Lithosphere–asthenosphere interactions beneath northeast China and the origin of its intraplate volcanism

Northeast China hosts one of the largest Cenozoic intraplate volcanic regions in the world. However, the mechanisms that generate the volcanism, its spatial-temporal distribution, and compositional signatures remain highly debated due to the lack of high-resolution images of the mantle’s thermochemical structure. We jointly inverted new surface-wave dispersion data, surface heat flow, geoid height, and elevation data to image the fine-scale thermal and compositional structures beneath northeast China and infer regions of partial melting in the mantle. Our model reveals a complex circulation pattern in the asthenosphere and a highly variable lithospheric structure. Combining predictions from our model with independent geochemical data from recent basaltic volcanism, we demonstrate that the generation, location, and composition of intraplate volcanism in this region are controlled by the interaction between shallow asthenospheric circulation and lithospheric thickness. The modeling approach and correlations between basaltic composition and mantle state identified in our study are globally applicable to assessing mantle conditions over time in other continental regions.

Scaling laws for stagnant-lid convection with a buoyant crust

SUMMARY Stagnant-lid convection, where subduction and surface plate motion is absent, is common among the rocky planets and moons in our solar system, and likely among rocky exoplanets as well. How stagnant-lid planets thermally evolve is an important issue, dictating not just their interior evolution but also the evolution of their atmospheres via volcanic degassing. On stagnant-lid planets, the crust is not recycled by subduction and can potentially grow thick enough to significantly impact convection beneath the stagnant lid. We perform numerical models of stagnant-lid convection to determine new scaling laws for convective heat flux that specifically account for the presence of a buoyant crustal layer. We systematically vary the crustal layer thickness, crustal layer density, Rayleigh number and Frank–Kamenetskii parameter for viscosity to map out system behaviour and determine the new scaling laws. We find two end-member regimes of behaviour: a ‘thin crust limit’, where convection is largely unaffected by the presence of the crust, and the thickness of the lithosphere is approximately the same as it would be if the crust were absent; and a ‘thick crust limit’, where the crustal thickness itself determines the lithospheric thickness and heat flux. Scaling laws for both limits are developed and fit the numerical model results well. Applying these scaling laws to rocky stagnant-lid planets, we find that the crustal thickness needed for convection to enter the thick crust limit decreases with increasing mantle temperature and decreasing mantle reference viscosity. Moreover, if crustal thickness is limited by the formation of dense eclogite, and foundering of this dense lower crust, then smaller planets are more likely to enter the thick crust limit because their crusts can grow thicker before reaching the pressure where eclogite forms. When convection is in the thick crust limit, mantle heat flux is suppressed. As a result, mantle temperatures can be elevated by 100 s of degrees K for up to a few Gyr in comparison to a planet with a thin crust. Whether convection enters the thick crust limit during a planet’s thermal evolution also depends on the initial mantle temperature, so a thick, buoyant crust additionally acts to preserve the influence of initial conditions on stagnant-lid planets for far longer than previous thermal evolution models, which ignore the effects of a thick crust, have found.

Lithosphere–asthenosphere boundary beneath the Sea of Japan from transdimensional inversion of S-receiver functions

The evolution history of the Sea of Japan back-arc basin remains under debate, involving the opening of sub-basins such as the Japan and Yamato Basins. Detailed knowledge of the lithospheric structure will provide the key to understanding tectonic history. This study identifies the lithosphere–asthenosphere boundary (LAB) beneath the Sea of Japan back-arc basin using S-receiver functions (S-RFs). The study area, including the Japan and Yamato Basins, has been instrumented with broadband ocean-bottom seismometers (OBSs). S-RFs from these OBSs show negative Sp phases preceding the direct S arrivals, suggesting the LAB. The S-RFs also show abnormally reduced amplitudes. For further qualitative interpretation of these findings, we conduct transdimensional Bayesian inversion for S-wave velocity models. This less-subjective Bayesian approach clarifies that the low-velocity seafloor sediments and damped deconvolution contribute to the amplitude reduction, illuminating the necessity of such considerations for similar receiver function works. Inverted velocity structures show a sharp velocity decrease at the mantle depths, which we consider the LAB. The obtained LAB depths vary among sites: ~ 45 km beneath the Japan and Yamato Basins and ~ 70 km beneath the Yamato Rise, a bathymetric high between the two basins. The thick lithosphere beneath the Yamato Rise most likely reflects its continental origin. However, the thickness is still thin compared to that of eastern Asia, suggesting lithosphere extension by rifting. Notably, the Japan and Yamato Basins show a comparable lithospheric thickness, although the crustal thickness beneath the Yamato Basin is known to be anomalously thick. This consistency in the lithospheric thickness implies that both basins undergo similar back-arc opening processes.

Melting conditions and mantle source composition from probabilistic joint inversion of major and rare earth element concentrations

The chemical composition of erupted basalts provides a record of the thermo-chemical state of their source region and the melting conditions that lead to their formation. Here we present the first probabilistic inversion framework capable of inverting both trace and major element data of mafic volcanic rocks to constrain mantle potential temperature, depth of melting, and major and trace element source composition. The inversion strategy is based on the combination of i) a two-phase multi-component reactive transport model, ii) a thermodynamic solver for the evolution of major elements and mineral/liquid phases, (iii) a disequilibrium model of trace element partitioning and iv) an adaptive Markov chain Monte Carlo algorithm. The mechanical and chemical evolution of melt and solid residue are therefore modelled in an internally- and thermodynamically-consistent manner. We illustrate the inversion approach and its sensitivity to relevant model parameters with a series of numerical experiments with increasing level of complexity. We show the benefits and limitations of using major and trace element compositions separately before demonstrating the advantages of a joint inversion. We show that such joint inversion has great sensitivity to mantle temperature, pressure range of melting and composition of the source, even when realistic uncertainties are assigned to both data and predictions. We further test the reliability of the approach on a real dataset from a well-characterised region: the Rio Grande Rift in western North America. We obtain estimates of mantle potential temperature ($\sim$ 1340 $^o$C), lithospheric thickness ($\sim$ 60 km) and source composition that are in excellent agreement with numerous independent geochemical and geophysical estimates. In particular, this study suggests that the basalts in this region originated from a moderately hot upwelling and include the contribution from a slightly depleted source that experienced a small degree of melt or fluid metasomatism. This component is likely associated with partial melting of the lower portions of the lithosphere. The flexibility of both the melting model and inversion scheme developed here makes the approach widely applicable to assessing the thermo-chemical structure and evolution of the lithosphere-asthenosphere system and paves the way for truly joint geochemical-geophysical inversions.

Rheological inheritance controls the formation of segmented rifted margins in cratonic lithosphere

Observations from rifted margins reveal that significant structural and crustal variability develops through the process of continental extension and breakup. While a clear link exists between distinct margin structural domains and specific phases of rifting, the origin of strong segmentation along the length of margins remains relatively ambiguous and may reflect multiple competing factors. Given that rifting frequently initiates on heterogenous basements with a complex tectonic history, the role of structural inheritance and shear zone reactivation is frequently examined. However, the link between large-scale variations in lithospheric structure and rheology and 3-D rifted margin geometries remains relatively unconstrained. Here, we use 3-D thermo-mechanical simulations of continental rifting, constrained by observations from the Labrador Sea, to unravel the effects of inherited variable lithospheric properties on margin segmentation. The modelling results demonstrate that variations in the initial crustal and lithospheric thickness, composition, and rheology produce sharp gradients in rifted margin width, the timing of breakup and its magmatic budget, leading to strong margin segmentation.

Geoid and topography on Venus: Isostatic or dynamic?

<p>Venus is commonly described as Earth’s slightly smaller twin planet. However, the dynamics of plate tectonics present at Earth are not observed at Venus.  Gravity and topography are key observations to help understand the interior dynamics of a planet. On Earth, the long-wavelength geoid and total surface topography are not well correlated, with the interpretation that total surface topography is mainly due to the ocean-continent dichotomy whereas geoid reflects density anomalies deep in the mantle, mainly caused by subducted slabs. Dynamic surface topography is small compared to the total surface topography. On Venus, in contrast, the geoid and topography are well correlated, indicating a more direct connection between convection and the lithosphere and crust.</p> <p>For Venus, two end-member origins of geoid and topography variations have been proposed: 1) Deep-seated (i.e. below the lithosphere) density anomalies associated with mantle convection, which may require a recent global lithospheric overturn to be significant [1][2][3]. 2) Variations in lithosphere and crustal thickness that are isostatically compensated - the so-called "isostatic stagnant lid approximation" [4][5], which appears consistent with simple stagnant-lid convection experiments.</p> <p>Here we analyse 2-D and 3-D dynamical thermo-chemical models of Venus' mantle and crust that include melting and crustal production, multiple composition-dependent phase transitions and strongly variable viscosities to test whether variations in crust and lithosphere thickness explain most of the geoid signal [4][5], or whether it is caused mostly by density variations below the lithosphere, and thus, what we can learn about the crust, lithosphere and deeper interior of Venus from observations, as well as which tectonic mode is most likely to explain the observed geoid signal. Multiple input parameter sets are used to recreate the end-member scenarios of stagnant-lid and episodic-lid tectonics and to investigate the influence of the different rheological parameters. Characteristic snapshots of simulations showing end-member tectonic behaviour are analysed to determine the depth ranges of heterogeneities that are the predominant influence on topography and geoid variations. Findings will also guide future efforts to combine gravity and topography observations to infer lithosphere and crustal thickness and their variations (e.g. [6][7]).</p> <p><strong>References</strong></p> <p>[1] Armann, M., and P. J. Tackley (2012), Simulating the thermo-chemical magmatic and tectonic evolution of Venus' mantle and lithosphere: two-dimensional models, <em>J. Geophys. Res.</em>, <em>117</em>, E12003, doi:12010.11029/12012JE004231</p> <p>[2] King, S. D. (2018), Venus resurfacing constrained by geoid and topography, <em>J. Geophys. Res.</em>, <em>123</em>, doi:10.1002/2017JE005475.</p> <p>[3] Rolf, T., B. Steinberger, U. Sruthi, and S. C. Werner (2018), Inferences on the mantle viscosity structure and the post-overturn evolutionary state of Venus, <em>Icarus</em>, <em>313</em>, 107-123, doi:10.1016/j.icarus.2018.05.014.</p> <p>[4] Orth, C. P., and V. S. Solomatov (2011), The isostatic stagnant lid approximation and global variations in the Venusian lithospheric thickness, <em>Geochem. Geophys. Geosyst.</em>, <em>12</em>(7), Q07018, doi:10.1029/2011gc003582.</p> <p>[5] Orth, C. P., and V. S. Solomatov (2012), Constraints on the Venusian crustal thickness variations in the isostatic stagnant lid approximation, <em>Geochemistry, Geophysics, Geosystems</em>, <em>13</em>(11), n/a-n/a, doi:10.1029/2012gc004377</p> <p>[6] Jiménez-Díaz, A., J. Ruiz, J. F. Kirby, I. Romeo, R. Tejero, and R. Capote (2015), Lithospheric structure of Venus from gravity and topography, <em>Icarus</em>, <em>260</em>, 215-231, doi:10.1016/j.icarus.2015.07.020.</p> <p>[7] Yang, A., J. Huang, and D. Wei (2016), Separation of dynamic and isostatic components of the Venusian gravity and topography and determination of the crustal thickness of Venus, <em>Planetary and Space Science</em>, <em>129</em>, 24-31, doi:10.1016/j.pss.2016.06.001.</p>

The Depth to Magnetic Sources in the Arctic and Its Relationship with Some Parameters of the Lithosphere

––The depth to magnetic sources in twenty Arctic tectonic provinces is determined from azimuthally averaged Fourier power spectra of geomagnetic anomalies according to the EMAG2v3 and WDMAM 2.0 global models. The resulting depths to the centroid and bottom of the magnetic lithosphere are more reliable than the depth to the upper magnetic boundary. The depth to the bottom of magnetic sources, corresponding to the Curie point depth, varies from 25.3 to 38.1 km in different provinces. The Curie point depth estimates are correlated with several parameters of the lithosphere. They are directly proportional to the lithospheric thickness and inversely proportional to average upper mantle temperatures, but the relationship with the intensity of long-wavelength satellite magnetic anomalies and crustal thickness is poor. The magnetic sources are located at crustal depths in most of the provinces, but the upper mantle may be magnetic beneath deep-water oceanic basins and the Laptev Sea. The results for the Laptev Sea shelf support a passive mechanism of current lithospheric extension in the area.