Strength of the continental lithosphere: Time to abandon the jelly sandwich?

For the past two decades, many people (including me) have embraced the view that the continental lithosphere is like a jelly sandwich, with a weak lower crust lying between a strong upper crust and a strong uppermost mantle. A recent reassessment of earthquake depth distributions and gravity anomalies on the continents makes it difficult to maintain this view, suggesting instead that the seismogenic layer may be the only significant source of strength in the continental lithosphere, and that the upper mantle beneath the continents is relatively weak. This change of view, if it is correct, has several implications for continental geodynamics: (1) patterns of surface faulting on the scale of a few hundred kilometers are likely to be controlled by the anisotropic strength of crustal blocks and their intervening faults; (2) flexure of the Indian shield is likely to be a major support of the topography in the Himalaya and southern Tibet; and (3) transient lowercrustal flow, of the type associated with metamorphic core complexes, is likely to be controlled by the input of igneous melts and fluid into the lower crust. Overall, this new view suggests that continental tectonics and mechanics are controlled by strength that resides mainly in the crust, rather than in the mantle.