S. J. Shand (1916) first introduced the term pseudotachylite (variant spelling "pseudotachylyte") to describe an enigmatic dark, flinty rock that apparently intruded granites near the Vaal River in what is now known as the Vredefort impact structure. Shand's early chemical analyses showed that the dark intrusive veins and pockets are compositionally similar to the enclosing granite, but he was unable to give a convincing explanation of their origin.
After Shand's discovery, nearly a century of study has clarified some, but not all, of the mysteries surrounding these strange rocks. It is widely supposed that pseudotachylites form as a result of frictional melting. They are found both in impact structures (Reimold 1995, 1998) and in undoubted tectonic settings (Sibson 1975). At Vredefort, it is easy to find thin, pseudotachylite-filled veins across which 1 to 10 cm offsets are clearly visible (Fig. 1). 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 between more coherent regions may have produced friction melt at the interface. Langenhorst et al. (2002) argue that melt veins in shocked single olivine crystals are almost certainly the result of shear melting, not fluctuations in shock pressure.
The problem is, that not all occurrences of pseudotachylite are narrow veins, as one would expect if they were directly produced by friction between adjacent blocks of rock (Figs. 2 and 3). Shand (1916) described large pockets with angular and even rounded inclusions of country rock on scales up to meters wide, a type of occurrence well illustrated by Killick and Reimold (1990) and Reimold and Colliston (1994). Similar occurrences of thick melt zones, in some cases up to 500 m wide and 11 km long, of so-called pseudotachylite, have been described at the Sudbury impact structure (Spray and Thompson 1995). As I argue below, the occurrence of such large melt masses poses a serious problem for any theory of frictional melting.
Lambert (1981) focused on the dike-like character of pseudotachylites. He distinguished two basic types of pseudotachylite occurrences, as did later work by Martini (1991) which was restricted to Vredefort. 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). Note, however, that Reimold (1995; 1998) disputes Lambert's identification of Lambert's type A and A1 breccia types as true pseudotachylites, arguing instead that they are impact melts. 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. These have been attributed to frictional sliding along very large displacement faults (Spray 1997) that formed well after shock compression ceased.
This interpretation, particularly of the thick, type B, pseudotachylites, raises a serious mechanical problem. Friction between large blocks of rock is certainly capable of raising the temperature of the interface to the melting point (Anderson 1951; Jeffreys 1942; McKenzie and Brune 1972; Spray 1995). However, once melt appears in abundance, it lubricates the interface and decreases the friction coefficient to the point that no further melt is produced (it is a pleasure to note that Harold Jeffreys (1942) was well aware of this problem, but offered no solution). So how are we to account for the reported occurrences of thick zones of pseudotachylites?
The goal of this paper is not to provide a definitive answer to this problem, but instead to define the physical constraints on frictional pseudotachylite formation. There are many aspects of rapid rock deformation that have not been explicitly considered in previous work. In a sense, this paper is a prolegomenon to a theory of pseudotachylite formation in which the main boundary conditions are set. I will also suggest possible directions in which answers to the principal problems may lie.
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