Experimental generation of shock‐induced pseudotachylites along lithological interfaces

Abstract

Abstract-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 camed out on samples consisting of two distinct lithologies (dunite and quartzite). It was possible to generate melt veins of 1-6 pm width along lithological interfaces at moderate 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 natural S-type pseudotachylites. The geometry of the veins depends on the orientation of the interface with respect t o the shock front and includes strong variations in thickness, formation of melt pockets and injection veins, sudden changes in vein orientation, and sharp vein margins. Two types of melt were observed: vesicle-free and vesicular melts. Dense vesicle-free melt rock is likely to represent high-pressure melts. Vesicular melts were also generated during shock compression, but they remained in a molten state during pressure release and continued shearing. Intermingling of comminuted olivine and melt suggests that ultracataclasis of olivine induced by a dynamic tensile failure is a precursor stage to frictional melting. Shock wave interferences at the lithological interface provide the necessary stress conditions to start dynamic failure of olivine. The composition of the frictional melts ranges from olivinenormative to enstatite-normative and is, thus, largely determined by olivine melting. The validity of the sequence of friction melting susceptibilities of rock-forming minerals inferred from tectonically-produced pseudotachylites is confirmed and can now be applied to ultra-high strain rates during shock compression.

Magnetic properties of fault pseudotachylytes in granites

“…[47] In general, fault pseudotachylytes form by ultracataclasis, frictional melting or a combination of both during seismic slip [e.g., Spray, 1987;Lin and Shimamoto, 1994;Kenkmann et al, 2000;Di Toro et al, 2006;Lin, 2008]. The pseudotachylytes examined in this study were all formed by frictional melting in felsic composition host rock [Wenk et al, 2000;Fabbri et al, 2000;Zechmeister et al, 2007].…”

“…Both materials are typically produced by high velocity friction, on the order of m/s. Shock pseudotachylytes, such as those formed during meteorite, asteroid or ballistic impacts [Grieve and Therriault, 2000;Kenkmann et al, 2000;van der Bogert et al, 2003] are excluded because they are the products of very high confining pressures.…”

Shock synthesis of quasicrystals with implications for their origin in asteroid collisions

“…The presence of other high-pressure phases (ahrensite and stishovite) (10) associated with shocks, adjacent to icosahedrite and decagonite in the Khatyrka CV3 carbonaceous chondrite, together with the static high-pressure stability of the quasicrystalline phase, are suggestive of an origin during the high-pressure pulse of a shock event. However, there are numerous differences between the static high-pressure, high-temperature conditions studied by Stagno et al (12) and shock-induced conditions at nominally similar peak (P, T) conditions-including heterogeneous and rapidly time-varying pressure and temperature fields (13,14), typical adiabatic decompression rather than temperature quench at high pressure (15), and hypersonic turbulent shear flows (16)(17)(18). A direct demonstration of synthesis of an Al-Cu-Fe icosahedral quasicrystal from discrete starting materials in a laboratory shock recovery experiment would offer substantial new evidence in favor of the shock-induced origin of the natural example and potentially may provide uniquely precise constraints on the shock conditions experienced in the Khatyrka meteorite.…”

The Mechanics of Pseudotachylite Formation in Impact Events

“…They are found both in impact structures (Reimold 1995) and in undoubted tectonic settings (Sibson 1975). Although experimenters have succeeded in producing melts by direct high-speed frictional sliding (Spray 1995), concern still exists that some melt may be produced by shock compression and release (Reimold 1995), a concern that is bolstered by the formation of pseudotachylite-like melts in shock compression experiments (Fiske et al 1995, Kenkmann et al 2000. Even in these experiments, however, there is a strong possibility that relative sliding among more coherent regions may have produced friction melt at the interface.…”