Abstract
Indications of natural catastrophes and crises are preserved in some geologic strutures and in the stratigraphic record, emphasizing the dynamic nature of the Earth's evolution. The crises may result either from exogenic or endogenic energy sources. Distinguishing between these sources is a complex, multidisciplinary problem in the Earth sciences. Shock deformation by impact has been well-studied both in shock recovery experiments and in known impactites, but only recently has the possibility of generation of shock waves by high-temperature internal explosions been seriously considered. An optically based reconaissance survey of microstructures in quartz and feldspar in some explosive silicic volcanics suggests that they may have been induced by dynamic deformation at high temperature; in some respects the character of the microstructures differs greatly from those induced by room temperature shock, whereas in others the differences are subtle. Results from recent shock recovery experiments on quartzite preheated to 440 ° and shocked to 28 GPa support this contention. For two classic Cretaceous/Tertiary sections at Gubbio, Italy, it is shown that shock mosaicism and shock lamellae occur over a 4 m interval, bisected by the boundary, in a pattern approximately correlatable with an independently established iridium enrichment. These data are incompatible with any single event and are most plausibly interpreted as reflecting a period (∼ 0.5 my) of intense, probably episodic, volcanism with a culmination at the K/T transition. Multiple dynamic events may also account for macroscopic and microscopic structures developed during the evolution of the Vredefort structure, South Africa.
Published Version
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