Publications
32 results found
Hazzard JAN, Richards FD, 2024, Antarctic Geothermal Heat Flow, Crustal Conductivity and Heat Production Inferred From Seismological Data, Geophysical Research Letters, Vol: 51, ISSN: 0094-8276
Geothermal heat flow is a key parameter in governing ice dynamics, via its influence on basal melt and sliding, englacial rheology, and erosion. It is expected to exhibit significant lateral variability across Antarctica. Despite this, surface heat flow derived from Earth's interior remains one of the most poorly constrained parameters controlling ice sheet evolution. To obtain a continent-wide map of Antarctic heat supply at regional-scale resolution, we estimate upper mantle thermomechanical structure directly from VS. Until now, direct inferences of Antarctic heat supply have assumed constant crustal composition. Here, we explore a range of crustal conductivity and radiogenic heat production values by fitting thermodynamically self-consistent geotherms to their seismically inferred counterparts. Independent estimates of crustal conductivity derived from VP are integrated to break an observed trade-off between crustal parameters, allowing us to infer Antarctic geothermal heat flow and its associated uncertainty.
Lloyd AJ, Crawford O, Al-Attar D, et al., 2024, GIA imaging of 3-D mantle viscosity based on palaeo sea level observations – part I: sensitivity kernels for an Earth with laterally varying viscosity, Geophysical Journal International, Vol: 236, Pages: 1139-1171, ISSN: 0956-540X
A key initial step in geophysical imaging is to devise an effective means of mapping the sensitivity of an observation to the model parameters, that is to compute its Fréchet derivatives or sensitivity kernel. In the absence of any simplifying assumptions and when faced with a large number of free parameters, the adjoint method can be an effective and efficient approach to calculating Fréchet derivatives and requires just two numerical simulations. In the Glacial Isostatic Adjustment problem, these consist of a forward simulation driven by changes in ice mass and an adjoint simulation driven by fictitious loads that are applied at the observation sites. The theoretical basis for this approach has seen considerable development over the last decade. Here, we present the final elements needed to image 3-D mantle viscosity using a dataset of palaeo sea-level observations. Developments include the calculation of viscosity Fréchet derivatives (i.e. sensitivity kernels) for relative sea-level observations, a modification to the numerical implementation of the forward and adjoint problem that permits application to 3-D viscosity structure, and a recalibration of initial sea level that ensures the forward simulation honours present-day topography. In the process of addressing these items, we build intuition concerning how absolute sea-level and relative sea-level observations sense Earth’s viscosity structure and the physical processes involved. We discuss examples for potential observations located in the near field (Andenes, Norway), far field (Seychelles), and edge of the forebulge of the Laurentide ice sheet (Barbados). Examination of these kernels: (1) reveals why 1-D estimates of mantle viscosity from far-field relative sea-level observations can be biased; (2) hints at why an appropriate differential relative sea-level observation can provide a better constraint on local mantle viscosity and (3) demonstrates that sea-level observations have
Richards FD, Coulson SL, Hoggard MJ, et al., 2023, Geodynamically corrected Pliocene shoreline elevations in Australia consistent with mid-range projections of Antarctic ice loss, Science Advances, Vol: 9, ISSN: 2375-2548
The Mid-Pliocene represents the most recent interval in Earth history with climatic conditions similar to those expected in the coming decades. Mid-Pliocene sea level estimates therefore provide important constraints on projections of future ice sheet behavior and sea level change but differ by tens of meters due to local distortion of paleoshorelines caused by mantle dynamics. We combine an Australian sea level marker compilation with geodynamic simulations and probabilistic inversions to quantify and remove these post-Pliocene vertical motions at continental scale. Dynamic topography accounts for most of the observed sea level marker deflection, and correcting for this effect and glacial isostatic adjustment yields a Mid-Pliocene global mean sea level of +16.0 (+10.4 to +21.5) m (50th/16th to 84th percentiles). Recalibration of recent high-end sea level projections using this revised estimate implies a more stable Antarctic Ice Sheet under future warming scenarios, consistent with midrange forecasts of sea level rise that do not incorporate a marine ice cliff instability.
Rovere A, Pico T, Richards F, et al., 2023, Influence of reef isostasy, dynamic topography, and glacial isostatic adjustment on sea-level records in Northeastern Australia, Communications Earth & Environment, Vol: 4, ISSN: 2662-4435
Understanding sea level during the peak of the Last Interglacial (125,000 yrs ago) is important for assessing future ice-sheet dynamics in response to climate change. The coasts and continental shelves of northeastern Australia (Queensland) preserve an extensive Last Interglacial record in the facies of coastal strandplains onland and fossil reefs offshore. However, there is a discrepancy, amounting to tens of meters, in the elevation of sea-level indicators between offshore and onshore sites. Here, we assess the influence of geophysical processes that may have changed the elevation of these sea-level indicators. We modeled sea-level change due to dynamic topography, glacial isostatic adjustment, and isostatic adjustment due to coral reef loading. We find that these processes caused relative sea-level changes on the order of, respectively, 10 m, 5 m, and 0.3 m. Of these geophysical processes, the dynamic topography predictions most closely match the tilting observed between onshore and offshore sea-level markers.
Stephenson SN, Ball PW, Richards FD, 2023, Destruction and regrowth of lithospheric mantle beneath large igneous provinces, Science Advances, Vol: 9, ISSN: 2375-2548
Large igneous provinces (LIPs) are formed by enormous (i.e., frequently >106 km3) but short-lived magmatic events that have profound effects upon global geodynamic, tectonic, and environmental processes. Lithospheric structure is known to modulate mantle melting, yet its evolution during and after such dramatic periods of magmatism is poorly constrained. Using geochemical and seismological observations, we find that magmatism is associated with thin (i.e., ≲80 km) lithosphere and we reveal a striking positive correlation between the thickness of modern-day lithosphere beneath LIPs and time since eruption. Oceanic lithosphere rethickens to 125 km, while continental regions reach >190 km. Our results point to systematic destruction and subsequent regrowth of lithospheric mantle during and after LIP emplacement and recratonization of the continents following eruption. These insights have implications for the stability, age, and composition of ancient, thick, and chemically distinct lithospheric roots, the distribution of economic resources, and emissions of chemical species that force catastrophic environmental change.
Richards F, 2023, Quantifying Plio-Pleistocene Global Mean Sea Level Variation, Encyclopedia of Quaternary Science, Editors: Elias, Publisher: Elsevier
Hazzard JAN, Richards FD, Goes SDB, et al., 2023, Probabilistic assessment of Antarctic thermomechanical structure: impacts on ice sheet stability, Journal of Geophysical Research: Solid Earth, Vol: 128, ISSN: 2169-9313
Uncertainty in present-day glacial isostatic adjustment (GIA) rates represents at least 44% of the total gravity-based ice mass balance signal over Antarctica. Meanwhile, physical couplings between solid Earth, sea level and ice dynamics enhance the dependency of the spatiotemporally varying GIA signal on three-dimensional variations in mantle rheology. Improved knowledge of thermomechanical mantle structure is therefore required to refine estimates of current and projected ice mass balance. Here, we present a Bayesian inverse method for self-consistently mapping shear-wave velocities from high-resolution adjoint tomography into thermomechanical structure using calibrated parameterisations of anelasticity at seismic frequency. We constrain the model using regional geophysical data sets containing information on upper mantle temperature, attenuation and viscosity structure. Our treatment allows formal quantification of parameter covariances, and naturally permits propagation of material parameter uncertainties into thermomechanical structure estimates. We find that uncertainty in steady-state viscosity structure at 150 km depth can be reduced by 4–5 orders of magnitude compared with a forward-modeling approach neglecting covariance between viscoelastic parameters. By accounting for the dependence of apparent viscosity on loading timescale, we find good agreement between our estimates of mantle viscosity beneath West Antarctica, and those derived from satellite GPS. Direct access to temperature structure allows us to estimate lateral variations in lithosphere-asthenosphere boundary (LAB) depth, geothermal heat flow (GHF), and associated uncertainties. We find evidence for shallow LAB depths (63 ± 13 km), and high GHF (76 ± 7 mW m−2) beneath West Antarctica that, combined with low asthenospheric viscosities, indicate a highly dynamic response to ice mass loss.
Huston DL, Champion DC, Czarnota K, et al., 2023, Zinc on the edge—isotopic and geophysical evidence that cratonic edges control world-class shale-hosted zinc-lead deposits, Mineralium Deposita, Vol: 58, Pages: 707-729, ISSN: 0026-4598
The North Australian Zinc Belt is the largest zinc-lead province in the world, containing three of the ten largest known individual deposits (HYC, Hilton-George Fisher, and Mount Isa). The Northern Cordillera in North America is the second largest zinc-lead province, containing a further two of the world’s top ten deposits (Red Dog and Howards Pass). Despite this world-class endowment, exploration in both mineral provinces during the past 2 decades has not been particularly successful, yielding only two significant discoveries (Teena, Australia, and Boundary, Canada). One of the most important aspects of exploration is to choose mineral provinces and districts within geological belts that have the greatest potential for discovery. Here, we present results from these two zinc belts that highlight previously unused datasets for area selection and targeting. Lead isotope mapping using analyses of mineralized material has identified gradients in μ (238U/204Pb) that coincide closely with many major deposits. Locations of these deposits also coincide with a gradient in the depth of the lithosphere-asthenosphere boundary determined from calibrated surface wave tomography models converted to temperature. Furthermore, gradients in upward-continued gravity anomalies and a step in Moho depth correspond to a pre-existing major crustal boundary in both zinc belts. A spatial association of deposits with a linear mid- to lower-crustal resistivity anomaly from magnetotelluric data is also observed in the North Australian Zinc Belt. The change from thicker to thinner lithosphere is interpreted to localize prospective basins for zinc-lead mineralization and to control the gradient in lead isotope and geophysical data. These data, when combined with data indicative of paleoenvironment and changes in plate motion at the time of mineralization, provide new exploration criteria that can be used to identify prospective mineralized basins and define the most favorable parts of the
Hollyday A, Austermann J, Lloyd A, et al., 2023, A revised estimate of early Pliocene global mean sea level using geodynamic models of the Patagonian slab window, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 24, ISSN: 1525-2027
Paleoshorelines serve as measures of ancient sea level and ice volume but are affected by solid Earth deformation including processes such as glacial isostatic adjustment (GIA) and mantle dynamic topography (DT). The early Pliocene Epoch is an important target for sea-level reconstructions as it contains information about the stability of ice sheets during a climate warmer than today. Along the southeastern passive margin of Argentina, three paleoshorelines date to early Pliocene times (4.8–5.5 Ma), and their variable present-day elevations (36–180 m) reflect a unique topographic deformation signature. We use a mantle convection model to back-advect present-day buoyancy variations, including those that correspond to the Patagonian slab window. Varying the viscosity and initial tomography-derived mantle buoyancy structures allows us to compute a suite of predictions of DT change that, when compared to GIA-corrected shoreline elevations, makes it possible to identify both the most likely convection parameters and the most likely DT change. Our simulations illuminate an interplay of upwelling asthenosphere through the Patagonian slab window and coincident downwelling of the subducted Nazca slab in the mantle transition zone. This flow leads to differential upwarping of the southern Patagonian foreland since early Pliocene times, in line with the observations. Using our most likely DT change leads to an estimate of global mean sea level of 17.5 ± 6.4 m (1σ) in the early Pliocene Epoch. This confirms that sea level was significantly higher than present and can be used to calibrate ice sheet models.
Richards FD, Hoggard MJ, Ghelichkhan S, et al., 2023, Geodynamic, geodetic, and seismic constraints favour deflated and dense-cored LLVPs, EARTH AND PLANETARY SCIENCE LETTERS, Vol: 602, ISSN: 0012-821X
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Davies DR, Ghelichkhan S, Hoggard MJ, et al., 2023, Observations and Models of Dynamic Topography: Current Status and Future Directions, Dynamics of Plate Tectonics and Mantle Convection, Pages: 223-269, ISBN: 9780323885867
The slow creeping motion of Earth's mantle drives transient changes in surface topography across a variety of spatial and temporal scales. Recent decades have seen substantial progress in understanding this so-called “dynamic topography,” with a growing number of studies highlighting its fundamental role in shaping the surface of our planet. In this review, we outline the current frontiers of geodynamical research into dynamic topography. It begins with a summary of ongoing observational, theoretical, and computational efforts that aim to quantify the present-day expression of dynamic topography, including its geographical distribution and sensitivity to different components of the mantle's flow regime. Next, observational constraints that shed light on how dynamic topography has changed over time are summarized, and compared with predictions from a range of geodynamical modeling studies, to highlight our current understanding of its evolution through the geological past. Although many model predictions can be reconciled with the available observational constraints, these comparisons demonstrate that there remain inconsistencies, particularly at shorter spatial and temporal scales. These discrepancies allow us to isolate the shortcomings of existing modeling approaches and identify pathways toward improving future reconstructions of dynamic topography through space and time. Such reconstructions are vital if we are to robustly connect the evolution of Earth's surface environments to the processes that are occurring deep within its interior.
Bowman DM, Richards F, Maunder M, et al., 2022, Stay in love with your PhD, Astronomy and Geophysics, Vol: 63, Pages: 32-35, ISSN: 0035-8738
Get this: PhDs are hard. But PhDs can be hard for the wrong reasons. A recent event held by the RAS's Early Career Network Committee identified a host of unnecessary energy-drains that should be avoided for a more profitable PhD experience. By Dominic M. Bowman, Fred Richards, Megan Maunder, Áine O'Brien and Douglas Boubert.
Maunder M, O'Brien A, Reid J, et al., 2022, Generation Covid, Astronomy and Geophysics, Vol: 63, Pages: 22-27, ISSN: 0035-8738
Austermann J, Hoggard MJ, Latychev K, et al., 2021, The effect of lateral variations in Earth structure on Last Interglacial sea level, Geophysical Journal International, Vol: 227, Pages: 1938-1960, ISSN: 0956-540X
It is generally agreed that the Last Interglacial (LIG; ∼130–115 ka) was a time when global average temperatures and global mean sea level were higher than they are today. However, the exact timing, magnitude and spatial pattern of ice melt is much debated. One difficulty in extracting past global mean sea level from local observations is that their elevations need to be corrected for glacial isostatic adjustment (GIA), which requires knowledge of Earth’s internal viscoelastic structure. While this structure is generally assumed to be radially symmetric, evidence from seismology, geodynamics and mineral physics indicates that large lateral variations in viscosity exist within the mantle. In this study, we construct a new model of Earth’s internal structure by converting shear wave speed into viscosity using parametrizations from mineral physics experiments and geodynamic constraints on Earth’s thermal structure. We use this 3-D Earth structure, which includes both variations in lithospheric thickness and lateral variations in viscosity, to calculate the first 3-D GIA prediction for LIG sea level. We find that the difference between predictions with and without lateral Earth structure can be metres to 10s of metres in the near field of former ice sheets, and up to a few metres in their far field. We demonstrate how forebulge dynamics and continental levering are affected by laterally varying Earth structure, with a particular focus on those sites with prominent LIG sea level records. Results from four 3-D GIA calculations show that accounting for lateral structure can act to increase local sea level by up to ∼1.5 m at the Seychelles and minimally decrease it in Western Australia. We acknowledge that this result is only based on a few simulations, but if robust, this shift brings estimates of global mean sea level from these two sites into closer agreement with each other. We further demonstrate that simulations with a suitable radial v
Bowman DM, Maunder M, Richards F, et al., 2021, Hear it through the grapevine, Astronomy and Geophysics, Vol: 62, Pages: 4.12-4.14, ISSN: 0035-8738
Dominic M Bowman and the ECN Committee share their perspective of the RAS Early Career Network's first career event: postdoctoral career advice from the community
Austermann J, Hoggard M, Latychev K, et al., 2021, The effect of lateral variations in Earth structure on Last Interglacial sea level, Publisher: EarthArxiv
It is generally agreed that the Last Interglacial (LIG; ∼130–115 ka) was a time when global average temperatures and global mean sea level were higher than they are today. However, the exact timing, magnitude, and spatial pattern of ice melt is much debated. One difficulty in extracting past global mean sea level from local observations is that their elevations need to be corrected for glacial isostatic adjustment (GIA), which requires knowledge of Earth’s internal viscoelastic structure. While this structure is generally assumed to be radially symmetric, evidence from seismology, geodynamics, and mineral physics indicates that large lateral variations in viscosity exist within the mantle. In this study, we construct a new model of Earth’s internal structure by converting shear wave speed into viscosity using parameterisations from mineral physics experiments and geodynamical constraints on Earth’s thermal structure. We use this 3D Earth structure, which includes both variations in lithospheric thickness and lateral variations in viscosity, to calculate the first 3D GIA prediction for LIG sea level. We find that the difference between predictions with and without lateral Earth structure can be meters to 10s of meters in the near field of former ice sheets, and up to a few meters in their far field. We demonstrate how forebulge dynamics and continental levering are affected by laterally varying Earth structure, with a particular focus on those sites with prominent LIG sea level records. Results from four 3D GIA calculations show that accounting for lateral structure acts to increase local sea level by up to ∼1.5 m at the Seychelles and minimally decrease it in Western Australia. We acknowledge that this result is only based on a few simulations, but if robust, this shift brings estimates of global mean sea level from these two sites into closer agreement with each other. We further demonstrate that simulations with a suitable radial vi
Richards F, Hoggard M, Ghelichkhan S, et al., 2021, Geodynamic, geodetic, and seismic constraints favour deflated and dense-cored LLVPs, Publisher: EarthArXiv
Two continent-sized features in the deep mantle, the large low-velocity provinces (LLVPs), influence Earth's supercontinent cycles, mantle plume generation, and its geochemical budget. Seismological advances have steadily improved LLVP imaging, but several fundamental questions remain unanswered, including: What is their vertical extent? And, are they purely thermal anomalies, or are they also compositionally distinct? Here, we investigate these questions using a wide range of observations. The relationship between measured geoid anomalies and long-wavelength dynamic surface topography places an important upper limit on LLVP vertical extent of ~900 km above the core-mantle boundary (CMB). Our mantle flow modelling suggests that anomalously dense material must exist at their base to simultaneously reproduce geoid, dynamic topography, and CMB ellipticity observations. We demonstrate that models incorporating this dense basal layer are consistent with independent measurements of semi-diurnal Earth tides and Stoneley modes. Our thermodynamic calculations indicate that a ~100 km-thick layer of early-formed, chondrite-enriched basalt is the chemical configuration most compatible with these geodynamic, geodetic and seismological constraints. By reconciling these disparate datasets for the first time, our results demonstrate that, although dominantly thermal structures, basal sections of LLVPs represent a primitive chemical reservoir that is periodically tapped by upwelling mantle plumes.
Ghelichkhan S, Fuentes JJ, Hoggard MJ, et al., 2021, The precession constant and its long-term variation, Icarus, Vol: 358, ISSN: 0019-1035
The dynamical flattening of the Earth, H, related to the precession constant, is a fundamental astro-geodetic parameter that appears in studies of the Earth's rotation and orbital evolution. We present numerical predictions and observations of the variation in H over time scales ranging from tens of millions of years to decades. The geophysical processes controlling this variation include solid-state convection in the rocky mantle of the Earth that drives plate tectonics, isostatic adjustments due to ice age loading, and ice-ocean mass transfer linked to modern global climate change. The time dependence of H is complex and non-linear, and thus, in contrast to previous suggestions, cannot be captured by a constant rate parameter.
O'Brien A, Boubert D, Bowman D, et al., 2021, Pandemic posters, Astronomy and Geophysics, Vol: 62, Pages: 1.19-1.19, ISSN: 0035-8738
Richards F, Hoggard M, Crosby A, et al., 2020, Structure and dynamics of the oceanic lithosphere-asthenosphere system, Physics of the Earth and Planetary Interiors, Vol: 309, Pages: 1-35, ISSN: 0031-9201
The thermochemical evolution of oceanic lithosphere and its interaction with the underlying asthenosphere exerts a fundamental control on the dynamics of the Earth system. Since the 1960s, the range, accuracy and spatial coverage of geophysical and geochemical datasets have increased substantially. These additional constraints have helped to elucidate aspects of the lithosphere-asthenosphere system, but some apparently contradictory observations have presented additional interpretational challenges. Here, we summarise the merits, limitations and ambiguities of available observational constraints on the thermomechanical evolution of oceanic upper mantle. Newly developed cooling models are generally compatible with these constraints, although there is evidence for systematic differences in behaviour between different oceanic basins. Subsidence, magnetotelluric and seismological observations from the Pacific Ocean are consistent with plate rather than half-space cooling models, whereas results from the Atlantic and Indian Oceans are more equivocal. We provide an overview of proposed mechanisms for seafloor flattening and we show that regional deviations from globally averaged trends can be attributed to asthenospheric temperature variation and to changes in lithospheric thickness. Although the plate cooling model generally provides a good description of available observations, it is probably a crude approximation of the dynamic processes operating within the thermal boundary layer that underlies oceanic basins. By incorporating mantle density structure inferred from surface wave tomography into more sophisticated convection simulations, we show that the plate model can match the age-dependent behaviour of bathymetric and gravity fields. While the results presented here suggest a unified understanding of the lithosphere-asthenosphere system is within reach, unambiguous evidence for small-scale convection at the base of the lithosphere remains elusive. As a result, the p
Klöcking M, Hoggard MJ, Rodríguez Tribaldos V, et al., 2020, A tale of two domes: Neogene to recent volcanism and dynamic uplift of northeast Brazil and southwest Africa, Earth and Planetary Science Letters, Vol: 547, Pages: 1-13, ISSN: 0012-821X
Topographic domes that are distant from active plate boundaries are often characterised by rapid, youthful uplift, contemporaneous mafic volcanism, radial drainage patterns, and positive long-wavelength gravity anomalies. There is increasing evidence that they are underlain by anomalously low sub-plate seismic velocities. Despite their well-known geomorphological expression, the origin of these epeirogenic features remains enigmatic and is much debated. Here, we investigate potential mechanisms for rapid regional uplift by combining disparate observations from the Borborema and Angolan plateaux that straddle the Brazilian and southwest African margins, respectively. Oceanic residual depth measurements, drainage analysis, stratigraphic architecture, emergent marine terraces and basement denudation are used to constrain their regional uplift histories. In both cases, the bulk of topographic growth occurred within the last 30 Ma in the absence of significant tectonic deformation. We estimate present-day mantle temperature and lithospheric thickness from Neogene to recent volcanic trace element compositions and upper mantle shear wave velocities. Volcanic geochemistry in northeast Brazil is compatible with decompression melting of warm asthenosphere and potentially a minor contribution from metasomatised lithospheric mantle. In Angola, melting of metasomatised lithosphere is probably triggered by injection of small-degree asthenospheric-derived melts. We find no evidence for an asthenospheric thermal anomaly >50 °C above ambient beneath either region. Present-day lithospheric thickness is ∼100 km beneath Angola and could be as thin as 60 km in the Borborema Province. For Angola, thermobarometry on mantle xenocrysts from Cretaceous kimberlites is used to estimate palaeogeothermal gradients. Results indicate a pre-existing gradient in lithospheric thickness between the edge of the Congo craton and the centre of the Angolan dome at ∼120 Ma. This gradient lik
Richards F, Hoggard M, White N, et al., 2020, Quantifying the relationship between short-wavelength dynamic topography and thermomechanical structure of the upper mantle using calibrated parameterization of anelasticity, Journal of Geophysical Research. Solid Earth, Vol: 125, Pages: 1-36, ISSN: 2169-9356
Oceanic residual depth varies on urn:x-wiley:jgrb:media:jgrb54314:jgrb54314-math-0001 5,000 km wavelengths with amplitudes of ±1 km. A component of this short‐wavelength signal is dynamic topography caused by convective flow in the upper ∼300 km of the mantle. It exerts a significant influence on landscape evolution and sea level change, but its contribution is often excluded in geodynamic models of whole‐mantle flow. Using seismic tomography to resolve buoyancy anomalies in the oceanic upper mantle is complicated by the dominant influence of lithospheric cooling on velocity structure. Here, we remove this cooling signal from global surface wave tomographic models, revealing a correlation between positive residual depth and slow residual velocity anomalies at depths <300 km. To investigate whether these anomalies are of sufficient amplitude to account for short‐wavelength residual depth variations, we calibrate an experimentally derived parameterization of anelastic deformation at seismic frequencies to convert shear wave velocity into temperature, density, and diffusion creep viscosity. Asthenospheric temperature anomalies reach +150°C in the vicinity of major magmatic hot spots and correlate with geochemical and geophysical proxies for potential temperature along mid‐ocean ridges. Locally, we find evidence for a ∼150 km‐thick, low‐viscosity asthenospheric channel. Incorporating our revised density structure into models of whole‐mantle flow yields reasonable agreement with residual depth observations and suggests that ±30 km deviations in local lithospheric thickness account for a quarter of total amplitudes. These predictions remain compatible with geoid constraints and substantially improve the fit between power spectra of observed and predicted dynamic topography. This improvement should enable more accurate reconstruction of the spatiotemporal evolution of Cenozoic dynamic topography.
Hoggard MJ, Czarnota K, Richards FD, et al., 2020, Global distribution of sediment-hosted metals controlled by craton edge stability, Nature Geoscience, Vol: 13, Pages: 504-510, ISSN: 1752-0894
Sustainable development and the transition to a clean-energy economy drives ever-increasing demand for base metals, substantially outstripping the discovery rate of new deposits and necessitating dramatic improvements in exploration success. Rifting of the continents has formed widespread sedimentary basins, some of which contain large quantities of copper, lead and zinc. Despite over a century of research, the geological structure responsible for the spatial distribution of such fertile regions remains enigmatic. Here, we use statistical tests to compare deposit locations with new maps of lithospheric thickness, which outline the base of tectonic plates. We find that 85% of sediment-hosted base metals, including all giant deposits (>10 megatonnes of metal), occur within 200 kilometres of the transition between thick and thin lithosphere. Rifting in this setting produces greater subsidence and lower basal heat flow, enlarging the depth extent of hydrothermal circulation available for forming giant deposits. Given that mineralization ages span the past two billion years, this observation implies long-term lithospheric edge stability and a genetic link between deep Earth processes and near-surface hydrothermal mineral systems. This discovery provides an unprecedented global framework for identifying fertile regions for targeted mineral exploration, reducing the search space for new deposits by two-thirds on this lithospheric thickness criterion alone.
Czarnota K, Hoggard M, Richards F, et al., 2020, Minerals on the edge: Sediment-hosted base metal endowment above steps in lithospheric thickness, Exploring for the Future: Extended Abstracts, Publisher: Commonwealth of Australia (Geoscience Australia)
To meet the rising global demand for base metals – driven primarily by the transition to cleaner-energy sources – declining rates of discovery of new deposits need to be countered by advances in exploration undercover. Here, we report that 85% of the world’s sediment-hosted base metals, including all giant deposits (>10 Mt of metal), occur within 200 km of the edge of thick lithosphere, irrespective of the age of mineralisation. This implies long-term craton edge stability, forcing a reconsideration of basin dynamics and the sediment-hosted mineral system. We find that the thermochemical structure of thick lithosphere results in increased basin subsidence rates during rifting, coupled with low geothermal gradients, which ensure favourable metal solubility and precipitation. Sediments in such basins generally contain all necessary lithofacies of the mineral system. These considerations allow establishment of the first-ever national prospectus for sediment-hosted base metal discovery. Conservative estimates place the undiscovered resource of sediment-hosted base metals in Australia to be ~50–200 Mt of metal. Importantly, this work suggests that ~15% of Australia is prospective for giant sediment-hosted deposits; we suggest that exploration efforts should be focused in this area.
Huston D, Champion D, Czarnota K, et al., 2020, Lithospheric-scale controls on zinc–lead–silver deposits of the North Australian Zinc Belt: evidence from isotopic and geophysical data
Mitrovica JX, Austermann J, Coulson S, et al., 2020, Dynamic topography and ice age paleoclimate, Annual Review of Earth and Planetary Sciences, Vol: 48, Pages: 585-621, ISSN: 0084-6597
The connection between the geological record and dynamic topography driven by mantle convective flow has been established over widely varying temporal and spatial scales. As observations of the process have increased and numerical modeling of thermochemical convection has improved, a burgeoning direction of research targeting outstanding issues in ice age paleoclimate has emerged. This review focuses on studies of the Plio-Pleistocene ice age, including investigations of the stability of ice sheets during ice age warm periods and the inception of Northern Hemisphere glaciation. However, studies that have revealed nuanced connections of dynamic topography to biodiversity, ecology, ocean chemistry, and circulation since the start of the current ice-house world are also considered. In some cases, a recognition of the importance of dynamic topography resolves enigmatic events and in others it confounds already complex, unanswered questions. All such studies highlight the role of solid Earth geophysics in paleoclimate research and undermine a common assumption, beyond the field of glacial isostatic adjustment, that the solid Earth remains a rigid, passive substrate during the evolution of the ice age climate system.
Richards F, 2019, Global Analysis of Predicted and Observed Dynamic Topography
While the bulk of topography on Earth is generated and maintained by variations in the thickness and density of crust and lithosphere, a significant time-variable contribution is expected as a result of convective flow in the underlying mantle. For over three decades, this dynamic topography has been calculated numerically from inferred density structure and radial viscosity profiles. Resulting models predict ±2 km of long wavelength (i.e., ~ 20,000 km) dynamic topography with minor contributions at wavelengths shorter than ~ 5,000 km. Recently, observational studies have revealed that, at the longest wavelengths, dynamic topography variation is ~ 30% that predicted, with ±1 km amplitudes recovered at shorter wavelengths. Here, the existing database of water-loaded basement depths is streamlined, revised and augmented. By fitting increasingly sophisticated thermal models to a combined database of these oceanic basement depths and corrected heat flow measurements, the average thermal structure of oceanic lithosphere is constrained. Significantly, optimal models are consistent with invariable geochemical and seismological constraints whilst yielding similar values of mantle potential temperature and plate thickness, irrespective of whether heat flow, subsidence or both are fit. After recalculating residual depth anomalies relative to optimal age-depth subsidence and combining them with continental constraints from gravity anomalies, a global spherical harmonic representation is generated. Although, long wavelength dynamic topography increases by ~ 40% in the revised observation-based model, spectral analysis confirms that a fundamental discrepancy between observations and predictions remains. Significantly, residual depth anomalies reveal a ~4,000 km-scale eastward tilt across the Indian Peninsula. This asymmetry extends onshore from the high-elevation Western Ghats in the west to the Krishna-Godavari floodplains in the east. Calibrated inverse modelling
Czarnota K, Hoggard M, Richards F, et al., 2019, Gigayear stability of cratonic edges controls global distribution of sediment-hosted metals, Publisher: EarthArXiv
Sustainable development and transition to a clean-energy economy is placing ever-increasing demand on global supplies of base metals (copper, lead, zinc and nickel). Alarmingly, this demand is outstripping the present rate of discovery of new deposits, with significant shortfalls forecast in the coming decades. Thus, to maintain growth in global living standards, dramatic improvements in exploration success rate are an essential goal of the geoscience community. Significant quantities of base metals have been deposited by low-temperature hydrothermal circulation within sedimentary basins over the last 2 billion years. Despite over a century of research, relationships between these deposits and geological structures remain enigmatic. Here, for the first time, we show that 85% of sediment-hosted base metals, including all giant deposits (>10 megatonnes of metal), occur within 200 km of the edges of thick lithosphere, mapped using surface wave tomography and a parameterisation for anelasticity at seismic frequencies. This remarkable observation implies long-term lithospheric edge stability and a genetic link between deep Earth processes and near-surface hydrothermal mineral systems. This result provides an unprecedented global framework for identifying fertile regions for targeted mineral exploration, reducing the search-space for new deposits by two-thirds on this lithospheric thickness criterion alone.
Huston DL, Champion DC, Czarnota K, et al., 2019, Crustal-scale controls on zinc-lead-silver deposits of the North Australian Zinc Belt: evidence from lead isotope geochemistry and surface wave tomography, 15th SGA Biennial Meeting on Life with Ore Deposits on Earth, Publisher: SOC GEOLOGY APPLIED MINERAL DEPOSITS-SGA, Pages: 1192-1195
Richards FD, Hoggard MJ, Cowton LR, et al., 2018, Reassessing the thermal structure of oceanic lithosphere with revised global inventories of basement depths and heat flow measurements, Journal of Geophysical Research: Solid Earth, Vol: 123, Pages: 9136-9161, ISSN: 2169-9313
Half-space cooling and plate models of varying complexity have been proposed to account forchanges in basement depth and heat flow as a function of lithospheric age in the oceanic realm. Here, werevisit this well-known problem by exploiting a revised and augmented database of 2,028 measurementsof depth to oceanic basement, corrected for sedimentary loading and variable crustal thickness, and 3,597corrected heat flow measurements. Joint inverse modeling of both databases shows that the half-spacecooling model yields a mid-oceanic axial temperature that is>100∘C hotter than permitted by petrologicconstraints. It also fails to produce the observed flattening at old ages. Then, we investigate a suiteof increasingly complex plate models and conclude that the optimal model requires incorporation ofexperimentally determined temperature- and pressure-dependent conductivity, expansivity, and specificheat capacity, as well as a low-conductivity crustal layer. This revised model has a mantle potentialtemperature of1300±50∘C, which honors independent geochemical constraints and has an initial ridgedepth of2.6±0.3km with a plate thickness of135±30km. It predicts that the maximum depth of intraplateearthquakes is bounded by the700∘C isothermal contour, consistent with laboratory creep experiments onolivine aggregates. Estimates of the lithosphere-asthenosphere boundary derived from studies of azimuthalanisotropy coincide with the1175±50∘C isotherm. The model can be used to isolate residual depth andgravity anomalies generated by flexural and sub-plate convective processes.
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