Upwelling plumes, superswells and true polar wander

Abstract

S U M M A R YThe geological evolution of the rotational axis of the Earth is most likely controlled by internal mass redistribution within the mantle. Palaeomagnetic observations suggest that it is episodic in nature, with periods of quasi-standstill alternating with periods of faster wander. Here, we investigate two models for the influence of mantle plumes that vary at different spatial wavelengths on the time variations of the rotational axis (true polar wander; TPW). In the first model, we represent an upwelling plume as a sphere whose radius varies as a function of the flux of material in the conduit and that traverses the mantle at the Stokes velocity. Such a plume produces very little wander of the rotational axis. We then study the effects of two superswells that mimic the ones observed with seismic tomography and conclude that a doming regime within the mantle involves significant polar wander. Some of the features of this TPW that are directly linked to the periodicity of doming are reminiscent of observed phases of slow and fast TPW, with similar peak velocities.True polar wander (TPW) is the coherent motion of Earth with respect to its rotation axis. In view of the angular momentum theorem, it may be interpreted physically as the motion of the rotational axis with respect to the fixed terrestrial frame described by hotspots. On geological timescales, this motion is most likely controlled by internal mass redistribution within the mantle.Palaeomagnetic evidence from recent Earth history suggests that this motion has been small. Palaeomagnetic studies combine apparent polar wander based on palaeomagnetic data (the palaeomagnetic reference frame) with motions of hotspots with respect to lithospheric plates (the hotspot reference frame). Given the relative motions between the plates, the position of these plates with respect to the rotation axis (provided by palaeomagnetic results), and the relative motions between plates and a reference frame attached to the mantle, TPW can be defined as the motion of the mantle with respect to the rotation axis. A recent study (Besse & Courtillot 2002), hereafter referred to as BC02, confirms earlier findings that TPW appears to be episodic in nature, with periods of quasi-standstill alternating with periods of faster TPW (Fig. 1). Specifically, the path back to 200 Ma shows a standstill at 160-130 Ma, then a circular track from 130 to 70 Ma and finally a standstill at 50-10 Ma. As a result of the uncertainties in models of hotspot kinematics prior to 130 Ma, the behaviour of the TPW prior to that time is doubtful. The major event since 130 Ma is the end of the 130-60 Ma period of relatively fast polar wander (where the TPW rate averages 30 km Ma −1 ), with a standstill (i.e. no or little TPW) from 50 Ma (possibly as early as 80 Ma because of larger 95 per cent confidence circles) to 10 Ma. It seems that TPW may have been negligible for approximately 50 Ma, but accelerated a few millions years ago, with a velocity of the order of 100 km Ma −1 ; this recent acceleration ...

Reorientation of planets with lithospheres: The effect of elastic energy

“…Finally, we note that in studies of the Earth's long-term rotational stability (e.g., Ricard et al, 1993;Steinberger and O'Connell, 1997;Richards et al, 1999;Greff-Lefftz, 2004) it has been assumed that the rotational bulge will ultimately (i.e., in the secular limit of the governing equations) adjust to any new orientation of the rotation axis. That is, it has been assumed, explicitly or implicitly, that there is no stabilization by a remnant rotational bulge.…”

True Polar Wander: A Key Indicator for Plate Configuration and Mantle Convection During the Late Neoproterozoic

“…In this study, we used the simple approach described in Rouby et al (2010) and Greff-Lefftz andBesse (2012, 2014) to model the large-scale pattern of mantle dynamics and calculate induced TPW through time. The structure of our Earth model is as follows: the mantle and the core are assumed to be homogeneous in density (4,455.75 and 10,988 kg/m 3 ); the lithosphere and the upper and lower mantles are viscous (1.1 × 10 22 , 10 21 , and 40 × 10 21 Pa·s), and the core is a homogeneous inviscid fluid.…”

Long-term polar motion on a quasi-fluid planetary body with an elastic lithosphere: Semi-analytic solutions of the time-dependent equation

“…Instead of the linear approximation, the present formulation is mainly grounded on the quasi-fluid approximation (Spada et al, 1992a(Spada et al, , 1996aRicard et al, 1993;Richards et al, 1997Richards et al, , 1999Greff-Lefftz, 2004;Tsai and Stevenson, 2007;Rouby et al, 2008). I extend this theory in the absence of a fossil shape to apply to a planetary body with an elastic lithosphere that retains a fossil shape.…”

“…The resultant behavior for an internal load would be basically similar even if the driving force of TPW is not a load on a surface but one inside a deeper part beneath the lithosphere, such as the case of subducting slabs (e.g., Spada et al, 1992a;Richards et al, 1997) or upwelling plumes (e.g., Greff-Lefftz, 2004;Rouby et al, 2008). For an internal origin, we have to estimate the internal load Love number instead of the surface one (e.g., Ricard et al, 1993;Spada et al, 1993Spada et al, , 1996a and then apply the present theory only by replacing the MoI tensor perturbation due to surface loading with that due to internal loading.…”