A review of observations and models of dynamic topography

TLDR: This paper showed that there is good agreement between published residual topography fields, including the one described here, and present-day dynamic topography predicted from mantle flow models, including a new one, showing peak-to-peak amplitudes of roughly ± 2 km and a dominant degree two pattern with high values for the Pacific Ocean, southern Africa, and the North Atlantic and low values for South America, western North America, and Eurasia.

Abstract: The topography of Earth is primarily controlled by lateral differences in the density structure of the crust and lithosphere. In addition to this isostatic topography, flow in the mantle induces deformation of its surface leading to dynamic topography. This transient deformation evolves over tens of millions of years, occurs at long wavelength, and is relatively small ( 5000 km), we show that there is good agreement between published residual topography fields, including the one described here, and present-day dynamic topography predicted from mantle flow models, including a new one. Residual and predicted fields show peak-to-peak amplitudes of roughly ±2 km and a dominant degree two pattern with high values for the Pacific Ocean, southern Africa, and the North Atlantic and low values for South America, western North America, and Eurasia. The flooding of continental interiors has long been known to require both larger amplitudes and to be temporally phase-shifted compared with inferred eustatic changes. Such long-wavelength inferred vertical motions have been attributed to dynamic topography. An important consequence of dynamic topography is that long-term global sea-level change cannot be estimated at a single passive margin. As a case study, we compare the results of three published models and of our model to the subsidence history of well COST-B2 offshore New Jersey. The <400 ± 45 m amount of anomalous subsidence of this well since 85 Ma is best explained by models that predict dynamic subsidence of the New Jersey margin during that period. Explicitly including the lithosphere in future global mantle flow models should not only facilitate such comparisons between model results and data, but also further constrain the nature of the coupling between the mantle and the lithosphere.

Figures

TABLE 1. AMPLITUDE OF RESIDUAL AND DYNAMIC TOPOGRAPHY FIELDS

TABLE 2. STRENGHTS AND LIMITATIONS OF DYNAMIC TOPOGRAPHY MODELS

Full text

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!
1!
A review of observations and models of dynamic 1!
topography 2!
3!
Nicolas Flament
1
, Michael Gurnis
2
, and R. Dietmar Müller
1
4!
1
EarthByte Group, School of Geosciences, University of Sydney NSW 2006, Australia 5!
2
Seismological Laboratory, California Institute of Technology, Pasadena CA, USA
6!
7!
ABSTRACT 8!
The topography of the Earth is primarily controlled by lateral differences in the 9!
density structure of the crust and lithosphere. In addition to this isostatic topography, 10!
flow in the mantle induces deformation of its surface leading to dynamic topography. 11!
This transient deformation evolves over tens of millions of years, occurs at long 12!
wavelength and is relatively small (< 2 km) in amplitude. Here, we review the 13!
observational constraints and modeling approaches used to understand the amplitude, 14!
spatial pattern and time-dependence of dynamic topography. The best constraint on the 15!
present-day dynamic topography induced by sub-lithospheric mantle flow is likely the 16!
residual bathymetry calculated by removing the isostatic effect of oceanic lithospheric 17!
structure from observed bathymetry. Increasing knowledge of the thermal and chemical 18!
structure of the lithosphere is important to better constrain present-day mantle flow and 19!
dynamic topography. Nevertheless, at long wavelengths (> 5,000 km), we show that 20!
there is good agreement between published residual topography fields, including one 21!
described here, and present-day dynamic topography predicted from mantle flow models, 22!
including a new one. Residual and predicted fields show peak-to-peak amplitudes of 23!

!
2!
roughly ± 2 km and a dominant degree two pattern with high values for the Pacific 24!
Ocean, southern Africa and the North Atlantic and low values for South America, 25!
western North America and Eurasia. The flooding of continental interiors has long been 26!
known to require both larger amplitudes and to be temporally phase-shifted compared 27!
with inferred eustatic changes. Such long wavelength inferred vertical motions have been 28!
attributed to dynamic topography. An important consequence of dynamic topography is 29!
that long-term global sea level change cannot be estimated at a single passive margin. As 30!
a case study, we compare the results of three published models and of our model to the 31!
subsidence history of well COST-B2 offshore New Jersey. The < 400 ± 45 m of 32!
anomalous subsidence of this well since 85 Ma are best explained by models that predict 33!
dynamic subsidence of the New Jersey margin during that period. Explicitly including the 34!
lithosphere in future global mantle flow models should not only facilitate such 35!
comparisons between model results and data, but also further constrain the nature of the 36!
coupling between the mantle and the lithosphere. 37!
38!
INTRODUCTION!!39!
! Knowledge of the effect of mantle flow on surface topography has considerably 40!
increased over the last 30 years. The rapid improvement in computational algorithms and 41!
computing resources has facilitated the modeling of global mantle flow at increasing 42!
resolution, that now achieve Earth-like convective vigor. The dramatic expansion of 43!
global and regional seismic data sets and numerical methods have lead to commensurate 44!
improvements in the resolution and global coverage of mantle tomographic images (e.g. 45!
Romanowicz, 2008). In turn these have lead to an improved understanding of the 46!
thermo-chemical structure of the mantle (Grand, 2002; Masters et al., 2000; Simmons et 47!

!
3!
al., 2007). Independently, global plate reconstructions that are used to constrain the 48!
evolution of mantle flow and to interpret seismic images of the mantle have likewise 49!
improved and such reconstructions are now available back to the Triassic in million year 50!
increments (Seton et al., 2012). As a consequence, plate kinematics and seismic 51!
tomography can now be assimilated into both forward and backward mantle flow models. 52!
Exploiting such improvements, a range of known long-wavelength vertical motions of 53!
continental plates have been attributed to mantle flow and hence used as constraints on 54!
the spatial character, amplitude and time-dependence of mantle flow. Examples include 55!
the Cenozoic uplift of Southern Africa (Gurnis et al., 2000), the late Cenozoic uplift of 56!
the Colorado Plateau (Moucha et al., 2009), the Cretaceous subsidence and subsequent 57!
uplift of the interior of North America (Liu et al., 2008; Mitrovica et al., 1989), the tilt of 58!
northern South America to the East during the Miocene (Shephard et al., 2010), the tilt of 59!
Australia since the late Cretaceous (DiCaprio et al., 2009) and the vertical motions of the 60!
Slave and Kaapvaal Cratons since the Paleozoic (Zhang et al., 2012). An important 61!
concept that derives from such studies is that there is likely “no such thing as a stable 62!
continental platform” (Moucha et al., 2008a) which in turn implies that long-term global 63!
sea level (eustasy) change cannot be defined based on the detailed analysis of the 64!
stratigraphic record in a single area, as has been argued previously (e.g. Miller et al., 65!
2005). 66!
Nevertheless, multiple challenges still limit our understanding of global dynamic 67!
topography, including different definitions of the meaning of the terms “dynamic 68!
topography”, inaccurate estimates of present-day dynamic topography in the absence of a 69!
detailed global model of the structure of the lithosphere, and contradicting model 70!

Frequently asked questions

Q1. What are the contributions mentioned in the paper "A review of observations and models of dynamic topography" ?

Here, the authors review the observational constraints and modeling approaches used to understand the amplitude, spatial pattern, and time dependence of dynamic topography. Nevertheless, at long wavelengths ( > 5000 km ), the authors show that there is good agreement between published residual topography fields, including the one described here, and presentday dynamic topography predicted from mantle flow models, including a new one. As a case study, the authors compare the results of three published models and of their model to the subsidence history of well COST-B2 offshore New Jersey. This journal article is available at Research Online: https: //ro. uow. edu. au/smhpapers/4274 Explicitly including the lithosphere in future global mantle flow models should not only facilitate such comparisons between model results and data, but also further constrain the nature of the coupling between the mantle and the lithosphere.