Description: The deepest geoid low globally w.r.t. hydrostatic equilibrium is in the Ross Sea area. Nearby in West Antarctica is a residual topography high. Both are in a region with thin lithosphere, where a mantle plume has been suggested. Hence upper mantle viscosity could be regionally reduced, allowing for faster rebound than elsewhere upon melting of the West Antarctic Ice Sheet, one of the global climate system's tipping elements. To study possible causes of the geoid low / topography high combination, we compute the effects of disk-shaped density anomalies. With -1% density anomaly, geoid low and topography high can be explained with disk radius ~10° and depth range ~150-650km. Alternatively, there may be two separate disks somewhat laterally displaced, one just below the lithosphere and mainly causing a dynamic topography high and one below the transition zone causing the geoid low. In order to test the feasibility of such density models, we perform computations of a plume that enters at the base of a cartesian box corresponding to a region in the upper mantle, as well as some whole-mantle plume models, with ASPECT. However, these plume models have typically a narrow conduit and the plume tends to only become wider as it spreads beneath the lithosphere, typically shallower than ~300km, hence they would tend to rather under-predict the amplitude of the geoid compared to dynamic topography. We discuss how to possibly overcome the discrepancy between what is required to explain geoid and dynamic topography, and the outcome of numerical forward models.

Description: Seismic anisotropy is an observation that is believed to yield information on the flow pattern in the mantle. There are many studies of anisotropy in the upper mantle; however, the lower mantle is still underexplored, due to problems in seismic imaging and complexities of modelling of flow laws of different minerals. In this study, we modelled the radially anisotropic behavior of two different geodynamic setups, one is the rising of a mantle plume from the core-mantle boundary to the surface, and another is subduction of a slab reaching the lowermost mantle. We use ASPECT for modelling large scale mantle flow and ECOMAN to simulate the development of lattice preferred orientation of mantle fabric. We then couple the results from ASPECT to ECOMAN for modelling the radial anisotropy and maximum shear wave splitting direction. We show that in the part of the lowermost mantle surrounding the plume horizontally polarized shear waves (Vsh ) are faster than the vertically polarized ones (Vsv ) while the inside of the plume tail shows opposite signature. However, Vsh becomes greater than Vsv when the plume flattens out at the surface. We also find that the maximum splitting direction is horizontal outside the base of the plume and it becomes vertical inside the plume tail and again becomes horizontal at the surface. Moreover, our result for the slab setup reveals that as the slab reaches the lowermost mantle, Vsh becomes higher than Vsv and maximum splitting is horizontal at the base of the slab.

Description: Space geodetic data contain a glacial isostatic adjustment (GIA) signal, which is most prominent in formerly glaciated areas with present-day crustal uplift rates exceeding 10 mm/year in NE Canada and Central Sweden. Employing GNSS, VLBI, DORIS, and SLR data ingested into the latest international terrestrial reference frame (ITRF2020) we create a global dataset of GIA present-day uplift rates. We employ a multi-analysis-centre ensemble of GNSS station and geocentre motion coordinate solutions. Tectonic and weather signatures were reduced in estimating GNSS-derived velocities, and the trend signal is extracted from these GNSS time series with the STL method (seasonal-trend decomposition based on Loess). In addition, we develop a validation method for GIA model – data comparisons. As the geodetic stations are unevenly distributed, we employ a weighting scheme that involves network density and cross-correlation of the stations’ displacement time series. As measures of agreement for global and regional cases, we employ the weighted root mean square error (RMSE) and the weighted mean absolute error (MAE). We apply the validation method to a large suite of GIA model simulations capturing uncoupled and coupled Solid Earth – Ice Sheet models, as well as laterally homogeneous and heterogeneous viscosity structures of the Earth’s mantle, which are derived from a broad spectrum of geophysical data. The results suggest constraints on global and regional GIA model parameterisations in view of the considered observational data.

Description: Hotspots are regions of intraplate volcanism or especially strong volcanism along plate boundaries, and many of them are likely caused by underlying mantle plumes – localized hot upwellings from deep inside the Earth. It is still uncertain, whether all plumes or just some of them rise from the lowermost mantle, and to what extent and where they entrain chemically different materials. Also, large uncertainties exist regarding their size. Some plumes (such as Hawaii) create linear hotspot tracks, as the plate moves over them and can therefore serve as reference frames for plate motions, whereas others (such as Iceland) show a more complicated distribution of volcanic rocks due to variable lithosphere thickness and plume-ridge interaction. Plumes may also weaken plate boundaries and hence influence plate motions. They may influence surface features such as ice sheets, and therefore climate, but we are just beginning to study and understand processes jointly involving solid earth, hydrosphere and atmosphere.

Description: In order to test the feasibility of density and viscosity models suitable to explain geoid and dynamic topography in West Antarctica, we perform computations of a thermal plume that enters at the base of a cartesian box corresponding to a region in the upper mantle, as well as some whole-mantle thermal plume models, as well as some instantaneous disk models, with ASPECT. The plume models have typically a narrow conduit and the plume tends to only become wider as it spreads beneath the lithosphere, typically shallower than ~300 km. These results are most consistent with a shallow disk model with reduced uppermost mantle viscosity, hence providing further support for such low viscosities beneath West Antarctica. The data are a supplement to the following article: Steinberger, B., Grasnick, M.-L. & Ludwig, R., Exploring the Origin of Geoid Low and Topography High in West Antarctica: Insights from Density Anomalies and Mantle Convection Models, Tektonika, https://doi.org/10.55575/tektonika2023.1.2.35

Description: The ocean basins contain numerous volcanic ridges, seamounts and large igneous provinces (LIPs). Numerous studies have focused on the origin of seamount chains and LIPs but much less focus has been applied to understanding the genesis of large volcanic structures formed from a combination or series of volcanic drivers. Here we propose the term Oceanic Mid-plate Superstructures (OMS) to describe independent bathymetric swells or volcanic structures that are constructed through superimposing pulses of volcanism, over long time periods and from multiple sources. These sources can represent periods when the lithosphere drifted over different mantle plumes and/or experienced pulses of volcanism associated with shallow tectonic drivers (e.g. plate flexure; lithospheric extension). Here we focus on the Melanesian Border Plateau (MBP), one example of an OMS that has a complex and enigmatic origin. The MBP is a region of shallow Pacific lithosphere consisting of high volumes of volcanic guyots, ridges and seamounts that resides on the northern edge of the Vitiaz Lineament. Here we reconcile recently published constraints to build a comprehensive volcanic history of the MBP. The MBP was built through four distinct episodes: (1) Volcanism associated with the Louisville hotspot likely generating Robbie Ridge and some Cretaceous seamounts near the MBP. (2) Construction of oceanic islands and seamounts during the Eocene when the lithosphere passed over the Rurutu-Arago hotspot. (3) Reactivation of previous oceanic islands/seamounts and construction of new volcanos in the Miocene when the lithosphere passed over the Samoa hotspot. (4) Miocene to modern volcanism driven by lithospheric deformation and/or westward entrainment of enriched plume mantle due to toroidal mantle flow driven by the rollback of the Pacific plate beneath the Tonga trench. The combination of these processes is responsible for ∼222,000 km2 of intraplate volcanism in the MBP and indicates that this OMS was constructed from multiple volcanic drivers.

Description: The geoid is an equipotential surface that broadly mimics the mean sea level. The difference between the geoid and the reference spheroid at any location is referred to as a geoid anomaly. The geoid ‘highs’ (positive) or ‘lows’ (negative) are primarily associated with mass anomalies, thereby could offer important information about compositional and thermal properties in the Earth's interior. The maximum geoidal surplus (+85 m) is observed to the east of New Guinea whereas the largest deficit (−106 m) is observed in the Indian Ocean south of Sri Lanka – commonly known as the Indian Ocean geoid low (IOGL). On a global geoid map, the IOGL anomaly covers an extensive circular area spanning 〉2000 km in diameter (Fig. 1). Several different hypotheses have been put forth to explain this enigmatic anomaly. These include effects of isostatically uncompensated crust (Ihnen and Whitcomb, 1983), depression in the core-mantle boundary (Negi et al., 1987), slab graveyards in the mantle (Spasojevic et al., 2010), anomalous variations in the mantle transition zone (Reiss et al., 2017; Rao et al., 2020) and presence of a very low-velocity material arising from the African large low shear velocity province (LLSVP) or simply known as the African superplume (Ghosh et al., 2017). Most of these hypotheses rely upon either very sparse seismological observations, numerical modelling or remote sensing data. Global seismic tomographic models provide first-order information about the Earth's interior (Simmons et al., 2010, Simmons et al., 2012, Simmons et al., 2015). However, the uneven distribution of seismological networks has stymied production of high-resolution sub-surface images. In search of concrete causative mechanisms behind the IOGL anomaly, deep seismological observations from the Indian Ocean have been awaited for a long time. Between 2015 and 2020, the Ministry of Earth Sciences (MoES) India deployed a focused linear broadband passive seismological array comprising 17 ocean bottom seismometers (OBS) for two successive seasons comprising 14 months each (Fig. 1). These OBS stations thus continuously recorded local and teleseismic events for 〉28 months (Pandey, 2017). Besides, some recent studies also carried out active OBS deployments in this region to evaluate crustal and upper mantle structures (Pandey et al., 2022; Ningthoujam et al., 2022; Altenbernd-Lang et al., 2022). This special issue was conceived to present a compilation of new field observations as well as numerical modelling studies to infer potential mass anomalies within the crust and mantle beneath the IOGL region. A collection of nine papers presented in this volume explore the role of causative sources at varying depths to explain the IOGL anomaly. In summary, scientific contributions in this special issue suggest minimal crustal contributions towards the spectacular IOGL anomaly. On the other hand, new seismological studies suggest that the IOGL anomaly can be reasonably explained by a combination of positive mass anomalies in the lower mantle and/or negative mass anomalies in the upper mantle. Varied outcomes further stress upon the need to carry out more long-term seismological observations in order to image precise mantle structure beneath the IOGL region.

Description: The West Siberian Seaway connected the Tethys to the Arctic Ocean in the Paleogene and played an important role for Eurasian-Arctic biogeography, ocean circulation, and climate. However, the paleogeography and geological mechanisms enabling the seaway are not well constrained, which complicates linking the seaway evolution to paleoenvironmental changes. Here, we investigate the paleogeography of the Peri-Tethys realms for the Cenozoic time (66–0 Ma), including the West Siberian Seaway, and quantify the influence of mantle convection and corresponding dynamic topography. We start by generating continuous digital elevation models for Eurasia, Arabia, and Northern Africa, by digitizing regional paleogeographic maps and additional geological information and incorporate them in a global paleogeography model with nominal million-year resolution. Then we compute time-dependent dynamic topography for the same time interval and find a clear correlation between changes in dynamic topography and the paleogeographic evolution of Central Eurasia and the West Siberian Seaway. Our results suggest that mantle convection played a greater role in Eurasian paleogeography than previously recognized. Mantle flow may have influenced oceanic connections between the Arctic and global ocean providing a link between deep mantle convection, surface evolution, and environmental changes. Our reconstructions also indicate that the Arctic Ocean may have been isolated from the global ocean in the Eocene, even if the West Siberian Seaway was open, as the Peri-Tethys – Tethys connection was limited, and the Greenland-Scotland Ridge was a landbridge.

Description: Age-progressive seamount tracks generated by lithospheric motion over a stationary mantle plume have long been used to reconstruct absolute plate motion (APM) models. However, the basis of these models requires the plumes to move significantly slower than the overriding lithosphere. When a plume interacts with a convergent or divergent plate boundary, it is often deflected within the strong local mantle flow fields associated with such regimes. Here, we examined the age progression and geometry of the Samoa hotspot track, focusing on lava flow samples dredged from the deep flanks of seamounts in order to best reconstruct when a given seamount was overlying the mantle plume (i.e., during the shield-building stage). The Samoan seamounts display an apparent local plate velocity of 7.8 cm/yr from 0 to 9 Ma, 11.1 cm/yr from 9 to 14 Ma, and 5.6 cm/yr from 14 to 24 Ma. Current fixed and mobile hotspot Pacific APM models cannot reproduce the geometry of the Samoa seamount track if a long-term fixed hotspot location, currently beneath the active Vailulu’u Seamount, is assumed. Rather, reconstruction of the eruptive locations of the Samoan seamounts using APM models indicates that the surface expression of the plume migrated ~2° northward in the Pliocene. Large-scale mantle flow beneath the Pacific Ocean Basin cannot explain this plume migration. Instead, the best explanation is that toroidal flow fields—generated by westward migration of the Tonga Trench and associated slab rollback—have deflected the conduit northward over the past 2–3 m.y. These observations provide novel constraints on the ways in which plume-trench interactions can alter hotspot track geometries.