Dynamical analysis of continental overflow

Recent field and laboratory studies indicate that the viscosity of the midd le and lower continental crust may be 10(18) Pa s or less in regions of hig h heat flow. Such strikingly low viscosities must strongly facilitate gravi tational collapse of overthickened crust (thicker than would be possible wi thout lateral tectonic support). In such areas, the ductile zone is so weak that lateral flow in it can relax all lateral pressure gradients in a geol ogically short time. The ductile or "fluid" zone is, for geological purpose s, a fluid in hydrostatic equilibrium, floating a brittle cap of upper crus t above it. A continent built in this way can collapse by overflow onto adj acent ocean basins if the continental elevation exceeds a critical threshol d related to the mean geothermal gradient. This model permits relatively si mple identification of the terms of the energy budget of gravitational coll apse. Here I show that the budget is likely to be dominated by the balance between liberated gravitational energy (a source) and normal fault friction (a sink). Examination of this budget provides a stability criterion for gr avitational collapse by continental overflow onto ocean basins: that crusta l elevation above the ocean basin beyond an amount between a third and a ha lf of the brittle cap thickness should drive a propagating overflow front. (C) 2001 Elsevier Science Ltd. All rights reserved.