The Elastic Thickness Controversy
Earthquakes in mid-plate settings rarely occur below 40 km depth, indicating that the physical and chemical conditions prevailing in deeper rocks cannot sustain brittle failure. In support of this observation, the equivalent elastic thickness of the continents inferred from free-air gravity data is typically less than 40 km. In contrast, estimates of flexural loading of the lithosphere require elastic conditions to prevail to depths of 40-100 km over time periods of many thousands of years. Hence the paradox – how is it possible for the Earth to support loads elastically at great depth and over long periods when the crust fails seismically at shallow depth and at short periods?

Flow in the Lower Crust
For many years, laboratory measurements of high-temperature creep of rock-forming minerals have been used to infer that crustal minerals should deform more readily than olivine at the same temperature. This has led to the "jam sandwich" image of a brittle upper crust and weak ductile lower crust, and a stronger, if also ductile, upper mantle. Topography and the distribution of deformation near the earth’s surface concur with this image, at least for regions like the Basin and Range Province and Tibet. At the same time, space geodetic measurements suggest that viscous deformation within or adjacent to the lithosphere is fast, and a weak lower crust does not participate. So, to what extent does a jam sandwich emulate the rheology of continental lithosphere?