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A centennial reappraisal of the Vredefort pseudotachylytes; shaken, not stirred by meteorite impact

A. A. Garde and Martin B. Klausen
A centennial reappraisal of the Vredefort pseudotachylytes; shaken, not stirred by meteorite impact
Journal of the Geological Society of London (June 2016) 173 (6): 949-953

Abstract

Major pseudotachylyte zones constitute a spectacular component of the renowned c. 2.023 Ga Vredefort impact structure, South Africa, but it has always been difficult to explain how they were formed. In his original account, in 1916, Shand interpreted the pseudotachylyte as due to cataclasis and frictional heating but pointed out two enigmas that have remained since: there were no associated major faults, and the pseudotachylyte volumes he observed were far greater than in similar rocks located within faults elsewhere on Earth. New observations show that the Vredefort pseudotachylyte zones were indeed formed by cataclasis and frictional heating, not by faulting but owing to impact-induced seismic shaking initiated around temporarily loosened blocks in dendritic fracture systems. Progressive cataclasis of such loose blocks by intense, high-frequency oscillations of the country rock at the beginning of the cratering process led to size reduction, rounding and comminution, and frictional melting of feldspars and biotite in the comminuted parts. Most pseudotachylyte was thus not injected from anywhere but produced in situ The process of seismic shaking is well known from impacts on the Moon and asteroids, terrestrial earthquakes and nuclear tests but has largely been overlooked in terrestrial cratering, except in the theoretical concept of acoustic fluidization.


ISSN: 0016-7649
EISSN: 2041-479X
Coden: JGSLAS
Serial Title: Journal of the Geological Society of London
Serial Volume: 173
Serial Issue: 6
Title: A centennial reappraisal of the Vredefort pseudotachylytes; shaken, not stirred by meteorite impact
Affiliation: Geological Survey of Denmark and Greenland, Copenhagen, Denmark
Pages: 949-953
Published: 20160610
Text Language: English
Publisher: Geological Society of London, London, United Kingdom
References: 58
Accession Number: 2016-087204
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, geol. sketch map
S27°06'00" - S26°55'00", E27°19'00" - E27°25'00"
Secondary Affiliation: Stellenbosch University, ZAF, South Africa
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data from The Geological Society, London, London, United Kingdom
Update Code: 201643
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