MANTLE DRIPS AND THEIR IMPACT ON MOHO TOPOGRAPHY AND DEFORMATION OF THE LOWER CRUST

Gravitational instability of the continental mantle lithosphere has been proposed as an explanation for the evolution of dynamic topography, seismic structure and magmatic history in locations as diverse as Tibet, the Carpathians, and the western USA. The signals are subtle perhaps because the lithospheric mantle drips that develop as part of a Rayleigh-Taylor instability may be decoupled from the surface by a low viscosity layer in the lower crust. The question of whether continental crust is strong relative to the mantle lithosphere has recently generated some controversy, and indeed there is good evidence that in some places it is strong. There are also observations, however, which suggest that a low-viscosity channel is ubiquitous in the lower continental crust, consistent with our understanding of the thermal activation of ductile flow laws. We describe new numerical calculations of Rayleigh-Taylor instability of the lithospheric mantle layer and its impact on lower crustal deformation and Moho topography. The Moho in general is pulled downward above a mantle drip. In the absence of a low-viscosity lower crustal layer, the Moho topography varies smoothly with an amplitude that depends on the ratio of crust/mantle viscosity, density and thickness. When a low viscosity layer is present above the Moho, however, significant lateral displacements of the lower crust may be induced as a mantle drip develops, and a cuspate depression of the Moho may form. After the culmination of the instability, the load that created the Moho topography diminishes, and the topography relaxes back to near-equilibrium. As it does so, the lower crustal material enclosed by the cusp in the Moho surface is pushed upward, potentially setting up the conditions for formation of a metamorphic core complex. We examine the circumstances (and specifically the viscosity-depth function) for which these cuspate features may be expected to form on the Moho, and how they may be detected.