The Siljan structure in south-central Sweden (centered at N61°02.196′; E014°54.467′; Fig. 1) is the largest verified impact structure in Western Europe with a commonly quoted size of 52 km (Grieve 1982, 1988). An impact origin for Siljan was suggested by Wickman et al. (1963) and Fredriksson and Wickman (1963) based on morphological features of the structure. Later, planar deformation features (PDFs) in quartz grains and observations of shatter cones gave unambiguous evidence that the structure is in fact impact-derived (Svensson 1971, 1973). Siljan is poorly studied despite its size, and basic facts regarding, for example, the original and present size of the structure remain elusive. Investigations about the distribution of shocked quartz and shock barometry at Siljan are also lacking. Some information about shocked quartz can be retrieved from publications based on few data, and limited data presentation (Svensson 1971, 1973; Tamminen and Wickman 1980; Åberg and Bollmark 1985; see also Collini 1988; Juhlin et al. 1991).
Previous studies of the shock level distribution at impact structures based on field studies and laboratory shock experiments have shown that PDFs oriented along different crystallographic planes develop at different shock pressures (e.g., Hörz 1968; Müller and Défourneaux 1968; Robertson et al. 1968; Grieve and Robertson 1976; Huffman and Reimold 1996). Rocks from impact structures can therefore be divided into different stages of shock metamorphism and be assigned specific shock pressures, defined by the PDF orientations observed in different samples (Robertson et al. 1968; Robertson 1975; Grieve and Robertson 1976). A method based on these early studies (an overview of this method is provided below) has been used for shock barometry studies of, for example, the Charlevoix, Slate Islands, and Manicouagan impact structures (e.g., Robertson 1975; Robertson and Grieve 1977; Dressler et al. 1998).
The present study is the first detailed characterization of the shock level distribution across the Siljan impact structure by studying shock deformation features in quartz grains. Shocked quartz is essential in investigations of shock distribution at impact structures because of the well-documented effects of shock metamorphism on quartz, the simple optical features, the stability of the mineral, and the frequent occurrences in continental rocks. Quartz is particularly useful in shock barometry studies because it develops shock effects over a wide pressure range. Here, we investigate the PDF distribution pattern to determine where the shock center of Siljan is located, and what shape the pattern of lateral shock attenuation has, to discuss whether these features are comparable with other terrestrial complex impact structures. As Siljan is deeply eroded, the distribution of shocked minerals might reveal new information about the features of complex impact structures at depth. With these data, we hope also to better constrain the original dimensions and form of Siljan.
Constraining the original size of Siljan has become even more relevant as the age of the structure was revised from 368±1.1 Ma (Bottomley et al. 1990) to 377±2 Ma by Reimold et al. (2005). This age corresponds within error to the newly revised age of the Frasnian/Famennian boundary (374.5±2.6 Ma; House and Gradstein 2004), and the associated extinction event, as discussed in Reimold et al. (2005). To date, there is no evidence for a connection between the formation of the Siljan structure and the Late Devonian extinction event, or even a connection between any other large impact event and a major mass extinction other than at the Cretaceous/Paleogene boundary (Alvarez 2003). However, the combination of several large impact events (see Earth Impact Database for details on structures with similar ages as Siljan) might be capable of disturbing the ecosystem (Rampino 2002 and references therein; Schmitz et al. 2008).