Abstract:
On a large scale, seismic waves travel faster through old continental shields than through Paleozoic platforms. On a smaller scale, the continental lithosphere appears to be composed of blocks with different velocities. Sharp vertical contacts between different blocks of lithosphere are revealed by short-wavelength offsets in P-vertical travel times; some contacts have been preserved for more than one billion years. These observations cannot be explained by temperature differences alone and imply persistent mineralogical differences within the lithosphere. To reconcile global and local seismological observations, a bimodal model is required: the continental lithosphere appears to consist of cores of depleted, refractory lithosphere surrounded by and/or superposed on younger, more fertile lithosphere. The seismic characteristics and mineralogical compositions correlate on a large scale with the age of the lithosphere and reflect episodic growth of the continents. Temperatures in the lithosphere of stable continental regions vary less than commonly assumed and may be close to the geotherms proposed by Jaupart and Mareschal [Lithos 48 (1999) 93–114].