Artificial generation of pseudotachylyte using friction welding apparatus: simulation of melting on a fault plane |
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| Institution: | 1. University of York, Department of Archaeology, The King''s Manor, York YO1 7EP, UK;2. Istituto di Geologia Ambientale e Geoingegneria, CNR, Rome, Italy;3. Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy;4. Department of History, Culture and Society, University of Rome ‘Tor Vergata’, Via Columbia 1, 00163 Rome, Italy;5. Department of Geoscience, University of Wisconsin-, Madison, USA;1. Institute of Precambrian Geology and Geochronology, nab. Makarova 2, St. Petersburg, 199034, Russia;2. Geological Institute of the Kola Science Center, Russian Academy of Sciences, ul. Fersmana 14, Apatity, Murmansk Region, 184209, Russia;3. Tellur North-East LLC, ul. Soyuza Pechatnikov 8, St. Petersburg, 199000, Russia;1. Department of Epidemiology, School of Public Health, University of Alabama at Birmingham, Birmingham, AL, USA;2. Division of Biostatistics and Epidemiology, Cincinnati Children''s Hospital Medical Center, Cincinnati, OH, USA;3. Department of Obstetrics and Gynecology, School of Medicine, University of Alabama at Birmingham, Birmingham, AL, USA;4. Biosciences Division, Center for Cancer & Metabolism, SRI International, USA;5. Department of Nutrition Sciences, School of Nutrition, University of Alabama at Birmingham, Birmingham, AL, USA |
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| Abstract: | A 150 μm thick fused layer of rock has been produced by rotating two metadolerite core faces against each other at 3000 r.p.m. under an axial load of 330 kg for 11 s using friction welding apparatus. Scanning electron microscopy and electron microprobe analysis reveal that the melt layer comprises sub-angular to rounded porphyroclasts of clinopyroxene, feldspar and ilmenite (>20 μm diameter), derived from the host metadolerite, set within a silicate glass matrix. Thermal calculations confirm that melting occurred at the rock interface and that mean surface temperatures in excess of 1400°C were attained. The fused layer shows many textural similarities with pseudotachylyte described from fault zones. Morphologically, the fused layer consists of a series of stacks of porphyroclasts welded together by melt to form ‘build-ups’ oriented at right angles to the friction surface. There is also evidence of gouging, ploughing and plucking, as well as transfer and adhesion of material having occurred between the rock faces. The mean surface velocity attained by the metadolerite (0.24 m s−1) and duration of the experiment are comparable with velocities and rise times of typical single jerk earthquakes occurring during stick-slip seismic faulting within brittle crust (i.e. slip rates of 0.1-0.5 m s−1 for, say, 1–10s). In these respects the experiment successfully simulated frictional fusion on a fault plane in the absence of an intergranular fluid. Power dissipation during the experiment was about MW m−2, comparable only to very low values for earthquakes (e.g. 1–100 MW m−2 for displacement rates of 0.1-0.5 m s−1 at shear stresses of 100–1000 bars). This indicates that melting on fault planes during earthquakes should be commonplace. Field evidence, however, does not support this contention. Either pseudotachylyte is not being recognized in exhumed ancient seismic fault zones or melting only occurs under very special circumstances. |
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