Folding of Viscous Plumes Impinging On A Density Or Viscosity Interface
No TL;DR found
Abstract
SUMMARY The possibility that slabs of subducted oceanic lithosphere may buckle and fold under longitudinal compressive stresses when they encounter the major seismic discontinuity near a depth of 670 km in the mantle has led us to explore experimentally the behaviour of very viscous plumes falling onto a density interface or a viscosity step. We begin with the case of axisymmetric and planar streams falling through air onto the free surface of a viscous fluid which has the same or smaller density, and observe the conditions under which they coil or fold. In order to remove surface tension and to introduce effects of a viscous environment we study plumes falling onto a density interface between two (generally very viscous) liquid layers. Stability to buckling is found to depend largely on a geometrical criterion based on the length to thickness ratio of plumes, but also the tendency to buckle is greater for large density differences across the interface and larger viscosity contrasts between plume and upper layer. When plumes are unstable, coils or folds are laid down with a horizontal scale in a fixed proportion to the thickness of the plume, and the frequency of folding is determined by the velocity and thickness of the slab as it enters the region of compressive stress. Entrainment of upper layer fluid can have a marked effect on the fate of the folded plume material, which may spread at the interface or continue to descend into the lower layer. Similar behaviour is observed at a viscosity interface. Application of results to the mantle depends on the nature of the 670 km discontinuity and is therefore inconclusive. However, cool slabs are likely to be unstable if mantle circulation involves two layers separated by a density interface or if whole-mantle circulation involves a large viscosity increase with depth near 700 km.