Jointed blocks of peridotite xenoliths in basalts and mantle dynamics

Abstract

In considering the formation of fracture in the upper mantle of the Earth, several authors^1,2 have pointed out that the major problem in the formation of cracks is the fact that tensile stresses in the upper mantle region, particularly in the lithospheric plate, are too small compared with the hydrostatic overburden pressure. From theoretical considerations, Anderson² demonstrated that cracks can nucleate in the lithospheric plates by the accumulation of low density fluid at the base of the plate in the presence of tensile stresses parallel to the plate surface. However, there has been no direct geological observation that supports this conclusion. This letter reports geological observations of planar features in ultramafic inclusions which may have important implications on the existence of fracture in an ambient upper mantle condition.