Holographic measurements of surface topography in laboratory models of mantle hotspots

Abstract

SUMMARY The evolution of surface topography produced by the rise of a buoyant droplet (or diapir) towards the free surface of a very viscous fluid in laboratory experiments is monitored using holographic interferometry. Such experiments enable us to investigate implications for surface topography of one possible unsteady model for intraplate hotspots: the arrival of mantle thermals or diapirs near the base of the Earth's lithosphere. Our model is possibly of most direct relevance to the interaction with the Earth's surface and lithosphere of large spherical caps that are expected to rise at the head of new mantle plumes. To previous experimental results for the axial height of topography (Olson & Nam) we add further information on the height, shape and width of the surface swell, and on the evolution of the diapir itself. When the ambient fluid has uniform density and viscosity (no lithosphere), surface topography is determined by the diameter, density anomaly and depth of the droplet. As the diapir approaches the surface a broad axisymmetric surface swell appears, and initially increases in height while decreasing in width. When the leading edge of the diapir is 0.2 diapir diameters below the surface, the height passes through a maximum and the width through a minimum. The swell then proceeds to subside and increase in width as the diapir spreads beneath the surface. In separate experiments the lithosphere is modelled by a discrete surface layer of more viscous fluid whose thickness and viscosity contrast with the mantle are treated as independent parameters. Effects of lithosphere buoyancy relative to the mantle, a property which may influence continental hotspot swells, are also studied. Within the parameter range used, the maximum swell height is independent of the lithosphere viscosity contrast but decreases with increasing lithosphere thickness and with decreasing lithosphere density. Surface uplift produced by the rise of two consecutive diapirs is shown to be more complex. Comparison of results with the characteristics of hotspot swells is attempted and should assist in determining the nature of hotspot plumes.