Pseudotachylites Generated in Shock Experiments: Implications for Impact Cratering Products and Processes

Abstract

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...

The Modification Stage of Crater Formation

“…Pseudotachylitic breccias are breccias containing a melted rock matrix that resembles pseudotachylite but where the genetic origin of the melt is unclear (Reimold and Gibson, 2005). Potential mechanisms for producing the melt in these breccias include friction melting (Spray and Thompson, 1995), shock melting (Fiske et al, 1995), decompression melting (Reimold and Gibson, 2005) or drainage from the melt sheet above (Lieger et al, 2009, Riller et al, 2010. Pseudotachylitic breccias are abundant in the central uplift of the Vredefort structure and are particularly pervasive in the central core rocks, which were uplifted furthest, from the greatest depth (Reimold and Gibson, 2006).…”

Impact‐induced frictional melting in ordinary chondrites: A mechanism for deformation, darkening, and vein formation

“…As a result of that deformation, tiny pseudotachylyte-like veins form. The density of these veins increases in direct relation to shock pressure, thereby suggesting that shock veins are shockrelated frictional melt features (Fiske et al 1995). More recently, Kenkmann et al (2000) found frictional melting necessary for vein formation at moderate to low shock pressures.…”

“…To investigate the effects of shock-induced shear experimentally, Fiske et al (1995) conducted a shock experiment that allowed variation of strain within a sample of quartz. Upon shock pressure release, lateral deformation, similar to excavation stage deformation upon decompression and gravitational collapse (e.g., Spray and Thompson 1995), allows shear to develop within the sample.…”

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.…”

“…He distinguished two basic types of pseudotachylite occurrences. The first, type A, consist of a network of thin (less than 1 mm to several mm) veins that may have formed during the initial shock compression (Fiske et al 1995). Type B pseudotachylites are much larger dike or sill-like bodies that range from cm to several hundred m wide and one or more km long.…”