Lateral Density Variations Research Articles

The lithosphere–asthenosphere system beneath Ireland from integrated geophysical–petrological modeling II: 3D thermal and compositional structure

The lithosphere–asthenosphere boundary (LAB) depth represents a fundamental parameter in any quantitative lithospheric model, controlling to a large extent the temperature distribution within the crust and the uppermost mantle. The tectonic history of Ireland includes early Paleozoic closure of the Iapetus Ocean across the Iapetus Suture Zone (ISZ), and in northeastern Ireland late Paleozoic to early Mesozoic crustal extension, during which thick Permo-Triassic sedimentary successions were deposited, followed by early Cenozoic extrusion of large scale flood basalts. Although the crustal structure in Ireland and neighboring offshore areas is fairly well constrained, with the notable exception of the crust beneath Northern Ireland, the Irish uppermost mantle remains to date relatively unknown. In particular, the nature and extent of a hypothetical interaction between a putative proto Icelandic mantle plume and the Irish and Scottish lithosphere during the Tertiary opening of the North Atlantic has long been discussed in the literature with diverging conclusions. In this work, the present-day thermal and compositional structure of the lithosphere in Ireland is modeled based on a geophysical–petrological approach (LitMod3D) that combines comprehensively a large variety of data (namely elevation, surface heat flow, potential fields, xenoliths and seismic tomography models), reducing the inherent uncertainties and trade-offs associated with classical modeling of those individual data sets. The preferred 3D lithospheric models show moderate lateral density variations in Ireland characterized by a slightly thickened lithosphere along the SW-NE trending ISZ, and a progressive lithospheric thinning from southern Ireland towards the north. The mantle composition in the southern half of Ireland (East Avalonia) is relatively and uniformly fertile (i.e., typical Phanerozoic mantle), whereas the lithospheric composition in the northern half of Ireland (Laurentia) seems to vary from moderately depleted to fertile, in agreement with mantle xenoliths erupted in northwestern Ireland.

The effect of lateral density variations on the state of stress in the uppermost crust in Finland

The effect of lateral density variations on the state of stress in the uppermost crust in Finland

A New Idea of NREGS and its Application to Oil Exploration, Case Study: Detecting Geological Salt Structures in Northern Coasts of Persian Gulf

If Correlation of Gravitational Data with Topography is minimized correctly and coherently, Interpretation of Land-Based Gravitational Data will be a valuable clue leading us to detecting Lateral Variations of underground Mass-Density Anomalies particularly for Oil Exploration Purposes. For this reason, the author recommends a New Idea based on new mathematical definition of ?Nearest Reference Equi-Gravitational Surface (NREGS)? for making uncorrelated gravitational data with topography. NREGS is Reference Equi-Gravitational Surface in exterior of earth where the gravitational data will be upward-continued to and interpreted, too. The Mathematical Theory of the Idea explains that gravitational data on NREGS should be the most uncorrelated with topography. Hence, gravitational data on it can easily illustrate lateral density variation of geological substructure with a lot of clear contrasts being so useful for horizontally detecting underground density anomalies e.g. salt structures. In a case study based on 6350 gravitational observations in Coastal Fars of Iran, the author applied NREGS Idea for exploration of salt structures. There, NREGS idea detects all outcropped salt structures and also, it shows some other Low Anomalies which can be underground hidden salt structures. In addition, the case study shows that the gravitational data on NREGS is statistically more uncorrelated with topography than other data e.g. Bouguer Anomaly, Free-Air Anomaly, etc. It means that the case study practically proves the theory of the New Idea, too.

Rotational motion of Phobos

Context. Phobos is in synchronous spin-orbit resonance around Mars, like our Moon around the Earth. As a consequence, the rotational period of Phobos is equal in average to its orbital period. The variations of its rotational motion are described by oscillations, called physical librations, which yield information of its interior structure. The largest libration of Phobos rotational motion was first detected in 1981 and the determination of this libration has recently been improved using Mars EXpress observations. Aims. The objective of this paper is to present the spectrum of Phobos’ librations by using recent orbital ephemerides and geophysical knowledge of this Martian satellite. The analysis of the librational spectrum highlights the relationship between dynamical and geophysical properties of the body, but is also useful for cartographic and geodetic purposes for future space missions dedicated to Phobos. Methods. We developed a numerical model of Phobos’ rotation that includes the point-mass Mars acting on the dynamical shape of Phobos, expanded to the third degree, and the e ect of Mars’ oblateness. The forced librations spectrum is extracted through a frequency analysis. Results. We find that the libration in longitude presents a quadratic term that coincides with the secular acceleration of Phobos falling onto Mars. The primary libration in longitude has a period equal to the anomalistic mean motion, whereas the primary libration in latitude has a period equal to the draconic mean motion (node to node). Both librations have amplitudes of about one degree leading to a surface displacement of about 200 m. These two components dominate the libration spectrum by a factor one hundred. Phobos’ third degree gravity harmonics and Mars’ oblateness a ect the librations amplitude at 10 4 degree. This is small but detectable from long-term tracking of a lander. The determination of the librational spectrum would bring strong constraints on the principal torques acting on the Martian moon, as well as on the possible presence of lateral variations in density predicted by certain geophysical models of the Stickney crater formation. We also investigate the obliquity variations of Phobos and find that their amplitudes are larger than the mean value of the obliquity. Conclusions. Phobos exhibits a rich and varied set of librational oscillations. The main librations and the librations close to the proper frequencies are the most sensitive to the interior structure. On the other hand, the superimposed e ect of large amplitude oscillations is likely to make the determination of the mean obliquity challenging.

Experiment Research of the Lateral Properties and Density Variation of Loess Subgrade to Dynamic Compaction for Mountainous Highway

To study the applicability of dynamic compaction for loess subgrade, the model test had been done. According to the test results, some conclusions were drawn out as follows: dynamic compaction can improve the density of loess in a short time, the lateral effective compaction width of loess was narrowed with the depth of loess deepened. In the same depth of soil, the density was improved with the tamping times, and the lateral compaction width was relevant to the distance between tamping pit and discussing spot.

Local-strain mapping on Ag(111) islands on Nb(110)

The local in-plane strain of Ag(111) islands on the Nb(110) surface has been derived from the lateral variation of the electronic density of states measured by scanning tunneling spectroscopy. The onset energy Ess of the Shockley-type surface-state band is shifted to higher energies compared to the bulk due to thermal strain arising from the difference in thermal expansion between Nb and Ag and cooling by 565 K. A quantitative dependence of Ess from the strain is obtained by density-functional theory calculations. This allows a mapping of the local strain of individual Ag islands on the nanometer scale.

Development of salt minibasins initiated by sedimentary topographic relief

Can sedimentary topographic relief (here termed uneven sedimentation) explain the enigmatic early initiation of salt minibasins at rifted continental margins when the sediment fill is less dense than the underlying salt? Using isostatic balancing arguments for compacting sediments, we first demonstrate that minibasin-scale thickness variations in a weak sedimentary overburden can cause lateral pressure differences large enough to initiate salt minibasins. These contribute toward the minibasin attaining the Rayleigh–Taylor threshold, the point at which sediments sink into an underlying salt layer. Secondly, using two-dimensional plane-strain finite element models, we show that uneven sediment loading on a viscous salt channel can initiate salt minibasins even when sediments have finite strength. Uneven sedimentation (sedimentary topographic relief) is idealized in the models as uniform aggradation of compacting sediment with a superimposed smooth sinusoidal topographic perturbation, wavelength λ, maximum height hl.Our main goal is to estimate the minimum duration that uneven sedimentation must persist, tP, such that the strength of the sediments is overcome and the minibasin continues to develop and mature (a successful minibasin) when uneven sedimentation is replaced by uniform aggradation. We investigate models with widths, W, equal to λ (confined salt models), and greater than λ (unconfined salt models). Our results suggest that for models that develop successful minibasins there is an optimal value of λ, λOPT, for which tRT, the time to attain the theoretical Rayleigh–Taylor threshold based on isostasy arguments, and tP are the smallest. This wavelength selection exists when the sediments have moderate (e.g., shale) to high (e.g., carbonates) density but is not prominent for low density sediments. For confined salt models at λOPT, tP varies from 8.0 to 2.25Ma for low to high density sediments, respectively, when hl ranges from 40 to 80m. In unconfined models, where salt can be expelled over larger distances than λ, tP ranges from 7 to 1.5Ma. For a given sediment density and compaction model, there is a lower limit of λ below which uneven sedimentation alone does not initiate minibasins. However, successful minibasins develop over a wider range of λ in unconfined models and when W≫λ, uneven sedimentation in one location can indirectly lead to the development of outboard basins.Uneven sediment may alternatively be deposited on a uniformly thick, compacting pre-kinematic sediment layer. Under these circumstances tP is further reduced to 9.0–0.5Ma depending on the density of the sediments. Weak in-plane contraction can further reduce tP to 3.5–0.375Ma even for low density sediments. Under optimal conditions a period of uneven sedimentation of this type in an otherwise uniformly aggrading sequence need only have a topographic amplitude of 40–80m and persist for 0.05–2Ma in order to seed the development of a successful minibasin. Our idealized uneven sedimentation model is interpreted as a proxy for channel–levee systems, turbidite lobe complexes, and/or lateral sediment density variations that cause equivalent sediment loading, and which occur both in the down-dip and along-strike directions of a rifted margin.

Density variations within the south polar layered deposits of Mars

The south polar layered deposits (SPLD) constitute the largest known reservoir of water on Mars. Previous studies solved for the best fit uniform density of the deposits using a forward approach. Here we invert for the lateral density variations in the layered deposit using gravity data from radio tracking of Mars Reconnaissance Orbiter, topography from MOLA on board Mars Global Surveyor, and radar sounding data from MARSIS on board Mars Express. We use the gravity anomalies outside the SPLD to construct a Wiener filter, which is applied to the gravitational signature of the SPLD to remove the short‐wavelength anomalies over the SPLD that are spectrally consistent with an origin in the crust or mantle. We then use a constrained inversion for the vertically averaged density within the SPLD as a function of position. The results suggest significant density variations within the SPLD. An inverse relationship between the density and thickness of the SPLD suggests that thicker portions of the cap contain less dust. Alternatively, the Dorsa Argentea Formation may extend beneath the SPLD and result in the observed high gravity anomaly in the marginal area of the SPLD. We find these conclusions to be robust against the choice of inversion constraint and perturbations to the applied filter. A synthetic test is also performed to verify the recoverability of the density variation in our approach.

VS and density structure beneath the Colorado Plateau constrained by gravity anomalies and joint inversions of receiver function and phase velocity data

The Colorado Plateau is a physiographic province in the western US with an average elevation of ∼1.9 km where, in contrast to neighboring provinces, there is little evidence of large scale tectonic deformation or magmatism. Recent availability of Earthscope/USArray seismic data allow us to better examine the crust and upper mantle structure beneath the region and test proposed explanations for the plateau's uplift and relative stability. Using phase velocities for fundamental mode Rayleigh waves and P receiver functions, we perform over 800 joint inversions for 1‐D shear wave velocity VS profiles sampling the plateau and surrounding regions down to 150 km depth. We image a sharp change in crustal thickness at the western edge of the Colorado Plateau with a more gradual increase eastward moving into the Rocky Mountains. A relatively thick (≳100 km) lithosphere beneath the plateau extends into the Rocky Mountains to the north. We use empirical scaling relations to estimate densities from our VS results, and predict the associated gravity anomalies, which are inconsistent with the observed distribution of the Bouguer gravity anomalies. We somewhat reconcile the prediction and observations by assuming that lateral density variations below 50 km can be ignored and the lithospheric root is therefore neutrally buoyant. While there is some evidence for small scale convection and lithospheric removal at its edges, the shape of the lithospheric mantle anomaly is consistent with a large scale uplift of the plateau by heating since removal of the Farallon slab. We conclude that the lithospheric root is key to the long term stability of the Colorado Plateau, leading to a colder, stronger crust.

On the accuracy of the bathymetry-generated gravitational field quantities for a depth-dependent seawater density distribution

In geophysical studies investigating the lithosphere structure, the gravitational field generated by the ocean density contrast (i.e., bathymetry-generated gravitational field) represents a significant amount of the signal to be modelled and subsequently removed from the Earth’s gravity field. The ocean density contrast is typically calculated as the difference between the mean density values of the Earth’s crust and seawater. The approximation of the actual seawater density distribution by its mean value yields relative errors up to about 2% in computed quantities of the gravitational field. To reduce these errors, a more realistic model of the seawater density distribution is utilized based on the analysis of existing oceanographic data of salinity, temperature, and pressure (depth). We study the accuracy of the bathymetry-generated gravitational field quantities formulated for a depth-dependent model of the seawater density distribution. This density distribution approximates the seawater density variations due to an increasing pressure with depth, whereas smaller lateral density variations caused by salinity, temperature, and other oceanographic factors are not taken into consideration. The error analysis reveals that the approximation of the seawater density by the depth-dependent density model reduces the maximum errors to less than 0.6%. The corresponding depth-averaged errors are below 0.1%. The depth-dependent seawater density model is further facilitated in expressions for computing the bathymetry-generated gravitational field quantities by means of the spherical bathymetric (ocean bottom depth) functions. The numerical realization reveals large differences in the results obtained with and without consideration of the depth-dependent seawater density distribution. The maxima of absolute differences reach 201 m2/s2 and 16.5 mGal in computed values of the potential and attraction, respectively. The application of the depth-dependent seawater density model thus significantly improves the accuracy in the forward modelling of the bathymetric gravitational field quantities.

Structure of the Hellenic Subduction Zone from Gravity Gradient Functions and Seismology

The Normalized Full Gradient (NFG) method has widespread applications in the analysis of potential fields, especially the gravity and magnetic fields. This method is used to identify the lateral and horizontal density variations in the crust and lithosphere. In this study, the NFG method was applied to the gravity data of the Cretan Arc and its surroundings. Because of the tectonic features of the eastern Mediterranean, the Cretan Arc and the neighboring areas are seismically very active. Especially the subduction zone and the complicated crustal features have been defined applying many different geophysical methods. In this study, first the NFG method is tested with synthetic prisms (two cubes). After that, the NFG method was applied to the Bouguer gravity data of the Cretan Arc and its subduction zone (Hellenic subduction zone) and Hellenic subduction zone was defined with the foci depth data (USGS) along the south–north direction. Thus, geometry of the focal depth distribution has been created to determine probable media depths and their localizations. According to the NFG results, vertical structural transitions were observed at a depth ranging between 10 and 180 km. Also, these results were compared with the foci depth model and the other results of the related publications. Finally, some considerations in vertical solution with the NFG method have been presented and locations of the different structures at horizontally have been defined with application of the NFG method.

Anisotropic scattering of surface state electrons at a point defect on Bi(111)

Scanning tunneling microscopy was applied to study the lateral variation of the local density of electronic states on the Bi(111) surface in the vicinity of a point defect. At an energy close to the Fermi level a characteristic pattern with a threefold symmetry is found. The pattern can be attributed to the scattering between two electronic surface states which are split by spin orbit coupling. The observation is well described by the superposition of three monochromatic waves. The phase of the waves relative to the center of the defect leads to a reduction to a threefold symmetry.

The new quasi-geoid model IRQG09 for Iran

Iran is a mountainous country with large lateral density variations of its crust. Constant density value is commonly used to determine the geoid models as well as topographic corrections. The effect of lateral density variation in the geoid can reach up to 14 cm in Iran which is not negligible in a precise geoid modelling. Also, the current height datum of Iran is based on the orthometric system but the effect of gravity variation was not applied in height parameter. Furthermore, the height systems of most neighbouring countries are defined as normal height. Connection of networks can be useful for the unification of height datum, geodynamics researches and optimal adjustment of levelling network. The new quasi-geoid model based on a recent EGM2008 global geo-potential model was created to solve the mentioned problem. The main purpose of the present study is to discuss the results of a research project in which a gravimetric quasi-geoid model for Iran was computed based on the least-squares modification of Stokes' formula. The evaluation is made using 475 GPS/levelling height anomalies covering the major parts of the country except the mountainous areas to the North and West. After a 7-parameter fit, the most promising attempt achieved a RMS value of 19 cm for the residuals based on the GPS/levelling data. ► Lateral density variation in the geoid can be reach up to decimeter level in Iran. ► Unification of height systems by normal height is useful for optimal adjustment of heights in Iran. ► Using of normal heights system is useful for tectonic deformation analysis. ► The least-squares modification of Stokes' formula gives the optimum results in quasi-geoid modelling in Iran.

Estimation of the deep density distribution in the lithosphere of central and Southern Asia using data about free mantle surface depth

The results of the selection of a model of the deep density distribution in the lithosphere of Central and Southern Asia, which explains the previously revealed dependence of the free mantle surface depth on the thickness of the crust [Artemjev, 1975], are described. It is shown that this dependence can be caused by variations in the mantle’s density with depth. Models of the continental and oceanic mantles with an increase in the linear density over depth are selected for the region of Asia. The level of the free surface depth in the oceanic mantle is higher than in the continental mantle. The observed dependence on the crustal thickness can also be used for determining nonlinear density variations with depth under the assumption that lateral density variations in this dependence are of a random character.

The Caribbean plate: Pulled, pushed, or dragged?

Mechanical coupling between the lithosphere and the asthenosphere remains a controversial topic in the geosciences. Beneath the Caribbean plate, shear wave splitting measurements indicate EW strain in the asthenosphere, which can be interpreted as mantle flow driving or resisting motion of the overlying lithosphere. Here, we constrain the average shear traction on the base of the Caribbean plate by balancing all torques. These torques result from body forces that act on the Caribbean (slab pull, ridge push, lateral density variations), from plate boundary friction and from basal shear tractions. We obtain a range of physically realistic torque solutions, which we examine further by computing the corresponding stresses and rotations within the Caribbean plate for comparison with observations. The deformation field for the Caribbean is particularly sensitive to the amount of friction on intraplate faults. Representative models have a good fit with observations and are characterized by (1) a near‐zero basal shear traction (≤0.3 MPa), (2) (lithosphere‐averaged) plate boundary friction ≤ 10 MPa, (3) local forces due to indenters and trench pull, (4) a net pull by the Caribbean slab and (5) intraplate fault shear stresses on the order of tens of megapascals. We conclude that the mechanical coupling of the Caribbean plate to the underlying asthenosphere is small.

Density inhomogeneities of the Earth’s mantle and core

Lateral density variations are determined at depth and are characterized by the anomalies of the seismic waves velocities, which are consistent with both the anomalies of the external gravity field and with the data from the analysis of Earth’s free-oscillation minimizing the deviations of the internal Earth structure from a hydrostatically equilibrated structure. It is shown that the obtained anomalous structures cause anomalies of the gravity field inside the Earth, which may be conducive to convective movements in the mantle and core.

Multiple phases of basin formation along the Stateline fault system in the Pahrump and Mesquite Valleys, Nevada and California

Two phases of deformation are needed to describe the Cenozoic tectonic evolution of the Pahrump and Mesquite basins in the southern Great Basin and eastern Mojave Desert, United States. By interpreting seismic reflection and gravity observations along with bedrock and surficial mapping, we infer an extensional phase of basin formation followed by a transtensional phase, in this area straddling the border of southern Nevada and southeastern California. We reprocessed ∼220 line km of industry seismic reflection data from the Pahrump and Mesquite Valleys to emphasize reflections in the basin fill, and combined these results with analysis of gravity data. The seismic lines portray the complex geometry of the Stateline fault system, a major Neogene dextral strike-slip system that passes through these valleys, and provide evidence for multiple ages of faulting along structures that bound the Pahrump basin. Locally thick sequences of preextensional Tertiary sedimentary rocks are cut by large-offset, relatively high-angle normal faults that record a phase of extensional basin formation that preceded transtension. The existence of preextensional basins beneath the Pahrump and Mesquite Valleys bears on tectonic reconstruction of the region and suggests that tilted ranges blocks to the west of these valleys need not restore to positions immediately adjacent to the Spring Mountains to the east. Subsequent dextral offset on the Stateline fault system resulted in the formation of steep-sided basins, local arching and tectonic inversion, and the burial of earlier-formed normal faults with coarse clastic detritus. Gravity models that are constrained to match the basin architecture observed in the seismic lines require lateral variations in basin-fill and bedrock density, and they confirm that the Paleozoic outcrop of Black Butte, a topographic high separating the Pahrump and Mesquite Valleys, is unrooted to underlying bedrock.

A multiphase model of core formation

The differentiation of solid planets with segregation of metal from silicates happens during the Hadean time while the planet is still growing by accretion. The separation of metal occurs when at least the metallic phase is liquid and proceeds by a combination of transport by diapiric instabilities and by more diffuse percolation flow. In this paper we develop a formalism derived from Bercovici et al. that can handle simultaneously two components, silicates and metal, and where the metal can be present both in solid and liquid states. The mechanical equations are non-Boussinesq as the lateral density variations are of the same order as the density itself. When the metal is solid, the metal and the silicates are locked together and we treat their mixture as a single-phase fluid where density is function of composition (iron–silicate proportions). When metal is liquid, it can separate from the silicates and the two phases interact through shear stress (e.g. Darcy flow) and normal stress. The evolution of the volume proportion of liquid iron is controlled by the difference of pressure between the solid and liquid phases. The energy conservation equation takes into account the different mechanisms by which the gravitational energy is dissipated as heat. The 2-D Cartesian numerical code that we implemented to solve these equations makes use of numerical techniques that have not been previously used in geophysical two-phase modelling; we discuss the numerical aspects and benchmark the solutions. We present simulations of core–mantle differentiation showing that the first impact that melts the iron phase near the surface is potentially able to trigger the whole core–mantle segregation in a runaway phenomenon. The threshold of this instability in terms of the impactor and planetary size and the initial planetary temperature is investigated. The segregation of the metal occurs by a mechanism that was not suggested before and which is intermediate between the usual diapir instability and a porosity wave. Although we cannot explore the whole parameter space of our numerical model, we show various simulations that clarify the role of the most important parameters, such as the solid and metal viscosities or the depth dependence of gravity.

Rift induced delamination of mantle lithosphere and crustal uplift: a new mechanism for explaining Rwenzori Mountains’ extreme elevation?

With heights of 4–5 km, the topography of Rwenzori Mountains, a large horst of old crustal rocks located inside a young passive rift system, poses the question “Why are the Rwenzori Mountains so high?”. The Cenozoic Western Rift branch of the East African Rift System is situated within the Late Proterozoic mobile belts between the Archean Tanzania Craton and Congo Craton. The special geological setting of the massif at a rift node encircled by the ends of the northern Western Rift segments of Lake Albert and Lake Edward suggests that the mechanism responsible for the high elevation of the Rwenzoris is related to the rifting process. Our hypothesis is based on the propagation of the rift tips, surrounding the stiff old lithosphere at Rwenzori region, thereby triggering the delamination of the cold and dense mantle lithosphere (ML) root by reducing viscosity and strength of the undermost lower crust. As a result, this unloading induces fast isostatic pop-up of the less dense crustal Rwenzori block. We term this RID—“rift induced delamination of Mantle Lithosphere”. The physical consistency of the RID hypothesis is tested numerically. Viscous flow of 2D models is approximated by a Finite Difference Method with markers in an Eulerian formulation. The equations of conservation of mass, momentum and energy are solved for a multi-component system. Based on laboratory data of appropriate rock samples, a temperature-, pressure- and stress-dependent rheology is assumed. Assuming a simple starting model with a locally heated ML, the ML block between the weakened zones becomes unstable and sinks into the asthenosphere, while the overlying continental crust rises up. Thus, RID seems to be a viable mechanism to explain geodynamically the extreme uplift. Important conditions are a thermal anomaly within the ML, a ductile lower crust with visco-plastic rheology allowing significant strength reduction and lateral density variations. The special situation of a two-sided rifting or offset rift segments to decouple the ML laterally from the surrounding continental lithosphere seems to be most decisive. Further support for the RID mechanism may come from additional crustal thickness and an extensive stress field. Some parameters, such as the excess temperature and yield stress, are very sensitive, small changes determine whether delamination takes place or not.

The effect of lateral density variations of crustal and topographic masses on GOCE gradiometric data — A study in Iran and Fennoscandia

The topographic and atmospheric masses influence the satellite gravity gradiometry data, and it is necessary to remove these effects as precise as possible to make the computational space harmonic and simplify the downward continuation of such data. The topographic effects have been formulated based on constant density assumption for the topographic masses. However in this paper we formulate and study the effect of lateral density variation of crustal and topographic masses on the satellite gravity gradiometry data. Numerical studies over Fennoscandia and Iran show that the lateral density variation effect of the crust on GOCE data can reach to 1.5 E in Fennoscandia and 1 E in Iran. The maximum effect of lateral density variation of topography is 0.1 E and 0.05 E in Iran and Fennoscandia, respectively.