The Mechanics of Pseudotachylite Formation in Impact Events

  • H. Jay Melosh
Part of the Impact Studies book series (IMPACTSTUD)


This paper presents a discussion of the basic constraints controlling the formation of pseudotachylites in the rapidly sheared rocks in the vicinity of a large meteorite impact. The prevailing opinion among many geologists is that pseudotachylites are formed by friction melting of rocks and/or shearing associated with differential shock compression of adjacent rock types. Several physical studies of friction melting have shown that, in theory, small amounts of movement (centimeters or less) are capable of producing very thin veins of melted rock. More realistic models suggest that irregularities on the sliding surface of the order of the grain size may still create primary melt veins up to a few millimeters thick. The principal mystery of pseudotachylite formation is not that friction can cause melting, but that it seems to form thick masses of it, meters to tens of meters wide. However, such thick masses ought to preclude melting by reducing the friction between sliding rock masses. I propose that one possible solution to this conundrum is that the melt produced by sliding on narrow shear zones is extruded into the adjacent country rock, thus keeping the sliding surfaces narrow, while creating thick accumulations of melt in adjacent low pressure zones that open at the end of the shear zones. For this mechanism to operate, the melted rock must be fluid enough to extrude from the shear zone during the time available during crater collapse. This places strong constraints on the viscosity and temperature of the melt. This model may be tested by future careful investigation of the geometry of pseudotachylite occurrences.


Shear Zone High Strain Rate Impact Event Fault Slip Impact Crater 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. Anderson EM (1951) The Dynamics of Faulting. Oliver and Boyd, Edinburgh, 206 ppGoogle Scholar
  2. Ashby MF, Sammis CG (1990) The damage mechanics of brittle solids in compression. Pure and Applied Geophysics 133: 489–521Google Scholar
  3. Batchelor GK (1970) An Introduction to Fluid Dynamics. Cambridge University Press, Cambridge, 615 ppGoogle Scholar
  4. Bombolakis EG (1973) Study of the brittle fracture process under uniaxial compression. Tectonophysics 18: 231–248CrossRefGoogle Scholar
  5. Byerlee J (1978) Friction of Rocks. Pure and Applied Geophysics 116: 615–626Google Scholar
  6. Collins GC, Melosh HJ, Ivanov BA (2004) Damage and deformation in numerical impact simulations. Meteoritics and Planetary Science 39: 217–231Google Scholar
  7. Fiske PS, Nellis WJ, Lipp M, Lorenzana H, Kikuchi M, Syono Y (1995) Pseudotachylites generated in shock experiments: Implications for impact cratering products and processes. Science 270: 281–283Google Scholar
  8. Grocott J (1981) Fracture geometry of pseudotachylite generation zones: A study of shear fractures formed during seismic events. Journal of Structural Geology 3: 169–178CrossRefGoogle Scholar
  9. Groves GW, Kelly A (1963) Independent slip systems in crystals. Philosophical Magazine 8: 877–887Google Scholar
  10. Gruntfest IJ (1963) Thermal feedback in liquid flow: Plane shear at constant stress. Transactions of the Society of Rheology 7: 195–207CrossRefGoogle Scholar
  11. Hanks TC (1977) Earthquake stress drops, ambient tectonic stresses and stresses that drive plate motion. Pure and Applied Geophysics 115: 441–458Google Scholar
  12. Horii H, Nemat-Nasser S (1986) Brittle failure in compression: Splitting, faulting and brittle-ductile transition. Philosophical Transactions of the Royal Society of London A 319: 337–374Google Scholar
  13. Jaeger JC, Cook NGW (1969) Fundamentals of Rock Mechanics. Chapman and Hall, London, 515 ppGoogle Scholar
  14. Jeffreys H (1942) On the mechanics of faulting. Geological Magazine 79: 291–295CrossRefGoogle Scholar
  15. Kenkmann T, Hornemann U, Stöffler D (2000) Experimental generation of shock-induced pseudotachylites along lithological interfaces. Meteoritics and Planetary Science 35: 1275–1290Google Scholar
  16. Killick AM (1990) Pseudotachylite generated as a result of a drilling “burn-in”. Tectonophysics 171: 221–227CrossRefGoogle Scholar
  17. Killick AM, Reimold WU (1990) Review of the pseudotachylites in and around the Vredefort ‘Dome’, South Africa. South African Journal of Geology 9: 350–365Google Scholar
  18. Lachenbruch AH (1961) Depth and spacing of tension cracks. Journal of Geophysical Research 66; 4273–4292Google Scholar
  19. Lambert P (1981) Breccia dikes: Geological constraints on the formation of complex craters. In: Schultz PH, Merrill RB (eds) Multi-ring Basins, Proceedings of Lunar and Planetary Science 12A. Pergamon Press, New York, pp 59–78Google Scholar
  20. Langenhorst F, Poirier J-P, Deutsch A, Hornemann U (2002) Experimental approach to generate shock veins in a single crystal olivine by shear melting. Meteoritics and Planetary Science 37: 1541–1553Google Scholar
  21. Lawn BR, Wilshaw TR (1975) Fracture of Brittle Solids. Cambridge University Press, New York, 204 ppGoogle Scholar
  22. Martini JEJ (1991) The nature, distribution and genesis of coesite and stishovite associated with the pseudotachylite of the Vredefort Dome, South Africa. Earth and Planetary Science Letters 103: 285–300CrossRefGoogle Scholar
  23. Maxwell DE (1977) A simple model of cratering, ejection, and the overturned flap. In: Roddy DJ, Pepin RO, Merrill RB (eds) Impact and Explosion Cratering, pp 1003–1008. Pergamon, New YorkGoogle Scholar
  24. McBirney AR, Murase T (1984) Rheological properties of magmas. Annual Reviews of Earth and Planetary Science 12: 337–357Google Scholar
  25. McKenzie D, Brune JN (1972) Melting on fault planes during large earthquakes. Geophysical Journal of the Royal Astronomical Society 29: 65–78Google Scholar
  26. Melosh HJ (1977) Crater modification by gravity: A mechanical analysis of slumping. In: Roddy DJ, Pepin RO, Merrill RB (eds) Impact and Explosion Cratering, pp. pp. 1245–1260. Pergamon Press, New YorkGoogle Scholar
  27. Melosh HJ (1979) Acoustic fluidization: A new geologic process? Journal of Geophysical Research 84: 7513–7520Google Scholar
  28. Melosh HJ (1989) Impact Cratering: A Geologic Process. Oxford University Press, New York. 245 ppGoogle Scholar
  29. Melosh HJ (2003) Shock viscosity and rise time of explosion waves in geologic media. Journal of Applied Physics 94: 4320–4325CrossRefGoogle Scholar
  30. Melosh HJ, Ivanov BA (1999) Impact crater collapse. Annual Reviews of Earth and Planetary Science 27: 385–415Google Scholar
  31. Moore HE, Sibson RH (1978) Experimental thermal fragmentation in relation to seismic faulting. Tectonophysics 49: T9–T17CrossRefGoogle Scholar
  32. O’Keefe JD, Ahrens TJ (1999) Complex craters: Relationship of stratigraphy and rings to the impact conditions. Journal of Geophysical Research 104: 27,091–27,104Google Scholar
  33. Ohnaka M (1995) A shear failure strength law of rock in the brittle-plastic transition regime. Geophysical Research Letters 22: 25–28CrossRefGoogle Scholar
  34. Palmer F (1949) What about friction? American Journal of Physics 17: 181–187Google Scholar
  35. Petford N (2003) Rheology of granitic magmas during ascent and emplacement. Annual Reviews of Earth and Planetary Science 31: 399–427CrossRefGoogle Scholar
  36. Reches Z (1978) Analysis of faulting in three-dimensional strain field. Tectonophysics 47: 109–129CrossRefGoogle Scholar
  37. Reimold WU (1995) Pseudotachylite in impact structures — generation by friction melting and shock brecciation?: A review and discussion. Earth Science Reviews 39: 247–265CrossRefGoogle Scholar
  38. Reimold WU (1998) Exogenic and endogenic breccias: A discussion of major problematics. Earth Science Reviews 43: 25–47CrossRefGoogle Scholar
  39. Reimold WU, Colliston WP (1994) Pseudotachylites of the Vredefort Dome and the surrounding Witwatersrand Basin, South Africa. In: Dressler BO, Grieve RAF, Sharpton VL (eds) Large Meteorite Impacts and Planetary Evolution Geological Society of America Special Paper 293, Boulder, CO, pp. 177–196Google Scholar
  40. Rudnicki JW, Rice JR (1975) Conditions for the localization of deformation in pressuresensitive dilatent materials. Journal of the Mechanics and Physics of Solids 23: 371–394CrossRefGoogle Scholar
  41. Scholz CH (1990) The Mechanics of Earthquakes and Faulting. Cambridge University Press, Cambridge. 439 ppGoogle Scholar
  42. Segall P, Pollard DD (1983) Nucleation and growth of strike slip faults in granite. Journal of Geophysical Research 88: 555–568Google Scholar
  43. Shand SJ (1916) The pseudotachylyte of Parijs (Orange Free State) and its relation to ‘trapshotten gneiss’ and ‘flinty crush-rock’. Quarterly Journal of the Geological Society of London 72: 198–221Google Scholar
  44. Sibson RH (1975) Generation of pseudotachylyte by ancient seismic faulting. Geophysical Journal of the Royal Astronomical Society 43: 775–794Google Scholar
  45. Sibson RH (1977) Fault rocks and fault mechanisms. Journal of the Geological Society of London 133: 191–213CrossRefGoogle Scholar
  46. Spray JG (1995) Pseudotachylyte controversy: Fact or friction? Geology 23: 1119–1122CrossRefGoogle Scholar
  47. Spray JG (1997) Superfaults. Geology 25: 579–582CrossRefGoogle Scholar
  48. Spray JG, Thompson LM (1995) Friction melt distribution in a multi-ring impact basin. Nature 373: 130–132CrossRefGoogle Scholar
  49. Stesky RM, Brace WF, Riley DK, Bobin PY (1974) Friction in faulted rock at high temperature and pressure. Tectonophysics 23: 177–203CrossRefGoogle Scholar
  50. Swegle JW, Grady DE (1985) Shock viscosity and the prediction of shock wave arrival times. Journal of Applied Physics 58: 603–701CrossRefGoogle Scholar
  51. Taylor GI (1938) Plastic strain in metals. Journal of the Institute of Metals 62: 307–324Google Scholar
  52. Tsutsumi A, Shimamoto T (1997) High-velocity frictional properties of gabbro. Geophysical Research Letters 24: 699–702CrossRefGoogle Scholar
  53. Turcotte DL, Schubert G (1982) Geodynamics: Applications of Continuum Physics to Geological Problems. John Wiley and Sons, New York. 450 ppGoogle Scholar
  54. Turtle EP, Pierazzo E (1998) Constraints on the size of the Vredefort impact crater from numerical modeling. Meteoritics and Planetary Science 33: 483–490CrossRefGoogle Scholar
  55. Zel’dovich YB, Raizer YP (1967) The Physics of Shock Waves and High Temperature Hydrodynamic Phenomena. Academic Press, New YorkGoogle Scholar
  56. Zoback MD, Zoback ML, Mount VS, Suppe J, Eaton JP, Healy JH, Oppenheimer D, Reasenberg P, Jones L, Raleigh CB, Wong IG, Scotti O, Wentworth W (1987) New evidence for the state of stress on the San Andreas fault system. Science 238: 1105–1111Google Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 2005

Authors and Affiliations

  • H. Jay Melosh
    • 1
  1. 1.Lunar and Planetary LaboratoryUniversity of ArizonaTucsonUSA

Personalised recommendations