Elastic Thickness Research Articles

Elastic thickness of the Iranian lithosphere from gravity and seismic data

We estimate the (effective) elastic thickness of the Iranian lithosphere (and adjoining tectonic plates) by using the approach that combines the Vening Meinesz-Moritz's (VMM) regional isostatic principle with the isostatic flexural model formulated based on solving a flexural differential equation for a thin elastic shell. To model the response on a load more realistically, we also consider the lithospheric density structure. The resulting expression describes a functional relation that links gravity field and mechanical properties of the lithosphere. The Young modulus and the Poisson ratio are computed from seismic velocity data in prior of estimating the lithospheric elastic thickness. The presented results reveal that the estimated elastic thickness closely resembles a regional tectonic configuration associated with the extensional tectonism along the Red Sea-Gulf Rift System, the continental collision of the Arabian and Eurasian plates, and the subduction along the Makran Subduction Zone. Seismically and volcanically active convergent tectonic margins of the Zagros and Kopeh Dagh Fold and Thrust Belts further extending along the Makran Accretionary Complex are characterised by a low lithospheric strength, with the elastic thickness typically less than ∼30km. These small values of the elastic thickness are in a striking contrast to much larger values within most of the Central Iranian Blocks. According to our estimate, local maxima there reach ∼70km in the Tabas micro-block. The elastic thickness of the Turan and Arabian Platforms reaches maxima of ∼100km. These results generally support the hypothesis that tectonically active zones and orogens have a relatively low strength, resulting in a significant response of the lithosphere on various tectonic loads, compared to a significant strength of old cratonic formations. Interestingly, however, we observe a striking contrast between a low strength of the Arabian Shield compared to a high strength of the Arabian Platform. A possible explanation of this finding could be given by a different thermal regime of the Arabian lithosphere, controlled mainly by a mantle upwelling and a consequent extensional tectonism along the Red Sea-Gulf Rift System.

Residual Stress Effect on the Strength Estimate of Base-Coating Systems

The effect of manufacturing-induced residual stresses in layered isotropic composites of basecoating type on the latter strength is studied. The conventional method, based on the principle of additivity of the initial properties of layers of a symmetric composite, is applicable to uniaxial tension or compression, but not to bending, where the latter principle is not valid. Therefore, in the latter case, stress calculation has to be based on the method of equivalent bending stiffness linking the derived strength characteristic with operating stresses in the composite layers. Residual stresses in strength assessment are taken into account by substituting their analytical expressions into respective calculation models. The reliability of mostly used relationships for determining residual stresses is analyzed. To this purpose, an independent derivation of engineering formulas for axial residual stresses was carried out, varying elastic parameters, thickness, and number of layers of the composite. The substitution of the above formulas into the additivity relation for an isotropic symmetric system under uniaxial loading does not change the latter form, due to self-balancing of residual stresses and, therefore, residual stresses do not affect the strength estimate. As an example, the axial tensile strength of an aluminum alloy clad with corrosion-resistant steel was calculated for various ratios of cladding layer-to-steel layer thickness. The results on the composite hardening are presented. It is shown that, in contrast to the uniaxial tension of a composite, residual stresses in bending loading, significantly affect the strength assessment results. Numerical results on such an assessment are presented for a case study. A scheme for determining the endurance limit under cyclic bending was elaborated based on the Goodman hypothesis and experimentally verified. All analytical derivations implied the assumptions of homogeneity and isotropy of the physicomechanical properties of the composite constituents, their ideal adhesion, and elastic stress-strain state described by Hooke’s law.

TRANSFER OF LOADS FROM A FINITE NUMBER OF ELASTIC OVERLAYS WITH FINITE LENGTHS TO AN ELASTIC STRIP THROUGH ADHESIVE SHEAR LAYERS

This article deals with the problem of an elastic infinite strip, which is strengthened along its free boundary by a finite number of finite overlays with different elastic characteristics and small constant thicknesses. The interaction between the strip and the overlays is mediated by adhesive shear layers. The overlays are deformed under the action of horizontal forces. The problem of determination of unknown stresses acting between the strip and overlays are reduced to a system of Fredholm integral equations of the second kind for a finite number of unknown functions defined on different finite intervals. It is shown that in the certain domain of variation of the characteristic parameter of the problem this system of integral equations in Banach space may be solved by the method of successive approximations. Particular cases are discussed and the character and behaviour of unknown shear stresses are investigated.

Dynamic mechanisms controlling the topography of Longmenshan area

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.

The Gravitational Signature of Martian Volcanoes

AbstractBy modeling the elastic flexure of the Martian lithosphere under imposed loads, we provide a systematic study of the old and low‐relief volcanoes (>3.2 Ga, 0.5 to 7.4 km) and the younger and larger prominent constructs within the Tharsis and Elysium provinces (<3 Ga, 5.8 to 21.9 km). We fit the theoretical gravitational signal to observations in order to place constraints on 18 volcanic structures. Inverted parameters include the bulk density of the load, the elastic thickness required to support the volcanic edifice at the time it was emplaced (Te), the heat flow, the volume of extruded lava, and the ratio of volcanic products that form within (Vi) and above the preexisting surface (Ve). The bulk density of the volcanic structures is found to have a mean value of 3,206±190 kg/m3, which is representative of iron‐rich basalts as sampled by the Martian basaltic meteorites. Te beneath the small volcanoes is found to be small, less than 15 km, which implies that the lithosphere was weak and hot when these volcanoes formed. Conversely, most large volcanoes display higher values of Te, which is consistent with the bulk of their emplacement occurring later in geologic history, when the elastic lithosphere was colder and thicker. Our estimates for the volumes of volcanic edifices are about 10 times larger than those that neglect the flexure of the lithosphere. Constraints on the magnitude of subsurface loads imply that the ratio Vi/Ve is generally 3:5, which is smaller than for the Hawaiian volcanoes on Earth.

Sensitivity of elastic thickness to water in the Martian lithosphere

Ancient Mars likely hosted oceans similar to those on Earth; however, such water is not presently observed on Mars. One possible explanation for the lack of present-day oceans is that surface water was transported into and stored within the interior of Mars throughout the geological history. As water can influence the rheological structure of the Martian lithosphere, we investigated the sensitivity of the elastic thickness of the lithosphere to water using recent laboratory data. Calculations indicate that the presence of water results in a significant decrease in elastic thickness relative to dry conditions at a given thermal structure. Gravity and topographic data acquired by the Mars Global Surveyor and other orbiters indicate a temporal change in elastic thickness during the geological history of Mars. The extremely thin elastic layer during the planet’s early history can be explained by a water-rich rheological model, whereas a dry rheology can account for the relatively thick elastic layer inferred during the later evolution of Mars. Although thermal evolution of Mars has a large uncertainty, the strong sensitivity of elastic lithospheric thickness to water suggests possible sequestered water into deeper levels during the early history of Mars.

Kinematic, flexural, and thermal modelling in the Central Andes: Unravelling age and signal of deformation, exhumation, and uplift

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.

Isostasy and crustal structure of the Chagos–Laccadive Ridge, Western Indian Ocean: Geodynamic implications

The Chagos–Laccadive Ridge (CLR), a prominent linear aseismic ridge in the Western Indian Ocean is believed to be a trace of the Réunion hotspot. In order to understand the mode of emplacement of this ridge and the nature of the underlying crust, we carried out three-dimensional (3D) flexural modelling and coherence analysis of satellite-derived gravity and bathymetry data along the ridge. The analysis revealed variations in Effective Elastic Thickness ( $$T_{\mathrm{e}}$$ ) along the CLR. While the northernmost part of CLR is associated with low $$T_{\mathrm{e}}$$ of 3 km with a subsurface to surface loading ratio (f) of 1, towards the south, the Maldive Ridge and the Chagos Bank have a fairly uniform $$T_{\mathrm{e}}$$ of 8–10 km with a very low loading ratio f of 0.1–0.2. We consider the Laccadive Ridge as a continental sliver possessing underplated magmatic rocks caused by the Réunion hotspot volcanism. The Maldive Ridge and the Chagos Bank appear to have emplaced on a lithosphere of intermediate strength possibly on the flanks of the Central Indian Ridge.

Selenophysical parameter inversion in the Lunar Southern Hemisphere Highland based on mutant particle swarm optimization

Selenophysical parameters can be estimated based on admittance between gravity and topography, especially high-precision GRAIL gravity and LOLA topography data. These parameters include load ratio between subsurface and surface loads, crustal thickness, crustal density, and effective elastic thickness. Considering non-negligible membrane stress, the lunar lithosphere is best modeled as a thin and elastic spherical shell. Taking into account of the nonlinearity of the governing equation of the shell and premature convergence of PSO, we introduced an updated algorithm of MPSO which considers a self-adaptive inertia weight and a mutation operator as commonly used in Genetic Algorithms (GA). Results indicate that MPSO is relatively more flexible in the choice of tuning parameters than the general algorithm of PSO. A low mutation probability of 0.005 is found to promote globally optimized convergence, and a low value of 0.002 is employed in parameters estimation for 20 areas in southern hemisphere highland. Using the updated algorithm of MPSO, it is found that a well-constrained crustal density for most of the studied regions is less than or equal to the mean crustal density (2550 kg·m−3) of the entire lunar highland. The relatively small crustal density is likely a result of high porosity of southern hemisphere crust. The elastic thickness at studied locations far from mare basins is found to be around 7 km, corresponding to the thickness at the stage of formation of highland formed from the crystallization of ancient ferroan anorthosite. A large elastic thickness is on the contrary discovered surrounding mare basins. The ancient lunar lithosphere developed quite thick as the Moon becoming cool and it was able to elastically support new loads during the formation of mare basins. The difference of elastic thickness indicates the development of lithospheric thickness at the distinct lunar evolutionary history.

Distinct Responses of Intraplate Sedimentation to Different Subsidence Mechanisms: Insights From Forward Landscape Evolution Simulations

AbstractBoth the surface processes associated with large intracontinental basins and their underlying geodynamic mechanisms remain unclear. Proposed models include flexural deformation due to orogenic loading, isostatic subsidence in response to lithosphere extension, and dynamic subsidence arising from sublithospheric mantle downwelling. Here we use forward landscape evolution modeling to quantitatively investigate the surface process response to various types of tectonic subsidence during intraplate sedimentation. We find that isostatic subsidence causes continuous accumulation of sediment through time with a largely stable pattern of fluvial drainage. In contrast, dynamic subsidence results in fast intraplate sedimentation followed by regional unconformities and continental‐scale reorganization of fluvial networks. Our quantitative simulations reveal that the traditional 1‐D isostatic backstripping approach may lead to erroneous estimates of the past tectonic subsidence from the preserved sedimentary records: for lithospheric elastic thickness (Te) < 50 km, the flexural effect leads to larger (smaller) apparent subsidence at the center (rim) of the basin, compared to the true subsidence; for Te > 50 km, broadscale flexural support leads to underestimated subsidence throughout the basin. Subparallel tilted strata are a unique stratigraphic feature associated with spatially migratory dynamic subsidence followed by dynamic rebound. Their characteristic slope provides a quantitative constraint on the spatiotemporal pattern of dynamic subsidence. The Cretaceous Western Interior Seaway provides an example of this type of basin fill. We demonstrate that quantitative, forward landscape evolution modeling can help decipher the relationship between intraplate sedimentation and the underlying tectonic drivers.

Competing Effects of Mountain Uplift and Landslide Erosion Over Earthquake Cycles

AbstractLarge earthquakes can construct mountainous topography by inducing rock uplift but also erode mountains by causing landslides. Observations following the 2008 Wenchuan earthquake show that landslide volumes in some cases match seismically induced uplift, raising questions about how the actions of individual earthquakes accumulate to build topography. Here we model the two‐dimensional surface displacement field generated over a full earthquake cycle accounting for coseismic deformation, postseismic relaxation, landslide erosion, and erosion‐induced isostatic compensation. We explore the related volume balance across different seismotectonic and topographic conditions and revisit the Wenchuan case in this context. The ratio (Ω) between landslide erosion and uplift is most sensitive to parameters determining landslide volumes (particularly earthquake magnitudeMw, seismic energy source depth, and failure susceptibility, as well as the seismological factor responsible for triggering landslides), and is moderately sensitive to the effective elastic thickness of lithosphere,Te. For a specified magnitude, more erosive events (higher Ω) tend to occur at shallower depth, in thicker‐Telithosphere, and in steeper, more landslide‐prone landscapes. For given landscape and seismotectonic conditions, the volumes of both landslides and uplift to first order positively scale withMwand seismic momentMo. However, higherMwearthquakes generate lower landslide and uplift volumes per unitMo, suggesting lower efficiency in the use of seismic energy to drive topographic change. With our model, we calculate the long‐term average seismic volume balance for the eastern Tibetan region and find that the net topographic effect of earthquakes in this region tends to be constructive rather than erosive. Overall, destructive events are rare when considering processes over the full earthquake cycle, although they are more likely if only considering the coseismic volume budget (as was the case for the 2008 Wenchuan earthquake where landsliding substantially offset coseismic uplift). Irrespective of the net budget, our results suggest that the erosive power of earthquakes plays an important role in mountain belt evolution, including by influencing structures and spatial patterns of deformation, for example affecting the wavelength of topography.

An upper bound on Pluto's heat flux from a lack of flexural response of its normal faults

The topography of rifts on icy bodies can be used to probe their internal properties. Uplifted and curved rift flanks occur when the elastic thickness (te) of the crust is low, indicative of high heat flow out of the body. Stereo topography of Pluto shows no evidence of rift-flank uplift associated with large extensional graben to the west of Sputnik Planitia. Modeling the amount of topographic deflection expected from varying elastic thicknesses yields a conservative lower bound on te of 8 km at the time of deformation; the maximum implied heat flux since graben formation is 66–85 mWm−2. This upper bound is consistent with the predicted paleo-heat fluxes from radioactive decay (~6 mWm−2), but likely only marginally consistent with nearby apparently viscously-relaxed impact craters. Additionally, we also analyze the shear stresses on these faults and find that Pluto's faults only need to support low stresses in the range of 100–300 kPa to explain their dimensions.

Evolving paleotopography and lithospheric flexure of the Pyrenean Orogen from 3D flexural modeling and basin analysis

We present the results of a numerical modeling study of the Pyrenees Mountains investigating the spatio-temporal variation in lithospheric flexure in response to the developing orogen, with the aim of setting constraints on paleotopography. We employ a finite-element method to model the 3D flexural deformation of the lithosphere beneath the Pyrenean orogen since the onset of convergence in the Late Cretaceous. Using subsurface and structural data, we describe the evolving geometry of both the northern Aquitaine and southern Ebro foreland basins at the Paleocene (early orogenic phase), the end- and mid-Eocene (peak orogenic phase), the Oligocene (late orogenic phase), and the present (post-orogenic phase). The flexural modeling provides insight into how both the rigidity of the lithosphere and the paleotopographic load have varied over the course of orogenesis to shape the basin geometry. Employing a 3D continuous-plate model, we find that the overriding European plate has slightly higher rigidity than the underthrusting Iberian plate. The best-fit model results for the modern setting produce a root-mean-square error (RMSE) of 458 m using effective elastic thickness (Te) values of 23 and 16 km for the European and Iberian plates, respectively. We also test a broken-plate model, which produces a similar RMSE but predicts higher Te (32 km for the European plate and 26 km for the Iberian plate). The broken-plate model results indicate minimal vertical plate-boundary forces are necessary to reproduce the observed basin geometry. Flexural modeling results suggest that the topographic load doubled from the Paleocene to the Oligocene, and achieved modern topography by the end of the Eocene. The topography remained relatively stable throughout the Oligocene and Neogene, with only limited growth and decay. These results have implications for surface processes and foreland-basin development of the Pyrenean Orogen, tectonic inheritance, and the geodynamic evolution of Western Europe.

Outer trench slope flexure and faulting at Pacific basin subduction zones

SUMMARY Flexure and fracturing of the seafloor on the outer trench wall of subduction zones reflect bending of the lithosphere beyond its elastic limit. To investigate these inelastic processes, we have developed a full nonlinear inversion approach for estimating the bending moment, curvature and outer trench wall fracturing using shipboard bathymetry and satellite altimetry-derived gravity data as constraints. Bending moments and downward forces are imposed along curved trench axes and an iterative method is used to calculate the nonlinear response for 26 sites in the circum-Pacific region having seafloor age ranging from 15 to 148 Ma. We use standard thermal and yield strength envelope models to develop the nonlinear moment versus curvature relationship. Two coefficients of friction of 0.6 and 0.3 are considered and we find that the lower value provides a better overall fit to the data. The main result is that the lithosphere is nearly moment saturated at the trench axis. The effective elastic thickness of the plate on the outer trench slope is at least three times smaller than the elastic thickness of the plate before bending at the outer rise in agreement with previous studies. The average seafloor depth of the unbent plate in these 26 sites matches the Parsons & Sclater depth versus age model beyond 120 Ma. We also use the model to predict the offsets of normal faults on the outer trench walls and compare this with the horst and graben structures observed by multibeam surveys. The model with the lower coefficient of friction fits the fault offset data close to the trench axis. However, the model predicts significant fracturing of the lithosphere between 75 and 150 km away from the trench axis where no fracturing is observed. To reconcile these observations, we impose a thermoelastic pre-stress in the lithosphere prior to subduction. This pre-stress delays the onset of fracturing in better agreement with the data.

Rheological stratification and spatial variations in the effective elastic thickness of the lithosphere underneath the central Anatolian region, Turkey

In this study, the regional components of global model EGM08 Bouguer anomalies obtained by low pass filtering were inverted to map the geometries of Moho and Lithosphere-Asthenosphere Boundary (LAB) of the central Anatolian region. It was determined the Moho and LAB depths in the region to be 35.8–41.2 km and 67–91 km, respectively. The results from rheological modeling indicate mechanical decoupling of the crust and uppermost lithospheric mantle in eastern part and coupling in the western part of the study area. We also compare the rheological stratification with the focal depth distribution of earthquakes to examine the possible discrepancies between the brittle-ductile transition zone and the maximum depths of earthquakes along the selected profiles. The spatial variations of effective elastic thicknesses (EET) of the lithosphere have been estimated from the strength of the crust and lithospheric mantle by implying deformation gradient at Moho and LAB. The EET values vary in the range of 19–24.3 km. Although the EET values are relatively high in the eastern part of the region, lower EET values are directly underlain by thinned lithosphere of northwestern and southwestern part of central Anatolian region. We also analyze the crustal rheologies obtained from the lithospheric strength by delineating the pattern of crustal seismic activities.

Neogene Epeirogeny of Iberia

AbstractThe origin of Iberia's topography is examined by combining gravity, magmatic, topographic, and seismological observations with geomorphic considerations. We have four principal results. First, the highest coherence between free‐air gravity and topography is at wavelengths 250 km where admittance indicates that elastic thickness of Iberia's plate is 20 ± 3 km. These results imply that flexural and subplate support of Iberian topography could be expressed at wavelengths of O(100) km. Second, P‐to‐S receiver functions and simple isostatic calculations indicate that while crustal thickness variations and flexural loading (e.g., as a result of plate shortening) partially explain the elevation of Pyrenean, Betics, Cantabrian, Spanish Central System, and Iberian Chain topography, they fail to explain the elevation of large parts of Iberia. Third, a new full waveform shear wave tomographic model and velocity to temperature conversions suggest that the asthenosphere beneath Iberia is anomalously slow and has excess temperatures of up to 162 ± 14 °C. Simple isostatic calculations indicate asthenospheric support of topography of up to 1 km. Neogene‐Recent (∼23–0 Ma) extrusive magmatism (e.g., Calatrava, Catalan) sit atop many of the slow shear wave velocity anomalies. Finally, biostratigraphic data, combined with inversion of 3,217 river profiles, show that most of Iberia's topography grew during the last ∼30 Ma at rates of up to 0.3 mm/year. Best‐fitting theoretical rivers have a low residual root‐mean‐square misfit (=0.96) and calculated uplift is consistent with an independent inventory of stratigraphic and biostratigraphic observations. We suggest that Neogene‐Recent growth of most of central Iberia's topography was a result of asthenospheric support.

Lithospheric elastic thickness estimates in central Eurasia

We estimate the elastic thickness of a continental lithosphere by using two approaches that combine the Vening Meinesz-Moritz (VMM) regional isostatic principle with isostatic flexure models formulated based on solving flexural differential equations for a thin elastic shell with and without considering a shell curvature. To model the response of the lithosphere on a load more realistically, we also consider lithospheric density heterogeneities. Resulting expressions describe a functional relation between gravity field quantities and mechanical properties of the lithosphere, namely Young's modulus and Poisson's ratio that are computed from seismic velocity models in prior of estimating the lithospheric elastic thickness. Our numerical study in central Eurasia reveals that both results have a similar spatial pattern, despite exhibiting also some large localized differences due to disregarding the shell curvature. Results show that cratonic formations of North China and Tarim Cratons, Turan Platform as well as parts of Siberian Craton are characterized by the maximum lithospheric elastic thickness. Indian Craton, on the other hand, is not clearly manifested. Minima of the elastic thickness typically correspond with locations of active continental tectonic margins, major orogens (Tibet, Himalaya and parts of Central Asian Orogenic Belt) and an extended continental crust. These findings generally support the hypothesis that tectonically active zones and orogens have a relatively small lithospheric strength, resulting in a significant respond of the lithosphere on various tectonic loads, compared to a large lithospheric strength of cratonic formations.

Ancient subsurface structure beneath crater Clavius: constraint by recent high-precision gravity and topography data

With the increasing precision of the GRAIL gravity field models and topography from LOLA, it is possible to investigate the substructure beneath crater Clavius. An admittance between gravity and topography data is commonly used to estimate selenophysical parameters, including load ratio, crustal thickness and density, and elastic thickness. Not only a surface load, but also a subsurface load is considered in estimation. The algorithm of particle swarm optimization (PSO) with a swarm size of 400 is employed as well. Results indicate that the observed admittance is best-fitted by the modeled admittance based on a spherical shell model, which was proved to be unsatisfactory in the previous study. The best-fitted load ratio f is around −0.194. Such a small load ratio conforms to the direct proportion between the nearly uncompensated topography and its corresponding negative gravity anomaly. It also indicates that a surface load dominates all the loads. Constrained within 2σSTD, a small crustal thickness (∼30 km) and a crustal density of ∼2587 kgm−3 are found, quite close to the results from previous GRAIL research. Considering the well constrained crustal thickness and density, the best-fitted elastic thickness (∼7 km) is rational. This result is slightly smaller than the previous study (∼12 km). Such difference can be attributed to the difference in crustal density used and the precision of gravity and topography data. Considering that the small difference between the modeled gravity anomaly and observations is quite small, a parameter inversed here could be an indicator of the subsurface structure beneath Clavius.

Crustal and Mantle Structure Beneath the Southern Payenia Volcanic Province Using Gravity and Magnetic Data

AbstractThe Auca Mahuida volcanic field lies on the southernmost Payenia Volcanic Province, one of the broadest retroarc volcanic plateaux in the southern Central Andes (~38°S). This voluminous basaltic flooding of Quaternary age was originated from a deep asthenospheric source, interpreted as a mantle plume product of changing slab dynamics. The geometry of this source is deduced from magnetotelluric data, but the limited spatial coverage of this array does not allow a detailed resolution of this anomaly. In order to present a detailed geometry of the conductive anomaly and related crustal magmatic bodies, we used multiple data sources. We combined Magnetic and Bouguer anomalies, Curie isotherm depth (Tc), Elastic Thickness (Te) and Moho depth derived from the Global Earth Magnetic Anomaly Grid (EMAG2) and terrestrial gravity measurements, all together in a holistic geophysical analysis. The magnetic data depict a nearly 200‐km‐in‐diameter circular anomaly that would correspond to a dense body according to the Bouguer anomaly. Geoid data from the Gravity Field Model (EIGEN‐6c4) have been filtered in order to isolate deeper mass influences and visualize the asthenospheric upwelling previously described from magnetotelluric data. Moho inversion yields a crustal attenuation at 36‐ to 32‐km depth coinciding with Te below 20‐km depth and a shallow Tc (≤15‐km depth) at the site where Geoid positive undulation was calculated. Finally, surface analysis allowed defining a topographic swell, compatible with the dimensions of the identified magnetic anomaly, where the main rivers deviated, potentially due to a recent base level change.

Flexural subsidence analysis of the Laxmi Basin, Arabian Sea and its tectonic implications

AbstractTwo-dimensional flexural backstripping and thermal modelling (assuming laterally variable stretching) is applied along regional depth-converted interpreted seismic profiles from the Laxmi Basin in the Arabian Sea. Results from reverse post-rift flexural modelling reveal considerable basin-wide subsidence in response to the crustal geodynamics during and after the last extensional phase. Unloading of the stratigraphy allows us to estimate the degree of laterally varying extension, assuming thermal subsidence and pure shear. High degrees of extension in the basin centre predict considerable water depths at the time of rift cessation, consistent with deep drilling data. We suggest that regional extension prior to Paleocene time could have fuelled variable subsidence in the Laxmi Basin but that extension is less than seen in typical oceanic lithosphere. Volcanic loading by the seamounts shortly after extension has flexed the basin and implies an effective elastic thickness (Te) at that time of ∼6 km. Reconstruction of the seamount top near sea level at the end of emplacement indicates no major transient uplift potentially linked to the Deccan mantle plume activity. Backstripping of post-rift sediments from interpreted seismic profiles supports the presence of a hyper-thinned crust underneath the Laxmi Basin, with β factors reaching >7 in the basin centre and ∼3 across much of the basin width. Computations of decompacted sediment accumulation rates in light of new results from IODP Expedition 355 show that basin sedimentation peaked during early–middle Miocene time, possibly coeval with uplift and erosion of the Himalayan–Tibetan Plateau driven by strong summer monsoon rains.