Standard models of the stress that can be supported by the continental lithosphere (encapsulated in the so-called Christmas-tree diagram) suggest that the deep continental crust is relatively weak compared to the mantle lithosphere, primarily because of the effect of thermal activation. Such models, however, are based on laboratory measurements that must be extrapolated over roughly seven orders of magnitude, in both spatial and temporal scales, to say nothing of the effects of variable crustal composition and, in particular, varying concentration of water. Direct measures of crustal strength, however, may be obtained from geodetic measurements of strain rate and stress estimates that are calibrated against the effect of gravity on an inferred density structure. Crustal strength in this case is defined by apparent viscosity: the ratio of stress difference to strain-rate. Such measurements, though less precise than laboratory measurements, are directly applicable on the time and length scales on which lithosphere deforms. When lithospheric deformation is driven by internal buoyancy forces in tectonically active areas the implied viscosity of the mantle lithosphere is on the order of 1021 Pa s or less. In such systems, deformation of the crust follows that of the mantle lithosphere, and the resulting spatial variations in crustal thickness and surface topography provide direct constraints on the relative crustal viscosity. The general stability of continental lithosphere suggests that it is relatively strong compared to typical intra-plate deviatoric stress, but may be weakened by stress, temperature, or fluids. Analysis of tectonic systems from a number of regions also shows that there are very large regional differences in the apparent strength of the crust. In the Transverse Ranges of California, for example, steep topographic gradients imply that the crust is relatively strong compared to the mantle. In contrast the relatively uniform elevation of the Tibetan Plateau is often taken to imply relatively low viscosity in the lower levels of the Tibetan crust. Steep gradients of crustal thickness cannot be maintained when the lower crust is weak; they are rapidly equilibrated by flow in the mantle and crustal layers.

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