Effects of compositional and rheological stratifications on small‐scale convection under the oceans: Implications for the thickness of oceanic lithosphere and seafloor flattening

Abstract

[1] Pressure-release melting at mid-ocean ridges generate compositional and rheological layering in the oceanic mantle that may control the evolution of the oceanic lithosphere. We use dynamic models coupled with melting and petrological models to explore 1) the influence of this layering on the development of small-scale convection under the oceans, 2) its role in determining the thickness of oceanic lithosphere, and 3) its feasibility as responsible for the deviations of seafloor and surface heat flow from predictions by conductive models in mature oceanic lithosphere. Here we show that the existence of smallscale convection is entirely compatible with experimental creep parameters and flow laws, and that the viscosity stratification due to melt extraction (i.e., H 2 O removal) is the main factor controlling the plate's thermal evolution, its asymptotic thickness, and the flattening of seafloor and surface heat flow at ages^70 Ma. The effects of Al-rich phase transitions and compositional layering are minor.

Mantle temperature as a control on the time scale of thermal evolution of extensional basins

“…This is consistent with the parameterization of Phipps Morgan (1997), and approximately consistent with the viscosity-depletion increase employed by Afonso et al (2008) even though the latter only assumes the effect of dehydration of viscosity. Together with temperature, stress and pressure, viscosities are given by:…”

“…The success of this simple model in explaining the evolution of the oceanic lithosphere is due to the rheological change caused by melt extraction during the formation of the oceanic crust: Removal of melt in the upper 60-80 km during decompression of the ascending mantle leads to a viscosity increase of 1-2 orders of magnitude due to removal of water and increasing Mg content, as well as a density reduction by up to 1% (Afonso et al, 2008;Phipps Morgan, 1997). This restricts convection to occur below this chemical boundary layer, and the thermal relaxation becomes plate-like.…”

LitMod3D: An interactive 3‐D software to model the thermal, compositional, density, seismological, and rheological structure of the lithosphere and sublithospheric upper mantle

“…In the buffer zone, the temperature varies linearly from the value at the bottom of the lithosphere (i.e., Ta = 1600 K) to T buffer = 1673 K at the bottom of the buffer layer. In order to be consistent with both the adiabatic temperature profile at the reference MOR column [Afonso et al, 2008a] and mantle convection models [Solomatov and Moresi, 2000;Zaranek and Parmentier, 2004;Afonso et al, 2008b], the thickness of the buffer layer varies automatically to maintain the heat balance (i.e., energy conservation) between the basal heat input (assumed constant), internal heat generation, and surface heat release in the system.…”

The hydrothermal power of oceanic lithosphere

“…Following Crosby et al (2006) and Hillier and Watts (2005), we consider it likely that seafloor around 100-130 Ma (10-11.5 Ma 1/2 ) is anomalous. Of particular note, the subsidence rate becomes negative around these ages, which cannot be accomplished by passive cooling processes and is not known to occur by means of small-scale convection beneath old seafloor Afonso et al, 2008). Thus, we consider models GH and GHC to better reflect the normal behavior of oceanic lithosphere.…”

“…Since heat flux may be calculated if the effective thermal expansivity and volume heat capacity are known (Eq. 9), future efforts should focus on linking detailed models of near-ridge environments (e.g., Cherkaoui et al, 2003;Maclennan, 2008;Craft and Lowell, 2009;Theissen-Krah et al, 2011) with comprehensive mineral physics and hydrogeological models of the crust and lithosphere (e.g., Afonso et al, 2007Afonso et al, , 2008Hasterok, 2010;Goutorbe and Hillier, 2013;Grose and Afonso, 2013).…”