Subcrustal density inhomogeneities of Northern Eurasia as derived from the gravity data and isostatic models of the lithosphere

For the territory of Northern Eurasia (6°E–165°W; 30–75°N) the distribution of anomalous masses in the lithosphere has been estimated in accordance with the lithosphere isostatic model. The method of model construction is based on the admittance technique. The experimental admittance presents a relation between the part of the outer load uncompensated by the Moho undulations and the residual gravity field and is used to select the best model. The 1 × 1° averaged values of topography elevations, basement and Moho depths, sedimentary cover density and gravity anomalies have been used as initial data. According to the correlation equation relating the outer load and Moho depths, the mean density contrast between the lower crust and the subcrustal lithosphere is 0.43 g/cm³, but the Moho undulation can not provide complete isostatic equilibrium. In some areas, the part of the outer load uncompensated by Moho undulations may be as large as 10⁷ kg/m² and the residual gravity field is as intensive as + 260 mGal. Assuming that for loads of wavelength > 200 km, local isostatic compensation is valid, in accordance with the admittance analysis, the anomalous masses compensating for the part of the outer load, which is not compensated by Moho undulations, have to be located partly in the lower crust and in the subcrustal layer. The regional trend of anomalous compensating masses is negative under Western Europe, the Mediterranean, Eastern Asia and adjacent marginal seas, and positive under the East European Platform and Western and Central Asia. The local compensating masses correspond to particular tectonic units. The isostatic gravity anomalies of Northern Eurasia have been determined and the long-wave component of the field reflecting anomalous masses under the isostatic compensation level has been evaluated.     

龙门山晚新生代均衡反弹隆升的定量研究

龙门山位于青藏高原东缘与四川盆地的交接部位,是青藏高原周边山脉中地形梯度变化最大的山脉,其隆升过程和机制一直是国际地学界关注的焦点。晚新生代经过大量的滑坡、泥石流等快速剥蚀作用,龙门山的高程却不断升高。讨论了龙门山构造隆升的3种地球动力学机制,即下地壳通道流机制、地壳挤压缩短变形机制、地壳均衡反弹机制。晚新生代龙门山的隆升与剥蚀引起的均衡反弹作用相关,剥蚀作用使得地壳岩石逐步被移去,剥蚀区重力损失,岩石圈或地壳卸载作用导致山脉顶峰的隆升。结合数字高程模型数据研究表明,巨大地震的长期同震构造变形以及滑坡、泥石流等引起的快速剥蚀所导致的地壳均衡反弹,可能是龙门山晚新生代构造隆升的地球动力学新机制。龙门山地区现今高程受构造作用与剥蚀引起的均衡反弹作用的共同影响,其中剥蚀引起的均衡反弹作用对龙门山隆升的影响贡献率约占30%。     

喜马拉雅中东部镜像均衡重力异常的形成研究

青藏高原南缘处于重力不均衡状态,由北向南可依次分为高原近重力均衡区、喜马拉雅山正均衡异常区和山前盆地负均衡异常区,正、负异常呈现壮观的镜像分布。本文选取喜马拉雅中东部的均衡重力异常数据,结合地貌高程、地壳厚度、降雨量、冰川及山前沉积等的分布状况,探讨地貌分异与均衡重力异常分布的相互关系。由上述资料获得3条跨越喜马拉雅山的综合剖面,结果显示喜马拉雅中东部正均衡重力异常的分布与冰川、河流等代表的地表剥蚀作用存在明显的空间耦合关系,而与降雨量无直接联系,山前盆地负均衡重力异常与沉积厚度的分布也存在很好的耦合。利用数值模型计算得到了喜马拉雅地区的均衡调整时间域在1 Ma左右的时间尺度内。通过与地貌响应时间域相对比,以及对地表剥蚀厚度的估计,认为山脉地区的正均衡异常主要由地壳厚度补偿不足引起(侧重Airy假说),而山前盆地的负均衡异常主要由低密度沉积层的分布引起(侧重Pratt假说),由于地貌响应时间快于均衡调整时间,在大约5～2 Ma以来,地壳的均衡调整始终延迟于山脉的持续剥蚀和山前的持续沉积,使得岩石圈朝着"反均衡"方向演变,最终形成了喜马拉雅现今壮观的镜像均衡重力异常分布。     

攀西地区重力均衡异常的研究

研究表明,康滇地轴及其东部地区属均衡补偿平衡区,西部属强烈上升的异常区。上升的原动力为板块碰撞的构造力与均衡调整力的综合。     

龙门山均衡重力异常及其对青藏高原东缘山脉地壳隆升的约束

青藏高原东缘处于不均衡状态，自西而东可分为青藏高原弱负均衡重力异常区、龙门山正均衡重力异常区和四川盆地负均衡重力异常区，表明该区的不均衡状态并未导致Airy均衡运动的产生，即龙门山没有均衡下降，而处于不断的隆升状态，显示该地区反均衡运动的构造抬升是导致龙门山隆升的主因。本次采用似三度体重力异常计算方法对该区的正均衡重力异常进行模拟和反演，研究了大尺度地貌分异与均衡重力异常分区之间的相互关系，结果表明，龙门山的下地壳顶面抬升了11．2～12．6km，造成了龙门山的正均衡异常，揭示了构造抬升和剥蚀作用在相似的时间尺度上和空间尺度上控制着龙门山地貌的形成，龙门山的表面隆升是构造隆升和剥蚀作用相叠加的产物。     

The Northern and Southern Scandes — structural differences revealed by an analysis of gravity anomalies, the geoid and regional isostasy

We investigate the Scandes mountain range by analysing the gravity field, the geoid heights and the degree of isostatic compensation of the lithosphere. Topographically, the Scandes mountain range can be divided in the Northern and Southern Scandes. Comparisons between the present topographic expression and the gravity field and the geoid show that the axis of highest elevation in the Northern Scandes is shifted eastwards compared to the minimum of the Bouguer anomaly, while the two coincide perfectly in the Southern Scandes. Geoid heights reduced by the effect of topographic masses show a large-scale minimum in the Northern Scandes, but no anomaly in the Southern Scandes.Regional, flexural isostatic calculations yield a flexural rigidity of D = 10²³ Nm for the lithosphere of the Southern Scandes and the isostatic gravity and geoid residuals point to additional isostatic support by low-density rocks below the Moho. On the other side, for the lithosphere in the Northern Scandes no significant flexural rigidity can be resolved. Here, the Bouguer anomaly is best modelled with a small flexural rigidity, indicating nearly Airy isostatic behaviour. Local subsurface loading and horizontal tectonic forces overprint the isostatic compensations and increase the tectonic complexity of the Northern Scandes. These distinctive features of the Scandes cannot be explained by currently existing models of the present and Neogene uplift and the isostatic mechanism of the Scandes.     

地壳均衡假说与湘西北地壳均衡失调探讨

由湖南省1:100万10′×15′平均均衡重力异常图知,湘西北地区呈重力高;除此以外的广大地区,当消除花岗岩对应的局部重力低外,均衡异常基本呈零值特征。笔者认为这是湖南全省的地壳均衡失调区,并从5个方面对该问题进行了分析。     

Global earth isostatic model using smoothed Airy-Heiskanen and Vening Meinesz hypotheses

Isostatic hypotheses are used for different purposes in geophysics and geodesy. The Erath crustal thickness modelling is more complicated than the classical isostatic models. In this study we try to modify Airy-Hesiskanen model, utilizing a smoothing factor, to a model with regional or global isostatic model through a modern solution of the gravimetric-isostatic Vening Meinesz model and CRUST.0. In Airy-Hesiskanen’s theory there is no correlation between neighbouring crustal columns, while this must be the case in reality due to the elasticity of the Earth. The idea is to keep the simplicity of the Airy-Heiskanen model, because it needs only the topographic data, and change the model which becomes to a model with regional/global isostatic model. The isostatic assumption for compensating the topographic potential is incomplete, as there are other geophysical phenomena which should be considered. Using the isostatic hypothesis for determining the depth of crust causes some disturbing signals in the gravity anomaly (approximately 285 mGal), which influence the crustal thickness determination. In this paper a simple method use for removing these effects. Spherical harmonic potential coefficients of the topographic compensation masses are used for modifying Airy-Heiskanen’s model in a least-square adjustment procedure by estimating smoothing factor. The numerical analysis shows that below degree 10, the modified Airy-Hesiskanen and Vening Meinesz models are close together. Smoothing factors for modifying the Airy-Hesiskanen model vary from 0.75 to 0.64 between degrees 200 and 2159.     

Course and origin of the Fennoscandian uplift: the case for two separate mechanisms

The classical glacial isostatic uplift paradigm of Fennoscandia is in need of a thorough revision as to its course and origin. The total uplift amounts to 830 m (not 300 m). The centre of uplift started to rise 13,000 BP (not 9300 BP). The uplift appears to be composed of two separate mechanisms; one true glacial isostatic rebound, and one linear uplift factor possibly representing a phase boundary adjustment. Calculations and modelling must treat these two factors independently (which has not been the case).     

Graben generation of major sedimentary basins

Regional isostatic adjustment to the load that fills the trough of a large graben will produce a surface depression that extends far beyond the boundaries of the load. Sediment infilling of this depression can lead to the development of a major sedimentary basin as the regional adjustment mechanism approaches isostatic equilibrium.     

Postglacial temperature anomalies and glacial isostasy

The isostatic depression of an ice-free land surface, originally caused by glacial ice, ought to be converted into a change in mean annual temperature according to the usual value for the environmental lapse rate, 6.5°C/1000 m. Calculations are undertaken to show the temperature changes, relative to present values at a site, that can be expected from this effect during the retreat of a major continental ice sheet. It is concluded that when restrained rebound is taken into account temperatures relative to the present could vary from 0°C at the start of deglaciation to about +2.32°C at the final disappearance of the Laurentide Ice Sheet. The decay of the warmer-than-present temperatures towards their current values is controlled, in this model, by isostatic recovery of the land surface. Variance estimates are built into the model and suggest that the anomaly might be as small as 0.75°C or as large as 4.0°C depending on the choice of values for crucial components such as maximum ice thickness, the proportion of isostatic deformation, and the amount of restrained rebound achieved by the time a site become ice free.     

均衡重力异常计算方法

本文提出了采用埃利均衡补偿模式计算5ˊ×5ˊ平均均衡重力异常的一种方认。计算是使用5ˊ×5ˊ平均高程值,将A～O带分为近、中、远三个区:近区用1ˊ40〞×1ˊ40〞的柱体,中区用5ˊ×5ˊ的柱体,远区用15ˊ×15ˊ的柱体,分别采用矩形柱体、质线和质点的正演公式计算地形一均衡校正值。从华南某地区的均衡重力异常图可以看出,根据均衡重力异常可以消除由地壳厚度变化而产生的区域场,突出局部异常场。     

利用相关分析划分重力异常

本文从理论上对相关分析法区分重力异常的可行性进行了论述，并从一元线性回归方程着手，对大地水准面上的重力值与相应的地形高程作了相关分析，通过分析，认为在区域性小比例主均衡补偿基本完善的条件下，重力值与高程有极好的线性相关关系；当计算窗口较大时，地下的局部不均匀体，不足以破坏重力值与高程的相头发一，所以，在实际工作中，利用相关分析处理重力资料的叠加场，可以收到较好的效果，且方法简单易行。     

Geomorphological evidence from the Lleyn Peninsula constraining models of the magnitude and rate of isostatic rebound during deglaciation of the Irish Sea Basin

A stepped series of sand and gravel terraces on the Lleyn peninsula of North Wales is used to test the magnitude and rate of isostatic depression required by the recently proposed glacimarine model of deglaciation of the Irish Sea Basin. A relative sea-level fall of 70 m is required while the ice remained pinned at the north Lleyn coast. Even taking the maximum known rate of isostatic uplift, the margin would have to remain stationary for 1400 years. It seems more reasonable to interpret the Lleyn terraces, and similar features around the Irish Sea Basin, as glacifluvial and glacilacustrine.     

Gravity field and isostatic state of Ethiopia and adjacent areas

Over 35,000 onshore and offshore gravity stations have been compiled in order to test isostatic models against geologic structures over a part of the Afro–Arabian shield. The area of Ethiopia covers an important part of this system because it contains the major section of the ≈5000 km Afro–Arabian rift and includes the transition between the Arabo-Nubian-Shield (ANS) and the Mozambique Belt (MB).Isostatic residual anomalies have been calculated using both Airy and Vening-Meinesz (flexural rigidity D = 10²² Nm) models. The isostatic residual anomalies outline the major Precambrian belts, the Cenozoic rifts and associated major structures. Positive residual anomalies associated with the main Ethiopian Rift (MER) and Kenyan rift systems could be the expressions of an axial intrusive body and swarms of local faults and fractures. The residual anomalies indicate relative stability in the MER and increased tectonic activity in the areas of the Red Sea, Gulf of Aden and Afar. Near-zero isostatic residuals flank the MER and Kenya rifts and are found within the Danakil Alps and some plateau regions.The small mean isostatic residual anomaly (about 8 mGal) and the isostatic analysis show a slight positive bias indicating under compensation. The undercompensation may imply that there are upper crustal features that are not compensated regionally (probably supported by the rigidity of the lithosphere) and isostatic disequilibrium in the region. Therefore, the high topography of Ethiopia and East African plateau is partly compensated by thicker crust (broad negative isostatic regional anomaly) and partly by dynamic forces.The results of the qualitative interpretation form the basis of continuing three-dimensional gravity modelling and quantitative analysis that also integrates data from eastern Sudan.     

Exhumation patterns along shallow low‐angle normal faults: an example from the Altotiberina active fault system (Northern Apennines,Italy)

A multi‐method approach (palaeothermal and thermochronological analyses; thermal modelling) is applied to reconstruct the exhumation history of the Altotiberina Fault (ATF), a representative example of crustal‐scale active low‐angle normal faulting in the Northern Apennines (Italy). Thermal maturity and thermochronological data yield similar burial histories but different exhumation patterns for the sedimentary successions in the hangingwall and the footwall of the ATF. Since 3.8 Ma, the ATF footwall has exhumed at rates of 0.90 mm a^?1. Exhumation led to bending and deactivation of the ATF uppermost portion as a result of tectonic unloading and isostatic adjustment, followed by migration of extension and the development of a set of domino‐like, east‐dipping normal faults, rooting on the buried portion of the ATF. ATF activity and isostatic rebound exhumed Triassic rock units from depths of about 4 km. We suggest that isostatic instability is accommodated at shallow crustal levels, in a similar way to what is observed on larger structures at mid‐low crustal levels.     

Syn-convergence exhumation of the Central Alps

Abstract

The exhumation of rocks in a plate convergence setting is commonly related to erosion and/or tectonic denudation accompanied by isostatic adjustment. Isostatic compensation is the physical response to denudation. It leads to unroofing of deep levels of the crust. A new model for producing topographic relief is proposed which explains well the rapid exhumation of high-temperature rocks in the Central Alps via erosion and tectonic denudation (i.e. gravitational collapse and normal faulting). It is shown that the forward motion of the cold and rigid Adriatic indenter into the European crust is twofold. Firstly, horizontal compression led to the vertical extrusion of the deepest ductile European basement into shallower levels. This tectonic process induced heat transfer through the southern steep belt as well as heat advection together with the extruded material, resulting in the metamorphic aureole observed in the Central Alps. Secondly, the lower part of the Adriatic crust protruded into the warm European crust as a result of continuous forward motion. Geophysical data suggest that the isostatic response to indentation (i.e. deepening of the alpine root) has been inhibited by the mechanical strength of the cold and rigid Adriatic crust. Then, the indentation process induced a deviation from isostatic equilibrium by creating a tremendous topographic relief. This relief disappeared rapidly, possibly as fast as it forms, by enhanced erosion and tectonic denudation leading to rapid exhumation of the metamorphic dome.     

Relationship of gravity anomalies and tectonics in Assam, India

Gravity data from Assam compiled on Bouguer, Hayford and Airy isostatic anomaly maps have been interpreted in terms of tectonics of the area. The gravity anomalies suggest that the Dauki fault is very deep-seated. A gravity high of about 60 mGal near Haflong is interpreted as being the expression of an intrusive body with a density contrast of about + 0.15 g/cm₃ with respect to the surroundings. From isostatic considerations, approximate crustal thicknesses over the Shillong Plateau, the Upper Assam valley and the Surma valley are estimated to be 40, 29 and 22 km respectively, suggesting a sharp change in crustal thickness from the Shillong Plateau to the Surma valley across the Dauki fault.     

Study on the Gravity Field and Deep-Seated Crustal Structure at the North Margin of the Qinghai-Tibet Plateau

1.I~ductionThenorthernmarginoftheQinghai-TibetplateauincludestheAltllnMis.,theQilianMis.,KunlunMis.,theQaidambasinandthesouthernTarimbasin.ThisareaistCctonicallycharacterizedbyintensiveCenozoicdeformationwithcomplicateddeformationalmechedsm(Molnaretal.,1987;Zheng,1991;Culetal.,1994;Ding,1995andXuetal.,1996).Thedeformationalmechanismsincludethrust-napping,strike-slipping,extensionandblockrotation,aswellassimultaneousupliftingandtypicalbasin-rangetectonics(CulandXu,1996).IntermsofCenozoi…     

重力异常与青藏高原地壳构造

1987年在青海格尔木—西藏亚东地区开展了1:50万重力路线扑点工作。在总长度1620km内,布设201个测站。利用上述资料计算了研究区的莫霍面深度及均衡异常(普拉特模式),分析了地壳与上地幔内的均衡信息。利用布格异常形态与测点高程的相关统计,在工区内划分了9个地体及13条较大的断裂。