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Hotspots are anomalous regions of volcanism at Earth's surface that show no obvious association with tectonic plate boundaries. Classic examples include the Hawaiian-Emperor chain and the Yellowstone-Snake River Plain province. The majority are believed to form as Earth's tectonic plates move over long-lived mantle plumes: buoyant upwellings that bring hot material from Earth's deep mantle to its surface. It has long been recognized that lithospheric thickness limits the rise height of plumes and, thereby, their minimum melting pressure. It should, therefore, have a controlling influence on the geochemistry of plume-related magmas, although unambiguous evidence of this has, so far, been lacking. Here we integrate observational constraints from surface geology, geochronology, plate-motion reconstructions, geochemistry and seismology to ascertain plume melting depths beneath Earth's longest continental hotspot track, a 2,000-kilometre-long track in eastern Australia that displays a record of volcanic activity between 33 and 9 million years ago, which we call the Cosgrove track. Our analyses highlight a strong correlation between lithospheric thickness and magma composition along this track, with: (1) standard basaltic compositions in regions where lithospheric thickness is less than 110 kilometres; (2) volcanic gaps in regions where lithospheric thickness exceeds 150 kilometres; and (3) low-volume, leucitite-bearing volcanism in regions of intermediate lithospheric thickness. Trace-element concentrations from samples along this track support the notion that these compositional variations result from different degrees of partial melting, which is controlled by the thickness of overlying lithosphere. Our results place the first observational constraints on the sub-continental melting depth of mantle plumes and provide direct evidence that lithospheric thickness has a dominant influence on the volume and chemical composition of plume-derived magmas.

[1] We present a new computational modeling framework, Fluidity, for application to a range of two-and three-dimensional geodynamic problems, with the focus here on mantle convection. The approach centers upon a finite element discretization on unstructured simplex meshes, which represent complex geometries in a straightforward manner. Throughout a simulation, the mesh is dynamically adapted to optimize the representation of evolving solution structures. The adaptive algorithm makes use of anisotropic measures of solution complexity, to vary resolution and allow long, thin elements to align with features such as boundary layers. The modeling framework presented differs from the majority of current mantle convection codes, which are typically based upon fixed structured grids. This necessitates a thorough and detailed validation, which is a focus of this paper. Benchmark comparisons are undertaken with a range of two-and three-dimensional, isoviscous and variable viscosity cases. In addition, model predictions are compared to experimental results. Such comparisons highlight not only the robustness and accuracy of Fluidity but also the advantages of anisotropic adaptive unstructured meshes, significantly reducing computational requirements when compared to a fixed mesh simulation.Components: 9800 words, 9 figures, 3 tables.Keywords: mesh adaptivity; geodynamics; mantle convection; finite element methods; benchmark; model validation and verification.Index Terms: 0545 Computational Geophysics: Modeling (1952, 4255, 4316); 1213 Geodesy and Gravity: Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120); 8120 Tectonophysics: Dynamics of lithosphere and mantle: general (1213). Davies, D. R., C. R. Wilson, and S. C. Kramer (2011), Fluidity: A fully unstructured anisotropic adaptive mesh computational modeling framework for geodynamics, Geochem. Geophys. Geosyst., 12, Q06001,

Earth's surface topography is a direct physical expression of our planet's dynamics. Most is isostatic, controlled by thickness and density variations within the crust and lithosphere, but a significant proportion arises from forces exerted by underlying mantle convection. This dynamic topography directly connects the evolution of surface environments to Earth's deep interior, but predictions from mantle flow simulations are often inconsistent with inferences from the geological record, with little consensus about its spatial pattern, wavelength and amplitude. Here, we demonstrate that previous comparisons between predictive models and observational constraints have been biased by subjective choices. Using measurements of residual topography beneath the oceans, and a hierarchical Bayesian approach to performing spherical harmonic analyses, we generate a robust estimate of Earth's oceanic residual topography power spectrum. This indicates power of 0.5 ± 0.35 km 2 and peak amplitudes of ∼0.8 ± 0.1 km at long-wavelength (∼10 4 km), decreasing by roughly one order of magnitude at shorter wavelengths (∼10 3 km). We show that geodynamical simulations can only be reconciled with observational constraints if they incorporate lithospheric structure and its impact on mantle flow. This demonstrates that both deep (long-) and shallow (shorter-wavelength) processes are crucial, and implies that dynamic topography is intimately connected to the structure and evolution of Earth's lithosphere. Between Earth's crust and core lies the mantle, a 2,900 km-thick layer of hot rock that constitutes greater than 80% of Earth's volume. Carrying heat to the surface, the convecting mantle is the 'engine' that drives our dynamic planet: it is directly or indirectly responsible for almost all large-scale tectonic and geological activity [1]. As the mantle flows, it transmits normal stresses to the lithosphere-Earth's rigid outermost shell-that are balanced by gravitational stresses arising through topographic deflections of Earth's surface [2, 3, 4, 5, 6, 7, 8, 9]. This so-called dynamic topography is transient, varying both spatially and temporally in response to underlying mantle flow. As a result, it is more challenging to isolate than isostatic topography. The relative importance of dynamic versus isostatic topography varies according to setting: for example, the elevation of the Himalaya is principally isostatic, due to the presence of Earth's thickest continental crust; but the broad excess elevation of the stable South African craton has been

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Numerical simulations of thermal convection in the Earth's mantle often employ a pseudoplastic rheology in order to mimic the plate-like behavior of the lithosphere. Yet the benchmark tests available in the literature are largely based on simple linear rheologies in which the viscosity is either assumed to be constant or weakly dependent on temperature. Here we present a suite of simple tests based on nonlinear rheologies featuring temperature, pressure, and strain rate-dependent viscosity. Eleven different codes based on the finite volume, finite element, or spectral methods have been used to run five benchmark cases leading to stagnant lid, mobile lid, and periodic convection in a 2-D square box. For two of these cases, we also show resolution tests from all contributing codes. In addition, we present a bifurcation analysis, describing the transition from a mobile lid regime to a periodic regime, and from a periodic regime to a stagnant lid regime, as a function of the yield stress. At a resolution of around 100 cells or elements in both vertical and horizontal directions, all codes reproduce the required diagnostic quantities with a discrepancy of at most $3% in the presence of both linear and nonlinear rheologies. Furthermore, they consistently predict the critical value of the yield stress at which the transition between different regimes occurs. As the most recent mantle convection codes can handle a number of different geometries within a single solution framework, this benchmark will also prove useful when validating viscoplastic thermal convection simulations in such geometries.

Cenozoic intraplate volcanism is widespread throughout much of eastern Australia and manifests as both age‐progressive volcanic tracks and non‐age‐progressive lava fields. Various mechanisms have been invoked to explain the origin and distribution of the volcanism, but a broad consensus remains elusive. We use results from seismic tomography to demonstrate a clear link between lithospheric thickness and the occurrence, composition, and volume of volcanic outcrop. Furthermore, we find that non‐age‐progressive lava fields overlie significant cavities in the base of the lithosphere. Based on numerical simulations of mantle flow, we show that these cavities generate vigorous mantle upwellings, which likely promote decompression melting. However, due to the intermittent nature of the lava field volcanics over the last 50 Ma, it is probable that transient mechanisms also operate to induce or enhance melting. In the case of the Newer Volcanics Province, the passage of a nearby plume appears to be a likely candidate. Our results demonstrate why detailed 3‐D variations in lithospheric thickness, plate motion, and transient sources of mantle heterogeneity need to be considered when studying the origin of non age‐progressive volcanism in continental interiors.