Continental and oceanic geotherm families parametric in surface heat flow intersect the mantle solidus at a depth coincident with the top of the seismic low-velocity zone, thus allowing surface heat-flow variations to be used to map the thickness of the lithosphere on a global scale. Our thermal models predict a lithospheric thickness of a few tens of kilometres in young oceans and continental orogenic provinces and more than 300 km in shield areas. The variable thermal structure of the mantle implies a greater viscosity beneath shields, which offers an explanation for the observed retarded motions of plates that bear shields.

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