An analysis of young ocean depth, gravity and global residual topography

Abstract

S U M M A R YThe variation of ocean depth with age in the absence of crustal thickening and dynamic support places valuable constraints on the thermal and rheological properties of the lithosphere and asthenosphere. We have attempted to estimate this variation using a global data set of shiptracks, with particular emphasis on young ocean floor. In this respect, this paper extends a previous study published in this journal by the same authors, which concentrated on the older parts of the ocean basins. We find that, prior to 80 Ma, subsidence patterns are reasonably consistent, with gradients of 325 ± 20 m Ma −1/2 and zero-age depths of 2600 ± 200 m. There is a strong inverse correlation between zero-age depth and the gradient of depth with the square root of age which is unrelated to local variations in dynamic support. Global depth-age trends to 160 Ma are not significantly different to those for the individual ocean basins. Within corridors of similar basement age, gravity-topography correlations are consistently 30 ± 5 mGal km −1 . Simple isostatic theory and numerical modelling of mantle plumes suggests that, if the minimum depth of convection is defined by the base of the mechanical boundary layer, the admittance should be a function of plate age. The observation that it is not implies that the active convective upwelling beneath young lithosphere ceases at the same depth as it does beneath old oceanic plates. This result is consistent with geochemical modelling of melts near mid-ocean ridges. We have examined the relationship between residual topography and gravity worldwide, and have found that good spatial correlations are restricted to the Atlantic, North Pacific and youngest Indian ocean basins. By contrast, residual topography and gravity are poorly or negatively correlated in the South and young North Pacific Ocean and in the older Indian Ocean. Away from regions of thick crust and flexure, histograms of residual topography and gravity have symmetric distributions about zero. We then use this residual topography to estimate the volume and buoyancy flux of seven major plume swells. In Hawaii, the clear correlation between melt and swell volumes in discrete age corridors is evidence that the horizontal velocity of the hot plume material far downstream from the plume is similar to the plate spreading velocity and that the plume pulses over time. Finally, comparison with seismic tomographic models suggests that the long-wavelength (>2000 km) residual topographic and gravity anomalies have an origin deeper than 250 km. This result is consistent with observations that the admittance is approximately constant at wavelengths longer than 800 km.

Double layering of a thermochemical plume in the upper mantle beneath Hawaii

“…van Ark and Lin, 2004;Robinson and Eakins, 2006) and supports a hotspot swell of height ∼1 km and width ∼1200 km (cf. Wessel, 1993;Crosby and McKenzie, 2009).…”

Small-scale convection beneath oceans and continents

“…Seismic velocity at the asthenosphere-lithosphere transition [31] and closely spaced parallel hotspot tracks [32,33] provide constraints that I discuss in subsection 5.3 and section 6 after discussing physics of convection. Gravity data confirm that the systematic relationship between depth and plate age as well as regional variations in depth result from density differences at lithospheric and asthenospheric depths [29,34]. Such data, however, provide little resolution on the lithosphere-asthenosphere transition.…”

“…Convection is not in the stagnant-lid regime. Some caveats apply to (34). It does not apply as steady state is approached, as the geotherm is significantly different than the half-space geotherm (7).…”

Toolbox for Analysis of Flexural Isostasy (TAFI)—A MATLAB toolbox for modeling flexural deformation of the lithosphere

“…On continents, elastic plate models have also been applied to analyze isostasy in continental rifts, mountain belts, and sedimentary basins (e.g., Karner and Watts, 1983;Watts et al, 1982;Chase and Wallace, 1988;Forsyth, 1985;McNutt et al, 1988;Flemings and Jordan, 1989;Weissel and Karner, 1989;Stern and ten Brink, 1989;ten Brink et al, 1997). Elastic plate models have also been used to assess glacial isostatic rebound (e.g., Stern et al, 2005) and to analyze isostasy on scales spanning continents and ocean basins (Banks et al, 1977;Watts and Burov, 2003;McKenzie and Bowin, 1976;Watts, 1978;McNutt and Menard, 1982;Lowry and Smith, 1994;Djomani et al, 1995;McKenzie and Fairhead, 1997;Petit and Ebinger, 2000;Flück et al, 2003;Crosby and McKenzie, 2009).…”