AGE-DEPENDENT LARGE-SCALE FABRIC OF THE MANTLE LITHOSPHERE AS DERIVEDFROM SURFACE-WAVE VELOCITY ANISOTROPY

Systematic variations of the seismic radial anisotropy xi to depths of 200-250 km in North America and Eurasia and their surroundings are re lated to the age of continental provinces, and typical depth dependenc es of xi(R) are determined. The relative radial anisotropy xi(R) in th e mantle lithosphere of Phanerozoic orogenic belts is characterized by nu S-H > nu(SV), with its maximum depth of about 70 km, on the averag e, while beneath old shields and platforms, it exhibits a maximum devi ation from ACY400 model (MONTAGNER and ANDERSON, 1989) at depths of ab out 100 km with nu(SV)greater than or equal to nu v(SH) signature. An interpretation of the observed seismic anisotropy by the preferred ori entation of olivine crystals results in a model of the mantle lithosph ere characterized by anisotropic structures plunging steeply beneath o ld shields and platforms, compared to less inclined anisotropies benea th Phanerozoic regions. This observation supports the idea derived fro m petrological and geochemical observations that a mode of continental lithosphere generation may have changed throughout earth's history.