Rheology and thermal structure of the lithosphere beneath the Hawaiian Ridge inferred from gravity data and models of plate flexure

2020 ◽  
Vol 222 (1) ◽  
pp. 207-224 ◽  
Author(s):  
Alexandra Pleus ◽  
Garrett Ito ◽  
Paul Wessel ◽  
L Neil Frazer
Keyword(s):  
Brittle Failure ◽  
Subduction Zones ◽  
Flexural Rigidity ◽  
Thermal Structure ◽  
Gravity Data ◽  
Mantle Xenoliths ◽  
Model Parameters ◽  
Plate Flexure ◽  
Hawaiian Ridge ◽  
Flow Laws

SUMMARY We examine the rheology and thermal structure of the oceanic lithosphere, expressed in situ by plate flexure beneath the Hawaiian Ridge, where volcanoes of variable sizes have loaded seafloor of approximately the same age, and thus where the lithosphere is expected to have had an approximately uniform age-dependent thermal structure at the time of loading. Shipboard and satellite-derived gravity, as well as multibeam bathymetry data are used in models of plate flexure with curvature-dependent flexural rigidity, the strength of which is limited, in the shallow lithosphere, by brittle failure, and in the deeper lithosphere, by low-temperature plasticity (LTP). We compute relative likelihoods and posterior probabilities for four model parameters: average crustal density ρc, friction coefficient for brittle failure ${\mu _f}$, a pre-exponential weakening factor F controlling the strength of LTP and lithospheric geotherm age t. Results show that if the lithosphere temperatures were as is expected for normal (t = ) 90-Myr-old seafloor at the time of volcano loading, the rheology must be significantly weaker than expected. Specifically, weak brittle strengths (μf ≤ 0.3) show relatively high probabilities for three of the six published LTP flow laws examined. Alternatively, moderate-to-large brittle strengths (μf ≥ 0.5) require all LTP flow laws to be substantially weakened with F = 102 to > 108 or, equivalently, activation energy reduced by 10–35 per cent. In contrast, if the lithosphere has been moderately reheated by the Hawaiian hotspot, represented by geotherms for t = 50–70 Myr, then the flow laws of Evans & Goetze, Raterron et al. and Krancj et al. require little or no weakening. Such modest thermal rejuvenation is allowed by heatflow constraints, supported by regional mantle seismic tomography imaging as well as compositions of mantle xenoliths, and reconciles previously noted discrepancies between the LTP strengths of lithosphere beneath Hawaii versus that entering the Pacific subduction zones.

scholarly journals Rheology of the Indian and Tarim plates in the Karakoram continent-to-continent collision zone

1997 ◽  
Vol 40 (5) ◽  
Author(s):  
A. Caporali
Keyword(s):  
Flexural Rigidity ◽  
Gravity Data ◽  
Tien Shan ◽  
Indian Plate ◽  
Model Parameters ◽  
Western Himalayas ◽  
Central Himalaya ◽  
Ganga Basin ◽  
Lateral Extrusion ◽  
Oscillatory Pattern

Bouguer gravity anomalies in the region of Western Himalayas, Karakoram and Tien Shan show large negative values, but classical isostatic models are insufficient to account for the detailed pattern of the observed anomalies. In the past years the gravimetric surveys in the Karakoram done by Marussi, Caputo and others in 1954 have been extended and intensified. The full body of available gravimetric data, including the pendulum observations by De Filippi and Hedin at the beginning of this century, have been re-analyzed. Terrain corrections have been computed systematically for all available data using a unique algorithm and Digital Terrain Model. The isostatic anomalies along a profile from the Indo-Gangetic foredeep, across the Karakoram range and terminating in the Tarim basin show the oscillating values already noted by Marussi. It is here proposed that this oscillatory pattern can be explained by a model in which the convergent boundaries of the Indian and Tarim plates deform by elastic flexure, besides isostasy. The gravity data constrain the numerical values of the model parameters, particularly the flexural rigidity of the plates. For the Indian plate the best fitting value of the flexural rigidity is D = 5 1024 N m, a value very similar to those reported in Central Himalaya. The flexural rigidity of the Tarim plate turns out to be considerably larger D = 7 1025 N m, which makes the Tarim more rigid than the neighboring Central Tibet. Both plates are loaded by an estimated shear stress of 7 1012 N m-1 located in a region corresponding to the Nanga Parbat Haramosh syntaxis. It is concluded that the Indo-Asian continental collision in the Western Himalaya and Karakoram resulted in the development of flexural basins on both sides, unlike the Central Himalaya where the collision produced a flexural basin, the Ganga basin, to the south and, to the north, the indentation of an isostatically supported Tibetan block with possible rheological layering and eastward lateral extrusion.

The influence of plate strength on dynamic topography estimates

2020 ◽  
Author(s):  
Jon Kirby
Keyword(s):  
Seismic Data ◽  
Flexural Rigidity ◽  
Gravity Data ◽  
Moho Depth ◽  
Dynamic Topography ◽  
Mantle Material ◽  
Plate Flexure ◽  
Isostatic Equilibrium ◽  
Effective Elastic Thickness ◽  
Geometric Analogue

<p>A common method used to evaluate dynamic topography amplitudes begins with an estimate of Moho depth, usually from seismic data but sometimes - or also - from the inversion of gravity data. Then the principles of Airy isostasy are applied: surface topography is assumed to be in isostatic equilibrium, buoyantly supported by the displacement of high-density mantle material by the low-density crustal ‘root’ that compensates the surface topographic mass. Hence, the actual relief of the Moho yields an ‘isostatic topography’ which will depart from the actual, observed topography by a component that, in theory, must arise from convective support or subsidence. Notwithstanding the fact that the errors on the seismic Moho may be larger than the topography itself, there is another source of uncertainty, that of the flexural rigidity of the lithosphere. Airy isostasy is essentially an end-member of plate flexure models, one in which the flexural rigidity is zero. However there are very few places on Earth where the flexural rigidity, usually represented by its geometric analogue the effective elastic thickness (Te), is indeed zero. In most environments, the rigidity of the plate will act to resist flexure, with the implication that the ‘Airy isostatic topography’ and therefore the dynamic topography will be in error. Here several scenarios will be presented illustrating these issues, and paths for remediation recommended.</p>

scholarly journals Bayesian inversion of surface heat flow in subduction zones: a framework to refine geodynamic models based on observational constraints

2020 ◽  
Vol 222 (1) ◽  
pp. 103-109
Author(s):  
M Morishige ◽  
T Kuwatani
Keyword(s):  
Heat Flow ◽  
Subduction Zones ◽  
Thermal Structure ◽  
Surface Heat ◽  
Dislocation Creep ◽  
Model Parameters ◽  
Limited Information ◽  
Surface Heat Flow ◽  
Trade Offs ◽  
Geodynamic Models

SUMMARY Surface heat flow has been widely used to constrain the thermal structure of subduction zones. However, the forward modelling approaches in previous geodynamic studies have only provided limited information on the model parameters controlling the thermal structure, which makes model validation difficult. Here we apply a probabilistic inversion technique based on Bayes’ theorem to surface heat flow data from Tohoku in Japan and Cascadia to simultaneously infer five model parameters that appear to have the greatest influence on the thermal structure of subduction zones. The surface heat flow is predicted via 2-D steady-state thermomechanical modelling. The Metropolis algorithm is used to obtain the posterior probability distributions. A comparison of our results with previous estimates indicates that our activation energy for the shear viscosity of dislocation creep is lower in both regions, and our radiogenic heat production rate in the upper continental crust is lower in Cascadia. These findings suggest that our geodynamic models cannot explain the surface heat flow observations with the acceptable ranges of model parameter values. We therefore need to refine the models by including, for example, the effects of recent backarc extension, vigorous thermal convection beneath the overriding plate and fluid circulation in the uppermost part of the oceanic crust. The approach presented here also allows us to determine trade-offs between the parameters. This study provides a framework to validate and refine geodynamic models based on various types of observations.

Magnetite (U-Th-Sm)/He dating: analytical challenges and application

2021 ◽  
Author(s):  
Marianna Corre ◽  
Martine Lanson ◽  
Arnaud Agranier ◽  
Stephane Schwartz ◽  
Fabrice Brunet ◽  
...  
Keyword(s):  
Laser Ablation ◽  
Subduction Zones ◽  
Major Elements ◽  
Western Alps ◽  
Planetary Science ◽  
Mantle Xenoliths ◽  
Transform Faults ◽  
Geological Processes ◽  
History Of ◽  
Geochronological Constraints

<p>Magnetite (U-Th-Sm)/He dating method has a strong geodynamic significance, since it provides geochronological constraints on serpentinization episodes, which are associated to important geological processes such as ophiolite obductions, subduction zones, transform faults and fluid circulations. Although helium content that range from 0.1 pmol/g to 20 pmol/g can routinely be measured, the application of this dating technique however is still limited due to major analytical obstacles. The dissolution of a single magnetite crystal and the measurement of the U, Th and Sm present at the ppb level in the corresponding solution, remains highly challenging, especially because of the absence of magnetite standard. In order to overcome these analytical issues, two strategies have been followed, and tested on magnetite from high-pressure rocks from the Western Alps (Schwartz et al., 2020). Firstly, we purified U, Th and Sm (removing Fe and other major elements) using ion exchange columns in order to analyze samples, using smaller dilution. Secondly, we performed in-situ analyzes by laser-ablation-ICPMS. Since no solid magnetite certified standard is yet available, we synthetized our own by precipitating magnetite nanocrystals. The first quantitative results obtained by LA-ICP-MS using this synthetic material along with international glass standards, are promising. The laser-ablation technique overcomes the analytical difficulties related to sample dissolution and purification. It thus opens the path to the dating of magnetite (and also spinels) in various ultramafic rocks such as mantle xenoliths or serpentinized peridotites in ophiolites.</p><p>Schwartz S., Gautheron C., Ketcham R.A., Brunet F., Corre M., Agranier A., Pinna-Jamme R., Haurine F., Monvoin G., Riel N., 2020, Unraveling the exhumation history of high-press ure ophiolites using magnetite (U-Th-Sm)/He thermochronometry. Earth and Planetary Science Letters 543 (2020) 116359.</p>

scholarly journals Disjoint interval bound constraints using the alternating direction method of multipliers (ADMM) for geologically constrained inversion: Application to gravity data

Geophysics ◽  
2020 ◽  
pp. 1-45
Author(s):  
Vitaliy Ogarko ◽  
Jérémie Giraud ◽  
Roland Martin ◽  
Mark Jessell
Keyword(s):  
Gravity Data ◽  
Alternating Direction Method ◽  
Rock Properties ◽  
Model Parameters ◽  
Method Of Multipliers ◽  
Geological Modeling ◽  
Bound Constraints ◽  
Data Inversion ◽  
Alternating Direction ◽  
Geological Information

To reduce uncertainties in reconstructed images, geological information must be introduced in a numerically robust and stable way during the geophysical data inversion procedure. In the context of potential (gravity) data inversion, it is important to bound the physical properties by providing probabilistic information on the number of lithologies and ranges of values of possibly existing related rock properties (densities). For this purpose, we introduce a generalization of bounding constraints for geophysical inversion based on the alternating direction method of multipliers (ADMM). The flexibility of the proposed technique enables us to take into account petrophysical information as well as probabilistic geological modeling, when it is available. The algorithm introduces a priori knowledge in terms of physically acceptable bounds of model parameters based on the nature of the modeled lithofacies in the region under study. Instead of introducing only one interval of geologically acceptable values for each parameter representing a set of rock properties, we define sets of disjoint intervals using the available geological information. Different sets of intervals are tested, such as quasi-discrete (or narrow) intervals as well as wider intervals provided by geological information obtained from probabilistic geological modeling. Narrower intervals can be used as soft constraints encouraging quasi-discrete inversions. The algorithm is first applied to a synthetic 2D case for proof-of-concept validation and then to the 3D inversion of gravity data collected in the Yerrida basin (Western Australia). Numerical convergence tests show the robustness and stability of the bound constraints we apply, which is not always trivial for constrained inversions. This technique can be a more reliable uncertainty reduction method as well as an alternative to other petrophysically or geologically constrained inversions based on more classical “clustering” or Gaussian-mixture approaches.

Inversion and uncertainty estimation of gravity data using simulated annealing: An application over Lake Vostok, East Antarctica

Geophysics ◽  
2005 ◽  
Vol 70 (1) ◽  
pp. J1-J12 ◽  
Author(s):  
Lopamudra Roy ◽  
Mrinal K. Sen ◽  
Donald D. Blankenship ◽  
Paul L. Stoffa ◽  
Thomas G. Richter
Keyword(s):  
Simulated Annealing ◽  
Gravity Data ◽  
A Priori ◽  
Model Space ◽  
Uncertainty Estimation ◽  
East Antarctica ◽  
Airborne Gravity ◽  
Model Parameters ◽  
Data Set ◽  
Lake Vostok

Interpretation of gravity data warrants uncertainty estimation because of its inherent nonuniqueness. Although the uncertainties in model parameters cannot be completely reduced, they can aid in the meaningful interpretation of results. Here we have employed a simulated annealing (SA)–based technique in the inversion of gravity data to derive multilayered earth models consisting of two and three dimensional bodies. In our approach, we assume that the density contrast is known, and we solve for the coordinates or shapes of the causative bodies, resulting in a nonlinear inverse problem. We attempt to sample the model space extensively so as to estimate several equally likely models. We then use all the models sampled by SA to construct an approximate, marginal posterior probability density function (PPD) in model space and several orders of moments. The correlation matrix clearly shows the interdependence of different model parameters and the corresponding trade-offs. Such correlation plots are used to study the effect of a priori information in reducing the uncertainty in the solutions. We also investigate the use of derivative information to obtain better depth resolution and to reduce underlying uncertainties. We applied the technique on two synthetic data sets and an airborne-gravity data set collected over Lake Vostok, East Antarctica, for which a priori constraints were derived from available seismic and radar profiles. The inversion results produced depths of the lake in the survey area along with the thickness of sediments. The resulting uncertainties are interpreted in terms of the experimental geometry and data error.

Architecture and dynamics of the magmatic system feeding the 2018 offshore Mayotte eruption from satellite gravity data

2021 ◽  
Author(s):  
Hélène Le Mével ◽  
Craig A. Miller ◽  
Yan Zhan
Keyword(s):  
Gravity Data ◽  
System Structure ◽  
Constant Thickness ◽  
Gravity Inversion ◽  
Model Parameters ◽  
Low Density ◽  
Submarine Eruption ◽  
Magmatic System ◽  
Magma Flow ◽  
Marine Gravity

<p>In May 2018, a submarine eruption started offshore Mayotte (Comoros archipelago, Indian Ocean), and was first detected as a series of earthquake swarms. Since then, at least 6.4 km<sup>3</sup> of lava has erupted from a newly mapped volcanic edifice (MAYOBS campaigns), about 50 km east of Mayotte island. Since the onset of the eruption, GNSS stations on the island have recorded subsidence (up to 17 cm) and eastward displacement (up to 23 cm). We combine marine gravity data derived from satellite altimetry with finite element models to examine the magmatic system structure and its dynamics. First, we calculate the Mantle Bouguer Anomaly (MBA) by taking into account the gravitational effect of the bathymetry and the Moho interfaces, assuming a crust of constant thickness of 17.5 km and correction densities of 2.8 g/cm<sup>3</sup> and 3.3 g/cm<sup>3</sup> for the crust and mantle, respectively. We then invert the MBA to determine the anomalous density structures within the lithosphere, using the mixed Lp-norm inversion and Gauss-Newton optimization implemented in the SimPEG framework. The gravity inversion reveals two zones of low density, east of Mayotte island. The first is located NE of Petite Terre island between ~15 and 35 km depth, and the second is located further east, south of La Jumelle seamounts and extends from ~25 to 35 km depth. We interpret these low density regions as regions of partial melt stored in the lithosphere and estimate the volume of stored magma. Finally, we use the newly imaged low density bodies to constrain the magma reservoir geometry and simulate magma flow from this reservoir to the eruptive vent in a 3D, time-dependent, numerical model. The model parameters are adjusted by minimizing the misfit between the modeled surface displacement and that measured at the 6 GPS sites, between May 2018 and 2020. The deformation modeling reveals the temporal evolution of the magma flux during the eruption, and the resulting stress distribution in the crust explains the patterns of recorded seismicity. Together with the existing seismic and geodetic studies, the gravity data analysis and FEM models bring new constraints on the architecture of the magma plumbing system and the magmatic processes behind the largest submarine eruption ever documented.</p>

scholarly journals Three-Dimensional Regularized Focusing Migration: A Case Study from the Yucheng Mining Area, Shandong, China

Minerals ◽  
2020 ◽  
Vol 10 (5) ◽  
pp. 471
Author(s):  
Yidan Ding ◽  
Guoqing Ma ◽  
Shengqing Xiong ◽  
Haoran Wang
Keyword(s):  
Large Scale ◽  
Mineral Exploration ◽  
Gravity Data ◽  
Three Dimensional ◽  
Mining Area ◽  
Model Parameters ◽  
Step Size ◽  
Migration Direction ◽  
Migration Imaging ◽  
Imaging Results

Gravity migration is a fast imaging technique based on the migration concept to obtain subsurface density distribution. For higher resolution of migration imaging results, we propose a 3D regularized focusing migration method that implements migration imaging of an entire gravity survey with a focusing stabilizer based on regularization theory. When determining the model parameters, the iterative direction is chosen as the conjugate migration direction, and the step size is selected on the basis of the Wolfe–Powell conditions. The model tests demonstrate that the proposed method can improve the resolution and precision of imaging results, especially for blocky structures. At the same time, the method has high computational efficiency, which allows rapid imaging for large-scale gravity data. It also has high stability in noisy conditions. The developed novel method is applied to interpret gravity data collected from the skarn-type iron deposits in Yucheng, Shandong province. Migration results show that the depth of the buried iron ore in this area is 750–1500 m, which is consistent with the drilling data. We also provide recommendations for further mineral exploration in the survey area. This method can be used to complete rapid global imaging of large mining areas and it provides important technical support for exploration of deep, concealed deposits.

Gravity interpretation using Fourier transforms and simple geometrical models with exponential density contrast

Geophysics ◽  
1993 ◽  
Vol 58 (8) ◽  
pp. 1074-1083 ◽  
Author(s):  
D. Bhaskara Rao ◽  
M. J. Prakash ◽  
N. Ramesh Babu
Keyword(s):  
Los Angeles ◽  
Fourier Transforms ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Sedimentary Basins ◽  
Density Contrast ◽  
Model Parameters ◽  
Exponential Density ◽  
Exponential Density Contrast ◽  
Gravity Interpretation

The decrease of density contrast in sedimentary basins can often be approximated by an exponential function. Theoretical Fourier transforms are derived for symmetric trapezoidal, vertical fault, vertical prism, syncline, and anticline models. This is desirable because there are no equivalent closed form solutions in the space domain for these models combined with an exponential density contrast. These transforms exhibit characteristic minima, maxima, and zero values, and hence graphical methods have been developed for interpretation of model parameters. After applying end corrections to improve the discrete transforms of observed gravity data, the transforms are interpreted for model parameters. This method is first tested on two synthetic models, then applied to gravity anomalies over the San Jacinto graben and Los Angeles basin.

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