Oceanic residual depth measurements, the plate cooling model, and global dynamic topography

Abstract

Convective circulation of the mantle causes deflections of the Earth's surface that vary as a function of space and time. Accurate measurements of this dynamic topography are complicated by the need to isolate and remove other sources of elevation, arising from flexure and lithospheric isostasy. The complex architecture of continental lithosphere means that measurement of present‐day dynamic topography is more straightforward in the oceanic realm. Here we present an updated methodology for calculating oceanic residual bathymetry, which is a proxy for dynamic topography. Corrections are applied that account for the effects of sedimentary loading and compaction, for anomalous crustal thickness variations, for subsidence of oceanic lithosphere as a function of age and for non‐hydrostatic geoid height variations. Errors are formally propagated to estimate measurement uncertainties. We apply this methodology to a global database of 1936 seismic surveys located on oceanic crust and generate 2297 spot measurements of residual topography, including 1161 with crustal corrections. The resultant anomalies have amplitudes of ±1 km and wavelengths of ∼1000 km. Spectral analysis of our database using cross‐validation demonstrates that spherical harmonics up to and including degree 30 (i.e., wavelengths down to 1300 km) are required to accurately represent these observations. Truncation of the expansion at a lower maximum degree erroneously increases the amplitude of inferred long‐wavelength dynamic topography. There is a strong correlation between our observations and free‐air gravity anomalies, magmatism, ridge seismicity, vertical motions of adjacent rifted margins, and global tomographic models. We infer that shorter wavelength components of the observed pattern of dynamic topography may be attributable to the presence of thermal anomalies within the shallow asthenospheric mantle.

Reassessing the Thermal Structure of Oceanic Lithosphere With Revised Global Inventories of Basement Depths and Heat Flow Measurements

“…Anomalies calculated using our optimal plate model are symmetric about zero irrespective of plate age with negligible skewness and a standard deviation of ±0.65 km (Figure c). In this case, the pattern, amplitude and wavelength of residual depth anomalies is similar to those determined by Hoggard et al ().…”

“…Oceanic age‐depth database. (a) Map showing global distribution of 2,028 water‐loaded depths to oceanic basement from Hoggard et al (); circles = data with both sedimentary and crustal corrections; upward/downward pointing triangles = lower/upper estimates of depth for which only sedimentary corrections are applied; yellow lines offshore northwest Africa and offshore east India = location of example seismic reflection profiles shown in (b) and (c), respectively; light/dark gray background shading = young/old oceanic plate age. (b) Seismic reflection profile offshore Guinea‐Bissau, northwest Africa, courtesy of Spectrum Geo.…”

“…Thermal models that predict the development of oceanic lithosphere must be consistent with independent constraints on axial temperature structure derived from either the thickness of oceanic crust or the geochemistry of mid‐ocean ridge basalts (McKenzie et al, ). Global compilations of marine seismic experiments yield an average crustal thickness of 6.9 ± 2.2 km (Hoggard et al, ; White et al, ). Within our melting parameterization, this range of thickness is produced when the potential temperature is 1331 ± 35°C.…”

On the Scales of Dynamic Topography in Whole‐Mantle Convection Models

“…These processes produce dynamic topography over a range of spatial scales that cannot yet be taken into account in 3-D global models because of computational limitations. The discrepancy between numerical models of dynamic topography and observation-derived present-day dynamic topography remains debated (e.g., Hoggard et al, 2017Hoggard et al, , 2016Molnar et al, 2015;Steinberger et al, 2017;Watkins & Conrad, 2018;Yang & Gurnis, 2016;Yang et al, 2017). The dynamic topography spectra that we obtained (Figure 7) are comparable to those derived by recent inversions of present-day mantle structures (Yang & Gurnis, 2016) to simultaneously fit the long-wavelength geoid, free-air gravity anomalies, gravity gradients, and residual topography point data (Hoggard et al, 2016), and of a high-resolution upper mantle tomography model (Steinberger et al, 2017).…”

“…Our models predict significant intermediate to small scales of dynamic topography, with a spectral distribution similar to the global residual topography spectrum of Hoggard et al (2016) at spherical harmonic degrees larger than 5 (Figure 7). Indeed, Hoggard et al, (2016Hoggard et al, ( , 2017 proposed from models of residual topography based on seismic profiles in the oceanic realm and free-air gravity anomalies in the continental domain that large-scale dynamic topography has an amplitude of ±300 m at most. Nevertheless, we note a significant discrepancy at low spherical harmonic degrees (1-4) between global model dynamic topography spectra, including ours, and the residual topography spectrum of Hoggard et al (2016), for which power is 10 times lower (Figure 7).…”