Abstract

The elastic-wave velocities in three spinel lherzolite xenoliths from the Massif Central rift zone (France) indicate that the high field seismic velocity (8.4 km/s) found parallel to the rift, at a depth of 40 km in the upper mantle beneath the Massif Central, can only be explained by a preferred orientation of the olivine a axis parallel to the rift. This is not predicted by two-dimensional models of mantle flow beneath a rift. Horizontal asthenospheric flow in lithospheric fractures associated with rifting would explain the olivine orientation and the high upper mantle velocity parallel to the rift axis.

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