Small-scale gravitational instabilities under the oceans: Implications for the evolution of oceanic lithosphere and its expression in geophysical observables

Abstract

Sublithospheric small-scale convection (SSC) is thought to be responsible for the flattening of the seafloor depth and surface heat flow observed in mature plates. Although the existence of SSC is generally accepted, its ability to effectively produce a constant lithospheric thickness (i.e. flattening of observables) is a matter of debate. Here we study the development and evolution of SSC with a 2D thermomechanical finite-element code. Emphasis is put on (i) the influence of various rheological and thermophysical parameters on SSC, and (ii) its ability to reproduce geophysical observables (i.e. seafloor depth, surface heat flow, and seismic velocities). We find that shear heating plays no significant role either in the onset of SSC or in reducing the lithospheric thickness. In contrast, radiogenic heat sources and adiabatic heating exert a major control on both the vigour of SSC and the thermal structure of the lithosphere. We find that either dislocation creep, diffusion creep, or a combination of these mechanisms, can generate SSC with rheological parameters given by laboratory experiments. However, vigorous SSC and significant lithospheric erosion are only possible for relatively low activation energies. Well-developed SSC occurs only if the first $300 km of the mantle has an average viscosity of &10 20 Pa s; higher values suppress SSC, while lower values generate unrealistic high velocities. Seismic structures predicted by our models resemble closely tomography studies in oceanic mantle. However, the fitting to observed seafloor topography and surface heat flow is still unsatisfactory. This puts forward a fundamental dichotomy between the two datasets. This can be reconciled if most of the observed flattening in seafloor topography is influenced by processes other than SSC.

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

“…[11] Previous studies have demonstrated the important role of radiogenic heat production in controlling the thermal thickness of OL and the cooling of the sublithospheric upper mantle by SSC [e.g., Jarvis and Peltier, 1980;Huang and Zhong, 2005;Zlotnik et al, 2008], and therefore we do not address this point here. We note, however, that all our simulations include a realistic radiogenic heat production value of 1.8 Â 10 À8 W m À3 [Labrosse and Jaupart, 2007].…”

“…Neglecting this term increases the density difference between upwellings and downwellings, and consequently the vigour and eroding capacity of SSC (Figure 2c). This may result in underestimations of the steady-state thermal thickness of OL by as much as 25-50 km [see also Zlotnik et al, 2008].…”

“…They also demonstrate that under most circumstances, the viscosity increase due to mantle dehydration at MORs has first-order effects in the vigour of SSC, on its ability to thermally erode the OL, and hence on determining its final thermal structure and associated geophysical observables (Figure 2b). Unfortunately, one must acknowledge that dynamic systems such as the one described in this work are extremely dependent on their viscosity structure [e.g., Zlotnik et al, 2008], which in turn depends on experimental parameters that are subject to large uncertainties. This partially may be the cause why other studies on SSC under oceans failed to slow down seafloor subsidence and produce a near constant surface heat flow [Korenaga and Jordan, 2002].…”

“…Despite this general acceptance, most of the studies on SSC have been focused on the general conditions for the existence of it and on its associated onset times [e.g., Davaille and Jaupart, 1994;Solomatov and Moresi, 2000;Huang et al, 2003;Dumoulin et al, 2005, and references therein]. Specific numerical models devised to asses its feasibility as responsible for the flattening of seafloor and surface heat flow are only a few [Dumoulin et al, 2001;Korenaga and Jordan, 2002;Huang and Zhong, 2005;Zlotnik et al, 2008]. These studies consider the OL to be the cold thermal boundary layer above a convective adiabatic mantle, and therefore all physical properties are mainly controlled by the thermal structure.…”

The thermochemical structure of the lithosphere and upper mantle beneath south China: Results from multiobservable probabilistic inversion

“…The base of the lithosphere (i.e., conduction‐dominated layer) is identified as the 1250°C isotherm. This is consistent with numerical simulations of small‐scale convection with realistic viscosities [e.g., Zlotnik et al ., ; Afonso et al ., ; Ballmer et al ., ] indicating that isotherms above ~1250°C tend to be dominated by advection. The sublithospheric thermal structure is obtained by linear interpolation between (inverted) values of the main nodes T buffer , T inter , and T bottom (Figure ).…”