The crustal structure of the Eastern Alps from a combination of 3D gravity modelling and isostatic investigations

The transition between European and Adriatic crust is an important feature related to the plate collision that formed the European Alps. The diversity of seismic and geological information allows the construction of two alternative 3D density models, which both match the observed gravity field. Different seismic experiments suggest a thickness for the Adriatic crust between 30 and 40 km. The thick crust model requires an unusually dense lower crust (>3050 kg/m³) to reproduce the observed Bouguer anomaly. To evaluate the two alternative models, the isostatic implications of the geometry and density distribution within both 3D models are investigated, using local (Airy) and regional (Vening Meinesz) isostasy.Airy isostatic investigations show that the Eastern Alps are not isostatically compensated and the residuals correlate strongly with exposed geological formations. Subsequently, subsurface loading is an important factor controlling isostatic processes. The different geometry and densities in the two 3D models imply different loading at the crust–mantle boundary. The subsurface loads calculated from the 3D density models were used to estimate regional isostasy by a convolution method. In general, small rigidity values (D<10×10²¹ Nm) are determined for the Eastern Alpine lithosphere. In the model with a 40-km-thick Adriatic crust, high flexural rigidities are inferred for the Adriatic plate (>100×10²¹ Nm), but these values are unusual for an active orogenic region. The results point to the interfingering of European and Adriatic crust that results in the squeezing of European crust between Adriatic crust and mantle with additional contamination by mantle material.     

Spectral harmonic analysis and synthesis of Earth’s crust gravity field

We developed and applied a novel numerical scheme for a gravimetric forward modelling of the Earth’s crustal density structures based entirely on methods for a spherical analysis and synthesis of the gravitational field. This numerical scheme utilises expressions for the gravitational potentials and their radial derivatives generated by the homogeneous or laterally varying mass density layers with a variable height/depth and thickness given in terms of spherical harmonics. We used these expressions to compute globally the complete crust-corrected Earth’s gravity field and its contribution generated by the Earth’s crust. The gravimetric forward modelling of large known mass density structures within the Earth’s crust is realised by using global models of the Earth’s gravity field (EGM2008), topography/bathymetry (DTM2006.0), continental ice-thickness (ICE-5G), and crustal density structures (CRUST2.0). The crust-corrected gravity field is obtained after modelling and subtracting the gravitational contribution of the Earth’s crust from the EGM2008 gravity data. These refined gravity data mainly comprise information on the Moho interface and mantle lithosphere. Numerical results also reveal that the gravitational contribution of the Earth’s crust varies globally from 1,843 to 12,010 mGal. This gravitational signal is strongly correlated with the crustal thickness with its maxima in mountainous regions (Himalayas, Tibetan Plateau and Andes) with the presence of large isostatic compensation. The corresponding minima over the open oceans are due to the thin and heavier oceanic crust.     

Signature of the upper mantle density structure in the refined gravity data

The gravitational signal of the upper mantle density structures is investigated in the refined gravity data which are corrected for the gravitational contributions of the crust density structures and the Moho geometry. The gravimetric forward modeling is applied to compute these refined gravity data globally on a 1 × 1 arcdeg grid using the global geopotential model (EGM2008), the global topographic/bathymetric model (DTM2006.0) including the ice-thickness data, and the global crustal model (CRUST2.0). The characteristics of the upper mantle density structures are further analyzed in association with the Moho parameters (i.e., Moho depths and density contrast). The 1 × 1 arcdeg global data of the Moho parameters are estimated by applying the combined least-squares approach based on solving Moritz’s generalization of the Vening–Meinesz inverse problem of isostasy. The refined gravity data exhibit mainly the mantle lithosphere structures attributed to the global mantle convection. A significant correlation found over oceans between the refined gravity data and the Moho density contrast is explained by the increasing density of the oceanic lithosphere with age. Despite the lithosphere structures attributed to the global mantle convection are confirmed also in the refined gravity data over continents, the significant correlation between the refined gravity data and the Moho parameters is in this case absent. Instead, the significant proportion of lateral variations of the Moho density contrast within the continental lithosphere is attributed to the depth-dependant density changes due to pressure and thermal gradient.     

A comparison of three gravity inversion methods for crustal thickness modelling in Tibet plateau

Crustal thickness can be determined by gravimetric methods based on different assumptions, e.g. by isostatic hypotheses. Here we compare three gravimetric inversion methods to estimate the Moho depth. Two Moho models based on the Vening Meinesz-Moritz hypothesis and one by using Parker-Oldenburg’s algorithm, which are investigated in Tibet plateau. The results are compared with CRUST2.0, and it will be presented that the estimated Moho depths from the Vening Meinesz-Moritz model will be better than the Parker-Oldenburg’s algorithm.     

Formation of sedimentary basins of graben type by extension of the continental crust

A mechanism for causing graben-like subsidence by crustal stretching in response to tension is suggested, based partly on previous hypotheses of Vening Meinesz, Artemjev and Artyushkov, Bott and Fuchs. The mechanism requires rheological subdivision of the crust into a brittle upper layer about 10–20 km thick overlying a ductile lower crust. The brittle layer responds to tension by normal faulting and wedge subsidence; the ductile layer responds by local or regional thinning and by lateral flow of material from beneath the subsiding wedge causing complementary uplift by horst formation or elastic upbending. A graben width of between 30 and 60 km is predicted in absence of basement inhomogeneity. Computations of the energy budget indicate that sedimentary basins of more than 5 km thickness can form by the mechanism provided that water pressure reduces the friction on the faults. The mechanism can explain relatively rapid beginning and end of subsidence, and spasmodic sinking may occur. A wide variety of observed graben-like basins can be explained by the hypothesis, including classical rift valleys and the Mesozoic basins of UK and the North Sea, but it is inapplicable to broad unfaulted cratonic or shelf subsidence.     

Geophysical Study of the Valle Fértil Lineament Between 28°45′S and 31°30′S: Boundary Between the Cuyania and Pampia Terranes

A gravimetric and magnetometric study was carried out in the north-eastern portion of the Cuyania terrane and adjacent Pampia terrane. Gravimetric models permitted to interpret the occurrence of dense materials at the suture zone between the latter terranes. Magnetometric models led to propose the existence of different susceptibilities on either side of the suture. The Curie temperature point depth, representing the lower boundary of the magnetised crust, was found to be located at 25 km, consistent with the lower limit of the brittle crust delineated by seismic data; this unusually thick portion of the crust is thought to release stress producing significant seismicity.

Moho depths determined from seismic studies near western Sierras Pampeanas are significantly greater than those obtained from gravimetric crustal models.

Considering mass and gravity changes originated by the flat-slab Nazca plate along Cuyania and western Pampia terranes, it is possible to reconcile Moho thickness obtained either by seismic or by gravity data. Thus, topography and crustal thickness are controlled not only by erosion and shortening but by upper mantle heterogeneities produced by: (a) the oceanic subducted Nazca plate with “normal slope” also including asthenospheric materials between both continental and oceanic lithospheres; (b) flat-slab subducted Nazca plate (as shown in this work) without significant asthenospheric materials between both lithospheres. These changes influence the relationship between topographic altitudes and crustal thickness in different ways, differing from the simple Airy system relationship and modifying the crustal scale shortening calculation. These changes are significantly enlarged in the study area. Future changes in Nazca Plate slope will produce changes in the isostatic balance.     

The isostatic state of the southern Urals crust

The Uralide orogen, in Central Russia, is the focus of intense geoscientific investigations during recent years. The international research is motivated by some unusual lithospheric features compared with other collisional belts including the preservation of (a) a collisional architecture with an orogenic root and a crustal thickness of 55–58 km, and (b) large volumes of very low-grade and non-metamorphic oceanic crust and island arc rocks in the upper crust of a low–relief mountain belt. The latter cause anomalous gravity highs along the thickened crust and the isostatic equilibrium inside the Uralides lithosphere as well as the overthrust high-metamorphic rocks. The integrated URSEIS '95 seismic experiment provides fundamentally new data revealing the lithospheric architecture of an intact Paleozoic collisional orogen that allows the construction of density models. In the Urals' lithosphere different velocity structures resolved by wide-angle seismic experiments along both the URSEIS '95- and the Troitsk profile. They can be used to constrain lithospheric density models: a first model consists of a deep subducted continental lower crust which has been highly eclogitized at depths of 60–90 km to a density of 3550 kg/m³. The second model shows a slightly eclogitized lower crust underlying the Uralide orogen with a crustal thickness of 60 km. The eclogitized lower crust causes a too-small impedance contrast to the lithospheric mantle resulting in a lack of reflectors in the area of the largest crustal thickness. Both models fit the measured gravity field. Analyzing the isostatic state of the southern Urals' lithosphere, both density models are in isostatic equilibrium.     

国外景观演化研究的最新进展

景观演化与全球构造、生物演变和主要气候变化具有相同的时间尺度 ,是长期全球变化的一个重要方面。最近 ,地表过程在地壳变形中的作用已经引起了广泛的注意 :一方面努力从景观中模拟地形演化或演绎地壳过程 ;另一方面试图通过地壳变形和地表过程的型式和速度来研究隆起、海平面变化、侵蚀、气候和地势之间的动力学关系。文中旨在介绍和评述国外关于会聚边缘和裂开边缘山脉尺度景观演化研究 ,包括高原边缘地形演化、前陆盆地的沉积型式、景观切割、古地形复原以及质量流动和平衡等方面的最新进展 ,以及地表过程和地壳过程相互作用中的反馈机制、均衡效应和岩石圈强度效应以及调整和平衡等几个理论问题。目前已经普遍认为 ,把地表过程纳入地球动力学模型之中 ,无疑对于认识造山带的发育和裂开边缘的演化是一个巨大的进步。     

http://www.sciencedirect.com/science/article/pii/S1674987112001302

We present relative sea level （RSL） curves in Antarctica derived from glacial isostatic adjustment （GIA）predictions based on the melting scenarios of the Antarctic ice sheet since the Last Glacial Maximum （LGM）given in previous works.Simultaneously,Holocene-age RSL observations obtained at the raised beaches along the coast of Antarctica are shown to be in agreement with the GIA predictions.The differences from previously published ice-loading models regarding the spatial distribution and total mass change of the melted ice are significant.These models were also derived from GIA modelling; the variations can be attributed to the lack of geological and geographical evidence regarding the history of crustal movement due to ice sheet evolution.Next,we summarise the previously published ice load models and demonstrate the RSL curves based on combinations of different ice and earth models.The RSL curves calculated by GIA models indicate that the model dependence of both the ice and earth models is significantly large at several sites where RSL observations were obtained.In particular,GIA predictions based on the thin lithospheric thickness show the spatial distributions that are dependent on the melted ice thickness at each sites.These characteristics result from the short-wavelength deformation of the Earth.However,our predictions strongly suggest that it is possible to find the average ice model despite the use of the different models of lithospheric thickness.By sea level and crustal movement observations,we can deduce the geometry of the post-LGM ice sheets in detail and remove the GIA contribution from the crustal deformation and gravity change observed by space geodetic techniques,such as GPS and GRACE,for the estimation of the Antarctic ice mass change associated with recent global warming.     

The structure, isostasy and gravity field of the Levant continental margin and the southeast Mediterranean area

A 3-D layered structure of the Levant and the southeastern Mediterranean lithospheric plates was constructed using interpretations of seismic measurements and borehole data. Structural maps of three principal interfaces, elevation, top basement and the Moho, were constructed for the area studied. This area includes the African, Sinai and Arabian plates, the Herodotus and the Levant marine basins and the Nile sedimentary cone. In addition, an isopach map of the Pliocene sediments, as well as the contemporaneous amount of denuded rock units, was prepared to enable setting up the structural map of the base Pliocene sediment. Variable density distributions are suggested for the sedimentary succession in accord with its composition and compaction. The spatial density distribution in the crystalline crust was calculated by weighting the thicknesses of the lower mafic and the upper felsic crustal layers, with densities of 2.9 g/cm³ and 2.77 g/cm³, respectively. Results of the local (Airy) isostatic modeling with compensation on the Moho interface show significant deviations from the local isostasy and require variable density distribution in the upper mantle. Moving the compensation level to the base of the lithosphere ( 100 km depth) and adopting density variations in the mantle lithosphere yielded isostatic compensation (± 200 m) over most of the area studied. The spatial pattern obtained of a density distribution with a range of ± 0.05 g/cm³ is supported by a regional heat flux. Simulations of the flexure (Vening Meinesz) isostasy related to the Pliocene to Recent sedimentary loading and unloading revealed concentric oscillatory negative and positive anomalies mostly related to the Nile sedimentary cone. Such anomalies may explain the rapid subsidence in the Levant Basin and the arching in central Israel, northern Sinai and Egypt during Pliocene–Recent times. Comparison between the observed (Bouguer) gravity and the calculated gravity for the constructed 3-D lithospheric structure, which has variable density distributions, provided a good match and an independent constraint for the large-scale structure suggested and confirmed an oceanic nature for the Levant Basin lithosphere.     

Thermal and chemical variations in subcrustal cratonic lithosphere: evidence from crustal isostasy

The Earth's topography at short wavelengths results from active tectonic processes, whereas at long wavelengths it is largely determined by isostatic adjustment for the density and thickness of the crust. Using a global crustal model, we estimate the long-wavelength topography that is not due to crustal isostasy. Our most important finding is that cratons are generally depressed by 300 to 1500 m in comparison with predictions from pure crustal isostasy. We conclude that either: (1) cratonic roots may be 50 to 300 °C colder than previously suggested by thermal models, or (2) cratonic roots may be, on average, less depleted than suggested by studies of shallow mantle xenoliths. Alternatively, (3) some combination of these conditions may exist. The thermal explanation is consistent with recent geothermal studies that indicate low cratonic temperatures, as well as seismic studies that show very low seismic attenuation at long periods (150 s) beneath cratons. The petrologic explanation is consistent with recent studies of deep (>140 km) mantle xenoliths from the Kaapvaal and Slave cratons that show 1–2% higher densities compared with shallow (<140 km), highly depleted xenoliths.     

A seismic landslide hazard analysis with topographic effect,a case study in the 99 Peaks region,Central Taiwan

It has been known that ground motion amplitude will be amplified at mountaintops; however, such topographic effects are not included in conventional landslide hazard models. In this study, a modified procedure that considers the topographic effects is proposed to analyze the seismic landslide hazard. The topographic effect is estimated by back analysis. First, a 3D dynamic numerical model with irregular topography is constructed. The theoretical topographic amplification factors are derived from the dynamic numerical model. The ground motion record is regarded as the reference motion in the plane area. By combining the topographic amplification factors with the reference motions, the amplified acceleration time history and amplified seismic intensity parameters are obtained. Newmark’s displacement model is chosen to perform the seismic landslide hazard analysis. By combining the regression equation and the seismic parameter of peak ground acceleration and Arias intensity, the Newmark’s displacement distribution is generated. Subsequently, the calculated Newmark’s displacement maps are transformed to the hazard maps. The landslide hazard maps of the 99 Peaks region, Central Taiwan are evaluated. The actual landslide inventory maps triggered by the 21 September 1999, Chi-Chi earthquake are compared with the calculated hazard maps. Relative to the conventional procedure, the results show that the proposed procedures, which include the topographic effect can obtain a better result for seismic landslide hazard analysis. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Flexural and gravity modelling of the Mérida Andes and Barinas–Apure Basin, Western Venezuela

The kinematic evolution of the Barinas–Apure Basin and the southern Mérida Andes from Lower Miocene to the Present is numerically modelled using flexural isostatic theory and geophysical and geological data. Two published regional transects are used to build up a reference section, which is then used to constrain important parameters (e.g. shortenings and sedimentary thicknesses) for the flexural modelling. To control the location of the main fault system in the flexural model earthquake information is also used. The estimated flexural elastic thickness of the South American lithosphere beneath the Barinas–Apure Basin and the Mérida Andes Range is 25 km. The value for the final total shortening is 60 km. The flexural isostatic model shows that the Andean uplift has caused the South American lithosphere subsidence and the development of the Barinas–Apure Basin.In addition, gravity modelling was used to understand deep crustal features that could not be predicted by flexural theory. Consequently, the best-fit flexural model is used to build a gravity model across the Mérida Andes and the Barinas–Apure Basin preserving the best-controlled structural features from the flexural modelling (e.g. basin wavelength and depth) and slightly changing the main bodies density values and deep crustal structures. The final gravity model is intended to be representative of the major features affecting the gravity field in the study area. The predicted morphology in the lower crustal level of the final gravity model favours the hypothesis of a present delamination or megathrust of the Maracaibo crust over the South American Shield. This process would use the Conrad discontinuity as a main detachment surface within an incipient NW dipping continental subduction.     

Relative sea‐level changes,glacial isostatic modelling and ice‐sheet reconstructions from the British Isles since the Last Glacial Maximum

While contributing <1 m equivalent eustatic sea‐level rise the British Isles ice sheet produced glacio‐isostatic rebound in northern Britain of similar magnitude to eustatic sea‐level change, or global meltwater influx, over the last 18 000 years. The resulting spatially variable relative sea‐level changes combine with observations from far‐field locations to produce a rigorous test for quantitative models of glacial isostatic adjustment, local ice‐sheet history and global meltwater influx. After a review of the attributes of relative sea‐level observations significant for constraining large‐scale models of the isostatic adjustment process we summarise long records of relative sea‐level change from the British Isles and far‐field locations. We give an overview of different global theoretical models of the isostatic adjustment process before presenting intercomparisons of observed and predicted relative sea levels at sites in the British Isles and far‐field for a range of Earth and ice model parameters in order to demonstrate model sensitivity and the resolving power available from using evidence from the British Isles. For the first time we show a good degree of fit between relative sea‐level observations and predictions that are based upon global Earth and ice model parameters, independently derived from analysis of far‐field data, with a terrain‐corrected model of the British Isles ice sheet that includes extensive glaciation of the North Sea and western continental shelf, that does not assume isostatic equilibrium at the Last Glacial Maximum and keeps to trimline constraints of ice surface elevation. We do not attempt to identify a unique solution for the model lithosphere thickness parameter or the local‐scale detail of the ice model in order to provide a fit for all sites, but argue that the next stage should be to incorporate an ice‐sheet model that is based on quantitative, glaciological model simulations. We hope that this paper will stimulate this debate and help to integrate research in glacial geomorphology, glaciology, sea‐level change, Earth rheology and quantitative modelling. Copyright © 2006 John Wiley & Sons, Ltd.     

Effective Elastic Thickness of Southeast Part of Arctic Ocean—Eurasia Continent—Pacific Ocean Geoscience Transect

The effective lithospheric elastic thickness of the continent is an important parameter for examination of the large-scale structure and analyses of the mechanism of isostatic compensation within the plate, and a parameter standing for the strength of the lithosphere.The Te values along Quanzhou-Heishui,the southeast section of the Arctic Ocean-Eurasian Continent-Pacific Ocean geoscience transect, are estimated by using the coherence technique developed by Forsyth.Studies of the feature of the coher-ence between gravity and topography suggest that at short wavelengths(6.6-100km)for each data box that is used to estimate Te,the plate is strong enough to support topographic loads and gravity and topog-raphy are uncorrelated.At long wavelengths where the plate is deflected by surface and subsurface loads are compensated by the flexure model.Sichuan land-stone with low heat-flow values has high Te values whereas in Ninghua,Datian,land-stone with high heat-flow values has low Te values,which reflects a correlation,low heat-flow values corresponding to high Te values and high heat-flow values corresponding to low Te values.Te values can be divided into two sections:northwest high section and southeast low sec-tion.There is roughly a positive correlation between crustal thickness and effective elastic thickness of the lithosphere.     

Numerical modeling of eastern Tibetan-type margin: Influences of surface processes,lithospheric structure and crustal rheology

The eastern Tibetan margin is characterized by a steep topographic gradient and remarkably lateral variations in crustal/lithospheric structure and thermal state. GPS measurements show that the surface convergence rate in this area is strikingly low. How can such a mountain range grow without significant upper crustal shortening? In order to investigate the formation mechanism of the eastern Tibetan-type margins, we conducted 2D numerical simulations based on finite difference and marker-in-cell techniques. The numerical models were constrained with geological and geophysical observations in the eastern Tibetan margin. Several major parameters responsible for topography building, such as the convergence rate, the erosion/sediment rate, and the presence of partially molten crust, were systematically examined. The results indicate that the presence of partially molten material in the middle/lower crust can make a positive contribution to the formation of steep topography, but it is not a necessary factor. A steep topographic gradient may be a characteristic feature when a thin lithosphere with thick crust converges with a thick lithosphere with thin crust. In the context of a high erosion rate, the Longmen Shan range still gains and maintains its steep high topography to the present. This could be explained by exerting a large push force on Tibet side. Our numerical experiments suggest that topographic characteristic across the eastern Tibetan-type margins is mainly derived from isostatic equilibration forces and intensive convergence between two continental lithospheres with totally different rheological properties.     

Evaluation of crustal and upper mantle structures using receiver function analysis: ISM broadband observatory data

A three-component broadband seismograph is in operation since January 2007 at the Indian School of Mines (ISM) campus, Dhanbad. We have used the broadband (BB) seismograms of 17 teleseismic events (M ≥ 5.8) recorded by this single BB station during 2008–09 to estimate the crust and upper mantle discontinuities in Dhanbad area which falls in the peninsular India shield. The converted wave technique and the Receiver function analysis are used. A 1-D velocity model has been derived using inversion. The Mohorovicic (Moho) discontinuity (crustal thickness) below the ISM observatory is estimated to be ∼41 km, with an average Poisson ratio of ∼0.28, suggesting that the crust below the Dhanbad area is intermediate to mafic in nature. The single station BB data shed new light to the estimate of crustal thickness beneath the eastern India shield area, which was hitherto elusive. Further, it is observed that the global upper mantle discontinuity at 410 km is delayed by ∼0.6 sec compared to the IASP-91 global model; this may be explained by a slower/hotter upper mantle; while the 660 km discontinuity is within the noise level of data.     

Lateral density inhomogeneities of the continental and oceanic lithosphere and their relationship with the Earth’s crust formation

This paper presents the results of the study of the free mantle surface (FMS) depth beneath continents and oceans. The reasons for the observed dependence of the FMS depth on the crustal thickness in the continental lithosphere are discussed. The influence of radial variations in the mantle’s density is evaluated. The calculations performed have indicated that the observed dependence of the FMS depth on the crustal thickness is caused mostly by lateral inhomogeneities in the lithospheric mantle, and the size of these inhomogeneities is proportional to the thickness of the crust. The origin of such inhomogeneities can be related to the process of continental crust formation.     

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.     

北冰洋-欧亚大陆-太平洋地学断面东南段岩石圈有效弹性厚度

大陆岩石圈有效弹性厚度（Te）是表示岩石圈强度的参数，计算该参数对研究岩石圈大规模构造，分析大陆板块内的均衡补偿机制有一定意义。利用Forsyth提出的相关技术计算的北冰洋－欧亚大陆－太平洋地学断面东南段自黑水到泉州的Te值，并分析了重力和地形波长的相关性特征，初步认为：计算Te值所选的每个数据块在短波长（6.6-100km)内岩石圈板块的强度足以平衡地形负载，重力和地形不相关；在长波长（100－250km)内，地形及地下负载由弯曲模型补偿，岩石圈板块在地形及地下负载作用下而挠曲。断面通过地段具有较低热流密度值的陆核有较高的Te值，具有较高热流密度值的宁化，大田地区有较低的Te值，反映了较高的热流密度值对应较低的Te值，较低的热流密度值对应较高Te 值的关系，Te可分为南东低值段和北西高值段，地壳厚度大体上与Te值呈正相关关系。