44 results found
Ball PW, Roberts GG, Mark DF, et al., 2023, Geochemical and geochronological analysis of Harrat Rahat, Saudi Arabia: An example of plume related intraplate magmatism, Lithos, Vol: 446-447, ISSN: 0024-4937
Harrat Rahat is the largest volcanic field in Saudi Arabia and has been active from ∼10 Ma to the present day. Due to its proximity to population centers, recent eruptions at Harrat Rahat— the Medinah lava flows (<1.7 Ma)— have been extensively studied to identify volcanic risk. However, evolution of Harrat Rahat's most extensive and oldest lava flows, known collectively as the Shawahit Basalt (>2.5 Ma), is poorly understood. In this study, we collected, dated and geochemically analyzed lavas from Harrat Rahat, primarily targeting the under-sampled Shawahit unit. We obtained dates of between 9.4 and 2.7 Ma using 40Ar/39Ar analyses of 23 Shawahit samples. Over the lifetime of Harrat Rahat, we observe a geochemical transition from predominantly subalkalic to alkalic eruptions coupled with a counter-intuitive decrease in incompatible element concentrations. We attribute these changes to a decrease in melt productivity and a reduction in contamination by enriched lithospheric melts, respectively. Thermobarometric analysis of basalts from Harrat Rahat indicates that they were generated by melting of asthenospheric mantle with a potential temperature of ∼1456-32+50°C beneath lithosphere that is 50–60 km thick. These results indicate that volcanic activity at Harrat Rahat was initiated by the arrival of a mantle plume beneath lithosphere thinned by a combination of rifting of the Red Sea and thermal erosion. Furthermore, we propose that this plume, either acting alone or in combination with a number of other plumes, is responsible for the formation of the Arabian swell, as well as much of the Neogene-recent intraplate volcanic activity observed across western Arabia. Our conclusions are consistent with a wide range of geochemical, seismologic, gravimetric, thermochronologic and geomorphic observations.
Lipp AG, de Caritat P, Roberts GG, 2023, Geochemical mapping by unmixing alluvial sediments: An example from northern Australia, Journal of Geochemical Exploration, Vol: 248, ISSN: 0375-6742
Alluvial sediments have long been used in geochemical surveys as their compositions are assumed to be representative of areas upstream. Overbank and floodplain sediments, in particular, are increasingly used for regional to continental-scale geochemical mapping. However, during downstream transport, sediments from heterogeneous source regions are carried away from their source regions and mixed. Consequently, using alluvial sedimentary geochemical data to generate continuous geochemical maps remains challenging. In this study we demonstrate a technique that numerically unmixes alluvial sediments to make a geochemical map of their upstream catchments. The unmixing approach uses a model that predicts the concentration of elements in downstream sediments, given a map of the drainage network and element concentrations in the source region. To unmix sedimentary chemistry, we seek the upstream geochemical map that, when mixed downstream, best fits geochemical observations downstream. To prevent overfitting we penalise the roughness of the geochemical model. To demonstrate our approach we apply it to alluvial samples gathered as part of the Northern Australia Geochemical Survey. This survey gathered samples collected over a ∼500,000 km2 area in northern Australia. We first validate our approach for this sample distribution with synthetic tests, which indicate that we can resolve geochemical variability at scales greater than 0.5–1° in size. We proceed to invert real geochemical data from the total digestion of fine-grained fraction of alluvial sediments. The resulting geochemical maps for two elements of potential economic interest, Cu and Nd, are evaluated in detail. We find that in both cases, our predicted downstream concentrations match well against a held-out, previously ‘unseen’ subset of the data, as well as against data from an independent geochemical survey. By performing principal component analysis on maps generated for all 46 available
Eschenfelder J, Lipp AG, Roberts GG, 2023, Quantifying excess heavy metal concentrations in drainage basins using conservative mixing models, Journal of Geochemical Exploration, Vol: 248, ISSN: 0375-6742
High concentrations of heavy metals and other pollutants in river sediments can have detrimental effects on the ecosystem and humans. The composition of river sediments throughout drainage basins therefore provides important information for environmental monitoring. An obvious first step for using river sediment compositions for monitoring is to quantify natural baseline concentrations. Once baselines have been quantified, it is straightforward to compare them to observations to identify excesses generated by, for example, anthropogenic inputs. In this study a new strategy for mapping element concentrations along rivers from discrete geochemical observations upstream is presented. We demonstrate our approach in a case study of the Clyde drainage basin in western Scotland, UK. First, continuous baselines are generated using simple forward models that conservatively mix source region concentrations along drainage networks. 1185 measurements of elemental concentrations from first-order streams are used to parameterise the source region. The calculated baselines are then compared to concentrations measured at 60 localities along the main channel of the Clyde river. For a range of major and trace elements (e.g., Mg, Sr, K, Mn), the downstream observations are in close agreement with baseline concentrations predicted by conservative mixing models. However, some heavy metal concentrations (Pb, Cu, Zn) tend to exceed predicted baseline concentrations. Therefore, the second part of our approach calculates element concentrations in source areas required to match the observed Pb, Cu and Zn concentrations measured along the river. An inverse approach is used to ‘unmix’ the observed concentrations utilising, again, a conservative mixing model. Model resolution is determined by the spatial distribution of the data. Resultant calculated natural baselines and heavy metal concentrations along the river can easily be compared to estimate excesses. We tentatively suggest t
Okwara IC, Hampson GJ, Whittaker AC, et al., 2023, Source‐to‐sink mass‐balance analysis of an ancient wave‐influenced sediment routing system: Middle Jurassic Brent Delta, Northern North Sea, offshore UK and Norway, Basin Research, ISSN: 0950-091X
Sediment mass-balance analysis provides key constraints on stratigraphic architecture and its controls. We use the data-rich Middle Jurassic Brent Delta sediment routing system in the proto-Viking Graben, Northern North Sea, to estimate sediment budgets and mass-balance between source areas and depositional sinks. Published studies are synthesised to provide an age-constrained sequence stratigraphic framework, consisting of four previously defined genetic sequences (J22, J24, J26, J32). Genetic sequence J32 (3.9 Myr) records transverse progradation of basin-margin deltas, sourced from the Shetland Platform to the west and Norwegian Landmass to the east. Genetic sequences J24 (1.1 Myr) and J26 (0.9 Myr) record the rapid progradation and subsequent aggradation of the Brent Delta along the basin axis, sourced from the uplifted Mid-North Sea High to the south, and the western and eastern source regions. Genetic sequence J32 (2.2 Myr) records the retreat of the Brent Delta. Sediment budgets for the four genetic sequences are estimated using palaeogeographical reconstructions, isopach maps, and sedimentological analysis of core and well-log data. The estimated net-depositional sediment budget for the mapped Brent Delta system is 2.0–2.8 Mt/year. Temporal variations in net-depositional sediment budget were driven by changes in tectonic boundary conditions, such as the onset of uplift before the deposition of genetic sequence J24. Over the same time period, the Shetland Platform, Norwegian Landmass and Mid-North Sea High source regions are estimated to have supplied 2.3–5.6, 5.0–14.1, and 2.8–9.4 Mt/year of sediment, respectively, using the BQART sediment load model and independent geometrical reconstruction of eroded volumes, which are constrained by isostatic uplift estimates based on the geochemistry of syn-depositional volcanic rocks. The net-depositional sediment budget in the sink is an order-of-magnitu
Roberts G, Stucky De Quay G, 2022, Geodynamic generation of a Paleocene-Eocene landscape buried beneath North Bressay, North Sea, Journal of the Geological Society, Vol: 180, Pages: 1-10, ISSN: 0016-7649
Histories of vertical lithospheric motions provide important clues about geodynamic processes. We present evidence of an ancient (∼58–55 Ma) landscape that likely uplifted and subsided rapidly during incipience of the Icelandic plume. Now buried beneath ∼0.4–0.8 km of rock in the North Bressay region in the North Sea, this landscape is located within a sedimentary basin on the margin of the North Atlantic Ocean. We use high-resolution 3D seismic reflection data to map this ancient surface. Correlation of stratigraphy with a survey in the Bressay region constrains age and depositional environment. The landscape contains excellent evidence of meandering fluvial channels, some of which record avulsions, and terminate against a coastline to the east where deltaic landforms are identified. The landscape was depth-converted and decompacted to generate a digital elevation model from which river profiles were extracted. Their geometries indicate that the landscape was generated by three phases of uplift. This history of uplift and subsidence is analogous to similar-aged landscapes in the Judd area ∼400 km to the west and Bressay ∼30 km to the south, and appears to be another manifestation of lithospheric motions generated by the passage of warm thermal anomalies away from the Icelandic plume.
Alqahtani SA, Collier REL, Paton DA, et al., 2022, Uplift evolution along the Red Sea continental rift margin from stream profile inverse modeling and drainage analysis, Journal of African Earth Sciences, Vol: 192, Pages: 1-17, ISSN: 1464-343X
Continental rifted margins can have complex uplift histories related to different processes including footwall uplift by mechanical unloading, dynamic uplift and interaction with transfer margins. Deciphering uplift histories along rift flanks is integral to understanding the margin evolution as a whole. Here, a combination of drainage analysis and stream profile inverse modeling is utilized to estimate the rift flank uplift along the north-eastern Red Sea onshore margin. The drainage network was extracted from an ASTER DEM (∼30 x 30 m-horizontal resolution) and the uplift history was calculated using an inverse model, which builds on the relationship between uplift, erosion and stream profile shape. Local relief, minimum erosion volumes and minimum erosion volume:catchment area ratios (Rva) were also calculated and compared to uplift estimates. Within the study area, small catchments represent footwall drainage and larger catchments are mostly associated with pre-rift structures and syn-rift accommodation zones. Uplift initiated in the southern part during early rifting (21-15 Ma) before shifting northward (12-0 Ma). This uplift distribution is reflected in Rva and relief maps. Early-rift uplift is interpreted as a record of early-rift faulting with possible additional mantle support, whereas later uplift was driven by fault linkage and mantle upwelling (12-6 Ma) as well as transform tectonics (6-0 Ma). These modeling results are largely in agreement with other independent data (low-temperature thermochronology and dated carbonate terraces). Our workflow benefits from its utilization of ubiquitous drainage data. The combination of drainage analysis and inverse modeling proves to be more discerning than either one method in isolation, and may have application to analysis of other margins.
OMalley CPB, Roberts GG, Mannion PD, et al., 2022, Scale-dependent coherence of terrestrial vertebrate biodiversity with environment, Publisher: Cold Spring Harbor Laboratory
Disentangling contributions from environmental variables is crucial for explaining global biodiversity patterns. We use wavelet power spectra to separate wavelength-dependent trends across Earth’s surface. Spectra reveal scale- and location-dependent coherence between species richness and topography (E), annual precipitation (Pn), temperature (Tm) and temperature range (ΔT). > 97% of richness of carnivorans, bats, songbirds, hummingbirds and amphibians resides at wavelengths ≳ 103 km. 30–69% is generated at scales ≳ 104 km. At these scales, richness across the Americas is anti-correlated with E and ΔT, and positively correlated with Pn and Tm. Carnivoran richness is incoherent with ΔT, suggesting insensitivity to temperature seasonality. Conversely, amphibian richness is anti-correlated with ΔT at large scales. At scales ≲ 103 km, richness is highest within the tropics. Terrestrial plateaux exhibit coherence between carnivoran richness and E at scales ~ 103 km, reflecting contributions of orogeny/epeirogeny to biodiversity. Similar findings result from transects across other continents. Scale-dependent sensitivities of vertebrate populations to climate are revealed.
OMalley CP, White NJ, Stephenson SN, et al., 2021, Large‐scale tectonic forcing of the African landscape, Journal of Geophysical Research: Earth Surface, Vol: 126, Pages: 1-37, ISSN: 2169-9003
Successful inverse modeling of observed longitudinal river profiles suggests that fluvial landscapes are responsive to continent-wide tectonic forcing. However, inversion algorithms make simplifying assumptions about landscape erodibility and drainage planform stability that require careful justification. For example, precipitation rate and drainage catchment area are usually assumed to be invariant. Here, we exploit a closed-loop modeling strategy by inverting drainage networks generated by dynamic landscape simulations in order to investigate the validity of these assumptions. First, we invert 4,018 African river profiles to determine an uplift history that is independently calibrated, and subsequently validated, using separate suites of geologic observations. Second, we use this tectonic forcing to drive landscape simulations that permit divide migration, interfluvial erosion and changes in catchment size. These simulations reproduce large-scale features of the African landscape, including growth of deltaic deposits. Third, the influence of variable precipitation is investigated by carrying out a series of increasingly severe tests. Inverse modeling of drainage inventories extracted from simulated landscapes can largely recover tectonic forcing. Our closed-loop modeling strategy suggests that large-scale tectonic forcing plays the primary role in landscape evolution. One corollary of the integrative solution of the stream-power equation is that precipitation rate becomes influential only if it varies on time scales longer than ∼1 Ma. We conclude that calibrated inverse modeling of river profiles is a fruitful method for investigating landscape evolution and for testing source-to-sink models.
Lipp A, 2021, Source region geochemistry from unmixing downstream sedimentary elemental compositions, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 22, Pages: 1-25, ISSN: 1525-2027
The geochemistry of river sediments is routinely used to obtain information about geologic and environmental processes occurring upstream. For example, downstream samples are used to constrain chemical weathering and physical erosion rates upstream, as well as the locations of mineral deposits or contaminant sources. Previous work has shown that, by assuming conservative mixing, the geochemistry of downstream samples can be reliably predicted given a known source region geochemistry. In this study, we tackle the inverse problem and “unmix” the composition of downstream river sediments to produce geochemical maps of drainage basins (i.e., source regions). The scheme is tested in a case study of rivers draining the Cairngorms, UK. The elemental geochemistry of the urn:x-wiley:15252027:media:ggge22639:ggge22639-math-0001 μm fraction of 67 samples gathered from the beds of channels in this region is used to invert for concentrations of major and trace elements upstream. A smoothed inverse problem is solved using the Nelder-Mead optimization algorithm. Predictions of source region geochemistry are assessed by comparing the spatial distribution of 22 elements of different affinities (e.g., Be, Li, Mg, Ca, Rb, U, V) using independent geochemical survey data. The inverse approach makes reliable predictions of the major and trace element concentration in first order river sediments. We suggest this scheme could be a novel means to generate geochemical baselines across drainage basins and within river channels.
Roberts G, 2021, Emergence of simplicity despite local complexity in eroding landscapes, Geology (Boulder), Vol: 49, Pages: 1322-1326, ISSN: 0091-7613
Much understanding of continental topographic evolution is rooted in measuring and predicting rates at which rivers erode. Flume tank and field observations indicate that stochasticity and local conditions play important roles in determining rates at small scales (e.g. < 10 km, thousands of years). Obversely, preserved river profiles and common shapes of rivers atop uplifting topography indicate that erosion rates are predictable at larger scales. These observations indicate that the response of rivers to forcing can be scale dependent. Here I demonstrate that erosional thresholds can provide an explanation for why profile evolution can be very complicated and unique at small scales yet simple and predictable at large scales.
Wapenhans I, Fernandes VM, O'Malley C, et al., 2021, Scale‐dependent contributors to river profile geometry, Journal of Geophysical Research: Earth Surface, Vol: 126, ISSN: 2169-9003
A range of complex hydraulic and geomorphic processes shape terrestrial landscapes. It remains unclear how these processes act to generate observed drainage networks across scales of interest. To address this issue, we transform observed and synthetic longitudinal river profiles into the spectral domain with a view to interrogating the different scales at which fluvial landscapes are generated. North American river profiles are characterized by red noise (i.e. spectral power, ϕ ∝ k−2, where k is wavenumber) at wavelengths > 100 km and pink noise (ϕ ∝ k−1) at shorter wavelengths. This observation suggests that river profile geometries are scale-dependent and using small-scale observations to develop a general understanding of large-scale landscape evolution is not straightforward. At wavelengths > 100 km, river profile geometries appear to be controlled by smoothly varying patterns of regional uplift and slope-dependent incision. Landscape simulations, based upon stream power, that are externally forced by regional uplift do not exhibit a spectral transition from red to pink noise because these simulations do not incorporate heterogeneous erodibility. Spectral analysis of erodibility extracted from patterns of lithologic variation along river profiles suggests that the missing spectral transition is accounted for by heterogeneous substrates which are characterized by white or blue noise (ϕ ∝ k0 or k1). Our results have implications for the way by which rivers record large-scale tectonic forcing whilst incising through complex lithologic patterns. [226/250 words]
Lipp A, Roberts G, Whittaker A, et al., 2021, Unmixing river sediments for the elemental geochemistry of their source regions
QuyeSawyer J, Whittaker AC, Roberts GG, et al., 2021, Fault throw and regional uplift histories from drainage analysis: evolution of southern Italy, Tectonics, Vol: 40, Pages: 1-26, ISSN: 0278-7407
Landscapes can record elevation changes caused by multiple tectonic processes. Here, we show how coeval histories of spatially coincident normal faulting and regional uplift can be deconvolved from river networks. We focus on Calabria, a tectonically active region incised by rivers containing knickpoints and knickzones. Marine fauna indicate that Calabria has been uplifted by >1 km since ∼0.8–1.2 Ma, which we used to calibrate parameters in a stream power erosional model. To deconvolve the local and regional uplift contributions to topography, we performed a spatiotemporal inversion of 994 fluvial longitudinal profiles. Uplift rates from fluvial inversion replicate the spatial trend of rates derived from dated Mid-Late Pleistocene marine terraces, and the magnitude of predicted uplift rates matches the majority of marine terrace uplift rates. We used the predicted uplift history to analyze long-term fault throw, and combined throw estimates with ratios of footwall uplift to hanging wall subsidence to isolate the nonfault related contribution to uplift. Increases in fault throw rate—which may suggest fault linkage and growth—have been identified on two major faults from fluvial inverse modeling, and total fault throw is consistent with independent estimates. The temporal evolution of nonfault related regional uplift is similar at three locations. Our results may be consistent with toroidal mantle flow generating uplift, perhaps if faulting reduces the strength of the overriding plate. In conclusion, fluvial inverse modeling can be an effective technique to quantify fault array evolution and can deconvolve different sources of uplift that are superimposed in space and time.
Lipp AG, Roberts GG, 2021, Scale‐dependent flow directions of rivers and the importance of subplate support, Geophysical Research Letters, Vol: 48, Pages: 1-12, ISSN: 0094-8276
Large rivers play crucial roles in determining locations of civilization, biodiversity, and efflux to the oceans. The paths they take across Earth's surface vary with scale. At long‐wavelengths rivers can have simple flow paths. At smaller scales, in meanders for example, their paths change rapidly as a consequence of lithology, biota, and other environmental variables. It is not straightforward to identify the scales at which river planforms are set. We overcome these issues by developing a spectral (wavelet) methodology to map flow‐directions as a function of distance and scale. This methodology allows short‐wavelength features (e.g., meanders) to be filtered from river flow‐paths. With short‐wavelength structure removed, the flow‐directions of rivers in Western USA correlate with long‐wavelength gravity anomalies suggesting control by subplate support. This relationship is replicated by an ensemble of landscape evolution models. These results combined suggest that drainage at large scales, O(103) km, is set by subplate support.
Quye-Sawyer J, Whittaker AC, Roberts GG, 2020, Calibrating fluvial erosion laws and quantifying river response to faulting in Sardinia, Italy, Geomorphology, Vol: 370, Pages: 1-14, ISSN: 0169-555X
It is now widely accepted that rivers modify their erosion rates in response to variable rock uplift rates, resulting in changes in channel slope that propagate upstream through time. Therefore, present-day river morphology may contain a record of tectonic history. The simple stream power incision model can, in principle, be used to quantify past uplift rates over a variety of spatial and temporal scales. Nonetheless, the erosional model's exponents of area and slope (m and n respectively) and ‘bedrock erodibility’ (k) remain poorly constrained. In this paper, we will use a geologically and geomorphically well constrained Plio-Pleistocene volcanic landscape in central Sardinia, Italy, to calibrate the stream power erosion equation and to investigate the slip rate of faults that have been seismically quiescent in the historic past. By analysing digital elevation models, geological maps and Landsat imagery, we have identified the geomorphic expression of several volcanic features (eruption centres and basaltic lava flows) and three normal faults with 6 to 8 km fault traces within the outcrop. Downstream, river longitudinal profiles show a similar transient response to relative base level fall, probably as a result of relief inversion at the edge of the volcanic outcrop. From measurements of incision, local slope and upstream catchment area across eight different rivers, we calculate n ≈ 1, m = 0.50 ± 0.02 and, using a landscape age from literature of 2.7 Ma, bedrock erodibility k = 0.10 ± 0.04 m(1−2m) Myr−1. There are also knickpoints on rivers upstream of two normal faults, and we used numerical inverse modelling of the longitudinal profiles to predict the slip rate of these faults since 2.7 Ma. The results from the inverse model show that the erosional parameter values derived in this study can produce theoretical longitudinal profiles that closely resemble observed river profiles upstream of the faults. The lowest misfit
Lipp AG, Roberts GG, Whittaker AC, et al., 2020, River sediment geochemistry as a conservative mixture of source regions: observations and predictions from the Cairngorms,, UK, Journal of Geophysical Research: Earth Surface, Vol: 125, ISSN: 2169-9011
The elemental composition of sediments in rivers is the product of physical and chemical erosion of rocks, which is then transported across drainage networks. A corollary is that fluvial sedimentary geochemistry can be used to understand geologic, climatic, and geomorphic processes. Here, we predict elemental compositions of river sediments using drainage networks extracted from digital elevation data and erosional models. The Geochemical Baseline Survey of the Environment was used to quantify substrate (i.e., source region) chemistry. Sedimentary compositions in rivers downstream are predicted by formally integrating eroding substrates with respect to distance downstream. Different erosional models, including the Stream Power model and uniform incision rates, are tested. Predictions are tested using a new suite of compositions obtained from fine grained (<150 μm) sediments at 67 sites along the Spey, Dee, Don, Deveron, and Tay rivers, Cairngorms, UK. Results show that sedimentary geochemistry can be predicted using simple models that include the topography of drainage networks and substrate compositions as input. The concentration of numerous elements including Magnesium, Rubidium, Uranium, Potassium, Calcium, Strontium, and Beryllium can be accurately predicted using this simple approach. Predictions are insensitive to the choice of erosional model, which we suggest is a consequence of broadly homogeneous rates of erosion throughout the study area. Principal component analysis of the river geochemical data suggests that the composition of most Cairngorms river sediments can be explained by mafic/felsic provenance and conservative mixing downstream. These results suggest that the elemental composition of river sediments can be accurately predicted using simple erosional models and digital elevation data.
Fernandes VM, Roberts GG, 2020, Cretaceous to Recent net continental uplift from paleobiological data: Insights into sub-plate support, GSA Bulletin, Vol: 133, Pages: 1217-1236, ISSN: 0016-7606
There are many geoscience problems for which constraining histories of uplift or subsidence of Earth’s surface is of direct or indirect importance, for example reconstructing tectonics, mantle convection, geomorphology, sedimentary and chemical flux, biodiversity, glacio-eustasy, and climate change. The least equivocal constraints on timing and amplitude of vertical motions on geological timescales come from the distribution of rock formed in shallow marine environments. However, obtaining enough observations at sufficiently large spatial and temporal scales (∼100−10,000 km, ca. 1−100 Ma) to constrain histories of regional topographic evolution remains challenging. To address this issue, we adapted modern inventories of paleobiological and paleoenvironmental data to generate a new compilation of >24,000 spot measurements of uplift on all continents and numerous oceanic islands. Uncertainties associated with paleobathymetry, post-deposition compaction, and glacio-eustasy are assessed. The compilation provides self-consistent and, in places, high-resolution (<100-km-length scale, <1 Ma) measurements of Cretaceous to Recent (post-deposition) net uplift across significant tracts of most continents. To illustrate how the database can be used, records from western North America and eastern South America are combined with geophysical observations (e.g., free-air gravity, shear, and Pn-wave tomography) and simple isostatic calculations to determine the origins of topography. We explore how lithospheric thinning and mantle thermal anomalies may generate uplift of the observed wavelengths and amplitudes. The results emphasize the importance of large inventories of paleobiological data for understanding histories of tectonic and mantle convective processes and consequently landscapes, climate, and the environment.
Brewer C, Hampson G, Whittaker A, et al., 2020, Comparison of methods to estimate sediment flux in ancient sediment routing systems, Earth-Science Reviews, Vol: 207, ISSN: 0012-8252
The need to predict accurately the volume, timing and location of sediments that are transported from an erosional source region into a basin-depocentre sink is important for many aspects of pure and applied sedimentological research. In this study, the results of three widely used methods to estimate sediment flux in ancient sediment routing systems are compared, using rich input datasets from two systems (Eocene South Pyrenean Foreland Basin, Spain and late-Pleistocene-to-Holocene Gulf of Corinth Rift Basin, Greece) for which mapped, dated sediment volumes provide an independent reference value of sediment accumulation rates. The three methods are: (1) the empirical BQART model, which uses values of drainage basin area, relief, temperature, lithology and water discharge; (2) empirical scaling relationships between characteristic geomorphological parameters of sediment-routing-system segments; and (3) the “fulcrum” model, which uses the palaeohydrological parameters of trunk river channels to estimate downsystem sediment discharge. The BQART model and empirical geomorphological scaling relationships were originally developed using modern sediment routing systems, and have subsequently been applied to ancient systems. In contrast, the “fulcrum” model uses hydrological scaling relationships from modern systems, but was developed principally for application in ancient systems.Our comparative analysis quantifies the sensitivity of the three methods to their input parameters, and identifies the data required to make plausible estimates of sediment flux for ancient sediment routing systems. All three methods can generate estimates of sediment flux that are comparable with each other, and are accurate to at least one order of magnitude relative to independent reference values. The BQART model uses palaeoclimatic and palaeocatchment input data, which are accurate for sub-modern systems but may be highly uncertain in deep-time systems. Corresponding
Lipp A, Shorttle O, Syvret F, et al., 2020, Major-element composition of sediments in terms of weathering and provenance: Implications for crustal recycling, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 21, ISSN: 1525-2027
The elemental composition of a sediment is set by the composition of its protolith and modified by weathering, sorting, and diagenesis. An important problem is deconvolving these contributions to a sediment's composition to arrive at information about processes that operate on the Earth's surface. We approach this problem by developing a predictive and invertible model of sedimentary major element composition. We compile a data set of sedimentary rock, river sediment, soil, and igneous rock compositions. Principal component analysis of the data set shows that most variation can be simplified to a small number of variables. We thus show that any sediment's composition can be described with just two vectors of igneous evolution and weathering. We hence define a model for sedimentary composition as a combination of these processes. A 1:1 correspondence is observed between predictions and independent data. The log ratios urn:x-wiley:ggge:media:ggge22195:ggge22195-math-0001 and urn:x-wiley:ggge:media:ggge22195:ggge22195-math-0002 are found to be simple proxies for, respectively, the model's protolith and weathering indices. Significant deviations from the model can be explained by sodium‐calcium exchange. Using this approach, we show that the major element composition of the upper continental crust has been modified by weathering, and we calculate the amount of each element that it must have lost to modify it to its present composition. By extrapolating modern weathering rates over the age of the crust, we conclude that it has not retained a significant amount of the necessarily produced weathering restite. This restite has likely been subducted into the mantle, indicating a crust‐to‐mantle recycling rate of 1.33 ± 0.89 ×1013 kg·year−1.
Morris M, Fernandes VM, Roberts GG, 2020, Extricating dynamic topography from subsidence patterns: Examples from Eastern North America's passive margin, Earth and Planetary Science Letters, Vol: 530, Pages: 1-13, ISSN: 0012-821X
Global sea-level (eustatic) histories generated by backstripping stratigraphy are predicated upon the lithosphere having a well understood tectonic history. However, sub-plate processes play a role in governing lithospheric vertical motions with timescales and amplitudes akin to eustasy, which are difficult to predict. We examine how stratigraphic and geophysical observations combined with simple isostatic models can be used to disentangle histories of sub-plate support and eustasy. We focus on the passive margin of Eastern North America, where a generally accepted history of eustasy has been estimated. Negative long wavelength free-air gravity anomalies, residual ocean-age depth estimates, fast upper mantle shear wave velocities, and geodynamic models suggest that Cenozoic evolution of this passive margin has been influenced by upper mantle drawdown. We build on existing analyses to backstrip sixteen wells, which, combined with seismic data, constrain timing and extent of Cenozoic subsidence. Results indicate up to ∼1000 m of water-loaded subsidence between ∼20–0 Ma centered on the Baltimore Canyon Trough. Seismic data from the trough shows Neogene aggrading clinoforms. There is little evidence for faulting or stratigraphic growth, which indicates that Neogene lithospheric strain rates were low. Amplitude and spatial extent of Neogene subsidence are difficult to explain by glacio-eustasy or glacio-isostatic adjustment. Instead, sub-plate support calculated from conversion of shear wave velocities to temperature and isostatic calculations indicate that upper mantle drawdown was responsible for subsidence of the margin. Because mantle convection is vigorous such observations are expected throughout the stratigraphic archive.
Lodhia B, Roberts G, Fraser A, et al., 2019, Observation and simulation of solid sedimentary flux: examples from northwest Africa, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 20, Pages: 4613-4634, ISSN: 1525-2027
The sedimentary archive preserved at passive margins provides important clues about the evolution of continental topography. For example, histories of African uplift, erosion, and deposition of clastic sedimentary rock provide information about mantle convection. Furthermore, relating histories of uplift and erosion from regions where sediment is generated to measurements of efflux is important for understanding basin evolution and the distribution of natural resources. We focus on constraining Mesozoic to Recent solid sedimentary flux to northwest Africa's passive margin, which today is fed by rivers draining dynamically supported topography. Histories of sedimentary flux are calculated by mapping stratigraphy using seismic reflection and well data courtesy of Tullow Oil Plc and TGS. Stratigraphic ages, conversion from two‐way time to depth and compaction, are parameterized using biostratigraphic and check‐shot records from exploration, International Ocean Discovery Program and Deep Sea Drilling Project wells. Results indicate that Late Cretaceous to Oligocene (∼100–23 Ma) sedimentary flux decreased gradually. A slight increase in Neogene sedimentary flux is observed, which is concomitant with a change from carbonate to clastic sedimentation. Pliocene to Recent (∼5–0 Ma) flux increased by an order of magnitude. This history of sedimentary flux and facies change is similar to histories observed at other African deltas. To constrain sources of sedimentary flux, 14,700 longitudinal river profiles were inverted to calculate a history of continental uplift. These results were used to parameterize a simple “source‐to‐sink” model of fluvial erosion and sedimentary efflux. Results suggest that increased clastic flux to Africa's deltas from ∼30 Ma was driven by denudation induced by dynamic support.
Fernandes VM, Roberts GG, White N, et al., 2019, Continental-scale landscape evolution: a history of North American topography, Journal of Geophysical Research: Earth Surface, Vol: 124, Pages: 2689-2722, ISSN: 2169-9011
The generation and evolution of continental topography are fundamental geologic and geomorphic concerns. In particular, the history of landscape development might contain useful information about the spatiotemporal evolution of deep Earth processes, such as mantle convection. A significant challenge is to generate observations and theoretical predictions of sufficient fidelity to enable landscape evolution to be constrained at scales of interest. Here, we combine substantial inventories of stratigraphic and geomorphic observations with inverse and forward modeling approaches to determine how the North American landscape evolved. First, stratigraphic markers are used to estimate postdepositional regional uplift. Present‐day elevations of these deposits demonstrate that >2 km of long‐wavelength surface uplift centered on the Colorado‐Rocky‐Mountain plateaus occurred in Cenozoic times. Second, to bridge the gaps between these measurements, an inverse modeling scheme is used to calculate the smoothest spatiotemporal pattern of rock uplift rate that yields the smallest misfit between 4,161 observed and calculated longitudinal river profiles. Our results suggest that Cenozoic regional uplift occurred in a series of stages, in agreement with independent stratigraphic observations. Finally, a landscape evolution model driven by this calculated rock uplift history is used to determine drainage patterns, denudation, and sedimentary flux from Late Cretaceous times until the present day. These patterns are broadly consistent with stratigraphic and thermochronologic observations. We conclude that a calibrated inverse modeling strategy can be used to reliably extract the temporal and spatial evolution of the North American landscape at geodynamically useful scales.
Roberts G, 2019, Scales of similarity and disparity between drainage networks, Geophysical Research Letters, Vol: 46, Pages: 3781-3790, ISSN: 0094-8276
At large scales ( urn:x-wiley:grl:media:grl58789:grl58789-math-000110 km, urn:x-wiley:grl:media:grl58789:grl58789-math-00021 Ma) drainage networks appear to have a synchronized response to uplift and erosional processes. At smaller scales erosion generates complex landforms. Here, cross wavelet spectral transformation of longitudinal river profiles is performed to develop a framework that unifies these scale‐dependent views of landscape evolution. Distance‐elevation and time‐elevation profiles are transformed using a continuous wavelet approach to determine where signal power resides and appropriate scaling regimes. Cross wavelet spectral power is then calculated to determine scales of similarity and disparity between river channels. Spectral power of rivers draining Angola are compared to lithology, biota, precipitation, and gravity data to examine origins of river shapes and commonalities. Most power and commonalities reside at long wavelengths and timescales (>100 km, >1 Ma). The presence of commonalities between drainage networks is expected for systems in which large signals (e.g., uplift) are forced through random media (e.g., lithology, biota).
Roberts G, Mannion P, 2019, Timing and periodicity of Phanerozoic marine biodiversity and environmental change, Scientific Reports, Vol: 9, ISSN: 2045-2322
We examine how the history of Phanerozoic marine biodiversity relates to environmental change. Our focus is on North America, which has a relatively densely sampled history. By transforming time series into the time-frequency domain using wavelets, histories of biodiversity are shown to be similar to sea level, temperature and oceanic chemistry at multiple timescales. Fluctuations in sea level play an important role in driving Phanerozoic biodiversity at timescales >50 Myr, and during finite intervals at shorter periods. Subsampled and transformed marine genera time series reinforce the idea that Permian-Triassic, Triassic-Jurassic, and Cretaceous-Paleogene mass extinctions were geologically rapid, whereas the Ordovician-Silurian and Late Devonian ‘events’ were longer lived. High cross wavelet power indicates that biodiversity is most similar to environmental variables (sea level, plate fragmentation, δ18O, δ13C, δ34S and 87Sr/86Sr) at periods >200 Myr, when they are broadly in phase (i.e. no time lag). They are also similar at shorter periods for finite durations of time (e.g. during some mass extinctions). These results suggest that long timescale processes (e.g. plate kinematics) are the primary drivers of biodiversity, whilst processes with significant variability at shorter periods (e.g. glacio-eustasy, continental uplift and erosion, volcanism, asteroid impact) play a moderating role. Wavelet transforms are a useful approach for isolating information about times and frequencies of biological activity and commonalities with environmental variables.
Conway-Jones BW, Roberts GG, Fichtner A, et al., 2019, Neogene epeirogeny of Iberia, Geochemistry, Geophysics, Geosystems, Vol: 20, Pages: 1138-1163, ISSN: 1525-2027
The origin of Iberia's topography is examined by combining gravity, magmatic, topographic and seismological observations with geomorphic considerations. We have four principal results. First, highest coherence between free‐air gravity and topography is at wavelengths ≲250 km where admittance indicates that elastic thickness of Iberia's plate is 20 ± 3 km. These results imply that flexural and sub‐plate support of Iberian topography could be expressed at wavelengths of O(100) km. Secondly, P‐to‐S receiver functions and simple isostatic calculations indicate that whilst crustal thickness variations and flexural loading (e.g. as a result of plate shortening) partially explain the elevation of Pyrenean, Betics, Cantabrian, Spanish Central System and Iberian Chain topography, they fail to explain the elevation of large parts of Iberia. Thirdly, a new full waveform shear wave tomographic model and velocity to temperature conversions suggest that the asthenosphere beneath Iberia is anomalously slow and has excess temperatures of up to 162 ± 14°C. Simple isostatic calculations indicate asthenospheric support of topography of up to 1 km. Neogene‐Recent (∼23–0 Ma) extrusive magmatism (e.g. Calatrava, Catalan) sit atop many of the slow shear wave velocity anomalies. Finally, biostratigraphic data, combined with inversion of 3217 river profiles, show that most of Iberia's topography grew during the last ∼30 Ma at rates of up to 0.3 mm yr−1. Best‐fitting theoretical rivers have a low residual rms misfit (=0.96) and calculated uplift is consistent with an independent inventory of stratigraphic and biostratigraphic observations. We suggest that Neogene‐Recent growth of most of central Iberia's topography was a result of asthenospheric support.
Roberts GG, White N, Lodhia BH, 2019, The generation and scaling of longitudinal river profiles, Journal of Geophysical Research: Earth Surface, Vol: 124, Pages: 137-153, ISSN: 2169-9003
The apparent success of inverse modeling of continent‐wide drainage inventories is perplexing. An ability to obtain reasonable fits between observed and calculated longitudinal river profiles implies that drainage networks behave simply and predictably at length scales of O(102–103) km and timescales of O(100–102) Ma. This behavior suggests that rivers respond in a predictable way to large‐scale tectonic forcing. On the other hand, it is acknowledged that stream power laws are empirical approximations since fluvial processes are complex, non‐linear, and probably susceptible to disparate temporal and spatial shocks. To bridge the gap between these different perceptions of landscape evolution, we present and interpret a suite of power spectra for African river profiles that traverse different climatic zones, lithologic boundaries, and biotic distributions. At wavelengths ≳ 102 km, power spectra have slopes of −2, consistent with red noise, demonstrating that profiles are self‐similar at these length scales. At wavelengths ≲ 102 km, there is a cross‐over transition to slopes of −1, consistent with pink noise, for which power scales according to the inverse of wavenumber. Onset of this transition suggests that spatially correlated noise, perhaps generated by instabilities in water flow and by lithologic heterogeneities, becomes more prevalent at wavelengths shorter than ∼100 km. At longer wavelengths, this noise gradually diminishes and self‐similar scaling emerges. Our analysis is consistent with the concept that complexities of river profile development are characterized by an adaptation of the Langevin equation, by which simple advective models of erosion are driven by a combination of external forcing and noise.
Stucky de Quay G, Roberts GG, Rood DH, et al., 2019, Holocene uplift and rapid fluvial erosion of Iceland: a record of post-glacial landscape evolution, Earth and Planetary Science Letters, Vol: 505, Pages: 118-130, ISSN: 0012-821X
In actively deforming regions fluvial systems are strongly regulated by uplift. River geometries record histories of vertical motions that can be used to examine the driving forces generating topographic relief. Iceland's rapidly evolving landscapes provide an opportunity to disentangle histories of uplift generated by postglacial rebound, volcanism, dynamic support, and plate spreading. Broad knickzones observed along Iceland's large rivers, and its powerful waterfalls and deep canyons, hint that regional processes have generated significant relief. We combine high-resolution drone photogrammetry and cosmogenic 3He dating of fluvial terraces to measure the erosional history of one of Iceland's largest knickzones, Jökulsárglúfur, in the northeast part of the island. Progressive younging of terraces indicates knickpoint propagation rates of up to ∼70 cm a−1 during the last 8 ka. Knickpoint velocities appear to be controlled partly by toppling of basalt columns. These rates were used to calibrate a model that inverts Iceland's drainage networks for uplift rate histories. Calculated uplift and isostatic calculations indicate that rifting, sub-plate support, and isostatic adjustment resulted in tens to hundreds of meters of regional Holocene uplift. Our results suggest regional uplift and fluvial erosion can rapidly generate hundreds of meters of relief in post-glacial landscapes.
Roberts GG, White N, Hoggard M, et al., 2018, A neogene history of mantle convective support beneath Borneo, Earth and Planetary Science Letters, Vol: 496, Pages: 142-158, ISSN: 0012-821X
Most, but not all, geodynamic models predict 1–2 km of mantle convective draw-down of the Earth's surface in a region centered on Borneo within southeast Asia. Nevertheless, there is geomorphic, geologic and geophysical evidence which suggests that convective uplift might have played some role in sculpting Bornean physiography. For example, a long wavelength free-air gravity anomaly of +60 mGal centered on Borneo coincides with the distribution of Neogene basaltic magmatism and with the locus of sub-plate slow shear wave velocity anomalies. Global positioning system measurements, an estimate of elastic thickness, and crustal isostatic considerations suggest that regional shortening does not entirely account for kilometer-scale regional elevation. Here, we explore the possible evolution of the Bornean landscape by extracting and modeling an inventory of 90 longitudinal river profiles. Misfit between observed and calculated river profiles is minimized by smoothly varying uplift rate as a function of space and time. Erosional parameters are chosen by assuming that regional uplift post-dates Eocene deposition of marine carbonate rocks. The robustness of this calibration is tested against independent geologic observations such as thermochronometric measurements, offshore sedimentary flux calculations, and the history of volcanism. A calculated cumulative uplift history suggests that kilometer-scale Bornean topography grew rapidly during Neogene times. This suggestion is corroborated by an offshore Miocene transition from carbonate to clastic deposition. Co-location of regional uplift and slow shear wave velocity anomalies immediately beneath the lithospheric plate implies that regional uplift could have been at least partly generated and maintained by temperature anomalies within an asthenospheric channel.
Roberts GG, Lodhia B, Fraser A, et al., 2018, Continental margin subsidence from shallow mantle convection: example from West Africa, Earth and Planetary Science Letters, Vol: 481, Pages: 350-361, ISSN: 0012-821X
Spatial and temporal evolution of the uppermost convecting mantle plays an important role in determining histories of magmatism, uplift, subsidence, erosion and deposition of sedimentary rock. Tomographic studies and mantle flow models suggest that changes in lithospheric thickness can focus convection and destabilize plates. Geologic observations that constrain the processes responsible for onset and evolution of shallow mantle convection are sparse. We integrate seismic, well, gravity, magmatic and tomographic information to determine the history of Neogene-Recent (<23 Ma) upper mantle convection from the Cape Verde swell to West Africa. Residual ocean-age depths of +2 km and oceanic heat flow anomalies of +16 ± 4 mW m−2 are centered on Cape Verde. Residual depths decrease eastward to zero at the fringe of the Mauritania basin. Backstripped wells and mapped seismic data show that 0.4–0.8 km of water-loaded subsidence occurred in a ∼500 × 500 km region centered on the Mauritania basin during the last 23 Ma. Conversion of shear wave velocities into temperature and simple isostatic calculations indicate that asthenospheric temperatures determine bathymetry from Cape Verde to West Africa. Calculated average excess temperatures beneath Cape Verde are View the MathML source °C providing ∼103 m of support. Beneath the Mauritania basin average excess temperatures are View the MathML source °C drawing down the lithosphere by ∼102 to 103 m. Up- and downwelling mantle has generated a bathymetric gradient of ∼1/300 at a wavelength of ∼103 km during the last ∼23 Ma. Our results suggest that asthenospheric flow away from upwelling mantle can generate downwelling beneath continental margins.
Tribaldos VR, White NJ, Roberts GG, et al., 2017, Spatial and temporal uplift history of South America from calibrated drainage analysis, Geochemistry, Geophysics, Geosystems, Vol: 18, Pages: 2321-2353, ISSN: 1525-2027
A multidisciplinary approach is used to analyze the Cenozoic uplift history of South America.Residual depth anomalies of oceanic crust abutting this continent help to determine the pattern of present-day dynamic topography. Admittance analysis and crustal thickness measurements indicate that the elasticthickness of the Borborema and Altiplano regions is 10 km with evidence for sub-plate support at longerwavelengths. A drainage inventory of 1827 river proﬁles is assembled and used to investigate landscapedevelopment. Linear inverse modeling enables river proﬁles to be ﬁtted as a function of the spatial and tem-poral history of regional uplift. Erosional parameters are calibrated using observations from the BorboremaPlateau and tested against continent-wide stratigraphic and thermochronologic constraints. Our results pre-dict that two phases of regional uplift of the Altiplano plateau occurred in Neogene times. Regional uplift ofthe southern Patagonian Andes also appears to have occurred in Early Miocene times. The consistencybetween observed and predicted histories for the Borborema, Altiplano, and Patagonian plateaux impliesthat drainage networks record coherent signals that are amenable to simple modeling strategies. Finally,the predicted pattern of incision across the Amazon catchment constrains solid sedimentary ﬂux at the Fozdo Amazonas. Observed and calculated ﬂux estimates match, suggesting that erosion and deposition weretriggered by regional Andean uplift during Miocene times.
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