Nicky White

Dynamic topography changes as mantle convection evolves, making its past surface expression difficult to reconstruct. Widespread Neogene‐Quaternary intraplate basaltic magmatism across the Western Mediterranean Region allows us to test whether dated basalts preserve the mantle thermal structure responsible for regional dynamic support. We combine major, trace, and rare earth element (REE) analyses and modeling from six volcanic provinces with calibrated earthquake tomographic models and residual topographic and bathymetric measurements. The geochemical data set includes both newly acquired and published analyses. Rare earth element inverse modeling of samples least affected by lithospheric‐mantle influence indicates that melting occurred beneath lithosphere typically 60–70 km thick and at asthenospheric potential temperatures ranging from approximately ambient to 50°C above ambient mantle. These estimates agree with independent tomographic constraints. Dynamic support calculated from the inferred thermal structure is also consistent with measured residual anomalies. Lithospheric thinning accounts for most of the largest positive anomalies of approximately 0.5 km without requiring large asthenospheric temperature anomalies. The agreement demonstrates that Western Mediterranean intraplate basaltic magmatism records upper‐mantle thermal structure capable of generating regional surface support and provides a basis for reconstructing past mantle conditions and transient dynamic topography from dated basalts.

Mantle plumes are a fundamental component of the Earth’s convective regime, but their long-term behaviour remains poorly understood. The Iceland plume, which is bisected by the Mid-Atlantic Ridge, presents an opportunity to constrain plume evolution. Here, we reconstruct its influence upon seafloor spreading by exploiting the geochemistry of basalts drilled south of Iceland during International Ocean Discovery Program (IODP) Expedition 395. Trace element and isotopic measurements, combined with contextual geophysical observations, demonstrate that plume influence waned after continental break-up at ~ 55 Ma, collapsed rapidly at ~ 38 Ma, and then was progressively re-established to the present day. Recovered ~ 32 Ma basalt samples have rare earth element compositions equivalent to mid-Atlantic Ridge dredge samples located south of the present-day plume influence. These compositions can be modelled by passive upwelling and melting of depleted MORB mantle with a potential temperature of ~ 1300 ^∘ C. In contrast, basalts recovered from younger (i.e. 0–14 Ma) sites show unequivocal evidence for plume influence. Together, these results imply dramatic changes in the extent of plume-ridge interaction across the North Atlantic region, providing key chemical constraints for geodynamic models of plume evolution and its imprint upon the geological record.

Thermal pulsing is thought to be a characteristic process of major mantle convection cells. Seafloor features near Iceland, known as the "V-Shaped Ridges" (VSRs), may comprise the best record of thermal plume pulsing. However, a satisfactory test of this thermal plume pulsing model has been compromised by the lack of suitable geophysical and geochemical datasets from the VSRs. Here, we present the first full crustal seismic image of multiple complete VSR cycles. In 2024, the IMPULSE experiment acquired an approximately 400 km long profile that straddles the Reykjanes Ridge spreading axis and several V-Shaped Ridge/Trough cycles spanning over 18 million years. Traveltime picks for crustal and upper mantle refractions and PmP wide-angle Moho reflections were inverted using the TOMO2D software package to obtain crustal thickness as well as crustal and upper mantle seismic velocity. The results show crustal thickness variations that correlate with VSR geometry. They also reveal seismic velocity variations which indicate fluctuations in mineralogy of the lower crustal cumulates that correlate with the VSRs. Mid-ocean ridge basalts sampled by International Ocean Drilling Program Expedition 395 at five sites along the seismic profile show trace element variations that correlate with the VSRs. Significantly, we have imaged both conjugate flanks of the spreading axis along a plate spreading flowline. Comparison of conjugate crustal thickness and structure permits us to disentangle primary melt supply processes from asymmetric crustal accretion processes. The combined geophysical and geochemical dataset supports a model in which the VSRs form when thermal plume pulsing causes fluctuations in the volume and composition of magma supplied to the mid-oceanic ridge, and crustal accretion processes related to oblique spreading at variable rate then modify VSR morphology in different locations.

It is recognized that mantle plumes play a direct role in generating regional uplift and producing immense volumes of basaltic magmatism, both of which can influence paleoclimate. The Icelandic Plume, beneath the North Atlantic Ocean, is of particular importance due to its size and position at a significant paleoceanographic gateway. It is transected by a mid‐oceanic ridge system, which has generated a series of V‐shaped ridges and troughs that flank the Reykjanes Ridge south of Iceland. The origin of these diachronous features is debated—do they reflect thermal fluctuations within the plume head or have they formed as a result of compositional variations within the buoyant convecting mantle? To address these and other hypotheses, the International Ocean Discovery Program (IODP) carried out three drilling expeditions, which recovered basalt cores from a sequence of V‐shaped ridges and troughs. Here, we show that the petrology and geochemistry of fifty whole‐rock samples taken from boreholes that penetrate different ridges and troughs reveal systematic differences in major, trace and rare earth element concentrations. By combining forward and inverse modeling based upon polybaric fractional melting, we show that these geochemical variations can be explained by varying melt fraction as a function of depth for plausible mantle source compositions. Our results suggest that temperature differences of 25–30C between cooler troughs and hotter ridges play a dominant role. We conclude that the drilled basaltic rocks reveal a chronology of resolvable temperature perturbations that should help to elucidate the fluid dynamics of flow within this major plume.

Global age‐depth and heat flow observations provide constraints for cooling and subsidence of oceanic plates. Numerous studies have addressed this problem, which has a bearing upon the calibration of shear‐wave tomographic models and upon lithospheric rheology. The robustness of these results depends upon the quality and spatial distribution of both age‐depth and heat flow measurements. Here, we revisit the plate cooling model for two reasons. First, a database of 10,863 age‐depth measurements that are distributed throughout the oceanic realm has been constructed. This database is combined with 3,573 heat flow measurements. Second, we wish to explore a range of analytical and numerical plate models that incorporate the temperature‐ and pressure‐dependence of conductivity, of expansivity, and of specific heat capacity. Our goal is to identify plate models that jointly fit observational constraints, whilst honoring laboratory‐based estimates of key thermal parameters. Both simple analytical and comprehensive numerical parameterizations recover an equilibrium plate thickness of or km with a temperature of C. This recovered temperature is consistent with independent petrologic constraints. Spatial analysis of age‐depth measurements demonstrates that previously invoked transient plate shallowing is not globally observed. This observation implies that the possible onset of a convective instability, which has been proposed as a mechanism to stabilize equilibrium plate thickness, might act on shorter length scales than that implied by previously reported transient shallowing. Finally, our revised plate model is used to track lithospheric thermal structure as a function of time and to calculate residual depth anomalies.

There is considerable interest in developing quantitative methods for analyzing present‐day fluvial landscapes with a view to extracting information about tectonic forcing and drainage evolution, together with the influence of lithologic substrates and of paleoclimatic variations. In view of the multifactorial nature of this complex problem, it has previously been proposed that natural geomorphic experiments could play a significant role in developing a quantitative understanding of landscape growth and decay. Here, we describe and analyze a stacked sequence of five buried transient landscapes that punctuate marine strata along the fringes of the North Atlantic Ocean. We propose that these landscapes constitute a suite of natural experiments, which illuminate significant aspects of quantitative fluvial geomorphology. Our preliminary analysis of four of these buried landscapes suggests that the amplitude of external tectonic forcing plays a significant role in fluvial landscape evolution. In future, we hope that this suite of natural experiments will be further exploited by the fluvial community with a view to identifying the most appropriate analytical techniques.

Continental topography is dominantly controlled by a combination of crustal thickness and density variations. Nevertheless, it is clear that some additional topographic component is supported by the buoyancy structure of the underlying lithospheric and convecting mantle. Isolating these secondary sources is not straightforward, but provides valuable information about mantle dynamics. Here, we estimate and correct for the component of topographic elevation that is crustally supported to obtain residual topographic anomalies for the major continents, excluding Antarctica. Crustal thickness variations are identified by assembling a global inventory of 26,725 continental crustal thickness estimates from local seismological data sets (e.g., wide‐angle/refraction surveys, calibrated reflection profiles, receiver functions). In order to convert crustal seismic velocity into density, we develop a parametrization that is based upon a database of 1,136 laboratory measurements of seismic velocity as a function of density and pressure. In this way, 4,120 new measurements of continental residual topography are obtained. Observed residual topography mostly varies between ±1 and 2 km on wavelengths of 1,000–5,000 km. Our results are generally consistent with the pattern of residual depth anomalies observed throughout the oceanic realm, with long‐wavelength free‐air gravity anomalies, and with the distribution of upper mantle seismic velocity anomalies. They are also corroborated by spot measurements of emergent marine strata and by the global distribution of intraplate magmatism that is younger than 10 Ma. We infer that a significant component of residual topography is generated and maintained by a combination of lithospheric thickness variation and sub‐plate mantle convection. Lithospheric composition could play an important secondary role, especially within cratonic regions.

Topography and bathymetry are principally supported by some combination of crustal and sub‐crustal density variations. However, dynamic topography is generated by vertical deflection of the Earth's surface as a result of mantle convection. Isolating and quantifying observable dynamic topography yields valuable information about mantle processes. Here, we investigate global dynamic topography by calculating residual depth anomalies throughout the oceanic realm and residual topographic anomalies across the continents. We correct for sedimentary and crustal loading by exploiting a variety of seismologic datasets that include seismic reflection profiles, wide‐angle/refraction surveys, and receiver functions. In this way, an extensively revised and augmented global compilation of 10,874 oceanic residual depth measurements and 3,777 continental residual topographic measurements is constructed. In the oceanic realm, the methodology has been revised to improve accuracy and resolution. First, quartz/clay content of the sedimentary column is adjusted to remove minor skewness of residual depth anomalies as a function of plate age. Secondly, variation of bulk density as a function of crustal thickness is taken into account. Our global compilation is used to generate spherical harmonic representations of observable dynamic topography out to degree 40 (i.e., ∼1,000 km). Resultant spectra demonstrate that dynamic topographic power varies linearly with inverse wavenumber. The spectral slope directly reflects the way by which dynamic topography is generated by Stokes' flow. Our global results are consistent with independent and diverse geologic markers of uplift and subsidence together with Neogene‐Quaternary intraplate basalt magmatism.