Adam A. Garde and Martin B. Klausen
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The Vredefort pseudotachylytes – a centennial reappraisal of Shand (1916): shaken, not stirred by meteorite impacting Adam A. Garde1 and Martin B. Klausen2 1Geological Survey of Denmark and Greenland, Copenhagen, Denmark. E-mail: [email protected] 2Stellenbosch University, Private Bag X1, Matieland 7602, South Africa 1 2 3 4 Pseudotachylyte, Salvamento quarry, Photograph of outcrop with arrested Shock-induced dendritic fractures, Distribution of pseudotachylyte, Interpretation of further pseudotachylyte Full-blown pseudotachylyte zone, and coauthor Martin B. Klausen pseudotachylyte formation traced from photograph of outcrop traced from photograph of outcrop formation with continued seismic shaking Leeukop quarry (prior to impact-induced seismic shaking) Pseudotachylyte zone (as mapped Background and objectives by Dressler & Reimold 2004) Conclusions – see also Garde & Klausen (2016) ★ Observations and samples, this study R • Major pseudotachylyte zones constitute a spectacular component of the renowned, c. 2.023 Ga Vredefort LLA • Observations in the host rocks are the key to understand the pseudotachylytes. Their development was CO RT impact structure in South Africa, but it has always been difficult to explain how they were formed. EFO initiated by a shock-induced, dendritic fracture system that penetrated the granitic host rocks and ED VR temporarily loosened individual blocks with sizes from centimetres to metres. • In his famous original account of the pseudotachylytes, S. J. Shand (1916) interpreted them as due to ★Salvamento ★ cataclasis and frictional heating. Shand [v] also pointed out two enigmas which have remained unsolved: ★Leeukop Otavi ★ Vaal river island • After the initial fracturing, intense impact-induced seismic shaking of the now loosened blocks destroyed • 1) Shand realised that the volumes of pseudotachylyte he observed at Vredefort greatly exceeded those of Vaal Parys their margins by cataclasis , whereby the blocks gradually became smaller and smaller and more and more other pseudotachylytes located within faults elsewhere on Earth, rounded. The cataclasis led to frictional heating and eventually incomplete melting (see box below). VREDEFORT DOME • 2) At Vredefort no associated major faults were identified. • The seismic shaking punched the rock into a very fine powder and heated it by friction, whereby it 27°00’S Vredefort ★ became the material we call pseudotachylyte. It was produced in situ and not injected from anywhere. • Today we know that the Vredefort pseudotachylytes are impact-related, but how were they Granophyre dyke actually formed? 27°30’E • The investigation of the Vredefort pseudotachylytes was prompted by the discovery of Seismic shaking: an important but overlooked cratering process Esperanza multiply repeated cataclasis by intense, impact-induced seismic shaking in the Maniitsoq ★ Johannesburg • Shocks induce seismic waves, as can be observed in any seismogram. Impact-induced seismic shaking has structure, West Greenland (observations in 2010, 2011 and 2016, Garde et al. 2012, 2014). ★ Vredefort been described from the Moon (Kreslavsky & Head 2012) and from asteroids (Richardson et al. 2005). South • Further inspiration was provided by observations of endogenic, earthquake-induced seismic shaking in a 45 10 km Africa Surprisingly, it has been overlooked in the context of terrestrial impacting except in the theoretical concept km long linear belt in the footwall of the Insubric Line, Southern Alps in Italy (Garde et al. 2015). of acoustic fluidisation of the crater floor (Melosh 1979). Progressive formation of pseudotachylyte in situ Host rocks were also affected by the cataclasis Microstructures: mainly cataclasis, not full melting Shock-induced dendritic Superficially almost intact The pseudotachylyte fracturing with beginning orthogneiss adjacent to matrix is composed of cataclasis in the Esperanza Dendritic fractures and cataclasis pseudotachylyte at the numerous small, equant to quarry (see the map for Salvamento quarry (sample Pl Pl elongate fragments of ★ location). The host rock in 524307). quartz and plagioclase, ★ this quarry is granite. besides tiny laths and ★ On closer inspection, a Pl patches of microcline and ★ Asterisks mark nodes dendritic microfracture minor biotite (both of Impact-induced with progressive formation system with beginning Qtz which may have dendritic of pseudotachylyte. cataclasis is seen traversing crystallised from mineral fracture system the centre of the slab Qtz Qtz melts). Pegmatite Note near-absence of (arrows). displacement along the Backscattered electron fractures (as recorded by SEM image and the thin oblique pegmatite photomicrograph (right ★ ★ vein left). part of the same area). 2 cm 1 mm 1 mm ★ ★ Photomicrograph of the Pseudotachylyte matrix same sample collected Pl consisting of intensely adjacent to the comminuted quartz and 1 m pseudotachylyte. All Pl + Kfs plagioclase and tiny laths of feldspar grains are Qtz K-feldspar. fractured, and original Arrested cataclasis and quartz grains are now Kfs The K-feldspar laths have a pseudotachylyte formation fine-grained mosaics. preferred planar along shock-induced Qtz orientation and wrap dendritic fractures in the Note the absence of around the quartz Salvamento quarry. penetrative shear strain or fragments. This suggests overall lateral displacement. Pl overall compression with Note the complete Qtz orientation lower left – absence of faulting and The cataclasis is upper right relative to the displacements. Pl heterogeneous, with a Qtz image frame. better preserved area in Most fractures with the upper left and a Bt Note absence of simple cataclasis are discordant strongly comminuted area shear textures. The to lithological boundaries, in the right of the image. textures are compressional, others follow them (black reflecting pure shear. arrow). 2 mm Photomicrograph with 1 mm crossed polarisers. BSE-SEM image. The host rocks are orthogneiss and pegmatite. Enlargement of cataclastic grain of plagioclase from Significance of wall rock Interpretation of the microstructures the host rock with observations: a mixture of fragments of intensely comminuted quartz combined syn- and The pseudotachylyte is not a melt in the petrological and plagioclase and minute laths and irregular patches of 50 cm antithetic microfaults. Although the major pseudotachylyte sense, where melt is produced by chemical reactions K-feldspar. The latter may have crystallised from K-feldspar Small blocky feldspar zones might appear as distinct veins, between several mineral (± fluid) phases. It is essentially melt created by frictional heating to above 1300°C. fragments have been there is a gradual transition from host Arrested cataclasis and displaced in opposite rock that is still coherent but intensely Relaxation joint pseudotachylyte formation directions (arrow). affected by cataclasis and into pseudo- along shock-induced tachylyte. The latter is mainly a finely dendritic fractures in the comminuted rock rather than a melt. Tectonic joints Esperanza quarry. Impact-induced In his original definition of pseudotachylyte, Our interpretation of the Vredefort dendritic ★ Asterisks mark nodes Shand (1916) did not imply melting but Discussion fracture system with progressive formation only aphanitic, almost glassy appearance. pseudotachylytes as the results of of pseudotachylyte. 0.5 mm Hence the prefix ‘pseudo’. The observation that pseudotachylyte impact-induced seismic shaking is simple Older tectonic joints and a melting is preceded by cataclasis is not and adequately solves these problems. The young subhorizontal ★ relaxation joint are also new, as shown in experiments by Spray comminution and concomitant frictional ★ present. (1987). Both these experiments and an heating would continue for as long as the Note absence of faulting influential paper by Sibson (1975) seismic shaking persisted – probably for a ★ and displacements beyond Some previous models and their shortcomings assumed that pseudotachylyte formation few minutes after the impact, and at a ★ a few centimetres. was caused by frictional heating between frequency of around 1 Hz (J.H. Melosh, pers. Pseudotachylyte formation by fric- sliding surfaces in faults and were comm., 2013). This allows for hundreds of tional heating along slip surfaces? to Shand (1916), have repeatedly shown that controlled by simple shear. violent oscillations during which individual 1 m This is the classical interpretation which the compositions of the pseudotachylytes and components of the rock would be banging goes all the way back to Shand (1916) – their host rocks are the same. Furthermore, However, it was shown by Melosh (2005) against each other. Detail of fracturing and excellent candidates for genuine Tectonic ★ pseudotachylyte formation. who was nevertheless aware of one of its that the frictional heating between sliding Impact-induced joint main limitations, namely the absence of the allochthonous melts intruded from above surfaces will cease as soon as a melt is The main stress component was pure shear, The white asterisks mark dendritic necessary faults. See also the adjacent box exist in the form of the so-called granophyre rather than simple shear as previously fracture system sites with diminishing and produced, hence the copious amounts of rounding of individual small ‘Discussion’. dykes, which are well exposed in the southern pseudotachylyte at Vredefort could not assumed with presumed fault-bound blocks. part of the Vredefort dome. These dykes are have been produced