Experimental generation of shock-induced pseudotachylites along lithological interfaces

To understand the mechanism of formation of shock-induced pseudotachylites and particularly the role that rock heterogeneities and interfaces play in their formation, shock recovery experiments were carried out on samples con sisting of two distinct lithologies (dunite and quartzite). It was possible to generate melt veins of 1-6 mum width along lithological interfaces at m oderate shock pressures (6 to 34 GPa). The magnitudes of-displacement along the interface, strain rate, and the kinetic heat production indicate that friction is an important heat source that largely contributes to the energy budget of the melt veins. The experimentally produced veins resemble natur al S-type pseudotachylites. The geometry of the veins depends on the orient ation of the interface with respect to the shock front and includes strong variations in thickness, formation of melt pockets and injection veins, sud den changes in vein orientation, and sharp vein margins. Two types of melt were observed: vesicle-free and vesicular melts. Dense vesicle-free melt ro ck is likely to represent high-pressure melts. Vesicular melts were also ge nerated during shock compression, but they remained in a molten state durin g pressure release and continued shearing. Intermingling of comminuted oliv ine and melt suggests that ultracataclasis of olivine induced by a dynamic tensile failure is a precursor stage to frictional melting. Shock wave inte rferences at the lithological interface provide the necessary stress condit ions to start dynamic failure of olivine. The composition of the frictional melts ranges from olivine-normative to enstatite-normative and is, thus, l argely determined by olivine melting. The validity of the sequence of frict ion melting susceptibilities of rock-forming minerals inferred from tectoni cally-produced pseudotachylites is confirmed and can now be applied to ultr a-high strain rates during shock compression.