Seismic properties of an asthenospherized lithospheric mantle: constraintsfrom lattice preferred orientations in peridotite from the Ronda massif

Above a mantle plume, the lithosphere is thermally 'eroded'. It is however not clear whether heating and partial melting of the lithosphere may erase the mineral lattice preferred orientation (LPO) inherited from previous tec tonic events or if, in the absence of large-scale flow, this fabric is pres erved. To evaluate the effect of heating and partial melting on the seismic properties of the lithospheric mantle, we have measured the LPO and comput ed the seismic properties of peridotites from the Ronda massif (Spain). In this massif, a narrow (less than or equal to 400 m) coarsening front separa tes a porphyroclastic peridotite domain, interpreted as old lithospheric ma ntle, from a coarse-granular peridotite domain produced by annealing and li mited partial melting (< 6.5%) of the porphyroclastic peridotites. The oliv ine LPO in the porphyroclastic peridotites is moderate. The [100] and [001] axes are distributed within the foliation with a maximum of [100] parallel to the lineation, and the [010] axes are concentrated close to the normal to the foliation. The olivine LPO does not vary drastically across the coar sening front: the LPO strength decreases slightly and symmetry of the patte rn progressively turns more orthorhombic. On the other hand, the strength o f the orthopyroxene LPO increases. The consistency of olivine LPO translate s to similar seismic properties of peridotites in the two domains. Especial ly, the anisotropy of both compressional and shear waves (P- and S-waves) r emains almost unchanged across the entire massif. These results support tha t heating and partial melting (asthenospherization) of the lithospheric man tle do not necessarily obliterate the minerals LPO inherited from previous tectonic events. The 'structural memory' of the lithosphere may therefore b e preserved even in the 'asthenospherized' mantle. In a region of asthenosp here-lithosphere interaction, tomography studies would indicate a largely a ttenuated lithosphere from the presence of a shallow low-velocity anomaly w hile S-wave splitting measurement yields delays between arrivals of the fas t and slow split waves requiring a larger lithosphere thickness. This appar ent discrepancy may be resolved considering the existence of a 'ghost litho sphere' having lithospheric characteristics regarding anisotropy studies an d asthenospheric properties regarding seismic waves velocities. (C) 2001 El sevier Science BY. All rights reserved.