Dynamic Topography Estimates Research Articles

The Influence of Mantle Structure on Dynamic Topography in Southern Africa

AbstractDue to relatively high terrain and negligible active tectonics, the southern Africa region boasts over 30 independent estimates of dynamic topography. These published estimates display a wide variance due to both the variety of methods used in computation and a lack of constraints on the regional mantle structure. Here we show that a focus on regional mantle structure is important to generate models of lithospheric and mantle dynamics. Global average mantle properties are not representative of a particular region, and it is necessary to generate viscosity profiles specific to a region of interest. We develop a Bayesian inversion using dynamic geoid kernels, existing seismic tomography models, and Slepian functions to invert for a localized radial viscosity profile that best explains the geoid in southern Africa. With an understanding of viscosity uncertainty, we constrain dynamic topography in southern Africa to lie between 1,000 and 2,000 m. Additionally, we model vertical displacements from 112 Global Navigation Satellite System stations across our region to examine the long‐term, long wavelength pattern of present‐day vertical motion, suggesting that a mean of 1.5 mm/yr (1: 0.8–2.0 mm/yr) of vertical motion may be related to ongoing dynamic topography. Our study demonstrates the utility of dynamic geoid kernels in local nonlinear inversions of non‐unique geophysical data. Furthermore, we present evidence that the mantle beneath southern Africa is generating significant dynamic support for and vertical displacement of the lithosphere in this region.

Effects of using the consistent boundary flux method on dynamic topography estimates

SUMMARY Dynamic topography is defined as the deflection of Earth's surface due to the convecting mantle. ASPECT (Advanced Solver for Planetary Evolution, Convection, and Tectonics) is a continually evolving, finite element code that uses modern numerical methods to investigate problems in mantle convection. With ASPECT version 2.0.0 a consistent boundary flux (CBF) algorithm, used to calculate radial stresses at the model boundaries, was implemented into the released version of ASPECT. It has been shown that the CBF algorithm improves the accuracy of dynamic topography calculations by approximately one order of magnitude. We aim to evaluate the influence of the CBF algorithm and explore the geophysical implications of these improved estimates of dynamic topography changes along the East Coast of the United States. We constrain our initial temperature conditions using the tomography models SAVANI, S40RTS and TX2008, and combine them with a corresponding radial viscosity profile (2 for TX2008) and two different boundary conditions for a total of eight experiments. We perform simulations with and without the CBF method, which takes place during post-processing and does not affect the velocity solution. Our dynamic topography calculations are spatially consistent in both approaches, but generally indicate an increase in magnitude using the CBF method (on average ∼15 and ∼76 per cent absolute change in present-day instantaneous and rate of change of dynamic topography, respectively). This enhanced accuracy in dynamic topography calculations can be used to better evaluate the effects of mantle convection on surface processes including vertical land motions, sea level changes, and sedimentation and erosion. We explore results along the US East Coast, where a Pliocene shoreline has been deformed by dynamic topography change. An increased accuracy in estimates of dynamic topography can improve Pleistocene and Pliocene sea level reconstructions, which allow for a better understanding of past sea level changes and ice sheet stability.

Mantle Structure Beneath the Damara Belt in South‐Central Africa Imaged Using Adaptively Parameterized P‐Wave Tomography

AbstractMany seismic tomography studies have indicated that the African Large Low Velocity Province (LLVP) extends from the lower mantle beneath southern Africa into the upper mantle beneath eastern Africa; however, it has been questioned whether the LLVP structure may also extend to the north or northwest beneath south‐central Africa. Debates regarding the upper mantle structure beneath the Damara Belt contribute to this uncertainty. Some studies suggest the Damara Belt is underlain by thermally perturbed upper mantle; however, other studies indicate the region is not associated with anomalous structure. Here, we use a comprehensive P‐wave travel‐time data set and an adaptive model parameterization to develop a new tomographic model for the Damara Belt and surrounding regions. Our results show that seismically slow structure beneath the Damara Belt is relegated to depths greater than ∼1,200 km, indicating that the LLVP is not significantly affecting this region. However, further to the northeast, the LLVP structure obliquely rises and crosses the mantle transition zone near the Irumide Belt, where it then extends into the upper mantle. The seismic structure beneath the Damara Belt and neighboring areas in our model correlates well with tectonic observations at the surface, including variations in heat flow, the distribution of geothermal features, the locations of rifts, and estimates of dynamic topography.

Shipborne GNSS-Determined Sea Surface Heights Using Geoid Model and Realistic Dynamic Topography

With an increasing demand for accurate and reliable estimates of sea surface heights (SSH) from coastal and marine applications, approaches based on GNSS positioning have become favored, to bridge the gap between tide gauge (TG) and altimetry measurements in the coastal zone, and to complement offshore altimetry data. This study developed a complete methodology for jointly deriving and validating shipborne GNSS-determined SSH, using a geoid model and realistic dynamic topography estimates. An approach that combines the properties of hydrodynamic models and TG data was developed to obtain the latter. Tide gauge data allow estimating the spatiotemporal bias of a hydrodynamic model and, thus, linking it to the used vertical datums (e.g., a novel geoid-based Baltic Sea Chart Datum 2000). However, TG data may be erroneous and represent different conditions than offshore locations. The qualities of spatiotemporal bias are, hence, used to constrain TG data errors. Furthermore, a rigid system of four GNSS antennas was used to ensure SSH accuracy. Besides eliminating the vessel’s attitude effect on measurement data, the rigid system also provides a means for internal validation, suggesting a 4.1 cm height determination accuracy in terms of standard deviation. The methodology also involves eliminating the effect of sea state conditions via a low-pass filter and empirical estimation of vessel sailing-related corrections, such as the squat effect. The different data validation (e.g., examination of residual values and intersection analyses) results, ranging from 1.8 cm to 5.5 cm in terms of standard deviation, indicate an SSH determination accuracy of around 5 cm.

Modelling the role of dynamic topography and eustasy in the evolution of the Great Artesian Basin

AbstractWidespread flooding of the Australian continent during the Early Cretaceous, referred to as the Eromanga Sea, deposited extensive shallow marine sediments throughout the Great Artesian Basin (GAB). This event had been considered ‘out of sync’ with eustatic sea level and was instead solely attributed to dynamic subsidence associated with Australia's passage over eastern Gondwanan subducted material. However, mantle convection models previously used to explain this event have since been shown to overestimate dynamic topography amplitude by a factor of two compared with residual topography estimates. Previous models were also based on a Cretaceous eustatic sea level peak at ca. 90 Ma in conventional eustatic sea level curves; however, more recent estimates of global sea level from ocean basin volume (OBV) suggest this peak may have occurred earlier at ca. 120 Ma. Our work links time‐dependent erosion and deposition with dynamic topography and eustasy to test their contribution to basin development using the landscape evolution code pyBadlands. Our results show that the lower amplitude estimates of dynamic topography derived from pseudo‐compressible mantle flow models better reflect the Cretaceous vertical motions of the Australian continent (ca. 100 m) compared with their incompressible counterparts (ca. 200–400 m). Additionally, our models include the Neogene north‐eastward tilting of Australia, elusive in most previously published geodynamic models. In conjunction with an OBV‐derived sea level curve, our preferred landscape evolution model broadly matches the Cretaceous inundation patterns and first‐order sedimentary sequences in the GAB. The results highlight that the Early Cretaceous inundation of the Australian continent is likely a combination of high global sea levels and the regional effects of dynamic subsidence. Our work provides a framework for a new generation of evolving paleogeographic models at continental scales, while also providing key insights into the viability of existing sea level curves and dynamic topography estimates for reproducing topographic and basin evolution.

Geodynamic and Surface Process Evolution of New Guinea Since the Jurassic

Regional scale flooding of New Guinea has occurred episodically since the Jurassic. The most recent flooding event during the Miocene occurred despite falling long-term eustatic sea levels. Recent work has suggested dynamic topography, the long-wavelength low-amplitude topographic response to mantle flow, as a factor in the emergence and flooding of this region, and therefore influencing the depositional history of New Guinea basins. The link between deep Earth and surface processes has not yet been explored for this region. We use forward numerical models coupling plate kinematics, mantle convection, paleogeography and eustasty to investigate the time-dependent topographic response of the New Guinea margin. Dynamic topography estimates derived from mantle convection models are then coupled with surface process modelling code Badlands to study the landscape evolution of New Guinea and the adjacent Australian continent. Reproducing the inundation history of New Guinea, our models show that continental scale dynamic topography plays a significant role in the development of drainage systems, and erosion-deposition regimes. Our work demonstrates the necessity in linking geological processes that operate across wide spatial and temporal scales to better understand how the interplay between deep Earth and surface processes control the source to sink evolution of basins.

Emergence/Subsidence Histories Along the Carnegie and Cocos Ridges and Their Bearing Upon Biological Speciation in the Galápagos

AbstractUsing plate motion reconstructions for the Nazca, Cocos, and South American plates in relation to the Galápagos hotspot, we find that age‐depth dependence of bathymetry, Galápagos plume dynamic topography, crustal relaxation, and magmatic production permit reasonable estimates for the subsidence of former islands along the Carnegie and Cocos Ridges. Our dynamic topography estimates are partially based upon geodynamic theory (fluid mechanics and elasticity) and also upon detailed bathymetric observations and analysis. For three different target areas over Carnegie Ridge and two over Cocos Ridge, we infer subaerial landmasses over the past ~20 Ma. We provide rough estimates of the surface areas of these paleo‐archipelagos, which were comparable to that of the present‐day Galápagos archipelago. We construct a history of emerged (paleo‐island) topography along the conjugate pair Carnegie‐Cocos hotspot tracks and infer that significant subaerial landscapes existed for at least the past ~20 Ma. The genetic ancestors of endemic faunal species in Galápagos (e.g., marine iguanas) migrated away from South America, genetically diverging from their land counterparts much earlier than the age of the present Galápagos Islands (~3 Ma). At 16.5 Ma, the total emerged landmass on the Carnegie Ridge alone may have been ~2.5 times that of the current Galápagos archipelago. (The conjugate landmass along the Cocos Ridge has been subducted.) We propose that this large landmass, at least one‐fourth the size of Iceland, be called Darwinia in recognition of its importance in the evolution of some Galápagos endemic flora and fauna made famous by Darwin's explorations.

Geochemical and geophysical constrains on the dynamic topography of the Southern African Plateau

AbstractThe deep mantle African Superswell is considered to contribute to the topographic uplift of the Southern African Plateau, but dynamic support estimates vary wildly depending on the approach and data used. One reason for these large disparities is that the role of lithospheric structure, key in modulating deep dynamic contributions to elevation, is commonly ignored or oversimplified in convection studies. We use multiple high‐quality geophysical data coupled with xenolith‐based geochemical constraints to compute the isostatic lithospheric contribution to the elevation of the Plateau, facilitating isolation of the current dynamic component from the total observed elevation. We employ a multiobservable stochastic algorithm to invert geoid anomaly, surface‐wave dispersion data, magnetotelluric data, and surface heat flow to predict elevation in a fully thermodynamically and internally‐consistent manner. We find that a compositionally layered 230 ± 7 km thick lithosphere is required to simultaneously fit all four data types, in agreement with abundant independent xenolith evidence. Our stochastic modeling indicates a lithospheric contribution to elevation of the order of 670 m, which implies dynamic support arising from the convecting sublithospheric mantle of ∼650 m. Our results have important implications for the understanding of lithospheric‐deep mantle feedback mechanisms and for calibrating dynamic topography estimates from global convection studies.

Mean dynamic topography estimates purely based on GOCE gravity field models and altimetry

The quality of mean dynamic topography (MDT) models derived from an altimetric mean sea surface and a gravity field model mainly depends on the spatial resolution and accuracy of the particular gravity field model. We use an integrated approach which allows for estimating the MDT and its (inverse) covariance matrix on a predefined grid which is one of the requirements for ocean data assimilation. The quality and accuracy of the MDT directly reflects the quality and accuracy of the used gravity field model. For the first time, MDT estimates along with its full error covariance matrix based on Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) data can be provided. We demonstrate the progress accomplished with GOCE processing and the valuable contribution of the GOCE gravity field models regarding the estimation of the MDT by showing results based on altimetric observations of Jason-1 and Envisat in combination with different GOCE gravity field models for the North Atlantic.

Spatial and temporal patterns of Cenozoic dynamic topography around Australia

Despite its importance, the spatial and temporal pattern of dynamic topography generated by mantle convective circulation is poorly known. We present accurate estimates of dynamic topography from oceanic basins and continental margins surrounding Australia. Our starting point is measurement of residual depth anomalies on the oldest oceanic floor adjacent to the continental shelf. These anomalies were determined from a combined dataset of ~200 seismic reflection and wide‐angle images of well‐sedimented oceanic crust. They have amplitudes of between −1 km and +0.5 km, and their spatial variation is broadly consistent with long‐wavelength free‐air gravity and shallow seismic tomographic anomalies. Along the Northwest Shelf, a regional depth anomaly of −300 to −700 m intersects the adjacent continental shelf. The temporal evolution of this anomaly was determined by analyzing the stratigraphic architecture of an extensive carbonate platform, which fringes the shelf and records a dramatic switch from progradation to aggradation during Neogene times. Three‐dimensional seismic mapping calibrated by boreholes was used to calculate water‐loaded subsidence histories at rollover points of clinoforms along the shelf. At 9 ± 3 Ma, the rate of subsidence increases from 5 to up 75 m Myr−1, generating a subsidence anomaly of −300 to −700 m. The amplitude of this anomaly varies along the shelf and cannot be generated by glacio‐eustatic sea‐level variation. Instead, we propose that a regional subsidence episode, which affects both the proximal shelf and the distal oceanic basin, was generated by convective drawdown. By combining our results with other published estimates of uplift and subsidence, a map of Australia, which shows the spatial and temporal pattern of dynamic topography is presented. Most, but not all, of Australia's epeirogeny can be attributed to rapid northward motion of the Australian plate over a pre‐existing pattern of convective circulation.

Impact of Accurate Geoid Fields on Estimates of the Ocean Circulation

Abstract The impact of new geoid height models on estimates of the ocean circulation, now available from the Gravity Recovery and Climate Experiment (GRACE) spacecraft, is assessed, and the implications of far more accurate geoids, anticipated from the European Space Agency’s (ESA) Gravity and Ocean Circulation Explorer (GOCE) mission, are explored. The study is based on several circulation estimates obtained over the period 1992–2002 by combining most of the available ocean datasets with a global general circulation model on a 1° horizontal grid and by exchanging only the EGM96 geoid model with two different geoid models available from GRACE. As compared to the EGM96-based solution, the GRACE geoid leads to an estimate of the ocean circulation that is more consistent with the Levitus temperature and salinity climatology. While not a formal proof, this finding supports the inference of a substantially improved GRACE geoid skill. However, oceanographic implications of the GRACE model are only modest compared to what can be obtained from ocean observations alone. To understand the extent to which this is merely a consequence of a not-optimally converged solution or if a much more accurate geoid field could in principle play a profound role in the ocean estimation procedure, an additional experiment was performed in which the geoid error was artificially reduced relative to all other datasets. Adjustments occur then in all elements of the ocean circulation, including 10% changes in the meridional overturning circulation and the corresponding meridional heat transport in the Atlantic. For an optimal use of new geoid fields, improved error information is required. The error budget of existing time-mean dynamic topography estimates may now be dominated by residual errors in time-mean altimetric corrections. Both these and the model errors need to be better understood before improved geoid estimates can be fully exploited. As is commonly found, the Southern Ocean is of particular concern.

The geoid constraint in global geodynamics: viscosity structure, mantle heterogeneity models and boundary conditions

SUMMARY The Earth's non-hydrostatic gravity field, or geoid, provides a first-order constraint on mantle density structure and dynamics. Geodynamic models for the geoid have proliferated since the advent of seismic mapping of mantle heterogeneity structure (tomography) because the geoid offers perhaps the best-measured independent constraint on mantle density heterogeneity. However, dynamic geoid models involve a number of questionable physical assumptions and uncertainties whose effects need to be evaluated before geodynamic inferences based upon the geoid can be considered sound. Troubling issues include the appropriate surface boundary conditions (free-slip, no-slip, plates?) and parametrization of radial viscosity variations (how many layers can be resolved?), in addition to lateral viscosity variations, possible chemical layering of the mantle, phase transitions, etc. There are also uncertainties in the density heterogeneity models used as ‘input’ to dynamic geoid models, most of which are derived from seismic tomography and require weakly constrained, empirical conversion factors to go from seismic velocity variations to density variations. Here we address several of the most straightforward problems inherent in geoid modelling, namely the issues of viscosity structure resolution, uncertainties in appropriate boundary conditions, and differences among mantle heterogeneity models. A robust feature of all models is a lower-mantle viscosity at least a factor of 30 greater than that of the upper mantle, but there is little resolution with regard to finer details such as lithospheric or uppermost mantle (‘low-viscosity zone’) viscosity. Ironically, free-slip boundary conditions result in the best fits to the geoid in all cases, but all boundary conditions exhibit predictable trade-offs with the uppermost-mantle viscosity. Models with a single viscosity layer representing the lower mantle yield similar dynamic topography estimates of the order of 700–1000 m in amplitude, regardless of the finer details of upper-mantle viscosity structure, boundary conditions or input heterogeneity models. Comparing mantle heterogeneity models based on two independent seismological determinations (Harvard and Berkeley models) and on the history of subduction, we find that these models are virtually indistinguishable regarding inferences of mantle viscosity structure and amplitude of dynamic topography, and in terms of the effects of different boundary conditions. Uncertainties concerning which type of boundary condition is appropriate are much more important than which mantle heterogeneity model is chosen. Given other uncertainties in modelling the geoid, particularly the strong effects due to lateral viscosity variations for intermediate (<10 000 km) wavelengths, we conclude that the class of dynamic geoid models explored so far cannot reliably elucidate the details of upper-mantle viscosity structure.

Extension and Validation of a Gulf Stream Geosat Synthetic Geoid

An extended synthetic geoid for the western North Atlantic Ocean was constructed by employing Geosat altimeter data, concurrent dynamic model forecasts, and climatology. Estimates of the absolute dynamic topography from the altimeter were compared to estimates of the dynamic topography computed from independent in situ temperature measurements. The rms difference between the two topographic estimates was 0.104 m in the Gulf Stream meander and ring region, 0.081 m in the Sargasso Sea, and 0.098 m overall in this portion of the western North Atlantic ocean. The position of the mean Gulf Stream axis, the 1-σ width of the meander envelope, and the extremes of the meander envelope were determined from the altimetric data. The yearly mean AVHRR- (Advanced Very High Resolution Radiometer) derived surface north walls for 1987 and 1988 were approximately 30 km north of the yearly mean surface maximum velocity axes for the same years. This 30-km offset, however, is affected by the different satellite sampling schemes and the AVHRR data processing techniques. Separation distances derived from individual comparisons of nearly concurrent Geosat, AVHRR, and AXBT (air expendable bathythermograph) datasets result in an average (rms) offset of 17 (±12) km between the axis and the surface north wall, and an 11- (±8) km offset between the axis and the subsurface north wall. The individual axis/surface north wall offsets and their variability were found to increase with increasing anti-cyclonic curvature, consistent with the theoretical effect of centripetal acceleration on a meandering constant potential vorticity jet. This synthetic geoid, validated in portions of the western North Atlantic within a vertical accuracy of about 0.10 m, can be used with the planned Geosat Follow-On mission in the late 1990s to calculate the absolute dynamic topography in near real time.

Measurements of dynamic topography during synop utilizing a Geosat synthetic geoid

Dynamic topography estimates were obtained for the Gulf Stream region employing a previously derived synthetic geoid and Geosat radar altimeter measurements. Ascending tracks passed through the central and eastern arrays of the Synoptic Ocean Prediction experiment. By utilizing the geoid estimate instead of the mean altimetric signal, the axis position and velocity can be computed. A time series of the dynamic height profiles crossing the Gulf Stream and a table of the positions of the axis of the Stream along two tracks are given. Time series of the maximum velocity perpendicular to the ground track were computed for both arrays. It was found that there was a seasonal cycle in this time series with a maximum in November.

Global circulation from SEASAT altimeter data [SST and dynamic topography estimates] : Engelis, Theo, 1985 Mar. Geod., 9(1):45–69 Dept. of Geod. Sci. and Surveying, Ohio State Univ., Columbus, OH, USA

Global circulation from SEASAT altimeter data [SST and dynamic topography estimates] : Engelis, Theo, 1985 Mar. Geod., 9(1):45–69 Dept. of Geod. Sci. and Surveying, Ohio State Univ., Columbus, OH, USA

Global circulation from SEASAT altimeter data

Abstract A set of time‐averaged sea surface heights at 1° intervals, derived from the adjusted SEASAT altimeter data, and the GEML2 gravity field are used to estimate the long‐wavelength stationary sea surface topography. In order to reduce the leakage of energy in the estimated sea surface topography, the GEML2 field is augmented by the Rapp81 gravity field to generate geoidal undulations with wavelengths consistent with the ones of sea surface heights. These undulations are subtracted from the sea surface heights, and the resulting differences are subjected to filtering in order to recover sea surface topography with minimum wavelengths of 6000 km and an estimated accuracy of 20–25 cm. These estimates agree well with oceanographic and other satellite‐derived results. The direction of current flow can be computed on a global basis using the spherical harmonic expansion of sea surface topography. This is done not only for the SEASAT/GEML2 estimates, but also using the recent dynamic topography estimates o...