Introduction

Rocks of the Earth's crust may experience a variety of catastrophic processes that can lead to the formation of new rocks characterized by fragments of the original rock(s) hosted in a much finer-grained, aphanitic, or even glassy matrix. Such breccias may owe their origin, inter alia, to violent, often volatile-assisted, stopping of wallrocks adjacent to magma chambers, more passive intrusion of melts into country rocks in magmatic and high-temperature metamorphic environments, post-tectonic or volcanic impregnation of cataclasites with deposits from hydrothermal solutions, slip and frictional sliding along fault or shear zones, or the catastrophic deformation processes associated with impact cratering.

In essence, shock compression is capable of melting minerals and whole rocks, at shock pressures above circa 40 GPa. This process results in strongly heterogeneous distribution of shock pressure and post-shock temperature (potentially, as at Vredefort, coupled with crustal heat inherent to the target rock), whereby mixtures of shock melted and crystalline remnants a breccia is generated. These impact melt breccias may be sharply separated from crystalline crater basement, at least partially resulting in a vein pattern, or grade into more and more massive crystalline material. Injection of shock melts into less shocked basement is also possible - resulting also in a distinct vein pattern that may closely resemble injection veins of friction melt (pseudotachylite) from tectonic settings. Dark-matrix breccia veining can also result from injection of impact melt breccia into the crater floor - as described from Puchezh-Katunki, where such impact melt rock injections have been interpreted to depths of >4000 m.

It is actually necessary to quantify the term "dark-matrix breccia" further. This term has been elected in this context, as the vast majority of veins and dikes that have been termed "pseudotachylite" in the impact cratering literature are either blackish or dark-grey; however, a number of descriptions of "pseudotachylitic breccia" have been reported (e.g., Fletcher and Reimold 1989; Killick and Reimold 1990; Reimold 1991; Reimold and Colliston 1994) of different color breccias from the Witwatersrand Basin. These workers have discussed that composition/nature of precursor material may determine the actual color of the breccia. In addition, secondary alteration can also strongly

Impact Craters Simple And Complex
Fig. 1. Oblique view of the 110 km diameter lunar impact crater Theophilus (Apollo 16, NASA AS16-0692). The image displays a complex central peak area and a series of distinct terraces along the inner crater wall.

change the color of such material - with many altered pseudotachyltiic breccias in the Vredefort Dome being of light-grey or beige color.

To complicate the options for pseudotachylite-like vein formation further, increasing recognition has been given to the possibility that friction melting and shock melting could actually go hand-in-hand (Kenkmann et al. 2000a; Langenhorst et al. 2002). Friction melting can further take place during the crater modification stage - involving uplift formation and collapse, first under the extreme high-strain rate uplift conditions, and then due to extensional collapse involving sliding and thrusting of rock masses against and over each other, and also by mass sliding along large listric faults at the crater margin, which would result in the distinct terracing observed along the inner margin of large impact structures (Fig. 1). In this setting, shock pressure decay has been completed by the time this process evolves, so that pure friction melting would take place.

Despite their superficial - and maybe even microscopic - similarities, such breccias signify markedly different geological environments and processes, and the question is whether they have distinctive features that can aid in elucidation of their origin(s). This problem is typified by "pseudotachylite" - a term that was originally introduced to describe breccias in an impact setting (at Vredefort - although the setting was unknown at the time), but which was subsequently extended to morphologically similar breccias found in nonimpact settings and which has, consequently, taken on a new meaning; but which is still used indiscriminately by impact workers. This review aims to explain why the present indiscriminate use of this term hampers, rather than aids, understanding of impact processes on Earth and why alternative terminology is needed for impact settings.

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