Laboratory hypervelocity impact experiments in which quartz was shock-loaded from 42 to 56 gigapascals imply that type A pseudotachylites form by strain heating and contribute to the loss of strength of rocks in the central uplift of large impact structures. Shock impedance-matched aluminum sample containers, in contrast to steel containers, produced nearly single-wave pressure loading, and enhanced deformation, of silicate samples. Strain heating may act with shock heating to devolatilize planetary materials and destroy extraterrestrial organic material in an impact.M e t e o r i t e impact is the most cataclysmic planetary geologic process operating in our solar system. Lluring the formation of a n impact crater, rocks are subjected to enormous transient pressure and deformation causing phase transitions, melting, and vaporization. Large impacts produce a translent cavity in the crust that illltnediately rebounds, forming a flat-floored crater with a central peak, or in the case of the largest impacts, a m~~l t i -r i n g basin. Some i m p a c t -~n d~~c e d features that occur in rocks from both large and slllall craters have been synthesized 111 laboratory shock experiments (1). Other features found in abundance only in larger impact structures, s~l c h as shatter cones and pseudotachylites (a fine-grained, dark gray or black rock that resembles tachylite, a volcanic glass) ( 2 ) , are diffic~llt to reprod~l c e in laboratory shock experiments, suggesting a significant dependence o n scale. Although the pressures experienced by rocks in small and large impacts are comparable, the princ~pal difference is in the amount of strain to which the rocks are sul~jected. Laboratory experiments have simulated the pressure but not the strain prod~lced by impact. T o investigate the origin of strain heating and the production of pseudotachylites, we have c o n d~~c t e d a series of experitnents using shock recovery systems that enable us to vary the strain experienced by the salllple during shock loading. W e produced glassy veins of black material in quartz that are analogous t o pse~ldotachylites found in the central portion of large ilupact craters.Pse~~dotachylites are bodies of melted and pulverized rock formed in situ by frictional melting and are found in large impact structures (3) and along some tectonic faults (4). Workers have identified two varieties of impact pse~~dotachylites that differ III size and tlming of formation (5). Type B cseuciotachvlites are large dlke-or sill-l~ke u tnasses (centimeters t o several hundred meters in width and up t o kilometers in length) and cotnlllonly occur along dlscrete faults. They are analogous to, t h o~~g h LISLIallv lareer than, tectonic nseudotachylites anh are' formed by frictio~ial heating'as a result of large ~Imounts of slip along a fault surface. Type B pseudotachylites have been prod~lced experimentally with the use of fr~ctlon-welding devices (6), denlollstrating that friction melts in rock can be ~r o d u c e d by high-speed s11p along a fault surface. A t the...
