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The Qinling Orogenic Belt (QOB) is an important tectonic belt in eastern Asia, which is believed to have been formed by the intracontinental collision between the North China Craton and South China Block. By analyzing radial P-wave receiver functions recorded by 239 broadband stations, we investigate the crustal thickness and Vp/Vs ratio maps under the QOB and its adjacent regions using the H-k and common-conversion-point (CCP) stacking methods. The crustal thickness (H) and Vp/Vs ratios (k) showed significant lateral variations. The assessment of crustal isostatic compensation reveals that the topography of the Dabashan and central QOB are compensated by crustal thickness, although the later may not be fully compensated. The crust of the western and eastern QOB are possibly in a non-Airy-type isostasy state, and the low-density uppermost mantle may play an important role in compensating their elevations. The Dabashan and the region to the south have thick crustal thickness and high Vp/Vs ratios, while the east QOB have thin crustal thickness and low Vp/Vs ratios. The CCP images reveal two strong positive amplitudes at depths consistent with the Moho interface near the Ankang fault. These observations indicate complex lower crustal structure and suggest the existence of eclogitized lower crust beneath Dabashan and lower crust delamination beneath east QOB. We also observe a continuous area with thick crust and high Vp/Vs ratios that covers the Dabashan, Hannan–Micang Dome, Shennong–Huangling Dome and their surrounding regions. This area shows consistency in location with a rapidly uplifted area since approximately 15 Ma, which—with other geological studies—implies the asthenospheric flux from the Tibetan Plateau. The asthenosphere flow may have eroded the lithosphere beneath the Dabashan and promoted the lower crustal eclogization.

This study reconstructs the coastal subsidence over the past 1300 years in a mangrove region along the coast of the Ganges–Brahmaputra Delta, an area not affected by anthropogenic interference. The relative sea level (RSL) history is based on radiocarbon and luminescence ages measured on 108 submerged kilns and in situ mangrove stumps. While the regional, long-term average subsidence rate is calculated to be 2.7 ± 0.3 mm/yr, modern RSL (including eustacy, isostasy, ocean level, and subsidence) rises by 8.7 ± 0.4 mm/yr. This rate has been balanced by natural sediment accretion so far. A reduction in sediment supply by engineering projects along rivers and coasts may, however, accelerate coastal inundation and retreat. Subsidence has accelerated during specific episodes since the eighth century. A major land-sinking event happened in the eighteenth century, with a downward displacement of 1–2 m, depending on geographic area. We propose that the subduction-related 1762 Arakan earthquake caused this sudden lowering. Prior to this event, RSL was nearly stable for 900 years. An earlier major subsidence event occurred around 900 CE, when the land suddenly sank by about 1 m, which also coincided with a major earthquake along the Arakan coast. Event-driven, sudden, significant subsidence, thus, needs to be considered a potential major hazard for coastal Bangladesh.

SUMMARY The (effective) elastic thickness of the lithosphere defines the strength of the lithosphere with respect to a load on it. Since the lithosphere is buoyant on a viscous mantle, its behaviour with respect to a load is not fully elastic, but rather viscoelastic. Fennoscandia is a well-known area in the world where the lithosphere has not yet reached its isostatic equilibrium due to the ongoing uplift after the last glacial period at the end of the Pleistocene. To accommodate for this changing property of the lithosphere in time, we present the flexural model of isostasy that accommodates temporal variations of the lithospheric flexure. We then define a theoretical model for computing the elastic thickness of the lithosphere based on combining the flexural and gravimetric models of isostasy. We demonstrate that differences between the elastic and viscoelastic models are not that significant in Fennoscandia. This finding is explained by a relatively young age of the glacial load when compared to the Maxwell relaxation time. The approximation of an elastic shell is then permissible in order to determine the lithospheric structure and its properties. In this way, the elastic thickness can be estimated based on combining gravimetric and flexural models of isostasy. This approach takes into consideration the topographic and ocean-floor (bathymetric) relief as well as the lithospheric structural composition and the post-glacial rebound. In addition, rheological properties of the lithosphere are taken into consideration by means of involving the Young modulus and the Poisson ratio in the model, both parameters determined from seismic velocities. The results reveal that despite changes in the Moho geometry attributed to the glacial isostatic adjustment in Fennoscandia are typically less than 1 km, the corresponding changes in the lithospheric elastic thickness could reach or even exceed ±50 km. The sensitivity analysis confirms that even small changes in input parameters could significantly modify the result (i.e. the elastic thickness estimates). The reason is that the elastic thickness estimation is an inverse problem. Consequently, small changes in input parameters can lead to large changes in the elastic thickness estimates. These findings indicate that a robust estimation of the elastic thickness by our method is possible if comprehensive information about structural and rheological properties of the lithosphere as input parameters are known with a relatively high accuracy. Otherwise, even small uncertainties in these parameters could result in large errors in the elastic thickness estimates.

In modern geodynamics, isostasy can be viewed either as the static equilibrium of the crust that minimizes deviatoric stresses, or as a dynamic process resulting from the viscous relaxation of the non-hydrostatic crustal shape. Paper I gave a general formulation of Airy isostasy as an elastic loading problem solved with Love numbers, and applied it to the case of minimum stress isostasy. In this sequel, the same framework is used to study Airy isostasy as the long-time evolution of a viscoelastic shell submitted to surface and internal loads. Isostatic ratios are defined in terms of time-dependent deviatoric Love numbers. Dynamic isostasy depends on the loading history, two examples of which are the constant load applied on the surface in the far past and the constant shape maintained by addition or removal of material at the compensation depth. The former model results in a shape decreasing exponentially with time and has no elastic analog, whereas the latter (stationary) model is equivalent to a form of elastic isostasy. Viscoelastic and viscous approaches are completely equivalent. If both load and shape vary slowly with time, isostatic ratios look like those of the stationary model. Isostatic models thus belong to two independent groups: the elastic/stationary approaches and the time-dependent approaches. If the shell is homogeneous, all models predict a similar compensation of large-scale gravity perturbations. If the shell rheology depends on depth, stationary models predict more compensation at long wavelengths, whereas time-dependent models result in negligible compensation. Mathematica and Fortran codes are available for computing the isostatic ratios of an incompressible body with three homogeneous layers., Comment: 51 pages, 9 figures, 5 tables; revised version submitted to Geophysical Journal International

Since marine seismic studies are relatively sparse and unevenly distributed, detailed tomographic images of the Moho geometry under large parts of the world’s oceans and marginal seas are not yet available. Marine gravity data is, therefore, often used to detect the Moho depth in these regions. Alternatively, Airy’s isostatic theory can be applied for this purpose. In this study, we compare different isostatic and gravimetric methods for a Moho recovery under the oceanic crust and continental margins, particularly focusing on a numerical performance of Airy, Vening Meinesz–Moritz (VMM), direct gravity inversion, and generalized (for the Earth’s spherical approximation) Parker–Oldenburg methods. Numerical experiments are conducted to estimate the Moho depth beneath the Indian Ocean. Results reveal that, among these investigated methods, the VMM model is probably the most suitable for a gravimetric Moho recovery beneath the oceanic crust and continental margins, when taking into consideration the lithospheric mantle density information. This method could to some extent model realistically a Moho geometry beneath mid-oceanic spreading ridges, oceanic subductions, most of oceanic volcanic formations, and marine sediment deposits. Nonetheless, this model still cannot fully reproduce a gradual Moho deepening caused by a conductive cooling and a subsequent isostatic rebalance of the oceanic lithosphere, which can functionally be described by a Moho deepening with the increasing ocean-floor age. Results also indicate that the Airy method typically overestimates the Moho depth under oceanic volcanic formations, while the direct gravity inversion and generalized Parker–Oldenburg methods could not reproduce more detailed features in the Moho geometry. Since Pratt’s theory better describes a large-scale isostatic mechanism of the oceanic lithosphere by means of compensation density variations, but does not account for additional changes in compensation depth (i.e., Moho depth) that are caused by these density changes, we tested a possibility of combining Pratt and Airy’s isostatic theories in order to estimate the Moho depth under the oceanic crust. Even this combined model cannot fully reproduce a gradual Moho deepening with the increasing ocean-floor age.

Earth's mantle undergoes convection on million-year timescales as heat is transferred from depth to the surface. Whilst this flow has long been linked to the large-scale horizontal forces that drive plate tectonics and supercontinent cycles, geologists are increasingly recognising the signature of convection through transient vertical motions in the rock record, known as "dynamic topography". A significant component of topography is supported by lithospheric isostasy, and changes in lithospheric thermal structure are sometimes included in the definition of dynamic topography. An additional component arises from active flow within the underlying convecting mantle, and this process causes dynamic topography that has lengthscales varying from 10,000 km down to 500 km and typical amplitudes of ±1 km. Transient uplift and subsidence events are often slow, but might evolve at rates as fast as 500 m/Myr over cycles as short as ~3 Myr, leading to periodic overwriting of the geological record that results in complex interpretational challenges. Despite these difficulties, a growing number of observational and computational studies have highlighted the important role of dynamic topography in fields as diverse as intraplate magmatism, sedimentary stratigraphy, landscape evolution, paleo-shorelines, oceanic circulation patterns, and ice sheet stability. This review provides a brief overview of our current understanding of the topic and explores some basic insights that can be gained from simple three-dimensional numerical simulations of mantle convection under different convective regimes. We summarise a suite of observational techniques used to estimate dynamic topography, and finish by laying out some key unanswered questions to stimulate debate and inspire future studies.

The northwestern sub-basin of South China Sea (SCS) is a unique tectonic unit formed in the early spreading of the SCS. The northwestern Sub-basin has a series of complex geological structures such as seamounts and fault zones surrounded by the Xisha Trough, the Zhongsha Massif, and the Pearl River Valley. These extensional structures and magmatic activity in the northwestern sub-basin are closely related to the lithospheric structure and its deformation. However, details of the deep lithosphere structure are still poorly known. Here, we obtained detailed data of water and Moho depth using sonar buoys, Extended Spread Profiles (ESP), Ocean Bottom Seismometer (OBS), both Multi-beam and land-sea joint seismic surveys in the northwestern sub-basin and its surrounding areas. Then we adopted a thermal isostasy method to calculate the depth of the Lithosphere-Asthenosphere Boundary (LAB) in the northwestern sub-basin of the SCS and its surrounding regions. Results show that the range of LAB depth is ~25–110 km. The shallowest burial depth is 25–60 km occurring in the ocean basin. The depth increases to 60–110 km toward the continental margin. The lithospheric structure on the north and south sides of the Xisha Trough is symmetrical and shows the deep structure and thermal features of aborted rifts. The LAB depth in the Zhongsha Trough and the Zhongsha Massif increased from 60 to 70 km southwestwards, consistent with the trend of surface morphology. The LAB depth to the west side of the Pearl River Valley is 60–80 km, and the thinning of the lithosphere is related to the distribution of faults, depressions and the magmatic activity. The LAB depth in the northwestern sub-basin and the eastern sub-basin is less than 60 km with the thinnest part being less than 46 km. Combining ocean drilling, seismic investigation, and seafloor topography, we show that the ocean basin of the northwestern sub-basin of the SCS locates within the 46 km isobath of the LAB. The formation of the rifted valleys and discrete blocks surrounding the ocean basins is both controlled by the regional tectonic movement and the deep thermal state, where their lithospheric structures show strong heterogeneity.

When and how the Tibetan Plateau formed and maintained its thick crust and high elevation on Earth is continuing debated. Specifically, the coupling relationship between crustal thickening and corresponding paleoelevation changing has not been well studied. The dominant factors in crustal thickness changing are crustal shortening, magmatic input and surface erosion rates. Crustal thickness change and corresponding paleoelevation variation with time were further linked by an isostatic equation in this study. Since 120 Ma crustal shortening, magmatic input and surface erosion rates data from the central Tibetan Plateau are took as input parameters. By using a one-dimensional isostasy model, the authors captured the first-order relationship between crustal thickening and historical elevation responses over the central Tibetan Plateau, including the Qiangtang and Lhasa terranes. Based on the modeling results, the authors primarily concluded that the Qiangtang terrane crust gradually thickened to ca. 63 km at ca. 40 Ma, mainly due to tectonic shortening and minor magmatic input combined with a slow erosion rate. However, the Lhasa terrane crust thickened by a combination of tectonic shortening, extensive magmatic input and probably Indian plate underthrusting, which thickened the Lhasa crust over 75 km since 25 Ma. Moreover, a long-standing elevation >4000 m was strongly coupled with a thickened crust since about 35 Ma in the central Tibetan Plateau. ©2021 China Geology Editorial Office.

Antarctic geothermal heat flow (GHF) affects the temperature of the ice sheet, determining its ability to slide and internally deform, as well as the behaviour of the continental crust. However, GHF remains poorly constrained, with few and sparse local, borehole-derived estimates and large discrepancies in the magnitude and distribution of existing continent-scale estimates from geophysical models. We review the methods to estimate GHF, discussing the strengths and limitations of each approach; compile borehole and probe-derived estimates from measured temperature profiles; and recommend the following future directions. (1) Obtain more borehole-derived estimates from the subglacial bedrock and englacial temperature profiles. (2) Estimate GHF from inverse glaciological modelling, constrained by evidence for basal melting and englacial temperatures (e.g. using microwave emissivity). (3) Revise geophysically derived GHF estimates using a combination of Curie depth, seismic, and thermal isostasy models. (4) Integrate in these geophysical approaches a more accurate model of the structure and distribution of heat production elements within the crust and considering heterogeneities in the underlying mantle. (5) Continue international interdisciplinary communication and data access.

The maximum positive combined effect on the earth's gravity field of all geologic sources present in a given area, including any isostatic compensation, is limited by the ability of the lithosphere to support load. Bouguer gravity is one possible proxy for this effect. Bouguer maxima close to +350 mGal have been measured on Chichijima Island and Penrhyn Atoll in the Pacific Ocean. These may be the highest values anywhere on the planet. Suggestions of even higher values in the northernmost segment of the Colombian Andes rely on extrapolation of trends established in areas surrounding the Santa Marta Massif and must be treated with caution.

Gravity anomalies across the Indian region depict most of the geological and tectonic domains of the Indian continental lithosphere, which evolved through Archean cratonic nucleation, Proterozoic accretion, Phanerozoic India-Eurasia plate convergence, and modification through many thermal perturbations and rifting. Integrated analysis of gravity and geoid anomalies together with topographic and heat flow data led to deciphering the mechanism of isostatic compensation of topographic and geological loads, lithospheric structure, and composition. This study discusses the nature of gravity (free-air, Bouguer and Isostatic) and geoid anomalies in relation to the topography, geology, and tectonics, and presents a lithospheric density model across the peninsular India and Himalaya. Southern peninsular Indian region shows relatively low Bouguer gravity anomalies compared to the northern region. The mobile belts are generally observed to have relatively higher Bouguer gravity anomalies, e.g., Eastern Ghats Mobile Belt compared to the shield regions. The gravity lows are observed over topographic features like the Western Ghats and Himalaya, while some of the topographic highs like Aravalli show positive gravity anomaly. The Indian Ocean Geoid Low varies from -82 m over Dharwar Craton to -98 m over the Southern Granulite Terrain and finally reaches a significant low of -106 m in the Indian Ocean. Flexural isostatic compensation with variable Effective Elastic Thickness (EET) ~10 km to 50 km prevails over the stable continental region. The lithospheric thickness varies from 80 km along the coastal region to 120-130 km beneath the Saurashtra Plateau, the Southern Granulite Terrain, and the Eastern Indian Shield, and reaches to more than 200 km under the Himalayan orogenic belt in the north. From Dharwar Craton to Bundelkhand Craton in central India, the lithospheric thickness varies between 160 and 180 km.

To determine the lateral and vertical variations in crustal structure and their influence on the seismicity of the Western North China Craton, the Trans-North China Orogen, and the surrounding regions, the wavelet multi-scale structures, Moho depth, crustal density structures, and isostatic state are modelled using Bouguer gravity anomaly data, topography, and earthquake focal mechanisms. We obtained homogeneous crustal densities and deviations of

The Scandes mountain range along the western rim of the Archean Baltic craton with elevation up to 2500 m forms an exceptional setting as the orogeny terminated 420 Ma ago and the Caledonides were deeply eroded afterwards. Since this region lacks recent compressional tectonic forces, a comprehensive explanation for the topography, which shows north-south and lateral variations along the Scandes, is missing. In my dissertation, I use earthquake surface waves and ambient noise to image the crustal and mantle structure aiming to provide new clues about the topography’s origin. The focus is also on exploring structural differences between the various tectonic domains. Here, I benefit from the seismic recordings by the ScanArray network supplemented by permanent and previous projects, distributed over entire Scandinavia. First, I performed a beamforming of Rayleigh surface waves which yielded average phase velocities for the study region and several of its sub-regions. An unusual 360° or sin(1θ) phase velocity variation with propagation azimuth is observed in northern Scandinavia and southern Norway/Sweden but not in the central area. For periods >35 s, a 5% variation between the maximum and minimum velocities was measured for opposite backazimuths of 120° and 300°, respectively. Such a variation is incompatible with the intrinsic azimuthal anisotropy and the path average approximation made in tomography. I assumed an eastward dipping lithosphere-asthenosphere boundary (LAB) to be the causing structure, inspired by some preliminary velocity models and observations made in previous studies. To test this hypothesis, I carried out 2D full-waveform modeling of the Rayleigh wave propagation. The models include a steep gradient at the LAB in combination with a pronounced reduction in the shear velocity below the LAB. The synthetic results are consistent with the observations: Faster phase velocities are obtained for propagation towards the thinning lithosphere, and lower ones for propagation in the direction of deepening LAB. The interference of reflected surface wave energy at the steep LAB with the forward propagating fundamental mode probably causes this peculiar effect. Second, the joint inversion of Rayleigh surface waves and ambient noise provides structural imaging down to 250 km depth. Resultant from my velocity model, I derive a new crustal model from which maps of the Moho depth as well as of the high-density lower crustal layer (LCL) are obtained. I observe crustal thickening from west to east below the Precambrian low-topography terranes, which is mainly a consequence of eastward thickening of the LCL. In contrast to the southern Scandes, with the overall highest topography (2,500 m), a crustal root below the northern Scandes (max. 2,100 m) is seen which diminish towards the central Scandes (max. 1,000 m). The LAB below the Scandes is deepening from west to east. The sharp steps in the LAB and strong velocity reductions both in the south (90–120 km LAB depth with 5.5% Vsv contrast) and the north (150 km LAB depth with 9% Vsv contrast) surprisingly correlate with the Caledonian mountain front. Whereas smoother laterally varying structures (150–170 km LAB depth with 4% Vsv contrast) are found below the central Scandes. The correlation of the lithosphere thickening with the Caledonian front might be related to metasomatism as result of the orogeny and/or the passive margin rifting. In Precambrian Scandinavia, low-velocity areas below 150 km depth are observed beneath the Archean Karelia craton in northern Finland. At mantle depth, the Paleoproterozoic Norrbotten craton can be separated from the Karelia craton, Caledonides and Paleoproterozic Svecofennian likely due to different degrees of metasomatism. Based on the structural differences, I conclude that different mechanisms are responsible for the compensation of the topography. The northern Scandes are likely compensated by a combined Airy-Pratt isostasy as implied by low-density rocks in the shallow crust (10 km LCL thickness) and the mountain root. The strongly reduced velocities at sub-lithospheric depth additionally suggest an uplift contribution from the upper mantle. Since the southern Scandes lacks these crustal attributes, they experience mainly mantle-driven buoyancy. In both cases, however, I assume the influence of small-scale edge-driven convections (EDC) that can arise at sharp LAB gradients. EDC emplaces thereby low-density material at sub-lithospheric depths by the upwelling of hot asthenosphere which implies additional buoyancy of the lithosphere. Moreover, the lateral topography differences along the Scandes can be explained by varying EDC cell dimensions. Primarily, Pratt isostasy compensates the low topography central Scandes, but a contribution from dynamic support could act as well. Ultimately, I see the strong gradients at the LAB below the southern and northern Scandes as the cause of the observed 1θ phase velocity variation. While the smoother velocity structure in the central study area explains the absence of the 1θ effect.

In the paper, an attempt has been made to describe seismic processes by means of classical mechanics. Dynamic parameters such as energy, impulse, and forces that determine the movement of masses are estimated. An earthquake model is regarded with consideration of the Earth gravity field influence on distribution of masses composing the Earth crust. The Earth crust is considered as a complex of geological bodies which is ordered by the planets gravity field. Due to isostasy the difference in density of the equal matter volumes leads to formation of mountain structures specific for the surface. When a base of such structures collapses a rock block shifts down and acquires mechanical impulse. Authors assume that this acquired mechanical impulse determines the earthquake’s magnitude. Impulse propagates in medium as a seismic wave. Front of the wave generates pressure effected on an environment that is not active yet. It determines macro-processes of the earthquake measured as earthquake intensity. Authors consider mass (weight) of a rocks block, magnitude (mechanical impulse acquired during an impact), seismic emission, which is a distribution of a momentum and a pressure of wave front (product of impulse by time), as parameters of a seismic process. To estimate dynamic parameters of earthquake processes, such as shifted mass, mechanical impulse, pressure of wave front, a nomogram is presented. Magnitude of a seismic event has dimension of a mechanical impulse, seismic intensity (earthquake intensity) has a dimension of pressure. Nomogram consists of several parts. The first one, allows analyzing the source of the emission. It shows the relation between emitted momentum, mass of a shifted body and energy. The next part of the nomogram represents momentum density per unit of area of the wavefront, which decreases in proportion to the square of a distance from the source. It characterizes the spherical propagation of seismic radiation in an isotropic medium. The third part of the nomogram helps to estimate the value of an average seismic pressure of the wavefront. The last part of the nomogram allows rating the empirical characteristics of a seismic event. A relation between a scale of seismic pressure and standardized scale of seismic intensity MSK-64 is established. As an example a Spitaka earthquake, December 7 th , 1988, was analyzed.

Isostasy is a key concept in geoscience in interpreting the state of mass balance between the Earth’s lithosphere and viscous asthenosphere. A more satisfactory test of isostasy is to determine the depth to and density contrast between crust and mantle at the Moho discontinuity (Moho). Generally, the Moho can be mapped by seismic information, but the limited coverage of such data over large portions of the world (in particular at seas) and economic considerations make a combined gravimetric-seismic method a more realistic approach. The determination of a high-resolution of the Moho constituents for marine areas requires the combination of gravimetric and seismic data to diminish substantially the seismic data gaps. In this study, we estimate the Moho constituents globally for ocean regions to a resolution of 1° × 1° by applying the Vening Meinesz-Moritz method from gravimetric data and combine it with estimates derived from seismic data in a new model named COMHV19. The data files of GMG14 satellite altimetry-derived marine gravity field, the Earth2014 Earth topographic/bathymetric model, CRUST1.0 and CRUST19 crustal seismic models are used in a least-squares procedure. The numerical computations show that the Moho depths range from 7.3 km (in Kolbeinsey Ridge) to 52.6 km (in the Gulf of Bothnia) with a global average of 16.4 km and standard deviation of the order of 7.5 km. Estimated Moho density contrasts vary between 20 kg m-3 (north of Iceland) to 570 kg m-3 (in Baltic Sea), with a global average of 313.7 kg m-3 and standard deviation of the order of 77.4 kg m-3. When comparing the computed Moho depths with current knowledge of crustal structure, they are generally found to be in good agreement with other crustal models. However, in certain regions, such as oceanic spreading ridges and hot spots, we generally obtain thinner crust than proposed by other models, which is likely the result of improvements in the new model. We also see evidence for thickening of oceanic crust with increasing age. Hence, the new combined Moho model is able to image rather reliable information in most of the oceanic areas, in particular in ocean ridges, which are important features in ocean basins.

Its extraordinary level of geologic activity, its potential for habitability, and the prospects of returning samples from its plume of erupting water ice make Saturn's small (∼500 km diameter) moon Enceladus a high priority target for future exploration and a key to our developing understanding of icy ocean worlds. The structure of its outer ice shell is particularly important as it relates to the global heat budget, the global-scale response to tidal forces, and the nature of the ongoing eruptions. It is also diagnostic of how and where heat is dissipated internally. Here, using the most recent shape model and a new approach to modeling isostasy, we obtain a shell structure that simultaneously accommodates the shape, gravity, and libration observations and suggests that tidal dissipation near the base of the ice shell is likely an important mode of internal heating. The implied conductive heat loss is greater than the heat loss associated with the eruptions but is nevertheless compatible with the condition of steady state.

Serpentinized mantle peridotites form prominent mountains, including the highest elevations of the Troodos ophiolite in Cyprus (Mount Olympus, 1,952 m), but to date, only qualitative mechanisms have been proposed to explain the uplift of mantle rocks to high altitudes. Serpentinization reactions between mantle rocks and water result in profound changes to the rheology and physical properties of peridotites including significant density reduction (∼900 kg/m 3). Field observations, density measurements, and isostatic uplift and erosional modeling provide new constraints on the contribution of serpentinization to the uplift of the Troodos Mantle Sequence. Different serpentinization styles have resulted in two distinct serpentinite domains with contrasting densities. Our modeling shows that the Troodos Mountains can form within the geologically constrained uplift time frame (∼5.5 Myr) exclusively through partial serpentinization reactions. We interpret the serpentinite domains as two nested diapirs that formed due to different extents of serpentinization and density reduction. Differential uplift and exhumation have decoupled the two serpentinite diapirs from the originally overlying ocean crustal rocks. Once at high altitudes the incursion of meteoric water reinforced coupled deformation-alteration-recrystallization processes in the shallow subsurface producing a localized low density completely serpentinized diapir. A second decoupling between the contrasting serpentinite diapirs results in localized differential uplift and exhumation, extruding deep materials to the east of Mount Olympus. Application of our modeling to other serpentinite massifs (e.g., St. Peter and St. Paul Rocks, New Idria, California) highlights the contribution of isostasy to the uplift of serpentinized massifs.

Lithospheric deformation is a fundamental process in plate tectonics. It is, therefore, critical to determine how the lithosphere responds to geological loads to better understand tectonic processes. The lithosphere can be modelled as the flexure of a thin, elastic plate over long-term (> 105 yr) geological timescales. The partial differential equation for the flexure of an orthotropic plate is used indirectly to calculate theoretical admittance and coherence, which are then compared against the observed admittance and coherence to invert for the non-uniform flexural rigidity (or effective elastic thickness, Te) of the plate. However, the process for accurately recovering variable lithospheric flexure remains unresolved, as the classical lithospheric model may overestimate the deflection of the plate. Here we adopt the classic lithospheric model with applied external and internal loads at the surface and Moho, respectively, and assume that the compensation material is denser than the mantle material beneath the Moho. The lithospheric flexure errors are derived mainly from the Te and Moho recovery errors in this lithospheric model. Synthetic modelling is then performed to analyse the influence of the Te and Moho errors. The analysis of synthetic modelling shows that: (1) the Te error-induced flexure errors exhibit a rippling pattern, and the rippling pattern is broader in high Te regions; (2) the Moho error-induced flexure errors mainly occur in the low Te regions, and applying Airy isostasy theory in low Te regions may still greatly overestimate the lithospheric deformation amplitude; and (3) the lithospheric flexure errors are dominated by the Te and Moho errors in the high and low Te regions, respectively.

Quantifying age, rate, and lateral variation of deformation and exhumation in convergent systems relies on integration of geologic map patterns, ag, and locations of reset thermochronometer systems, and synorogenic sediment distribution. The central Bolivian Andes provide an ideal location to examine the influence of variations in shortening and stratigraphic architecture on the structural evolution of the mountain range due to differential age and rates of shortening and distinct sedimentary basin geometries along strike. To quantify age and rate of shortening, we link thermokinematic modelling of sequentially deformed, forward-modelled, balanced cross-sections to synorogenic and thermochronologic histories. The preserved basin history in the Altiplano and Eastern Cordillera argues for an early fold-and-thrust belt located in the now-Western Cordillera, with subsequent propagation of shortening eastward around 40–50 Ma. Flexural modelling incorporating isostasy and erosion requires multiple basement thrust sheets with 35–97 km of displacement. A temporally evolving effective elastic thickness, as well as imposed subsidence in the foreland and uplift in the hinterland, is required to reproduce the surface geology, increase Subandean foreland basin depth, limit Altiplano sedimentation, and facilitate Altiplano uplift to modern elevation. Thermokinematic modelling is compatible with initiation of deformation at 50–40 Ma in a marked increase in Subandean velocities from ~5.5 to 8–10 mm/yr from ~12–10 Ma to present. Out-of-sequence thrusting at the westernmost limit of the Subandes is required to match measured young and partially reset zircon helium ages. Out-of-sequence faulting is supported by high Ksn values, indicative of active uplift, and was likely promoted by the abrupt eastern edge of the Paleozoic basin rocks, which limited forward propagation of structures, and/or increased erosion due to focused precipitation. Our results highlight the importance of incorporating detailed structural modelling in differentiating the geometry, kinematics, and timing of deformation to reproduce thermochronologic ages and basin histories.

The Longmenshan fault, which defines the eastern edge of the Tibetan Plateau, is one of the steepest margins of the plateau with a sharp elevation drop of about 4 km over a distance less than 100 km across the Longmenshan fault. The mechanism which is responsible for controlling and maintaining the elevation difference is highly debated. Using multiple observations including seismic velocity model, Moho depth, effective elastic thickness of the lithosphere, we conducted a quantitative study for elucidating the contributions from crust and lithospheric mantle by an integrated analysis of lithospheric isostasy and flexure. It is shown that the topography of the Longmenshan fault is supported by both lithospheric isostasy and flexure statically, and lower crustal channel flow and mantle convection dynamically. Different mechanisms have different weights for contribution to the topography of the Songpan-Ganzi block and the Sichuan Basin. The static and dynamic support contribute roughly the same to the topographic difference of ~4 km between the two sides of the Longmenshan fault. The static topographic difference of ~2 km is mainly resulted from the lithospheric isostasy, while the dynamic one of ~2 km is contributed by the uprising of the accumulated material in the lower crust beneath the Songpan-Ganzi block and the downward drag force caused by the upper mantle convection under the Sichuan Basin. It is thus suggested that the lower crustal flow and upper mantle convection are dynamic forces which should be taken into account in the studies on the dynamics in the Longmenshan and surrounding regions.

Elevated topography is evident across the continental margins of the Atlantic. The Cumberland Peninsula, Baffin Island, formed as the result of rifting along the Labrador–Baffin margins in the late Mesozoic and is dominated by low-relief high-elevation topography. Apatite fission-track (AFT) analysis of the landscape previously concluded that the area has experienced a differential protracted cooling regime since the Devonian; however, defined periods of cooling and the direct causes of exhumation were unresolved. This work combines the original AFT data with 98 apatite new (U–Th)/He (AHe) ages from 16 samples and applies the newly developed ‘broken crystals’ technique to provide a greater number of thermal constraints for thermal history modelling to better constrain the topographic evolution. The spatial distribution of AFT and AHe ages implies that exhumation has been significant toward the SE (Labrador) coastline, and results of thermal modelling outline three notable periods of cooling: in the pre-rift stage (460–200 Ma), from synrift stage to present (120–0 Ma) and within the post-rift stage (30–0 Ma). Pre-rift cooling is interpreted as the result of exhumation of Laurentia and synrift cooling as the result of rift-flank uplift to the SE and differential erosion of landscape, whereas the final post-rift period is probably an artefact of the modelling process. These results suggest that the source of the Cumberland Peninsula9s modern-day elevated topography is uplift during rifting in the Cretaceous and the isostatic compensation following continuous Mesozoic and Cenozoic differential erosion. This work highlights how interaction of rift tectonics and isostasy can be the principal source for modern elevated continental margins, and also provides insight into the pre-rift exhumational history of central Laurentia. Supplementary material: Thermal histories are available at: https://doi.org/10.6084/m9.figshare.c.4528409

The distribution of masses within topography is extremely important for the evaluation of a rigorous geoid model. Most present day models of topographical density are either determined via gravity field inversions or have too coarse a resolution to be useful on a regional level. The ideal solution would be to have a 3-dimensional (3D) density model but such a dataset does not currently exist and would be quite complicated and expensive to develop. As a substitute, a 2-dimensional (2D) topographical density model is presented here through the application of a high resolution lithological map. By assigning probable surface density values and error estimates to the lithologies based on geological data, the Global Lithology Model (GLiM) is transformed into a laterally varying topographical density model (UNB_TopoDens). This model is then validated against present-day global and regional topographical density models. Apart from some water and ice covered regions, the UNB_TopoDens model agrees with all regional and global models at our disposal with the differences all falling within the 95% confidence region associated with the UNB_Topodens model. UNB_TopoDens is the first laterally varying topographical density model with associated error estimates that spans the globe at a 30″ x 30″ resolution and is therefore extremely useful for investigations of the crust and mantle, isostasy, gravimetric studies and various other disciplines.

[Deformation, isostasy and erosion for one 8 km increment of thrust motion (479 km total). Top panel, fault displacement and isostatic loading following 8 km of displacement. Middle panel, new estimated topography and erosion. Bottom panel, estimated cross section geometry and topography following erosion and deposition. , Abstract Forward modeled, balanced cross sections that account for the flexural response to thrust loading and erosional unloading can verify and refine the kinematic sequence of deformation in fold‐thrust belts as well as help assess the validity of a balanced cross section. Results from flexural‐kinematic reconstructions that indicate either the cross section, the kinematic order or both are invalid include: (a) a predicted final topography that is dramatically different from the actual topography; (b) large normal fault or thrust fault bounded synorogenic basins that are not present in the mapped geology; and/or (c) an exhumation history that is not consistent with provenance records in the basin or measured thermochronometers. Where detailed measured foreland basin sections exist, flexural‐kinematic modeling of fold‐thrust belt deformation, including out‐of‐sequence (OOS) faults can predict a foreland basin evolution that can be compared to measured data. The modeling process creates a “pseudostratigraphy” in the modeled foreland. The pseudostratigraphy and predicted provenance of each modeled stratigraphic increment can be directly compared to measured stratigraphic sections. We present a case study using two cross sections through the Himalaya of far western Nepal (Api and Simikot) to assess the validity of the section geometries and the resulting kinematic histories, displacement rates, flexural wave response and predicted provenance for both sections. Insights from combining the flexural‐kinematic models with existing stratigraphic data include: (a) Changing the order of proposed OOS and normal faults to earlier in the evolution of the fold‐thrust belt was necessary to reproduce the foreland provenance data. We argue that OOS thrust and normal faults in the Api section occurred between 11 and 4 Ma. (b) Published shortening estimates for the Simikot cross section are too high (>50 km), resulting in unrealistic shortening rates up to 80 mm/yr between 25 and 20 Ma. (c) Flexural forward models with and without an additional sediment loading modeling step indicate that while sediment loading does not have a measurable effect on the magnitude and location of erosion within the fold‐thrust belt, it does have a small effect on accumulation rates and thus the predicted age of stratigraphic boundaries when compared to measured stratigraphic thicknesses and age. Thickness difference range from 0.2 to 0.5 km and can result in predicted age differences of ca. 1 Ma. Accounting for both flexural isostacy and erosion can eliminate unviable kinematic sequences and when combined with provenance data from measured stratigraphic sections, can provide insight into the order, age and rate of deformation.]

The origin of the high topography within the western United States has been attributed to either crustal/lithospheric isostasy or dynamic topography, but their relative contributions remain unconstrained. Here we investigate this problem using gravity, residual topography and geodynamic modeling. We first evaluate two end-member scenarios of isostatic balance: crustal isostasy and lithospheric isostasy. Both cases lead to prominent negative mantle residual gravity within the tectonically active western U.S. and unrealistic crustal/lithospheric density structures, requiring the presence of low-density mantle underneath. The negative mantle residual gravity is consistent with both the estimated positive residual topography and calculated dynamic uplift due to the presence of hot asthenospheric mantle underneath. Geodynamic modeling further reveals that this landward migrating dynamic uplift originates from the eastward intrusion of the hot Pacific mantle through tears and edges of the Juan de Fuca slab since middle Miocene. The estimated paleotopography maps by combining dynamic topography and lithosphere isostasy over the western U.S. are consistent with several observational constraints, including episodic uplifts of the Sierra Nevada, post-mid-Miocene uplift of the Idaho batholith, the sustaining subsidence within most of the B&R, and the largely stable topography of central Colorado Plateau since 20 Ma.

Intra-basinal highs within classic ‘block and basin’ style tectonic frameworks, in areas such as northern England, are underpinned by large granite bodies. This is widely believed to relate to the relative ‘rigidity’ and ‘buoyancy’ of granite in relation to accommodating basement. It has been suggested that during periods of tectonic extension, normal faulting around the peripheral regions of granite batholiths permits granite-cored blocks to resist subsidence, thus forming stable areas during periods of widespread faulting-induced subsidence. However, one-dimensional modelling indicates that relatively less dense crust is incapable of resisting subsidence in this way. Instead, when local isostasy is assumed, the occurrence of granite-cored, intra-basinal highs relates to initial isostatic compensation following granite emplacement. Differential sediment loading during extensional tectonism exaggerates this profile. An integrated two-dimensional lithospheric numerical modelling approach highlights the role of flexural rigidity in limiting the amplitude whilst increasing the wavelength of isostatic deflection. In light of these models, it is suggested that such a response leaves residual second-order stresses associated with the under-compensated buoyancy of the granite body and flexural tension. The observed basin geometries of the Carboniferous North Pennine Basin can be replicated by incorporating a density deficiency within the crust, flexural rigidity, simple shear deformation within the shallower subsurface and pure shear deformation within the deeper subsurface. In adopting this technique, the regional flexural profile in response to underlying granite bodies and large extensional faults can be reproduced and thus, to an extent, validated. It is proposed that the interaction of three factors dictate the tectonic framework within a partially granitic, brittle-ductile lithosphere and the occurrence of inter-basinal highs: 1) non-tectonic, ‘second-order’ stresses such as the flexural response of the lithosphere and residual, under-compensated buoyancy forces in relation to granite bodies; 2) extensional tectonic stress and importantly; 3) inherited basement fabric.

The assumption of isostatic equilibrium is often used in geophysics; e.g., the depth of the lithosphere–asthenosphere boundary (LAB) is determined using data regarding the gravity field and topography, together with the assumption of isostasy. However, isostasy implies a hydrostatic state, which is contrary to the mantle convection hypothesis, which states that most of the mantle matter is in motion. We therefore discuss herein the question of when the assumption of isostasy can be used. It is suggested that isostasy may be used for parts of oceanic plates (except for subduction zones, hotspots, and oceanic ridges) and for many continental regions (except for postglacial regions and regions of intensive volcanic or tectonic activity). Moreover, using the results of a numerical model of convection and calculations of dynamic topography, it is shown that some generalization of isostasy is possible in the form of deep dynamic isostasy (DDI). It is also indicated that, for some regions without isostatic equilibrium (e.g., postglacial regions), it is possible to use the “isostatic” method with some corrections to the results.

The crustal structure in the interior of Greenland is largely unknown because of its remote location below the up to 3.4-km-thick ice sheet. We present a model of the crustal velocity structure in central-eastern Greenland based on simultaneous inversion of P and S receiver functions for data acquired at 23 broadband stations between the coast and the center of the ice sheet. The area is believed to mainly include Precambrian basement and includes a part covered by Tertiary volcanic rocks and some sedimentary basins. Our results show a westward deepening Moho from less than 20 km at the coast to 50 km below central Greenland. Crustal S wave velocities are generally 3.75 km/s through the whole crust which may be relatively small for Precambrian areas, and V p /V s is generally around 1.73, although slightly higher in central Greenland. In the coastal area we observe anomalously low velocities at the top of the crust. In the volcanic area south of Scoresbysund Fjord this layer has very high V p /V s (>2), which indicates a high mafic content and the presence of water-filled cracks in the basaltic material. In the north, outside the volcanic area, V p /V s is normal and the low-velocity layer probably is instead related to the presence of sedimentary basins. At stations in the center of our study area we find low V s and high V p /V s in the lower crust. Based on the Moho topography, our results do not support Airy type isostasy as explanation of the high topography in eastern Greenland.

Terrestrial heat flow is a crucial parameter to indicate the thermal state within the Earth, containing integrated information of the ground temperature, thermal conductivity, and crustal/mantle heat flow. We present an updated heat flow dataset and map in continental China. The data increases from 862 to 1230 observations since the year 1999 and the sites cover the major tectonic units, although their spatial distribution remains uneven. Excluding the local anomalies related to hydrothermal activities, the background heat flow values display a range from 30 to 140 mW m–2 with a mean of 60.4 ± 12.3 mW m–2. The updated heat flow map exhibits a heat flow pattern consisted of four heat flow provinces in continental China: eastern mantle-induced high flow province, southwestern crust-induced high heat flow province, central normal heat flow province and northwestern low heat flow province. The major heat flow provinces correspond to the Cenozoic lithosphere-scale tectonic units. The heat flow shows good correlation with crustal thickness and elevation, agreeing with the thermal isostasy. The heat flow distribution also corresponds with the Meso-Cenozoic tectonic activities. The present heat flow-tectonic pattern in continental China is formed by the Meso-Cenozoic geodynamic processes, including the Cenozoic India-Eurasian collision and the Mesozoic-Cenozoic western subduction of the Pacific Plate.

A new method for modeling the thermal structure of the lithosphere, termed thermal isostasy, is presented and used to calculate the lithosphere thickness and to predict geothermal heat flux in Europe and the adjacent off-shore regions. The method is based on analysis of topography deviations from the expected correlation between the Moho depth and topography. Anomalous topography is interpreted as caused by thermal anomalies in the lithosphere and by variations in the lithosphere thermal thickness. The new method allows for recovering the lithosphere thermal structure with a high lateral resolution and for testing the results by comparing the predicted and measured heat flux. The results for Europe, constrained by the regional seismic crustal model EUNAseis, demonstrate that the lithosphere thermal structure is controlled by both the geodynamic setting and the lithosphere age. The lithosphere of the East European craton ranges in thickness from ca. 140-180 km in the paleorifts to 200-220 km on average with local lithosphere roots down to ca. 300 km depth, such as in the south-central Proterozoic Finland, and local thinning to

Lithosphere thermal structure in Greenland is poorly known and models based on seismic and magnetic data are inconsistent, while growing awareness in the fate of the ice sheet in Greenland requires reliable constraints on geothermal heat flux (GHF) from the Earth's interior in the region where conventional heat flux measurements are nearly absent. The lithosphere structure of Greenland remains controversial, while its geological evolution is constrained by direct observations in the narrow ice-free zone along the coasts. The effect of the Iceland hotspot on the lithosphere structure is also debated. Here I describe a new thermal isostasy method which I use to calculate upper mantle temperature anomalies, lithosphere thickness, and GHF in Greenland from seismic data on the Moho depth, topography and ice thickness. To verify the model results, the predicted GHF values are compared to available measurements and show a good agreement. Thick (200–270 km) cratonic lithosphere of SW Greenland with GHF of ca. 40 mW/m2 thins to 180–190 km towards central Greenland without a clear boundary between the Archean and Proterozoic blocks, and the deepest lithosphere keel is observed beneath the largest kimberlite province in West Greenland. The NW-SE belt with an anomalously thin (100–120 km) lithosphere and GHF of 60–70 mW/m2 crosses north-central Greenland from coast to coast and it may mark the Iceland hotspot track. In East Greenland this anomalous belt merges with a strong GHF anomaly of >100 mW/m2 in the Fjordland region. The anomaly is associated with a strong lithosphere thinning, possibly to the Moho, that requires advective heat transfer such as above active magma chambers, which would accelerate ice basal melting. The anomaly may extend 500 km inland with possibly a significant contribution of ice melt to the ice-drainage system of Greenland.

The Cambro–Ordovician Alum Shale Formation on Bornholm, Denmark, is in total 26.7 to ≥ 34.9 m thick in nine boreholes, but may be up to ~39 m thick. The well sections are correlated using gamma-ray logs supplemented in some boreholes with resistivity and sonic logs. The gamma radiation of the ‘hot’ Alum Shale Formation primarily reflects the uranium content, which is moderately high in the Miaolingian (≈ middle Cambrian) and Tremadocian (Lower Ordovician), and very high in the Furongian (≈ upper Cambrian). The log pattern is calibrated with the detailed biozonation established in the Gislövshammar-1 and -2 wells in south-eastern Skåne, Sweden. Except for the Eccaparadoxides oelandicus Superzone, all superzones known from the Alum Shale in Scandinavia are also developed on Bornholm, but not all zones. On Bornholm, the Miaolingian interval is 7.2–11.9 m thick, the Furongian is 16.4–22.8 m thick and the Tremadocian is 2.5–4.0 m thick. The Miaolingian strata exhibit no systematic thickness variations across southern Bornholm, whereas the Furongian Parabolina, Peltura and Acerocarina Superzones and, less pronounced, the Tremadocian, show increased condensation towards the south-east. In comparison with Skåne, the Alum Shale Formation is overall strongly condensed on Bornholm, but different stratigraphic levels show variable developments. The Miaolingian Paradoxides paradoxissimus Superzone is thus extremely condensed and incomplete, whereas the Paradoxides forchhammeri Superzone has almost the same thickness as in Skåne, and locally is even thicker. The Furongian Olenus and Parabolina Superzones are slightly thinner than in Skåne while the Protopeltura, Peltura and Acerocarina Superzones are half as thick or less. The Tremadocian is also much thinner on Bornholm. The Furongian Olenus scanicus–O. rotundatus and Parabolina brevispina Zones seem to be developed on Bornholm, and a thin ‘Leptoplastus neglectus’ Zone is also possibly present. The ‘Parabolina megalops’ Zone in the upper part of the Peltura Superzone appears to be absent. It is impossible to distinguish the individual thin zones in the lower part of the Acerocarina Superzone using wireline logs. A thin veneer of the Lower Ordovician Tøyen Formation, hitherto considered absent on Bornholm, is described from the Billegrav-2 core. It may also be present in the uncored Sømarken-3 and -4 wells. The Middle Ordovician Komstad Limestone Formation thins from c. 4.0–4.7 m in the Læså area to 0.1– c. 2.5 m in the Øleå area. The general decrease in thickness of Cambro–Ordovician strata from Skåne to Bornholm and also within Bornholm from the Læså to the Øleå area is inferred to reflect isostatic uplift of the southern margin of Baltica commencing with the terminal 'early' Cambrian Hawke Bay Event and lasting until the Late Ordovician. In detail, several uplift and subsidence phases can be discerned. The isostatic adjustments are surmised to reflect stress changes related to ongoing plate tectonic processes in the adjacent closing Tornquist Sea.

The fluvio-deltaic Triassic Mungaroo Formation, North West Shelf (NWS) of Australia, hosts vast resources of hydrocarbons. However, the mechanisms that generated its 4-6 km monotonous infill archit...

Elastic thickness (Te) is a parameter representing the lithospheric strength with respect to the loading. Those places, having large values of elastic thickness, flexes less. In this paper, the on-orbit measured gravitational gradients of the Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission are used for determining the elastic thickness over Africa. A forward computational method is developed based on the Vening Meinesz-Moritz (VMM) and flexural theories of isostasy to find a mathematical relation between the second-order derivative of the Earth’s gravity field measured by the GOCE satellite and mechanical properties of the lithosphere. The loading of topography and bathymetry, sediments and crystalline masses are computed from CRUST1.0, in addition to estimates of laterally-variable density of the upper mantle, Young’s modulus and Poisson’s ratio. The second-order radial derivatives of the gravitational potential are synthesised from the crustal model and different a priori values of elastic thickness to find which one matches the GOCE on-orbit gradient. This method is developed in terms of spherical harmonics and performed at any point along the GOCE orbit without using any planar approximation. Our map of Te over Africa shows that the intra-continental hotspots and volcanoes, such as Ahaggar, Tibesti, Darfur, Cameroon volcanic line and Libya are connected by corridors of low Te. The high values of Te are mainly associated with the cratonic areas of Congo, Chad and the Western African basin. First Online: 06 November 2018Funders: Czech Science Foundation, GA18-06943S

The Cameroon Volcanic Line (CVL) is one of the major features of the African Plate which consists of volcanic islands of the Gulf of Guinea in the Atlantic Ocean, volcanoes and plutonic complexes in the continent trending NE-SW. Volcanic activities of the CVL are still current with eruptions of Mount Cameroon (the latest in 2000) and emission of deadly gases from lakes Monoun and Nyos in 1984 and 1986 respectively. Based on Airy-Heiskanen local compensation model, this work computes isostatic gravity anomalies to study the geodynamics of the lithosphere beneath the CVL using the Earth Gravitational Model EGM2008 and the digital topographic data ETOPO1. Isostatic regional anomalies obtained outline two major crustal zones corresponding to two distinct quasi-ellipsoidal shape zones oriented NE-SW. The southern part of the CVL including Mount Cameroon, coastal and oceanic zones is characterized by the highest values, while the northern part comprising Kumbo zone, lakes Monoun and Nyos shows lower magnitudes. Successive and located positive anomalies observed on the isostatic residual anomaly map line up towards the CVL direction. These findings bring forward the fluctuations inside the crust and the lithosphere in conformity with the CVL direction. Upper mantle with higher density than asthenosphere and crust may influence the highly positive gravity anomalies observed in the study area. In conformity with the principle of isostasy, most of the mountains of the study area may be located in zones where the conditions of complete isostatic compensation are not satisfied. Asthenospheric uplift would therefore be the cause of upward flow of materials from depths through volcano vents or fractures in both Mount Cameroon and Kumbo regions, aiming at restoring the isostatic equilibrium.

The relationship between sea surface height (SSH) and seawater density anomalies, which affects the pressure on the seafloor (PSF) anomalies off the southeastern coast of Hokkaido, Japan, was analyzed using the eddy-resolving spatial resolution ocean assimilation data of the JCOPE2M for the period 2001–2018. On an interannual (i.e., year-to-year) timescale, positive SSH anomalies of nearly 0.1 m appeared off the southeastern coast of Hokkaido, Japan, in 2007, associated with a warm-core ring (WCR), while stronger SSH anomalies (~ 0.2 m) related to a stronger WCR occurred in 2016. The results show that the effects of such positive SSH anomalies on the PSF are almost canceled out by the effects of negative seawater density anomalies from the seafloor to the sea surface (SEP; steric effect on pressure on the seafloor) due to oceanic baroclinic structures related to the WCRs, especially in offshore regions with bottom depths greater than 1000 m. This means that oceanic isostasy is well established in deep offshore regions, compared with shallow coastal regions. To further verify the strength of the oceanic isostasy, oceanic isostasy anomalies (OIAs), which represent the barotropic component of SSH anomalies, are introduced and analyzed in this study. OIAs are defined as the sum of the SSH anomalies and SEP anomalies. Our results indicate that the effect of oceanic fluid changes due to SSH and seawater density anomalies (i.e., OIAs) on PSF changes cannot be neglected on an interannual timescale, although the amplitudes of the OIAs are nearly 10% of those of the SSH anomalies in the offshore regions. Therefore, to better estimate the interannual-scale PSF anomalies due to crustal deformation related to slow earthquakes including afterslips, long-term slow slip events, or plate convergence, the OIAs should be removed from the PSF anomalies.

The stacking of thrust sheets and mass transfer of sediment during fold and thrust belt accretion imposes a load on the basement and underlying mantle. This load induces an isostatic adjustment through a flexural response, which may also contribute to the overall architecture of the fold and thrust belt. Whereas plate kinematics imposes its tempo to evolving fold and thrust belts, the rheology of the mantle controls the tempo of the isostatic flexure. Using two-dimensional high-resolution numerical experiments, we explore how the interplay between the tectonic compressional rate and the isostatic flexural rate influences the structural evolution and final architecture of fold and thrust belts. We run a suite of numerical experiments using the well-tested code Underworld. Our geological model is mapped over a 42 km by 16 km numerical grid, with a cell resolution of 80 m. The geological model consists from top to bottom of ‘sticky air’, 4 km of sediment that alternates in competence at 500 m intervals, a 3 km thick basement, and a basal layer which - in combination with a basal kinematic boundary condition - controls the amount of isostatic flexure. Materials have a mechanical behavior that results from elasto-visco-plastic rheology. The pressure at the base of the model is held constant, and the vertical velocity is updated at each timestep. The amount of material entering or exiting the model at each point along the base scales with the density of the basal layer, which is used to control the isostatic rate. Sedimentation and erosion are self-consistent through mechanical erosion and a hillslope diffusion law. Our models show that as the ratio between tectonic and flexural rates decreases (i.e. flexure gets faster), fold and thrust belts become narrower, lower in elevation, and structurally more complex. We compare these results with natural analogs including the Cordilleran and Jura fold and thrust belts.

The nature and contribution of flexural isostatic compensation to subsidence and uplift of passive margin deltas remains poorly understood. We performed a series of simulations to investigate flexural isostatic responses to high frequency fluctuations in water and sediment load associated with climatically-driven sea-level changes. We use a parallel basin and landscape dynamics model, BADLANDS, (an acronym for BAsin anD LANdscape DynamicS) that combines erosion, sedimentation, and diffusion with flexure, where the isostatic compensation of the load is computed by flexural compensation. We model a large drainage basin that discharges to a continental margin to generate a deltaic depocenter, then prescribe synthetic and climatic-driven sea-level curves of different frequencies to assess flexural response. Results show that flexural isostatic adjustments are bidirectional over 100-1000 kyr time-scales and mirror the magnitude, frequency, and direction of sea-level fluctuations, and that isostatic adjustments play an important role in driving along-strike and cross-shelf river-mouth migration and sediment accumulation. Our findings demonstrate that climate-forced sea-level changes set up a feedback mechanism that results in self-sustaining creation of accommodation into which sediment is deposited and plays a major role in delta morphology and stratigraphic architecture.

The opening of internally-drained (endorheic) sedimentary basins often leads to a major drainage change, re-excavation of the basin sedimentary infill, and transient landscape. The timing of such basin openings can be dated only in exceptional cases in which the youngest sedimentary infill remains preserved. For this reason, the processes and timing involved in their transient landscape evolution are poorly known. We explore the role of erodibility, basin geometry and flexural isostasy during the capture of internally-drained basins by means of numerical modelling techniques constrained by recent terrace cosmogenic dating and geomorphological analysis, addressing the issue as to why the Duero and Ebro rivers, draining two Cenozoic sedimentary basins in N Iberia with similar geographical dimensions and drainage histories, have undergone a markedly different erosion evolution leading to distinctly different present morphology. To evaluate how these intrinsic parameters affect the transient landscape evolution, we design a synthetic scenario inspired by those basins. The results show that, once a basin becomes externally drained, its drainage integration and erosion rates are strongly dependent on (1) the basin elevation above the base level; (2) the width of the topographic barrier, (3) its erodibility; and (4) the rigidity of the lithosphere. The results show that transient landscape evolution can last for tens of millions of years even in absence of tectonic activity and changes in base level or climate. Basins isolated by wide and resistant barriers such as the Duero Basin may undergo a multi-million-year time lag between drainage opening and basin-wide incision. In the case of the Duero Basin, this delay may explain the paradoxical time lag between the last lacustrine bulk sedimentation dated at 9.6 Ma and the onset of widespread basin incision variously estimated at 3.7 to 1 Ma., L’ouverture de bassins sédimentaires à drainage interne (endoréique) conduit souvent à un changement majeur du drainage, à la ré-excavation du remplissage sédimentaire du bassin et à un paysage transitoire. Le moment auquel ces ouvertures de bassin se produisent ne peut être daté que dans des cas exceptionnels où le plus jeune remplissage sédimentaire reste préservé. Pour cette raison, les processus et le calendrier impliqués dans l’évolution transitoire du paysage des bassins sont mal connus. Nous explorons le rôle de l’érodibilité, de la géométrie du bassin et de l’isostasie flexural lors de la capture de bassins endoréiques au moyen de techniques de modélisation numérique contraintes par les récentes datations cosmogéniques de terrasses fluviatiles et l’analyse géomorphologique. La question abordée c’est pourquoi les fleuves du Duero et de l’Ebro, drainant deux bassins sédimentaires cénozoïques du nord de l’Ibérie avec des dimensions géographiques et des histoires de drainage semblables, ont subi une évolution de l’érosion nettement différente conduisant à une morphologie actuelle nettement plus érodée au bassin de l’Ebre. Pour évaluer comment ces paramètres intrinsèques affectent l’évolution transitoire du paysage, nous concevons un scénario synthétique inspiré de ces bassins. Les résultats montrent que, une fois qu’un bassin à drainage interne est capturé et son drainage devient externe, l’intégration de son drainage et ses taux d’érosion dépendent fortement de (1) l’élévation du bassin au-dessus du niveau de base ; (2) la largeur de la barrière topographique ; (3) son érodabilité ; et (4) la rigidité de la lithosphère. Les résultats montrent que l’évolution transitoire du paysage peut se prolonger pendant des dizaines de millions d’années même en l’absence d’activité tectonique et de changements de niveau de base ou de climat. Les bassins isolés par des barrières larges et résistantes comme le bassin du Duero peuvent subir un décalage de plusieurs millions d’années entre l’ouverture du drainage et l’incision à l’échelle du bassin. Dans le cas du bassin du Duero, ce retard peut expliquer le paradoxal décalage temporel entre la dernière étendue sédimentation lacustre datée de 9,6 Ma et le début de l’incision généralisée du bassin estimée de différentes manières à 3,7–1,0 Ma., This work is supported by MITE (CGL2014-59516), SUBTETIS (PIE-CSIC-201830E039) and ALPIMED (PIE-CSIC-201530E082) projects, funded by the Spanish Government. L.R.-R. was funded by the Marie Curie- Clarín COFUND program financed jointly by the 7th WP of the European Union-Marie Curie Actions and Gobierno del Principado de Asturias (Reference ACA-17-19). The work by A.J.-D. was supported by a Juan de la Cierva-Formación postdoctoral contract (ref. FJCI-2016-28878) from the Spanish Ministry of Science, Innovation and Universities.

The research presented in this thesis seeks to address meaningful geodynamic problems related to the viscosity structure of the Earth’s interior. Isostatic adjustment is a process which is dependent upon the mechanical properties of the lithosphere and mantle. By performing computational simulations of the isostatic response for various surface-loading scenarios and numerous viscosity structures, insight can be gained into the mechanical structure of the Earth and geodynamic processes related to that structure. The modelled isostatic signal for a given set of Earth model parameters can be compared to real-world observational data in order to identify valid Earth model configurations. In Chapter 2, the “Transition Zone Water Filter” theory is tested by modelling the geophysical effects of a low-viscosity melt-rich layer atop the 410 km discontinuity. The thickness and viscosity of this layer, and the surrounding mantle, is constrained using observations of relative sea level and the geodetic J ̇_2 parameter, as well as multiple ice-loading scenarios by which the isostatic adjustment process is driven. The relative sea level data, being most sensitive to the upper mantle and the theorized melt-rich layer it contained, constrain layer properties more effectively than the J ̇_2 observation, which is strongly dependent on the lower mantle. Constraints on the viscosity of the melt-rich layer vary according to thickness, with thicker layers requiring stiffer viscosities to satisfy observations. For instance, a 20 km thick layer would require a viscosity of 10^17 Pas or greater, but any of the considered viscosities could be possible for a 1 km thick layer. Similarly, a broad range of upper mantle viscosities are possible, but they must be balanced by variations in the lower mantle. However, J ̇_2 results show a strong preference for a high-viscosity lower mantle (≥10^22 Pas). For every evaluated Earth model parameter, there is evidence of ice-model sensitivity in the inversion results. Although the results of this study demonstrate that observables related to glacial isostatic adjustment can provide constraints on the properties of this theorized melt-rich layer, the confounding effect of parameter trade-off prevents a more definitive test of this model of mantle geodynamics. The purpose of the study presented in Chapter 3 is to analyze the nature of solid-Earth deformation beneath the Lower Mississippi River, most crucially in the Mississippi Delta region where subsidence is an ongoing and costly problem. The study uses the displacement of the long profile of the Lower Mississippi River over the last 80 kyr to constrain isostatic deformation and determine constraints on the mechanical structure of both the mantle and lithosphere. Deformation recorded in the northern portion of the long profile is dominated by the effect of glacial isostatic adjustment, whereas the southern portion is governed by sediment isostatic adjustment. However, the southern portion is also potentially affected by past fault displacement, and to account for this the observational data are corrected using two distinct faulting scenarios. Displacement of the long profile is modelled using either an entirely elastic lithosphere or a lithosphere with internal viscoelastic structure, the latter of which is derived from two end-member geothermal profiles. Between the elastic and viscous lithosphere models, the viscous models are better able to replicate the observational data for each faulting scenario – both of which prefer a viscous lithosphere corresponding to the warmer geotherm. The chosen faulting scenario exerts no control over the optimal mantle model configuration, however the optimal mantle for the viscous lithosphere models is much stiffer than was determined for their elastic counterparts, reflecting significant parameter trade-off between mantle and lithosphere mechanical structure. These study results demonstrate the utility of the long profile displacement data set for constraining Earth viscosity structure, as well as the importance of considering more-complex models of lithosphere mechanical structure when addressing surface-loading problems similar to those encountered in the Mississippi Delta region.

Investigation of deep crustal and lithospheric structures is essential to understand the nature of geodynamical processes beneath the Himalaya and Tibetan plateau of the India-Eurasia collision zone. Our density cross sections across the Himalaya-Eurasia collision zone using integrated 2-D modeling of gravity, topography, and geoid data incorporating constraints from seismic information supports the above contention. Analysis of gravity, geoid, and elevation data over the interior of the Tibetan plateau predicts complete isostatic compensation, whereas margins of the plateau, having large topographic gradients, show lack of isostatic compensation as the Airy Moho differs from flexural Moho and seismic Moho beneath the Himalaya. Our 2-D modeled lithospheric cross sections show thick crust (~75 km) and thick lithosphere (~240 km) beneath the Himalayas and southern Tibetan plateau and relatively thin crust (~60 km) and thin lithosphere (~140 km) beneath the northern Tibetan plateau. Therefore, depth of lithosphere-asthenosphere boundary (LAB) mimics the Moho relief. Thinner crust and thin lithosphere under northern Tibetan plateau suggest the importance of the mantle isostasy where the temperature is anomalously high. This corroborates with the presence of recent potassic volcanism, inefficient Sn propagation, east and southeast oriented global positioning system displacements, and large shear wave splitting anisotropy (>2 s). Excellent correlation between effective elastic thickness and lithospheric thickness predicts hot and deformable lithosphere in the northern Tibet and underthrusting of cold Indian mantle beneath the Himalayas.

Relative sea level is the difference in height between the coast and sea surface. It changes across space and through time in response to many different physical processes operating on a range of scales. Using an example from North Carolina, USA, we introduce some of these key processes and explore how their importance varies on timescales from hours (e.g., hurricanes and tides) to millennia (e.g., ongoing subsidence of the coast). Below the salt marshes of North Carolina (and elsewhere) are sequences of sediment that accumulated over thousands of years and record relative sea-level changes. Geologists collect this sediment, establish its age and interrogate it to understand when and how relative sea level changed long before people began to make systematic measurements. This research shows that the rate of rise during the twentieth century was without precedent in ~2000 years.

Plateaus, located far away from the plate boundaries, play an important role in understanding the deep-rooted geological processes responsible for the epeirogenic uplift and dynamics of the plate interior. The Karnataka plateau located in the Dharwar craton, southern India, is a classic example for the plateau uplift. It is explored using orthogonal deep crustal seismic reflection studies, and a mechanism for the epeirogenic uplift is suggested. A pseudo three-dimensional crustal structure derived from these studies suggests a regionally extensive 10 km thick magmatic underplating in the region. It is further constrained from active-source refraction and passive-source seismological data. We interpret the Marion and Reunion mantle plume activities during 88 Ma and 65 Ma on the western part of Dharwar craton are responsible for the magmatic underplating, which caused epeirogenic uplift. Flexural isostasy related to the onshore denudational unloading and offshore sediment loading is also responsible for the persisting uplift in the region. Plate boundary forces are found to be contributing to the plateau uplift. The present study provides a relationship between the mantle plumes, rifting, development of continental margins, plateau uplift, and denudational isostasy. Combination of exogenic and endogenic processes are responsible for the plateau uplift in the region.

We applied a 2-dimensional, non-spectral technique to investigate the spatial variations of the lithospheric effective elastic thickness (Te) in the eastern Bayan Har block and its adjacent areas. Te values were determined by comparison of observed and flexural modelled Bouguer gravity anomalies. The results suggested that Te vary significantly from eastern Tibet to the Sichuan basin. The eastern Bayan Har block and northern Sichuan-Yunan block had low Te (0

In this study, we analysed 2708 receiver functions (RFs) using data recorded by 53 seismographic stations that surround Mt. Merapi and Mt. Merbabu – two volcanos in Central Java - to map the boundary between Earth's crust and upper mantle. We observe that a number of RFs from this new dataset have complex signals and do not exhibit typical RF characteristics; in particular, where the converted Ps signal from the Moho discontinuity is the clearest and strongest amplitude arrival following the P onset. This effect may be related to complex shallow velocity structure due to the presence of magmatic rocks and sediments. Further analysis of the RF results using the H-κ method suggests that Moho depth varies between 27 and 32 km beneath the array, with no apparent correlation between crustal thickness and surface topography, as one might expect from Airy isostacy. For instance, the Moho is quite shallow beneath Mt. Merapi (up to 27 km depth), despite its elevation of nearly 3 km. This may be a consequence of dynamic support from an active upper mantle coupled with erosion and/or weakening of the lower crust due to the active volcanic plumbing system. To the north of Mt. Merapi, the Moho is deeper (30–31 km depth) below Mt. Merbabu. Vp/Vs ratio estimates from the H-κ method are relatively high (~1.9) beneath the Mt. Merapi and Kendeng Basin area, which may indicate the presence of a zone of hydrous and active partial melting in the underlying crust. Lower Vp/Vs ratios (~1.7) are found beneath Mt. Merbabu, which may be due to its relative lack of volcanic activity compared to Mt. Merapi.