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Variations in the structure and rheology of the lithosphere. J. Jackson (1*) (1) University of Cambridge, Bullard Laboratories, Madingley Road, Cambridge CB3 0EZ, UK *Corresponding authorː [email protected] Differences in the structure, composition and rheology of the lithosphere between the oceans, young orogenic belts and the ancient Precambrian shields are responsible for first-order variations in tectonic history seen at the Earth’s surface over geological time. The last decade has seen a number of developments in the understanding of the lithosphere, some of which have challenged previously accepted views, and opened up many new directions in research. A coherent picture is now emerging that reconciles observations from fields as diverse as seismology, gravity, heat flow, rock mechanics, metamorphic petrology and geochemistry. The principal points of this new view are as follows: 1) Earthquakes in the mantle are confined to regions colder than about 600oC. 2) With very few exceptions, earthquakes everywhere are confined to a single seismogenic layer which, in the oceans is limited by the 600oC isotherm, in young orogenic belts is typically limited to the upper crust (~350oC), and in ancient shields may include the whole crust (in material as hot as 600oC. An apparent exception is in the Himalaya, where the seismogenic lower crust of India underthrusts the seismogenic upper crust of Tibet, giving a bimodal depth distribution, but one that is not in steady-state and has no generic significance for continental rheology. 3) Where it is well resolved, the elastic thickness is everywhere less than the seismogenic thickness, and nowhere do the data require it to exceed the seismogenic thickness. This observation is consistent with long-term strength residing in the seismogenic layer, and regions of active deformation, the mantle generally plays no role in the long-term support of loads on the continents. 4) Lateral strength changes in the continents between ancient shields and young orogenic regions are important and cannot be represented by a laterally uniform continental rheology. They allow mountains to be supported by their adjacent forelands without requiring the mantle beneath the forelands to be strong. 5) The great strength of the ancient shields, responsible for lower crustal earthquakes and larger elastic thickness than in younger continental lithosphere, is related to the composition of the lower continental crust, which is probably dominated by a dry granulite facies mineral assemblage. 6) The cause of the granulite metamorphism is likely to be continental collision, resulting in extreme crustal thickening and internal heat production, causing melting and the removal of water by the extraction of granite. This was probably common in the Archean, when radiogenic production was greater, but may be happening today in Tibet. 7) Many of the ancient shields of the continents are characterized by extreme lithosphere thicknesses, which may reach 250 or more km. Surface-wave seismic tomography now allows us to map the distribution of such regions. The stability of the thick lithosphere is related to its chemical depletion during melting, which occurred prior to its subsequent thickening. 8) The stability and survival of the ancient shields and cratons over geological time is thus related to both their strength and buoyancy, neither of which can easily be changed.