Roundness of survivor clasts as a discriminator for melting and crushing origin of fault rocks: A reappraisal

  • Arindam Sarkar
  • Anupam ChattopadhyayEmail author
  • Tusharika Singh


Roundness of survivor clasts (mineral/rock fragments) in fault rocks (e.g., pseudotachylyte and cataclasite/gouge) has been used by some workers to distinguish melt-origin from crush-origin of such rocks. Keeping in view the large overlap in the published data on the roundness of fault rocks, the reliability of such a discriminator appears somewhat uncertain. The present study attempts to reappraise the aforesaid criterion through roundness analysis of quartz and feldspar clasts in melting-dominated pseudotachylyte (M-Pt), crushing-dominated pseudotachylyte (C-Pt) and fault-related cataclasite (F-Ct) collected from Sarwar–Junia Fault Zone in Rajasthan and from Gavilgarh–Tan Shear Zone in central India. Our analysis shows that roundness of clasts cannot reliably distinguish between fault rocks of melt-origin and crush-origin (especially M-Pt and F-Ct) as the roundness values overlap and a distinct limit of roundness value for each rock type cannot be established. While the roundness of clasts in M-Pt and C-Pt may be enhanced due to melt-induced rounding off of the initially angular clasts, rounding of clasts can also occur by abrasion during rolling of crushed material in F-Ct. Furthermore, anomalous thermal expansion of clasts in melt-origin pseudotachylyte may cause disintegration of larger clasts into smaller angular fragments, thereby increasing the percentage of angular clasts in melt-origin fault rocks. Therefore, roundness of survivor clasts cannot be solely used as a discriminator between melt-origin and crush-origin fault rocks.


Pseudotachylyte cataclasite survivor clasts roundness melting 



This work was supported by a UGC-MRP Grant (F. No. 42-66/2013 (SR)) and partially by a Delhi University Faculty R&D Grant (No. RC/2014/6820) awarded to AC. AS was supported through the aforesaid UGC project fellowship. Microscopy and software facility were provided by the Department of Geology, University of Delhi.


  1. Behera B M, Thirukumaran V, Soni A, Mishra P K and Biswal T K 2017 Size distribution and roundness of clasts within pseudotachylytes of the Gangavalli Shear Zone, Salem, Tamil Nadu: An insight into its origin and tectonic significance; J. Earth Syst. Sci. 126(4) 46.CrossRefGoogle Scholar
  2. Chattopadhyay A and Khasdeo L 2011 Structural evolution of Gavilgarh–Tan Shear Zone, central India: A possible case of partitioned transpression during Mesoproterozoic oblique collision within central Indian tectonic zone; Precamb. Res. 186(1) 70–88.CrossRefGoogle Scholar
  3. Chattopadhyay A, Khasdeo L, Holdsworth R E and Smith S A F 2008 Fault reactivation and pseudotachylite generation in the semi-brittle and brittle regimes: Examples from the Gavilgarh–Tan Shear Zone, central India; Geol. Mag. 145(6) 766–777.CrossRefGoogle Scholar
  4. Chester F M, Evans J P and Biegel R 1993 Internal structure and weakening mechanisms of the San Andreas Fault; J. Geophys. Res. 100 13,007–13,020.Google Scholar
  5. Cowan D S 1999 Do faults preserve a record of seismic slip? A field geologist’s opinion; J. Struct. Geol. 21(8) 995–1001.CrossRefGoogle Scholar
  6. Di Toro G and Pennacchioni G 2004 Superheated friction-induced melts in zoned pseudotachylytes within the Adamello tonalites (Italian Southern Alps); J. Struct. Geol. 26 1783–1801.CrossRefGoogle Scholar
  7. Gupta S N, Arora Y K, Mathur R K, Iqbaluddin B P, Sahai, T N, Sharma S B and Murthy M V N 1980 Lithostratigraphic map of Aravalli region, southern Rajasthan and northeastern Gujarat; Geol. Surv. India, Calcutta.Google Scholar
  8. Heron A M 1953 The geology of central Rajputana; Gov. India Press 79(1) 1–389.Google Scholar
  9. Kirkpatrick J D, Dobson K J, Mark D F, Shipton Z K, Brodsky E E and Stuart F M 2012 The depth of pseudotachylyte formation from detailed thermo-chronology and constraints on coseismic stress drop variability; J. Geophys. Res. Solid Earth 117B6.Google Scholar
  10. Krumbein WC 1941 Measurement and geologic significance of shape and roundness of sedimentary particles; J. Sedim. Petrol. 11 64–72.CrossRefGoogle Scholar
  11. Legros F, Cantagrel J M and Devouard B 2000 Pseudotachylyte (frictionite) at the base of the Arequipa volcanic landslide deposit (Peru): Implications for emplacement mechanisms; J. Geol. 108(5) 601–611.CrossRefGoogle Scholar
  12. Lin A 1994 Glassy pseudotachylyte veins from the Fuyun fault zone, northwest China; J. Struct. Geol. 16(1) 71–83.CrossRefGoogle Scholar
  13. Lin A 1996 Injection veins of crushing-originated pseudotachylyte and fault gouge formed during seismic faulting; Eng. Geol. 43(2) 213–224.CrossRefGoogle Scholar
  14. Lin A 1999 Roundness of clasts in pseudotachylytes and cataclastic rocks as an indicator of frictional melting; J. Struct. Geol. 21(5) 473–478.CrossRefGoogle Scholar
  15. Lin A 2008 Seismic slip in the lower crust inferred from granulite-related pseudotachylyte in the Woodroffe Thrust, central Australia; Pure Appl. Geophys. 165(2) 215–233.CrossRefGoogle Scholar
  16. Lin A and Shimamoto T 1998 Selective melting processes as inferred from experimentally generated pseudotachylytes; J. Asian Earth Sci. 16(5) 533–545.CrossRefGoogle Scholar
  17. Lofgren G 1980 Experimental studies on the dynamic crystallization of silica melts; In: Physical magmatic process (ed.) Hargraves R B, Princeton University Press, Princeton, New Jersey, pp. 487–551.Google Scholar
  18. Maddock R H 1983 Melt origin of fault-generated pseudotachylytes demonstrated by textures; Geology 11(2) 105–108.CrossRefGoogle Scholar
  19. Magloughlin J F 1992 Microstructural and chemical changes associated with cataclasis and frictional melting at shallow crustal levels: The cataclasite–pseudotachylyte connection; Tectonophys. 204(3–4) 243–260.CrossRefGoogle Scholar
  20. Magloughlin J F and Spray J G 1992 Frictional melting processes and products in geological materials: Introduction and discussion; Tectonophys. 204(3–4) 197–204.CrossRefGoogle Scholar
  21. McKenzie D and Brune J N 1972 Melting on fault planes during large earthquakes; Geophys. J. Roy. Astron. Soc. 29(1) 65–78.CrossRefGoogle Scholar
  22. Ray S K 1999 Transformation of cataclastically deformed rocks to pseudotachylyte by pervasion of frictional melt: Inferences from clast-size analysis; Tectonophys. 301(3) 283–304.CrossRefGoogle Scholar
  23. Ray S K 2004 Melt–clast interaction and Power-law size distribution of clasts in pseudotachylytes; J. Struct. Geol. 26(10) 1831–1843.CrossRefGoogle Scholar
  24. Reimold W U and Gibson R L 2005 ‘Pseudotachylite’ in large impact structures; In: Impact tectonics (eds) Koeberl C and Henkel H, Impact Studies Series, No. 6, Springer-Verlag, pp. 1–53.Google Scholar
  25. Sammis C, King G and Biegel R 1987 The kinematics of gouge deformation; Pure Appl. Geophys. 125(5) 777–812.CrossRefGoogle Scholar
  26. Shand S J 1916 The pseudotachylyte of Parijs (Orange free State), and its relation to ‘Trap-Shotten Gneiss’ and ‘Flinty Crush-rock’; Quart. J. Geol. Soc. London 72(1–4) 198–221.CrossRefGoogle Scholar
  27. Shimamoto T and Nagahama H 1992 An argument against the crush origin of pseudotachylytes based on the analysis of clast-size distribution; J. Struct. Geol. 14(8–9) 999–1006.CrossRefGoogle Scholar
  28. Sibson R H 1975 Generation of pseudotachylyte by ancient seismic faulting; Geophys. J. Roy. Astron. Soc. 43(3) 775–794.CrossRefGoogle Scholar
  29. Sibson R H 1977 Fault rocks and fault mechanisms; J. Geol. Soc. London 133(3) 191–213.CrossRefGoogle Scholar
  30. Spray J G 1995 Pseudotachylyte controversy: Fact or friction? Geology 23(12) 1119–1122.CrossRefGoogle Scholar
  31. Tsutsumi A 1999 Size distribution of clasts in experimentally produced pseudotachylytes; J. Struct. Geol. 21(3) 305–312.CrossRefGoogle Scholar
  32. Wadell H A 1932 Volume, shape, and roundness of rock particles; J. Geol. 40 443–451.CrossRefGoogle Scholar
  33. Wenk H R 1978 Are pseudotachylites products of fracture or fusion? Geology 6(8) 507–511.CrossRefGoogle Scholar
  34. Wenk H R and Weiss L E 1982 Al-rich calcic pyroxene in pseudotachylite: An indicator of high pressure and high temperature? Tectonophys. 84(2–4) 329–341.CrossRefGoogle Scholar

Copyright information

© Indian Academy of Sciences 2019

Authors and Affiliations

  • Arindam Sarkar
    • 1
  • Anupam Chattopadhyay
    • 1
    Email author
  • Tusharika Singh
    • 1
  1. 1.Department of GeologyUniversity of DelhiDelhiIndia

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