Mechanical erosion and reheating of the lithosphere: A numerical model for hotspot swells

Abstract

It is currently debated if either thermal erosion of the lithosphere or dynamical support is the source of topography and geoid anomalies. The origin of tins controversy lies probably in the difficulty to model simultaneously these two effects. For this purpose we have studied the time dependent behavior of two‐dimensional convection with a temperature and pressure dependent viscosity. The use of a control volume method allows us to define a rigid zone simulating the mechanical lithosphere. The interface between the lithosphere and the convective mantle is determined by a viscosity cutoff. First, some experiments model the rise of a plume below the lithosphere in order to observe the evolution of the uplift and thus to appreciate the various processes involved in the swell formation. Before the plume reaches the base of the thermal lithosphere, an uplift a few hundred meters in amplitude develops which can only be ascribed to a pure dynamical support. The major uplift occurs when the ductile part of the lithosphere, the convective boundary layer, is squeezed by the plume. The reheating of the mechanical lithosphere takes place after this transient stage of dynamical erosion. However, this late process is very slow but can magnify the amplitude of the swell if the lithosphere stays long enough above the plume. These results shed some light on the different mechanisms occurring during the swell formation, but the configuration modeled does not correspond to the one expected for actual hotspot swells. They feature plume rising up to the lithosphere while natural situations correspond to lithosphere drifting above preexisting plumes. An experiment with a moving lithosphere was run and shows that thermal erosion does not affect significantly a moving lithosphere even for relatively slow drifting velocities (few centimeters/year). Indeed, the thermal structure of the lithosphere is not modified above the 800°C isotherm except for a motionless plate. In this case the resulting swell should be greater: this could explain why Azores, Crozet or Cap Verde swells are so high. On the other hand, the shape of a swell over a moving lithosphere is strikingly reminiscent of the Hawaiian swell.

Viscosity structure of the oceanic lithosphere inferred from the differential late Quaternary sea-level variations for the southern Cook Islands

“…Several recent numerical and laboratory studies, however, indicate that thermomechanical erosion of the lithosphere by secondary asthenospheric convection associated with a mantle plume seems to be a main factor in the formation of hotspot swells such as the Hawaiian swell (Yuen & Fleitout, 1985;Olson et al 1988; Robinson & Parsons 1988;Ceuleneer et al 1988; Monnereau & Cazenave 1990;Monnereau et al 1993;Davies 1994;Sleep 1994). Thus it may be reasonable to consider local internal loads in the upper mantle, as is the case here.…”

“…Differential crustal movement in this region may then provide constraints on the initial stage of the lithospheric erosion leading to crustal uplift. The convective process in the asthenosphere is related to the squeezing of the ductile part of the oceanic lithosphere numerically simulated by Yuen & Fleitout (1985) and Monnereau et al (1993).…”

“…The other model examined here has an mantle, which corresponds to the small-scale and secondary convection in the asthenosphere leading to thermomechanical lithospheric erosion. In particular, h e n & Fleitout (1985),Olson et al (1988) and Monnereau et a/. (1993) find the Relationship between the relative displacement during the past 125 kyr and relative mass ratio (density anomaly) for the earth model with a depth-dependent density structure for the LPL.…”

“…) and small-scale convection in the asthenosphere (e.g Yuen & Fleitout 1985;Monnereau et al 1993;Davies 1994…”

“…qu: upper mantle viscosity, qa: asthenospheric viscosity, ha: asthenospheric thickness, '11: LPL viscosity, ht LPL thickness, Hi: thickness of internal load, &: maximum uplift during past 125 kyr, Api : density momaly of internal load, m&: relative mass ratio erosion (e.g Yuen & Fleitout 1985;Olson et al 1988;Monnereau et al 1993)…”