Small-scale convection in a plume-fed low-viscosity layer beneath a moving plate

Abstract

Two-dimensional simulations using a thermomechanical model based on a finite-difference method on a staggered grid and a marker in cell method are performed to study the plumelithosphere interaction beneath moving plates. The plate and the convective mantle are modelled as a homogeneous peridotite with a Newtonian temperature-and pressure-dependent viscosity. A constant velocity, ranging from 5 to 12.5 cm yr −1 , is imposed at the top of the plate. Plumes are generated by imposing a thermal anomaly of 150 to 350 K on a 50 km wide domain at the base of the model (700 km depth); the plate atop this thermal anomaly is 40 Myr old. We analyse (1) the kinematics of the plume as it impacts the moving plate, (2) the dynamics of time-dependent small-scale convection (SSC) instabilities developing in the low-viscosity layer formed by spreading of hot plume material at the base of the lithosphere and (3) the resulting thermal rejuvenation of the lithosphere. The spreading of the plume material at the base of the lithosphere, characterized by the ratio between the maximum down-and upstream horizontal (dimensionless) velocities in the plume-fed sublithospheric layer, Pe up /Pe down depends on the ratio between the maximum plume upwelling velocity and the plate velocity, Pe plume /Pe plate . For fast plate velocities and sluggish plumes (low Pe plume /Pe plate ), plate motion drags most plume material and downstream flow is dominant. As Pe plume /Pe plate increases, an increasing part of the plume material flows upstream. SSC systematically develops in the plume-fed sublithospheric layer, downstream from the plume. Onset time of SSC decreases with the Rayleigh number. For vigorous plumes, it does not depend on plate velocity. For more sluggish plumes, however, variations in the plume spreading behaviour at the base of the lithosphere result in a decrease in the onset time of SSCs with increasing plate velocity. In any case, SSC results in uplift of the isotherm 1573 K by up to 20 km relative to its initial equilibrium depth at the impact point.

Lithospheric Thinning by Mantle Plumes

“…Farnetani & Richards, 1994;1995;Moore et al 1999;Herzberg et al 2007) In this study we use the recently-developed computational modelling framework, Fluidity (Davies et al 2011;Kramer et al 2011) to explore the dynamics and efficiency of SSC above a mantle plume, in 2-D and 3-D simulations with realistic Earth-like material properties and a sub-km local mesh resolution. Specifically, we will build on and complement earlier studies by, for example, Huang et al (2003), van Hunen et al (2003) and Agrusta et al (2013;. This project is undertaken as part of the completion of Honours at the Australian National University for the year 2016.…”

“…Study the effect, in 2-D and 3-D, of the morphology and onset time of SSC produced from shearing under a moving plate first without, then with, a plume. The behaviour of SSC in 2-D and 3-D is likely to be distinctive: longitudinal roll ('Richter rolls') structures, with their axes aligned parallel to plate motion are favoured in 3-D, whilst transverse roll structures, with their axis perpendicular to plate motion, are the only instabilities that can be simulated in 2-D Agrusta et al 2013;Davies et al, 2016). Also, faster onset time of SSC is expected in 3-D in comparison to 2-D.…”

“…Analytical predictions from Davies (1994) suggest that the lower the plume viscosity, the faster should be the thinning, and that narrower plume conduits (<100 km) would produce faster thinning, while, plume heads would not cause any significant thinning, which is inconsistent with an active rifting scenario (Courtillot et al 1999). Furthermore, the plume vigour, quantified by the Rayleigh number, is believed to negatively impact the onset time of SSC (Agrusta et al 2013). …”

“…Above plumeheads, thinning is thought to have been significant enough to produce substantial crust-derived melts and induce rifting (Campbell & Griffiths, 1990). The thermo-mechanical thinning of overlying lithosphere by a mantle plume is believed to be the result of the combined effect of mechanical removal by: (i) sub-lithospheric small-scale convection (SSC) (see Figure 1); (ii) lithospheric delamination; or (iii) lateral shear stresses resulting from horizontal flow in the asthenosphere away from the plume impact point and, to a lesser extent, of thermal rejuvenation due to heat conduction from anomalously hot plume (Detrick & Crough, 1978;Davies, 1994;Elkins-Tanton, 2005;Agrusta et al 2013). …”

Terrestrial aftermath of the Moon-forming impact

“…This parameter is not precisely constrained. For reference, Agrusta et al [30] At the low-viscosity end, we consider slurries, which designate a material that could pass through a water main pipe if cool. As described in §4c, we can calculate the viscosity of a convecting slurry to be 2 × 10 9 Pa s, which implies a shear modulus of 2 × 10 5 Pa for a Maxwell time of 10 4 s. This value for the shear modulus is too low, and thus the Maxwell model is unlikely to represent slurry.…”