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

Abstract

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 tectonic events or if, in the absence of large-scale flow, this fabric is preserved. To evaluate the effect of heating and partial melting on the seismic properties of the lithospheric mantle, we have measured the LPO and computed the seismic properties of peridotites from the Ronda massif (Spain). In this massif, a narrow (≤400 m) coarsening front separates a porphyroclastic peridotite domain, interpreted as old lithospheric mantle, from a coarse-granular peridotite domain produced by annealing and limited partial melting (<6.5%) of the porphyroclastic peridotites. The olivine 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 coarsening front: the LPO strength decreases slightly and symmetry of the pattern progressively turns more orthorhombic. On the other hand, the strength of the orthopyroxene LPO increases. The consistency of olivine LPO translates to similar seismic properties of peridotites in the two domains. Especially, the anisotropy of both compressional and shear waves (P- and S-waves) remains almost unchanged across the entire massif. These results support that heating and partial melting (asthenospherization) of the lithospheric mantle do not necessarily obliterate the minerals LPO inherited from previous tectonic events. The ‘structural memory’ of the lithosphere may therefore be preserved even in the ‘asthenospherized’ mantle. In a region of asthenosphere–lithosphere interaction, tomography studies would indicate a largely attenuated lithosphere from the presence of a shallow low-velocity anomaly while S-wave splitting measurement yields delays between arrivals of the fast and slow split waves requiring a larger lithosphere thickness. This apparent discrepancy may be resolved considering the existence of a ‘ghost lithosphere’ having lithospheric characteristics regarding anisotropy studies and asthenospheric properties regarding seismic waves velocities.