低的主要原因.本文获得的地壳均衡特征及岩石圈有效弹性强度结果,加深了对巴颜喀拉东部及邻区岩石圈构造演化过程的认识. 相似文献
12.
S.Thomas Crough 《Physics of the Earth and Planetary Interiors》1977,14(4):365-377
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). 相似文献
13.
Nafiz Maden 《Pure and Applied Geophysics》2012,169(1-2):235-248
The eastern Pontides orogenic belt is one of the most complex geodynamic settings in the Alpine–Himalayan belt due to the lack of systematical geological and geophysical data. In this study, 1D crustal structure and P-wave velocity distribution obtained from gravity modeling and seismological data in the area has been used for the development of the thermal model of the eastern Pontides orogenic belt. The computed temperature-depth profiles suggest a temperature of 590?±?60°C at a Moho depth of 35?km indicates the presence of a brittle-ductile transition zone. This temperature value might be related to water in the subducted crust of the Tethys oceanic lithosphere. The Curie temperature depth value of 29?km, which may correspond to the crustal magma chambers, is found 5–7?km below the Moho depth. Surface heat flow density values vary from 66.5 and 104.7?mW?m?2. High mantle heat flow density value of 48?mW?m?2 is obtained for the area should be related to melting of the lithospheric mantle caused by upwelling of asthenosphere. 相似文献
14.
A linear relationship between the sea floor depth and the square root of age has been found for ocean lithosphere spreading from mid-ocean ridges. The asymptotic solution of depth as a function of age for the thermally contracting lithosphere predicts a linear dependence of depth ontwith a proportionality involving the initial lithosphere temperature, the thermal diffusivity, and the isostatic expansion coefficient averaged to include any temperature dependent phase changes. Empirical depth observations, when plotted as a function of the square root of age, bear out this prediction well, but there is a variation in the gradient,ht, along the ridge on a fine scale (up to 20% over 200 km). This implies a fundamental variation of the contraction parameter over the same scale, most probably of compositional origin. Details of a more complete cooling model near the ridge crest, including a crust of different thermal parameters than those of the mantle, predict a crestal height about 0.2 km below that of the simplified model. Individual profiles from the southeast Pacific show no such crestal deviation, and it is concluded that by quickly cooling the new crust, hydrothermal circulation may remove any effects of the crust which would be seen in the topography of a lithosphere cooled totally by conduction. The straightness of depth versust for older ocean data (to 80 m.y.) precludes any basal isothermal boundary shallower than 100 km. 相似文献
15.
High-resolution P wave tomography shows that the subducting Pacific slab is stagnant in the mantle transition zone and forms a big mantle wedge beneath eastern China. The Mg isotopic investigation of large numbers of mantle-derived volcanic rocks from eastern China has revealed that carbonates carried by the subducted slab have been recycled into the upper mantle and formed carbonated peridotite overlying the mantle transition zone, which becomes the sources of various basalts. These basalts display light Mg isotopic compositions(δ26 Mg = –0.60‰ to –0.30‰) and relatively low87 Sr/86 Sr ratios(0.70314–0.70564) with ages ranging from 106 Ma to Quaternary, suggesting that their mantle source had been hybridized by recycled magnesite with minor dolomite and their initial melting occurred at 300-360 km in depth. Therefore, the carbonate metasomatism of their mantle source should have occurred at the depth larger than 360 km, which means that the subducted slab should be stagnant in the mantle transition zone forming the big mantle wedge before 106 Ma. This timing supports the rollback model of subducting slab to form the big mantle wedge. Based on high P-T experiment results, when carbonated silicate melts produced by partial melting of carbonated peridotite was raising and reached the bottom(180–120 km in depth) of cratonic lithosphere in North China, the carbonated silicate melts should have 25–18 wt% CO2 contents, with lower Si O2 and Al2 O3 contents, and higher Ca O/Al2 O3 values, similar to those of nephelinites and basanites, and have higher εNdvalues(2 to 6). The carbonatited silicate melts migrated upward and metasomatized the overlying lithospheric mantle, resulting in carbonated peridotite in the bottom of continental lithosphere beneath eastern China. As the craton lithospheric geotherm intersects the solidus of carbonated peridotite at 130 km in depth, the carbonated peridotite in the bottom of cratonic lithosphere should be partially melted, thus its physical characters are similar to the asthenosphere and it could be easily replaced by convective mantle. The newly formed carbonated silicate melts will migrate upward and metasomatize the overlying lithospheric mantle. Similarly, such metasomatism and partial melting processes repeat, and as a result the cratonic lithosphere in North China would be thinning and the carbonated silicate partial melts will be transformed to high-Si O2 alkali basalts with lower εNdvalues(to-2). As the lithospheric thinning goes on,initial melting depth of carbonated peridotite must decrease from 130 km to close 70 km, because the craton geotherm changed to approach oceanic lithosphere geotherm along with lithospheric thinning of the North China craton. Consequently, the interaction between carbonated silicate melt and cratonic lithosphere is a possible mechanism for lithosphere thinning of the North China craton during the late Cretaceous and Cenozoic. Based on the age statistics of low δ26 Mg basalts in eastern China, the lithospheric thinning processes caused by carbonated metasomatism and partial melting in eastern China are limited in a timespan from 106 to25 Ma, but increased quickly after 25 Ma. Therefore, there are two peak times for the lithospheric thinning of the North China craton: the first peak in 135-115 Ma simultaneously with the cratonic destruction, and the second peak caused by interaction between carbonated silicate melt and lithosphere mainly after 25 Ma. The later decreased the lithospheric thickness to about70 km in the eastern part of North China craton. 相似文献
16.
Firstly, the new single and combined error models applied to estimate the cumulative geoid height error are efficiently produced by the dominating error sources consisting of the gravity gradient of the satellite-equipped gradiometer and the orbital position of the space-borne GPS/GLONASS receiver using the power spectral principle. At degree 250, the cumulative geoid height error is 1.769 × 10?1 m based on the new combined error model, which preferably accords with a recovery accuracy of 1.760 ×10?1 m from the GOCE-only Earth gravity field model GO_CONS_GCF_2_TIM_R2 released in Germany. Therefore, the new combined error model of the cumulative geoid height is correct and reliable in this study. Secondly, the requirements analysis for the future GOCE Follow-On satellite system is carried out in respect of the preferred design of the matching measurement accuracy of key payloads comprising the gravity gradient and orbital position and the optimal selection of the orbital altitude of the satellite. We recommend the gravity gradient with an accuracy of 10?13?10?15 /s2, the orbital position with a precision of 1-0.1 cm and the orbital altitude of 200-250 km in the future GOCE Follow-On mission. 相似文献
17.
郯庐断裂带是中国东部规模最大的构造活动带,有着复杂的形成演化历史,对中国东部的区域构造、岩浆活动、矿产资源的形成和分布以及现代地震活动都有重要控制作用.2010年在郯庐断裂带中南段的江苏宿迁市附近,采用深地震反射探测方法对郯庐断裂带及其两侧地块的岩石圈结构进行了解剖.结果表明,该区莫霍面和岩石圈底界均向西倾,其中,地壳厚度约为31~36km,岩石圈厚度约为75~86km,且岩石圈厚度在郯庐断裂带下方出现突变.郯庐断裂带在剖面上表现为由多条主干断裂组成的花状构造,其内部发育有断陷盆地和挤压褶皱,具有伸展、挤压和走滑并存的构造形迹,暗示郯庐断裂带的形成和演化经历了多期复杂的构造活动.这一断裂带错断了近地表沉积层,向下切割莫霍面和岩石圈地幔,属岩石圈尺度的深大断裂构造系统.软流圈高温高压热物质沿断裂带的上涌、岩浆底侵或热侵蚀作用造成岩石圈出现拉张伸展和岩石圈减薄,并可能使岩石圈组构及其物质成分发生改变.本项研究结果不但可进一步加深对郯庐断裂带深、浅部结构的认识,而且还可为分析研究华北克拉通东部的深部过程和浅部构造响应提供资料约束. 相似文献
18.
Seismological evidence for a lithospheric normal faulting — the Sanriku earthquake of 1933 总被引：1，自引：0，他引：1
The focal process of the Sanriku earthquake of March 2, 1933, is discussed in relation to the bending mechanism of the lithosphere. On the basis of the P times obtained at more than 200 stations, it is confirmed that the hypocenter of this earthquake is within the lithosphere beneath the Japan trench. The P wave fault plane solution, the amplitude of long-period (100 s) Love and Rayleigh waves and two near-field observations suggest, almost definitely, that the Sanriku earthquake represents a predominantly normal faulting on a plane dipping 45° towards N 90° W. A fault size of 185 × 100 km2, in agreement with the size of the aftershock area, is required to yield a slip dislocation of 3.3 m, a value consistent with the tsunami data. This result suggests that the fracture took place over the entire thickness of the lithosphere, thereby precluding the possibility that the Sanriku earthquake merely represents a surface tensile crack due to the bending of the lithosphere. This large scale lithospheric faulting is presumably due to a gravitational pull exerted by the cold sinking lithosphere. The fracture probably took place on an old fault plane which had once fractured and healed up. The existence of this fracture zone which decouples, to some extent, the oceanic lithosphere from the sinking lithosphere accounts for the sharp bend of the lithosphere beneath oceanic trenches and also the abrupt disappearance of seismic activity across oceanic trenches. The sharp bend of the lithosphere is therefore a result, not the cause, of great earthquakes beneath oceanic trenches. 相似文献
19.
V. P. Trubitsyn 《Izvestiya Physics of the Solid Earth》2012,48(6):467-485
The modern concepts of the rheology of viscous mantle and brittle lithosphere, as well as the results of the numerical experiments on the processes in a heated layer with a viscosity dependent on pressure, temperature, and shear stress, are reviewed. These dependences are inferred from the laboratory studies of olivine and measurements of postglacial rebound (glacial isostatic adjustment) and geoid anomalies. The numerical solution of classical conservation equations for mass, heat, and momentum shows that thermal convection with a highly viscous rigid lithosphere develops in the layer with the parameters of the mantle with the considered rheology under a temperature difference of 3500 K, without any special additional conditions due to the self-organization of the material. If the viscosity parameters of the lithosphere correspond to dry olivine, the lithosphere remains monolithic (unbroken). At a lower strength (probably due to the effects of water), the lithosphere splits into a set of separate rigid plates divided by the ridges and subduction zones. The plates submerge into the mantle, and their material is involved in the convective circulation. The results of the numerical experiment may serve as direct empirical evidence to validate the basic concepts of the theory of plate tectonics; these experiments also reveal some new features of the mantle convection. The probable structure of the flows in the upper and lower mantle (including the asthenosphere), which shows the primary role of the lithospheric plates, is demonstrated for the first time. 相似文献
20.
首先计算了欧亚地区均衡残差大地水准面．基于地幔热对流的内负荷理论和最新全球层析成像结果，探讨了欧亚地区中波长均衡残差大地水准面的地球动力学意义．研究结果表明，中波长均衡残差大地水准面主要受上地幔粘滞度和岩石层强度的影响，进而得出欧亚地区一些古老地盾和构造稳定地区的上地幔与年轻山脉及构造活动地区的上地幔结构存在着差异．这个差异主要是占老地盾和构造稳定地区，如波罗的海地盾、中西伯利亚地台、东欧等区域，冷却的上地幔已穿透地幔较深，上地幔与岩石层之间耦合较好;而年轻山脉和构造活动区，如帕米尔、天山、贝加尔活动带、青藏高原、日本海周围地区，在上地幔可能存在着热物质即粘滞度很低的软流层，上地幔与岩石层耦合程度较差，甚至有可能解耦．从欧亚地区上地幔属性的差异，可以解释该地区的一些地球动力学问题． 相似文献