Chapter 5

Rethinking Tibet

Continental lithosphere tomographytomographya technique for representing graphically a subsurface plane. has produced excellent agreement on continental structure down to 100 kilometres. “The astonishing thing is that the old parts of the continents are twice as thick as anywhere else. And we are now beginning to understand how this happens.” Tibet has the thickest lithosphere on the planet. McKenzie now thinks that, during shortening, the roots of the continents persisted. They were buoyant because they had been depleted by melting; the loss of elements like iron resulted in a lower density. “And that seems to be true of all the roots of all the old continents”, notes McKenzie. “They’re all depleted in this way completely contrary to a paper I’d written with two other people [Greg Houseman and Philip England] on Tibet, where we’d argued that the boundary layer had detached, and that was why Tibet had such high heat flow … that’s completely wrong.”